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Silicon Detectors XII ICFA School on Instrumentation Bogotá, Nov. 25th – Dec. 6th, 2013 Part 1 Manfred Krammer Institute of High Energy Physics, Vienna, Austria Silicon Detectors Content 1 Introduction 2 Basics 3 Detector Structures 4 Performance Parameter 5 Radiation Damage 6 Silicon Detectors in the LHC Experiments XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 1 1 Introduction XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 2 1 Introduction What is a silicon detector? NOT a gas detector! anode wire ionising particle cathode Charged particles ionise the gas. Electrons and positive ions drift to the electrodes. Large electric field close to the wire causes secundary ionisation and signal multiplication. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 3 1 Introduction What is a Silicon Detector? NOT a scintillator! Scintillators: Cristalls, Plastics, Liquids, Gases Penetrating charged particles excite atomic or molecular levels. When decaying photons are emitted. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 4 1 Introduction What is a Silicon Detector? A semiconductor detector! Also called a solid state detector. Through going charged particles create electron hole pairs. These charges drift to the electrodes. The drift generates a signal. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 5 1 Introduction Where are semiconductor detector used? Semiconductor detectors are used for: • Nuclear Physics Energy measurement of charged particles (MeV range), gamma spectroscopy (precise determination of photon energy) • Particle Physics: Tracking or vertex detectors, precise determination of particle tracks and decay vertices • Satellite Experiments Tracking detectors • Industrial Applications Security, Medicine, Biology,... XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 6 1 Introduction History of semiconductor detectors 1951: First detector was a germanium-pn-diode (McKay). 1960: p-i-n semiconductor detectors for - und -spectroscopie. (E.M. Pell) 1960ies: Semiconductor detectors from germanium but also silicon are more and more important for the energy measurement in nuclear physics. 1980: First silicon surface barrier micro strip detector (E. Heijne) 1983: First use of a planar silicon strip detector in a fix target experiment NA11 at CERN. (J. Kemmer) 1980ies and after: micro structured silicon detectors gain rapid importance for tracking detectors in high energy physics experiments! XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 7 1 Introduction Advantages of semiconductor detectors Semiconductor detectors have a high density large energy loss in a short distance Diffusion effect is smaller than in gas detectors resulting in achievable position resolution of less than 10 µm Low ionisation energy (few eV per e-hole pair) compared to gas detectors (20-40 eV per e-ion pair) or scintillators (400-1000 eV to create a photon). Large experience in industry with micro chip technology (silicon). Ô Rapid development of detectors, reduction of cost Easy integration with readout electronics due to identical materials used (silicon) Self supporting structure High intrinsic radiation hardness XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 8 1 Introduction Disadvantages of semiconductor detectors No internal amplifications (with the exception of APDs) High cost per surface XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 9 2 Basics XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 10 2.1 Materials Elemental semiconductors Germanium: Used in nuclear physics, due to small band gap (0.66 eV) needs cooling (usually done with liquid nitrogen at 77 K) Silicon: Standard material for vertex and tracking detectors in high energy physics, can be operated at room temperature, synergies with micro electronics industry. Diamond (CVD or single crystal): Large band gap, requires no depletion zone, very radiation hard, drawback is a low signal and high cost! XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 11 2.1 Materials Compound semiconductors Compound semiconductors consist of two (binary semiconductors) or more atomic element. Depending on the column in the periodic system of elements one differentiates between IV-IV- (e.g. SiGe, SiC), III-V-, und II-VI compounds important III-V compounds: – GaAs: Faster and probably more radiation resistant than Si. Drawback is less experience in industry and cost. – GaP, GaSb, InP, InAs, InSb, InAlP important II-VI compounds: – CdTe: High atomic numbers (48+52) hence very efficient to detect photons. – ZnS, ZnSe, ZnTe, CdS, CdSe, Cd1-xZnxTe, Cd1-xZnxSe XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 12 2.2 Material Properties Silicon, germanium und diamond are group IV elements. The crystal structure is the diamond lattice, consisting of 2 interpenetrating sublattices shifted by one quarter along the diagonal of the cube. Each atom is surrounded by four equidistant neighbours. Diamond lattice Zincblende lattice Most III-V semiconductors (e.g. GaAs) have a zincblende lattice. This lattice is similar to the diamond lattice, except that each sublattice consists of one element. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer Source: S.M. Sze, Semiconductor Devices , J. Wiley & Sons, 1985 Crystal structure of semiconductors 13 2.2 Material Properties Bond model of semiconductors Example of column IV elemental semiconductor (2dim projection) : T=0K T >0K Valence electron Conduction electron Each atom has 4 closest neighbors, the 4 electrons in the outer shell are shared and form covalent bonds. At low temperature all electrons are bound At higher temperature thermal vibrations break some of the bonds cause conductivity (electron conduction) The remaining open bonds attract other econduction) XII ICFA School on Instrumentation, Bogotá, 2013 free e- The “holes” change position (hole Manfred Krammer 14 2.2 Material Properties Energy bands: isolator–semiconductor–metal In an isolated atom the electrons have only discrete energy levels. In solid state material the atomic levels merge to energy bands. In metals the conduction and the valence band overlap, whereas in isolators and semiconductors these levels are separated by an energy gap (band gap). In isolators this gap is large. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 15 2.2 Material Properties Fermi distribution, fermi level Fermi distribution ƒ(E) describes the probability that an electronic state with energy E is occupied by an electron. 1 f (E) = 1+ e E -E F kT The fermi level EF is the energy at which the probability of occupation is 50%. For metals EF is in the conduction band, for semiconductors and isolators EF is in the band gap Fermi distribution function for different temperautures T4 > T3 > T2 > T1 > T0 = 0 K XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 16 2.2 Material Properties Intrinsic carrier concentration Due to the small band gap in semiconductors electrons already occupy the conduction band at room temperature. Electrons from the conduction band may recombine with holes. A thermal equilibrium is reached between excitation and recombination Charged carrier concentartion ne = nh = ni This is called intrinsic carrier concentration: In ultrapure silicon the intrinsic carrier concentration is 1.45·1010 cm-3. With approximately 1022 Atoms/cm3, about 1 in 1012 silicon atoms is ionised. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 17 2.2 Material Properties Drift velocity and mobility Drift velocity For electrons: and for holes: Mobility For electrons: and for holes: e mn , mp n , p … … … … electron charge external electric field effective mass of e- and holes mean free time between collisions for e- and holes Source: S.M. Sze, Semiconductor Devices , J. Wiley & Sons, 1985 XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 18 2.2 Material Properties Resistivity Resistivity of semiconductor material: ne , nh n , p … Charge carrier density for electrons and holes … Mobility for electrons and holes XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 19 2.2 Material Properties Resistivity – example silicon In silicon the mobilities are in good approximation constant below an electric field of ≈ 1 kV/cm. At T = 300 K: µn(Si, 300 K) ≈ 1450 cm2/Vs µp(Si, 300 K) ≈ 450 cm2/Vs The charge carrier concentration in pure silicon (i.e. intrinsic Si) for T = 300 K is: ne = nh ≈ 1.45 · 1010 cm-3 This yields an intrinsic resistivity of: ≈ 230 kcm XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 20 2.2 Material Properties Comparison of different semiconductor materials – 1 Si Ge GaAs GaP CdTe Diamond* Atomic number Z 14 32 31+33 31+15 48+52 6 Atomic mass A (amu) 28.086 72.61 69.72+74.92 Lattice constant a (Å) 5.431 5.646 5.653 5.451 6.482 3.567 Density (g/cm3) 2.328 5.326 5.32 4.13 5.86 3.52 Eg (eV) bei 300 K 1.11 0.66 1.42 2.26 1.44 5.47–5.6 Eg (eV) bei 0 K 1.17 0.74 1.52 2.34 1.56 ≈6 rel. permittivity r =/0 11.9 16.0 12.8 11.1 10.9 5.7 Melting point (°C) 1415 938 1237 1477 1040 3527 eff. e–-mass (mn /me) 0.98, 0.19 1.64, 0.08 0.067 0.82 0.11 0.2 eff. hole mass+ (mh /me) 0.16 0.044 0.082 0.14 0.35 0.25 69.72+30.97 112.4+127.6 12.011 *usually considered an isolator Material Source: http://www.ioffe.rssi.ru/SVA/NSM/Semicond/ ; S.M.Sze, Physics of Semicon. Devices , J. Wiley & Sons, 1981, J. Singh, Electronic & Optoelectronic Properties of Semiconductor Structures, Cambridge University Press, 2003 XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 21 2.2 Material Properties Comparison of different semiconductor materials – 2 Si Ge GaAs GaP eff. density of states in conduction band nCB (cm-3) 3 · 1019 1 · 1019 4.7 · 1017 2 · 1019 ≈ 1020 eff. Density of states in valence band nVB (cm-3) 1 · 1019 6 · 1018 7 · 1018 2 · 1019 ≈ 1019 Electron mobility µe bei 300 K (cm2/Vs) ~1450 3900 8500 < 300 1050 1800 Hole mobility µh bei 300 K (cm2/Vs) ~450 1900 400 < 150 100 1200 instrins. charge carrier density at 300 K (cm-3) 1.45 · 1010 2.4 · 1013 2 · 106 2 instrins. resistivity at 300 K (cm) 2.3· 105 47 ≈ 108 Breakdown field (V/cm) 3 · 105 ≈ 105 4 · 105 ≈ 106 Mean E to create an e–h+ pair (eV), 300 K 3.62 2.9 4.2 ≈7 *usually considered an isolator CdTe Diamond* Material ≈ 10-27 ≈ 109 ≥ 1042 3 · 107 4.43 13.25 Source: http://www.ioffe.rssi.ru/SVA/NSM/Semicond/ ; S.M.Sze, Physics of Semicon. Devices , J. Wiley & Sons, 1981, J. Singh, Electronic & Optoelectronic Properties of Semiconductor Structures, Cambridge University Press, 2003 XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 22 2.3 Constructing a Detector The ideal semiconductor detector One of the most important parameter of a detector is the signal to noise ratio (SNR). A good detector should have a large SNR. However this leads to contradictory requirements: Large signal low ionisation energy small band gap Low noise very few intrinsic charge carriers large band gap An optimal material has a band gap which is small enough to create a large number of e-h+ pairs through ionisation, but large enough to have an almost empty conduction band at room temperature, i.e. about Eg ≈ 6 eV. Such a material exist, it is Diamond. However even artificial diamonds (e.g. CVD diamonds) are too expensive for large area detectors. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 23 2.3 Constructing a Detector Estimate SNR in an intrinsic silicon detector How about Silicon? Let’s make a simple calculation: Mean ionization energy I0 = 3.62 eV, Mean energy loss per flight path (mip*) dE/dx = 3.87 MeV/cm, Intrinsic charge carrier density at T = 300 K ni = 1.45 · 1010 cm-3. Assuming a detector with a thickness of d = 300 µm and an area of A = 1 cm2. Signal (mip*) in such a detector: dE dx × d 3.87 ×10 6 eV cm× 0.03cm = » 3.2 ×10 4 e-h +-pairs I0 3.62eV Intrinsic charge carrier in the same volume (T = 300 K): ni d A =1.45 ×1010 cm-3 × 0.03cm×1cm2 » 4.35 ×108 e-h+-pairs Number of thermal created e–h+-pairs are four orders of magnitude larger than signal!!! Have to remove the charge carrier! Depletion zone in reverse biased pn junctions * minimum ionising particle XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 24 2.4 Doping pn junction needs doped materials A pn junction consists of n and p doped substrates: Doping is the replacement of a small number of atoms in the lattice by atoms of neighboring columns from the atomic table (with one valence electron more or less compared to the basic material). Typical doping concentrations for Si detectors are ≈1012 atoms/cm3 (1014 und 1018 atoms/cm3 for CMOS elements). These doping atoms create energy levels within the band gap and therefore alter the conductivity. An undoped semiconductor is called an intrinsic semiconductor A doped semiconductor is called an extrinsic semiconductor. In an intrinsic semiconductor for each conduction electron there exists the corresponding hole. In extrinsic semiconductors there is a surplus of electrons or holes. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 25 2.4 Doping Bond model: n-doping in Si Doping with an element 5 atom (e.g. P, As, Sb). The 5th valence electrons is weakly bound. The doping atom is called donor XII ICFA School on Instrumentation, Bogotá, 2013 The released conduction electron leaves a positively charged ion Manfred Krammer 26 2.4 Doping Bond model: n-doping in Si The energy level of the donor is just below the edge of the conduction band. At room temperature most electrons are raised to the conduction band. The fermi level EF moves up. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 27 2.4 Doping Bond model: p-doping in Si Doping with an element 3 atom (e.g. B, Al, Ga, In). One valence bond remains open. This open bond attracts electrons from the neighbor atoms. The doping atom is called acceptor. XII ICFA School on Instrumentation, Bogotá, 2013 The acceptor atom in the lattice is negatively charged. Manfred Krammer 28 2.4 Doping Bond model: p-doping in Si The energy level of the acceptor is just above the edge of the valence band. At room temperature most levels are occupied by electrons leaving holes in the valence band. The fermi level EF moves down. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 29 2.4 Doping Donor and acceptor levels in Si und GaAs Measured ionization energies for doping atoms in Si and GaAs. Levels above band gap middle are donators and are measured from the edge of the conduction band (exceptions denoted D). Levels below band gap middle are acceptors and are measured from the edge of the valence band (exceptions denoted A). Source: S.M. Sze, Semiconductor Devices , J. Wiley & Sons, 1985 XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 30 2.4 Doping Temperatur dependance of the carrier concentration At low temperatures the thermal energy is not sufficient to ionize all donors. Some e- are frozen at the donor level. Electron density as a function of temperature for a Si sample with a donor concentration of 1015 cm–3: Source: S.M. Sze, Semiconductor Devices , J. Wiley & Sons, 1985 As the temperature increases all donors become ionized (“extrinsic region”). At even higher temperature (kT ≈ Eg) the intrinsic carrier concentration becomes comparable to the donor concentration. Beyond this point the semiconductor becomes intrinsic. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 31 2.5 The p-n Junction Creating a p-n junction At the interface of an n-type and p-type semiconductor the difference in the fermi levels cause diffusion of surplus carries to the other material until thermal equilibrium is reached. At this point the fermi level is equal. The remaining ions create a space charge and an electric field stopping further diffusion. The stable space charge region is free of charge carries and is called the depletion zone. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 32 2.5 The p-n Junction Electrical characteristics XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 33 2.5 The p-n Junction Operation with forward bias p-n junction with forward bias Applying an external voltage V with the anode to p and the cathode to n e- and holes are refilled to the depletion zone. The depletion zone becomes narrower. The potential barrier becomes smaller by eV and diffusion across the junction becomes easier. The current across the junction increases significantly. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 34 2.5 The p-n Junction Operation with reverse bias p-n junction with reverse bias Applying an external voltage V with the cathode to p and the anode to n e- and holes are pulled out of the depletion zone. The depletion zone becomes larger. The potential barrier becomes higher by eV and diffusion across the junction is suppressed. The current across the junction is very small “leakage current”. That’s the way we operate our semiconductor detector! XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 35 2.5 The p-n Junction Width of the depletion zone Example of a typical p+-n junction in a silicon detector: Effective doping concentration Na = 1015 cm–3 in p+ region and Nd = 1012 cm–3 in n bulk. Without external voltage: Wp = 0.02 µm Wn = 23 µm Applying a reverse bias voltage of 100 V: Wp = 0.4 µm Wn = 363 µm Width of depletion zone in n bulk: with XII ICFA School on Instrumentation, Bogotá, 2013 p+n junction V … … … Neff … External voltage specific resistivity mobility of majority charge carriers effective doping concentration Manfred Krammer 36 2.5 The p-n Junction Current-voltage characteristics Typical current-voltage of a p-n junction (diode): exponential current increase in forward bias, small saturation in reverse bias. Ideal diode equation: I0 … reverse saturation current S.M. Sze, Semiconductor Devices , J. Wiley & Sons, 1985 XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 37 2.6 Detector Characteristics Leakage Current A silicon detector is operated with reverse bias, hence reverse saturation current is relevant (leakage current). This current is dominated by thermally generated e-h+ pair. Due to the applied electric field they cannot recombine and are separated. The drift of the e- and h+ to the electrodes causes the leakage current. Measured detector leakage current, CMS strip detector (measurement at room temperature): XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 38 2.6 Detector Characteristics Depletion Voltage The depletion voltage is the minimum voltage at which the bulk of the sensor is fully depleted. The operating voltage is usually chosen to be slightly higher (overdepletion). reverse bias voltage V [V] High resistivity material (i.e. low doping) requires low depletion voltage. Depletion voltage as a function of the material resistivity for two different detector thicknesses (300 µm, 500 µm). resistivity [kOhm cm] XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 39 2.6 Detector Characteristics Capacitance of a detector For a typical Si p-n junction (Na >> Nd >> ni) the detector capacitance is given as: … specific resistivity of the bulk e0er … mobility of majority charge carrier C= ×A V … bias voltage 2mr V A … detector surface Measured detector capacitance as a function of the bias voltage, CMS strip detector: XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 40 2.7 Manufacturing Si Detectors Wafer production Properties of Si bulk required for detectors: • Diameter: 6 inches (soon 8?) • Lattice orientation <111> or <100> • Resistivity 1–10 kΩcm Therefore, float-zone technique for ingot production is used* – technique moves a liquid zone through the mater • Result: single-crystal ingot *More recently wafers from other production processes are tried, e.g. Magnetic Czochralski. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 41 2.7 Manufacturing Si Detectors Planar process - 1 1. Starting Point: single-crystal n-doped wafer (ND ≈ 1–5·1012 cm-3) 2. Surface passivation by SiO2-layer (approx. 200 nm thick). E.g. growing by (dry) thermal oxidation at 1030 °C. 3. Window opening using photolithography technique with etching, e.g. for strips 4. Doping using either • Thermal diffusion (furnace) • Ion implantation - p+-strip: Boron, 15 keV, NA ≈ 5·1016 cm-2 - Ohmic backplane: Arsenic, 30 keV, ND ≈ 5·1015 cm-2 XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 42 2.7 Manufacturing Si Detectors Planar process - 2 5. After ion implantation: Curing of damage via thermal annealing at approx. 600°C, (activation of dopant atoms by incorporation into silicon lattice) 6. Metallization of front side: sputtering or CVD 7. Removing of excess metal by photolitography: etching of noncovered areas 8. Full-area metallization of backplane with annealing at approx. 450°C for better adherence between metal and silicon 9. Last step: wafer dicing (cutting) XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 43 2.7 Manufacturing Si Detectors Lithography exposure mask photoresist SiO2 developing etching Photoresist removal XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 44 2.7 Manufacturing Si Detectors Sensor mask design • Design tools like in commercial chip industry – ICStation from Mentor Graphics – Cadence • Design is not drawn but actually “programmed” – using simple programming language (C like) • Therefore, it is easy to change any parameter and re-create the full sensor within minutes – e.g. width of strips XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 45 Polysilicon pieces 2.6 Manufacturing Si Detectors Single crystal Silicon wafers with different diameter Wafers in a package box Electronic parts XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 46