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Nuclear Instruments and Methods in Physics Research A 552 (2005) 112–117

www.elsevier.com/locate/nima

Semi-3D silicon detector and initial results of its performance

Amitava Roya,�, Gino Bollaa, Daniela Bortolettoa, Zheng Lib

aPurdue University, West Lafayette, IN, USAbBrookhaven National Laboratory, Upton, NY, USA

Available online 23 June 2005

Abstract

A novel p+–n+/n/n+ semi-3D structure configuration has been developed and it is expected to improve the radiation

hardness of silicon sensors after space charge sign inversion (SCSI). A special configuration of semi-3D sensors

facilitates depletion from both sides of the sensors after SCSI and reduces the depletion voltage by half or more. The

reduction of depletion voltage will increase the ability of silicon detectors to operate in the presence of severe bulk

radiation damage expected at high-intensity colliders. Semi-3D sensors can be manufactured using only single-sided,

conventional planar processing. Electrical characterization of semi-3D test structures through I– V and C– V curves

before and after irradiation is presented here.

r 2005 Elsevier B.V. All rights reserved.

PACS: 29.40.Gx; 29.40.Wk; 61.80.�x; 71.55.Cn

Keywords: Semiconductor sensors; Semi-3D sensors; Radiation hardness

1. Introduction

Advances in sensor design and improved bulkmaterials during the last decade have pushed theradiation hardness of current silicon detectortechnology to impressive performance. It willallow operation of the tracking systems of theLarge Hadron Collider (LHC) experiments atnominal luminosity (1034 cm�2 s�1) for about

e front matter r 2005 Elsevier B.V. All rights reserve

ma.2005.06.016

ng author. Department of Physics, 525 North-

, West Lafayette, IN 47907-2036, USA. Tel.:

.

ss: amitroy@physics.purdue.edu (A. Roy).

10 years [1]. However, the predicted 1MeV neu-tron equivalent fluences of fast hadrons, rangingfrom 3� 1015 cm�2 at a radius of R ¼ 4 cm to3� 1013 cm�2 at R ¼ 75 cm for an integratedluminosity of 500 fb�1, will lead to substantialradiation damage of the sensors and degradationof their performance. A replacement of the inner-most silicon pixel layers may become necessarybefore an integrated luminosity of 500 fb�1 hasbeen reached. Recently, the option to extend thephysics reach of the LHC by increasing theluminosity to 1035 cm�2 s�1 after the year 2012 [2]has been discussed. In that case the silicon trackerswould face fast hadron fluences above 1016 cm�2

d.

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A. Roy et al. / Nuclear Instruments and Methods in Physics Research A 552 (2005) 112–117 113

(2500 fb�1) at R � 4 cm and current silicon detec-tors would be unable to cope with such anenvironment.

Research has been initiated to design moreradiation-tolerant silicon sensors to meet thechallenges of LHC upgrades or future high-luminosity hadron colliders. The focus of theseefforts can be divided into three fundamentalapproaches:

�

Material engineering includes the enrichment ofsilicon base material with oxygen, or otherimpurities or the development on new materialssuch as Czochralski silicon [3], SiC and amor-phous silicon.

�

Fig. 1. Schematic and biasing scheme of a p+–n+/n/n+

detector.

Device engineering aims at improving thepresent planar detector structures by e.g.modification of the electrode configuration suchas semi-3D sensors [4] or using thin silicon asbulk material and the development of newdetector geometries such as ‘‘3D detectors’’ [5].

�

Variation of detector operational conditions

optimizes the detector’s operational conditionssuch as the possible operation of silicondetectors at low temperatures or under forwardbias.

In this article, performance of semi-3D sensorswill be discussed. Leakage current and depletionvoltage of semi-3D sensors were measured at aprobe station before and after irradiation. Resultsof those measurements will be presented in thisarticle.

Fig. 2. Schematic of test structure.

2. Semi-3D sensors and test structures

The configuration of semi-3D sensors understudy is p+–n+/n/n+ as shown in Fig. 1. Thissensor configuration only requires single-sided,planar processing. Before Space Charge SignInversion (SCSI), the depletion zone developsfrom the p+–n+ side and goes laterally under-neath the n+ electrodes and towards the n+

electrode onto the other side. After SCSI thedepletion starts from both sides. Simulations ofthis configuration presented in Ref. [4] show that abias of 130V will deplete the detector, which is

about 2.5 times smaller than that of a standardconfiguration (320V) [4].The test structures used in our study have two

interwoven, p+ and n+, spirals on n-type bulkmaterial as shown in Fig. 2. The spirals are

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A. Roy et al. / Nuclear Instruments and Methods in Physics Research A 552 (2005) 112–117114

surrounded by three guard rings to intercept theleakage current from the edge before it reaches theelectrodes and facilitate the voltage drop betweenthe edge and the spirals in a controlled manner.The p+ spiral and n+ spiral in the test structureare separated by a p+ implant. There is one n+

contact on the backside. The sensors are 260 mmthick, have a resistivity of 4 kO cm and thesensitive area covered by the spirals is of diameter0.79mm.

3. Experimental setup and irradiation

The measurements on the test structures aredone at room temperature before irradiation andat �10 1C after irradiation. The measurement ofcurrent as a function of the reverse bias voltage(I– V) is used to determine the quality of theprocessing since it detects most fabrication defects.Before irradiation, test structures are biasedthrough the n+ side using a Keithley 237 SMUas shown in Fig. 3. We measure the leakagecurrent through a Keithley 617 electrometerconfigured as an ammeter. The measurement ofcapacitance as a function of voltage (C– V) is usedto determine the depletion voltage. The capaci-tance is measured using QUAD 7600. The highsurface current of the test structures makes itnecessary to use an external decoupler to decouplethe DC current of the test structures from an ACsignal of QUAD 7600.

After initial measurements test structures wereirradiated with a 200MeV proton beam at the

Fig. 3. Schematic of (a) biasing scheme for I– V and (b) bia

Indiana University Cyclotron Facility (IUCF) andwith a 24GeV proton beam at CERN. Teststructures received a total fluence of 5� 1014 neq/cm2. During the irradiation, the test structureswere kept at room temperature. After irradiationthe sensors were kept at a temperature of �10 1C.After irradiation, the n+ spiral was biased

separately during the measurement of I– V andC– V (Fig. 4) to test the effect of bias on n+ spiral.The Keithley 617 was used as a source to bias then+ spiral.

4. Results and discussion

The inverse of capacitance square is plotted as afunction of bias voltage in Fig. 5. The value ofdepletion voltage is extracted by fitting twostraight lines. Before irradiation a depletionvoltage of 59V is obtained, which is consistentwith 4 kO cm resistivity, as shown in Fig. 5.The measurement of the total leakage current

versus bias voltage is shown in Fig. 6. The devicedoes not show any breakdown up to 800V. Afterirradiation the total leakage current at �10 1C is1.4 mA (Fig. 7(a)). Fig. 7(b–d) shows the currentthrough n+ spiral, p+ spiral and backside asdifferent positive voltages are applied to the n+

spiral. As the voltage is applied at the n+ spiral,current flows from the n+ spiral to the backsideand the p+ spiral until a sufficient high voltage atbackside is reached. Once there is sufficient highvoltage at the backside and voltage at n+ spiral isless than 66V, electric field lines from backside end

sing scheme of C– V measurement before irradiation.

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Fig. 5. Measurement of depletion voltage before irradiation

Fig. 4. Schematic of (a) biasing scheme for I– V and (b) biasing scheme of C– V measurement after irradiation.

Fig. 6. Measurement of leakage current before irradiation.

A. Roy et al. / Nuclear Instruments and Methods in Physics Research A 552 (2005) 112–117 115

at n+ spiral and the p+ spiral and current flowsfrom the backside to the n+ spiral and the p+

spiral. Current flows from the backside and then+ spiral to the p+ spiral when sufficient highvoltage is reached at the backside and voltage atn+ spiral is more than 66V. In this condition allthe electric field lines end at the p+ spiral. Whenthe applied voltage at the n+ spiral reaches 88Vthe electric field between n+ spiral and the p+

spiral becomes too high and the current from then+ spiral to the p+ spiral exceeds the bulk leakagecurrent.

For the operation of the detector the chargescreated along the track must be collected by thereadout electronics. That is equivalent to collectingall the current through the p+ spiral in ourmeasurement setup. To determine in what condi-tions the detector should be operated we draw thephase space diagram shown in Fig. 8, where weplot the values of the voltage applied at thebackside and at the n+ spiral for which the totalcurrent is collected through the p+ spiral. Thevoltage applied at the n+ spiral has been limited tothe region where the current from the n+ spiral to

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Fig. 7. Measurement of leakage current. Total leakage current is 1.25mA. When the n+ spiral is floating all the current is collected at

the p+ spiral (a). With 30 and 50V at the n+ spiral, current initially flows from the n+ spiral to backside and the p+ spiral. As voltage

on the backside increases, current starts flowing from backside to the n+ spiral and the p+ spiral (b,c). With 70V at the n+ spiral and

sufficient high voltage at the backside, current flows from the backside and the n+ spiral to the p+ spiral (d).

A. Roy et al. / Nuclear Instruments and Methods in Physics Research A 552 (2005) 112–117116

the p+ spiral is less than the bulk leakage currentat depletion voltage (150V). It should be kept inmind that the phase space diagram stronglydepends on the distance between the p+ spiraland the n+ spiral.

The measurement of the capacitance shows thatthe sensor depletes around 150V after irradiation(Fig. 9), which is much lower than the fulldepletion voltage for Si detector with standardstructure (p+/n/n+) and same thickness and

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Fig. 8. Phase space diagram for operation of the detector.

When the detector is operated in the shaded region, all the

current bulk and the n+ spiral is collected at the p+ spiral. Line

AB denotes the depletion voltage and line CD denotes the value

of voltage at the n+ spiral above which the current from the n+

spiral is more than the bulk leakage current.

Fig. 9. Measurement of depletion voltage. Depletion voltage is

150V. Depletion voltage does not change as 0, 35 and 75V is

applied to the n+ spiral.

A. Roy et al. / Nuclear Instruments and Methods in Physics Research A 552 (2005) 112–117 117

radiation fluence (4300V) [4]. Applying voltageon n+ spiral, however, does not change thedepletion voltage any further (Fig 9). The built-in field under the n+ spiral is high enough thatfurther biasing is made insignificant for samplesirradiated to this medium–high fluence.

5. Conclusion

After receiving a total fluence of 5� 1014 neq/cm2, semi-3D test structures deplete at a greatlyreduced voltage of 150V as compared to adetector with standard structure. The detectorshould be operated within the shaded region inphase space diagram (Fig. 8) for optimal chargecollection after SCSI. Application of a voltage tothe n+ spiral has no effect on the depletionvoltage. More measurements have to be performedto further our understanding of the chargecollection performance of semi-3D sensors withthe readout chip.

Acknowledgements

We would like to thank the RD50 collaborationfor providing support for the production of thesesensors. We would also like to thank Michael Mollfor helping in irradiating the sensors.

References

[1] CMS—Technical Proposal; CERN/LHCC 94-38.

[2] F. Gianotti, et al., CERN-TH-2002-078; April 2002.

[3] J. Harkonen, et al., Nucl. Instr. and Meth. A 518 (2004)

346.

[4] Z. li, R. Beuttenmuller, W. Chen, D. Elliott, V. Radeka,

J. Takahashi, W.C. Zhang, Nucl. Instr. and Meth. A 478

(2002) 303.

[5] S.I. Parker, C.J. Kenney, J. Segal, Nucl. Instr. and Meth. A

395 (1997) 328.