P-bulk Silicon Microstrip Sensors and Irradiation

1. P-bulk Silicon Microstrip Sensors and Irradiation Y. Unno, S. Terada, T. Kohriki, Y. Ikegami (KEK) K. Hara, K. Inoue, A. Mochizuki (Univ. of Tsukuba) K. Yamamura, K. Sato (HPK) STD6 at Carmel, Sep 11-15, 2006

2. Introduction • Getting signals from the silicon microstrip sensor at SLHC • Expected fluence • At r=30cm • LHC: ~2x1014 1MeV-neq/cm2 (700fb-1) • ~8.6x1014 1MeV-neq/cm2 (3,000fb-1) or • ~1.7x10151MeV-neq/cm2(6,000fb-1) • Radiation damage by charged hadrons and neutrons • Dominance of radiation-induced acceptor states • A silicon bulk mutates to p-bulk • High full-depletion voltage, thus partially depleted operation • Start from the p-bulk and read out n-strips, n-in-p sensor • Continuation of an early study[STD2@Hiroshima, S.Terada et al., NIMA 383(1996) 159-165] STD6 at Carmel, Sep 11-15, 2006

3. N-in-p R&D Sensor Fabrication • Issues (for us) are • Industrial wafers • “High” resistivity • p-FZ (5~10 kΩcm, <111>), p-MCZ (600~1kΩcm, <100>) • CZ (n or p) are of order of 10 Ωcm and • n-MCZ 100Ωcm • High bias voltage operation • Holding the voltage up to the design target value of 800V • N-strip isolation • Much discussion of whether p-stop or/and p-spray • No microdischarge up to the design voltage • Investigation on performance • Pre- and post-Irradiation • Microdischarge on-set voltage • Full depletion voltage • N-strip isolation • Charge collection efficiency (CCE) STD6 at Carmel, Sep 11-15, 2006

4. N-in-p R&D Sensor Fabrication • ATLAS05 One main sensor 6.4 x 6.4 cm2 6 miniature sensors 1 x 1 cm2 STD6 at Carmel, Sep 11-15, 2006

5. N-strip isolation implementation • 6 zones • 1cm x 1cm miniature sensors are one zone-one chip • To test p-stop, p-stop+p-spray, p-spray, none • NS and AF have no structure in silicon surface NS IPSTP CPSTP AF IPSTPDF CPSTPDF STD6 at Carmel, Sep 11-15, 2006

6. I-V and Microdischarge • Pre-irradiation • Bias ring and associated structure could hold 1000V!! • Microdischarges observed • p-FZ >700V • p-MCZ >350V • +DC-field plate is worse in p-MCZ • Hot-electron analysis • Revealed weak spots p-FZ p-MCZ STD6 at Carmel, Sep 11-15, 2006

7. Mask Rework • ATLAS05M • Increased gaps • DC pads staggered • Other gaps also adjusted • Onset voltages • IPSTP, CPSTP improved • Gap widening works, but • +DC-field plate no difference • Hot electron analysis • Next weak spots… • Hot spot at STRIP side! • (no confirmation yet in p-FZ) ATLAS05 (MCZ) ATLAS05M (MCZ) STD6 at Carmel, Sep 11-15, 2006

8. Irradiation Facility • Cyclotron and Radioisotope Center (CYRIC), Tohoku University • AVF Cyclotron • Radius=930 mm (max. k.e.=130MeV) • Radio frequency: 11-22 MHz • Irradiation setup • Beamline 31-2 • Protons: 70 MeV • Current (max.): 500 nA • Beam spot: ~5 mm FWHM • Scanning stage • XY: 50 cm x 20 cm • Scanned area: 20 mm x 20 mm • Fluences • Dosimetory: Al foils • Low: 10nA, 0.7x1014 1MeV-neq/cm2 • High: 100nA, 0.7x1015 1MeV-neq/cm2 STD6 at Carmel, Sep 11-15, 2006

9. Post-Irradiation I-V High Low • High fluence • No MD, hold 1000V, in both FZ and MCZ • Low fluence • MD visible, currents scattered in low voltages (effect of mild(?) MD?) • Note the green line in MCZ… p-FZ p-MCZ STD6 at Carmel, Sep 11-15, 2006

10. C-V Measurements Non-irrad • p-FZ bulk • 1/C2 vs. V plots • Full depletion voltage (FDV) from the cross points • Non-irrad: 180V • Low flu.: 260V • High flu.: >500V High Low STD6 at Carmel, Sep 11-15, 2006

11. C-V Measurements • p-MCZ bulk • Full depletion voltage (FDV) from the kink points • Non-irrad: >1000V • Low flu.: 530V • High flu.: >300V(?) • cf. CCE Non-irrad High Low STD6 at Carmel, Sep 11-15, 2006

12. Charge Collection Efficiency (CCE) • Measurements by 1064nm laser • Two repeated meas. for reproducibility • ~10% max.(?) • FDV • CCE • p-FZ • Non-ir>Low>High at 400V e.g. • Low & High equal >800V • p-MCZ • Low>High>Non-ir at 400V e.g. • All equal >800V STD6 at Carmel, Sep 11-15, 2006

13. FDV and CCE Summary • RD50 FZ and MCZ from Fig. 1 (NIMA 546(05)99 • More samples and irradiations are required to establish the FDV values STD6 at Carmel, Sep 11-15, 2006

14. Strip Isolation • p-FZ • 5V between two strips • All p-stop work from near null bias voltage • IPSTP, CPSTP get worse at high fluence, yet isolated >400V • +DF are better • How about AF? Non-irrad High Low STD6 at Carmel, Sep 11-15, 2006

15. Strip Isolation • p-FZ-AF • Null gate voltage • Non-irrad. • No isolation near null bias voltage - no surprise • Isolated at Vbias>700V • Post-irradiation • Isolation gets better, at Vbias>400V • With gate voltages on • Vbias at 200V • Non-irrad. • Isolated at Vg>50V • Post irradiation • Isolated at Vg>10V STD6 at Carmel, Sep 11-15, 2006

16. Strip Isolation • p-MCZ • All structures work from near null bias voltages • Including AF(Null voltage) • Isolation at Vbias>100V at high fluence • Increase at Vbias>400V due to MD in non-irrad. Non-irrad Low High STD6 at Carmel, Sep 11-15, 2006

17. Strip Isolation - No Strucutre (Zone1) • Confirmation of isolation of “No Structure” zones in pre-irradiation • p-MCZ isolates with Vbias>50V • p-FZ does not up to 1000V • note: p-FZ does isolate with p-stop structures Cf. p-stops do isolate in p-FZ STD6 at Carmel, Sep 11-15, 2006

18. Discussion • We have found that p-MCZ does not require the structure to isolate the n-strips • Little electron accumulation layer • Hot spots are in the strip side • If there is electron accumulation layer in silicon under the Si-SiO2 interface, it shorts and the high field is at the p-stop edge. This shorting is why we need an isolation structure. • Very little voltage is required to isolate the n-strips for the voltage to the AC field plate • The hypotheses • 1stly because of <100>, I.e., less dangling bonds • Lower density than <111> • 2ndly because of low resistivity (~700 Ωcm) • Partially compensating the built-in positive fixed charges • Post irradiation • Still valid • Possible candidate sensor for SLHC • n-in-p in the industrial p-MCZ material (~700 Ωcm) • Issues • Initial low CCE at lower bias voltages • Compensated with higher S/N of electronics in initial phase? • Variation of density of electron accumulation layer • An isolation structure, e.g., low doping common p-stop (HPK preference?), for assureance? • Full depletion voltage and CCE up to 6000fb-1? STD6 at Carmel, Sep 11-15, 2006

19. Conclusions • We have fabricated n-in-p microstrip sensors in p-FZ and p-MCZ industrial wafers, with various n-strip isolation structures. • Compared the performance of null and post-irradiation • 0.7x1014, and 0.7x1015 1MeV-neq/cm2 fluences • Edge/bias structure holding 1000V has been established. • Full depletion voltages are evaluated with C-V and laser CCE methods. • p-FZ started at ~150V and went up ~600V at the high fluence. • p-MCZ starts at ~1000V, decreased to ~500V at low fluence and increased to ~800V at high fluence • CCE’s of p-FZ and p-MCZ were all nearly equal at the bias voltage of >800V in null and post-irradiations. • Microdischarge occurred at n-strip side in p-MCZ • in contrast to p-stop side of n-in-n sensors • No MD was observed in the AF structure (floating) up to 1000V • Specially in p-MCZ, I.e., no isolation structure in silicon • All isolation structures isolated n-strips with high bias voltages. • AF in p-FZ isolated the n-strips with gate voltage >50V • AF (floating) in p-MCZ isolated the n-strips STD6 at Carmel, Sep 11-15, 2006