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Nuclear Instruments and Methods in Physics Research A 535 (2004) 398–403

www.elsevier.com/locate/nima

Monolithic active pixel detector realized in silicon oninsulator technology

Antonio Bulgheronia, Massimo Cacciaa, Krzysztof Domanskib, Piotr Grabiecb,Miroslaw Grodnerb, Bohdan Jaroszewiczb, Tomasz Klatkac,Andrzej Kociubinskib, Michal Kozielc, Wojciech Kucewiczc,�,

Krzysztof Kucharskib, Stanislaw Kutac, Jacek Marczewskib, Halina Niemiecc,Maria Saporc, Michal Szelezniakc, Daniel Tomaszewskib

aDip. di Scienze, Univ. degli Studi dell’Insubria, Como, ItalybInstitute of Electron Technology, Warsaw, Poland

cAGH - University of Science and Technology, al. Mickiewicza 30, 30-059, Krakow, Poland

Available online 17 August 2004

Abstract

The paper concerns the development of a novel monolithic silicon pixel detector, which exploits Silicon on Insulator

substrates for the integration of the readout electronics and pixel detector. In the discussed solution, the readout CMOS

circuit is manufactured in the thin device layer over the buried oxide while the pixel matrix is created in the high resistive

handle wafer of the SOI substrate. Small test matrices of the SOI sensor have been recently manufactured and

preliminary tests with an infrared laser light and a radioactive source indicated the sensor sensitivity for the ionizing

radiation. The concept and design of the SOI detector together with the preliminary measurement results of the sensor

matrices are addressed in the paper.

r 2004 Elsevier B.V. All rights reserved.

PACS: 87.66.Pm

Keywords: Position silicon detector; Pixel detector

e front matter r 2004 Elsevier B.V. All rights reserve

ma.2004.07.160

ng author. Tel.: +48-12-617-3045; fax: +48-

ss: kucewicz@agh.edu.pl (W. Kucewicz).

1. Introduction

A common trend in the field of highly segmen-ted ionizing radiation detectors is the developmentof monolithic active pixel sensors. The integrationof a pixel detector and readout electronics in the

d.

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Fig. 1. A cross-section of the SOI pixel sensor.

A. Bulgheroni et al. / Nuclear Instruments and Methods in Physics Research A 535 (2004) 398–403 399

sensor cell allows avoiding the complicated hy-bridization process as well as obtaining thindevices and thus reducing the multiple scatteringeffect, important in the vertex detector applica-tions. One of the well-established approaches tothe monolithic detectors relies on bulk CMOStechnologies and thermal diffusion of the chargegenerated in a lightly doped epitaxial layertowards collecting N-wells. In such a solution,however, due to the small detector active volumethe charge generated by a minimum ionizingparticle is small (of the order of a few hundredsup to 1000 electrons) and only NMOS circuitrymay be implemented in the front-end electronics[1]. In this paper an alternative concept of amonolithic active pixel detector, which allowselectrical separation of the detector and thefront-end electronics active layers and the opera-tion in the full depletion region, is presented. Themain idea of the sensor relies on the utilization ofboth silicon layers (support and device layer) ofthe silicon on insulator (SOI) substrate for therealization of the pixel detector diodes and thereadout electronics, respectively. Fabrication of amonolithic active pixel detector in the SOIsubstrate may potentially lead not only to gooddetection efficiency, but also to improved circuitimmunity for single event radiation effects (SEE)and to increased design flexibility due to thepossibility of the use of transistors of both typesin the readout channel.

The SOI sensor project was started in 2001 andis partially supported by the European Commis-sion within the 5th Framework Program. One ofits goals is the development of a monolithic sensoroptimized for medical imaging applications [2].

Fig. 2. A microscopic photo of the pixel junction cavity.

2. Structure of the SOI sensor

The SOI monolithic active pixel detector isrealized in a wafer-bonded SOI substrate, whichallows the optimization of the resistivity of theelectronics and detector layers. The importantadvantages of the wafer-bonded substrates incomparison with other popular SOI substratesobtained in the SIMOX process are also the lowerlevel of the structural defects in both the device

layer and the handle wafer, absence of siliconinclusions and islands in the buried oxide (BOX),and lower temperature processing [3,4]. The SOIsensor is fabricated on commercially available SOIsubstrates since both the readout electronics andthe detector diodes are created after the bondingprocess and no wafer pre-processing is required.The structure of the SOI pixel sensor is

illustrated in Fig. 1. The pixel detector has aconventional form of a matrix of p þ�n junctionsand is fabricated in the high resistive ð4 kOcmÞ;300mm thick silicon layer below the buried oxide.The readout electronics, which may consist of bothNMOS and PMOS transistors, is implemented inthe low resistive active layer and it is monolithi-cally coupled with the detector by a connectionthat passes through the silicon film and buriedoxide. A microscopic photo of the pixel contactwindow, obtained by anisotropic etching ofthe silicon with orientation h1 0 0 i; is presentedin Fig. 2.

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An important feature of the proposed sensor isthe use of relatively thick insulator and devicelayers (about 1mm and 1:5 mm; respectively). It isexpected that such an approach, together with thedense substrate polarization, may lead to aneffective reduction of the cross-talk between theelectronics and the detector and that the noiseperformance of the readout circuit may benefitfrom the larger distance of the transistor channelsto the interface states on the surface of the buriedoxide.

Fig. 3. Schematic views of the detector cells implemented on

the SOI detector test structures.

3. Design and measurements of the SOI detector

test structures

In order to validate the concept of the SOIdetector a special test structure, including smallmatrices of readout channels with both integrateddetector diodes for tests of the complete sensorsand input pads for the electronics characterization,were designed and fabricated. The schematic viewsof the readout channels implemented on the teststructures are presented in Fig. 3. Both thepresented configurations have a structure similarto the typical three transistor cells, but contrary tothe conventional solutions, widely used in theCMOS sensors, the reset transistors of theopposite type than the input transistors are usedto achieve faster discharging of the integratingcapacitances and additional diodes are added toprotect the readout channel from the excessiveinput voltage coming from the detector polariza-tion. For the channel configuration presented inFig. 3(a), the single-transistor row-selection switchis also replaced by the transmission gate toimprove the linearity of the sensor response. Thedimensions of the sensor cells on the test structuresare 140mm� 140mm and 140mm� 122mm for theschemes presented in Figs. 3(a) and (b), respec-tively. Both presented sensor cells are optimizedfor high particle fluxes, expected in some objectiveapplications of the SOI detector [2]. The tests ofthe readout channels with an external voltagepulse signal indicated an input dynamic range upto 300 MIPs and 150 MIPs for the structures withthe transmission gate and the single-transistorswitch, respectively. An output rms, noise of

270mV; corresponding to the equivalent noisecharge (ENC) of about 990 e�; for a bare readoutchannel without detector diode was also measured[5].Since on the test structures no readout digital

control blocks were implemented, for the measure-ments of the complete SOI sensors (with mono-lithically coupled detector junctions) the matriceswith the readout cells presented in Fig. 3(b) werechosen to reduce the number of necessary externalcontrol signals. The sensor sensitivity for ionizingradiation was preliminarily demonstrated in testswith an infrared laser light and a radioactivesource. The linearity of the sensor response wasstudied by shining an infrared laser spot (withwavelength of 850 nm) on the backplane of thesensor. The tests were performed for a detectorpolarization of 60V (above full depletion voltage)and an integration time of 1ms. Different numberof 4ms wide light pulses simulated the particlespenetrating the detector active volume. For every

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Fig. 5. The Alpha particle emission spectra obtained with the

SOI detector.

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number of light pulses injected during the integra-tion time, 10 000 events were recorded and theresults were averaged. In order to overcome thelight reflection, the detector backplane was polar-ized by a metal mesh with rectangular holes. Themeasured values of the output signals (after CDSprocessing and pedestal subtraction) versus theinput charge are illustrated in Fig. 4. The detectorsensitivity for the ionizing radiation and the linearresponse as a function of the generated charge inthe investigated range of 80 MIPs was observed.

Further tests of the small area SOI detectorswere performed with Americium 241 (Am241)radioactive source. The source was placed at adistance of 1 cm from the detector backplane andthe emission spectrum of alpha particles with aninitial energy of 5.5MeV was measured at theintegration time of 720ms and a detector polariza-tion of 70V. During the measurements on-linecorrelated double-sampling (CDS) processing wasperformed by the data acquisition system in orderto suppress fixed pattern and kTC noises. Thespectrum of alpha particles after the pedestalsubtraction, the common mode suppression andthe cluster search is presented in Fig. 5. As isillustrated, a broad spectrum was obtained due tothe presence of air and the large distance betweendetector and source. The typical value of 200 ADCcorresponding to approximately half of the initialenergy of the alpha particles was measured at asignal to noise ratio of about 130.

Fig. 4. The Output signal of the SOI sensor versus the input

charge corresponding to a different number of MIPs.

4. Design of the fully functional SOI detector

Next stage of the SOI detector development isthe design of a fully functional sensor with anactive area of 2 cm� 2 cm: The simplified layout ofthe detector is presented in Fig. 6. The detectorconsists of four independent sub-segments withdimensions of 12mm� 12mm and 64� 64 read-out channels. Each sub-segment has its own set ofbiasing and control lines and its own analogueoutput. In order to avoid dead areas between sub-segments all the peripheral elements of the readoutcircuit as well as detector guardrings are designedonly from the two neighbouring sides of thereadout matrix. The extension of the sensordimensions beyond 24mm� 24mm is possible byplacing the basic segments of the detector next toeach other and building ladders with the length upto 72mm and small dead areas between theparticular segments.A single cell of the sensor matrix has the

dimensions of 150mm� 150 mm and the config-uration presented in Fig. 3(a). Such structure of areadout channel allows measurements of the

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Col_sel

ADC

9.6 mm

12 mm

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9.6

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12 m

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Fig. 6. Simplified layout of the large area SOI sensor; (a) layout

of a single sub-segment, (b) layout of a complex of four sub-

segments.

A. Bulgheroni et al. / Nuclear Instruments and Methods in Physics Research A 535 (2004) 398–403402

charge generated in the detector volume andintegrated on the pixel capacitance during a giventime period. Measurements of the integratedcharge, instead of the charge generated by a singleevent in the detector, are necessary for thedosimetry of radioactive sources, which is one of

the possible application fields of the SOI detectors[2].The readout method of the front-end circuit of

the SOI detector relies on the classical serialanalogue organization and the un-triggered modeof the operation. The implemented readoutsequence is similar to the rolling shutter technique[6], where the rows of the readout matrix are bothreseted and read out (after the integration time)one by one in the same sequence and at the samespeed. Contrary to this traditional technique,during the proposed readout sequence everychannel is accessed twice: immediately after thereset of the diode and after the integration time,which is equal to the readout time of the wholematrix. This method not only guarantees veryshort sensor dead time and well-defined integra-tion time, but also enables external CDS proces-sing. The proposed readout scheme has beenalready successfully implemented in the prototypefront-end circuit fabricated in a commercialtechnology. The detailed description of the read-out method and the test results obtained for theprototype circuit may be found in Ref. [7].

5. Conclusions

A novel solution of the monolithic active pixelsensor, which may offer good detection efficiencyand flexibility of the readout electronics design,has been proposed. Preliminary tests of the smallarea detector test structures proofed the sensorsensitivity for the ionizing radiation and validatethe concept of the device. Following the encoura-ging measurement results, a fully functional largerarea detector has been designed and will befabricated in the nearest future.

References

[1] J.D. Berst, Proceedings of 6th Workshop on Electronics for

LHC Experiments, Krakow, Poland, 2000, pp. 535–539.

[2] M. Caccia, et al., Nucl. Instr. and Meth. B 125 (2003)

133.

[3] A. Marshall, S. Natarajan, SOI Design: Analog, Memory

and Digital Techniques, Kluwer Academic Publishers,

Massachuselts, USA, 2001.

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[4] A. Chediak, et al., University of California, Berkeley,

electronic file available at www.mse.berkeley.edu/faculty/

sands/MSE125.

[5] H. Niemiec, et al., Monolithic pixel sensor in SOI

technology—latest results, ECFA study of physics and

detectors for a linear collider—1st Workshop, Montpellier,

France, 2003.

[6] Eastman Kodak Company, Application Notes, electronic

file available at http:www.kodak.com/global/plugins/acro-

bat/en/digital/ccd/applicationNotes/ShutterOperations.pdf,

2001.

[7] H. Niemiec, et al., Proceedings of 16th European Con-

ference on Circuit Theory and Design—ECCTD ’03,

Krakow, Poland, 2003, pp. II9–II12.