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Developement of Radiation Hard Silicon for Tracking Detektors

Developement of Radiation Hard Silicon for Tracking Detektors. Frank Hönniger University of Hamburg, Institut für Experimentalphysik. Student Seminar, 10. April 2006. Outline. Motivation Properties of Silicon Detectors Radiation Damage Experimental Methods Experimental Results

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Developement of Radiation Hard Silicon for Tracking Detektors

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  1. Developement of Radiation Hard Silicon for Tracking Detektors Frank Hönniger University of Hamburg, Institut für Experimentalphysik Student Seminar, 10. April 2006

  2. Outline • Motivation • Properties of Silicon Detectors • Radiation Damage • Experimental Methods • Experimental Results • Conclusion and Outlook

  3. p n n p n n p n n n p p n p p p p p p n q q Standard model of particle physics electromagnetic strong weak gravitation quarks: d, u, s, c, b, t --------- leptons: (e- e) (- ) (- ) Open questions: is there a universal force? (GUT?) what is the origin of mass (Higgs-boson?) unknown types of matter (dark matter, SUSY) HEP-Experiments towards higher energies

  4. LHC Experimental request Detector property Reliable detection of mips S/N ≈ 10 reachable with employing minimum minimum detectorthickness material budget High event rate excellent time- (~10 ns) and & high track accuracy position resolution (~10 µm) Complex detector design low voltage operation in normal ambients, (hybrid integration) Intense radiation field Radiation tolerance up to throughout operational 1015 1MeV eq. n/cm² period of 10 years low dissipation power moderate cooling Silicon pixel and microstrip detectors meet all requirements for LHC How about future developments? LHC properties Proton-proton collider Energy: 2 x 7 TeV Luminosity: 1034cm-2 s-1 Bunch crossing: every 25 nsec Rate: 40 MHz pp-collision event rate: 109/sec (23 interactions per bunch crossing) Annual operational period: 107 sec Expected total op. period: 10 years

  5. Radiation requirements for LHC-experiments • radiation damage for ATLAS inner detector • annual hadron fluence (1 MeV n equiv.) LHC:L=1e34 cm-2 s-1 technology for Si-detectors available, however serious radiation damage CERN-RD48 http://cern.ch/rd48/ vertex luminosity upgrade S-LHC:factor 10: L=1e35 cm-2 s-1 (R=4cm) ~ 1.6E16 cm-2 5 years • no technology for Si-detectors at • S-LHC available yet (thinner detectors?) • coordinated R&D needed • developement of radiation hard and cost-effctive detectors CERN-RD50 http://cern.h/rd50/

  6. Motivation for thinner detectors LHC: use of high resistivity FZ silicon Thickness: 300 m „type inversion“ Pixel: oxygen enriched silicon – DOFZ SCT: standard silicon - StFZ but all show „type inversion“ after 2*1013 p/cm2 S-LHC:to prevent type inversion • higher initial doping concentration (more n-type) • to maintain reasonable reverse bias during operation Si-detectors have to be thin • shorter charge collection time faster signal response • less charge loss through trapping better signals • thin detectors reduction of e.g. pixel area higher position resolution

  7. Motivation of radiation tests and annealing studies Radiation damage and annealing (thermal treatment) have a big influence on detector performance • use pad detectors with simple and cheap structures • irradiate them with different particles and fluences • simulation of the irradiation of the whole operation time for silicon detectors in short time • annealing studies at higher temperatures (60°C, 80°C) • acceleration of annealing (2 min@80°C  10 days@RT) • extract particle, fluence and time dependencies of the detector parameters • make predictions

  8. Use of silicon

  9. Semiconductor detector - principle p+nn+ - junction a solid state ionisation chamber • high energy- and position resolution • fast readout of the signals • large S/N-ratio (CCE=100%) • compact geometry possible • silicon: wide availibility, operation at RT, in ambient atmospheres and under low voltages fully depleted • segmented detectors by planar process (Kemmer 1984) microstrip-, pixel-detectors, CCD‘s various application in HEP, space, atomic & material physics, and in medicine

  10. p n    – –  + – – – + + + + + +   – +  + – – + + + + + – donor/acceptor concentration space charge distribution carrier distribution electric field electric potential 4 Parameters of silicon detectors • you can assume an abrupt p-n-junction • under reverse bias depletion region depletion voltage Vdep 1 most important: determines the bias supply capacitance C of depletion region 2 V>Vdep influence on S/N-ratio

  11. Operational parameters of silicon detectors • volume generation current (caused by • impurities and defects) + • diffusion current + • surface generation current • important for the S/N-ratio • and the power consumption • prevention by cooling (ATLAS: -10°C) leakage current 3 charge collection efficiency CCE CCE = Q/Q0 4 • ratio of measured charge to the induced charge • non-irradiated detector CCE = 1

  12. Radiation Damage in silicon classify into bulk damage (and surface damages) (PKA = Primäry knocked on atom) dislocation of Si -atoms impinging particle mobileVacancy V • Point defects: • they can have discrete energy levels in the band gap • can have electrical influence on detectors: • generation and recombination of e-h-pairs • carriers can be trapped • compensation of the initial doping • primäry point defects are V and I • secondary complex defects: VP, VO, V2O, CO • influence of oxygen: Theory (thus DOFZ-Silicon) • more O more VO (elect. not active at RT) • less O more V2O (elect. active at RT) CsCi mobile Interstitial I • Cluster: regions of dislocations • high energy PKA cascades of shifted atoms • can locally change the band structure • generation and recombination of e-h-pairs • not really understood yet impinging particle Cluster

  13. Experimental findings Defect states of the defect levels No influence on the macroscopic parameters Influencing doping and current Influencing only doping Influencing only current Reverse current: Effective doping: nt is large for a deep acceptor if sp >> sn Generation rate: VP-defect  „donor removal“

  14. NIEL - Theory How can I compare the irradiation effects of different particles and energies? • with PKA (>2keV), sort of irradiation defects • become independent of particle and energy • NIEL scales with radiation damage • D(E)  NIEL one can convert the particle fluence to 1 MeV n equivalent fluence hardness factor  24 GeV/c protons: 0.62 reactor neutrons: 0.91 10 MeV protons: 3.99 Li-ions: 50.7 eq=   Simulation (M. Huhtinen): Initial distribution of vacancies in (1µm)3 after 1014 particles/cm² charged particle: coulomb scattering neutrons: elastic scattering both, at higher energies: nuclear react. 10 MeV protons 24 GeV/c protons neutrons

  15. Damage induced changes of macroscopic properties Increase of leakage current Introduction of defects/clusters with near to mid-gap levels as generation centers, increase of noise and power consumption, thermal run-away I/V =    Change effective doping concentration  change of voltage for total depletion Vdep • Introduction of defects which are charged in the space charge region, (acceptor creation) • e.g.: V + P = VP (donor removal) „type inversion“ Degradation of charge collection efficiency due to increaseof charge carrier trapping1/eff,e,h =  e,h  

  16. Silicon detectors – used test structures Epitaxy CZ, FZ-process 25, 50, 75 μm EPI-diode standard diode

  17. CV/IV- measurements • easy determination of macroscopic properties • (depletion voltage, leakage current) • allows a fast check of detector functionality prober to pad prober to guard ring • measuring in dark box • bias supply to the bottom • guard ring is grounded

  18. TCT- measurements (transient current technique) • measurements of current pulses with • oscilloscope • induced from drift of free carriers • front and back illumination with 670 (3m) and 1060 nm (across) laser • penetration depth proportional to  • also exposure with  (23 m) or  (across) (get absolute values) cooling possible with nitrogen • investigation of • elect. field distribution • sign of the space charge • trapping probability, • separated for e and h • depletion voltage • CCE

  19. DLTS and TSC DLTS-spectrum after proton irradiation TSC-spectrum after neutron irradiation TSC current [A] T [K] • DLTS Method: • p/n junction is hold under reverse bias • Electrical or optical pulsefills traps inside • the SCR with carriers • Traps release carriers by thermal emission • Emission is monitored as a capacitance signal TSC Method: Traps filled at low temperature by electrical or optical injection Diode heated under reverse bias Current during the heating is monitored Trap concentration Nt is proportional to the peak height Trap concentration is proportional to the released charge

  20. DLTS principle [1] A bias pulse towards a smaller voltage will reduce the SCR [2] The junction capacitance is reduced because positive space charge is partially compensated by trapped electrons in the SCR [3] The process of carrier emission can be followed as a capacitance transient

  21. DLTS method The emission time constant can be evaluated:

  22. CERN Scenario Experiment-Depletion Voltage High energy protons max. bias supply • CERN Scenario Experiment: • consecutive irradiation steps • in between annealing for 4 min@80°C • after annealing CV/IV-measurements • quasi „online“ monitoring • annealing corresponds to • 20 days at 20 °C • closely related to stable damage 120V 160 V • large improvement for EPI-detectors • small change in depletion voltage for EPI up to very high fluences • no type inversion for EPI • limitation for StFZ, DOFZ and CZ for very high luminosity colliders Why is EPI radiation harder: shifted donor removal, because of higher initial donor concentration radiation induced acceptor creation compensated by radiation induced donors

  23. Material Parameters • Oxygen depth profilesSIMS-measurements after diode processing O diffusion from substrate into epi-layerinterstial Oi + dimers O2i [O] 25 µm > [O] 50 µm process simulation yields reliable [O] • Resistivity profilesSR before diode process, C-V on diodes SR coincides well with C-V method Excellent homogeneity in epi-layers

  24. Typical Annealing Curves Typical annealing behavior of EPI-devices: Vfd development:Inversion only(!) during annealing ()(100 min @ 80C ≈ 500 days @ RT)  EPI never inverted at RT, even for 1016

  25. Parameterization of Annealing Results Change of effective “doping“ concentration: Neff = Neff,0 – Neff (,t(T)) Standard parameterization: Neff = NA(,t(T)) + NC() + NY(,t(T)) Annealing components: • Short term annealing NA(,t(T)) • Stable damage  NC()NC = NC0(1-exp(-cΦeq) + gCΦeqgC negative for EPI (effective positive space charge generation!) • Long term (reverse) annealing:Two components: NY,1(,t(T)), first order process NY,2(,t(T)), second order processNY1, NY2~ Φeq, NY1+NY2 similar to FZ

  26. Stable Damage Component Neff(t0): Value taken at annealing time t0 at which Vfd maximum • No space charge sign inversion after proton and neutron irradiationIntroduction of shallow donors overcompensates creation of deep acceptors • Protons: Stronger increase for 25 µm compared to 50 µm higher [O] and possibly [O2] in 25 µm (see SIMS profiles) • Neutrons: Similar effect but not nearly as pronounced most probably due to less generation of shallow donors and as strong influence of deep acceptors (clusters)

  27. Shallow Donors, the real issue for EPI -Comparison of 25, 50 and 75 µm Diodes- ≈ 105 V (25 µm) ≈ 230 V (50 µm) ≈ 320V (75 µm) Defect spectroscopy after PS p-irradiation Generation of recently found shallow donors BD (Ec-0.23 eV) strongly related to [O] Possibly caused by O-dimers, outdiffused from Cz with larger diffusion constant dimers monitored by IO2 complex SIMS profiling: [O](25µm) > [O](50µm)>[O](75µm) Stable Damage: Neff(25µm) > Neff(50µm) > Neff(75µm) TSC Defect Spectroscopy: [BD](25µm) > [BD](50µm) >[BD](75µm) Strong correlation between [O]-[BD]-gC generation of O (dimer?)-related BD reason forsuperior radiation tolerance of EPI Si detectors

  28. DLTS (Deep Level Transient Spectroscopy) Φ(25 μm epi) = 1.2 ·1012 p/cm2 25 μm epi: defect at 67K TW = 200 ms, tp= 100 ms UR=-20V, UP=-0.1V 25 μm epi

  29. Charge Collection Efficiency • CCE measured with 244Cm -particles (5.8 MeV, R30 µm)Integration time window 20 ns • CCE degradation linear with fluence if the devices are fully depletedCCE = 1 –  , = 2.710-17 cm2 CCE(1016 cm-2) = 70 % • CCE measured with 90Sr electrons (mip’s), shaping time 25 ns • CCE no degradation at low temperatures ! CCE measured after n- and p-irradiation  CCE(Φp=1016 cm-2) = 2400 e (mp-value) trapping parameters = thos for FZ diodes for small Φ, For large Φ less trapping than expected !

  30. Charge Collection Efficiency • CCE mit 90Sr Elektronen (mip’s), Integrationtime 25 ns  CCE(Φp=1016 cm-2) = 2400 e (mp-Wert)

  31. Current Generation Result almost identical to FZ silicon:Current related damage rate α = 4.1·10-17 Acm-1 (Small deviations in short term annealing)

  32. S-LHCCERN scenario experiment Stable donor generation at high Φ would lead to larger Vfd, but acceptor generation during RT anneal could compensate this. Proposed Benefit: Storage of EPI-detectors during beam off periods at RT (in contrast to required cold storage for FZ) Check by dedicated experiment: • Experimental parameter: • Irradiation:fluence steps  2.21015 cm-2irradiation temperature  25°C • After each irradiation stepannealing at 80°C for 50 min,corresponding 265 days at 20°C • Simulation:reproducing the experimental scenario with damage parameters from analysis Excellent agreement between experimental data and simulated results  Simulation + parameters reliable!

  33. S-LHCoperational scenario simulation results S-LHC: L=1035cm-2s-1Most inner pixel layer operational period per year:100 d, -7°C, Φ = 3.48·1015cm-2beam off period per year265 d, +20°C (lower curves) -7°C (upper curves)  RT storage during beam off periods extremely beneficial Damage during operation at -7°C compensated by 100 d RT annealing Effect more pronounced for 50 µm: less donor creation, same acceptor component Depletion voltage for full SLHC period less than 300 V

  34. Summary • Thin low resistivity EPI diodes (grown on Cz) are extremely radiation tolerant • No type inversion observed up to Φeq = 1016 cm-2 for protons and neutrons • Radiation induced stable donor generation related most likely to O-dimers • Elevated temperature annealing results verified at 20°C • Dedicated CERN scenario experiment shows benefits of RT storage • Simulation of real SLHC operational scenario with RT storage demonstrated 50 µm EPI Detectors withstand 5y SLHC with Vop ≤ 300V (full depl.) Future plans: p-type epi, thicker n-type epi, thin Cz …..

  35. Destination To understand the nature of the universe !

  36. Experimental conditions at LHC Proton-proton collider Energy: 2 x 7 TeV Luminosity: 1034 cm-2 s-1 Bunch crossing: every 25 nsec Rate: 40 MHz pp-collision event rate: 109/sec (23 interactions per bunch crossing) Annual operational period: 107 sec Expected total op. period: 10 years LHC properties • Detector requirements • reliable detection of charged particles • material budget has to kept in mind, • because of big detectors • high event rate& high track accuracy • complex detector design • intense radiationfield in the whole operational • period of 10 years Radiation damage • negative influence on detector parameters • Silicon detector can handle with all requirements for LHC

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