Nuclear Instruments and Methods in Physics Research A 485 (2002) 84–88

Silicon microstrip detectors for the ATLAS SCT

D. Robinsona,*, P. Allportb, L. Andricekc, J. Bohmd, C. Buttare, J.R. Cartera,A. Chilingarovf, A.G. Clarkg, D. Ferr"ereg, J. Fusterh, C. Garciah, C. Grigsone,

L. Johanseni, G. Lutzc, M.C. Moroneg, R. Richterc, B. Stugui, N. Unnoj

aCavendish Laboratory, HEP Group, Madingley Road, CB3 0HE Cambridge, UKbOliver Lodge Laboratory, University of Liverpool, Liverpool, L69 7ZE, UK

c Max-Planck Institut f .ur Physik, Foehringer Ring 6, 80805 M .unchen, Germanyd Prague Academy of Science, Na Slovance 2, 18040 Praha 8, Czech Republic

e Department of Physics, University of Sheffield, Sheffield, S3 7RH, UKf Department of Physics, Lancaster University, Lancaster LA1 4YB, UK

gNuclear Physics Department, University of Geneva, 24 Rue Ernest Ansermet, CH-1211 Gen"eve 4, Switzerlandh IFIC, Centre Mixto Universidad de Valencia, CSIC, E-46100 Burjassot (Valencia), Spain

iDepartment of Physics, University of Bergen, Allegaten 55, N-5007 Bergen, NorwayjHigh Energy Accelerator Research Organisation (KEK), Oho 1-1, Tsukuba, Ibaraki 305, Japan

Abstract

The ATLAS Semiconductor Tracker at the Large Hadron Collider (LHC) will incorporate B20,000 individual

silicon microstrip sensors representing B60 m2 of silicon. Production and delivery of the sensors is already underway

and scheduled for completion by late 2002. The sensors have been optimised for operation in the harsh radiation

environment of the LHC, and subjected to an extensive qualification program in which their pre- and post-irradiation

characteristics have been evaluated. The sensor design features are reviewed, together with their electrical characteristics

and the Quality Control procedures adopted by ATLAS during production. r 2002 Elsevier Science B.V. All rights

reserved.

PACS: 29.40.Gx; 29.40.Wk

Keywords: ATLAS SCT; Silicon microstrip detectors; Irradiation; Quality control

1. Introduction

The harsh radiation environment of the LargeHadron Collider (LHC) and the complexity of theSemiconductor Tracker (SCT) has demanded

extremely resilient and robust designs for thesilicon sensors. The innermost regions of theSCT will be exposed to an equivalent fluence inexcess of 2 � 1014 p cm�2 during the lifetime of theexperiment. Furthermore, access to the sensorswill be severely restricted due to the necessity ofmaintaining them at low temperature to suppressreverse annealing. In these conditions the siliconsensors are required to maintain sufficient perfor-mance to ensure efficient tracking throughout the

*Corresponding author. Tel.: +44-1223-337240; fax: +44-

1223-353920.

E-mail address: robinson@hep.phy.cam.ac.uk

(D. Robinson).

0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 5 3 6 - 3

lifetime of the LHC. Sensors accepted for use inthe construction of the SCT have been subjected toan extensive qualification program. This processwas used to demonstrate that their electricalcharacteristics and tracking performance exceedminimum SCT requirements both before and afterirradiation, and that there is consistent behaviourfrom a number of sensors of the same design andprocessing from a given manufacturer. Followingqualification of their sensors and competitivetendering, three companies were awarded con-tracts to supply all the sensors, with series deliverystarting from the beginning of 2001 and due forcompletion by late 2002. The quality assurancestrategy adopted by the SCT is that deliveredsensors must have been fabricated with identicaldesign and processing as the qualified sensors.Quality Control (QC) tests are in place at SCTinstitutes not only to check the basic quality of thesensors, but also to monitor any potential changein processing during production.

2. The silicon detectors

The sensors are supplied by Hamamatsu [1], CiS[2] and Sintef [3], which are providing 79%, 17%and 4%1 of the total order, respectively. Thesensors have two basic shapes. The barrel region ofthe SCT uses B10,900 rectangular-shaped sensorswith parallel strips, and the end caps compriseB8700 wedge-shaped sensors with radial strips.Due to the varying coverage requirements in thedifferent end-cap wheels, there are five minor

variations in the size of the wedge sensors, referredto as W12, W21, W22, W31 and W32. Thegeometries of the sensors are summarised inTable 1. The distance from the cut edge to theactive area is B1 mm on all four sides of the sensors.

All the sensors are of p-strip on n-bulk design,with 768 AC-coupled readout strips. The stripimplants extend to within 5–10 mm of the bias railin order to limit the strip implant voltage in thecase of beam splash. Strip metal and implantwidths are within the range 16–22 and 16–20 mm;respectively, depending on the manufacturer. TheHV contact is to a metallised, unpassivated, n-implant on the rear of the sensor, with the groundcontact to the bias rail implant surrounding thestrips. Barrel and wedge sensors from the samemanufacturer share identical features, but sensorsfrom different manufacturers are fabricated usingtheir own commercially confident design rules andprocessing techniques. Detailed design issuesand the choice of substrate are therefore atthe discretion of the manufacturer, so long as thesensors meet the common requirements of thequalification program. The main design andprocessing differences between the three manufac-turers are summarised in Table 2.

3. QC during sensor production

Following the process of qualification, it is theresponsibility of the manufacturer to ensure thatno changes in their processing may occur whichmay modify any parameters relevant to SCTspecifications, or any pre- and post-irradiationelectrical behaviour. As consistency of processingis ensured, the r #ole of the SCT is mainly that of a

Table 1

Sensor geometries

Barrel W12 W21 W22 W31 W32

Length (mm) 64.000 61.060 65.085 54.435 65.540 57.515

Outer width (mm) 63.360 55.488 66.130 74.847 64.636 71.810

Inner width (mm) 63.360 45.735 55.734 66.152 56.475 64.653

Strip pitch (mm) 80 57–69 70–83 83–94 71–81 81–90

Interstrip angle (mrad) 0 207 207 207 161.5 161.5

All sensors are 285 mm thick.

1The Sintef contribution is subject to further qualification

tests at the time of submission of this paper.

D. Robinson et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 84–88 85

visual inspection and leakage current measurementas a basic check on quality. However, on a subset(B10%) of the delivered sensors, an extensiveevaluation of sensor characteristics is performed asa verification of processing consistency and themanufacturers own QC checks. Furthermore, asmall sample of delivered sensors (B1% or less)are regularly irradiated in order to verify that post-irradiation behaviour matches that observed dur-ing qualification.

3.1. QC tests on every sensor

The acceptance criteria for sensor electricalproperties are summarised in Table 3. Themanufacturer performs all checks necessary toensure consistency of processing and that char-acteristics match these acceptance criteria. Stripquality is checked by the manufacturer by probingall 768 strips on every sensor, a bad strip beingdefined as having an electrical short with 100 Vapplied across the strip dielectric, or a strip metalbreak or short to a neighbour. Upon receipt at theSCT institute, every sensor is visually inspectedand the leakage current measured up to 500 Vbias. The sensor is rejected if there are significantvisual defects (severe scratching, or edge chippingexceeds 50 mm) or if the current exceeds the specsin Table 3 or significantly disagrees with themanufacturer data. Typical characteristics forHamamatsu sensors are shown in Figs. 1(a)and (b), showing IV characteristics up to 500 Vand the current deviation over 24 h at 150 V;respectively.

3.2. QC tests on a subset

Around 10% of sensors are selected for moreextensive testing at the SCT institute. To minimisethe risk of damage from repeated handling andprobing, these sensors are secured in a supportframe with the two bias connections bonded out tosoldered leads. The most effective evaluation of asensor’s functionality is made by the Full StripTest, in which the impedance at low frequency(p100 Hz) between the strip metal and the biasrail is determined by probing every strip metal

Table 2

Substrate and design differences between manufacturers

Hamamatsu CiS Sintef

Sensor shapes All Wedges only Barrels only

Orientation /111S /111S /100SOxygenation None W12 only None

Biasing resistor Polysilicon Implant Polysilicon

Edge design Single guard 14-multiguard 11-multiguard

Strip dielectric Composite structure, depending on

manufacturer

Table 3

Acceptance criteria for sensor electrical properties

Leakage current at 201C o6 mA at 150 V; o20 mA at

350 V

Current stability at 201C DIo2 mA during 24 h at 150 V

Depletion voltage o150 V

Bias resistance 1:2570:75 MOCoupling capacitance X20 pF cm�1 at 1 kHz

Interstrip capacitance o1:1 pF cm�1 at 100 kHz

Strip metal resistance o15 O cm�1

No. of good strips > 98% per detector, mean of >99% in a batch

0255075

100125150175200225250

0 100 200 300 400 500

(a) (b)

Bias(Volts)

Cur

rent

(nA

)

0

5

10

15

20

25

-1 -0.5 0 0.5 1Current Deviation (MicroAmps)

No

of D

etec

tors

Fig. 1. (a) IV characteristics to 500 V; and (b) current deviation at 150 V over 24 h:

D. Robinson et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 84–8886

while the sensor is partially biased. The impedanceis modelled on a CR in series, with the measuredvalues of C and R corresponding to the couplingcapacitance and bias resistance, respectively.Therefore the coupling capacitance and biasresistance are determined for every strip, and thistest is sensitive to any defects to the strip metal,strip implant or bias resistor, as well as to anydeterioration in interstrip resistance. Fig. 2 showsthe results from a typical full strip test, showingthe deviation in C and/or R in the presence of animplant break (strip 738) and strip metal short(strips 41–42). Before the sensor is removed fromthe frame, further measurements are performed tocomplete the subset testing: depletion voltage,interstrip capacitance, metal strip resistance, and24-h leakage current stability.

3.3. Radiations

A small representative sample (B1%) of sensorsare regularly irradiated [4] to 3 � 1014 p cm�2

(B1:5 times anticipated dose at the LHC) duringproduction. After irradiation all sensors areuniformly annealed for 7 days at 251C: Thepurpose of the irradiations is to monitor anychange in post-irradiation behaviour that may beattributed to changes in processing during fabrica-tion. Such changes may give rise to microdischargeor to significant shifts in the bias required toachieve sufficient charge collection. Fig. 3 shows(a) typical post-irradiation leakage currents, and(b) a typical strip noise distribution at 500 V biasillustrating that the sensor is free from micro-discharge (small deviations in this plot are due tobonding defects and a hybrid defect).

Charge collection vs. bias and a search formicrodischarge are performed by bonding theirradiated sensor to analogue or binary electronicsrunning at the 40 MHz LHC readout speed.Figs. 4(a) and (b) show typical signal and noisedistributions respectively, as a function of sensorbias, using analogue readout. The data correspondto effectively 12 cm long strips.

0

50

0 100 200 300 400 500 600 700

(a)

0

300

0 100 200 300 400 500 600 700

(b)

0

2

0 100 200 300 400 500 600 700

(c)

Fig. 2. Typical full strip test results, showing: (a) current through strip dielectric at 100 V; (b) capacitance and (c) resistance.

0

50

100

150

200

0 100 200 300 400 500

(a) (b)

Bias(Volts)

Cur

rent

(mic

roA

mp)

0

1

2

3

4

150 200 250Strip No

Noi

se (

arbi

trar

y un

its)

Fig. 3. Typical post-irradiation: (a) IV characteristics at �181C; and (b) strip noise distribution at 500 V:

D. Robinson et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 84–88 87

4. Summary

The quality and robustness to radiation of theSCT microstrip sensors have been demonstrated ina qualification program. An extensive qualitycontrol program is in place at SCT institutes toverify that the characteristics and processingconsistency of the qualified sensors are maintainedthroughout production.

References

[1] Hamamatsu, 325-6, Sunayama-cho, Hamamatsu City,

Sizuoka Pref, 430-0193, Japan.

[2] CiS Institut f .ur Mikrosensorik fGmbH, Haarbergstrasse 61,

99097 Erfurt, Germany.

[3] Sintef Electronics and Cybernetics, OS Bragstad plass, N-

7465 Trondheim, Norway.

[4] C.M. Buttar, et al., Nucl. Instr. and Meth. A 447 (2000)

126.

0

50

100

150

0 200 400 600

(a)

Sensor Bias(Volts)

Sig

nal(A

DC

Cha

ns)

0

5

10

15

0 200 400 600

(b)

Sensor Bias(Volts)

Noi

se(A

DC

Cha

ns)

Fig. 4. Typical post-irradiation: (a) noise vs. bias, and (b) signal vs. bias.

D. Robinson et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 84–8888