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Titanium nitride thin-film bias resistors for AC coupled segmented silicon detectors Master’s thesis Jennifer Ott University of Helsinki Faculty of Science Department of Chemistry Laboratory of Radiochemistry Supervisor: Prof. h.c. Dr. Jaakko Härkönen Helsinki Institute of Physics Abstract [on separate form] Silicon detectors with spatial resolution, such as strip and pixel detectors, are essential for highenergy physics experiments, where they are exposed to very high luminosities and particle fluences. Radiation causes damage in silicon and deteriorates detector performance. The main consequences of radiation damage are the increase in depletion voltage, the increase in detector leakage current leading to a worse signal-to-noise ratio, and the decrease in charge collection length and efficiency. One solution for mitigating the effect of increased leakage current is the use of capacitive coupling, which permits the separation of a signal from the leakage current in the front-end electronics. This requires a bias resistor for each detector segment, with a suitable value of resistance in order to allow biasing of the detector while isolating the individual segments from each other. In this thesis, titanium nitride (TiN) is studied as potential bias resistor material in the form of very thin films with thicknesses of around 20 nm. Its resistivity at smaller resistor dimensions is expected to be sufficiently high to permit the fabrication of very small segments, for example pixels with an area of 25 × 25 µm2. This would compensate the decrease in charge collection length and efficiency caused by radiation damage. Furthermore, TiN is expected to be far less sensitive to radiation damage than the polycrystalline silicon commonly used as bias resistor, and it can be deposited at lower temperatures compatible with standard semiconductor processing. In order to deposit suitable TiN thin films with good accuracy and repeatability, a new process of plasma-enhanced atomic layer deposition (PEALD) was developed. Crystallinity, conformality, surface morphology, chemical composition and resistivity of the films obtained this way were studied. The new PEALD process allowed for the deposition of good-quality TiN thin films at 300 °C from the routinely used and easily available precursors TiCl4 and NH3. Two TiN films were processed into bias resistor test structures, which were then tested for radiation hardness by irradiation with protons and photons of different energies. As expected, the radiation hardness of the resistors was excellent and none of the performed irradiations had any effect on the resistors’ performance. It was discovered that relatively short annealing treatments, which are used routinely for sintering of aluminium, increased the resistance of the test structures significantly. This presents a very simple and straightforward way to tune the resistance of the TiN resistor structures to a desired value. It was concluded that TiN resistors fabricated in the way presented in this thesis have great potential for use in segmented silicon particle detectors. Tiivistelmä [erilliselle lomakkeelle] Paikkaresoluutiota tarjoavat pii-ilmaisimet, kuten pikseli- ja raitadetektorit, ovat keskeisiä korkeaenergiafysiikan kokeissa, joissa ne kuitenkin altistuvat suurelle hiukkasvuolle ja korkeille säteilyannoksille. Säteily aiheuttaa vaurioita piissä ja heikentää detektorin toimintakykyä. Säteilyvauroiden pääasialliset seuraukset ovat tyhjennysjännitteen nousu, korkeampi vuotovirta, joka huonontaa signaali-tausta-suhteen, ja heikkenevä varauskeruutehokkuus ja –matka. Yksi mahdollinen ratkaisu korkean vuotovirran kompensoimiseksi on kapasitiivinen kytkentä, jolla signaali voidaan erottaa vuotovirrasta detektorielektroniikan avulla. Tähän tarvitaan oma biasvastus jokaiselle detektorin segmentille, jolla on sopiva sekä detektorin tyhjennystä että segmenttien erottamista toisistaan salliva resistanssi. Tässä työssä tutkitaan titaaninitridiä (TiN) bias-vastusmateriaaliksi ohuiden, noin 20 nm paksuisten ohutkalvojen muodossa. Sen resistiivisyyden odotetaan olevan myös pienillä vastusten mitoilla riittävä mahdollistamaan hyvin pienten detektorisegmenttien tuottamista, esimerkiksi 25 × 25 µm2 pinta-alan pikseleitä. Tällä voitaisiin kompensoida säteilyn aiheuttamaa laskua varauskeruutehokkuudessa ja –matkassa. Lisäksi TiN:n oletetaan olevan selvästi vähemmän herkkä säteilyvaurioille kuin yleisesti bias-vastuksena käytetty monikiteinen pii (polycrystalline silicon eli poly-Si), ja sitä voidaan kasvattaa alhaisemmissa, tavallisen puolijohdeprosessoinnin kanssa yhteensopivissa lämpötiloissa. Sopivien TiN-ohutkalvojen tarkkaa ja toistettavaa kasvattamista varten kehitettiin uusi prosessi plasma-avusteiseen atomikerroskasvatukseen (plasma-enhanced atomic layer deposition, PEALD). Kasvatettujen kalvojen kiteisyys, konformaalisuus, morfologia, kemiallinen koostumus ja resistiivisyys tutkittiin. Kehitetty PEALD-prosessi mahdollistaa hyvälaatuisten TiN-ohutkalvojen kasvattamista 300:ssa asteessa käyttäen hyväksi yleisesti käytetyt ja helposti saatavat lähdeaineet TiCl4 ja NH3. Kahta TiN-ohutkalvoa prosessoitiin bias-vastuksiksi ja näiden säteilynkestävyyttä tutkittiin säteilyttämällä niitä erienergisillä protoneilla ja fotoneilla. Kuten odotettiinkin, vastusten säteilynkestävyys oli erinoimainen, eikä mikään suoritetuista säteilytyksistä vaikuttanut kalvojen rakenteeseen taikka vastusten toimivuuteen. Suhteellisen lyhyiden, rutiininomaisesti alumiinikontaktien sintrauksessa käytettyjen kuumennusten huomattiin nostavan vastusten resistanssia selkeästi. Tämä tarjoaa yksinkertaisen ja suoraviivaisen tavan nostaa TiN-vastusten resistanssin halutulle tasolle. Edellisten perusteella pääteltiin, että tässä työssä valmistetut TiN-ohutkalvovastukset ovat erittäin lupaavia käyttöön paikkaherkissä pii-ilmaisimissa. Table of contents 1 Introduction .................................................................................................................................................... 1 2 Background ..................................................................................................................................................... 4 2.1 Interaction of ionizing radiation with matter .......................................................................................... 4 2.1.1 Electrons ........................................................................................................................................... 5 2.1.2 Charged particles .............................................................................................................................. 6 2.1.3 Photons ............................................................................................................................................. 8 2.1.4 Radiation damage cascade and displacement damage in solids.................................................... 10 2.2 Silicon detectors .................................................................................................................................... 12 2.2.1 Operation principle......................................................................................................................... 12 2.2.2 Radiation damage in silicon detectors .......................................................................................... 17 2.2.3 Bias resistors: properties of polycrystalline silicon; alternative materials ..................................... 20 2.3 Titanium nitride ..................................................................................................................................... 26 2.3.1 Structure and applications of titanium nitride ............................................................................... 26 2.3.2 Radiation damage in titanium nitride............................................................................................. 30 2.4 Atomic layer deposition......................................................................................................................... 35 2.4.1 Introduction to atomic layer deposition ........................................................................................ 35 2.4.2 Plasma-enhanced atomic layer deposition .................................................................................... 37 2.4.3 Thermal and plasma-enhanced atomic layer deposition of titanium nitride thin films: focus on TiCl4 as titanium source ........................................................................................................................... 39 3 Introduction to the experimental section .................................................................................................... 50 4 Materials and methods................................................................................................................................. 52 4.1 Plasma-enhanced atomic layer deposition of TiN thin films ................................................................. 52 4.1.1 Setup and experiments................................................................................................................... 52 4.1.2 Film characterization ...................................................................................................................... 57 4.2 Processing of TiN films into bias resistor structures ............................................................................. 58 4.2.1 Preliminary tests ............................................................................................................................. 58 4.2.2 Photolithography of TiN ................................................................................................................. 59 4.2.3 Photolithography of Al probing pads ............................................................................................. 64 4.3 Resistance measurements ..................................................................................................................... 67 4.4 Annealing ............................................................................................................................................... 68 4.5 Irradiation with 10 MeV protons ........................................................................................................... 70 4.5.1 Calibration ...................................................................................................................................... 73 4.5.2 Experiments .................................................................................................................................... 76 4.6 Irradiation with 24 GeV/c protons......................................................................................................... 77 4.7 Irradiation with gamma rays ................................................................................................................. 79 4.8 Irradiation with X-rays ........................................................................................................................... 79 5 Results and discussion .................................................................................................................................. 82 5.1 TiN thin films deposited by plasma-enhanced atomic layer deposition ............................................... 82 5.1.1 Process parameters ........................................................................................................................ 82 5.1.2 Film density and crystallinity .......................................................................................................... 85 5.1.3 Surface morphology and film conformality with field-emission scanning electron microscopy ... 91 5.1.4 Surface morphology with atomic force microscopy....................................................................... 94 5.1.5 Analysis of chemical composition................................................................................................... 97 5.1.6 Resistivity of TiN films and the influence of chemical composition and parameters .................. 101 5.1.7 Observations on film adhesion and film color.............................................................................. 105 5.2 Resistance measurements: Homogeneity of resistors over a wafer half ............................................ 106 5.3 Annealing ............................................................................................................................................. 108 5.4 Irradiation experiments ....................................................................................................................... 113 5.4.1 Irradiation with 10 MeV protons .................................................................................................. 113 5.4.2 Irradiation with 24 GeV/c protons ................................................................................................ 118 5.4.3 Irradiation with gamma rays ........................................................................................................ 119 5.4.4 Irradiation with x-rays .................................................................................................................. 120 6 Conclusions ................................................................................................................................................. 121 7 References .................................................................................................................................................. 128 1 Introduction High-energy physics research aims to provide answers to essential questions of particle physics. In the 21st century, answers to many problems have been found, but always new questions arise. Central questions in modern physics are the validity of the Standard Model, the existence of physics beyond the Standard Model, the reasons for the imbalance of matter and antimatter and the identity of dark matter. The fast development of physics and technology offers previously unseen opportunities to experimentally verify (or belie) existing theories that could help understanding the laws of physics and our universe. The most famous example of this is the discovery of a particle compatible with the Higgs boson (ATLAS Collaboration 2012) which was predicted theoretically almost 50 years earlier (Higgs 1964). This heavy particle with a mass of ca. 126 GeV (ATLAS Collaboration 2012, Giardino et al. 2012) is one manifestation of the Higgs field, which provides mass to elementary particles without violating gauge invariance and could be responsible for the breaking of electroweak symmetry (ALEPH Collaboration et al. 2003, Giardino et al. 2012). However, glancing into the realm of subatomic particles requires very high energies to overcome the strong force binding them together. These are reached by accelerating and colliding particles, most commonly protons, and studying the fragments and decay products emerging from the collisions. Since the start of its operation in 2010, the largest and most powerful accelerator has been the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland, with a circumference of 27 km. In its first run between 2010 and 2013, which led to the discovery of the potential Higgs boson particle, proton beam energies of 4 TeV and thus center-of-mass energies of 8 TeV were reached (Jakobs 2011). After an upgrade with the goal of reaching energies of 14 TeV, the LHC was restarted in 2015. Already now more powerful accelerators for reaching even higher energies are envisioned (Reich 2013). 1 A central role in high-energy physics experiments is played by the detector technology, because the fragments produced in the particle collisions have to be characterized as completely and accurately as possible. Conclusions on the nature of the collision event and further the constitution of the original particles can be made by applying the laws of conservation of energy and momentum to the experimental data. By analyzing the path of a fragment in a strong, external magnetic field, information about its momentum is obtained, and its stopping in a different, calorimetric detector allows determination of its energy. The detectors in high-energy physics experiments are large, multilayer constructions, because different types of particles – photons, hadrons, electrons – require different detectors to be detected most accurately, and high-energy particles can be stopped only in a large amount of absorbing material. (Hartmann and Sharma 2012) The particles continue into more particle-type specific calorimeter detectors only after passing the position-sensitive detectors in the experiments’ inner layer, the so-called trackers. Position sensitivity is achieved by dividing the detector into segments: pixels for spatial resolution in two dimensions as innermost detector layers, and strips for resolution in one dimension outside the pixel detectors (Hartmann 2012). Resolution in the third dimension is reached by installing several layers of segmented detectors. As opposed to the calorimeters, the trackers should disturb the measured particles as little as possible (Moser 2009), and are therefore based on light materials. Silicon detectors enable precise tracking through high granularity, fast response and good radiation hardness – in addition to the inexpensiveness and processability of silicon, these are the reasons why all four large LHC experiments use silicon detectors in their central tracking systems. (Turala 2005, Moser 2009, Hartmann 2012) The world’s largest silicon detector is hosted by the Compact Muon Solenoid (CMS) experiment: an active silicon detector area of approximately 210 m2 (Turala 2005, Hartmann 2012) contains 66 million pixels with areas of 100 × 150 µm2 and almost 10 million 80 µm × 10 cm microstrips (Hartmann 2012). 2 The silicon tracker detectors are located in the immediate proximity of the beam collision point. This exposes them to the highest particle energies and the highest luminosities, both of which are planned to be increased further in the future. For example the High Luminosity (HL) LHC upgrade, proposed to be performed after ten years of LHC operation, has the goal of reaching a luminosity of 1035 cm-2 s-1 (Jakobs 2011). This increases the already very high levels of radiation the tracker detectors have to resist. Many researchers, among them the RD-50 collaboration on radiation hard semiconductor devices for very high luminosity colliders, are working on improving the radiation hardness of silicon or on developing new detector concepts, electronic solutions and materials for mitigating the effects of radiation (Moll 2003, Casse 2009, Dierlamm 2010). In this thesis, the optimization of the radiation hardness of segmented silicon detectors is approached from the side of detector layout and front-end readout electronics. The decrease in charge collection length and efficiency at high radiation fluences is sought to be compensated by increasing the granularity of the detector, i.d. decreasing the pixel size. In addition, the concept of AC coupling to isolate the leakage current from signals, which is already employed in e.g. the CMS microstrip detector (Hartmann 2012) is aimed to be extended to the pixel detectors. An essential component in an AC coupled detector is the bias resistor, which in order for the previous aims to be possible, needs to enable the fitting of a sufficiently high resistance density into a small space. In the experimental section of this thesis, titanium nitride thin-film bias resistors are deposited, characterized and tested for their radiation hardness. 3 2 Background In this chapter, background information about silicon detectors and the concept of AC coupling is given. The macroscopic effects of radiation damage in silicon detectors are summarized, based on the basic principles of the interactions of radiation with matter and especially solid matter. The method of choice for thin film deposition in this case, atomic layer deposition, is introduced, and literature on the deposition, structure and applications of titanium nitride is reviewed. 2.1 Interaction of ionizing radiation with matter The interaction of radiation with matter can be examined from many angles. In the following, the different types of interaction of fast electrons, light charged particles – in particular protons and alpha particles – and high-energy photons with atoms are summarized. These are further divided into interactions with electrons, which are responsible for the majority of energy loss of radiation in an absorber material, and interactions with the nucleus or the nuclear field. As neutral particles, neutrons do not interact with the atom’s electron cloud at all. In a material, neutrons lose energy by inelastic scattering (moderation) or are captured, which leads to nuclear reactions or even fission. Moderation is more effective for light elements, while neutron capture occurs more readily in heavy materials with a strong dependence on the resonance structure of the nuclei of individual isotopes. (Lamarsh and Baratta 2001) The behavior of neutrons is not covered in detail on a nuclear scale, but is discussed again in Chapter 2.1.4 for microscopic effects of radiation in an absorber. 4 2.1.1 Electrons Electrons do not participate in the strong interaction and therefore do not interact with the nucleas via nuclear excitation or nuclear reactions. However, electrons that are accelerated in the electric field of a nucleus emit bremsstrahlung radiation (Equation 1). 𝑑𝐸 𝑁𝐸𝑍(𝑍 + 1)𝑒 4 2𝐸 4 −( ) = (4 ln − ) 2 2 4 𝑑𝑥 𝑟 𝑚0 𝑐 3 137𝑚0 𝑐 (1) dE where − dx is the energy loss over a certain path length, v is the velocity and ze the charge of the primary particle. N is the number density and Z the atomic number of the absorber material, and I an experimentally determined parameter describing the excitation and ionization potential of the absorber. m0 is the mass of the electron and 1/137 the fine structure constant. (Knoll 2010) The energy loss of electrons by bremsstrahlung is central in heavy materials and for high electron energies. The total energy loss of an electron in an absorber material consists of radiative energy loss by bremsstrahlung emission and Coulomb interactions (Equation 2) with other electrons, in the form of excitation and ionization. (Choppin et al. 2002, Knoll 2010) −( 𝑑𝐸 2𝜋𝑒 4 ) = 𝑁 𝑍 (ln 𝑑𝑥 𝐶 𝑚0 𝑣 2 𝑚0 𝑣 2 𝐸 𝑣 2 𝑣 2 √ − (ln 2) (2 1 − ( ) − 1 + ( ) ) 𝑣 2 𝑐 𝑐 2 2𝐼 (1 − ( 𝑐 ) ) 2 𝑣 2 1 𝑣 2 + 1 − ( ) + (1 − √1 − ( ) ) ) 𝑐 8 𝑐 (2) v2 For non-relativistic particles, the terms containing c2 can be neglected, however, electrons reach relativistic velocities already at relatively low energies due to their small mass. 5 As fast electrons have the same (low velocities) or a similar mass (relativistic velocities) as the their counterparts in the absorber material, they can transfer more energy in a single collision. For the same reason, the direction of a fast electron can change more drastically in collisions than for heavier particles, which explains the relatively long and complicated paths characteristic for electrons. The presented interactions are the same for the positron, the electron’s positively charged antiparticle. In addition, after reaching thermal energies, positrons annihilate with absorber electrons, which gives rise to two characteristic 511 keV gamma photons. 2.1.2 Charged particles Protons, alpha particles and heavier charged particles, such as carbon ions, transfer energy to the oppositely charged electrons via Coulomb forces, either exciting or directly ionizing the absorber material’s atoms. The Bethe formula (Equation 3) describes the dE energy loss over a certain path length − dx (Knoll 2010): − 𝑑𝐸 4𝜋𝑒 4 𝑧 2 2𝑚0 𝑣 2 𝑣2 𝑣2 = 𝑁 𝑍 (𝑙𝑛 − ln (1 − ) − ) 𝑑𝑥 𝑚0 𝑣 2 𝐼 𝑐2 𝑐2 (3) Even though the energy loss of a high-energy charged particle in a single collision is small compared to the particle’s total kinetic energy, the literature does not speak of elastic scattering from electrons for charged particles the same way as for electrons; it is probably unlikely due to the large mass and charge of the particle radiation. However, the phenomenon of elastic scattering of charged particles from the nucleus, known after its discoverer as Rutherford scattering, has been central in the development of the atomic model with extremely small nuclei carrying most mass of an atom (Kónya and Nagy 2012). 6 Inelastic scattering from the atomic nucleus also occurs for charged particles. The occurrence of nuclear excitation, from which the nucleus can relax by internal conversion or by emitting a gamma ray, exhibits a complicated dependence on angle and center-ofmass energy of the collision, and on the wave functions of nucleus and incident particle. (Kantele 1995) The transfer of kinetic energy from the incident particle to a nucleus and thus to the atom is especially important in solids, where the displacement of an absorber atom from its lattice site (Equation 4) forms defects (more in Chapter 2.1.4). 𝑛𝑑𝑖𝑠𝑝 = 𝐸𝑡𝑟 2𝐸𝑑𝑖𝑠𝑝 (4) where ndisp is the number of displaced atoms, Etr the energy transferred by the primary particle, and Edisp is the energy required to for the displacement of one atom. (Choppin et al. 2002) Instead of being scattered elastically or inelastically, a charged particle can be captured by the nucleus to induce a nuclear reaction, where one or several particles are exchanged and the nucleus is transmuted into an often unstable daughter nucleus (activation). (Kantele 1995, Choppin et al. 2002) Normally, a nuclear reaction can only occur if the incident particle has sufficient energy to overcome the Coulomb barrier between itself and the likewise positively charged nucleus, which means e.g. energies of approximately 4.5 MeV for a proton or 8 MeV for an alpha particle, for a target nucleus mass number of 50 (Kantele 1995). Although tunnelling through the Coulomb barrier might occur, nuclear reactions are usually negligible for lower energies. There is no generally applying equation (in forms like e.g. Equation 3) for probabilities of nuclear reactions; they are instead described by numerical, experimentally determined reaction cross-sections. These depend on the individual isotope and the energy of the incident particle, and are characteristic for the nuclear reaction in question. The cross-sections for (p, n) and (p, α) reactions, for example, are different. 7 2.1.3 Photons Of the radiation types discussed here, gamma rays undergo the widest range of interactions with matter. Elastical scattering of gamma rays from electrons plays a role only in very heavy absorbers, such as lead, and low gamma energies of less than approximately 100 keV (Kantele 1995, Choppin et al. 2002). Scattering from bound electrons or entire atoms is called Rayleigh scattering, while scattering from free electrons, as found in metals, is known as Thomson scattering. Elastic scattering from a nucleus, also known as Delbrück scattering (Kantele 1995), is often overlooked in the literature. The probability of Rayleigh scattering is agreed to be proportional to Z2; however, both Thomson and Delbrück scattering are proportional to Z4 according to Kantele (1995), but only to Z according to Kónya and Nagy (2012). For photons with energies between 0.1 and 1 MeV, inelastic Compton scattering from electrons ist the primary interaction mechanism. The energy of an inelastically scattered photon depends on its angle according to Equation 5 (Knoll 2010). 𝐸𝛾′ = 𝐸𝛾 1+ 𝐸𝛾 (1 − cos 𝜃) 𝑚0 𝑐 2 (5) where Eγ is the energy of the photon before scattering, Eγ′ its energy after scattering, and θ the scattering angle. The probability of Compton scattering increases proportionally to Z and N, but decreases with increasing photon energy (Kantele 1995). Inelastic scattering from or absorption by the nucleus leads to the excitation of the latter. A special example of this is the Mössbauer effect, which describes the recoil-free emission and absorption of gamma rays by a an isotope/nuclear isomer system and is used for analysis of chemical composition (Choppin et al. 2002). Absorption of gamma rays by the nucleus can induce nuclear reactions, such as (γ, p) or (γ, n), but these demand photon energies that are extremely rare in natural decays, for example 10-20 MeV for some metals (Phillips et al. 1999). 8 Gamma rays can also transfer their entire energy to an electron, expelling it from its shell. The same is possible for any photon whose energy is at least the binding energy Eb of an electron, and is known as the photoelectric effect. The energy of the electron Ee is given by Equation 6 (Knoll 2010). 𝐸𝑒 = 𝐸𝛾 − 𝐸𝑏 (6) For gamma rays, the coefficient for the photoelectric effect is proportional to Z (Kónya and Nagy 2012) and to N5 and Eγ−3.5 (Kantele 1995). The perhaps most exotic property of gamma rays is the ability for pair production, the inverse process of positron-electron annihilation: theoretically, any photon with higher energy higher than 1.022 MeV (twice the rest mass of the electron) can transform into an electron and a positron upon interaction with a third body, usually a nucleus (Knoll 2010). In reality, pair production becomes significant only at higher energies, e.g. at 2 MeV in Al or 9 MeV in Cu (Kantele 1995). The probability for pair production increases in a slightly different way depending on the photon energy range, as seen in Equations 7a and 7b for energies E > 1 MeV and E >> 1 MeV, respectively (Kantele 1995). 𝜅 ∝ 𝑁 𝑍 (𝐸 − 1.022 𝑀𝑒𝑉) (7a) 𝜅 ∝ 𝑁𝑍 2 ln 𝐸 (7b) where E is the photon energy and κ the coefficient for pair production. 9 2.1.4 Radiation damage cascade and displacement damage in solids The effects of irradiation on any material are based on the entirety of the interaction processes described above. In the following, the microscopic mechanisms for radiation damage in solids are introduced. Free electrons generated in the interaction of a heavier primary particle with the absorber material can carry a substantial amount of kinetic energy, and cause a cascade of further excitation and ionization in the vicinity of the primary particle’s path. Such electrons are often referred to as delta rays. (Mozumder 1997, Was 2007) The events of an electron cascade are presented in Table 1. Table 1. The timescale of events and their results in an electron cascade (after Race et al. 2010 and lectures in Radiation Chemistry, M. Paronen and S. Makkonen-Craig 2014). Time (s) Event Result 10-17 - 10-16 10-13 - 10-12 Energy transfer from the incident electron Energy transfer from delta rays to other electrons Electron-phonon coupling ca. 10-10 Cooling of the lattice Excitation of electrons, ionization – Coulomb explosion Electron cascade, cooling of hot electrons Energy transfer to atoms, lattice heating, local melting – thermal spike Recombination of defects; defect stabilization, cluster formation 10-15 - 10-14 The same principle also applies when a high amount of kinetic energy is transferred to an entire atom, which is displaced from its site in the crystal lattice as primary knock-on atom (PKA). However, the PKA can initiate a cascade of other displaced atoms when its energy is reduced sufficiently for it to interact with other atoms and ions (Race et al. 2010). Displacement of an atom principally forms two defects: a vacancy at the atom’s original position, and a self-interstitial at the site the displaced atom stopped at. These pairs of self-interstitials and corresponding vacancies are called Frenkel pairs, and are representants of point defects. (Abromeit 1994, Was 2007) The events of a displacement cascade and their timescales are collected into Table 2. 10 Table 2. The timescale of events and their results in the displacement cascade (after Was 2007). Time (s) Event Result 10-18 Energy transfer from the incident particle Displacement of lattice atoms by the PKA Energy dissipation, spontaneous recombination and clustering Defect reactions by thermal migration Creation of a primary knockon atom (PKA) Displacement cascade 10-13 10-11 > 10-8 Stable Frenkel pairs (interstitial and vacancy) and defect clusters Interstitial and vacancy recombination, clustering, trapping, defect emission Individual point defects can move in the crystal lattice by diffusion, and have a tendency to accumulate together with other defects, forming defect complexes or even larger clusters. The formation of point or cluster defects depends to some extent on the radiation type: neutron and high-energy proton irradiation produces clusters, photons and electrons primarily point defects (Abromeit 1994, Luukka 2006). Radiation damage caused by neutrons is often simulated by irradiation with charged particles due to the inconveniences associated with accessing facilities for neutron irradiation, the year-long scale of the manifestation of neutron radiation damage and the difficulty to vary irradiation parameters (Abromeit 1994). Damage caused by various types of particle radiation is often scaled to damage done by 1 MeV neutrons (e.g. Moll et al. 1999, Luukka 2006), collecting contributing factors such as the particle type, energy and fluence, as well as properties and thermal history of the absorber material, into a correlation coefficient known as the hardness factor. The quantity correlated is the number of displaced atoms, which implies that only nonionizing energy loss (NIEL) is taken into account. Due to the essential differences between different types of radiation, the comparison between neutrons and for example charged particles should be taken with caution. (Abromeit 1994, Moll 1999, Moser 2009) 11 2.2 Silicon detectors Chapter 2.2.1 briefly explains the operation principle of silicon detectors relying, unless otherwise indicated, on the excellent review articles of Dijkstra (2002), Moser (2009) and Hartmann (2012). Chapter 2.2.2 presents the effects of radiation damage on silicon detectors based mainly on Lindström et al. (1999), Moll (1999), Lindström (2003) and Luukka (2006); for a more detailed analysis, the reader is directed to these references. 2.2.1 Operation principle Silicon is an intrinsic semiconductor with a band gap of 1.1 eV. In order to define its electrical properties, most importantly carrier concentration and thus conductivity, silicon is implanted (doped) with ions. These introduce additional energy levels into the band gap of silicon, and are divided into two groups according to their electron structure. Group-III dopants, such as indium or boron, have one electron less than silicon and therefore act as electron acceptors or alternatively hole donors. Acceptors form energy levels close to the valence band, with ionization energies of approximately 0.05 eV. Silicon doped with group-III atoms is referred to as p-type silicon. The opposite of this is n-type silicon, which is achieved by doping with group-V atoms that have an additional electron compared to silicon. Examples of these electron donor dopants are phosphorus and arsenic. Their energy levels are close to the conduction band and have even lower ionization energies, around 0.025 eV. (Sze and Ng 2007) The operation of silicon detectors is based on the properties of the boundary between pand n-doped silicon. Typically, a detector consists of an n-type bulk with a smaller, but more heavily doped p-type region, denoted as p+. The backside often contains a thin n+ layer. When p-and n-type regions are brought into contact with each other, the respective charge carriers diffuse to some extent into the other region: holes diffuse from p- into n-type, electrons in the opposite direction. This leads to an annihilation of charge carriers at the p-n junction, leaving a space charge region, also called depletion zone, without mobile carriers. The stationary dopant atoms remain there as positive (n-type) or negative (p-type) ions, creating an electric field. 12 By applying a reverse bias, i.e. a positive voltage to the n-type region and negative voltage to the p-type region, more charge carriers are drawn towards the electrodes and the space charge region is widened. The thickness d of the space charge region is expressed according to Equation 8: 𝑑= 2𝜖 𝑉𝑟𝑒𝑣 𝑞𝑁 (8) where 𝜖 is the dielectric constant of silicon, q the elementary charge, Vrev an applied reverse bias, and N the doping concentration. Full depletion, when the space charge region extends through an entire silicon device with thickness D, is achieved by applying a sufficiently high voltage Vfd (Equation 9). 𝑉𝑓𝑑 𝑞𝑁𝐷2 = 2𝜖 (9) At this point, the silicon sensor can be compared to a capacitor (Equation 10) with a capacitance C, area A and distance d between the electrodes: 𝐶= 𝐴𝜖 𝑑 (10) A particle or photon with enough energy can excite electrons from the valence to the conduction band. This leaves a hole in the valence band and effectively creates an electron-hole pair. The energy required for the formation of such a pair in silicon is ca. 3.6 eV. Due to the electric field generated by the dopant ions in the space charge region, the new charge carriers drift towards the electrodes (Equation 11) with a velocity v ⃗: 𝑣 = 𝜇𝐸⃗ (11) where ⃗E is the electric field and µ is the mobility of the charge carrier. The mobility in silicon is almost three times higher for electrons than for holes. From the detector, the signal is transferred to the readout electronics. Perhaps the most critical part of these is the preamplifier, which can hamper the detector performance and add to the background noise if it is not optimized. Detailed information about the synergy between actual detector and preamplifier is provided by De Geronimo et al. (2001). 13 In strip and pixel detectors, the p+ region into isolated smaller segments. Holes generated by ionizing radiation are drawn towards a strip or pixel by its individual electric field, as indicated in Figure 1. Spatial resolution is obtained by reading out each segment separately through its own preamplifier and further readout channel. Figure 1. Schematic cross-section through a strip detector (Hartmann 2012). For both readout and biasing of the detector, different arrangements are possible, but at least in silicon strip detectors the standard is AC coupling. AC coupling, also known as capacitive coupling, can be used to transfer an AC signal between circuit nodes while still keeping them separated from each other at different DC potentials. This is achieved by a coupling capacitor that blocks the DC component of an incident current – a DC current can charge a capacitor, but not pass through it – but transfers the AC component. The propagation of the AC signal can be visualized as a periodic charging-discharging of the capacitor, which is, due to the nature of a capacitor, noticeable at both electrodes, not only at the one receiving the signal. The amplitude of current and voltage of the transferred AC signal and their phase difference is determined by the capacitor’s impedance. The coupling capacitor is formed by the system of p+ implant and metallization, which act as electrodes, and the SiO2 dielectric between them. Biasing and transmission of the DC current blocked by the coupling capacitor occurs via the individual bias resistors of each strip, which are connected to a common bias line. A schematic of an AC coupled strip detector seen from above is presented in Figure 2. 14 Figure 2. Schematic of an AC coupled strip detector (Hartmann 2012). A picture of a similar AC coupled detector visualizes that each strip has its own bias resistor (Figure 3). Figure 3. Example of an AC coupled strip detector, courtesy of J. Härkönen. 15 As opposed to strip detectors, for example the pixels in the CMS tracker are DC coupled, and the metallization is in direct contact with the p+ region. In this configuration, socalled punchthrough resistors connected to an area of undoped silicon are used for biasing– these consume considerable space (ca. 30 × 30 µm2), and the undoped silicon does not contribute to signal detection (Figure 4). Figure 4. DC coupled pixels as found in the CMS Pixel Detector with punch-through resistors in the lower right corners. Courtesy of J. Härkönen. 16 2.2.2 Radiation damage in silicon detectors The interaction of radiation in silicon follows the mechanisms described in Chapter 2.1. The observable macroscopic effects of radiation damage on detector properties trace back to the defects formed during the displacement cascade caused by an incident energetic particle. In silicon, said defects exhibit certain electric properties that interfere with detector performance: radiation-induced defects form additional states in the semiconductor’s band gap. Depending on their position, more correctly energy, with respect to the valence and conduction bands, these defects act in different ways (Figure 5). Figure 5. Positions and effects of radiation-induced defects in the silicon band gap (Hartmann 2012). The behavior of radiation-induced charged defects is very similar to that of the dopant ions introduced into the silicon bulk intentionally (cf. Chapter 2.2.1). They occupy energy levels in the band gap close to the valence or conductivity bands, for acceptors and donors, respectively. Accordingly, this increases the doping concentration and therefore the voltage needed to fully deplete the detector, at high radiation fluences up to several hundreds of volts. High voltages are less convenient for both electronics and the detector itself, which can break down at too high voltages. 17 It has been observed that the majority of radiation-induced charged defects is of the acceptor type – for a common silicon detector, the initial n-type doping is slowly compensated until the Neff is zero, and further irradiation results in effective p-type doping. This process is referred to as type inversion (Figure 6). Figure 6. Effective doping concentration and depletion voltage as function of particle fluence, clearly visible type inversion (Moll 1999). Consequences of the production of positively charged defects is also visible at the detector surface. Irradiation leads to an accumulation of positive charge in the insulator oxide and the formation of silicon-oxide interface traps. In segmented detectors, this increases the inter-segment capacitance and can cause unwanted signal sharing. (Luukka 2006) The effect of neutral defects in detector silicon depends on their position in the band gap. Defects situated in midgap generate charge carriers without the influence of incident particles, which increases the detector’s leakage current even at full depletion. The leakage current is proportional to the fluence, with a material-dependent constant known as the α-parameter (Figure 7). 18 Figure 7. Leakage current as a function of fluence for different types of silicon (Moll 1999). Neutral radiation-induced defects at energy levels further from midgap, on the other hand, are significant due to the trapping of charge carriers. This does not compensate the increase in leakage current mentioned above, but instead concentrates on the “real” charge carriers generated by an incident particle. Due to trapping, the charge collection efficiency (CCE) decreases: fewer charge carriers can reach the electrodes and the counts in a signal are reduced. For the same reason the charge collection length (CCL), i.e. the path length charge carriers can move before becoming trapped, also decreases. This contributes to the reduction of the signal counts and furthermore causes a deterioration of resolution, as large parts of the detector are effectively rendered dead space. The decrease in CCE and CCL reducing the intensity of a signal, in combination with the abovementioned increase in leakage current (i.e. background noise), leads to a decrease in the signal-to-noise ratio, which makes both observation and identification of a signal more difficult. 19 2.2.3 Bias resistors: properties of polycrystalline silicon; alternative materials The standard material for bias resistors in segmented silicon detectors is polycrystalline silicon (poly-Si). Poly-Si has also been used in bipolar silicon applications (Enlow et al. 1991, Monizurraman et al. 1999), in thin-film and field-effect transistors (Inoue et al. 2003, Huang et al. 1983), for metal-oxide-semiconductor systems (Monizurraman et al. 1999, Chen et al. 2001, Inoue et al. 2003) and as gate electrode or interconnection layer (Huang et al. 1983, Mahan et al. 1983), and in static random-access memory devices (Mahan et al. 1983, Saito et al. 1985). As opposed to the monocrystalline silicon used as detector bulk material, poly-Si consists of many smaller crystallites with grain sizes between 10 and 1000 nm (Lu and Lu 1984), often with an average grain size of around 30 nm (Huang et al. 1983, Saito et al. 1985). The difference betwwn monocrystalline silicon and poly-Si is demonstrated well in Figure 8. The average grain size depends on many factors: the smoothness of the substrate (Moniruzzaman et al. 1999), the conditions of the chosen film deposition method and a possible crystallization process (Pereira et al. 2009), and doping, which has been found to increase grain size (Lu and Lu 1984, Rydberg and Smith 2000). Figure 8. Transition electron microscope image of a B doped poly-Si sample on a monocrystalline silicon substrate (Nakabayashi et al. 2002). 20 For applications in silicon detectors, poly-Si is deposited by low-pressure chemical vapor deposition (LPCVD) as product of the pyrolysis of silane (SiH4) or disilane (Si2H6) in H2 at temperatures of 425 °C (Inoue et al. 2003), 630-650 °C (Huang et al. 1983, Mahan et al. 1983, Saito et al. 1985, Lee et al. 1999, Rydberg and Smith 2000, Chen et al. 2001) up to 790 °C (Caccia et al. 1984). Other deposition methods are plasma-enhanced CVD at 300 °C by decomposition of a SiH4/SiF4 mixture (Moniruzzaman et al. 1999), atmosphericpressure CVD from SiH4 at 1050 °C (Lu and Lu 1984), and physical vapor deposition, such as sputtering (Pereira et al. 2009). Silicon films can be deposited at lower temperatures, but are then in an amorphous state and require crystallization, either by high temperatures of 600-900 °C (Pereira et al. 2009) or by crystallization with a laser (Inoue et al. 2003, Pereira et al. 2009). Film thicknesses are mostly around 500 nm (Huang et al. 1983, Mahan et al. 1983, Caccia et al. 1984, Saito et al. 1985, Ziock et al. 1991, Rydberg and Smith 2000), thinner films around 50 nm (Inoue et al. 2003) or thicker films around 1 µm are seen less frequently (Moniruzzaman et al. 1999, Chen et al. 2001). Similarly to monocrystalline detector silicon, poly-Si is implanted with ions in order to achieve the desired electrical properties and resistivity. The most typical dopant atoms are boron as electron acceptor and phosphorous or arsenic as electron donor dopants. (Lu and Lu 1984) Resistivity as a function of dopant concentration is presented in Figure 9. Figure 9. Dependence of the resistivity of poly-Si on dopant concentration (French 2002). 21 After their implantation into the poly-Si film, the dopants need to be activated. This is achieved by high-temperature annealing at 950-1050 °C (Mahan et al. 1983, Rydberg and Smith 2000, Chen et al. 2001). In order to establish ohmic contacts to the metallization and bias line, the ends of the poly-Si resistors are doped more heavily in an additional step (Huang et al. 1983, Saito et al. 1985). The electrical properties of poly-Si, most centrally resistivity and dependence of the dependence of it on temperature, are to the largest extent determined by the boundaries between the individual grains. The grain boundaries are often described as amorphous material between the crystallite grains, which essentially correspond to monocrystalline silicon. Charge carriers are transferred from one grain to another by thermoionic emission (Lee et al. 1999), tunnelling (Huang et al. 1983) or diffusion (Mahan et al. 1983), but can be trapped by dangling bonds and trapping states found at the grain boundaries. (French 2002) Dopants or unwanted impurity atoms, such as hydrogen from the LPCVD deposition process, interact explicitly at the grain boundaries. Hydrogen is bound by trapping states and dangling bonds and saturates them, but only loosely, which leads to an instability of resistivity over time due to the possible reformation of traps when hydrogen is removed. The breaking of Si-H bonds is also associated to poly-Si degradation by so-called self-heating (Inoue et al. 2003). When the poly-Si film is doped with arsenic or phosphorous (i.e. electron donor dopants), these atoms segregate to the grain boundaries and replace hydrogen (Lee et al. 1999, Mahan et al. 1983). This reduces resistivity, and renders the dopants inactive. Boron, however, does not interact with the grain boundaries to the same extent, and can be used to compensate the low resistivity caused by too high donor doping (Rydberg and Smith 2000). Simultaneous doping with boron and phosphorous can lead to the formation of BP complexes that act as traps for holes (Rydberg and Smith 2000). 22 Because of its use in segmented silicon detectors, the radiation hardness of poly-Si has been studied. Experiments indicate that the performance of poly-Si is strongly affected by both charged particles and photons, which cause clear changes in resistance and even lead to resistor failure (Zhang et al. 2011, Ziock et al. 1991, Enlow et al. 1991). Ziock et al. tested the radiation hardness of AC coupled silicon microstrip detectors with 8.2-8.9 MΩ poly-Si bias resistors, intended for use in the Superconducting Super Collider, by irradiation experiments with 800 MeV protons (9×1014 p/cm2), 1 MeV neutrons (3×1013 n/cm2), and Co-60 gamma rays of approximately 1.3 MeV. While neutron irradiation had no apparent effect on the poly-Si resistors, the authors noticed an increase in resistance of 7.5 % under proton irradiation. Gamma ray irradiation had an even more distinct impact on resistance, which increased 15 % already after a total dose of 0.2 Mrad, equivalent to 2 kGy. In addition, the difference between the resistance values of the resistors at opposite ends of the strip detector increased. The influence of the gamma irradiation was attributed to surface effects, which are suspected to have caused an accumulation of fixed charges in the oxide layers in contact with the poly-Si resistor, leading to a partial depletion of the poly-Si. Despite the deficient radiation hardness of the studied resistors, it was concluded that bulk damage in the detector silicon is the limiting factor for detector performance. However, the reduction of resistance to e.g. 250 kΩ is suggested, also in order to reduce the risk of punchthrough at high detector leakage currents. (Ziock et al. 1991) Zhang et al. investigated damage caused by 12 keV X-rays in silicon sensors and p-doped poly-Si resistors in AC coupled samples, in anticipation of the doses of 1 GGy expected within three years of operation at the X-ray Free-Electron Laser. Concurring with Ziock et al. (1991), the effects of X-rays were expected to consist mainly of oxide charge as well as interface trap formation. After X-rays doses of 1 and 10 MGy, the resistance of the poly-Si bias resistors increased from the original 0.5 MΩ to 0.6 and 1.0 MΩ, respectively. After 100 MGy, resistance exceeded 100 MΩ, corresponding to a resistor failure. The removal of free holes in the resistors’ low dose p+ implants due to the positive oxide and interface charges was presented as possible explanation for resistor failure. (Zhang et al. 2011) 23 The theories presented by Ziock et al. and Zhang et al. for the mechanisms of photoninduced damage are further supported by Enlow et al., who studied the gain degradation induced in microcircuit bipolar poly-Si structures by 60Co gamma radiation. Gain degradation, which was observed to be higher for low dose rates and to continue even during room temperature annealing, is also attributed to an increase of surface recombination velocity due to interface traps, and depletion caused by oxide trapped charge. (Enlow et al. 1991). Even though this is only briefly mentioned by Zhang et al. (2011), it is logical to assume that radiation damage affects polycrystalline silicon through similar mechanisms as are observed in monocrystalline bulk silicon. Most importantly, it is considered very probable that poly-Si is subject to a compensation of the effective doping by radiation-induced defects, which act as acceptors and to a lesser extent as donors. Since poly-Si resistors are already heavily doped to begin with, already low fluences can result in donor concentrations of 1019 cm-3, after which drastic changes in resistivity are observed (cf. Figure 9). The above indicates that poly-Si resistors are very sensitive to radiation damage and cannot be considered reliable at high fluences, especially not at the extreme luminosities and fluences planned in future high-energy physics experiments. One alternative for poly-Si as resistor material studied from the year 2011 onwards is tungsten nitride (WNx). When deposited by reactive sputtering at low temperatures, WNx is amorphous, making it assumably more radiation hard than poly-Si. However, repeatability of the film thickness between batches and uniformity of the deposited thin film over an entire wafer cannot be assured with reactive sputtering. Too clear differences in film thickness and therefore resistivity are not acceptable for the application in question, as they would lead to differences in the performance and response of the individual detector segments. 24 Tracker detectors should disturb the particles they measure as less as possible, but tungsten is a very heavy element (atomic number 74) and interacts much more strongly with radiation than silicon (atomic number 14), since the probability of interaction for all types of radiation increases with atomic number of the material (cf. Chapter 2.1). In addition, WNx has a high density of 7.7-17.7 g/cm3, depending on its stoichiometry (Haynes 2015). For the mentioned reasons, WNx deposited by reactive sputtering is not the first choice for a new bias resistor material. However, metallic bias resistors remain very interesting due to their doping-independent resistivity and generally better radiation hardness. Transition metal nitrides, which are mostly metallic materials, are preferred over instead of elemental metals: their deposition with chemical vapor deposition methods is easier, their resistivities are usually higher, and they are more resistant to temperature and oxidation or sulfurization than elemental metals. One of the most well-known metallic transition metal nitrides is titanium nitride (TiN). 25 2.3 Titanium nitride 2.3.1 Structure and applications of titanium nitride Titanium nitride, denoted as TiN, is an inorganic compound from the group of transition metal nitrides and consists of titanium – in its less common oxidation state +III – and nitrogen. Like most transition metal nitrides, TiN is described as an interstitial compound (Cotton and Wilkinson 1988) or a solid state solution of nitrogen in titanium (Atkins et al. 2010), where the smaller nitrogen atoms occupy octahedral holes in the original titanium lattice. TiN forms a typical sodium chloride structure with individual, alternating facecentered cubic lattices for titanium and nitrogen in which each atom has six neighbours at octahedral positions (Cotton and Wilkinson 1988, Figure 10). In an ideal case, the N/Ti ratio is 1:1, but both over- and understoichiometric films have been reported. TiN can tolerate a rather wide range of stoichiometry, with N/Ti ratios from 0.2 up to 1.6 (Sundgren 1985), but extreme stoichiometries can also be a sign of the presence of a different, usually more unstable and uncommon phase, such as Ti2N or Ti3N4. Figure 10. Crystal structure of titanium nitride, white: Ti, green: N (<https://upload.wikimedia.org/wikipedia/commons/thumb/c/c0/NaCl_polyhedra.png/700pxNaCl_polyhedra.png>, 6.10.15). 26 TiN shows an exceptional combination of properties, with characteristics of both metals and covalent compounds (Niyomsoan et al. 2002). TiN is electrically conductive with reported bulk resistivities as low as ca. 20 µΩcm (Elam et al. 2003, Sundgren 1985) and has a reflecting luster – both are typical properties of metals (Cotton and Wilkinson 1988). Its optical response is similar to gold and therefore gives it a gold-resembling yellowish colour (Sundgren 1985, Patsalas and Logothetidis 2001). However, TiN has a very high hardness of 9 on the Mohs and 1770 on the Knoop scale – compared to diamond with Mohs hardness 10 and Knoop hardness 7000 (Haynes 2015) – and is brittle, as opposed to the mostly ductile elemental metals. TiN also has a high melting point at 2947 °C, a boiling point is not defined due to decomposition of the material before vaporization (Haynes 2015), and is chemically rather inert (Wittmer et al. 1983, Cotton and Wilkinson 1988, Schubert and Hüsing 2005, Atkins et al. 2010). This combination of properties indicates that the bonds in TiN have both a metallic nature with close-packed atoms and delocalized electrons (Cotton and Wilkinson 1988) and a certain covalent character (Atkins et al. 2010). X-ray photoelectron spectroscopy revealed that contribution of Ti 3d, 4p and 4s states to the N 2p valence band is much stronger than for the corresponding O orbital in TiO 2 – this stronger interaction could offer an explanation for the metallic character of TiN (as opposed to the semiconductor/insulator TiO2) as well as the high hardness more typical for covalently bound compounds (Song et al. 1998). On the other hand, the Ti 3d orbital is not completely hybridized with the N 2p orbital (Patsalas et al. 2015), leaving 3d electrons to titanium that are not present in elemental metals. The Fermi level of TiN lies in the valence band of these Ti 3d electrons, explaining the electrical conductivity. (Patsalas and Logothetidis 2001). The very high hardness, good wear stability and chemical inertness of TiN make it widely used as protective coating for many kinds of tools, increasing their edge retention, corrosion resistance and wear resistance (Sundgren 1985, Chatterjee et al. 1992, Zhang and Zhu 1993). Due to its gold-like appearance, TiN can also serve for decorative purposes as coating on jewelery and watches (Sundgren 1985, Schubert and Hüsing 2005). 27 Its stability and non-toxicity allow the use of TiN as coating biomedical applications, such as for medical tools and prostheses, (Serro et al. 2009) and even in biomedical microelecromechanical systems (BioMEMS) (Birkholz et al. 2011). Protective coatings of TiN usually have thicknesses of 2-10 µm (Sundgren 1985), as thicker layers would be more susceptible to cracks or fissures due to the brittleness of the material. These coatings are traditionally deposited by gas phase methods, which can be divided into physical vapor deposition (PVD) and chemical vapor deposition (CVD). In PVD, a film is formed by transferring small amounts of material(s) from a source to the substrate by physical means. Chemical reactions are not used in the material transfer, but can still occur during or after it, for example in the formation of TiN from a Ti target and N2 gas or plasma. Examples of PVD methods are evaporation and sputtering, which are often ”activated”, i.e their performance is enhance by the use of substrate bias, plasma or a magnetic field (Hahn et al. 1987, Meng and dos Santos 1997). PVD processes are fast and can be performed at low temperatures, but have a less accurate control of film thickness and are directional methods, i.e. steps and other nonuniformities on the path or on the substrate block or reduce film deposition in the shadowed areas (Price et al. 1993, Zhao et al. 2000). The high kinetic energies reached by particles in sputtering processes can damage the film surface and alter film properties strongly (Wittmer et al. 1983). Therefore, PVD methods are more and more often replaced by chemical vapour deposition methods (Zhao et al. 2000). These deposit films by taking advantage of decomposition or gas phase reactions of volatile precursor chemicals. Especially decomposition-base processes, however, often require very high temperatures, making them more difficult to control and more sensitive to contamination (Tiznado and Zaera 2006, Jeon et al. 2000). Perturbations in the gas flow or temperature gradients can lead to nonuniform and nonconformal growth (Price et al. 1993, Leskelä and Ritala 2002, Profijt et al. 2010). 28 More recently, TiN films were introduced for various uses in for microelectronics, usually as thinner films with thicknesses on a nanometer scale. The most widely seen applications for TiN thin films in integrated circuits are diffusion barriers, which prevent the diffusion of metal atoms into silicon, while at the same time retaining an electric connection between metal and silicon (Sundgren 1985, Leskelä and Ritala 2002, George 2010, Miikkulainen et al. 2013). The metal in question is usually copper (Uhm and Jeon 2001, Elers et al. 2002), but also barrier layers for aluminium (Tompkins 1991) and lithium (Baggetto et al. 2008) have been reported. TiN can also be used as electrode in DRAM capacitors (Ahn et al. 2001, Kim et al. 2003, Xie et al. 2014), gate metal in CMOS transistors (Heil et al. 2006, Profijt et al. 2011, Van Bui et al. 2012) and assisting layer during the CVD of W (Zhao et al. 2000, Heil et al. 2006). Baturina et al. (2004), Hadacek et al. (2004) and Coumou et al. (2013) have studied superconducting TiN thin films. Coumou et al. focused on the possible applications of such films, while Baturina et al. and Hadacek et al. investigated the behavior of TiN at low temperatures and applied magnetic fields, with an emphasis on its superconductorinsulator transition. Baturina et al. used 5 nm TiN films deposited by ALD at 350 °C, which showed superconducting behavior at temperatures of approximately 1.5 K and below (Baturina et al. 2004). The films studied by Hadacek et al. were thicker (ca. 100 nm) and deposited by reactive DC magnetron sputtering, and exhibited superconductivity at 4.4 K, which is rather close to the critical temperature value of 4.7 K given for bulk TiN (Hadacek et al. 2004). For the relatively thick coatings on tools and comparable stubstrates, PVD and CVD are very suitable methods. However, since components of integrated circuits and other microelectronic systems have been the subject of constant decrease size downscaling (Kuhn 2009), many materials from metals to dielectrics are now deposited as very thin films with thicknesses on a nanometer scale. For such thin films, repeatability and thickness control at relatively low temperatures can be achieved by atomic layer deposition (ALD), which is introduced in Chapter 2.4. 29 2.3.2 Radiation damage in titanium nitride No literature could be found on radiation damage and its effects in TiN, except for the influence of neutron irradiation on superconducting TiN films (Dew-Hughes and Jones 1980), which is not of great interest for this work. The effects of plasma on TiN growth behavior and film properties in plasma-enhance thin film deposition methods have been studied, but the electron and ion energies in that kind of plasma are on the order of a few electronvolts (Profijt et al. 2011) and are not comparable to particles in high-energy physics experiments. TiN is considered a metallic material, so a logical starting point for investigating its behavior under irradiation would be examining the basic mechanisms and common effects of radiation damage in metals. Drawing detailed conclusions from these reports is risky, however, because it is probable that TiN as a compound metal reacts differently to radiation than elemental metals. Studies on other metallic transition metal nitrides would provide the best clues for the behavior of TiN under irradiation, but for the entire compound group literature on radiation damage is effectively nonexistent. However, some conclusions can be drawn from irradiation experiments on titanium carbide (TiC), which is also a binary compound and exhibits characteristics of both metallic and covalent bonds. Experiments with neutron and electron irradiations indicated that TiC can be approximated as diatomic metal under irradiation: its behavior was very similar to that of a metal in the sense that defects were presumably produced by nuclear collisions instead of interactions with electrons. However, TiC is expected to differ from a monoatomic metal in the process of recovery from radiation damage, due to the presence of two different types of interstitials and vacancies and their interactions with each other. (Morillo et al. 1981) Based on the above, it appears tolerable to treat TiN primarily as a metal when considering the mechanisms of radiation damage and their effect on electrical properties. This approach should be limited to metals that are similar to TiN in either composition, structure or properties: good candidates are elemental titanium or metals with a facecentered cubic structure, in which radiation-caused atom displacement could have similar effects as in TiN. 30 In metals, changes in mechanical properties, are the most frequently described consequences of radiation damage. Under irradiation, the hardness and yield stress of metals increase, while their ductility decreases (Abromeit 1994, Wirth et al. 2001, Choppin et al. 2002, Was 2007). In structural materials exposed to neutron irradiation, swelling is a major issue due to the formation of voids and bubbles of helium as consequence of the (n, α) reactions (Abromeit 1994). TiN differs from most elemental metals in the way that it is hard and brittle already under normal conditions, thus irradiation is expected to only intensify existing attributes and not to cause radical changes. For the application in particle detectors studied in this thesis, the mechanical properties of TiN are not decisive: no external mechanical stress is exerted on the bias resistors, and the detectors are carefully protected from scratching during all phases of production and installation. Intrinsic compressive or tensile stress that might affect other properties of thin films is expected for films deposited by PVD methods (Hultman 2000, Machunze and Jansen 2009); in CVD and especially ALD, the slower growth at elevated temperatures facilitates stress relief already during film growth (Uhm and Jeon 2001). It has to be considered that TiN is interesting for detector applications as very thin film with thicknesses of some tens of nanometers. The mechanical properties and response of such a thin film are not identical to the properties of the bulk material. Very thin films do not exhibit brittleness in the same way as bulk material, but are instead linked closely to the behavior of the substrate, if their adhesion is sufficient. Nanometer-scale thin films are sensitive to scratches, even if the bulk material is classified as very hard. Therefore, it is not certain if radiation damage affects the mechanical properties of TiN thin films. In general, the energy loss of radiation of any kind is expected to be very low in TiN thin films. The films have a moderate density (slightly below 5.2 g/cm3) and are extremely thin, therefore the number density in equations 1-3 and 7 is very small and the energy loss can occur only over a very short path. 31 Said equations also show that the interaction of radiation with an absorber material shows an even stronger dependence on atomic number; consequently, in TiN the majority of interactions occurs with Ti (Z = 22) and non-nuclear radiation interactions can be neglected for N (Z = 7). Ti is still a rather light metal and its interactions with radiation are fewer than e.g. for Fe and Cu. In addition, there are some remarks in the literature that the principle of ionization and excitation in the form of a cascade around the track of a high-energy particle might not apply to thin films. Mozumder states that in extremely thin metallic absorbers, energy transferred from an incident particle could lead to plasmon excitation (Mozumder 1999), i.e. collective oscillation of the free electron density. The excess energy would in this case be stored in the entire lattice, e.g. in the form of heat, instead of causing excitations of single atoms or chemical transformations (Mozumder 1999). Myers deduces the occurrence of plasmon excitations from the observation that electrons lose energy in metal thin films even though those are transparent for electrons (Myers 1997). Both Mozumder and Myers set a thickness of ca. 100 nm as the limit for this phenomenon. Since TiN thin films deposited by ALD for electronics applications are normally thinner than 100 nm, plasmon excitation might play a large role in their interaction with radiation. Changes in mechanical properties or surface structure of TiN thin films might not be detectable with common thin film characterization methods, such as X-ray diffraction or scanning electron microscopy, at low doses of proton or photon irradiation. The unstable nuclei formed in the activation of Ti under proton irradiation, however, should be visible already in very small quantities due to the radiation they emit. In terms of interactions of high-energy particles with the atomic nuclei, TiN can be approximated a combination of Ti and N, although it is warned that this approach may yield inaccurate predictions (Knoll 2010). The following focuses purely on the nuclear activation of Ti. 32 The strong (‘extreme’) activation of titanium, in this case by irradiation with deuterons, alpha particles and neutrons, was noticed already shortly after the discovery of artificial radioactivity (Walke 1937). A variety of radioisotopes formed this way was identified, mostly through their half-lives and if necessary chemical separation. The observed activation products and the reactions they are formed in are presented in Table 3. Table 3. Activation products of titanium produced under irradiation with deuterons, neutrons and alpha particles (after Walke 1937). Isotope Nuclear reactions 51 50 Ti Ti (n, γ) Ti (d, p) 48 Ti (d, α) 46 Ti (n, p) 46 Ti (d, α) Remarks 50 46 Sc 44 Sc 48 47 Ti (d, n) Ti (d,2n) 48 Ti (n, p) 48 Ti (n, α) V 48 48 Sc Ca 45 49,50 48,49 V Ti (d, n) Ti (α,p) 48,49,50 Ti (α, p) 46,47 51,52,53 V 49-53 Cr 46-50 Ti (α,n) 48 Ti (d,α) only for strongly activated samples. Unintense, not certain; probability for (d, α) low. High probability noticed for 48 V. Rather intense. Unintense, not certain; 52V improbable. Unintense, not certain. The high probability for the formation of 48V under irradiation of titanium with deuterons was noted (Walke 1937). This isotope is also formed in high activities under proton irradiation: for example Szelecsenyi et al. (2001) list cross-sections for reactions of natTi (p,γ) 48V between 5.7 and 17.9 MeV. Four reactions yielding 48V were identified, out of which the first is deemed least significant due to the low abundance of 47Ti: 47 Ti (p, γ) 48 Ti (p, n) 49 Ti (p, 2n) 50 Ti (p, 3n) 33 Cross-sections for the above reactions are around 360-390 mb for 10 MeV protons (Szelecsenyi et al. 2001).Tarkanyi et al. studied the same reaction and obtained identical results. In addition, the reactions natTi (3He, x) 48V and natTi (α, x) 51Cr were addressed. At 10 MeV, cross-sections for the former were with approximately 3.8 mb only one tenth of the proton activation cross-section, alpha or 2He activation was slightly more probable with a cross-section of 63 mb. Other activation products listed for 2He and 3He irradiation of natTi are scandium isotopes, namely 43,44,44m,46,47Sc. (Tarkanyi et al. 1992) An extensive list of cross-sections for the formation of titanium activation products by proton-induced nuclear reactions, as well as spallation, was assembled by Brodzinski et al. in 1971. Experimental data on reactions induced by extremely high-energy protons of up to 2.6 GeV in energy is presented by Michel et al. (1995). The interested reader is referred to these publications for more detailed information. Photonuclear reactions of titanium were studied for example by Sherwood and Turchinetz for photon energies from 14 to 31 MeV, obtained as bremsstrahlung radiation from a synchrotron. Similarly to 2He and 3He irradiation, the identified products were scandium isotopes, although in this case 44,46,47,48,49Sc obtained by (γ, p) or (γ, np) reactions. In addition, 45Ti was formed in the exceptional reactions 46Ti (γ, n) and 47Ti (γ, 2n). (Sherwood and Turchinetz 1962) 34 2.4 Atomic layer deposition 2.4.1 Introduction to atomic layer deposition Atomic layer deposition (ALD) is a modification of traditional CVD. It relies on reactions of the precursors occurring on the substrate surface instead of the gas phase. The first precursor is pulsed over a substrate and reacts with its surface groups in a complete, yet self-limiting way, which means that in an ideal case, the precursor will form no more and no less than one monolayer on the surface. The excess amount is then removed by purging the reactor chamber with an inert gas. After this, the second precursor is pulsed over the substrate and reacts with the first precursor, ideally again in a complete, selflimiting way and only via a reaction (or reactions) that yields the target material. A second purge removing the excess of the second precursor completes the ALD cycle, after which one layer of the desired material has been deposited on the substrate. The desired film thickness is obtained by repeating this cycle as many times as necessary. (George 2010, Leskelä and Ritala 2002) The film growth rate in ALD is normally expressed in units of thickness/cycle instead of thickness/time. In contrast to CVD processes, which usually are a strongly depending on temperature as energy source for the reactions leading to film deposition, the film growth rate in ALD should not depend on temperature. The temperature region where this is true is called the ALD window, where film deposition is caused only by self-limiting surface reactions, presented schematically in Figure 11. (George 2010, Leskelä and Ritala 2002) Outside the window, film growth can either decrease or increase with increasing temperature. A growth rate increasing with temperature is the consequence of incomplete reactions at lower and thermal decomposition of the precursors at higher temperatures. This is the case for most ALD processes, but especially processes based on metal halide precursors appear to be sensitive to condensation at low and desorption high temperatures (Mäntymäki et al. 2015). 35 Figure 11. Schematic of the growth rate behavior in ALD as function of temperature. (<http://www.frontiersin.org/files/Articles/113306/fmats-01-00018-HTML/image_m/fmats-0100018-g002.jpg>, 17.10.15) As film deposition occurs through chemical reactions of gaseous precursors, ALD is not affected by shadowing effects as PVD is and allows the coating of 3D structures. Due to relatively slow growth based on surface reactions instead of the decomposition in the gas phase, the conformality achievable with ALD is much better compared to conventional CVD. Ideally, the surface reactions are self-limiting and do not depend on small temperature or precursor flow gradients. All previous factors also account for the uniformity of ALD thin films over large substrates and its repeatability – the desired film thickness can be tuned accurately at a nanometer scale by adjusting the number of ALD cycles. 36 2.4.2 Plasma-enhanced atomic layer deposition The surface reactions characterizing ALD require energy. Even an exothermic chemical reaction usually has an energy wall, the activation energy, that needs to be overcome in order for the reaction to proceed. In traditional ALD, the required energy is supplied by heating the system, but there are also other ways to promote surface reactions. The most common of these is plasmaenhanced ALD (PEALD), also known as plasma-assisted or radical-assisted ALD. The only essential difference between thermal ALD and PEALD is the ignition of a plasma during the pulse of the second precursor, illustrated in Figure 12. (Kim 2011, Profijt et al. 2011) Figure 12. Schematic of an ALD cycle for thermal ALD and PEALD (Profijt et al. 2011). 37 The plasma is formed between two electrodes, usually a showerehead top electrode and the substrate holder. Both capacitive or inductive coupling systems are found. The applied electric field accelerates electrons, which in turn ionize and excite other gasphase species to form highly reactive radicals, free ions and electrons, as well as photons (Profijt et al. 2011). The plasma gas often works as precursor, for example in the PEALD of oxides and nitrides by using a plasma of O2 or NH3, respectively (Kim 2011). These processes often involve combinations of thermal and plasma-induced reactions (Profijt et al. 2011). Alternatively, the plasma gas can function as a reactant that is not intentionally incorporated into the thin film, the most prominent example of which is the deposition of metal thin films with a reducing H2 plasma (Kariniemi et al. 2011, Kim 2011). The introduction of radicals and other reactive species brings several possible advantages to an ALD process. The energy supplied by plasma can enable film growth at lower temperatures and can lead to higher growth rates. The energy as well as reactivity of the plasma species often widenes the ALD window of a process due to enhanced removal of precursor ligands, byproduct and also of condensed precursor molecules. All these are sources of impurities in thin films, and indeed PEALD is known for often producing purer films than thermal ALD. PEALD can even enable completely new processes by opening new reaction pathways. (Kim 2011, Profijt et al. 2011) A general disadvantage of PEALD is the more complicated process chemistry and consequently the difficulty to determine which factor is responsible for the observed behavior. Saturative film growth might be influenced by plasma power and exposure time in addition to precursor pulse length and temperature. The plasma species might alter the process chemistry significantly, but are more challenging to predict and to study than neutral gas molecules. Since film growth relies strongly on radicals, PEALD can have poorer film conformality on substrates with high aspect ratios, as radicals often recombine before they can reach the bottom of trenches and other structures. The probability of recombination depends on the radical and also on the surface. (Kim 2011, Profijt et al. 2011) 38 Significant for the upscaling of PEALD processes for industry is the requirement of modified or additional equipment. Profijt et al. (2011) remark that the advantages of PEALD over thermal ALD must be important for the application in question in order for this method to be considered for wider industrial use. 2.4.3 Thermal and plasma-enhanced atomic layer deposition of titanium nitride thin films: focus on TiCl4 as titanium source Precursors for ALD must display an often contradictory combination of fast and complete surface reactions while at the same time being thermally stable and non-reactive in the gas phase. Because the substrate is exposed only to small amounts of precursor vapors at a time, the choice of precursors is not limited to gases. As long as they are sufficiently volatile, liquids and even solids can be used as precursors, but the latter increase the risk of particle contamination of the film. (Leskelä and Ritala 2002, Schubert and Hüsing 2005). Atomic layer deposition of TiN has been dominated by the inorganic halide compound titanium tetrachloride (TiCl4). Unlike most transition metal halides, it is a liquid under standard conditions (m.p. -24.12 °C, b.p. 136.45 °C, Haynes 2015). The benefits of TiCl4 as an ALD precursor are its volatility, thermal stability, and the absence of carbon in the precursor molecule. In addition, it is a bulk chemical and therefore easily available and inexpensive. TiCl4 is not air sensitive as many other common precursors, but forms HCl (together with TiO2) upon contact to moisture, which classifies it as corrosive. Thermal ALD using the precursor pair of TiCl4 and NH3 is the most common ALD process for TiN thin films and one of the most investigated of all ALD processes (Miikkulainen et al. 2013). It was published first by Hiltunen et al. in 1988, who for the first time deposited several transition metal nitrides, among them TiN, by ALD from the corresponding metal chlorides and ammonia at 500 °C. Relatively slow growth rates of around 0.2 Å/cycle were reported for all deposited nitrides, but TiN was the only one to be found to contain significant chlorine impurities. 39 It was observed that the rather smooth TiN film deposited by ALD preferred the (200) orientation, while PVD processes produced either the (111) phase or a mixture of different phases (see also Patsalas and Logothetidis 2001). The resistivities of the films were not reported. (Hiltunen et al. 1988) Even though the process is frequently studied and applied for the deposition of TiN thin films for various purposes, its chemistry is not completely understood. Some questions still remaining open are the mechanism of the reduction of Ti, the identity of the byproduct of the corresponding oxidation, and the sources and the incorporation of contaminants (Tiznado and Zaera 2006). In the following, the chemistry of the TiCl4-NH3 process is discussed using publications on CVD for support. Even though CVD usually involves gas-phase and decomposition reactions, which are tried to be avoided in ALD, it is a chemical deposition method and some mechanisms that occur in CVD can be projected – with some reservations – also into ALD. The most thorough study of the TiCl4-NH3 process and its surface chemistry on a molecular level was conducted by Mochizuki et al. (1995). The process is labeled as CVD, but the authors regard it a series of two (subsequent) reactions and conclude that the growth proceeds in a layer-by-layer fashion, which would essentially correspond to ALD. Juppo et al. (2002b) studied surface reactions in the ALD of TiN films from TiCl 4 and ND3 between 300 and 400 °C with in situ mass spectrometry, Tiznado and Zaera (2006) studied the chemistry of the same process at 400 °C by in situ XPS. Titanium nitride deposition from TiCl4 and NH3 is used as an example of metal nitride CVD by Schubert and Hüsing (2005). For the initial growth of TiN, Mochizuki et al. suspect a dissociative adsorption at a dimeric Si site: the electron-deficient Ti in the TiCl4 molecule is donated electrons from one Si atom, while the other in turn receives electrons from the electron-rich Cl. During this process, only one Ti-Cl bond is broken. 40 The hypothesis of dissociative adsorption is supported by the observed formation of DCl also during the TiCl4 pulse (Juppo et al. 2002b) and the presence of Ti3+ signals in the first TiCl4 pulses(Tiznado and Zaera 2006). Also the next step of chlorine elimination involves donation - back donation, this time between the Ti 3d orbital and the lone electron pair of the ammonia nitrogen, and between chlorine and one ammonia hydrogen. The latter interaction produces HCl. The reactions proceed in a similar way also with H2 as reducing agent, but for H2 a less favorable activation energy of ca. 167 kJ/mol, compared to a theoretical 63 kJ/mol for NH3, was presented. The chlorine elimination is believed to be the rate-controlling step and slowest for the first chlorine atom to be abstracted, which would lead to a rather slow and therefore conformal layer-by-layer growth for TiCl4. For TiCl2, another possible Ti-supplying species, chlorine elimination is expected to be faster and thus to result in 3D CVD growth. TiCl3 is considered less important due to its dissociation into TiCl4 and TiCl2. (Mochizuki et al. 1995) Juppo et al. (2002b) state that the ND3 is the process’s ratelimiting step, supporting Mochizuki et al., if chlorine elimination is assumed to occur during the ND3 pulse. Equation 12 shows the commonly accepted total reaction equation leading to the deposition of TiN in CVD (Juppo et al. 2002b, Schubert and Hüsing 2005, Tiznado and Zaera 2006): 6 TiCl4 + 8 NH3 → 6 TiN + 24 HCl + N2 (12) According to Eq. 12, the reaction byproducts are gaseous nitrogen and hydrogen chloride. Nitrogen is an inert and non-toxic gas, but HCl is toxic and corrosive. However, it is produced in minute amounts that are disposed of through the pump oil and, more importantly, do not cause etching of the already deposited TiN film (Wittmer et al. 1983). 41 Equation 12 might represent the total reaction also in ALD, but the separate precursor pulses demand for it to be split in two half- reactions. Juppo et al. (2002b) propose the following half-reactions for the ND3 (13a) and TiCl4 (13b) pulses: -TiClx (s) + ND3 (g) → -TiND3-x (s) + x DCl (g) (13a) -NDy (s) + TiCl4 (g) → -NTiCl4-y (s) + y DCl (g) (13b) The main species adsorbing on the –NDy surface was found to change with increasing temperature from -TiCl through -TiCl2 towards -TiCl3. This might be explained by the loss of reactive -NDy groups or change from -ND2 to -ND groups at higher temperatures. (Juppo et al. 2002b) Schubert and Hüsing, among others, assume that ammonia acts as both nitrogen source and reducing agent in the deposition of TiN. Tiznado and Zaera (2006) do not rule this out, but present evidence that the reduction of titanium from oxidation state +IV in the precursor to +III in the nitride might occur during the titanium precursor pulse: results indicating the formation of a very thin (0.3 nm) layer of Ti3N4 on the surface during the TiCl4 pulse lead to the conclusion that reduction of Ti occurs during the following TiCl 4 pulse. For a reduction of Ti during the TiCl4 pulse, equation 14 was introduced as an alternative to Eq. 12. In this case, chlorine would be the oxidated byproduct more likely than N2 and desorb as Cl2. The formation of a chloroamine instead of Cl2 was mentioned as another alternative, but no equation was provided. (Tiznado and Zaera 2006) 2 TiCl4 (g) + 2 NH3 (g) → 2 TiN (s) + 6 HCl (g) + Cl2 (g) (14) It was admitted that at 400 °C, the reduction with TiCl4 would be thermodynamically less favorable (179.4 kJ/mol) than with NH3 (-48.8 kJ/mol), but it was also noted that the separation of the precursors, as well as the removal of byproducts and leftover precursors by the purge steps, might alter kinetics and mechanisms to be more favorable in reality (Tiznado and Zaera 2006). 42 Juppo et al. observed DCl as central byproduct formed during the pulses of both precursors, but did not observe formation of nitrogen, probably due to the strength of the Ti-N bond (over 335 kJ/mol) and the slowness of N2 formation at the lower temperatures used in ALD. This naturally questions the validity of equation 1 for ALD of TiN. Another reaction equation (Eq. 15), involving the production of hydrogen in addition to N2 and HCl, was proposed by Kim et al. (2003). None of the other authors mentions this possibility. TiCl4 (g) + 2 NH3 (g) → TiN (s) + 4 HCl (g) + H2 (g) + ½ N2 (g) (15) An important issue in the deposition of metal nitride films are impurities, especially chlorine, which is assumed to cause higher resistivities and influence device reliability (Ahn et al. 2001, Price et al. 1993). Schubert and Hüsing (2005) mention the formation of ammonium halides NH4X as a disadvantage described for several metal nitride CVD processes using titanium halides and ammonia. The halide forming in the TiCl 4 process, NH4Cl, is a potential source for chlorine impurities in the deposited films, as it is sublimable but not very volatile (Schubert and Hüsing 2005). The dissociation of NH4Cl or similar species is expected to yield HCl as toxic and corrosive byproduct, but this is a negligible concern compared to high impurity concentrations in the films. The equilibrium between the mentioned species is presented in Equation 16 (Schubert and Hüsing 2005): NH3 + HCl NH4Cl (16) As a rise in temperature shifts the equilibrium to the left side of the equation, i.e. promotes the dissociation reaction of NH4Cl into ammonia and HCl (Schubert and Hüsing 2005), the amount of chlorine impurities caused this way is expected to decrease with increasing temperature. Indeed a higher temperature has been confirmed to reduce resistivity, presumably by reducing chlorine impurities in TiCl4-NH3 thermal ALD (Elers et al. 2002, Jeon et al. 2000, Uhm and Jeon 2001). 43 The same effect has been observed also in TiCl4-H2/N2 PEALD (Heil et al. 2006), but since ammonia is not expected to be present in this process, it is concluded that chlorine residues are introduced into the TiN films also by a different mechanism than described by eq. 16. In the mass spetrometry study conducted by Juppo et al., the release of DCl increases at higher temperatures, especially during the ND3 pulse. The molecular adsorption of ND3 to solid TiClx species at lower temperatures to form solid TiNCl is presented as a possible explanation, which would prevent the ammonia from contributing to DCl formation and thus chlorine removal. (Juppo et al. 2002b) It was noticed that even at 400 °C and very high ammonia flows, some chlorine was left in the film (Tiznado and Zaera 2006). The literature on further investigation and application of the TiCl4 + NH3 process is more abundant than process mechanism studies. Below, parameters, such as growth rate, temperature and pulse length, as well as results on TiN thin film properties are presented. In the study of Tiznado and Zaera, the total growth rate was estimated to be 0.4 Å/cycle, though a CVD growth component of the process, caused by the re-desorption of TiCl4 (previously adsorbed on the reactor walls) during the NH3 pulse, was considered, unlike in any other TiN ALD article (Tiznado and Zaera 2006). Mochizuki et al. and Juppo et al. do not mention growth rates. A growth rate of 0.4 Å/cycle is also reported by Jeon et al. at temperatures between 350 and 450 °C with rather long, 5 s precursor pulses (Jeon et al. 2000). Kim et al. report the saturation of the growth rate at 420 °C even to 0.6 Å/cycle (Kim et al. 2003), but a number of other authors reaches only growth rates of around 0.17 Å/cycle at temperatures around 400 °C (Elers et al. 2002, Xie et al. 2014) or in a range of 350-500 °C (Ahn et al. 2001). These differences in growth rates might be explained by different ALD equipment or precursor pulse lengths – pulses were 4-8 s for Kim et al. (2003), but only 0.1-1 s for Xie et al. (2014). TiN films obtained by this ALD process were columnar and polycrystalline. Observed crystal orientations were (111), (200) and (220). Only Ahn et al. (2001) reports a (100) orientation. In the studies of Ahn et al. (2001), Kim et al. (2003) and Xie et al. (2014), the (111) orientation dominates, while Uhm and Jeon (2001) report more or less equal intensities. 44 Jeon et al. (2000), on the other hand, describes a domination of the (200) orientation, with the other orientations appearing at higher deposition temperatures. Film stoichiometries were mostly close to 1:1 (Jeon et al. 2000, Ahn et al. 2001, Kim et al. 2003). The resistivity of the TiN films was observed to decrease with increasing temperature. Since the same trend was observed for chlorine content, the higher resistivities at lower temperatures were mainly attributed to a higher chlorine content. Other factors that could have an impact on resistivity are other impurities besides chlorine, crystallinity and preferred orientation, as well as other microstructure or morphology (Jeon et al. 2000, Kim et al. 2003). However, none of the earlier were thoroughly investigated. Ahn et al. (2001) could not observe changes in chlorine content with increasing temperature, and therefore attribute changes in film resistivity to film density, orientation or both. Resistivities ranged from 300 µΩcm (Xie et al. 2014) over 200 µΩcm (Elers et al. 2002, Kim et al. 2003) to around 80 µΩcm for some films (Jeon et al. 2000, Ahn et al. 2001). Jeon et al. (2000) describe a decrease from 350 to 75 µΩcm over their temperature range of 350-450 °C. A 2.5 % chlorine content of the films at 350 °C was attributed to a thermodynamic calculation, according to which the energy for the exchange of Cl with NH3 was not high enough for complete chlorine removal (Jeon et al. 2000), although the formation of TiN should be thermodynamically favorable already at 320 °C (Elers et al. 2002). At around 400 °C, chlorine contents were 1.2 % (Elers et al. 2002) or below 0.5 % (Uhm and Jeon 2001, Kim et al. 2003), the latter appearing higher in the original figure. Other impurities were carbon (Uhm and Jeon 2001, Ahn et al. 2001) and oxygen (Ahn et al. 2001, Uhm and Jeon 2001, Kim et al. 2003), whose influence on film resistivity was considered, but not discussed extensively. 45 Due to the growing interest in TiN thin films, the thermal TiCl4-NH3 process was sought to be modified and optimized in order to enhance film quality, increase the growth rate and enable growth at lower temperatures. Ritala et al. slightly modified the thermal TiCl4-NH3 process by adding Zn as third precursor in order to facilitate the reduction of Ti from the +VI to the +III oxidation state and enhancing the electric properties of the film which had not been addressed by Hiltunen et al. By the addition of Zn, the resistivity of the TiN film could be lowered from 250 µΩcm to 50 µΩcm, but the growth rate remained low and the deposition temperature high. The TiN films were polycrystalline and thus rougher than comparable amorphous films, and were sensitive to surface oxidation. The article also assessed the conformality of the films, which was judged perfect. (Ritala et al. 1999) However, the exact aspect ratio of the trenches was not mentioned in the article and was estimated to be only around 1:5, based on SEM cross-section images. As shown by Miikkulainen et al. (2013), experiments on the exchange of either precursor of the TiCl4-NH3 process were also conducted. Dimethylhydrazine, allylamine and tertbutylamine were studied by Juppo et al. as alternative nitrogen sources for the deposition of nitride thin films together with TiCl4. Transition metal nitride films deposited with dimethylhydrazine showed good characteristics at 400 °C and could be deposited even at 200 °C, but the films had high carbon contents of around 10 % (Juppo et al. 2000). For both TiCl4 and TiI4, allylamine led to carbon and hydrogen impurities, and tert-butylamine as the only nitrogen source resulted in very low growth rates (Juppo et al. 2002a). TiI4 in combination with NH3 exhibited low growth rates with strong temperature dependence and oxygen concentrations of 10 % even at 500 °C (Ritala et al. 1998). All depositions with TiI4 were performed at relatively high temperatures of 400-500 °C. Even though e.g. dimethylhydrazine appeared promising as an alternative nitrogen source, the described variations of the TiCl4-NH3 process remained single publications and were not adopted for routine deposition of TiN films for applications. The use of metal reductive agents, such as Zn, is not compatible with most electronics and semiconductor applications (Leskelä and Ritala 2002). 46 Much more successful was the use of metalorganic alkylamino compounds, which in addition to functioning as titanium precursors also supplied part or all of the nitrogen necessary for the nitride formation. The by far most important of these newer precursors is tetrakis(dimethylamino)titanium, also known as TDMAT. This precursor has been studied as in thermal ALD together with NH3, as well as in PEALD. Two other molecules within this group reported as ALD precursors are tetrakis(diethylamino)titanium (TEAT) and tetrakis(ethylmethylamino)titanium (TEMAT), however only for thermal ALD in combination with NH3. The advantages of these metalorganic compounds are very high growth rates at low temperatures and the possibility to avoid chlorine contamination, which is seen as a risk to device performance in electronics (Jeon et al. 2000). Thermal ALD with NH 3 reaches growth rates of up to 4.4 Å/cycle for TDMAT and 5 Å/cycle for TEMAT (Elam et al. 2003, Musschoot et al. 2009), while the deposition temperatures can be kept below 300 °C or even 200 °C in PEALD (Miikkulainen et al. 2013). However, especially the thermal ALD processes have shown serious problems: the high growth rates, together with the absence of an ALD window and non-saturative growth during pulses (Elam et al. 2003), are signs of non-ALD growth by adsorption and thermal decomposition of the precursor. The absence of chlorine impurities is compensated with the appearance of significant amounts of carbon (6-13 % according to Miikkulainen et al. 2013) due to the incomplete removal of the ligands by NH3 (Elam et al. 2003). High resistivities of 10-50 mΩcm and extremely high oxygen concentrations of around 40 % were observed by both Elam et al. and Musschoot et al., probably due to the low densities of often under 3 g/cm3 and substantial porosity of over 40 % (for Elam et al.) of the deposited TiN films, which predispose them to oxidation. PEALD of TiN from metalorganic precursors is one of the rare cases where a clear decrease in growth rate is observed, for example from 4 to 0.8 Å/cycle for TDMAT and NH3 as precursors (Musschoot et al. 2009). This is attributed to the enhanced removal of condensated precursors by plasma species (Kim 2011). 47 In summary, there is no ideal process for thermal ALD of TiN. TiCl4 is a very common, thermally stable chemical, but it requires high temperatures and long growth times and leaves chlorine contamination in the film. TDMAT enables fast growth at lower temperatures, but the process is less reliable due to the non-ALD characteristics and results in less dense films with considerable carbon and oxygen impurities. Which method is in the end more suitable, i.e. which process type’s negative properties cause the least inconvenience, depends on the choice of substrate and the application of the desired TiN thin films. Both impurities due to incomplete precursor ligand removal and high process temperatures are characteristics that are often tried to improve by the use of plasmaenhanced ALD. This resulted in a large variety of PEALD processes for TDMAT: it has been reported in combination with NH3, NH3/H2, H2 and N2 (Miikkulainen et al. 2013). The use of plasma allowed for lower temperatures of 150-250 °C. For the TDMAT+NH3 process, the use of plasma improved the film purity, but did not completely remove impurities. The growth rate was slightly higher, but still no ALD window nor an otherwise saturating growth rate could be achieved. The resistivity of the films was lowered considerably to 180 µΩcm, and showed a correlation with plasma power and plasma exposure time. (Musschoot et al. 2009) PEALD also offered a means to improve TiN thin film deposition from TiCl4, for which a process with highly reactive H2/N2 as plasma gas mixture was developed (Heil et al. 2006, Langereis et al. 2006). This process permits depositions in a temperature range of 100400 °C. Half-reactions (Eq. 17a and 17b) for this process were proposed by Profijt et al. (2011): -TiNH (s) + TiCl4 (g) → -TiNTiCl3 (s) + HCl (g) (17a) -TiCl (s) +2 H (g) + N (g) → -TiNH (s) + HCl (g) (17b) This process appears to be the most promising of the newer TiN ALD and PEALD processes, as it allows the deposition of TiN thin films at low temperatures from TiCl4, avoiding the use metalorganic precursors. 48 However, even in this process the films with the least impurities and best quality, according to the authors, were deposited at temperatures of 300 °C and higher. Resistivities decreased from 209 to 71 µΩcm between 100 and 300 °C, respectively. (Heil et al. 2006) This confirms the theory that the amount of chlorine impurities depends most strongly on temperature, and that the effect of plasma, even highly reactive H2 plasma, on chlorine impurities is limited. Surprisingly, no literature could be found on a variation of the most common precursor pair in thermal ALD – a PEALD process combining TiCl4 with NH3 plasma has not been reported. Ammonia plasma was used only together with the metalorganic titanium precursors TDMAT or TEMAT, while TiCl4 was used with H2/N2 plasma. (cf. Miikkulainen et al. 2013) 49 3 Introduction to the experimental section The experimental part of this master’s thesis reports the testing of TiN for potential use as thin-film bias resistor in segmented silicon detectors. TiN has already been suggested as replacement for poly-Si, although for other applications (Wittmer et al. 1983, Tompkins 1991, Heil et al. 2006). TiN is expected to form better ohmic contacts to silicon (Wittmer et al. 1983) and could permit the deposition of a smaller components at lower temperature while simultaneously increasing the capacitance per area (Heil et al. 2006). TiN is hoped to be superior to the traditionally used polycrystalline silicon, as well as the tungsten nitride studied earlier, in several ways: 1. TiN is expected to have sufficient resistivity and to form ohmic contacts to aluminium without requiring additional doping as poly-Si does. The two doping steps and the corresponding masking layers for doping, as well as the etching of additional contact openings, are not necessary for TiN, which would make processing faster and thus cheaper. 2. The use of TiN bias resistors could enable AC coupling of pixels, which apart from the advantages of AC coupling mentioned earlier, would permit testing of the performance of individual segments and resistors without costly and lengthy wirebonding of the segments as is necessary in the case of DC coupled pixels. 3. TiN is expected to have sufficient resistivity to allow for a decrease in pixel size from 100 × 150 µm2 to 25 × 25 µm2, greatly enhancing the spatial resolution of pixel detectors. 4. The radiation hardness of TiN is assumed to be much higher compared to poly-Si, due to its metallic nature, lower crystallinity and its use as very thin film of clearly less than 100 nm. 5. Titanium is a much lighter element (Z = 22) and therefore interacts less with radiation than tungsten in WNx, making TiN more suitable for use in silicon tracker detectors. 50 In order for thin-film deposition and processing to be compatible with standard semiconductor processing, no temperatures higher than 400 °C should be necessary in the process, and the use of non-selective etchants and oxidizing or otherwise aggressive chemicals should be avoided. In the literature, there are many examples of TiN thin films being deposited by both thermal and plasma-enhanced atomic layer deposition (ALD) at temperatures of 400 °C (Chapter 2.4.3). ALD also enables deposition of very thin films – the optimal thickness of the TiN thin film was estimated to be between 15 and 30 nm – with high precision, uniformity and repeatability, and was therefore chosen as deposition method. The most traditional titanium precursor in ALD, titanium tetrachloride (TiCl4), was chosen as titanium precursor because of its availability at the Laboratory of Inorganic Chemistry and due to uncertainties associated with newer metalorganic titanium precursors (tendency to decompose, higher cost etc). Also the nitrogen source, NH3, was adopted from the traditional TiN ALD processes, but in order to achieve lower deposition temperatures, it was decided to try depositions by plasma-enhanced ALD with a mixture of NH3 and an inert gas as plasma gas. The precursor combination of TiCl4 + NH3 has not been published for PEALD, which explains a certain interest in studying this process also besides the specific application for the bias resistors studied here. However, this implied that virtually no information on the process’s basic characteristics, e.g. growth rate and its behavior under different conditions, could be taken from the literature, but had to be studied thoroughly before continuing towards bias resistor processing and testing. The same applied to the properties of the deposited films, most importantly crystallinity, chemical composition and resistivity. After their properties were studied and judged promising, two TiN films were processed into bias resistor structures, which were then subjected to different experiments in order to study their behavior and radiation hardness. These experiments consisted of annealing treatments and irradiations with protons and photons of different energies. The response of the resistors to several treatments was assessed by comparing their resistance after an experiment to the original resistance. For proton irradiations, the activation of TiN was also investigated. 51 4 Materials and methods In this chapter, the equipment, setups and methods used in the experimental work of this thesis are presented. First, methods used for PEALD of the TiN thin films are introduced. The processing of TiN thin films into bias resistor structures is reported next, as the experiments presented later – resistance measurements, annealing and irradiation with different types of radiation – focused on these resistor structures. Of the irradiation experiments, irradiations with 10 MeV protons received the most attention, because they were performed personally by the author with a non-standardized setup. The former also applies to the X-ray irradiations, but to a slightly lesser extent. 4.1 Plasma-enhanced atomic layer deposition of TiN thin films This chapter is divided into setup and experiments for the actual thin film depositions, and the methods used for subsequent characterization of the obtained films. 4.1.1 Setup and experiments Titanium nitride thin film depositions and characterizations were performed at the Laboratory of Inorganic Chemistry of the University of Helsinki in a Beneq TFS-200 ALD reactor (Figure 13) capable of holding silicon wafers with a diameter of up to 8” (203 mm). 52 Plasma gas inlet Deposition chamber TiCl4 source Figure 13. Picture of the Beneq TFS-200 ALD reactor, courtesy of M. Mäkelä. This reactor could be operated both at a thermal and at a remote plasma configuration, but the focus of this study lied on the latter configuration, presented as schematic in Figure 14. Plasma gas inlet Matching unit RF Showerhead top electrode Plasma TiCl4 precursor and carrier gas Grid Exhaust to pump Substrate Figure 14. Schematic drawing (simplified and not in scale) of the Beneq TFS-200 reactor at remote plasma configuration. 53 The plasma was generated by capacitive coupling with a 13.56 MHz RF power source. A metal grid inserted into the reaction chamber confined the plasma between itself and the upper electrode and retains part of the ions, photons and the generated radicals (Figure 15). This configuration is also referred to as triode configuration or direct plasma with grid (Profijt et al. 2011). The distance between this grid and the substrate was 4 cm. Figure 15. Schematic drawing of the electrode-grid-system in the Beneq TFS-200 reactors, courtesy of M. Mäkelä. In this study, films were deposited on several smaller substrates at the same time. Pieces of Si(100) from 6” (152 mm) wafers with a 1-2 nm native SiO2 layer served as standard substrates, on which film characterization, especially film thickness modelling by XRR, was the most straightforward. As SiO2 is the target substrate for the bias resistor experiments, it was used as a second substrate material. Normally smaller pieces for film characterization and resistivity measurements were used, but in a few selected batches TiN films were deposited on whole 6” wafers of oxidized silicon (oxide obtained by thermal oxidation at 1000 °C, thickness ca. 200 nm as determined by ellipsometry) for resistor processing. Soda lime glass (5 × 5 cm2) was used as additional insulating substrate for resistivity measurements. Both silicon and soda lime glass were also used for comparison of the TiN films to other batches and even other materials. TiN films were grown in most batches between B1067 and B1093 of the Beneq reactor and in individual later depositions. All information about the batches relevant for this thesis is found in Appendix A. 54 A new 2 l bottle of NH3 (6.0, AGA) was connected to the Beneq TFS-200 reactor via gas lines installed specially for this purpose. TiCl4 (≥ 99.0 %, Fluka) was evaporated from the external source, an airtight, ca. 200 ml steel/aluminium bottle that had been connected to the Beneq reactor earlier. The NH3 bottle was stored at at room temperature, the TiCl4 precursor at 19 °C. N2 (5.0, AGA) and Ar (5.0, AGA) were available as plasma and carrier gas at the Beneq reactor, and both were experimented with. It was observed that plasma ignition of a mixture of N2 and NH3 was difficult, often momentarily impossible, and therefore this mixture was not trusted to last for longer or repeated depositions. Ignition was easier and more reliable for the NH3/Ar mixture, which was consequently chosen for the majority of the depositions. The flows of argon and ammonia were optimized empirically to the lowest flow of Ar and highest flow of NH3 for which plasma ignition was reliable over a longer period of time, with the aim of keeping the total plasma gas flow at around 180 sccm. The flows determined for Ar and NH3 were 110 sccm and 40 sccm, respectively, and were kept constant during all variations of other process parameters. The pressure in the inner chamber of the reactor was around 5-10 mbar during the depositions. For all depositions, one PEALD cycle consisted of the following sequence: TiCl4 pulse – Purge – Wait – Plasma pulse – Purge. The Wait phase was established in order to let the NH3 flow rise to its level of 40 sccm before plasma ignition. The typical pulse lengths (in seconds) were 0.5 – 3 – 2 – 5 – 3, respectively. Film growth was studied at temperatures from 250 °C to 325 °C. In previous PEALD process studies, 300 °C had been the highest temperature used in the Beneq reactor at remote plasma configuration, and it was also the typical temperature during the following depositions. A plasma pulse length of five seconds was adopted as standard value from earlier depositions with the same reactor. In order to study the effect of plasma pulse length on film growth, the plasma pulse length was varied from 2.5 to 10 s at 300 °C. 55 The conformality of the TiN films deposited with the PEALD process was assessed with two types of vertical high aspect ratio substrates, referred to as G8 and G10 (Figure 16). For the conformality study, all pulse lengths were increased (3 – 10 – 5 – 10 – 15 s). The G10 substrate contained several different trenches with different widths and aspect ratios. The studied trench most likely had a width of 1 µm width and depth of 27 µm, which corresponds to an aspect ratio of 27:1. The substrates were broken by hand with a diamond pen and studied under the FESEM without further treatment, e.g. polishing. a b c d Figure 16. Top: a Picture of the G10 substrate, b Picture of the G8 substrate. Bottom: c FESEM image of a cross-section of the G10 substrate (courtesy of M. Mäkelä), d Optical microscope picture of the G8 substrate’s surface structure in 37.5-fold magnification. In order to evaluate the role of plasma ignition in the growth of TiN films at 300 °C, one film was deposited thermally. Except for the missing plasma ignition, all conditions – temperature, remote plasma configuration, pulse lengths – were kept exactly the same as described above for a standard PEALD deposition. 56 4.1.2 Film characterization The TiN films deposited on the Si(100) substrates were studied by X-ray reflectivity (XRR) with a PANalytical XPert PRO MPD X-ray diffractometer in parallel beam geometry, using characteristic Cu Kα radiation (λ = 1.5406 Å). Film thicknesses, as well as information about film roughness and density, were obtained by using the analysis software XPert Reflectivity: an X-ray reflectogram was simulated and its parameters, most importantly thickness, density and roughness of film, Si substrate and native SiO2 layer, were varied until suitable values were reached to make the model correspond to the experimental spectrum. The same instrument was also used for studying the crystallinity of the films by grazing incidence X-ray diffractometry (XRD). Comparing XRD patterns, measured at 2065 degrees 2θ with a step size of 0.08 degrees and step length of 3 s and analyzed by the HighScore Plus program, to a reference pattern was the routine method for confirming the films as TiN. The use of X-ray techniques for determination of chemical composition, e.g. by energydispersive X-ray spectroscopy (EDS), was not successful for the TiN films, as the low energy of the characteristic nitrogen Kα X-ray (0.3 keV) made a quantitative analysis of the nitrogen content practically impossible. A field-emission scanning electron microscope (FESEM, Hitachi S-4800) was used to study the surface morphologies of the TiN films deposited on planar substrates and to determine TiN film conformality from cross-sections of the abovementioned high aspectratio substrates. The substrates were also studied briefly under an Olympus BX51 optical microscope. In order to analyse and visualize surface roughness and morphology more accurately, atomic force microscopy (AFM) images were recorded from TiN films deposited on Si substrates using a Veeco Multimode V AFM with Nanoscope V controller. The images were captured in tapping mode in air using silicon probes with nominal tip radius of 8 nm and nominal spring constant of 40 N/m (RTESP from Bruker) or 3 N/m (VLFM from Bruker). Images were flattened to remove artefacts caused by sample tilt and scanner bow. 57 Roughness was calculated as a root-mean-square value (RMS or Rq) from 500 × 500 nm2 images (512 × 512 pixels) obtained at a scan rate of 1-2 Hz. Larger images up to 10 × 10 μm were also captured to analyse sample homogeniety. Image processing and analysis were done using the Bruker Nanoscope Analysis 1.5 program. Resistivities of the films were determined with the four-point probe method (Cascade Microtech four-point probe connected to a Keithley 2400 SourceMeter). Measurements were performed on films deposited on insulating glass or SiO2 substrate at several points to form a map of the film’s resistance. The chemical compositions of selected TiN films were analysed with time-of-flight elastic recoil detection analysis (ToF-ERDA) at the Accelerator Laboratory of the University of Helsinki. A 5 MV EGP-10-II tandem accelerator referred to as TAMIA was used to accelerate 79Br7+ ions to 40 MeV, after which the ions hit the TiN target at an angle of 20°. The time-of-flight energy detector measuring the bromine ions after their recoiling from the target was also positioned at an angle of 20°, giving a total scattering angle of 40°. The entire ToF-ERDA system was essentially the same as described by Jokinen et al. (1996), with slight modifications. 4.2 Processing of TiN films into bias resistor structures 4.2.1 Preliminary tests Processing took place in the class 10-100 cleanroom at Micronova in Espoo, starting on 6.3.2015 with TiN photolithography. Two wafers, from ALD runs B1076 and B1089, were chosen for the first processing. Both are 6’’ silicon wafers with a SiO2 layer (thickness ca. 200 nm) obtained by thermal oxidation. TiN thin films were deposited on both wafers using the PEALD described earlier. However, the films were not identical: thicknesses were 12.0 and 22.9 nm and resistivities 0.76104 and 1.244844 mΩcm for B1076 and B1089, respectively. 58 The wafers were rinsed with water and dried under a N2 flow.The etching behavior of the TiN thin films had not been tested earlier by our group, and the statements about this topic in the literature were somewhat contradictory. Naturally, a straightforward and selective etching method was wanted, if possible with a nonhazardous chemical. Due to earlier experiences with WNX films, which could be etched conveniently with hydrogen peroxide, this was also tried first for TiN. A piece of wafer B1087 (TiN thickness 36.4 nm) was held into a 30 % solution of H2O2 at 50 °C and checked after 2, 10 and ca. 23 minutes. After 2 minutes, no change in film color was visible, but after 10 minutes the part of the film held under the liquid surface had turned from pink to orange-yellow. After 23 minutes, the oxide underneath was revealed. Therefore, etching with 30 % H 2O2 at 50 °C for approximately 20-25 minutes was considered a suitable method for the processing of the TiN films. 4.2.2 Photolithography of TiN In processing the TiN resistor test structures, two mask layers were used: the BIAS mask for patterning the TiN layer and the METAL mask for patterning a sputtered aluminium layer. The mask layout of an entire wafer is presented in Figure 17. For clarity, each chip was assigned a code containing the batch code and the chip’s position on the wafer. The rows are assigned letters, starting from A with the top row under the heading. The columns are assigned numbers starting from 1, counted from the inside towards the edges of the wafer. The wafer is divided in two halves according to the positions of the mask names, i.e. the left half is referred to as the Metal (M) and the right the Bias (Bi) half. The significance of division is explained later in the text. As an example, the first chip on the top left of the wafer is named 76_M_A4. In an ideal case, the TiN layer should be uniform and therefore all chips with the same resistor structures should behave identically, making the tracking of their positions unnecessary, but this could not be trusted to be true for a PEALD process in such an early stage of optimization and upscaling. 59 A B K L M Figure 17. Wafer layout with two mask layers BIAS (red) and METAL (dark grey). The blue line shows where the wafer was split into the M and Bi halves, the letters and numbers indicate the position of each chip. Courtesy of J. Härkönen. Each chip contained only a single type of resistor out of the four that were produced by the BIAS mask. Depending on the size of the resistor structure, a chip consisted of 6-8 rows with 23 resistors. The code used for distinguishing the structures, in the upper center of each chip, simply states the structure’s width (W) and length (L) in micrometers. These values for each structure, as well as the resulting area and the layout of the structure, are presented in Table 4. Two of these structures were possible resistors for pixel detectors - the longer one was situated traditionally around a pixel (W4L700), the other was presumably intended to be winding directly on a much smaller pixel (W4L460). 60 The other two structures both represented resistors for strip detectors: one shorter structure with rounded turns (W5L600), and the largest structure with angular turns (W4L3550). All resistors had the form of a meandering line, with the length in Table 4 corresponding to the theoretical lenght if the resistor were stretched out into a straight line. Table 4. The dimensions and layouts of the four resistor test structures. Width (µm) 4 4 5 4 Length (µm) 460 700 600 3550 Area (µm2) 1840 2800 3000 14200 Description Winding on pixel Winding around pixel Strip, round turns Strip, angular turns The wafers were primed and coated with a monolayer of hexamethlysilazane (HMDS) in an oven at 150-160 °C to enhance photoresist adhesion. The wafers were then spincoated with a positive photoresist (AZ5214E). The resist layer’s thickness was ca. 1.4 µm. The coated wafers were soft-baked at 90 °C for 15 min in order to harden the photoresist. Afterwards the first mask (BIAS mask) was aligned manually under an optical microscope and the resist was irradiated with UV light for 3 s. The resist development took 70 s for B1076 and 75 s for B1089. The wafers were rinsed with water and dried. Microscope pictures were taken of the different structures of both wafers to have a first look at the pattern quality. Figure 18 shows an example. Figure 18. Comparison of structure W5L600. a (left): B1076, 12 nm; b (right): B1089, 22.9 nm. 61 All structures on both wafers were very uniform, and no adhesion problems were observed for neither the TiN film nor the photoresist. However, in some areas of wafer B1089, a small drop of resist had remained in the narrowest part of the smallest test structure W4L460 – this problem did not occur on B1076 (Figure 19). Figure 19. Test structure W4L460. Left: B1076; right: B1089, incomplete resist removal. The wafers were hard-baked at 120 °C for 75 min. After this, the exposed TiN was etched with 30 % H2O2 at 50 °C for 20 min (B1076) and 24 min (B1089). The wafers were subsequently rinsed in a water bath with N2 bubbles and dried. The results were checked with a microscope. Again, all structures except for some of the smallest on B1089 were excellent. Finally, the photoresist was removed by dissolving it in acetone 1 for 10 min, then rinsing in acetone 2 for 2 min, isopropanol for 2 min and water. As the TiN film thickness is not the same on the two wafers, they exhibit a slightly different color in the pictures – the thicker film B1089 has a reddish color, while the B1076 film is yellow . Figure 20, taken after TiN etching, shows a whole wafer with TiN resistor structures. 62 Figure 20. Wafer with TiN test structure chips. After drying, the final TiN resistor structures were again photographed under the optical microscope. Figure 21 shows that the residual droplets of photoresist in the W4L460 structures from Figure 19 have caused underetching in an entire corner. Figure 21. B1089, test structure W4L460. a (left): incompletely etched structure; b (right): intact structure. 63 Figure 22 shows examples of the other final resistor structures. All were flawless without any etching or adhesion problems. a b c d Figure 22. Top: a Test structure W5L600 on B1076; b Test structure W4L700 on B1089.Bottom: c and d Test structure W4L3550 on B1089. 4.2.3 Photolithography of Al probing pads Resistor test structure processing was continued on 12.3.2015. An approximately 300400 nm thick layer of aluminium had been sputtered on both wafers in order to provide the grid lines and description for the test structure chips as well as probe pads for the resistors (cf. Figure 17). The TiN structures were still sufficiently visible through the metal, which allowed a precise aligning of the second mask (METAL mask). The procedure (from priming to hard-baking the developed photoresist) were performed in the same way as described above for the BIAS mask, except for a slightly shorter photoresist development of 60 s. 64 The exposed aluminium was etched with a standard phosphoric acid solution (Honeywell 80-16-4(65), containing 74 % H3PO4 and 2.5 % HNO3 in water) at 30 °C for 70 s. The wafers were rinsed in a water bath with N2 bubbles and dried. A finished wafer is seen in Figure 23. Figure 23. A completed wafer with TiN bias resistors and Al contact pads. Again, pictures of the individual resistor structures were taken with a CCD camera integrated into an optical microscope (Figure 24). The brigth aluminium contact pads and the darkened SiO2 substrate – darkening was told to be a common consequence of the etchant – caused decreased contrast, but the now complete structures were clearly visible and could be seen to be in excellent condition. As expected, the Al etching solution did not attack the TiN structures. 65 a b c d Figure 24. Top: a Structure W5L600 on B1089, b Structure W4L3550 on B1076. Bottom: c Structure W4L460 on B1089, d Structure W4L700 on B1076. In order to see if the sintering usually employed in aluminium processes was necessary here and if it would have an effect on the resistors, the wafers were not annealed completely, but were cut in half with only the METAL side being annealed for 30 min at 400 °C under N2 atmosphere. The BIAS side was left without annealing. Two resistors of W4L3550 and W4L700 structure from each wafer half of B1076 were measured directly at the probe station in the Micronova cleanroom. It was observed that for both wafers and both measured structures, annealing had caused an increase in resistance of ca. 30 %. 66 4.3 Resistance measurements Resistances measurements were performed at Helsinki Institute of Physics and at CERN in March – August 2015. The measurements were performed in probe stations equipped with a microscope and probing needles connected to the necessary electronics and a computer (Figure 25). Probing needles Hole / sample Chuck Figure 25. a (left): the IV measurement setup, b (right): close-up of the IV measurement setup, microscope and needles. All measurements were performed the same way. A chip with processed TiN resistors was put onto the chuck, which was neither biased nor temperature-controlled for these measurements, and held in place by a vacuum pumped with an oil pump through a small hole in the chuck. The microscope was adjusted until the probing pads of the individual resistors were clearly visible. The two probing needles were placed over the opposite probing pads and were then lowered until a sufficient contact was established. The measurement itself was controlled by a computer, using Matlab at HIP or LabView software at CERN. A Keithley 2410 SourceMeter connected to the computer via GPIB served as both voltage source and ammeter. At CERN, an additional Keithley 485 Picoammeter was used to permit measurements of even lower currents. The voltage was ramped up in 0.5 V steps in a range of -5 – 5 V, each voltage being applied for 2 s with simultaneous current measurement. 67 Of each chip used in an experiment, several (5-20) resistors were measured before and after the experiment. Resistance values are given as average resistances with uncertainties corresponding to the standard deviation of the individual results. Single measurements giving clearly higher or lower values than the majority of the rest were excluded from the calculations. 4.4 Annealing Annealing experiments were performed with one sample of each resistor type from the BIAS halves of both wafers, giving a total of eight samples. They were annealed in 30 min steps at 400 °C under N2. Two different setups were used for annealing treatments: one at the Laboratory of Inorganic Chemistry at University of Helsinki, the other at the Isotope Separation On-line Device (ISOLDE) experiment at CERN. Both setups consisted of a quartz tube furnace (Carbolite ST15/450 at CERN, Figure 26 / CTF12/75/700 at UH), a pumping system and a N2 bottle (6.0 AZOTE / 5.0 AGA). In order to avoid oxidation of the TiN layer in ambient air at the elevated temperature, the samples were inserted into the center of the furnace at room temperature. The furnace tube was then first evacuated to a vacuum of ca. 1×10-3 mbar using an oil pump, and then filled with nitrogen up to a pressure slightly above environmental air pressure. Only after this the heating of the furnace was started, at a rate of 8-10 °C/min. A steady nitrogen flow of 4-10 l/h was maintained for the time of the entire treatment. The annealing time used in this text refers to as the interval during which the temperature has been held constant at 400 °C; however, the time the samples spent at elevated temperatures during heating and cooling of the furnace should also be taken into account. 68 Figure 26. Picture of the Carbolite furnace used in the annealing experiments at CERN. The resistors were measured before the experiments and after each annealing step, normally with n = 20. Measurements deviating clearly from the average were left out of calculations. In addition to the processed resistor structure chips, a piece of the 76.7 nm film was included in two half-hour annealing treatments and was later studied with AFM and ToFERDA for changes in morphology or chemical composition, respectively. 69 4.5 Irradiation with 10 MeV protons Proton irradiations were performed with 10 MeV protons in an IBA 5/10 cyclotron at the Laboratory of Radiochemistry at the University of Helsinki. This cyclotron is used mainly for the production of short-lived tracer isotopes for radiopharmaceutical research, such as 18F and 11C. Both are produced from liquid targets in chambers inside the outer layer of the cyclotron and their transport to a separate synthesis united is controlled remotely from the cyclotron control room. However, for less routinely performed irradiations of solid, potentially larger samples, such as the author’s thin film samples, the proton beam is directed into the external beam line (Figures 27 and 28). There, the beam shape can be adjusted by one dipole and three quadrupole magnets, which enables the uniform irradiation of larger targets. Furthermore, the easily accessible external beam line permits customization of the target chamber and e.g. the insertion of scattering foils into the beam line in order to reduce the energy of the protons and spread the beam, without influencing the more frequently used standard liquid targets mentioned above. A Keithley 6485 picoammeter connected to the external beam line’s target block is capable of measuring much lower currents than the cyclotron’s normal ammeter, which becomes accurate only in the region of microamperes. IBA 5/10 cyclotron Target block Target chamber Proton beam Keithley picoammeter Figure 27. The IBA 5/10 cyclotron external beam line from the front, just before irradiation with samples loaded on the target. 70 Target chamber Dipole Quadrupole 3 Quadrupole 2 Quadrupole 1 Proton beam Figure 28. The IBA 5/10 cyclotron beam line from the back. For all irradiations described in this thesis, an aluminium collimator was placed in front of the target block to absorb the proton beam except for a 2×2 cm 2 window under which the samples were placed (Figure 29). Therefore the current registered at the picoammeter ideally represented only the charge incident on the samples. Figure 29. Target used for irradiation of solid samples in the external beam line. a (left): Target block, b (right): Samples glued to the target block behind the collimator. 71 All samples were irradiated by pinning them to the aluminium target block with small pieces of double-sided carbon tape, which could be removed easily after irradiation and also provided both a thermal and an electrical connection between the samples and the target block. The irradiations were followed via a LabView program, which showed current, requested integrated current, requested total charge and the ratio of real and requested values in real time. In order to irradiate the entire sample area as homogeneously as possible, the proton beam had to be spread in both directions. This was achieved by adjusting the dipole and quadrupole magnets. The stripper and its angle with respect to the target were used mainly to transfer the position of the beam, but also to modify the beam shape to some extent. The beam shape was first studied in both horizontal and vertical direction with a fluorescent plate made of Al2O3. The plate was attached to the target, which was turned approximately 45 degrees towards the target chamber window. A camera positioned outside the window transmitted a live video to the cyclotron control room, showing the plate fluorescing red where hit by the proton beam. Due to its limited accuracy, this method was used only for crude observations and preliminary adjustments of the beam shape and position with the magnets. The spreading of the beam achieved by varying the current of the quadrupole magnets could be observed relatively clearly for the vertical direction, but not for the horizontal direction, due to the limited depth resolution of the camera and the turned target. Later, the homogeneity of the beam was studied by irradiating a piece of polyvinylfluoride (PVF) film (thickness 55 µm, density 1.35 g/cm3) with a low current (ca. 10 nA) and short irradiation time. Based on the film’s colour change from colourless to yellowish and brown on irradiation, the dose was observed to be not entirely homogeneous. This was, however, attributed to adjustments in stripper angle during the irradiation. 72 An oscilloscope was always used in addition to the other mentioned methods in order to obtain more accurate data on the beam width and height (Figure 30). It was observed that spreading the beam in the horizontal direction was easy and possible with all three quadrupole magnets, while the beam remained less homogeneous and narrower in the vertical direction. However, for the small target size of 2×2 cm2, the beam spreading was judged to be sufficient. Adjusting the beam position by varying the stripper angle was straightforward, but usually not necessary. Figure 30. Screenshot of the oscilloscope screen during beam monitoring. The left peak corresponds to the horizontal, the right to the vertical spread of the proton beam. 4.5.1 Calibration Beam dosimetry for the cyclotron is usually based on a simple current measurement with an integrated ammeter (for high currents in the internal targets, e.g. for 18F production) or with a picoammeter for the lower currents used in the external beamline. The proton dose per area is then easily calculated from the measured current and the known charge of the proton. For the experiments described here, however, a calibration of the proton beam was performed in order to obtain more accurate values – or to verify the sufficient accuracy of the dose determination by current measurement – for the proton doses. 73 The calibration method makes use of the increase in the diode’s leakage current under irradiation. Diodes from two 4” wafers of n-type Float Zone silicon (named Fz 2012 and Fz 2012 (2)) were used for calibration. The diodes were characterized before irradiation by measuring their capacitance-voltage (CV) and current-voltage curves (IV). The measurements before irradiation were used for familiarization with the properties of diodes and for sorting out unsuitable diodes with too high depletion voltages and/or abnormally high leakage currents. The depletion voltage (Vfd) of the diode was determined as demonstrated in Figure 32 for an irradiated diode: in a plot of the reciprocal of the capacitance squared as function of applied voltage (C-2V), straight lines were fitted to the data points of linear increase in C-2 and the area of constant C-2, and the depletion voltage was defined as the voltage corresponding to the interception point of the two lines. The leakage current was read from the IV curve at the voltage of full depletion. Even though there were leakage currents also in the unirradiated silicon diodes, the values measured for good quality diodes before irradiation were found to be negligibly low (some nA) compared to the values expected after irradiation (µA – mA) and were ignored. The diodes were irradiated in the same geometry as planned for the resistor sample chips. Each irradiation was performed for two diodes (one from each wafer) at the same time, in case one diode should break down under irradiation. The fluence was scaled to 1 MeV neutron equivalents using a hardness factor of 4.3 1 MeV neq / p+ for 10 MeV protons (Tuovinen et al. 2006). After irradiation, the CV and IV measurements were repeated, and the new full depletion voltage and leakage current were determined. Figure 31a shows a CV plot of the irradiated diodes from wafer Fz 2012 and Figure 31b a corresponding IV plot of the diodes from wafer Fz 2012 (2). With increasing fluence, a shift of the saturation of both capacitance and leakage current to higher voltages is visible, as well as an overall increase in leakage current. The differences in capacitance for the single diodes appear large in Figure 32, but this effect is increased by the squared capacitance and is not significant. 74 Figure 31. a (left): CV curves of irradiated silicon diodes from wafer Fz 2012, b (right): IV curves of irradiated silicon diodes from wafer Fz 2012 (2). Figure 32. C-2V plot of irradiated silicon diodes from wafer Fz 2012, with depletion voltage determined manually for one diode. From the results of the IV measurements, the leakage currents per volume in units of A/cm3 was calculated by dividing the measured leakage currents at Vfd by the volume of the diode, which was known to be 300 µm × 0.5 cm × 0.5 cm = 0.0075 cm3. These values were then plotted as function of the irradiation fluences to give the final calibration curve presented in Figure 33. A straight line was fitted to the data points of both wafers, of which wafer Fz 2012 showed a clearly better behavior. 75 Therefore, the experimental alpha parameter was defined as the slope of the straight line fitted to the data points of this wafer, which resulted in a value of 4.09×10-17 A/cm. Compared to literature values of 3.61 - 4.14 × 10-17 A/cm (Moll et al. 1999) for n-type Fz silicon, the alpha parameter determined here is surprisingly accurate despite the relatively poor linear fit. Figure 33. The leakage current of irradiated silicon diodes as function of fluence, with the experimental alpha parameter as slope of the line fitted to the data points of wafer Fz 2012 (black). 4.5.2 Experiments A 2×2 cm2 piece of TiN on silicon substrate from batch B1084 (76.7 nm) was irradiated with a fluence of 4.7×1014 p/cm2 in order to study the irradiated TiN film’s morphology with AFM and chemical composition with ToF-ERDA, and to compare the results to those obtained for an unirradiated film. 76 In addition, the activation of the TiN was studied very briefly and qualitatively – a quantitative measurement would have required known standards – by autoradiography, using a Fuji FLA-5000 scanner with a red 635 nm laser and a pixel size of 10×10 µm. A first irradiation of processed TiN resistors was performed on 22.5.2015 with a fluence of 1.56×1013 p/cm2. All four samples were irradiated simultaneously. After irradiation, the samples were kept in a freezer at -20 C. The total time spent outside the freezer for transport, measurements etc was estimated as 24 h. On 3.9.2015, the same samples were irradiated again to a total fluence of 5.0×1014 p/cm2. After this irradiation, gamma spectroscopy with the so-called GXRS232 detector (high-purity germanium, Canberra, model GX8021) and software Genie 2000 VDM was performed for all four samples simultaneously. 4.6 Irradiation with 24 GeV/c protons The irradiations with 24 GeV/c protons were performed at the CERN PS-IRRAD-1 facility (schematic in Figure 34). The total dose received by the samples was determined by adding a piece of aluminium foil with the same active area as the samples to each irradiation. The stable aluminium isotope 27Al undergoes two main nuclear reactions: 27Al (p, 3pn) 24Na and 27Al (p, 3p3n) 22 Na. The proton dose is calculated from the known cross-sections of the studied reactions and the activities of 24Na and 22Na measured with a NaI scintillation detector and a high-purity Ge detector, respectively. The hardness factor of 0.6 is used for 24 GeV/c protons (Tuovinen et al. 2006). An array of 10 samples was submitted for irradiation: 8 TiN resistor structure chips and 2 silicon diodes (n-type Fz silicon, wafer 331, detector development group at CERN). The samples were divided into two sets according to the resistor structure. Each set contained one resistor chip from each half-wafer (76_M, 76_Bi, 89_M and 89_Bi) as well as one silicon diode. 77 The sets were irradiated with different fluences following increasing resistor structure length: 1. W4L700: 5×1015 p/cm2 ≡ 3×1015 n eq./cm2 2. W4L3550: 2×1016 p/cm2 ≡ 1.2×1016 n eq./cm2 Figure 34. The CERN PS-IRRAD 1 facility (<http://ps irrad.web.cern.ch/images/ea_irrad_layout.jpg>, 16.10.15) In order to determine the activation products in the irradiated samples, gamma ray spectroscopy was performed with a high-purity germanium detector on 11.8.2015 on two samples irradiated with the second highest fluence of 5×1015 p/cm2. Since a slightly different chemical composition might result in a different distribution of activation products, samples from both wafers were measured. 78 4.7 Irradiation with gamma rays Two sets of four sample chips each were submitted for gamma irradiation: one set of TiN resistor structure chips, consisting of one chip for each resistor test structure from wafer B1089, and four older chips containing WNx resistors and capacitors. All eight samples were left in the minigrip bags they had been stored in and were packed into a thin-walled plastic jar. The irradiation was performed as batch irradiation at Scandinavian Clinics, Estonia, using a 60Co source with main gamma ray energies of 1173.2 keV (99.97 %) and 1332.5 keV (99.99 %) (Chu et al. 1999). The irradiation aimed for a dose of 100 kGy with a maximum dose rate of 5 kGy/h. Samples were submitted for irradiation on 26.5.2015, irradiated on 8.6.2015 and arrived back in Helsinki on 16.6.2015. From there they were sent to CERN, where they arrived on 25.6.2015. 4.8 Irradiation with X-rays The irradiations with X-rays were performed at DESY (Deutsches Elektronen-Synchrotron, “German Electron Synchrotron”) in Hamburg between 12.8. and 17.8.2015. Most of the irradiation setup was mounted inside an aluminium box which provided shielding from the X-rays. Figure 35 shows the most important parts of the setup, among others the sample holder and X-ray tube. A metal rail on the bottom of the aluminium box permitted irradiation at different distances from the X-ray source, i.e. different dose rates. The Xrays were produced by a PANalytical PW3830 X-ray generator, operated with 50 keV and 44 mA, using a molybdenum target with characteristic X-rays at 17.5 keV and 19.6 keV and weaker background from 10 keV to 28 keV. The rays were emitted horizontally, parallel to the rail. 79 Sample Sample holder X-ray tube Opening, collimator Mount Rail Figure 35. The X-ray irradiation setup inside the aluminium box. Calibration of the dose rate was performed using a passivated implanted planar silicon (PIPS) detector with a thickness of 300 µm and an area of 1×1 cm2. The detector, wirebonded to a ceramic circuit board, was mounted in the sample holder and its leakage current during irradiation was measured with at several distances. The samples were glued with double-sided Scotch tape to a similar ceramic board as the PIPS detector, but were not wire-bonded. Due to the limitations of the sample holder, the samples were glued as close as possible to each other, in order to receive a dose as uniform as possible for the surface area of 2×1 cm2 (Figure 36). 80 Ceramic Sample holder Samples Figure 36. The TiN resistor chips glued to the ceramic board and inserted into the sample holder. Two samples of W4L700 pixel resistor structures – one from each wafer, i.e. 89_Bi and 76_Bi – were irradiated to a total dose of 1 MGy in several steps (Table 5). For the lower doses the dose rate was left at 1 Gy/s, but for the final dose of 1 MGy the sample holder was moved closer to the X-ray collimator to allow a dose rate of 4 Gy/s. Table 5. Irradiation times, expected and actual doses received by the TiN resistor chips. Expected dose 100 Gy 900 Gy 9.2 kGy 90 kGy 900 kGy Expected total dose after irradiation 100 Gy 1 kGy 10 kGy 100 kGy 1 MGy Approximate irradiation time Actual dose after irradiation 100 s 15 min 153 min 15 h 60 h 70 Gy 800 Gy 10 kGy 100 kGy 1 MGy 81 5 Results and discussion The results obtained in the experimental work of this thesis are presented in the same order as introduced in Chapter 4. The investigation of the standard process parameters (growth rate, saturation, linearity of growth) are discussed first, after which the results of thin film characterization for different properties are presented. This chapter contains comparisons to the literature and already some of the author’s own thoughts on specific issues. A summary of the results, as well as wider evaluations, interpretations and conclusions, can be found in Chapter 6. 5.1 TiN thin films deposited by plasma-enhanced atomic layer deposition 5.1.1 Process parameters TiN film depositions were started at 250 °C. At this temperature, the growth rate of 0.085 Å/cycle was very low compared to the lowest growth rates of around 0.2 Å/cycle reported in the literature for TiN thermal ALD (Ahn et al. 2001, Elers et al. 2002, Ritala et al. 1999, Van Bui et al. 2012, Xie et al. 2014), and it was therefore concluded that the suitable process temperature or temperature window would be found at higher temperatures. Figure 37 shows the growth rate as a function of temperature from 250 to 325 °C. No clear ALD temperature window is visible for this process; instead, the growth rate increases with increasing temperature. 82 Figure 37. The growth rate of TiN films as a function of temperature. The absence of a temperature window of constant growth is rather typical TiN ALD processes (Elers et al. 2002, George 2010, Musschoot et al. 2009) and is not automatically contradictory to ALD film growth. As the process studied here is plasma-enhanced ALD, the focus lied on film growth behavior with plasma pulse length instead of temperature – a growth rate remaining constant even when the plasma pulse is prolonged would give the proof of self-limiting film growth. Figure 38 presents the film growth rate at 300 °C as a function of the plasma pulse length. It is seen clearly that the growth rate saturates at a plasma pulse length of five seconds, which confirms that the surface reactions involved in this process are indeed self-limiting and thus follow the most important principle of ALD. 83 Figure 38. The growth rate of TiN films as a function of plasma pulse length. The linearity of film growth, i.e. the linear increase of film thickness with increasing number of ALD cycles, was studied in order to obtain more evidence of the process’s ALD nature. Figure 39 shows that the thicknesses of all films deposited at 300 °C with a plasma pulse length of 5 s can be fitted very well with a straigth line. The slope of this line can be interpreted as growth rate, which in this case would be 0.194 Å/cycle. However, when calculating the average of the growth rates determined individually for each film, a growth rate of 0.183 Å/cycle is obtained both for plasma pulse lengths of 5 s and plasma pulse lengths from 5 to 10 s. As mentioned above, this growth rate concurs well with the literature on TiN ALD. The non-zero intercept (ca. 1.5 nm) of the straight line fitted to the data points is caused by nonlinear growth during the very first deposition cycles, a phenomenon characterized thoroughly by several authors (Van Bui et al. 2012, Satta et al. 2002, Langereis et al. 2006). 84 Figure 39. Thickness as a function of cycle number for TiN films deposited at 300 °C with plasma pulse lengths of 5 s. 5.1.2 Film density and crystallinity According to XRR, the TiN films had densities of 3.8-4.7 g/cm3. These values are clearly lower than the TiN bulk density of 5.2 g/cm3 (Haynes 2015), but agree very well with the densities between 3 and 4.9 g/cm3 reported in the literature for ALD TiN thin films (Miikkulainen et al. 2013). Heil et al. (2006) remark that this phenomenon is not limited to ALD, but that most chemical vapor deposition methods lead to thin film densities lower than bulk density, which Jeon et al. (2000) attribute to the difficulties of packing the atoms in the TiN lattice at the relatively low deposition temperatures. Whithin the group of TiN films deposited by PEALD and ALD, the lowest densities are observed for films deposited by thermal ALD, possibly due to densification caused by plasma exposure occurring in TiN (Zhao et al. 2000) or generally in transition metal nitrides (Kim 2011), and for films deposited with TDMAT and other metalorganic precursors (Elam et al. 2003, Heil et al. 2006). 85 In the XRD patterns recorded for the deposited films, peaks were visible at around 36.7, 42.6 and 61.8 degrees. The peaks clearly matched the TiN reference pattern (00-0381420, following the International Centre for Diffraction Data (ICDD), www.iccd.com). The positions of all peaks exhibited a small, systematic deviation of ca. 0.1 degree from the reference, but this was attributed to the measurement conditions, namely the substrate of the TiN thin films. The reference data on peak positions, intensities and corresponding crystal orientations, along with lattice parameters is presented in Table 6. According to this reference, the three peaks mentioned above correspond to crystal orientations of the cubic TiN osbornite phases (111), (200) and (220), respectively. Crystal orientations are marked in Figure 40, which shows TiN films with different thicknesses grown with 5 s plasma pulses at 300 °C. Table 6. Information about XRD peaks of ICDD TiN reference 00-038-1420, with peaks observed in the recorded XRD patterns in bold. Peak Index 1 2 3 4 5 6 7 8 9 10 111 200 220 311 222 400 331 420 422 511 Lattice parameter (Å) 2.44917 2.12071 1.49967 1.27892 1.22449 1.06042 0.97305 0.94848 0.86577 0.81637 Position 2θ (°) Intensity (%) 36.663 42.597 61.814 74.070 77.964 93.172 104.677 108.611 125.678 141.320 72 100 45 19 12 5 6 14 12 7 Figure 40 shows that crystallinity increases with increasing film thickness – this trend is observed frequently for ALD processes of many different materials (Miikkulainen et al. 2013) and was therefore expected also for TiN. 86 Figure 40. XRD patterns of TiN films with different thicknesses. The crystal orientations corresponding to the visible peaks are marked in the Figure. Not only crystallinity, but also crystal orientation appears to be influenced by film thickness: Figure 40 shows that the intensity of the always dominating (200) peak increases compared to the two other peaks with increasing film thickness. The peak intensity ratios at different film thicknesses are collected in Table 7 to visualize this effect. When comparing the intensity values in Table 6 and 7, it is noticed that the measured intensities do not correspond entirely to the reference. The difference is clearest for the (111)/(200) ratio of the 76.7 nm film, which is as low as 0.3, compared to the 0.72 given for the reference pattern. 87 Table 7. Peak intensities for different films; (200) set as 100 %. Thickness (nm) 12.0 18.2 27.0 38.9 76.7 69.1 nm thermal ALD Reference I (111) (%) I (220) (%) 75 87 66 47 31 68 72 30 58 46 44 29 36 45 The peak intensities measured from the thermally grown control film (69.1 nm), on the other hand, were rather close to the reference. Figure 41 points out the differences in crystal structure between the thermal film and the plasma-grown film closest in thickness (76.7 nm). This result indicates that the plasma-enhanced ALD of TiN using the process developed in this study provides the opportunity of depositing TiN films with a higher content of the (200) orientation – in the literature, a dominating (111) orientation is more common (Ahn et al. 2001, Kim et al. 2003, Xie et al. 2014). Figure 41. XRD patterns of a thermally deposited TiN film (69.1 nm) and a TiN film deposited with our PEALD process (76.7 nm). The crystal orientations corresponding to the visible peaks are marked in the Figure. 88 Two other parameters that might affect film crystallinity were the deposition temperature and the plasma pulse length. XRD patterns from TiN films grown at different plasma pulse lengths are presented in Figure 42. All films had thicknesses of 27.0-27.2 nm. The film deposited with a plasma pulse length of 7.5 s was left out, since it was only 22.0 nm thick and the strong effect of film thickness on crystallinity would make the comparison meaningless. No distinct increase in crystallinity nor changes in peak intensity ratios as function of plasma pulse length are seen. The film grown at 5 s plasma pulse lengths is the most crystalline, but crystallinity decreases slighly towards both 2.5 and 10 s. Figure 42. XRD patterns of TiN films grown at different plasma pulse lengths. 89 The effect of deposition temperature is shown in Figure 43 for films of 18.2-22.9 nm. Despite the thickness differences, it can be concluded that crystallinity increases with increasing deposition temperature. The (200) peak is the first to appear already at 250 °C, while peaks (111) and (220) follow only at 300 °C, a behavior described also by Jeon et al. (2000). All peak intensities increase towards 325 °C, but a change in intensity ratio of the other peaks and the (200) peak cannot be observed for the relatively small film thicknesses. Figure 43. XRD patterns of TiN films grown at different temperatures. 90 5.1.3 Surface morphology and film conformality with field-emission scanning electron microscopy Especially the earlier TiN films were studied with scanning electron microscopy to obtain information about their surface morphology and crystallinity; however, only very few expressive images were obtained. This can be interpreted either as an indication of relatively low conductivity of the films, leading to dark images with poor contrast, or simply a sign of very smooth films with small surface structures and crystal grains. Figure 44 shows two relatively good-quality images of 27 nm TiN films. Both images indicate that the TiN films in question are polycrystalline, with a rather narrow grain size distribution around a grain size of ca. 20 nm Figure 44. FESEM images of TiN films. a (left): B1072 on Si, b (right): B1068 on SiO2 (right). Figure 45 shows the cross-section of a TiN film on a normal substrate surface without structures. Grains are not distinguishable, but the film appears columnar. 91 TiN Si substrate Figure 45. TiN film on a silicon substrate. Of the high aspect ratio substrates, good FESEM images were obtained only for the G10 substrate. Figure 46a, taken from the opening of a trench on G10, shows the conformal covering of the edge by the 35 nm TiN film, which appears to be continuous with a slight columnar structure and clearly visible, rounded grains. The grain size is estimated to be between 50 and 100 nm, in any case several times larger than the film thickness and slightly larger than the grains seen in Figure 44. Figure 46b demonstrates that even a trench wall with step structures are covered conformally by the TiN film. 92 Figure 46. Uniform TiN films on the surfaces and walls of trenches on the G 10 substrate. a (left): a trench with smooth wall, b (right): a different trench wall with step structure. Figure 47a shows the same trench as in Figure 46a at a depth of 5 µm. The TiN film is uniform and has the same thickness as at the surface. Figure 47b shows the trench’s bottom area. The TiN is still uniform and has the same thickness and structure (faintly visible in the background) as on the surface, up to a distance of ca. 250 nm from the bottom of the trench, where the film rapidly becomes thinner and disappears from sight. Figure 47. a (left): TiN film near the bottom of a trench in a G10 substrate, b (right): TiN film at a depth of 5 µm in a trench in the G 10 substrate. Conformality studies show that the TiN film covers the walls of both smooth and structured trenches very well up to an aspect ratio of over 26:1. For a thin film deposited by plasma-enhanced ALD, this result is excellent and indicates that uniform and controlled PEALD of TiN films is possible even for very high aspect ratio substrates. 93 5.1.4 Surface morphology with atomic force microscopy Image recording and and modification were performed by Miika Mattinen. Three samples of different thicknesses (12.9, 22.9 and 76.7 nm), all deposited at 300 °C with 5 s plasma pulses, were imaged. 3D images (Figure 48) recorded over a 0.5 µm × 0.5 µm area show very smooth and uniform 12.0 nm and 76.7 nm films, with Rq values of 0.43 nm and 0.46 nm, respectively. Interestingly, the intermediate film of 22.9 nm appears more rough and coarse (Rq = 2.9 nm). Figure 48. 3D images of a 0.5 µm × 0.5 µm area. Left: 12.0 nm, middle: 22.9 nm, right: 76.7 nm. In a 2D image (Figure 49) over the same area, the 12.0 nm film appears finely granular, but the 76.7 nm film has a different appearance without clear grains. The mentioned higher roughness of the 22.9 nm film is explained by clearly visible, round crystal grains. Figure 49. 2D images of a 0.5 µm × 0.5 µm area. Left: 12.0 nm, middle: 22.9 nm, right 76.7 nm. Phase images (Figure 50) indicate that all samples consist of TiN without any alien phase being present, and visualize the crystal grains in the 12.0 and 22.9 nm films even better. 94 Figure 50. Phase images of a 0.5 µm × 0.5 µm area. Left: 12.0 nm, middle: 22.9 nm, right 76.7 nm. 3D (Figure 51) and 2D (Figure 52) images over a larger area of 10 µm × 10 µm reveal the presence of particles on the 76.7 nm film. Based on the substantial height of the particles relative to the film surface and their scarceness, it is concluded that the particles do not originate from the deposition process itself. They might be the result of contamination of the film surface, or irregularities on the substrate that were then covered by the TiN film. This theory is supported by the lack of any particles on the thinner films, which exhibit perfect uniformity and smoothness even over a larger area. Even the 76.7 nm film remains relatively smooth (Rq = 3.3 nm). Figure 51. 3D images of a 10 µm × 10 µm area. Left: 12.0 nm, middle: 22.9 nm, right 76.7 nm. The ”waves” seen in the 12.0 nm film are image artefacts. Figure 43. 2D images of a 10 µm × 10 µm area. Left: 12.0 nm, middle: 22.9 nm, right 76.7 nm. 95 Figure 53 shows a 3D and a 2D image over a 5 µm × 5 µm area for the thermally deposited 69.1 nm TiN film. The film is much rougher than the films deposited by PEALD, with a Rq value of 16.1 nm. Figure 53. 5 µm × 5 µm images of the thermally deposited TiN film. Left: 3 D image, right: 2 D image. The AFM results support the evidence of increasing crystallinity with increasing film thickness. The thinnest film of 12.0 nm is very smooth, but a higher roughness resulting from large crystal grains is observed when the film thickness is doubled. The 76.7 nm film is again smooth, but based on the XRD data, this film is more crystalline than the thinner ones, and a decrease in grain size after previous increase is rather improbable. Therefore it is concluded that the crystal grains have grown and coalesced even further, to form very large, flat grains, or even to form a nearly ideal, monocrystalline thin film. 96 5.1.5 Analysis of chemical composition Five TiN thin film grown on silicon substrates were analyzed by ToF-ERDA. Analysis, calculation of the results and their collection into depth profiles were performed by Kenichiro Mizohata. Due to the resolution limitations of the ToF-ERDA method, relatively thick samples (27-77 nm) were chosen in order to obtain more reliable depth profiles for the composition of the films. However, they only provide an indication of the distribution of different elements in the TiN films. The results for the overall element concentrations in at-% (Table 8) are accurate as such, but might give a too negative picture of the amount of impurities in the film, since these numbers include both impurities distributed throughout the films and impurities induced by surface oxidation or possible surface contamination. Table 8. Composition of samples analyzed with ToF-ERDA. Sample B1070 B1072 B1084 B1087 B1090 B1133 (thermal) Thickness (nm) 27.7 27 76.7 38.9 27.2 69.1 1.28 1.27 0.94 1.08 1.10 [Cl] (at-%) 7.20 3.62 6.59 6.51 7.42 [O] (at-%) 18.73 19.29 4.10 8.49 8.80 [H] (at-%) 4.28 3.19 2.34 2.63 3.14 [C] (at-%) 0.10 0.17 0.08 0.15 0.18 Resistivity (mΩ cm) 1.882 0.795 0.921 0.952 0.986 1.05 7.01 7.90 2.36 0.65 0.787 Ti/N Figure 54 shows the depth profile of the B1072 film with a thickness of 27 nm. The film is clearly confirmed as TiN with similar concentrations of titanium and nitrogen. However, there is also a very high amount of oxygen, with concentrations between ca. 12 at-% in the film and 20-30 at-% at the surface and substrate interface. Chlorine is present in smaller amounts of around 5 at-% throughout the film, but interestingly not at the surface. Carbon impurities are detected, but are very small compared to oxygen and chlorine. The same is true for hydrogen, except for a peak during the first nanometers of the film surface. 97 Figure 54. Depth profile of the chemical composition of the B1072 TiN film (27 nm). The perhaps most reliable and representative ToF-ERDA depth profile was obtained for the thickest film from batch B1084 (76.7 nm, Figure 55), for which one can differentiate best between substrate interface, film ”bulk” and surface. The film consists mainly of titanium and nitrogen in similar concentrations, the most abundant impurity is chlorine with around 5 at-%. The distribution of oxygen continues the trend observed for the thinner films: there is an oxygen peak directly at the surface and a flatter one at the substrate interface, but only a very low oxygen concentration in the film. 98 Figure 55. Depth profile of the chemical composition of the B1084 TiN film (76.7 nm). ToF-ERDA analysis proved that the samples, and therefore assumingly also the rest of the films deposited for this study, were TiN with a tendency to understoichiometry. Only B1084 (76.7 nm PEALD film) is slightly overstoichiometric, but is of all films closest to the ideal 1:1 stoichiometry. Titanium-rich films have been reported at short plasma pulse lengths of 5 s (Heil et al. 2006) similar to the ones used in most of the depositions in this study. Also plasma energy might have affected the stoichiometry (Langereis et a. 2006). The titanium richness of the films indicates that no Ti3N4 phase is present. Figures 45 and 46 indicate that titanium extends deeper than nitrogen towards the silicon substrate or is even mixing with it. The formation of a Ti-Si or Ti-Si-O interface layer is not ruled out, but the literature focuses instead on the formation of an oxynitride layer (Kim 2011, Langereis et al. 2006). This phenomenon might also be caused by inaccurate normalization of the profiles for single elements. The impurities seen in all films are chlorine, oxygen, hydrogen and carbon. Carbon is present only in negligible amounts, which are believed to be caused by contamination of the surface after deposition, since none of the precursors should contain carbon. 99 Hydrogen is present in small amounts throughout the film and more abuntantly on the surface. The peak at the surface can be the result of either contamination or simply the termination of the surface with –NHx or, in case of surface oxidation, –TiNxOyHz groups. Hydrogen inside the film is most likely an impurity left there by incomplete removal or trapping of the NH3 hydrogen atoms, or by readsorption a possibly formed H2 side product. All analyzed films contained a significant amount of chlorine. This impurity is left in the film due to the incomplete removal of the chlorine atoms from chemisorbed TiClx species due to the too low reactivity of NH3 and the various plasma species. At the relatively low process temperatures studied here, the formation of less volatile NH4Cl might also play a role, as indicated in the literature (Schubert and Hüsing 2005, Juppo et al. 2002b) The decrease of chlorine towards the surfaces, or at least the absence of a peak in chlorine concentration at the surface, and the even distribution throughout the film support the hypothesis that the chlorine impurities originate from the process chemistry rather than contamination or post-deposition events. All films also exhibited high levels of oxygen impurities. Unlike chlorine, these were found in higher amounts on the surface and at the film-substrate interface. Therefore it is believed that the oxygen is introduced by surface oxidation, to which TiN thin films were observed to be sensitive (Ernsberger et al. 1985, Heil et al. 2006, Xie et al. 2014, Niyomsoan et al. 2002), even though the bulk material is described as chemically stable. A contamination of the surface with water or moisture from ambient air might also explain part of the oxygen and hydrogen observed right at the film surface. The oxygen seen at the TiN-silicon interface results from the native oxide layer of the silicon substrates, which was not removed prior to TiN deposition. This interface oxygen might be at least partly responsible for oxygen impurities throughout the film distributed by diffusion, as even films that were not exposed to ambient air were found to contain high amounts of oxygen (Tiznado and Zaera 2006). 100 It appears that the earlier films, i.e. films from batches of under B1080, contain more oxygen. This can be due to a leakage in the reactor (maintenance was performed at batch numbers B1080 – B1082), or the smaller thickness of the analyzed films, allowing oxygen from ambient air to diffuse more easily into the films. The thermally deposited film contained slightly more oxygen throughout the film, as well as a broader oxygen peak at the TiN-substrate interface. The latter could be an indication that the native oxide layer of the silicon substrate (or a forming TixOy layer) is sputtered by the plasma to some degree. Despite all this, the thermally deposited film had a lower resistivity than the comparable 76.7 nm PEALD film, which shows that at least small variations in the oxygen content have no clear effect on film resistivity. The stoichiometry of the thermal film was very close to the ideal 1:1 stoichiometry, but, on the contrary to the PEALD film, slightly overstoichiometric. 5.1.6 Resistivity of TiN films and the influence of chemical composition and parameters Four-point probe measurements gave resistance values R (in Ω) as results. Resistances were measured at several points of a wafer to form a map, as schematically presented in Figure 56 for wafer B1089. Figure 56. Resistance map of wafer B1089 (22.9 nm), all values in Ω. 101 Higher resistivities at the lower edge were observed for most films, and for some a slightly different color was visible in the same region. The gradient might have been caused by processing-induced irregularities in the wafer, or differences in film thickness at the wafer edge closest to the TiCl4 precursor inlet (”leading” edge). An approximate average resistance value was used to calculate the film’s sheet resistance according to Equation 18, which could be used because the spacing between the four-point probe contacts is much smaller than the length and width of the measured samples (Heaney 2004): Rs = πR ln(2) ≈ 4.53 R (18) were Rs is the sheet resistance and R a measured resistance value. Inserting 120 Ω determined for B1089 gives: Rs (B1089) = 4.53 × 120 Ω = 543.6 Ω/sq The resistivity ρ was then calculated by multiplying the sheet resistance with the film thickness d, e.g. for B1089: ρ (B1089) = Rs (B1089) × d (B1089) (19) = 543.6 Ω/sq × 22.9×10-7 cm = 1.24 mΩ cm No dependency of resistivity on plasma pulse length or deposition temperature could be seen for the deposited films, but resistivity decreased nearly exponentially with increasing film thickness (Figure 57). This is a consequence of size effects (Mayadas and Shatzkes 1970, Langereis et al. 2006) and is often seen for TiN thin films (Ahn et al. 2001, Langereis et al. 2006, Musschoot et al. 2009, Xie et al. 2014). However, two relatively thin films had surprisingly low resistivities for unclear reasons. 102 Figure 57. Resistivity as function of film thickness. For TiN thin films deposited by PVD, a correlation of resistivity with crystal orientation has been observed – films with preferred (200) orientation had lower resistivities than films with (111) orientation (Hahn et al. 1987, Meng and dos Santos 1997). It appears that this does not apply to the new PEALD process, since the thermally deposited film, despite its higher (111)/(200) ratio, had a slightly lower resistivity than a comparable PEALD film. Most chemical impurities in TiN films – carbon, oxygen, chlorine – are associated with higher resistivities. The role of hydrogen is less clear, and high amounts of it in TiN films are rare. The influence of chlorine on film properties, namely resistivity, is not clear – often chlorine is simply assumed to be the reason for higher resistivities, but without direct proof. In this case, only a very small increase in resistivity was observed for increasing chlorine concentration (Figure 58). 103 Figure 58. Resistivity as a function of chlorine concentration. No clear pattern is visible for resistivity as a function of Ti/N ratio. The resistivity increases very slightly from a ratio of 0.95 towards 1.10, but the resistivity of the film with Ti/N ratio 1.27 was the lowest of all measured samples. It is concluded that moderate changes in stoichiometry do not affect the electric properties of the TiN films deposited during this study. The effect of oxygen on film resistivity is surprisingly small: a fourfold increase in oxygen concentration changes the resistivity only by less than 20 %, and is associated with a decrease, rather than increase, in resistivity (Figure 59). 104 Figure 59. Resistivity as function of oxygen concentration. The oxygen concentration depicted here contains both surface, TiN-Si interface and film oxygen, showing that even a considerable amount of oxygen does not lead to significantly higher resistivity. It is therefore concluded that no insulating TiO2 is formed inside the films – the oxygen in the films is either in impurity or interstitial form, or forms conducting Ti- and O- containing phases, such as TiO. 5.1.7 Observations on film adhesion and film color Adhesion of two TiN films (77 and ca. 14 nm) to a SiO2 substrate was evaluated by the Scotch tape test. Both films passed the test. Despite bulk TiN being known for its extreme hardness and wear-resistance, the thin films were easily scratched with steel pincettes. Microscope images have shown both larger and smaller scratches, from which the latter appear to be caused by smaller particles, maybe Si splinters. 105 The colour of the TiN films on glass was a light brown or grey, sometimes with a yellowish tone. Thinner films were transparent and even ”thicker” films (up to 77 nm) only sligthly reflective. On silicon substrates, film growth was only sparingly visible with the naked eye; the polished silicon surface turned darker grey and subsequently brownish as film thickness increased. A film of ca. 35-40 nm caused a clearly visible, light brown colour. The thickest films of 70-77 nm exhibited a dark blue-grey colour. On silicon oxide, i.e. a SiO2 layer of approximately 200 nm formed by thermal oxidation of silicon wafers with a light golden colour of its own, already very thin films (8-12 nm) of TiN were exposed by a change in colour from golden to yellowish. With increasing film thickness, the film color changed from more intense yellow through yellow- and orange-red to a pinkish red. The thickest films (70-77 nm), however, had a colour very similar to that on the silicon substrates. Compared to titanium oxide, the titanium nitride thin films’ colours on all substrates were much more shallow: for example, while films of TiO2 (ca. 54-60 nm) exhibited a shiny electric blue or deep violet colour, even the thickest TiN films’ blue color was subdued and closer to grey. 5.2 Resistance measurements: Homogeneity of resistors over a wafer half If the new TiN PEALD process is to be used in the serial fabrication of TiN bias resistors, it is crucial that the film is deposited uniformly over a substrate, which is expected to be a silicon wafer of at least 4” in diameter. The uniformity most important for this application, the uniformity of resistivity over the wafer, was assessed by plotting the average resistances of bias resistor chips from wafer half B1089_Bi as a function of the chips’ position on the wafer. Figure 60 shows the average resistances of different, notannealed resistor chips as a function of the row, presenting a side view of the wafer, with 1 corresponding to row A directly under the mask title (cf. Figure 17), etc. The figure indicates that resistivity has slight profile with an increase in resistance towards the bottom of the wafer, but the differences are small, especially for the smaller structures, and should not affect the resistor performance significantly. 106 Figure 60. Average resistance of different resistor structures from wafer half B1089_Bi as a function of their vertical position on the wafer. Figure 61 shows the corresponding plot of average resistance as a function of the position in a certain column of the wafer, 1 being the innermost column. Here, there is no visible gradient of resistance, however, the chips closest to the wafer edges (numbers 5-7) were not used. Figure 61. Average resistance of different resistor structures from wafer half B1089_Bi as a function of their horizontal position (column) on the wafer. 107 Judging from these preliminary results, the uniformity of the TiN thin film over the measured resistor chips is excellent. The higher resistivities measured for the front edge by four-point probe, however, indicate that the resistors at the lower edge of the wafer might have higher resistivities compared to those more at the center of the wafer. The resistors on the edges of the entire wafer were not yet used in order to minimize the effect of any processing faults on the results of the annealing or irradiation experiments. 5.3 Annealing As mentioned in Chapter 4.2.3, it was noticed that already a relatively short annealing of 30 min at 400 °C in a N2 atmosphere caused a clear increase in resistance for the processed TiN resistor structures. This treatment corresponds to the common process step of aluminium sintering necessary for establishing an ohmic contact between Al and Si or SiO2. The increase in resistance needed to be studied in more detail, since the resistance values of as-processed resistors were too low for the application as bias resistors in segmented detectors (250-330 kΩ for W4L3550 and ca. 60 kΩ for W4L700). Figures 62 and 63 present the development of the resistance for the different samples plotted as a function of annealing time at 400 °C for wafers B1076 and B1089, respectively. A quasi-exponential increase in resistance is observed for all resistor types and both wafers. 108 Figure 62. Resistance of different resistor structures processed from B1076_Bi (12 nm) as function of the annealing time at 400 °C under N2. Figure 63. Resistance of different resistor structures processed from B1089_Bi (23 nm) as function of the annealing time at 400 °C under N2. 109 It has to be noted that the resistance measurement becomes less reliable as the resistance values increase, even though the errors appear tolerable. The values of over 100 MΩ reached after annealing samples from B1076 for 90 min are included in the results, but should not be trusted. It was observed that the reliability of the resistors weakened especially for B1076 resistors after several annealing steps. At first, 20 resistors at random could be measured easily from each chip, while after 90 min of annealing, many resistors did not give useable results. After 150 min, no data could be obtained for B1076 resistors. The thicker film from wafer B1089 was more reliable, and results were obtained, though with difficulty, even after 150 min of annealing. Naturally, the reasons for this behavior were investigated. AFM could not detect any changes in surface morphology for the annealed 76.7 nm film. Table 9 shows the ToF-ERDA results for an annealed film in comparison to the asdeposited film, revealing several changes in chemical composition. Table 9. Chemical composition for the 76.7 nm TiN film as-deposited and annealed for 2x30 min at 400 °C. Film Ti/N [Cl] (at-%) [O] (at-%) [H] (at-%) [C] (at-%) 76.7 nm as-deposited 76.7 nm annealed 0.94 6.59 4.1 2.34 0.08 1.00 6.42 6.28 1.90 0.16 The corresponding depth profile, unfortunately in a different scale than the profile for the as-deposited film, is presented in Figure 64. 110 Figure 64. ToF-ERDA profile of a sample of the 76.7 nm TiN film annealed for 2x30 min at 400 °C. Both results and profile show that no changes have occurred in the chlorine content of the film or its distribution, implying that chlorine is not much affected by the elevated temperatures during annealing and does not evaporate from the film as was suspected. Higher chlorine content might cause a slightly higher resistivity of the film to begin with, but it cannot not to be responsible for the increase in resistivity observed for the resistor structures, as there is no possible source of additional chlorine in the environment used for annealing treatments. The carbon content was also found to increase during annealing. However, its relevance is estimated to be minuscule, as its values are low with less than 0.2 at-% for all PEALD TiN films. The Ti/N ratio is slightly higher in an annealed film and corresponds to ideal 1:1 stoichiometry. This is an indication of a loss of nitrogen during annealing until the equilibrium stoichiometry was reached. Hydrogen also appears to be partly evaporating under annealing, according to the profile mostly from the surface. The role of hydrogen content in TiN films is less well studied than for other impurities, and the consequences of the decrease in hydrogen are difficult to interpret. 111 Due to the rather low hydrogen concentrations (< 2.4 at-%) found in the TiN films deposited in this study, their influence on film resistivity is assumed to be negligible. It is most likely that the increase in resistance is connected to the oxygen in the TiN film. ToF-ERDA shows that the total oxygen content of the annealed film has increased from 4 to 6 at-%. A rudimentary visual examination of the depth profile shows a roughly unchanged oxygen concentration at the TiN-Si interface, but a decrease in surface oxygen and a slight increase of oxygen concentration in the film. The most probable cause for this is the diffusion of oxygen, rendered more mobile at elevated temperatures, from the surface into the film. There is no sign that significant additional oxidation would have occurred during annealing. Another aspect besides the total amount and distribution of oxygen is its chemical state: oxygen can be present in the film as interstitial impurity, replacing N atoms, or in a separate phase. The elevated temperatures might have caused phase separation, from impurities to a separate Ti-O phase, as well as transformations of one phase into another, most importantly the transition of conductive TiO or Ti2O3 phases (Ernsberger et al. 1985) into insulating or semiconducting TiO2. However, none of the techniques used in this study observes the chemical or oxidation state of an element, and the total concentration of oxygen is too low to form a separate, crystalline oxygen-containing phase in sufficient amounts for detection through changes in the films’ XRD patterns. The responsibility of chemical interactions of TiN with the aluminium used in the contact pads for the increase in resistance is seen as unlikely, as the onset of such interactions occurs at 550 °C (Wittmer et al. 1983), i.e. at a temperature far higher than used in the annealing treatments. 112 5.4 Irradiation experiments 5.4.1 Irradiation with 10 MeV protons No changes in resistivity were noted in preliminary measurements (n = 5 per chip) for any of the structures on 25.6.2015. Further measurements (overall n = 20 per chip) around 20.7.2015 confirmed that the resistance of all four resistor structures has remained unchanged after 10 MeV proton irradiation, as shown in Figure 65. The measurements also showed that this irradiation did not have long-term (1-2 months) effects on resistance. Figure 65. Resistance of resistor structures from wafer B1089 as function of resistor size before and after 10 MeV proton irradiation. 113 The autoradiogram (Figure 66) developed after an exposure time of two hours showed that the TiN film had been activated to some extent – in the darker areas marked in the autoradiogram, the sample was accidentally broken and pieces of the surface with silicon and the TiN film were removed, proving that the observed activity was caused by a nuclide in the TiN film. Also, the more short-lived activation products of silicon and aluminium were assumed to have already decayed at that time. 1 cm Figure 66. Autoradiogram of a 76.7 nm TiN film on a Si substrate irradiated with 10 MeV protons after 2 h of exposure. AFM could not show any any changes in the 76.7 nm film after irradiation. Table 10 shows the ToF-ERDA results for an irradiated film in comparison to the as-deposited film- The corresponding depth profile, unfortunately in a different scale than the profile for the asdeposited film, is presented in Figure 67. Table 10. Chemical composition for the 76.7 nm TiN film as-deposited and irradiated with 4.7×1014 p/cm2. Film Ti/N [Cl] (at-%) [O] (at-%) [H] (at-%) [C] (at-%) 76.7 nm as-deposited 0.94 6.59 4.10 2.34 0.08 76.7 nm irradiated 0.97 6.93 4.28 2.46 0.17 114 Like annealing, also proton irradiation appears to cause a slight loss of nitrogen that brings the film closer to ideal stoichiometry. The concentrations of all impurities appear to have increased, but at least for chlorine this is unlikely. The increase on carbon content is attributed to contamination, caused by more frequent handling of the sample. In total, no significant changes were caused by irradiation. Figure 67. ToF-ERDA profile of a sample of the 76.7 nm TiN film irradiated with 4.7×1014 p/cm2. Recording of signals for the gamma spectroscopy was started 20 minutes after the end of the irradiation and continued for 18167 s. The spectrum presented in Figure 68 was obtained as a result. 115 Figure 68. Gamma spectrum of irradiated TiN resistor chips. Identified spectral peaks are numbered. The radionuclides causing the observed spectral peaks were identified by comparing their energy to literature values in the Lund/LBNL Nuclear Data database (Chu et al. 1999). Table 11 lists all significant peaks and their origin. Table 11. Peaks observed in the gamma spectrum of proton-irradiated TiN resistor chips, their origin and literature values for energy and intensity. Peak 2 3 4 5 6 7 8 Energy in data report (keV) 75.11 77.34 242.07 295.18 351.82 510.87 609.14 943.88 983.29 9 10 1039.07 1119.78 1 Origin Bi Kα 2 Bi Kα 1 Pb-214 Pb-214 Pb-214 + e - e annihilation Bi-214 V-48 V-48 Sc-48 Bi-214 Bi-214 Literature value of energy (keV) 74.815 77.107 242.0 295.2 351.9 Literature value of intensity (%) 27.8 46.8 7.43 19.3 37.6 609.3 944.1 983.5 983.5 1038.0 1120.3 46.1 7.76 99.98 100 ? 15.1 116 Table 11 continues Peak 11 12 Energy in data report (keV) 1156.77 1311.95 13 1494.66 14 1764.45 15 1823.53 16 17 18 2204.1 2241.15 2296.36 19 2615.84 20 2753.19 Origin Sc-44 V-48 Sc-48 Coincidence of peaks 5 and 8 Bi-214 Literature value of energy (keV) 1157.0 1312.1 1312.1 Literature value of intensity (%) 99.9 97.5 100 1764.5 15.4 2204.2 2240.4 5.1 2.4 2754.0 99.9 Coincidence of peaks 5 and 12 Bi-214 V-48 Coincidence of peaks 8 and 12 Coincidence of two peaks 12 Na-24 214 Pb (t1/2 = 26.8 min) and its daughter 214Bi (t1/2 = 19.9 min) are naturally occurring isotopes that belong to the 238U decay chain. Although the two isotopes themselves are short-lived, their activities are sustained by their longer-lived mother nuclides which are ubiquitous in the granite bedrock of Finland. (Atwood 2010) 214Pb and 214Bi are often seen in gamma spectra measured with long recording times in the abovementioned detector. 48 V is the radioisotope with the highest activity of all nuclides identified from the gamma spectrum (peaks 7, 8, 12, 17). This is in good agreement with earlier spectroscopies of activated titanium from the cyclotron beam line, as well as the literature, where 48V is frequently mentioned as activation product of titanium due to the high cross-section of the 48Ti (p, n) reaction and the presence of several formation routes from almost all stable titanium isotopes (Walke 1937, Szelecsenyi et al. 2001). The intensity of the 48V gamma transitions is already sufficiently high to give rise to several sum peaks (peaks 5, 8, 12). This could be avoided by diluting the sample or increasing its distance to the detector, but both actions would drastically decrease the detection efficiency and prevent the detection of isotopes present in lower activities. 117 Already now, the only other isotope resulting from titanium activation that could be reliably identified was 44Sc (peak 11), the product of the (p,α) reaction of 48Ti. The same peak could also belong to 44mSc, but this isotope should have a far more intense transition at 271 keV, which is not visible. A distinction between 48V and 48Sc is not possible with gamma spectroscopy, as both decay back into the same stable isotope 48Ti (by positron emission and β- decay, respectively) and thus have identical gamma transitions. However, 48Sc is not expected to form in any of the common nuclear reactions induced by protons - for example, its formation via the (p,α) reaction would require the isotope 52Ti, which is not a stable titanium isotope and not expected to be formed in proton activation, either. Peak 10 as such could have originate also from 46Sc, the product of a (p,α) reaction of the stable 50Ti. Based on the lack of another peak at 889 keV, the first high-intensity gamma transition of 46Sc, peak 10 was attributed to 214Bi instead. 5.4.2 Irradiation with 24 GeV/c protons Due to delays in access to the irradiated samples, gamma spectroscopy was performed only about six weeks after the end of the irradiation. At that point, most possible activation products of titanium and all of nitrogen had already decayed. The only nuclides visible, 7Be and 22Na, are common radioisotopes in irradiated silicon, present here as the substrate material. Similarly to 10 MeV proton irradiation, no change in resistance could be observed for the samples irradiated with 24 GeV protons, as is seen in Figure 69. The four data points at each fluence correspond to the samples from the four different wafer halves. 118 Figure 69. Resistance of resistor structures from both wafers and both halves as function of resistors size before and after 24 GeV proton irradiation with doses of 5×1015 p/cm2 (W4L700) and 2×1016 p/cm2 (W4L3550). 5.4.3 Irradiation with gamma rays The final absorbed dose was 102.8 ± 5.14 kGy for each sample. The irradiation report by Scandinavian Clinics is attached to this thesis (appendix B). No activation was noticed, and preliminary measurements (n = 5) indicated that there were no changes in resistance. The samples were kept mostly at room temperature, after 25.6.2015 in the freezer at -20 C. The final measurements (n = 20) performed on 20.7.2015 confirmed that resistance had not changed due to gamma irradiation, and showed that gamma irradiation had no longterm (1-2 months) effects on the resistance. The resistances measured before and after gamma irradiation are shown in Figure 70. 119 Figure 70. Resistance of resistor structures from wafer B1089 as function of resistor size before and after 60Co gamma irradiation and a dose of 100 kGy. 5.4.4 Irradiation with x-rays The resistance of the TiN resistors was measured for each chip (n = 5) after each irradiation step. As shown in Figure 71, no change in resistance could be observed for either of the samples, not even after a dose of 1 MGy/sample. The uncertainties are much smaller for the chip from B1089 (22.9 nm) compared to the one from B1076 (12 nm), which concurs with the observation made during the annealing experiments that the resistors from B1089 are more reliable. 120 Figure 71. Resistance of W4L700 resistors from B1076 and B1089 as function of 15-18 keV X-ray dose. 6 Conclusions Titanium nitride thin films were deposited with a newly developed plasma-enhanced ALD process using TiCl4 as titanium precursor and a NH3/Ar mixture as nitrogen source and plasma gas. Film growth was studied at different temperatures, and its self-saturating nature and linearity were investigated. The properties of films of different thicknesses and deposited under different deposition conditions were studied. At 300 °C, the growth rate saturated at ca. 0.19 Å/cycle for plasma pulse lengths of 5 s and longer, and film thickness increased linearly as a function of ALD cycle number. The growth rate appears logical, considering that for thermal ALD of TiN, growth rates of around 0.17 Å/cycle are frequently reported and higher growth rates in TiCl4-based processes are only reached in some cases. 121 With the new process, TiN films could be deposited slightly, though not dramatically, faster than with most published thermal ALD processes, and at a temperature of 300 °C instead of the usually used 400-500 °C. Similarly to many TiN ALD processes, and ALD window was not observed for this process, either. The proof of self-limiting growth was instead given by the growth rate saturation at a certain plasma pulse length and supported by the linearity of growth. Film crystallinity increased noticeably with increasing thickness and slightly for increasing temperature and plasma pulse length, as expected. For all films, the dominating orientation was (200), concurring with the literature on TiN films deposited with chemical methods. Compared to the reference and a thermally deposited film, the PEALD TiN films showed clearly lower disposition for other orientations beside (200). It is not clear whether this has an effect on film resistivity, or whether it might be advantageous for any applications. Film conformality was assessed preliminarily and judged to be very good, especially for a PEALD process. Films of different thicknesses were very smooth and homogeneous over large areas, while large particles were found in the imaging of a thermally deposited film. The TiN films contained some oxygen, mostly due to surface oxidation and the native oxide layer of the used silicon substrates. Surface oxidation is reported to be very common, if not unavoidable, for TiN thin films, and did not appear to influence resistivity significantly. The chlorine concentration of the films was higher than in a large part of the literature on TiN ALD, even though PEALD is often praised for improving film purity. It is concluded that either chlorine content is almost unaffected by the use of a plasma and is influenced by temperature only (which was relatively low in the new process), or that the use of NH3 plasma was in this case the cause for higher chlorine concentration in the films. The latter is considered more probable, since the TiCl4-H2/N2 PEALD process reaches lower chlorine concentrations even at 100 °C. The NH3 plasma might, besides promoting desired surface reactions, also have contributed to the dissociation of adsorbed and already desorbed chlorine-containing byproducts, leading to increased chlorine amounts in the film. 122 Whether caused by chlorine or other factors, the resistivities of the TiN films deposited with the new PEALD process were high compared to most publications. No direct correlation of resistivity to any process parameter or impurity content could be observed, except for a slight increase with increasing chlorine content. High resistivities of TiN thin films are undesirable for most applications, but for the application as bias resistor studied in this work, the new PEALD process proved more promising than processes described in the existing literature. The only ALD TiN films with comparable or higher resistivities were deposited thermally from TDMAT and NH3, and these were of bad quality with low densities and 40 % oxygen. It is very likely, however, that other processes and approaches leading to high-resistivity TiN thin films of better quality have been discovered earlier, but were deemed unworthy of publishing, because the studies’ objective were films with low resistivities. Processing of TiN thin films into bias resistor structures was uncomplicated. The fabrication of TIN thin films into bias resistors involved thin film deposition, mask application, etching and resist stripping, then the same steps for the fabrication of aluminium contact pads, and finally aluminium sintering / resistor annealing. TiN could be etched with a relatively harmless 30 % H2O2 solution at 50 °C that does not attack the other materials, and was in its turn left undamaged by aluminium etching and photoresist stripping. Application, development and removal of the photoresist layer were unproblematic, except for part of the smallest structures on one wafer that remained underetched. This should be overcome by more careful (probably longer) photoresist development, and the immediate removal of bubbles from the wafer surface during TiN etching. Deposition of TiN thin films by PEALD is slower than by PVD or CVD methods, but this is compensated by the excellent film uniformity and conformality provided by ALD. Both aluminium and TiN photolithographies are selective for their respective materials and the obtained bias resistor structures were mostly flawless. All processing steps, except thermal oxidation of the silicon wafers, remain at temperatures of 400 °C or below. 123 The annealing does not require protection of the rest of the wafer surface nor exceptional conditions, it can be performed by simply prolonging the aluminium sintering step necessary in any case. TiN forms ohmic contacts with silicon and aluminium without additional doping, and the resistors can be tested immediately after processing with a simple IV measurement. The additional ion implantation and annealing steps for resistivity increasing and ohmic contact formation in poly-Si are not necessary for TiN resistors. This allows the removal of two mask layers, two doping steps and at least one annealing step from the overall processing, making it more straightforward and therefore both faster and more economical. It was noticed that the common process step of aluminium sintering at 400 °C under inert gas caused an increase in resistance. This behavior was studied further, and drastic increases in resistance of up to three orders of magnitude were observed for all resistor structures on both wafers. The resistance value desired for the bias resistors is not expected to exceed 10 MΩ, which does not extend into the area where both measurements and resistors would become unreliable. The described annealing procedure therefore appears to be a very straightforward way to raise the resistance of the TiN resistor structures to a desired value. Plausible explanations for this behavior are an increase in oxygen concentration, changes in the state of oxygen in the films or changes in film microstructure, such as crystal grain size. These theories could not be verified during this study due to the lack of suitable analysis methods, and require further investigation. Energy dispersive x-ray spectroscopy (EDX) or electron energy loss spectroscopy (EELS), integrated into a transmission electron microscope (TEM), are proposed for this purpose, as they would allow the determination of chemical composition of film and film-substrate interface from a cross-section sample with very high accuracy on a nanometer scale. X-ray photoelectron spectroscopy (XPS), a probably more easily available technique, could give indirect information about the identities of titanium-containing compounds and phases in the film through analyzing the oxidation states of titanium, which are different for e.g. TiO, TiO2 and TiN. However, XPS is limited to surface regions of 2-3 nm. 124 Even though the reason for the increase in resistivity under annealing could not be verified yet, it is assumed that the achievement of high resistivities can be repeated in the fabrication of real detectors, as the aluminium sintering and annealing conditions used there are very similar to the conditions in the annealing experiments performed in this study. An aspect that should receive attention in the future is the behavior of the resistivity of TiN with temperature, which was not studied in the presented work. Silicon detectors may be operated at lower temperatures than the -20 °C achievable with conventional IV measurement setups. For poly-Si, resistivity tuning occurs by implantation of dopant atoms like boron, phosphorus or arsenic instead of annealing. Doping is a routine procedure, but is still an additional processing step during which the device’s other materials must be protected by mask layers. In addition, the dopants are activated by annealing at very high temperatures of 900-1100 °C. The dopants make the electrical behavior of poly-Si more complicated and affect its long-term stability due to their interaction with grain boundaries and charge carrier traps. Even though it is not discussed in the literature, there is good reason to believe that radiation damage in poly-Si has similar effects as in monocrystalline detector bulk silicon, most importantly here the formation of positively charged acceptor defects that compensate donor dopants and finally an inversion from ntype to p-type. This should lead to a change – assumably an increase – in resistivity over time, hampering the reliability and operability of the entire detector. The processed TiN bias resistor chips were irradiated with 15 keV and 1.3 MeV photons as well as 10 MeV and 24 GeV protons. Since the application does not involve mechanical action and TiN is used as approximately 20 nm thin film, possible increases in stress and brittleness, the most common radiation-induced effects in metallic materials, are not of relevance. None of the performed irrradiations had any effect on the electrical performance of the resistors, i.e. their resistivity, which proves that their radiation hardness with respect to this application is excellent. However, irradiation experiments should be performed also with resistors annealed to their final desired resistance values. 125 As expected, no activation was observed after photon irradiation – much higher photon energies would be required for that. For X-ray irradiation, it is strongly recommended to extend the experiments presented here to higher doses, which could not be reached with the setup used for the work presented in this thesis. If TiN bias resistors could withstand X-ray irradiation with doses of e.g. 1 GGy without significant performance degradation, they would be significantly superior to poly-Si bias resistors, for which resistor failure after 100 MGy has been reported. Spectroscopy after 10 MeV proton irradiation revealed the presence of a few radioactive activation products, most importantly 48V. This isotope is ubiquitous in irradiated titanium, but is relatively short-lived with a half-life of 16 days, and does not present a problem even after irradiation with high fluences. All activation products of nitrogen are so short-lived that they decay completely within some minutes, and would have require an in situ gamma measurement setup in order to be observed. Further irradiation studies on TiN, preferably with fluences that induce measurable or visible changes in the material, might be interesting also from the point of view of basic research, as virtually no literature exists on them. Again, TEM could be suitably sensitive measurement method, in addition to gamma spectrometry. Nuclear reactions of titanium have been studied extensively, but most articles about this topic are from the 1970’s and thus relatively old. In summary, the potential of titanium nitride as thin-film bias resistors for future capacitive-coupled segmented silicon detectors was investigated. In order to deposit TiN thin films with good uniformity and conformality at temperatures under 400 °C, a new plasma-enhanced atomic layer deposition process was developed. Both the process and the films’ properties were studied thoroughly. The processing of TiN thin films into resistor structures was uncomplicated, and it was observed that the resistance of the resistor structures could be raised to sufficiently high values by simple annealing treatments at 400 °C under N2. Irradiations with photons and protons of different energies had no noticeable effect on the films’ properties. 126 It is concluded that TiN thin films deposited by the described PEALD process have outstanding potential for use as bias resistors in segmented silicon detectors and are expected to withstand even high particle fluences and luminosities. Whether TiN thin-film bias resistors are going to be seen in silicon particle detectors depends on their performance in further studies, total production cost in comparison to poly-Si, and their general acceptance by the scientific community. Even if the outcome of the previous aspects is favorable for TiN, the collider and detector design of future highenergy physics experiments will play a central role: also detectors achieving spatial resolution by other means than capacitively coupled pixel and strip detectors, for example n-in-p 3D detectors, are studied at this moment. The superconductivity of TiN thin films, which can set in below 5 K, is unproblematic in the present detector environment, but might interfere with the use of TiN in cryogenic silicon detecors. The mentioned factors involve many evaluations and decisions, the results of which cannot possibly be predicted by the author. 127 7 References Abromeit, C. Aspects of simulation of neutron damage by ion irradiation, J. Nucl. Mater. 1994, 216, 78–96 Ahn, C.H., Cho, S.G., Lee, H.J., Park, K.H., Jeong, S.H. Characteristics of TiN thin films grown by ALD using TiCl4 and NH3, Met. Mater. Int. 2001, 7, 621–625 ALEPH Collaboration, DELPHI Collaboration, L3 Collaboration, OPAL, Collaboration, The LEP Working Group for Higgs Boson Searches. Search for the Standard Model Higgs boson at LEP, Phys. Lett. B 2003, 565, 61–75 Atkins, P., Overton, T., Rourke, J.P., Weller, M.T., Armstrong, F.A. Shriver and Atkins’ Inorganic Chemistry, 5th ed., p. 625, Oxford University Press (2010), Oxford, UK ATLAS Collaboration. Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC, Phys. Lett. B 2012, 716, 1–29 Atwood, D.A. Radionuclides in the environment, 1st ed., John Wiley & Sons Ltd (2010), Chichester, West Sussex, UK Baggetto, L., Niessen, R.A.H., Roozeboom, F., Notten, P.H.L. High Energy Density All-SolidState Batteries: A Challenging Concept Towards 3D Integration, Adv. Funct. Mater. 2008, 18, 1057–1066 Baturina, T.I., Islamov, D.R., Bentner, J., Strunk, C., Baklanov, M.R. Satta, A. Superconductivity on the Localozation Threshold and Magnetic-Field-Tuned Superconductor-Insulator Transition in TiN films, JETP Lett. 2004, 79, 337–341 Birkholz, M., Ehwald, K.-E., Kulse, P., Drews, J., Fröhlich, M., Haak, U., Kaynak, M., Matthus, E., Schulz, K., Wolansky, D. Ultrathin TiN Membranes as a Technology Platform for CMOS-Integrated MEMS and BioMEMS Devices, Adv. Funct. Mater. 2011, 21, 1652– 1656 Brodzinski, R.L., Rancitelli, L.A., Cooper, J.A., Wogman, N.A. High-Energy Proton Spallation of Titanium, Phys. Rev. C 1971, 4, 1250–1257 Caccia, M., Evensen, L., Hansen, T.E., Horisberger, R., Hubbeling, L., Peisert, A., Tuuva, T., Weilhammer, P., Zalewska, A Si strip detector with integrated coupling capacitors, Nucl. Inst. Meth. Phys. Res. A 1987, 260, 124–131 128 Casse, G. Overview of the recent activities of the RD50 collaboration on radiation hardening of semiconductor detectors for the sLHC, Nucl. Instr. Meth. Phys. Res. A 2009, 598, 54–60 Chatterjee, S., Chandrashekar, S., Sudarshan, T.S. Review: Deposition processes and metal cutting applications of TiN coatings, J.Mater. Sci. 1992, 27, 3409–3423 Chen, C.-H., Fang, Y.-K., Yang, C.-W., Wang, T.-W., Hsu, Y.-L., Hsu S.-L. Nitrogen Implanted Polysilicon Resistor for High-Voltage CMOS Technology Application, IEEE Electron Device Letters 2001, 22, 542-526 Choppin, G.R., Liljenzin, J.-O., Rydberg, J. Radiochemistry and Nuclear Chemistry, 3rd edition, Butterworth-Heinemann (2002), Woburn, MA, USA Chu, S.Y.F., Ekström, L.P., Firestone, R.B. The Lund/LBNL Nuclear Data Search, Version 2.0, 1999, <http://nucleardata.nuclear.lu.se/toi/index.asp>, 1.6.15 Cotton, F. A., Wilkinson, G. Advanced Inorganic Chemistry, 5th ed., p. 4, 312, 654, John Wiley & Sons, Inc. (1988), New York City Coumou, P.C.J.J., Zuiddam, M.R., Driessen, E.F.C., de Visser, P.J., Baselmans, J.J.A., Klapwijk, T.M. Microwave properties of superconducting atomic-layer deposited TiN films, IEEE Transactions on Applied Superconductivity, 23 (2013), 3 De Geronimo, G., O’Connor, P., Radeka, V., Yu B. Front-end electronics for imaging detectors, Nucl. Instr. Meth. Phys. Res. A 2001, 471, 192–199 Dew-Hughes, D., Jones, R. The effect of neutron irradiation upon the superconducting critical temperature of some transition-metal carbides, nitrides, and carbonitrides, Appl. Phys. Lett. 1980, 36, 856 Dierlamm, A. Silicon detectors for the SLHC – An overview of recent RD50 results, Nucl. Instr. Meth. Phys. Res. A 2010, 624, 396–400 Dijkstra, H. Overview of silicon detectors, Nucl. Instr. Meth. Phys. Res. A 2002, 478, 37–45 Elam, J.W., Schuisky, M., Ferguson, J.D., George, S.M. Surface chemistry and film growth during TiN atomic layer deposition using TDMAT and NH3, Thin Solid Films 2003, 436, 145–156 Elers, K.-E., Saanila, V., Soininen, P. J., Li, W.-M., Kostamo, J. T., Haukka, S., Juhanoja, J., 129 Besling, W. F. A. Diffusion Barrier Deposition on a Copper Surface by Atomic Layer Deposition, Chem. Vap. Dep. 2002, 8, 149–153 Enlow, E.W., Pease, R.L., Combs, W., Schrimpf, R.D., Nowlin, R. N. Response of advanced bipolar processes to ionizing radiation, IEEE Transactions on Nuclear Science 1991, 38, 1342-1351 Ernsberger, C., Nickerson, J., Miller, A., Banks, D. Contact resistance behavior of titanium nitride, J.Vac. Sci.Technol. A 1985, 3, 2303–2307 French, P.J. Polysilicon: a versatile material for microsystems, Sens. Actuators A 2002, 99, 3–12 George, S.M. Atomic Layer Deposition: An Overview, Chem. Rev. 2010, 110, 111–131 Giardino, P.P., Kannike, K., Raidal, M., Strumia, A. Reconstructing Higgs boson properties from the LHC and Tevatron data, J.High Energy Phys. 2012, 117, 1-20 Hadacek, N., Sanquer, M., Villégier, J.-C. Double reentrant superconductor-insulator transition in thin TiN films, Phys. Rev. B 2004, 69, 02450-1–7 Hahn, B.H., Jun, J.H., Joo, J.H. Plasma conditions for the deposition of TiN by biased activated reactive evaporation and dependence of the resistivity on preferred orientation, Thin Solid Films 1987, 153, 115–122 Hartmann, F. Silicon tracking detectors in high-energy physics, Nucl. Instr. Meth. Phys. Res. A 2012, 666, 25–46 Hartmann, F., Sharma, A. Multipurpose detectors for high energy physics, an introduction, Nucl. Instr. Meth. Phys. Res. A 2012, 666, 1–9 Haynes, W.M. CRC Handbook of Chemistry and Physics, 96th ed. 2015-2016, <http://www.hbcpnetbase.com/>, 2.10.15 Heaney, M.B. Electrical Conductivity and Resistivity, 2004, CRC Press LLC, <https://www.academia.edu/2719392/Electrical_conductivity_and_resistivity>, 17.10.15 Heil, S.B.S., Langereis, E., Roozeboom, F., van de Sanden, M.C.M., Kessels, W.M.M. Lowtemperature deposition of TiN by plasma-assisted atomic layer deposition, J. Electrochem. Soc. 2006, 153, 956–965 130 Higgs, P.W. Broken symmetries, massless particles and gauge fields, Phys. Lett. 1964, 12, 132–133 Hiltunen, L., Leskelä, M., Mäkelä, M., Niinistö, L., Nykänen, E., Soininen, P. Nitrides of titanium, niobium, tantalum and molybdenum grown as thin films by the atomic layer epitaxy method, Thin Solid Films 1988, 166, 149–154 Huang, R.-S., Cheng, C.-H., Liu, N.C., Lee, M.K., Chen, C.T. Electrical measurements on ionimplanted LPCVD polycrystalline silicon films, Solid-State Electron. 1983, 26, 657–665 Hultman, L. Thermal stability of nitride thin films, Vacuum 2000, 57, 1–30 Inoue, S., Kimura, M., Shimoda, T. Analysis and Classification of Degradation Phenomena in Polycrystalline-Silicon Thin Film Transistors Fabricated by a Low-Temperature Process Using Emission Light Microscopy, Jpn. J. Appl. Phys. 2003, 42, 1168–1172 Jakobs, K. Physics at the LHC and sLHC, Nucl. Instr. Meth. Phys. Res. A 2011, 636, S1–S7 Jeon, H., Lee, J.-W., Kim, Y.-D., Kim, D.-S., Yi, K.-S. Study on the characteristics of TiN thin film deposited by the atomic layer chemical vapor deposition method, J.Vac. Sci.Technol. A 2000, 18, 1595–1598 Jokinen, J., Keinonen, J., Tikkanen, P., Kuronen, A., Ahlgren, T., Nordlund, K. Comparison of TOF-ERDA and nuclear resonance reaction techniques for range profile measurements of keV energy implants, Nucl. Instr. Meth. Phys. Res. B 1996, 119, 533–542 Juppo, M., Alén, P., Ritala, M., Sajavaara, T., Keinonen, J., and Leskelä, M. Atomic layer deposition of titanium nitride thin films using tert-butylamine and allylamine as reductive nitrogen sources, Electrochem. Solid-State Lett. 2002, 5, C4-C6 Juppo, M., Rahtu, A., Ritala, M. In Situ Mass Spectrometry Study on Surface Reactions in Atomic Layer Deposition of TiN and Ti(Al)N Thin Films, Chem. Mater. 2002, 14, 281–287 Juppo, M., Ritala, M., Leskelä, M. Use of 1, 1-Dimethylhydrazine in the Atomic Layer Deposition of Transition Metal Nitride Thin Films, J. Electrochem. Soc. 2000, 147, 3377– 3381 Kantele, J. Handbook of Nuclear Spectrometry, Academic Press Inc. (1995), San Diego, CA, USA 131 Kariniemi, M., Niinistö, J., Hataanpää, T., Kemell, M., Sajavaara, T., Ritala, M., Leskelä, M. Plasma-enhanced Atomic layer deposition of silver thin films, Chem. Mater. 2011, 23, 2901–2907 Kim, H. Characteristics and applications of plasma enhanced-atomic layer deposition, Thin Solid Films 2011, 519, 6639–6644 Kim, J., Hong, H., Oh, K., Lee, C. Properties including step coverage of TiN thin films prepared by atomic layer deposition, Appl. Surf. Sci. 2003, 210, 231–239 Knoll, G.F. Radiation detection and measurement, 4th edition, John Wiley & Sons Inc. (2010), Hoboken, NJ, USA Kónya, J., Nagy, N.M. Nuclear and Radiochemistry, 1st edition, Elsevier (2012), Waltham, MA, USA Kuhn, K.J. Moore's Law past 32nm: Future Challenges in Device Scaling, IEEE 13th International Workshop on Computational Electronics 2009, 1–6 Lamarsh, J.R., Baratta, A.J. Introduction to Nuclear Engineering, 3rd Edition, p. 53-79, Prentice Hall Inc. (2001), Upper Saddle River, NJ, USA Lane, W.A., Wrixton, G.T. The Design of Thin-Film Poly silicon Resistors for Analog IC Applications, IEEE Transactions on electron devices 1989, 36, 738–744 Langereis, E., Heil, S.B.S., van de Sanden, M.C.M., Kessels, W.M.M. In situ spectroscopic ellipsometry study on the growth of ultrathin TiN films by plasma-assisted atomic layer deposition, J. Appl. Phys. 2006, 100, 023534-1–10 Lee, D.W., Roh, T.M., Park, H.S., Kim, J., Koo, J.G., Kim, D.Y. Fabrication technology of polysilicon resistors using novel mixed process for analogue CMOS applications, Electron. Lett. 1999, 35, 603–604 Leskelä, M., Ritala, M. Atomic Layer Deposition (ALD): from precursors to thin film structures, Thin Solid Films, 2002, 409, 138–146 Lindström, G. Radiation damage in silicon detectors, Nucl. Instr. Meth. Phys. Res. A 2003, 512, 30–43 Lindström, G., Moll, M., Fretwurst, E. Radiation hardness of silicon detectors – a challenge for high-energy physics, Nucl. Instr. Meth. Phys. Res. A 1999, 426, 1–15 132 Lu, N.C.-C., Lu, C.-Y. Characteristics of polysilicon resistors at high electric field and the non-uniform conduction mechanism, Solid-State Electron. 1984, 27, 797–805 Luukka, P.-R. Characterization of Czochralski silicon detectors, Doctoral thesis, 2006, Helsinki Institute of Physics, Helsinki Machunze, R., Janssen, G.C.A.M. Stress and strain in titanium nitride thin films, Thin Solid Films 2009, 517, 5888–5893 Mahan, J.E., Newman, D.S., M.R. Gulett, Gigaohm-Range Polycrystalline Silicon Resistors for Microelectronic Applications, IEEE Transactions on electron devices 1983, 30, 45–51 Mäntymäki, M., Heikkilä, M. J., Puukilainen, E., Mizohata, K., Marchand, B., Räisänen, J. Ritala, M., Leskelä, M. Atomic Layer Deposition of AlF3 Thin Films Using Halide Precursors, Chem.Mater. 2015, 27, 604–611 Mayadas, A.F., Shatzkes, M. Electrical-Resistivity Model for Polycrystalline Films: the Case of Arbitrary Reflection at External Surfaces, Phys. Rev. B 1970, 1, 1381–1389 Meng , L.-J. , dos Santos , M.P. Characterization of titanium nitride films prepared by d.c. reactive magnetron sputtering at different nitrogen pressures, Surf. Coat.Technol.1997, 90, 64–70 Michel, R., Gloris, M., Lange, H.-J., Leya, I., Lüpke, M., Herpers, U., Dittich-Hannen, B., Rösel, R., Schiekel, Th., Filges, D., Dragovitsch, P., Suter, M., Hofmann, H.-J., Wölfli, W., Kubik, P.W., Baur, H., Wieler, R. Nuclide production by proton-induced reactions on elements (6 < Z < 29) in the energy range from 800 to 2600 MeV, Nucl. Instr. Meth. Phys. Res. B 1995, 103, 183–222 Miikkulainen, V., Leskelä, M., Ritala, M., Puurunen, R.L. Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends, J. Appl. Phys. 2013, 113, 021301-1–101 Mochizuki, Y., Okamoto, Y., Ishitani, A., Hirose, K. On the Reaction Scheme for Ti/TiN Chemical Vapor Deposition (CVD) Process Using TiCl4, Jpn. J. Appl. Phys. 1995, 34, 326– 329 Moll, M. Radiation damage in silicon particle detectors – microscopic defects and macroscopic properties, Doctoral thesis, 1999, DESY, Hamburg 133 Moll, M. Development of radiation hard sensors for very high luminosity colliders—CERNRD50 project, Nucl. Instr. Meth. Phys. Res. A 2003, 511, 97–105 Moll, M., Fretwurst, E., Lindström, G. Leakage current of hadron irradiated silicon detectors – material dependence, Nucl. Instr. Meth. Phys. Res. A 1999, 426, 87–93 Moniruzzaman, S., Inokuma, T., Kurata, Y., Takenaka, S., Hasegawa, S. Structure of polycrystalline silicon films deposited at low temperature by plasma CVD on substrates exposed to different plasma, Thin Solid Films 1999, 337, 27–31 Morillo, J., de Novion, C.H., Dural, J. Neutron and electron radiation defects in titanium and tantalum monocarbides: An electrical resistivity study, Radiat. Eff., 1981, 55, 67–78 Moser, H.-G. Silicon detector systems in high energy physics, Prog. Part. Nucl. Phys. 2009, 63, 186–237 Mozumder, A. Fundamentals of radiation chemistry, p. 35–37, Academic Press Inc. (1999), San Diego, CA, USA Musschoot, J., Xie, Q., Deduytsche, D., Van den Berghe, S., Van Meirhaeghe, R.L., Detavernier, C. Atomic layer deposition of titanium nitride from TDMAT precursor, Microelectron. Eng. 2009, 86, 72–77 Myers, H.P., Introductory Solid-State Physics, 2nd edition, Taylor & Francis Inc. (1997), Bristol, PA, USA Nakabayashi , M., Ohyama, H., Simoen, E., Ikegami, M., Claeys, C., Kobayashi, K., Yoneoka, M., Miyahara, K. Effects of mechanical stress on polycrystalline-silicon resistors, Thin Solid Films 2002, 406, 195–199 Niyomsoan, S., Grant, W., Olson, D.L., Mishra, B. Variation of color in titanium and zirconium nitride decorative thin films, Thin Solid Films 2002, 415, 187–194 Patsalas, P., Kalfagiannis, N., Kassavetis, S. Optical Properties and Plasmonic Performance of Titanium Nitride, Materials 2015, 8, 3128–3154 Patsalas, P., Logothetidis, S. Optical, electronic, and transport properties of nanocrystalline titanium nitride thin films, J. Appl. Phys. 2001, 90, 4725–4734 134 Pereira, M.C., Martins, M.J., Bonnaud, O. Thin Film Transistors Gas Sensors: Materials, Manufacturing Technologies and Test Results, Electronics and Electrical Engineering 2009, 89, 39–44 Phillips, T.W., Cable, M.D., Cowan, T.E., Hatchett, S.P., Henry, E.A., Key, M.H., Perry, M.D., Sangster, T.C., Stoyer, M.A. Diagnosing hot electron production by short pulse, high intensity lasers using photonuclear reactions, Rev. Sci. Instrum. 1999, 70, 1213–1216 Price, J.B., Borland, J.O., Selbrede, S. Properties of chemical-vapor-deposited titanium nitride, Thin Solid Films 1993, 236, 311–318 Profijt, H. B., Potts, S.E., van de Sanden, M.C.M., Kessels, W.M.M. Plasma-Assisted Atomic Layer Deposition: Basics, Opportunities, and Challenges, J. Vac. Sci. Technol. A 2011, 29, 050801-1–26 Race, C.P., Mason, D.R., Finnis, M.W., Foulkes, W.M.C., Horsfield, A.P., Sutton, A.P. The treatment of electronic excitations in atomistic models of radiation damage in metals, Rep. Prog. Phys. 2010, 73, 116501-1–40 Reich, E.S. Physicists plan to build a bigger LHC, Nature 2013, 503, 177 Ritala, M., Leskelä, M., Dekker, J.-P., Mutsaers, C., Soininen, P. J., Skarp, J. Perfectly Conformal TiN and Al2O3 Films Deposited by Atomic Layer Deposition, Chem. Vap. Dep. 1999, 5, 7–9 Ritala, M., Leskelä, M., Rauhala, E., Jokinen, J. Atomic Layer Epitaxy Growth of TiN Thin Films from Til4 and NH3, J. Electrochem. Soc. 1998, 145, 2914–2920 Rydberg, M., Smith, U. Long-Term Stability and Electrical Properties of Compensation Doped Poly-Si IC-Resistors, IEEE Transactions on electron devices 2000, 47, 417–426 Saito, Y., Mizushima, I., Kuwano, H. Conduction mechanism of high-resistivity polycrystalline silicon films, J. Appl. Phys. 1985, 57, 2010–2013 Satta, A., Schuhmacher, J., Whelan, C.M., Vandervorst, W., Brongersma, S.H., Beyer, G.P., Maex, K., Vantomme, A., Viitanen, M.M., Brongersma, H.H., Besling, W.F.A. Growth mechanism and continuity of atomic layer deposited TiN films on thermal SiO2, J. Appl. Phys. 2002, 92, 7641–7646 Schubert, U., Hüsing, N. Synthesis of Inorganic Materials, 2nd ed., p. 71-108, Wiley-VCH Verlag GmbH & Co. KGaA (2005), Weinheim, Germany 135 Serro, A.P., Completo, C., Colaço, R., dos Santos, F., Lobato da Silva, C., Cabral, J.M.S., Araújo, H., Pires, E., Saramago, B. A comparative study of titanium nitrides, TiN, TiNbN and TiCN, as coatings for biomedical applications, Surf. Coat. Technol. 2009, 203, 3701– 3707 Sherwood, T.R., Turchinetz, W.E. Some photo-disintegration reactions in the titanium isotopes, Nucl. Phys. 1962, 29, 292–299 Song, B., Nakamatsu, H,. Sekine, R., Mukoyama, T., Taniguchi, K. Valence band structures of titanium nitride and titanium carbide calculated with chemically complete clusters, J. Phys. Condens. Matter 1998, 10, 9443–9454 Sundgren, J.-E. Structure and properties of TiN coatings (Metallurgical and protective coatings), Thin Solid Films 1985, 128, 21–44 Sze, S.M., Ng, K.K. Physics of semiconductor devices, 3rd edition, John Wiley & Sons Inc. (2007), Hoboken, NJ, USA Szelecsenyi, F., Tarkanyi, F., Takacs, S., Hermanne, A., Sonck, M., Shubin, Y., Mustafa, M.G., Youxiang, Z. Excitation function for the natTi(p, x)48V nuclear process: Evaluation and new measurements for practical applications, Nucl. Instr. Meth. Phys. Res. B 2001, 174, 47-64 Tarkanyi, F., Szelecsenyi, F., Kopecky, P. Cross section data for proton, 3He and α-particle induced reactions on natNi, natCu and natTi for monitoring beam performance, in Qaim, S.M. (ed.) Nuclear Data for Science and Technology, 529-532, Springer-Verlag (1992), Berlin Tiznado, H., Zaera, F. Surface Chemistry in the atomic layer deposition of TiN films from TiCl4 and ammonia, J. Phys. Chem. B 2006, 110, 13491–13498 Tompkins, H. G. Oxidation of titanium nitride in room air and in dry O2, J. Appl.Phys. 1991, 70, 3876 Tuovinen, E., Härkönen, J., Luukka, P., Tuominen, E., Verbitskaya, E., Eremin, V., Ilyashenko, I., Pirojenko, A., Riihimäki, I., Virtanen, A., Leinonen, K. Czochralski silicon detectors irradiated with 24 GeV/c and 10 MeV protons, Nucl. Instr. Meth. Phys. Res. A 2006, 568, 83–88 136 Turala, M. Silicon tracking detectors—historical overview, Nucl. Instr. Meth. Phys. Res. A 2005, 541, 1–14 Uhm, J., Jeon, H. TiN diffusion barrier grown by atomic layer deposition method for Cu metallization, Jpn. J. Appl. Phys. 2001, 40, 4657–4460 Van Bui, H., Kovalgin, A. Y., Wolters, R. A. M. Growth of Sub-Nanometer Thin Continuous TiN Films by Atomic Layer Deposition, ECS J. Solid State Sci.Technol. 2012, 1, 285–290 Walke, H. The Induced Radioactivity of Titanium and Vanadium, Phys. Rev. 1937, 52, 777– 787 Was, G.S. Fundamentals of Radiation Materials Chemistry. Metals and Alloys. SpringerVerlag (2007), Berlin Wirth, B.D., Caturla, M.J., Diaz de la Rubia, T., Khraishi, T., Zbib, H. Mechanical property degradation in irradiated materials: A multiscale modelling approach, Nucl. Instr. Meth. Phys. Res. B 2001, 180, 23–31 Wittmer, M., Noser, J. R., Melchior, H. Characteristics of TiN gate metal-oxidesemiconductor field effect transistors, J. Appl. Phys. 1983, 54, 1423–1428 Xie, S., Cai, J., Wang, Q., Wang, L., Liu, Z. Properties and Morphology of TiN films deposited by atomic layer deposition, Tsinghua Sci. Technol. 2014, 19, 144–149 Zhang, J., Fretwurst, E., Klanner, R., Perrey, H., Pintilie, I., Poehlsen T., Schwandt J. Study of X-ray radiation damage in silicon sensors, J. Inst. 2011, 6, C11013-1–10 Zhang, S., Zhu, W. TiN coating of tool steels: a review, Journal of Materials Processing Technology 1993, 39, 165–177 Zhao, J., Garza, E.G., Lam, K., Jones, C.M. Comparison study of physical vapor-deposited and chemical vapor-deposited titanium nitride thin films using X-ray photoelectron spectroscopy, Appl. Surf. Sci. 2000, 158, 246–251 Ziock, H.J., Milner, C., Sommer, W.F., Cartiglia, N., DeWitt, J., Dorfan, D., Hubbard, B., Leslie, J., O'Shaughnessy, K.F., Pitzl, D., Rowe, W.A., Sadrozinski, H.F.-W., Seiden, A., Spencer, E., Tennenbaum, P., Ellison, J., Jerger, S., Lietzke, C., Wimpenny, S.J., Ferguson, P., Giubellino, P. Tests of the Radiation Hardness of VLSI Integrated Circuits and Silicon Strip Detectors for the SSC Under Neutron, Proton, and Gamma Irradiation, IEEE Transactions on Nuclear Science 1991, 38, 269–276 137 Appendix A. Table of all TiN thin film depositions in the Beneq TFS-200 ALD reactor relevant for this thesis. Batch code Number of cycles Thicknes Growth Roughness Density Temperature Resistivity Plasma pulse s acc. to rate acc. to XRR acc. to (°C) (mΩcm) length (s) XRR (nm) (Å/cycle) (nm) XRR B1067 2000 17.0 0.085 1.1 4.0 250 3.46545 5 B1068 ca. 1500 27.0 0.18 0.6 4.7 300 2.4462 5 B1069 1000 18.2 0.182 0.8 4.1 300 3.545178 5 B1070* 2000 27.7 0.1385 0.7 4.3 275 1.882215 5 B1071 1000 21.7 0.217 0.8 4.4 325 1.081311 5 B1072* 1500 27.0 0.18 0.6 4.6 300 0.795015 10 B1075 ca. 1050 13.6 0.13 1.2 4.1 300 1.293768 2.5 B1076 640 12 0.1875 0.9 4.1 300 0.76104 5 B1078 980 14.2 0.145 0.9 4.1 300 12.8652 2.5 B1079 1000 10.8 0.108 0.6 5.0 300 no data 7.5 B1080 2000 9.7 0.0485 0.6 4.9 300 15.37935 5, N2 plasma B1081 4000 24.3 0.06 0.7 4.0 300 1.4200191 5 B1082 513 9.8 0.1910 0.7 3.7 300 887.88 7.5 B1083 1200 20.7 0.1725 1.0 3.9 300 3.75084 5 B1084* 4000 76.7 0.19175 0.7 4.2 300 0.9207452 5 B1086 1200 22.0 0.1833 1.1 3.9 300 1.1959 7.5 B1087* 2000 38.9 0.1945 0.8 4.2 300 0.9515718 5 B1088 1000 15.4 0.154 0.9 4.1 300 1.430121 5. NH3 20 sccm B1089 1300 22.9 0.176 1.0 4.0 300 1.244844 5 B1090* 1700 27.2 0.16 0.9 4.0 300 0.985728 2.5 B1092 1400 19.6 0.14 0.9 3.9 275 2.042124 5 B1093 800 13.2 0.165 0.9 4.0 300 1.255716 5 B1132 3800 69.1 0.182 1.4 3.6 300 0.7870875 Red: abnormal results; Blue: deposited on full-size wafer, green: conformality substrates added; * : sample for ToF-ERDA Runs: 3/0,5/3/2/5 (purge – TiCl4 – purge – wait – plasma); B1115: 15/3/10/5/10 Plasma power (W) 100 100 100 100 110 100 100 100 100 100 150 50 50 50 50 100 100 100 100 100 100 100 thermal Appendix B. Irradiation certificate for the 60Co gamma irradition of TiN resistor samples.