STD6 at Carmel, Sep 11-15, 2006 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 sen

sor 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]

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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)

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N-in-p R&D Sensor Fabrication• ATLAS05

One main sensor6.4 x 6.4 cm2

6 miniature sensors1 x 1 cm2

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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

IPSTP CPSTP

IPSTPDF CPSTPDF AF

NS

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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

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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)

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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

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Post-Irradiation I-V

• 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

Low High

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C-V Measurements• 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

Non-irrad

Low High

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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

Low High

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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

Wafer Non-ir Low High

p-FZ ~170V ~200V ~600V

p-MCZ >1000V ~480V ~810V

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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

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Strip Isolation• p-FZ

– 5V between two strips– All p-stop work from near null bias vo

ltage• IPSTP, CPSTP get worse at high flu

ence, yet isolated >400V• +DF are better

– How about AF?

Non-irrad

Low High

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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

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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

LowHigh

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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

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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?

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Conclusions• We have fabricated n-in-p microstrip sensors in p-FZ and p-MCZ indust

rial 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 t

o ~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