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Semiconductor detectors An introduction to semiconductor detector physics as applied to particle physics PPE S/C detector lectures Dr R. Bates 1 Contents 4 lectures – can’t cover much of a huge field Introduction Fundamentals of operation The micro-strip detector Radiation hardness issues PPE S/C detector lectures Dr R. Bates 2 Lecture 1 - Introduction What do we want to do Past, present and near future Why use semiconductor detectors PPE S/C detector lectures Dr R. Bates 3 What we want to do - Just PPE Track particles without disturbing them Determined position of primary interaction vertex and secondary decays Superb position resolution Large signal Small amount of energy to crate signal quanta Thin Highly segmented high resolution Close to interaction point Low mass Minimise multiple scattering PPE S/C detector lectures Detector Readout Cooling / support Dr R. Bates 4 Ages of silicon - the birth J. Kemmer * Fixed target experiment with a planar diode* Later strip devices -1980 Larger devices with huge ancillary components J. Kemmer: “Fabrication of a low-noise silicon radiation detector by the planar process”, NIM A169, pp499, 1980 PPE S/C detector lectures Dr R. Bates 5 Ages of Silicon - vertex detectors LEP and SLAC ASIC’s at end of ladders Minimise the mass inside tracking volume Minimise the mass between interaction point and detectors Minimise the distance between interaction point and the detectors Enabled heavy flavour physics i.e. short lived particles PPE S/C detector lectures Dr R. Bates 6 ALEPH PPE S/C detector lectures Dr R. Bates 7 ALPEH – VDET (the upgrade) 2 silicon layers, 40cm long, inner radius 6.3cm, outer radius 11cm 300mm Silicon wafers giving thickness of only 0.015X0 S/N rF = 28:1; z = 17:1 srf = 12mm; sz = 14mm PPE S/C detector lectures Dr R. Bates 8 Ages of silicon - tracking paradigm CDF/D0 & LHC Emphasis shifted to tracking + vertexing Only possible as increased energy of particles Cover large area with many silicon layers Detector modules including ASIC’s and services INSIDE the tracking volume Module size limited by electronic noise due to fast shaping time of electronics (bunch crossing rate determined) PPE S/C detector lectures Dr R. Bates 9 ATLAS PPE S/C detector lectures Dr R. Bates A monster ! 10 ATLAS barrel PPE S/C detector lectures Dr R. Bates 2112 Barrel modules mounted on 4 carbon fibre concentric Barrels, 12 in each row 1976 End-cap modules mounted on 9 disks at each end of the barrel region 11 What is measured Measure space points Deduce Vertex location Decay lengths Impact parameters PPE S/C detector lectures Dr R. Bates 12 Signature of Heavy Flovours Stable particles t > 10-6 s ct n 2.66km m 658m Very long lived particles t > 10-10 s p, K±, KL0 2.6 x 10-8 7.8m KS0, E±, D0 2.6 x 10-10 7.9cm Long lived particles t > 10-13 s t± 0.3 x 10-12 91mm Bd0, Bs0, Db 1.2 x 10-12 350mm p0, h0 8.4 x 10-17 0.025mm r,w 4 x 10-23 10-9mm!! Short lived particles PPE S/C detector lectures Dr R. Bates 13 Decay lengths E.g. B J/Y Ks0 L Secondary vertex Primary vertex L = p/m c t By measuring the decay length, L, and the momentum, p, the lifetime of the particle can be determined Need accuracy on both production and decay point PPE S/C detector lectures Dr R. Bates 14 Impact parameter (b) b = distance of closest approach of a reconstructed track to the true interaction point b beam PPE S/C detector lectures Dr R. Bates 15 Impact parameter Error in impact parameter for 2 precision measurements at R1 and R2 measured in two detector planes: 2 b s b a c 2 p 2 a=f(R1 & R2) and function of intrinsic resolution of vertex detector b due to multiple scattering in detector c due to detector alignment and stability PPE S/C detector lectures Dr R. Bates 16 Impact parameter sb = f( vertex layers, distance from main vertex, spatial resolution of each detector, material before precision measurement, alignment, stability ) Requirements for best measurement Close as possible to interaction point Maximum lever arm R2 – R1 Maximum number of space points High spatial resolution Smallest amount of material between interaction point and 1st layer Good stability and alignment – continuously measured and correct for 100% detection efficiency Fast readout to reduce pile up in high flux environments PPE S/C detector lectures Dr R. Bates 17 Impact parameter* Blue = 5mm Black = 1mm (baseline) Effect of extra mass and distance from the interaction point Green = 0.5 mm Red = 0.1 mm Lower Pt GR Width Flux increase(%) to silicon Improvement of the IPres. wrt 1mm(%) -44 -38.10.9 0.5mm +14.1 +5.8 0.7 0.1mm +27.7 +10.0 0.7 5mm *Guard Ring Width Impact on d0 Performances and Structure Simulations. A Gouldwell, C Parkes, M Rahman, R Bates, M Wemyss, G Murphy, P Turner and S Biagi. LHCb Note, LHCb-2003-034 Why Silicon Semiconductor with moderate bandgap (1.12eV) Thermal energy = 1/40eV Little cooling required Energy to create e/h pair (signal quanta)= 3.6eV c.f Argon gas = 15eV High carrier yield better stats and lower Poisson stats noise Better energy resolution and high signal no gain stage required PPE S/C detector lectures Dr R. Bates 19 Why silicon High density and atomic number Higher specific energy loss Thinner detectors Reduced range of secondary particles High carrier mobility Fast! Better spatial resolution Less than 30ns to collect entire signal Industrial fabrication techniques Advanced simulation packages Processing developments Optimisation of geometry Limiting high voltage breakdown Understanding radiation damage PPE S/C detector lectures Dr R. Bates 20 Disadvantages Cost Area covered Detector material could be cheap – standard Si Most cost in readout channels Material budget Radiation length can be significant Effects calorimeters Tracking due to multiple scattering Radiation damage Replace often or design very well – see lecture 4 PPE S/C detector lectures Dr R. Bates 21 Radiation length X0 High-energy electrons predominantly lose energy in matter by bremsstrahlung High-energy photons by e+e- pair production The characteristic amount of matter traversed for these related interactions is called the radiation length X0, usually measured in g cm-2. It is both: the mean distance over which a high-energy electron loses all but 1=e of its energy by bremsstrahlung the mean free path for pair production by a high-energy photon 1 2 4N A Z Z 1re log 183Z 3 1 X0 A PPE S/C detector lectures Dr R. Bates 22 Lecture 2 – lots of details Simple diode theory Fabrication Energy deposition Signal formation PPE S/C detector lectures Dr R. Bates 23 Detector = p-i-n diode Near intrinsic bulk Highly doped contacts Apply bias (-ve on p+ contact) Radiation creates carriers Deplete bulk High electric field n+ contact ND=1018cm-3 ND~1012cm-3 signal quanta Carriers swept out by field Induce current in external circuit signal PPE S/C detector lectures Dr R. Bates p+ contact NA=1018cm-3 24 Why a diode? Signal from MIP = 23k e/h pairs for 300mm device Intrinsic carrier concentration ni = 1.5 x 1010cm-3 Si area = 1cm2, thickness=300mm 4.5x108 electrons 4 orders > signal Need to deplete device of free carriers Want large thickness (300mm) and low bias But no current! Use v.v. low doped material p+ rectifying (blocking) contact PPE S/C detector lectures Dr R. Bates 25 p-n junction (1) p+ n (5) (2) Carrier density Electric field (6) (3) Dopant concentration (4) Space charge (7) density PPE S/C detector lectures Dr R. Bates Electric potential 26 p-n junction 1) 2) take your samples – these are neutral but doped samples: p+ and nbring together – free carriers move two forces drift and diffusion o In stable state Jdiffusion (concentration density) = Jdrift (e-field) o 3) p+ area has higher doping concentration (in this case) than the n region PPE S/C detector lectures Dr R. Bates 27 p-n junction Fixed charge region Depleted of free carriers 4) 5) o o o o Called space charge region or depletion region Total charge in p side = charge in n side Due to different doping levels physical depth of space charge region larger in n side than p side Use n- (near intrinsic) very asymmetric junction 6) Electric field due to fixed charge 7) Potential difference across device o dV r E dx Constant in neutral regions. PPE S/C detector lectures Dr R. Bates 28 Resistivity and mobility Carrier DRIFT velocity and E-field: v mE mn = 1350cm2V-1s-1 : mp = 480cm2V-1s-1 Resistivity 1 r qm n n m p p p-type material n-type material PPE S/C detector lectures r 1 r qm p N D 1 qm N A Dr R. Bates 29 Depletion width Depletion Width depends upon Doping Density: 2V 1 1 W q ND N A For a given thickness, Full Depletion Voltage is: qN DW 2 V fd 2 W = 300mm, ND 5x1012cm-3: Vfd = 100V PPE S/C detector lectures Dr R. Bates 30 Reverse Current Diffusion current From generation at edge of depletion region Negligible for a fully depleted detector j gen Generation current From generation in the depletion region Reduced by using material pure and defect free 1 ni q W 2 t0 high lifetime Must keep temperature low & controlled Eg n NC NV exp kT 2 i PPE S/C detector lectures j gen T 32 1 exp 2kT Dr R. Bates j gen 2 for DT 8K 31 Capacitance Capacitance is due to movement of charge in the junction Fully depleted detector capacitance defined by geometric capacitance Strip detector more complex Inter-strip capacitance dominates dQ qN A N D qN D C dV 2N A N D V 2V 2mrV W PPE S/C detector lectures Dr R. Bates 32 Noise Depends upon detector capacitance and reverse current Depends upon electronics design Function of signal shaping time Lower capacitance lower noise Faster electronics noise contribution from reverse current less significant PPE S/C detector lectures Dr R. Bates 33 Fabrication Use very pure material High resistivity Low bias to deplete device Low defect concentration Easy of operation, away from breakdown, charge spreading for better position resolution No extra current sources No trapping of charge carriers Planar fabrication techniques Make p-i-n diode pattern of implants define type of detector (pixel/strip) extra guard rings used to control surface leakage currents metallisation structure effects E-field mag limits max bias PPE S/C detector lectures Dr R. Bates 34 Fabrication stages Stages dopants to create p- & n-type regions passivation to end surface dangling bonds and protect semiconductor surface metallisation to make electrical contact n- Si Starting material Phosphorous diffusion PPE S/C detector lectures Dr R. Bates Usually n- P doped poly n+ Si 35 Fabrication stages Deposit SiO2 Grow thermal oxide on top layer Photolithography + etching of SiO2 PPE S/C detector lectures Dr R. Bates Define eventual electrode pattern 36 Fabrication stages Form p+ implants PPE S/C detector lectures Boron doping thermal anneal/Activation Removal of back SiO2 Al metallisation + patterning to form contacts Dr R. Bates 37 Fabrication Tricks for low leakage currents low temperature processing simple, cheap marginal activation of implants, can’t use IC tech gettering very effective at removal of contaminants complex PPE S/C detector lectures Dr R. Bates 38 Energy Deposition Charge particles Bethe-Bloch Bragg Peak Not covered Neutrons Gamma Rays Rayleigh scattering, Photo-electric effect, Compton scattering, Pair production PPE S/C detector lectures Dr R. Bates 39 Charge particles - concentrating on electrons At 3 dE/dx minimum independent of absorber (mip) Electrons mip @ 1 MeV E>50 MeV radiative energy loss dominates Momentum transferred to a free electron at rest when a charged particle passes at its closest distance, d. integrate over all possible values of d PPE S/C detector lectures Dr R. Bates 40 Well defined range at end of range specific energy loss increases particle slows down deposit even more energy per unit distance Bragg Peak E = 5 MeV in Si: (increasing charge) p 16O PPE S/C detector lectures R (mm) 220 25 4.3 Useful when estimating properties of a device Dr R. Bates 41 Energy Fluctuation Electron range of individual particle has large fluctuation Energy loss can vary greatly Landau distribution Close collisions (with bound electrons) rare energy transfer large ejected electron initiates secondary ionisation Delta rays - large spatial extent beyond particle track Enhanced cross-section for K-, L- shells Distance collisions common M shell electrons - free electron gas PPE S/C detector lectures Dr R. Bates 42 e/h pair creation Create electron density oscillation - plasmon De-excite almost 100% to electron hole pair creation Hot carriers requires 17 eV in Si thermal scattering optical phonon scattering ionisation scattering (if E > 3/4 eV) Mean energy to create an e/h pair (W) is 3.6 eV in Si (Eg = 1.12 eV 3 x Eg) W depends on Eg therefore temperature dependent PPE S/C detector lectures Dr R. Bates 43 Delta rays PPE S/C detector lectures Dr R. Bates a) Proability of ejecting an electron with E T as a function of T b) Range of electron as a 44 function of energy in silicon Displacement from d-electrons Estimate the error Assume 20k e/h from track 50keV d-electron produced perpendicular to track Range 16mm, produces 14k e/h Assume ALL charge created locally 8mm from particle’s track D PPE S/C detector lectures 20k 0mm 14k 8mm 3.3mm 20k 14k Dr R. Bates 45 Consequences of d-electrons Centroid displacement Resolution as function of pulse height 70 6 60 5 Resolution (microns) Probability (%) 50 40 30 20 10 0 4 3 2 1 0 0 2 4 6 8 10 Displacement (microns) PPE S/C detector lectures 0 1 2 3 4 5 6 Pulse height (mip) Dr R. Bates 46 Consequence of d-electrons 45º 45º 15 mm E.g. CCD 300 mm Most probable E loss = 3.6keV 10% proby of 5keV d pulls track up by 4 mm E.g. Microstrip Most probable E loss = 72keV 10% proby of 100keV d pulls track up by 87 mm PPE S/C detector lectures Dr R. Bates 47 Signal formation Signal due to the motion of charge carriers inside the detector volume & the carriers crossing the electrode Displacement current due to change in electrostatics (c.f. Maxwell’s equations) Material polarised due to charge introduction Induced current due to motion of the charge carriers See a signal as soon as carriers move PPE S/C detector lectures Dr R. Bates 48 Signal Simple diode Signal generated equally from movement through entire thickness Strip/pixel detector Almost all signal due to carrier movement near the sense electrode (strips/pixels) Make sure device is depleted under strips/pixels If not: Signal small Spread over many strips PPE S/C detector lectures Dr R. Bates 49 Lecture 3 – Microstrip detector Description of device Carrier diffusion Charge sharing Why is it (sometimes) good Cap coupling Floating strips Off line analysis Performance in magnetic field Details AC coupling Bias resistors Double sides devices PPE S/C detector lectures Dr R. Bates 50 What is a microstrip detector? p-i-n diode Patterned implants as strips One or both sides Connect readout electronics to strips Radiation induced signal on a strip due to passage under/close to strip Determine position from strip hit info PPE S/C detector lectures Dr R. Bates 51 What does it look like? P+ contact on front of n- bulk Implants covered with thin thermal oxide (100nm) Strips surrounded by a continuous p+ ring Forms capacitor ~ 10pF/cm Al strip on oxide overlapping implant Wirebond to amplifier The guard ring Connected to ground Shields against surface currents Implants DC connected to bias rail Use polysilicon resistors MW Bias rail DC to ground HV PPE S/C detector lectures Dr R. Bates 52 Resolution Delta electrons See lecture 2 Diffusion Strip pitch Capacitive coupling Read all strips Floating strips PPE S/C detector lectures Dr R. Bates 53 Carrier collection Carriers created around track Φ 1mm Drift under E-field p+ strips on n- bulk p+ -ve bias Holes to p+ strips, electrons to n+ back-plane Typical bias conditions 100V, W=300mm E=3.3kVcm-1 Drift velocity: e= 4.45x106cms-1 & h=1.6x106cm-1 Collection time: e=7ns, h=19ns PPE S/C detector lectures Dr R. Bates 54 Carrier diffusion Diffuse due to conc. gradient dN/dx Diffusion coefficient: kT D m q RMS of the distribution: s 2Dt coll Since D m & tcoll 1/m Gaussian x2 dN 1 dx exp N 4pDt 4 Dt Width of distribution is the same for e & h As charge created along a strip Superposition of Gaussian distribution Dr R. Bates PPE S/C detector lectures 55 Diffusion Example for electrons: Lower bias wider distribution For given readout pitch tcoll = 7ns; T=20oC s = 7mm wider distribution more events over >1 strip Find centre of gravity of hits better position resolution Want to fully deplete detector at low bias High Resistivity silicon required PPE S/C detector lectures Dr R. Bates 56 Resolution as a f(V) Spatial resolution as a function of bias Resolution (micros) 5 4 3 2 Vfd = 50V 1 0 0 20 40 60 80 100 Bias (V) V<50V charge created in undeleted region lost, higher noise V>50V reduced drift time and diffusion width less charge sharing more single strips PPE S/C detector lectures Dr R. Bates 57 Resolution due to detector design Strip pitch BUT Very dense Share charge over many strips Reconstruct shape of charge and find centre Signal over too many strips lost signal (low S/N) FWHM ~ 10mm Limited to strip pitch 20mm Signal on 1 or 2 strips PPE S/C detector lectures Dr R. Bates 58 Two strip events Track between strips Track mid way Q on both strips Find position from signal on 2 strips Use centre of gravity or Algorithm takes into account shape of charge cloud (eta, h) best accuracy Close to one strip Small signal on far strip Lost in noise PPE S/C detector lectures Dr R. Bates 59 Capacitive coupling Strip detector is a C/R network Cstrip to blackplace = 10x Cinterstrip Csb || Cis ignore Csb Fraction of charge on B due to track at A: K Ceff Cis Ceff 2Ceff C AC as C AC B Cis QB CB K QA QB QC C A C B CC C C CB is AC Ceff Cis C AC C AC Cis Cis K C AC Qs C AC Cis C AC A C K is small PPE S/C detector lectures Dr R. Bates 60 Floating strips 20mm strip pitch s=2.2mm Large Pitch (60mm) 1/3 tracks on both strips 60mm 20mm Intermediate strip 20mm 20mm 20mm PPE S/C detector lectures Assume s = 2.2mm 2/3 on single strips s = 40/12 = 11.5mm Overall: s = 1/3 x 2.2 + 2/3 x 11.5 = 8.4mm Capacitive charge coupling 2/3 tracks on both strips NO noise losses due to cap coupling 1/3 tracks on single strips s = 2/3 x 2.2 + 1/3 x 20/12 = 3.4mm Dr R. Bates 61 Off line analysis Binary readout No information on the signal size Large pitch and high noise Get a signal on one strip only s x2 <x> = 0 P(x) 12 2 x x P( x)dx 1 2 12 2 x P( x)dx 1 2 -½ pitch ½ pitch PPE S/C detector lectures s Dr R. Bates 1 Pitch 12 12 62 Centre of Gravity PHL Have signal on each strip Assume linear charge sharing between strips PHR X PH x PH i strips i strips i i i Q on 2 strips & x = 0 at left strip PH R P X PH L PH R e.g. PHL = 1/3PHR P x X PPE S/C detector lectures Dr R. Bates 1 3 0 3 4 P 3 P 1 33 4 4 63 Eta function PHL Non linear charge sharing due to Gaussian charge cloud shape PHR P More signal on RH strip than predicted with uniform charge cloud shape Non-linear function to determine track position from relative pulse heights on strips x PPE S/C detector lectures Dr R. Bates 64 Eta function Measured tracks as a function of incident particle flux PPE S/C detector lectures Dr R. Bates Measured and predicted particle position 65 Lorentz force Force on carriers due to magnetic force v F q E B c Perturbation in drift direction Charge cloud centre drifts from track position Asymmetric charge cloud No charge loss is observed Can correct for if thickness & B-field known vh E PPE S/C detector lectures H ve qL Dr R. Bates 66 Details Modern detectors have integrated capacitors Thin 100nm oxide on top of implant Metallise over this Readout via second layer Integrated resistors Realise via polysilicon Complex Punch through biasing Not radiation hard Back to back diodes – depleted region has high R PPE S/C detector lectures Dr R. Bates 67 Details Double sided detectors Both p- and n-side pattern Surface charge build up on n-side Trapped +ve charge in SiO Attracts electrons in silicon near surface Shorts strips together p+ spray to increase inter-strip resistance PPE S/C detector lectures Dr R. Bates 68 Lecture 4 – Radiation Damage Effects of radiation Microscopic Macroscopic Annealing What can we do? Detector Design Material Engineering Cold Operation Thin detectors/Electrode Structure – 3-D device PPE S/C detector lectures Dr R. Bates 69 Effects of Radiation Long Term Ionisation Effects Trapped charge (holes) in SiO2 interface states at SiO2 - Si interface Can’t use CCD’s in high radiation environment Displacement Damage in the Si bulk 4 stage process Displacement of Silicon atoms from lattice Formation of long lived point defects & clusters PPE S/C detector lectures Dr R. Bates 70 Displacement Damage Incoming particle undergoes collision with lattice PKA moves through the lattice produces vacancy interstitial pairs (Frenkel Pair) PKA slows, reduces mean distance between collisions clusters formed Thermal motion 98% lattice defects anneal knocks out atom = Primary knock on atom defect/impurity reactions Stable defects influence device properties PPE S/C detector lectures Dr R. Bates 71 PKA PPE S/C detector lectures Clusters formed when energy of PKA< 5keV Strong mutual interactions in clusters Defects outside of cluster diffuse + form impurity related defects (VO, VV, VP) e & don’t produce clusters Dr R. Bates 72 Effects of Defects EC e EV e h Generation h Recombination Leakage Current PPE S/C detector lectures e e h Trapping Charge Collection Dr R. Bates Compensation Effective Doping Density 73 Reverse Current -1 DI / V [A/cm3] 10 10-2 10-3 n-type FZ - 7 to 25 KWcm n-type FZ - 7 KWcm n-type FZ - 4 KWcm n-type FZ - 3 KWcm p-type EPI - 2 and 4 KWcm n-type FZ - 780 Wcm n-type FZ - 410 Wcm n-type FZ - 130 Wcm n-type FZ - 110 Wcm n-type CZ - 140 Wcm p-type EPI - 380 Wcm 10-4 10-5 10-6 11 10 1012 1013 Feq [cm ] -2 1014 1015 = 3.99 0.03 x 10-17Acm-1 after 80minutes annealing at 60C PPE S/C detector lectures I = FVolume Material independent linked to defect clusters Annealing material independent Scales with NIEL Temp dependence Dr R. Bates E I T T 2 exp g 2kT 74 Effective Doping Density 5 Neutron irradiation 3 300 Wacker Polovodice Wacker Topsil 250 200 150 2 100 1 0 Vdep [V] (300mm) |Neff| [1012 cm-3] 4 1.8 KWcm 2.6 KWcm 3.1 KWcm 4.2 KWcm Donor removal and acceptor generation 50 0.5 1 1.5 2 Feq [10 cm ] 14 -2 N eff F N eff 0 exp cF F type inversion: n p depletion width grows from n+ contact Increase in full depletion voltage V Neff = 0.025cm-1 measured after beneficial anneal PPE S/C detector lectures Dr R. Bates 75 Effective Doping Density Short-term beneficial annealing Long-term reverse annealing 10 6 NY, = gY Feq NA = ga Feq 4 NC gC Feq 2 NC0 0 1 10 100 1000 1000 annealing time at 60 C [min] RA 10000 o BA temperature dependent stops below -10C RA BA B A Neff = Zero 800 standard silicon operation voltage: 600 V 600 600 400 400 200 200 oxygenated silicon Increasing Radiation 0 PPE S/C detector lectures 1000 800 Vdep (250mm) [V] D Neff [1011cm-3] 8 Dr R. Bates 1 2 3 4 5 6 time [years] 7 8 9 10 76 Signal speed from a detector Duration of signal = carrier collection time Speed mobility & field Speed 1/device thickness PROBLEMS Post irradiation mobility & lifetime reduced mt lower longer signals and lower Qs Thick devices have longer signals PPE S/C detector lectures Dr R. Bates 77 Signal with low lifetime material Lifetime, t, packet of charge Q0 decays Q(t ) Qo e t t In E field charge drifts Time required to drift distance x: x x t v mE Remaining charge: Drift length, L mt mt is a figure of merit. Q( x) Q0e x mEt Q 0 e x L PPE S/C detector lectures Dr R. Bates 78 Induced charge Parallel plate detector: d d 1 1 Qs Q( x)dx Q0e x L dx d0 d0 L Qs Q0 1 e d L d Qs L L d : Q0 d In high quality silicon detectors: t 10ms, me = 1350cm2V-1s-1, E = 104Vcm-1 L 104cm (d ~ 10-2cm) Amorphous silicon, L 10mm (short lifetime, low mobility) Diamond, L 100-200mm (despite high mobility) CdZnTe, at 1kVcm-1, L 3cm for electrons, 0.1cm for holes PPE S/C detector lectures Dr R. Bates 79 What can we do? Detector Design Material Engineering Cold Operation Electrode Structure – 3-D device PPE S/C detector lectures Dr R. Bates 80 Detector Design n-type readout strips on n-type substrate post type inversion substrate p type depletion now from strip side high spatial resolution even if not fully depleted Single Sided Polysilicon resistors W<300mm thick limit max depletion V Max strip length 12cm lower cap. noise PPE S/C detector lectures Dr R. Bates 81 Multiguard rings Poly Guard rings Enhance high voltage operation Smoothly decrease electric field at detectors edge back plane strip bias bias V PPE S/C detector lectures Dr R. Bates 82 Substrate Choice Minimise interface states Substrate orientation <100> not <111> Lower capacitive load Independent of ionising radiation <100> has less dangling surface bonds PPE S/C detector lectures Dr R. Bates 83 Metal Overhang Used to avoid breakdown performance deterioration after irradiation 2 SiO2 p+ (1) (2) n n+ 4mm p+ 0.6mm Breakdown Voltage (V) 1 Strip Width/Pitch <111> after 4 x 1014 p/cm2 PPE S/C detector lectures Dr R. Bates 84 Material Engineering Do impurities influence characteristics? Leakage current independent of impurities Neff depends upon [O2] and [C] 1E+13 500 8E+12 Standard (P51) O-diffusion 24 hours (P52) O-diffusion 48 hours (P54) O-diffusion 72 hours (P56) Carbon-enriched (P503) -3 |Neff| [cm ] 7E+12 6E+12 5E+12 400 St = 0.0154 300 4E+12 [O] = 0.0044 0.0053 3E+12 2E+12 200 100 VFD for 300 mm thick detector [V] [C] = 0.0437 9E+12 1E+12 0 0 1E+14 2E+14 3E+14 4E+14 0 5E+14 Proton fluence (24 GeV/c ) [cm-2] PPE S/C detector lectures Dr R. Bates 85 O2 works for charged hadrons Neff unaffected by O2 content for neutrons Believed that charge particle irradiation produces more isolated V and I 7 standard FZ 400 neutrons pions protons 5 oxygen rich FZ neutrons pions protons 4 3 300 200 2 Vdep [V] (300mm) |Neff| [1012 cm-3] 6 100 1 0 0.5 1 1.5 2 2.5 3 3.5 V + O VO V + VO V2O V2O reverse annealing High [O] suppresses V2O formation Feq [10 cm ] 14 PPE S/C detector lectures -2 Dr R. Bates 86 Charge collection efficiency Oxygenated Si enhanced due to lower depletion voltage CCI ~ 5% at 300V after 3x1014 p/cm2 CCE of MICRON ATLAS prototype strip detectors irradiated with 3 1014 p/cm2 PPE S/C detector lectures Dr R. Bates 87 ATLAS operation Vdep (250mm) [V] Damage for ATLAS barrel layer 1 1000 1000 800 800 standard silicon operation voltage: 600 V 600 600 400 400 200 200 oxygenated silicon 0 1 2 3 4 5 6 7 8 9 Use lower resistivity Si to increase lifetime in neutron field Use oxygenated Si to increase lifetime in charge hadron field 10 time [years] PPE S/C detector lectures Dr R. Bates 88 Cold Operation Know as the “Lazarus effect” Recovery of heavily irradiated silicon detectors operated at cryogenic temps observed for both diodes and microstrip detectors PPE S/C detector lectures Dr R. Bates 89 The Lazarus Effect For an undepleted heavily irradiated detector: 2 t drift d CCE exp D t trap where d2 d active region 1 N eff T = 300 K e undepleted region D T = 77 K e conduction band conduction band electron trapping electron de-trapping e trap filled trap filled h hole trapping hole de-trapping valence band h h valence band Traps are filled traps are neutralized Neff compensation (confirmed by experiment) B. Dezillie et al., IEEE Transactions on Nuclear Science, 46 (1999) 221 PPE S/C detector lectures Dr R. Bates 90 Reverse Bias Measured at 130K - maximum CCE CCE falls with time to a stable value PPE S/C detector lectures Dr R. Bates 91 Cryogenic Results CCE recovery at cryogenic temperatures CCE is max at T ~ 130 K for all samples CCE decreases with time till it reaches a stable value Reverse Bias operation MPV ~5’000 electrons for 300 mm thick standard silicon detectors irradiated with 21014 n/cm2 at 250 V reverse bias and T~77 K very low noise Forward bias is possible at cryogenic temperatures No time degradation of CCE in operation with forward bias or in presence of short wavelength light same conditions: MPV ~13’000 electrons PPE S/C detector lectures Dr R. Bates 92 Electrode Structure Increasing fluence Reducing carrier lifetime Increasing Neff Higher bias voltage Operation with detector under-depleted Reduce electrode separation Thinner detector Reduced signal/noise ratio Close packed electrodes through wafer PPE S/C detector lectures Dr R. Bates 93 The 3-D device Co-axial detector Micron scale Dr R. Bates Readout each p+ column Strip device PPE S/C detector lectures USE Latest MEM techniques Pixel device Arrayed together Connect columns together 94 Operation SiO2 +ve -ve -ve +ve -ve -ve p+ h+ h+ Bulk n e- W3D n+ Equal detectors thickness W2D>>W3D E Carriers swept horizontally Travers short distance between electrodes PPE S/C detector lectures W2D e- E Dr R. Bates +ve Carriers drift total thickness of material 95 Proposed by S.Parker, Nucl. Instr. And Meth. A 395 pp. 328-343(1997). Advantages If electrodes are close Low full depletion bias Low collection distances Thickness NOT related to collection distance No charge spreading Fast charge sweep out PPE S/C detector lectures Dr R. Bates 96 A 3-D device Form an array of holes Fill them with poly-silicon Add contacts Can make pixel or strip devices Bias up and collect charge PPE S/C detector lectures Dr R. Bates 97 Real spectra At 15V Plateau in Q collection Fully active Very good energy resolution PPE S/C detector lectures Dr R. Bates 98 3-D Vfd in ATLAS 2000 s ta n d a rd s ilic o n 1500 1500 6 000 e fo r B -lay er 1000 1000 dep ( 2 00 m m ) [ V ] 2000 V opera tio n voltag e: 60 0 V 500 500 o x y g e n a te d s ilic o n 0 1 2 3 4 5 6 tim e [y e a rs ] 7 8 9 • 3D detector! 10 Damage projection for the ATLAS B-layer (3rd RD48 STATUS REPORT CERN LHCC 2000-009, LEB Status Report/RD48, 31 December 1999). PPE S/C detector lectures Dr R. Bates 99 Summary Tackle reverse current Cold operation, -20C Substrate orientation Multiguard rings Overcome limited carrier lifetime and increasing effective doping density Change material Increase carrier lifetime Reduce electrode spacing PPE S/C detector lectures Dr R. Bates 100 Final Slide Why? Where? How? A major type A major worry PPE S/C detector lectures Dr R. Bates 101