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Czochralski Silicon - a radiation hard material?

Czochralski Silicon - a radiation hard material?. Vertex 2005 November 7 – 11 Chuzenji Lake, Japan Alison Bates The University of Glasgow, UK. Cz Characterization – Overview. Main players: Ljubljana, CERN, SMART, CNM, Helsinki, BNL and Hamburg. Lab measurements study: CV IV

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Czochralski Silicon - a radiation hard material?

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  1. Czochralski Silicon- a radiation hard material? Vertex 2005 November 7 – 11 Chuzenji Lake, Japan Alison Bates The University of Glasgow, UK.

  2. Cz Characterization – Overview • Main players: • Ljubljana, CERN, SMART, CNM, Helsinki, BNL and Hamburg. Lab measurements study: CV IV Annealing studies TCT provides: Depletion voltage (QV) CCE Space charge determination Electric field profile Trapping times Defect characterization: Specific defect level concentrations Test beams: Towards detector grade components… CCE Resolution S/N,…..

  3. What is Czochralski Silicon? • A crystalline silicon growth method. • The growth method used by the IC industry. • Recent developments (~3 years) has meant that the silicon is now of sufficient purity to allow use for HEP detectors. • Pull Si-crystal from a Si-melt while rotating. • Cz Silicon has an intrinsically high level of oxygen. • MCz is Cz silicon grown in the presence of an magnetic field. • Cheap production. • Common production technique. • IRST (Italy), CNM (Spain), CiS (Germany), Helsinki Institute of Physics (Finland) and BNL (USA) have all successfully produced Cz detectors. Czochralski Growth

  4. Float Zone silicon (FZ)-the usual growth method used to make HEP detectors • Start with a polysilicon rod inside a chamber either in a vacuum or an inert gas • An RF heating coil melts ≈2 cm zone in the rod • The RF coil moves through the rod, moving the molten silicon region with it • This melting purifies the silicon rod • Oxygen can be diffused into the silicon – called Diffusion Oxygenated Float Zone (DOFZ) (done at the wafer level) Poly silicon RF Heating coil Single crystal silicon Float Zone Growth

  5. Why should Cz be any better?- Oxygen is important • Oin Diffusion Oxygenated FZ (DOFZ) ~ 1x1017 cm-3 • Oin magnetic Cz (MCz) ~ 2-5 x1017 cm-3 • DOFZ silicon has less variation in Vfd with radiation compared to FZ – more radiation hard • Adding carbonto silicon decreases the radiation hardness For hadron radiation only • DOFZ: Saturation of reverse annealing (24 GeV/c p - only little effect after neutron irradiation observed !)

  6. Why should Cz be any better?- Oxygen is important • Oin Diffusion Oxygenated FZ (DOFZ) ~ 1x1017 cm-3 • Oin magnetic Cz (MCz) ~ 2-5 x1017 cm-3 • DOFZ silicon has less variation in Vfd with radiation compared to FZ – more radiation hard • Adding carbonto silicon decreases the radiation hardness For hadron radiation only Reverse Annealing Component • DOFZ: Saturation of reverse annealing (24 GeV/c p - only little effect after neutron irradiation observed !)

  7. Cz Characterization – leakage current 300μm thick 5x5 mm2 p+n silicon diodes (all 1 kΩcm) have been characterized before and after 24 GeV/c proton irradiation at the CERN PS. Diodes processed at the Helsinki Institute of Physics from FZ, DOFZ and MCz silicon manufactured at Okmetric Oyj. The leakage current of MCz silicon after proton irradiation follows the same behaviour as FZ and DOFZ silicon

  8. Cz Characterization – radiation hard? STFZ DOFZ Cz • The irradiation experiments which have been performed with Cz/MCz are; • reactor neutrons • 23 GeV protons • 10, 20, 30 MeV protons • 190 MeV pions • 900 MeV electrons • Co60 gamma The gradient of the slope after the minimum is β, which is a measure of the radiation hardness β has been measured to be smaller for MCz than DOFZ, FZ silicon for 10MeV, 50 MeV and 24 GeV proton irradiations (E. Tuovinen, 4th RD50 workshop, May 2004) MCz is more radiation hard than DOFZ or FZ silicon (with charged irradiation)

  9. Cz Characterization – Transient Current Technique p+ Hole movement Electron collection n+ 660 nm laser light • Illuminate front (p+) or rear (n+) side of detector with 660 nm photons • Light penetrates only a few mm depth • Ramo’s theorem dictates signal will be dominated by one type of charge carrier • I(t)=q E(u(t)) u(t)drift • e.g. hole dominated current (hole injection) is produced by illuminating the rear (n+) side of detector High field Low field Time [ns] Need signal deconvoluted from electronic shaping

  10. Cz Characterization – Transient Current Technique Injection time Injection charge The effective trapping probability, 1/eff, is the probability that a carrier is lost due to trapping in the silicon. Measured charge from detector Corrected charge When the detector has been irradiated the drifting charge, Qe,h(t), will be lost with an exponential time dependence due to trapping in the defects To derive the electric field profile/SC sign you must take trapping effects into account Trapping compensation

  11. Cz Characterization – Transient Current Technique Charge Correction Method (CCM) • For V>Vfd, then: • constant Q collected if no trapping • try various teff values • correct teff value is when gradient of this line is zero Charge [arb.] Sqrt V [Sqrt V] Example of an electron injection signal collected before (dashed) and after (solid line) the correction for the trapped charge. Details: 15 kΩcm FZ 5.2x1013 24 GeV/c p/cm2 Vfd = 30 V measured at 90 V. CCM method resulted in eff,e= 37.8 ns.

  12. Type inversion in FZ siliconElectron injection High field High field I [V/50Ohms] I [V/50Ohms] Low field Low field Time [ns] Time [ns] F= 3.61x1014 24 GeV/c p F = 1.74x1013 24 GeV/c p

  13. TCT in MCz Hole injection Hole injection signal NO TYPE INVERSION IN CZ SILICON UP TO 5x1014 p/cm2 High field I [V/50Ohms] • MCz silicon always has the high electric field on the structured side of the detector even after high fluences using standard p+-on-n MCz detectors • Avoid the expensive double sided processing costs that arise from using n+-on-n silicon detectors Low field 5x1014 p/cm2 14 Alison Bates Time [ns]

  14. MCz @5x1014 p/cm2 Confirmed by Gregor Kramberger et. al

  15. Effective doping concentration - FZ Build up of negative space charge Cluster and V20 responsible Donor Removal ND*exp(-cΦ) Resultant Neff shape can be explained by the two processes

  16. Effective doping concentration - Cz Build up of negative space charge Cluster and V20 responsible Donor Removal ND*exp(-cΦ) Build up of positive space charge Due to radiation induced donors (linked to O2?) Resultant Neff shape can be explained by the three processes MCz has higher Oxygen content than FZ

  17. Cz Characterization –trapping • Trapping times • Effective trap introduction rate, βe, for electrons agree within experimental errors for FZ, DOFZ and Cz silicon. • Effective trap introduction rate, βh, for holes are 10-30% larger than βe for all of FZ, DOFZ and Cz.  Cz silicon has similar trapping to FZ and DOFZ silicon

  18. Cz Characterization – annealing Annealing studies - MCz shows excellent annealing behaviour FZ DOFZ Cz G. Pellegrini et. al “Annealing Studies of magnetic Czochralski silicon radiation detectors” NIM A, article in press

  19. Cz Characterization – test beam • Test beam at the CERN SPS of a MCz detector* before and after irradiation • LHC speed electronics (40MHz) (3 SCTA (analogue) chips) • p+-on-n MCz material • Area read out = 6.1 x 1.92 cm • 380 mm thick • 1150 Wcm (after processing) • 50 mm pitch parallel strips • Vdep measured = 420 V (CV) *Many thanks to the Helsinki Institute of Physics for the MCz detector NIM A 535 (2004) 428

  20. MCz test beam results Irradiated Detector Unirradiated Detector Signal [ADC Counts] Signal [ADC Counts] Bias Voltage [V]] Bias Voltage [V]] • 1.3 x 1014 24 GeV p/cm2 S/N = 15 • 4.3 x 1014 24 GeV p/cm2 S/N = 11 (under depleted) • 7.0 x 1014 24 GeV p/cm2 S/N = 7 (under depleted) • Depleted the detector (~550 V) (CV measured Vdep ~ 420 V) S / N > 23.5 + 2.5 (380 mm thick)

  21. Cz Characterization – Conclusions • Czochralski silicon is a cheap and standard industrial method for growing high purity silicon. • Cz silicon • shows increased radiation hardness when compared to FZ or DOFZ with charged irradiation • does not type invert with charged particle radiation (up to a 24 GeV/c proton fluence of 5.1014 p/cm2) • has the same trapping behaviour as FZ and DOFZ • has small variation in Neff with annealing time • Cz strip detector read out with LHC speed electronics shows promising results both before and after irradiation. • Is Czochralski silicon something to get excited about? Yes!

  22. Back up slides

  23. The evolution of the depletion voltage as determined by CV and IV methods for DOFZ (d1) silicon The evolution of the depletion voltage as determined by CV and IV methods for MCz silicon.

  24. Cz Characterization – procurement • IRST, CNM, CiS, HIP and BNL have successfully produced Cz detectors. • Sumitomo is no longer accessible and Okmetric Oyj require large orders (>1000 wafers per order)

  25. Cz Characterization – proton irradiation Confirmation of MCz depletion voltage behaviour of MCz after 24 GeV/c proton irradiation by G. Pellegrini et. al “Annealing Studies of magnetic Czochralski silicon radiation detectors” NIM A, article in press. FZ SCSI? DOFZ MCz SCSI

  26. Cz Characterization – pion irradiation Confirmation of MCz depletion voltage behaviour of MCz after 190 MeV/c pion irradiation by G. Lindstroem et. al 1st RD50 workshop, October 2002 MCz FZ DOFZ

  27. Cz Characterization – neutron irradiation Measurements after irradiation and before annealing SMART collaboration  The minimum of Vdep is reached at 1-1.5×1014 n/cm2.  Vdep is 650 at 1015 n/cm2.

  28. Cz Characterization – leakage current 5x5 mm2 p+n silicon diodes have been characterized before and after 24 GeV/c proton irradiation at the CERN PS. Diodes processed at the Helsinki Institute of Physics from FZ, DOFZ and MCz silicon manufactured at Okmetric Oyj. Depletion voltages for FZ, DOFZ and MCz diodes before irradiation Single guard ring was always connected.

  29. Cz Characterization – proton irradiation • Diodes measured after 24 GeV/c proton irradiation and 4mins/80oC annealing with IV and CV techniques. • Guard ring connected. • CV measurements made with 10kHz in parallel mode. FZ silicon SCSI

  30. Cz Characterization – proton irradiation • Diodes measured after 24 GeV/c proton irradiation and 4mins/80oC annealing with IV and CV techniques. • Guard ring connected. • CV measurements made with 10kHz in parallel mode. MCz silicon SCSI? SCSI

  31. TCT in MCz NO TYPE INVERSION IN CZ SILICON UP TO 5x1014 p/cm2 Hole injection signal High field 5x1014 p/cm2 I [V/50Ohms] High field I [V/50Ohms] • MCz silicon always has the high electric field on the structured side of the detector even after high fluences using standard p+-on-n MCz detectors Low field Low field 14 Confirmed by Gregor Kramberger et. al Time [ns] 14 Alison Bates

  32. Cu/Be spring contact to front pad CERN TCT set-up(1) • Easy detector mounting • Floating guard ring • Front and back illumination possible • Peltier cooled to ~-10oC • Temp. stability to +0.1oC • Flushed with N2 gas • Red 660 nm laser diode • IR 1060 nm laser diode • Amount of charge deposited can be tuned - laser diode output controlled by pulse generator signal Au PCB for ground plate Laser fibre for illuminating the top of the detector Water cooling and gas system

  33. CERN TCT set-up(2) • Custom written LabVIEW DAQ • ROOT analysis of data* *Data analysis software courtesy from the wonderful Gregor Kramberger Almost no detector shaping from electronics Rise time of signal 1.5 ns Pulse duration min 1.5 ns FWHM

  34. Signal treatment • Deconvolution of the true signal from the measured signal Measured signal = detector signal  transfer function Transfer function: I(t)=tTCT/R x dUosc(t)/dt + Uosc(t)/R R = 50W from input of preamp tTCT= RCd (Cd = detector capacitance) I [V/50W] Time [ns]

  35. Charge Correction Method • The CCM assumes three conditions: • There exists one dominant trapping time • The detrapping effects are negligible in the readout time • All the lost charge is due to trapping • The method requires no knowledge of the electric field profile in the detector or any information about the charge carrier distributions. All plots presented in this paper have been deconvoluted from the electronic transfer function of the TCT readout circuit and corrected for the trapped charge.

  36. b parameter summary Table 4. Comparison of βe and βh determined after 24 GeV/c proton irradiation. The top 4 rows are the values found in this work while the last four rows show data previously obtained by other groups. All values have been scaled to 5oC, for the temperature dependence of β (see section 4.5).

  37. What’s the limitations with FZ detectors • p+n detectors deplete from the front segmented side before irradiation • p+n FZ detectors type invert at a certain radiation level and then deplete from the back side of the detector • The Vfd is increasing every day hence at some point the detector will be operated under-depletion, in which case: • Charge spread – degraded resolution • Charge loss – reduced CCE p+n detector before type inversion and under-depleted.

  38. What’s the limitations with FZ detectors • p+n detectors deplete from the front segmented side before irradiation • p+n FZ detectors type invert at a certain radiation level and then deplete from the back side of the detector • The Vfd is increasing every day hence at some point the detector will be operated under-depletion, in which case: • Charge spread – degraded resolution • Charge loss – reduced CCE

  39. resolution Depletion fraction n+on-n Charge spread for p+on-n Si • n-on-n silicon, under-depleted: • Limited loss in CCE • Less degradation with under-depletion For LHCb n+on-n detectors are the technology choice

  40. Macroscopic changes Shockley-Read-Hall statistics (standard theory) e e charged defects Neff , Vdepe.g. donors in upper and acceptors in lower half of band gap Trapping (e and h) CCEshallow defects do not contribute at room temperature due to fast detrapping generation leakage currentLevels close to midgap most effective Effects the operating voltage Reduced Signal Increased Noise

  41. before inversion n+ p+ n+ after inversion Depletion voltage in FZ silicon • Neff – Effective doping concentration • Neff positive – n-type silicon (e.g. Phosphorus doped – donor) • Neff negative – p-type silicon (e.g. Boron doped – acceptor) • Donor removal and acceptor generation • type inversion: n  p • depletion width grows from n+ contact Neff(0) is the effective doping concentration before irradiation  = 0.025cm-1 measured after beneficial anneal

  42. Reverse current and Carrier Trapping • Defects located close to the middle of the bandgap can generate current. • Damage parameter  (slope) •  independent of eq and impurities used for fluence calibration T dependence Material independent • Defects can trap the charge carriers • CCE = Charge Collection Efficiency • CCE is reduced by radiation induced traps • Problems arise if the de-trapping time becomes less than 25ns for the LHC • t is the carrier transient time (e or h), βis a constant.

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