High spatial resolution silicon detectors

Sensors and Actuators,

4 (1983)

HIGH SPATIAL RESOLUTION J KEMMER

423

423 - 426

SILICON DETECTORS*

I

Physrk Department,

Technuwhe

R HOFMANNandG

Unwersztat

Munchen,

D-8046

Garchlng

(F R G )

LUTZ

Max-Planck-Instjtut fur Physzk D-8000 Munich 40 (F R G )

und Astrophyslk,

Werner-Helsenberg-Instztut

fur Physrk,

Abstract Posltlon-sensltlve silicon detectors conslstmg of 1200 diode stnps of 20 pm pitch have been developed and successfully used m an experiment with high-energy elementary particles A spatial resolution of 5 pm was achieved The design and fabncatlon will be described as far as plans for developing the detector electronics system

1. Introduction If a relatlvlstlc elementary particle interacts with the nucleus m a fixed target, the emergent particles move m a very narrow cone around the dlrectron of the mcommg particle due to the conservation of momentum To measure the tracks of the emergent particles m the regron close to the mteractlon pomt, a detector with high spatial resolution 1s needed The ACCMOR collaboration at the Super Proton Synchroton (SPS) at CERN m Geneva required such a detector for measurmg the hfetlme of short-lived (lo-l3 to IO-‘* s) particles [I] 2. Requirements for the detector The precision of the measured coordmate perpendicular to the beam dlrectlon should be better than 10 pm It should be possible to dlstmgulsh two particles down to a separation of about 100 pm The detector should also be capable of handling particle rates of 2 X 106/s. *Based on a Paper presented at Solid-State May 31 -June 3,1983 0250-6874/83/$3

00

Transducers 83, Delft, The Netherlands,

0 Elsewer Sequola/Prmted

m The Netherlands

424

3 The rmcrostrlp

detector

These requirements could not be met by the well-known multlwlre dnft chambers Therefore the ACCMOR collaboration, mamly the Mumch (MPI) group, introduced and developed a posltlon-sensltlve slllcon detector m cooperation with the Techmsche Umversltat Munchen The so-called Mlcrostrlp Detector (MSD) consists of 1200 diodes with 20 pm pitch and a length of 36 mm on one wafer of 2 mch diameter Figure 1 shows a cross-section of such a detector An elementary particle with high momentum traversing the fully depleted detector creates about 24 000 electron-hole pairs, which are collected at the electrodes The signal picked up at the readout stnp yields the mformatlon on the particle posltlons to a precision determmed by the pitch of the strips Due to space hmltatlons, only every third strip m the central and every sixth m the outer regon IS connected to the electronic readout The charge collected at a non-connected stnp IS reglstrated via capacitive charge dlvlslon [2] by the adjacent readout stnps

p+-Implantation

_ n+-lmphntatjon

t Boron 1

(Arsenrc 1

1 pm Alumwum

Fig

1

Cross

sectlon

of the mlcrostrlp

detector

4 Fabncatron The fabncatlon technology was developed by J Kemmer [3] The fabncatlon steps are as follows (1) n-type (111) &con, 3 ki2 cm, 2 mch diameter, 280 I.crnthick, (2) S10, passlvatlon by thermal oxldatlon at 1030 “C, (3) Implantation for p-doped strips urlth 5 X 1Ol4 boron 1ons/cm2, (4) lmplantatlon for ohmic rear contact with 5 X 1Ol5 arsenic ions/cm* , (5) thermal anneahng at 600 “C for 30 mm, (6) evaporation of alummmm on both sides, (7) etching of alummmm stnp pattern After a first test, a further step IS carned out

425

Fig 2 Photograph of a mounted mlcrostrlp detector

(8) sputtermg of a high ohmic &con stnp perpendicular to the diode stnps to avoid voltage drop between readout and mtermedlate stnps After a second test the two final steps are performed (9) glumg on a pnnted fanout board, (10) connecting the stnps mth the board by ultrasomc bonding In Fig 2 the detector card 1s shown, including the connectors to the preamplifiers 5. Detector charactenstlcs The important detector charactenstlcs are summarized below [l] Depletion voltage 120 v Mean number of produced electron-hole pairs 24 000 e ^=4 X 101’ Cb created by a mmlmum lonlzmg particle -1Ons Charge collection time - 1 nA/stnp, - 1 PA/detector Dark current 1O-4 Inefficiency 50% FWHM Signal vanatlon (Landau dlstnbutlon) Position resolution 43pm m 60 pm readout region 8.6 pm m 120 1-m readout reeon

426

An lrradlatlon with 5 X 1013 beam particles/cm* caused no deterioration m detector performance, except for an increase m dark current m one of the eight detectors

6 Future proJects The present ratlo of electronics to detector area 1s about 1000 to 1 Therefore, work IS m preparation to mtegrate the first electronic stage and the detector on to the same wafer mth readout for every strip After a fu-st amphficatlon the signals ~11 be stored m capacitors and readout will be serial This prodect will be done m collaboration wrth the Umversltat Dortmund, Lehrstuhl fur Bauelemente der Elektrotechmk

7 Summary The mlcrostnp detectors, which were assembled m a telescope with 1380 channels, have a preclslon of 5 pm and worked satlsfactonly m an Integration of some expenment urlth high-energy elementary particles electromcs and diode stnps on the same wafer will reduce area and costs Acknowledgment The detector was developed for the ACCMOR collaboration Many members of the collaboration and the technical staff of MPI Munich and CERN have contributed very slgmflcantly to this work References 1 B Hyams et al , A s&con counter telescope to study short-lived parlxles m highenergy hadromc mteractlons, Nucl Instr Mefh , 205 (1983) 99 2 J B A England et al, Capacltlve charge divlslon readout with a sllrcon strip detector, Nucl Instr Meth, 185 (1981) 43 3 J Kemmer, Fabrlcatlon of low noise sdlcon radiation detectors by the planar process, 169 (1980) 499 Nucl Znstr Meth,