noise studies of n-strip on n-bulk silicon microstrip detectors using fast binary readout...

Nuclear Instruments and Methods in Physics Research A 426 (1999) 28—33

Noise studies of n-strip on n-bulk silicon microstrip detectorsusing fast binary readout electronics after irradiation

to 3]1014 p cm~2

D. Robinson!,*, P.P. Allport#, J. Bizzell", C. Buttar$, A.A. Carter%, J.R. Carter!,M. Goodrick!, A. Greenall#, J.C. Hill!, D. Morgan$, D.J. Munday!, T. Ohsugi',

P.W. Phillips", P. Riedler), N.A. Smith#, S. Terada&, P.R. Turner#, Y. Unno&

!HEP Group, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK"Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK

#Oliver Lodge Laboratory, University of Liverpool, Liverpool L69 3BX, UK$Department of Physics, University of Sheffield, Sheffield S3 7RH, UK

%Queen Mary and Westfield College, University of London, London E1 4NS, UK&High Energy Accelerator Research Organisation (KEK), Oho 1-1, Tsukuba, Ibaraki 305, Japan

'Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 724, Japan)ECP Division, CERN, CH-1211 Geneva 23, Switzerland

Abstract

N-strip on n-bulk silicon microstrip detectors were irradiated at the CERN PS to 3]1014 p cmv2 and theirpost-irradiation performance evaluated using fast binary readout electronics. Strip noise measurements demonstrate thatdetectors using conventional p-stop strip isolation are vulnerable to microdischarge at high bias voltages afterirradiation. However, a novel isolation technique is shown to suppress microdischarge and lead to excellent post-irradiation characteristics. ( 1999 Elsevier Science B.V. All rights reserved.

1. Introduction

Silicon detectors incorporated into the ATLASSemiconductor Tracker [1] (SCT) are expected toundergo type inversion from n- to p-type early inthe experiment’s lifetime as a result of the unprece-dented levels of radiation anticipated at the LargeHadron Collider. N-strip on n-bulk technology of-fers considerable advantage in this environment

*Corresponding author. Tel.: #44-1223-337240; fax: #44-1223-353920: e-mail. [email protected].

because the bulk silicon depletes from the strip sidefollowing inversion, and signal collection is assuredeven if the bulk is only partially depleted. Thisoption is particularly attractive if reverse annealingis not suppressed effectively by cooling throughoutthe experiment’s lifetime, and the voltage requiredto fully deplete the silicon significantly increases.The high charge collection efficiency and sig-nal/noise ratio obtained from partially depletedn-strip on n-bulk detectors after type inversion hasnow been well-established. experimentally [2].However, strip losses due to irradiation also needed

0168-9002/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 1 4 6 7 - 3

Fig. 1. Shematic showing a detector bonded to the detachable binary module.

to be quantified, as the high-field regions at thep-stop isolation implants following inversion mayresult in microdischarge. In addition, high currentsinduced briefly in the detector by beam spills dur-ing irradiation make the strip oxides vulnerable topunchthrough in AC coupled detectors becausea significant fraction of the bias voltage is appliedbriefly across the strip oxides instead of across thedetector bulk.

2. Detectors

The detectors used in this study were fabricatedby Hamamatsu Photonics [3] as prototype de-tectors for the barrel region of the ATLAS SCT.The detectors were 300 lm thick with an overallsize of 64.0]63.6 mm, and an active area of62.0]61.6 mm. They comprised 770 AC coupledn-strips (768 readout strips plus 2 dummies) onn-bulk, with 80 lm pitch and polysilicon biassing.For the n-strip isolation, 7 detectors were fab-ricated with conventional individual p-stops, and

2 were fabricated with a novel isolation technique[4] which used polysilicon field plates in DC con-tact with a common p-frame. Typical pre-irradia-tion depletion voltages and leakage currents were&65 V and (1 lA at 150 V, respectively, for bothdetector designs.

The detectors were glued in each of their cornersto ceramic support frames and then irradiated [5]at the CERN Proton Synchrotron with 24 GeV/cprotons to 3]1014 p cm~2, which is the maximumfluence (with a systematic uncertainty of $50%)expected to be received by the SCT. Throughoutthe irradiation the detectors were cooled to !8°Cin a nitrogen atmosphere and biassed at 150 V toensure similar operating conditions to those an-ticipated for SCT detectors. For most detectors(except the novel p-stop design detectors and one ofthe individual p-stop detectors) the strip metalswere grounded during irradiation. Uniform irradia-tion across the full detector area was ensured byscanning the detectors across the beam with XYstage. After the irradiation (of duration &12 days)the detectors were immediately stored at !10°C

D. Robinson et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 28—33 29

I. SILICON DETECTORS

Fig. 2. Schematic detailing the detector bonding to pitch adaptor D, showing the bussing to allow for the readout of 6 and 12 cm strips.

Fig. 3. Post-irradiation leakage currents for the two detector designs after annealing at 20°C.

to suppress reverse annealing, although transportto the UK entailed an anneal time of &7 days at25°C equivalent temperature. In addition the novelp-stop detector had been annealed for a further 21days at 20°C equivalent temperature as part ofanneal studies [5]

3. Detector measurements

The detectors were coupled to binary readoutchips before and after irradiation in order to quan-tify losses of individual strips arising from radiationdamage. For the purpose of reading out thedetectors, special rebondable modules had beendeveloped which enabled all 768 detector strips tobe read out by 512 binary readout channels. The

modules comprised a hybrid populated four LBIC[6]-CDP [7] chipsets, which were bonded to thedetector via three pitch adaptors as illustratedschematically in Fig. 1. The LBIC is an amplifier-comparator chip with 22 ns shaping time, and theCDP is a pipeline chip. Pitch adaptor ‘D’ incorpor-ated bussing which allowed for two or the chipsetsto be bonded to 6 cm detector strips and two chip-sets to be bonded to effectively 12 cm detectorstrips as illustrated schematically in Fig. 2. Pitchadaptor ‘L’ consisted of long wide metal pads at100 lm pitch to allow for the repeated bonding andunbonding to the module of several detectors se-quentially. Once a detector was bonded to thehybrid, the noise per channel was extracted [8]from a measurement of the noise occupancy perchannel (the fraction of events for which the noise

30 D. Robinson et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 28—33

Fig. 4. Plot of noise (in electrons) vs. channel number for the unbonded hybrid and for an individual p-stop detector pre-irradiation andpost-irradiation at increasing bias.



Table 1Summary of strip quality measurements pre- and post-irradiation.

Detector Pre-irradiation Post-irradiation

No ofoxideshorts

150 V bias

NoisyChannels

No. ofoxideshorts

200 V bias

NoisyChannels

300 V bias

NoisyChannelsTotal (%) Total (%) Total (%)

A-21(*) 3 Not known Not known 8 2 1.3 2 1.3B-05 8 0 1.0 15 24 5.1 40 7.2B-01 6 0 0.8 9 18 3.5 38 6.1B-08 5 2 0.9 11 27 4.9 43 7.0B-03 5 0 0.7 5 37 5.5 48 6.9B-09(*) 4 Not known Not known 7 18 4.9 45 10.1

Detector type A corresponds to the novel p-stop design, and type B corresponds to conventional individual p-stop design. Detectorsmarked with an asterisk indicates irradiation performed with strips floating. These detectors did not have pre-irradiation noisemeasurements.The total columns include oxide shorts and noisy channels

of that channel exceeded the threshold) for a rangeof thresholds using data taken with 104—106 eventsper threshold.

Pre-irradiation current—voltage characteristicsand the strip quality (the percentage of strips withshorts through the AC-coupling oxide induced dur-ing detector fabrication) were provided by themanufacturer and verified on delivery by probesta-tion measurements. Pre-irradiation noise measure-ments with the modules were performed at roomtemperature at 150 V bias voltage (&2.5] fulldepletion). After irradiation all 770 strips on eachdetector were first probed to check for shortsthrough the AC coupling oxides which had beencaused by irradiation. The detectors were thenbonded again to the readout modules, and both IVcharacteristics and noise measurements were per-formed at !12°C to suppress further annealingand to limit detector currents.

4. Results and analysis

Detector depletion voltages and leakage currentshad significantly increased during irradiation asexpected. Post irradiation depletion voltages weretypically &300 V, and had been determined fromthe onset of the plateau in the plot of charge collec-tion efficiency vs. bias with the detector coupled toa FELIX analogue chip [9] and exposed to a Ru106

b-source. Typical current—voltage characteristicsfor the two detectors options are shown in Fig. 3.

Excellent results were obtained from the binaryreadout measurements, which provided clearand unambiguous identification of strip noiseanomalies arising from irradiation induced defectssuch as microdischarge and oxide punchthroughsas well as effects uncorrelated with the irradiationsuch as bonding or pitch adaptor shorts.Fig. 4 shows plot of noise vs. channel number forone of the individual p-stop detectors before andafter irradiation. The plot shows data from onechipset only (corresponding to 6 cm region) forclarity. Plot (a) is for the unbonded hybrid, showingdefects arising form the hybrid itself (though zeroson this plot indicate insufficient statistics). Plot (b)shows data from the hybrid bonded to the unir-radiated detector; there is an overall increase innoise due to the detector strip capacitance, and thedip is noise at channel 109 indicates a missing bond.Plots (c)—(f) show data from the irradiated detectorwith increasing bias from 50 to 300 V. Two catego-ries of radiation-induced defect can be observed: anoxide punchthrough at channel 122 characterisedby the channel noise decreasing to zero with in-creasing voltage (as the current increases the chan-nel is saturated and switches off), and the onset ofmicrodischarge centred on channels 3 and 97.Microdischarge is characterised by an increase innoise on individual channels at low bias, and as the

32 D. Robinson et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 28—33

bias increases the noise increases sharply and moreneighbouring channels are effected.

Table 1 summarises results from the binarymeasurements for all detectors. In general onlya small number of strip oxides (0—1%) had beenpunched through during the irradiation. Detectorswith the individual p-stop design showed largenumbers of excessively noisy strips as the bias wasincreased, which could be attributed entirely to theonset of microdischarge. The detector with thenovel p-stop design showed no evidence of micro-discharge, and maintained strip quality close to thepre-irradiation level. The effects of irradiation onthe strip quality is clearly independent on whetherthe strip metals are grounded or floating.

5. Conclusions

N-strip on n-bulk detectors with conventionalindividual p-stop n-strip isolation as well as a novelisolation technique [4] were irradiated to3]1014 p cm~2 and the strip losses due to irradia-tion were quantified. Detectors of both designsshowed a small ((1%) number of strips withoxides shorted following irradiation (though therobustness of the strip oxides is expected to bestrongly dependent on the manufacturer andmethods of processing). The detectors withconventional individual p-stops had excellent pre-irradiation characteristics but showed evidence ofmicrodischarge at high voltages following irradia-tion. The novel p-stop design effectively suppressesmicrodischarge, and detectors using this designhave demonstrated excellent pre- and post-irradia-tion characteristics, with a strip quality maintainedclose its pre-irradiation level at &99%. N-strip onn-bulk detectors have already been demonstratedelsewhere [2] to offer significant advantages overconventional designs following inversion due to

their high efficiencies at partial depletion. The n-strip on n-bulk detectors using the new isolationtechnique would therefore provide excellent de-tectors for highly radiative environments such asthe LHC where the possible need to operate at highvoltages due to reverse annealing may be of con-cern.

Acknowledgements

We would like to acknowledge the invaluabletechnical support for this project provided byB. Fromant at the Cavendish Laboratory andC. Grigson and R. Nicholson at the University ofSheffield. We also thank the PS staff at CERN forexcellent run conditions during the detector ir-radiation. We would like to thank E. Koffeman ofMPI, amongst others, for their contribution to theATLAS irradiation program.

References

[1] Atlas Technical proposal, CERN/LHCC/94-43.[2] P.P. Allport et al., Nucl. Instr. and Meth. A 418 (1998) 110.[3] Hamamatsu Photonics, 1126-1 Ichino-cho, Hamamatsu

435, Japan[4] Y. Unno et al., Novel p-stop structure for n-strip readout

detector, presented at the 3rd Int. Symp. Development andApplication of Semiconductor Tracking Detector, Mel-bourne, 9-12 Dec, 1997, Nucl. Instr. and Meth. to be sub-mitted.

[5] D. Morgan et al., Annealing study of irradiated prototypesilicon microstrip detectors, ATLAS INDET-NO-199.

[6] E. Spencer et al., IEEE Trans. Nucl. Sci. NS-42 (1995) 796.[7] J. DeWitt, A pipeline and bus interface chip for silicon strip

detector read-out, Proc. IEEE Nucl. Sci. Symp., San Fran-cisco, CA., Nov 1993.

[8] T.M. Pulliam, Noise studies on silicon microstrip detectors,Ph.D. Thesis, 1995, University of California, Santa Cruz.

[9] S. Gadomski, P. Weilhammer, Nucl. Instr. and Meth. A. 51(1994) 201.



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