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First investigation of a novel 2D position-sensitive semiconductor detector

concept

Article  in  Journal of Instrumentation · June 2011

DOI: 10.1088/1748-0221/7/02/P02005 · Source: arXiv

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A novel 2D position-sensitive semiconductordetector

D.Bassignanaa, M.Lozanoa, G.Pellegrinia, D.Quiriona, M.Fernandezb,R.Jaramillob, F.J.Munozb, I.Vilab

a Centro Nacional de Microelectrnica (IMB-CNM-CSIC),Campus Univ. Autnoma de Barcelona

08193 Bellaterra, Barcelona (Spain)b Instituto de Fsica de Cantabria IFCA (CSIC-UC),

Edificio Juan Jord, Avenida de los Castros, s/nE-39005 Santander (Spain)

Abstract

We present a novel 2D position-sensitive semiconductor detector manufactured using the conventionalplanar technology used in the production of single-side AC-coupled microstrips sensors. In the newdevice the upper coupling electrode is made of a slightly resistive material being read out at bothends. The balance between the recorded charges at both electrode ends is used to define an estimateof the position along the strip where the charge was created. A proof-of-concept sensor has beenmanufactured using strongly doped polycrystalline silicon as resistive material. The sensor responsewas characterized using a microspot infrared laser and a radioactive 90Sr source. Experimentalresults were compared against an electronic simulation of the sensor equivalent circuit. The spacialresolution achieved with these first sensors is of about 30 um (laser test-stand) and a Signal-to-noiseratio of around fifteen was determined using the radioactive source. We have demonstrated for thefirst time, the feasibility of this technological implementation of the charge-division concept in areal microstrip detector. keyword: 2D position sensitive detectors, Si microstrip detectors, Particle trackingdetectors, Detector design and construction technologies and materials

1 Introduction

In the last 30 years semiconductor sensors have been object of great interest as position-sensitivedetectors. Different segmentation of the electrodes and different principles of operation as well asdedicated read-out electronics have been developed to achieve high time and spatial resolution (fewmicrons in one or two dimensions).

2D position-sensitive devices give two coordinates of an ionizing event in a plane. This impliesdouble side processing (2D microstrip detectors and drift detectors) or the implementation of acomplex readout system with a large number of electronic channels (pixel detectors). These are im-portant parameters to take into account in a wide range of applications like particle trackers for highenergy or nuclear physics experiments, space experiments (Compton cameras) or instrumentationfor medical imaging. In all these applications operational efficiency, high compactness and low costof the components is needed. We propose a new microstrip single-sided detector for 2D positionmeasurements. A resistive material layer deposited on each strip and equipped with metal pads atthe terminals for the connection with the readout electronics allows the use of the resistive chargedivision concept to obtain spatial information of the event along the strip direction.

A description of the device and its working principle is given in this paper. We have bothsimulated the behavior of the new detector and produced proof-of-concept prototypes at IMB-CNMclean room facilities in Barcelona [1]. We have studied resistive charge division performance by meansof automated laser scans and we used a 90Sr radioactive source to determine the signal to noise figureof the detector.

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Figure 1: Schematic top view of the new microstrip detector (not to scale). Only a few strips have been representedto simplify the structure. It is possible to distinguish the Aluminium elements in black and the resistive electrodes onthe strips (striped regions). The Aluminium pads are connected each one to a channel of the read-out electronics (twofor each strip). When an ionizing particle crosses the detector, differently attenuated signals (S1 and S2) are inducedin the electronic channels. The X coordinate of the event can be reconstructed using the center of gravity method,whereas the Y coordinate is reconstructed comparing the signals at the ends of each strip.

2 The new device

2.1 Principle of operation

The device presented is an AC coupled microstrip detector with the upper electrodes of the couplingcapacitor made of resistive material ended by two Aluminium pads, for the connections to the read-out chips.

Figure 1 shows a schematic of the top view of the device. In a conventional microstrip detectorthe Aluminium contacts of the strips extend over almost all the length (L) of the implants and areconnected each one to a read-out channel. When an ionizing particle crosses the detector, the propa-gation of the induced signal along the coupling electrode does not introduce a significant attenuation,i.e., the signal amplitude does not depend on the particle’s impinging point along the electrode di-rection. When using, instead of Aluminium, a slightly resistive upper coupling electrode, the signalundergoes a significant attenuation during its propagation along the electrode direction. The longerthe propagation length is, the larger the signal attenuation becomes. By comparing the recordedsignal amplitude at both ends of the coupling electrode one can derive the particle’s impinging pointalong the electrode direction (see figure 1) In this way, for the first time, a conventional manufac-tured microstrip sensor provides the two-dimensional coordinates of the particle’s impinging point;the transversal coordinate derived thanks to the usual electrode’s transversal segmentation [2] andthe longitudinal coordinate determined by related signal amplitude at both ends of the electrodes.

2.2 A Spice model for the detectors

We developed a general SPICE equivalent model of the new sensors starting from the work presentedin reference [3] . Five p+-type strips (64 mm long, 10 µm wide) on a 300 µm thick n-type substrateare modeled by a periodic structure composed of 200 cells. The unit cell is a complex chain ofcapacitances and resistors representing a portion of the main electric characteristics of the device asthe substrate resistance and capacitance (Rsub and Csub), the interstrip resistance and capacitance(Rint and Cint), the p

+implant resistance (Rimpl), the coupling capacitance (CAC) and the resistanceof the resistive upper electrode (R). In figure 2 a detail of the model representing a strip cross sectionis shown. In the first simulations performed we set all the values of the circuital elements to the onesgiven in reference [3] and changed the value of R. We connected a pulse generator to different pointsof the central strip implant and recorded, for each position, the signals read at the entrance of the

Figure 2: SPICE model for the simulation. Schematic cross-section of the new microstrip detector modeled (top)and a detail of the circuital model of the same cross-section (bottom). Each unit cell (between two consecutive nodes)represents a portion of the strip by means of a chain of resistances and capacitances like the coupling capacitance(CAC), the substrate resistance and capacitance (Rsub and Csub), the p

+implant resistance (Rimpl) and the resistanceof the resistive electrode(R). In the simulation a pulsed current has been induced at different m-nodes.

amplifiers coupled to the ends of the strips. The attenuation of the signals induced at the entranceof the amplifiers is shown in figures 3 and 4 for R=0.2Ω/µm. When the pulse generator is connectedto the central node (position corresponding to 32mm) the signals result equally attenuated, due tothe simmetry of the configuration. We repeated the simulation for different values of R. Figure 5shows the position of the laser pulse as a function of the dimensionless variable S defined as:

S =S2 − S1S2 + S1

, (1)

with S1 and S2 the signal amplitudes read at the entrance of the preamplifers Ampl1 and Ampl2 infigure 2. A third degree polynomial fits the data with good accuracy.

Table 1: Results of the third degree polynomial fits. The parameters of the function y(x) = intercept + B1x +B2x2 + B3x3 and the coefficient of determination are displayed for the four values of the upper electrode resistanceconsidered in the simulations.

R (Ω/µm) intercept B1 B2 B3 R2

0.05 31.996 29.954 0.006 2.768 0.9990.2 32.002 28.661 -0.02 5.098 0.9991 31.936 19.187 0.085 14.004 0.9972 31.931 13.67 0.054 18.54 0.993

The results summarized in Table 1 suggest that the position along the strip can be calculatedusing this empirical method.

- 5 0 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0- 0 . 10 . 00 . 10 . 20 . 30 . 40 . 50 . 60 . 70 . 80 . 91 . 01 . 1

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nt pu

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, S1)

T i m e ( n s )

C u r r e n t p u l s e i n j e c t e d 0 m m 6 . 4 m m 1 2 . 8 m m 1 9 . 2 m m 2 5 . 6 m m 3 2 m m 3 8 . 4 m m 4 4 . 8 m m 5 1 . 2 m m5 7 . 6 m m 6 4 m m

Figure 3: Attenuation of the signal read at the en-trance of the amplifier Ampl1 for different positionsof the pulse generator along the strip, from position0 mm to position 64 mm (R=0.2Ω/µm).

- 5 0 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0- 0 . 10 . 00 . 10 . 20 . 30 . 40 . 50 . 60 . 70 . 80 . 91 . 01 . 1

C u r r e n t p u l s e i n j e c t e d 0 m m 6 . 4 m m 1 2 . 8 m m 1 9 . 2 m m 2 5 . 6 m m 3 2 m m 3 8 . 4 m m 4 4 . 8 m m 5 1 . 2 m m5 7 . 6 m m 6 4 m m

Curre

nt pu

lse sh

ape (

Ampl2

, S2)

T i m e ( n s )

Figure 4: Intensification of the signal read at theentrance of the amplifier Ampl2 for different positionsof the pulse generator along the strip, from position0 mm to position 64 mm (R=0.2Ω/µm).

2.3 Design and fabrication of a proof-of-concept prototype

Two kinds of proof-of-concept prototypes with different electrode resistivity have been designed andfabricated at the IMB-CNM clean room facilities in Barcelona using the conventional technology forp-on-n, AC coupled, silicon microstrip detectors. Each sensor consists of 34 p+ strips (20 or 40 µmwide), each 1.4 cm long and with a pitch of 160 µm. The resistive material used is highly dopedpolysilicon with R/µm=10 Ω/µm for the detector with the strips 40 µm wide and R/µm=20 Ω/µmfor the 20 µm wide.

As shown in figure 6 Aluminium tracks have been added parallel to the strips to route signal S2 toa pad in the same area of the one related to signal S1, for each strip. This solution allows to connectall the pads to the same chip of the readout electronics (see next section). Therefore the readout pitchresults to be 80 µm. The detectors have been electrically characterized in the IMB-CNM laboratorieswith the use of a probe station Cascade Microtech, two Keithleys 2410 Source/Meter and an Agilent4284A LCR Meter. The results are consistent with the ones of the standard microstrip detectorsincluded in the process run. The measured values are listed in table 2.

Table 2: Electric characterization: measured values of depletion voltage, breakdown voltage, bias resistance, interstripresistance and capacitance, coupling capacitance and polysilicon electrode resistance.

strip width Vdep [V] Vbd [V] Rbias [MΩ] Rint Cint [pF] CAC [pF] R [Ω/µm]

20 µm 40 >400 1.31 >GΩ 1,32 120 2040 µm 40 >200 1.37 >GΩ 1,60 240 10

3 Laser and radioactive source characterization

The prototype sensors were characterized using a laser test-stand and a radioactive 90Sr source. Withthe laser we determined the signal attenuation profile along the strip direction; with the radioactivesource we measured the signal-to-noise ratio. In the following sections we provide a description ofthe experimental arrangement and the obtained results.

Figure 5: Results of the simulation for four different values of the resistance of the upper resistive electrodes (0.05-0.2-1-2 Ω/µm). The value for the Aluminium electrode with the same geometry is about 2.2 mΩ/µm. The pulsegenerator position is related to the variable S through a 3rd degree polynomial function represented here for each valueof the resistance: solid line for R=0.05Ω/µm, dashed line for R=0.2Ω/µm, dashed and dotted line for R=1Ω/µm anddotted line for R=2Ω/µm.

3.1 Experimental arrangement

Two sensors, each with a different electrode resistance, where mounted in a PCB sensor carrierand read out using an ALIBAVA daq system [4]. The ALIBAVA is a dedicated daq system for thereadout of microstrip sensors based on the Beetle analog readout ASIC [5]. The Beetle integrates128 pipelined channels with low-noise charge-sensitive pre-amplifiers and shapers with peaking timeof 25ns.

In our arrangement, each sensor was wire bonded to a different Beetle chip. The pads relatedto the signals S1 were bonded to the Bettle even channels while the pads related to the signal S2were bonded to the Beetle odd channels. Unfortunately, after the bonding procedure one Beetle chipfailed. Therefore, hereafter all the results will only refer to the sensor with the higher resistivity(20 Ω/µm).

The laser test-stand consists of a pulsed DFB laser coupled to a monomode optical fiber whichfeeds a microfocusing optical head. The laser wavelength is centered at 1060 nm; the pulse widthis around one nanosecond and the gaussian beam spot width is smaller than five microns. Themicrofocusing optical head was moved by a 3D axis stage with a displacement accuracy better than10um on each axis. The vertical movement -transverse to the sensor plane- of the optical headallows us to precisely place the laser waist on the sensor surface. The precise transverse movement-parallel to the sensor plane- allow us to carry out the longitudinal scanning of the laser along thesensor electrode to determine the signal attenuation profile. We perform a longitudinal scan along apolysilicon electrode using the laser -contrary to the aluminium, polycristalline silicon is transparent.We scanned the whole electrode length (14mm) with a scanning step of 1 mm. On each scanningpoint, 60000 laser pulses were recorded.

The optical head can be replaced with a collimated 90Sr radioactive source. For this arrangement,the trigger signal comes from a photomultiplier tube placed underneath the sensor under test. Inspite of the small collimation aperture -below 1mm- the source-triggered signals were spread over

Figure 6: Schematic top view of the first prototypes (not in scale)(left). The black color refers to Aluminiumstructures while the striped elements represents the resistive electrodes on the strips. The strip implants pitch is160µm while the readout pitch is 80 µm due to the Aluminium tracks. The two photographs on the right side showthe actual prototype layout at the electrode ends.

all the sensor active area, making this arrangement unsuitable for a scanning analysis as the onedone with the laser. Nevertheless, the benefit comes from fact that the 90Sr radioactive source is anexcellent tool for estimating the actual sensor’s signal-to-noise ratio, something still quite complexto be correctly estimated with a laser test-stand.

3.2 Results and discussion

Processing the data from the laser longitudinal scan along the polycristalline electrode, we computethe value of the variable S defined in equation 1; on each scanning point the recorded signal distribu-tion is fitted to a gaussian taking its mean parameter as the pulse amplitude estimate (see figure 7and figure 8). Figure 9 shows the scanning point distance as a function of the S variable. As predictedby the simulation, the experimental dependence can be described by a third order polynomial. As

Figure 7: Signal S1 distribution and gaussian fit’sresults related to the position 11 mm of the laser alongthe electrode.

Figure 8: Signal S2 distribution and gaussian fit’sresults related to the position 11 mm of the laser alongthe electrode.

Figure 9: Results of the laser scan of one strip. Thevariable S has been calculated for each position of thelaser.

Figure 10: Residuals of the fit shown in the leftplotted versus the correspondent value of laser posi-tion. The RMS is equal to 0.028.

shown in figure 10, the randomly distributed residuals spread around the fitted value less than 30 um(RMS), this figure would be the position resolution one can achieve along the electrode longitudinalcoordinate when getting signals of similar amplitude to the laser one. The values of the measured

Figure 11: Comparison between the experimental and simulated results.

S values have been compared against the SPICE simulation, this comparison is shown in figure 11.We can observe an increasing discrepancy between the measured and simulated value as we movethe laser towards the pad related to signal S1 (position 0 mm). The origin of this bias in the sensorresponse could be found in the parasitic coupling capacities introduced by the aluminium tracks. Anew design of the devices has been developed without the Aluminium tracks to avoid this effect.

Using the 90Sr source we have measured at the centre of the electrode the signal-to-noise ratiowhich is around 15 as shown in figures 12 and 13.

Figure 12: Charge collected by the even channelwhen the radioactive source is placed in the middleof the strip.

Figure 13: Charge collected by the odd channelwhen the radioactive source is placed in the middleof the strip.

4 Conclusion

We have demonstrated the feasibility of the charge division concept in microstrip sensors to determinethe coordinate of an ionizing event along the strip length. In our prototype the spatial resolution inthe determination of the coordinate is better than 100 µm and the signal to noise ratio result equalto 15. We have used the standard technology to produce this genuine 2D single-sided strip detectorwhich can be coupled with the standard readout electronics for microstrip sensors. We propose thisnew device for a large range of applications like in the future outer tracker (trigger capable modules)in high energy physics experiments, in ions tracking systems, in neutron imaging (with the use of aconversion element) and in space applications in which the compactness of the tracking system is astrict requirement.

International patent pending- ref PCT/ES2011/070088 ”Semiconductor 2D position sensitivedetector”.

5 Acknowledgements

We thanks A.Candelori for the clarifications concerning the SPICE model of ref.[3]. We thank the”Development of particle detectors” group of IFIC-Valencia for the bonding of the sensors and boardsand for the help with the ALIBAVA system.

This work has been supported by the Spanish Ministry of Science and Innovation under grantFPA2007-66387 and through the GICSERV program ”Access to ICTS integrated nano-and microelectronics cleanroom” of the same ministery.

References

[1] Centro Nacional de Microelectrnica, Campus Universidad Autnoma de Barcelona. 08193 Bel-laterra (Barcelona), Spain (http://www.imb-cnm.csic.es/)

[2] ”The Methods for Coordinate Reconstruction in Silicon Microstrip detectors”, A.A.Kiryakov,V.N.Ryadovikov, A.V.Kubarovskii, V.V.Popov, Instruments and Experimental Techniques, Vol.47, No.5, 2004 pp.611-618

[3] ”SPICE Analysis of Signal Propagation in Si Microstrip Detectors” N. Bacchetta et al. , IEEETRANSACTIONS ON NUCLEAR SCIENCE, VOL. 42, NO. 4, August 1995.

http://www.imb-cnm.csic.es/

[4] ”A Portable Readout System for Microstrip Silicon Sensors (ALIBAVA)” Marco-Hernandez, Ri-cardo; Nuclear Science Symposium Conference Record, 2008. NSS ’08. IEEE 19-25 Oct. 2008Page(s):3201 - 3208.

[5] ”Beetle - a readout chip for LHCb” http://www.kip.uni-heidelberg.de/lhcb/

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http://www.kip.uni-heidelberg.de/lhcb/https://www.researchgate.net/publication/51933838

1 Introduction2 The new device2.1 Principle of operation2.2 A Spice model for the detectors2.3 Design and fabrication of a proof-of-concept prototype

3 Laser and radioactive source characterization3.1 Experimental arrangement3.2 Results and discussion

4 Conclusion5 Acknowledgements