Nuclear Instruments and Methods in Physics Research A 695 (2012) 354358

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Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/nima

Single photoelectron timing resolution of SiPM as a function of the biasvoltage, the wavelength and the temperature

V. Puill n, C. Bazin, D. Breton, L. Burmistrov, V. Chaumat, N. Dinu, J. Maalmi, J.F. Vagnucci, A. Stocchi

Laboratory of Linear Accelerator (LAL), CNRSIn2p3, 91898 Orsay, France

a r t i c l e i n f o

Available online 8 January 2012

Keywords:

Silicon photomultipliers (SiPM)

Single photoelectron timing resolution

(SPTR)

Time-of-flight (TOF)

Picoseconds level

02/$ - see front matter & 2012 Elsevier B.V. A

016/j.nima.2011.12.039

esponding author.

ail address: puill@lal.in2p3.fr (V. Puill).

a b s t r a c t

This work reports on Silicon Photomultipliers (SiPM) timing resolution measurements performed at the

picosecond level at Laboratory of Linear Accelerator (LAL), In2p3- CNRS.

The dependence of Single Photoelectron Timing Resolution (SPTR) with the applied voltage,

wavelength of the light and the temperature was measured for detectors from Hamamatsu Photonics,

AdvanSiD and Sensl with an active area of 1 and 9 mm2.

The SPTR improves with the bias voltage increase. No significant variation of SPTR was observed

with the temperature change. We also observed a weak variation of it as a function of the wavelength of

the light. The best SPTR measured was about 120 ps (FWHM).

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Time-of-flight (TOF) technique is used in High Energy Physicsexperiments to perform particle identification. TOF systems basedon SiPM detectors coupled to quartz Cherenkov radiators could bean option for upgrading the Particle Identification system cap-abilities. It is assumed that few photons (less than 10) reach thephotodetector at the quartz output; SiPM should then be eval-uated at a weak light level in order to determine its contributionto the total timing resolution of the detection chain.

In the present article, we report on the study of the singlephotoelectron timing resolution (SPTR) of different SiPMs. Thismeasurement is performed in blue light to match with thewavelength of Cherenkov detectors but also in red to study itsvariation with the wavelength of the light. We also study the SPTRat 0, 10 and 20 1C to determine if an accurate stabilization of thetemperature is mandatory to keep the SPTR at a constant level.

This work was carried out in the framework of the Forward PIDcollaboration of the SuperB experiment with funding of IN2P3and INFN.

2. Experimental

2.1. Description of the tested devices and experimental set-up

Devices from Hamamatsu Photonics (MPPC), Sensl (SPM) andAdvanSiD (ASD, produced by F.B.K) were characterized in the same

ll rights reserved.

experimental conditions. These detectors are listed in Table 1.The two MPPC with the references 10-50SBK-4S and 10-100FFSare prototypes (also called wide trace MPPC) designed to improvethe timing resolution of the standards MPPC [1].

All the characterizations were performed inside a climaticchamber that gives a high stability (70.1 1C) of the temperature.The temperature of the SiPM is monitored by a Pt100 sensormounted very close (2 mm) to it and read by an acquisition unit(Keithley 2700).

The Fig. 1 presents the experimental set-up used for the SPTRmeasurements. Optical pulses from Pilas laser diodes are sent viaa semi-reflective mirror on the SiPM and on a reference PMT(Hamamatsu R7400U-01) that checked the time stability of thelaser intensity. Three different laser diodes (405, 467 and 635 nm)are driven at a repetition rate of 500 kHz. The light intensity iscontrolled with neutral density filters placed between the semi-reflective mirror output and the SiPM. The spectral width of thelaser pulses does not exceed 3 nm and, depending on the laserdiode head, the pulse timing width is between 38 and 50 ps.

The read-out electronics for the SiPM signal consists in a500 MHz MITEQ voltage amplifier (gain350) with an inputimpedance of 50 O. The amplifier output is sent via a SMA cableand sampled either by a Wavepro 750ZI LeCroy digital oscillo-scope (40 GSamples/s, 4 GHz of analog bandwidth) or by the3.3 GSamples/s WaveCatcher ASIC-based waveform digitizerdeveloped at LAL [2].

2.2. Timing measurement method

A first set of measurements was performed in order to deter-mine the working range of each SiPM: its breakdown voltage (VBD),

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V. Puill et al. / Nuclear Instruments and Methods in Physics Research A 695 (2012) 354358 355

gain and dark count rate (DCR) were measured at 0, 15 and 20 1C.The temperature variation coefficient of the breakdown voltage ofeach device was then calculated. For more precisions about theemployed set-up and the principle of measurements, refer to [3].

Then, and for each measurement, we checked that the SiPMworks in single photo-electron mode: on the histogram of thesignal amplitude, we observe the pedestal peak with at least 80%of the events (signals with amplitude 0), then a second peak dueto signals with an amplitude of 1 p.e and between 15% and 20% ofthe population of this peak with amplitude 2 p.e (Fig. 2). Theevents of the 2 p.e peak include the cross-talk of the device.

Since the PDE SiPM increases with the bias voltage, weadapted the attenuation of the light when changing the biasvoltage (Vbias) of the SiPM in order to stay in the single photo-electron detection condition.

The timing resolution to single photon was studied by mea-suring the fluctuations of the difference in time (Dt) between theSiPM amplified signal and the laser driver synchronization output(laser trigger).

The tuning of the instruments (oscilloscope and Wavecatcher)is a very important matter:

TabCha

P

A

H

H

H

H

H

Se

Se

A

H

H

H

Se

as the incident flux is very low (1 or 2 photons/pulse) and theSiPM PDE is between 10 and 40% (depending on the device andthe wavelength), a lot of photons are not converted. In order to

le 1racteristics of the tested SiPMs (VBD: breakdown voltage).

roducer Ref SiPM Area(mm2)

Pixelsize (lm)

VBD (V)at 20 1C

dvanSiD ASD-SiPM1S-M-50 1 5050 29AMAMATSU S10262-11-25 1 2525 69.2AMAMATSU S10262-11-50 1 5050 68.3AMAMATSU S10262-11-100 1 100100 68.7AMAMATSU 10-50S BK-4S 1 5050 69.1AMAMATSU 10-100S FS 1 100100 69.1nsl SPM1020X13 1 2020 27nsl SPM1035X13 9 3535 27.5

dvanSiD ASD-SiPM3S-M-50 9 5050 31AMAMATSU S10262-33-25 9 2525 69.5AMAMATSU S10262-33-50 9 5050 69.5AMAMATSU S10262-33-100 9 100100 69.2nsl SPM1035X13 9 3535 27

Fig. 1. Experimental setup for the Si

avoid those lost events for timing resolution measurements,we performed a coincidence window of around 10 ns betweenthe laser trigger and the SiPM signal taken with a thresholdabove the electronic noise of the chain (around 20 mV). Thistuning improves the single photo-electron acquisition rate.

as the rising time of the SiPM signal changes when its amplitude

changes (e.g. due to cross-talk event, variation of Vbias), we use aconstant-fraction threshold instead of a fixed one to perform themeasurements of Dt. With this method, all the detectors arecharacterized in the same experimental conditions.This threshold is set to 50% of the peak of the SiPM signal and ofthe laser trigger one even if this last is very stable.

In all measurements discussed below, we report timing resolu-tions as the FWHM of the timing (Dt) distribution and refer tothem as SPTR. The SPTR is not corrected with the laser pulse width.

The contributions to the SPTR from the electronics weremeasured at LAL: it is about 8 ps in the case of the use of theWavecatcher and 1 ps for the Wavepro 750ZI. Optical contribu-tions were measured by Advanced Laser Diode Systems: as thelaser trigger jitter is around 3 ps, the main contribution from thelight source comes from the pulse width (3850 ps FWHM).

The systematic errors on the SPTR measurement were esti-mated at 75% (10% for the 9 mm2). The values of Dt measuredby the Wavecatcher and the oscilloscope are in agreementwithin 5%.

3. SiPM SPTR measurements

3.1. SiPM SPTR as a function of the bias voltage

The study of the SiPM SPTR with the bias voltage wasperformed at 467 nm and at a temperature of 20 1C. Fig. 3 showsthe results of these measurements as a function of the over-voltage DV (VbiasVbreakdown).

We observe an improvement of the timing resolution with anincreasing overvoltage for all the devices till a maximum valueof the bias voltage above which we cannot distinguish dark noisepulses from the true pulses due to the detection of light. For theMPPC, we observe an improvement of the SPTR with the pixel sizeincrease (at the same DV). The size of the pixel does not affect theSPTR of the SPM.

PM SPTR measurement.

Fig. 2. Laser and SiPM signals on the oscilloscope with the CFD ratios, coincidence window and histograms of the measurements (SiPM amplitude and Dt between theSiPM and the laser trigger signals). The SiPM signal, in this example, arrives before the laser trigger due to the delay put between the laser driver and the oscilloscope.

Fig. 3. 1 mm2 SiPM SPTR as a function of the over-voltage.

Fig. 4. 1 mm2 and 9 mm2 SiPM SPTR as a function of the temperature at constantDV and for different wavelengths.

V. Puill et al. / Nuclear Instruments and Methods in Physics Research A 695 (2012) 354358356

The best SPTR was measured on the detector ASD (FWHM120 ps), the wide trace MPPCs and the two SPMs show approxi-mately the same behavior (150160 ps). The wide trace MPPCsshow better SPTR than the standard one.

3.2. SiPM SPTR as a function of the temperature

The variation of the temperature is a critical parameter for thebehavior of SiPM as it implies the change of its breakdownvoltage, gain and DCR [4,5]. To study its effect on the timingresolution, we performed measurement between 0 and 20 1C atconstant DV (which is calculated at 20 1C for a bias voltage valuethat gives the best SPTR). Only 9 mm2 MPPC could be measured,the SPM and ASD showing too much dark count rate (E10 MHzat 20 1C).

Fig. 4 shows the results at 0, 10 and 20 1C and for differentwavelengths. As the MPPC and ASD show a stable behavior withthe temperature increase, we observe a small trend to thedegradation of the SPTR (1015%) for the SPM. We assume thatthis is due to the fact that the difference of temperature is toolow to affect in a significant way the mobility of the chargecarriers [6].

These variations with the temperature are independent ofthe pixel size and on the wavelength of the light detected bythe SiPM.

3.3. SiPM SPTR as a function of the wavelength

Fig. 5 shows the SPTR variations with the wavelength at atemperature of 20 1C. We ob serve 2 different behaviors: a trendto the improvement of the SPTR when the wavelength increasesfor the MPPCs whereas the contrary is observed for the SPMs andthe ASD. For all these detectors, SPTR and PDE (photon detectionefficiency) do not achieved their best value at the same wave-length [2] (as the MPPC PDE is best in blue and the SPMs is bestin red).

V. Puill et al. / Nuclear Instruments and Methods in Physics Research A 695 (2012) 354358 357

In the case of the SPMs, it seems that the wavelength increasehas a more important effect on larger pixel size (the SPTRvariation between 405 and 635 nm is negligible for the 20 mmwhereas it is about 16% for the 35 mm).

We observe also this phenomenon with the MPPCs: even if thevariation is very weak (almost within the measurement errors),the trend is stronger for the 100 mm than for the 25 mm for the9 mm2 for example.

To explain this observation is not trivial; for MPPC forexample, we would have expected a better SPTR in blue thanin red: if we consider the simple detector geometry shown inFig. 6 (p/n junction on a n-type substrate) and the absorptionlength of light in Silicon, blue photons are absorbed in half a mm

Fig. 5. 1 mm2 and 9 mm2 SiPM SPTR as a function of the wavelength at 20 1C.

Fig. 6. Simplified structure of a MPPC and absorption depth of photon in

and the red one are stopped deeper, at a depth of 23 mm.The electrons created by the absorption of blue photon reachquickly the high field region (if we consider that the junction is ata depth of 0.51 mm) whereas the holes created by the absorptionof red photon reach this region hundred of picoseconds later (asthey are farther and their drift velocity is twice less than that ofthe electron [7]). We would then have expected more importantfluctuations on the arrival time of these carriers due to thephenomenon of lateral spreading by diffusion [8] and thereforea higher SPTR value.

This reasoning is too simple and other parameters like the fieldprofiles and the depth of the junction have to be taken intoaccount in order to explain what we observe (these informationare not disclosed by the producers).

3.4. Delayed events

The timing profile (Dt histogram) in the single photon regimeis well fitted by a Gaussian function plus another one correspond-ing to delayed events with a mean value 100300 ps latter thanthe mean value of the main Gaussian fit (Fig. 7).

We observed this behavior, in a more or less pronounced way,with all the SiPMs, at the three different wavelengths (with 2 laserdrivers and 3 different laser diode heads), in two test bencheswith different geometries. The proportion of delayed eventsincreases with the overvoltage. We checked, cutting the eventswith an amplitude pulse of 2 photo-electrons and fitting theresulting timing distribution histogram that these events do notcome from the cross-talk inherent to the detector. They cannot beexplained either by after-pulses as we use a narrow coincidencewidow (between 5 and 10 ns). The study of this tail as a functionof the detector pixel size, the bias voltage and the wavelength ofthe light is on progress and its results will be reported in a futurearticle.

4. Conclusions

The single photoelectron timing resolution of different SiPMswas measured over each detector bias voltage range in blue light,then at 2 different wavelengths of pulsed light at a fixed over-voltage for temperatures from 0 to 20 1C.

The increase of the bias voltage improves significantly theSPTR. However, this effect is limited by the parallel increase of theDCR that prevents the detection of single photon when theovervoltage is too high. The best values of SPTR for MPPC andSPM are around 150 ps (FWHM) whereas the SPTR of ASD shows

intrinsic Si as a function of the wavelength (reproduced from [9]).

Fig. 7. Dt distribution of the MPPC S10362-33-100 at 2 bias voltages (left: 69.3 V, right: 70 V) at 20 1C and 467 nm.

V. Puill et al. / Nuclear Instruments and Methods in Physics Research A 695 (2012) 354358358

120 ps. MPPC SPTR is slightly better in red light than in bluewhereas the contrary (with amplitude of variation more pro-nounced) is observed on the SPM and ASD.

The variation of the temperature (from 0 to 20 1C) does notaffect in a significant way the SPTR; nevertheless, the cooling ofthe device can improve the single photon discrimination effi-ciency by decreasing the DCR.

More work is in progress to understand the shape of the timingresolution (delayed events) and the SPTR variations with thewavelength and the pixel size.

SiPMs exhibit good single photoelectron timing resolution forparticle identification system in comparison with MCP-PMTs(multi channels plate photomultiplier) with SPTR quoted at70 ps in sigma, around 160 ps in FWHM. However, their weakradiation hardness does not permit to use them, for the moment,in hostile environments.

References

[1] S. Kamakura, S. Ohsuka, K. Yamamura, K. Sato, Production and Developmentstatus of MPPC, PoS(PD09)017.

[2] J. Vavra, D. Breton, E. Delagnes, J. Maalmi, K. Nishimura, L.L. Ruckman,G. Varner, High resolution photon timing with MCP-PMTs: a comparison of

commercial constant fraction discriminator (CFD) with ASIC-based waveformdigitizers TARGET and WaveCatcher, NIM A 629 (2011) 123132.

[3] N. Dinu, Z. Amara, C. Bazin, V. Chaumat, C. Cheikali, G. Guilhem, V. Puill,C. Sylvia, NIM A 610 (2009) 423.

[4] N. Dinu, C. Bazin, V. Chaumat, C. Cheikali, A. Para, V. Puill, C. Sylvia,J.F. Vagnucci, NSS Conference Records (2010) 215.

[5] G. Collazuol, M.G. Bisogni, S. Marcatili, C. Piemonte, A. Del Guerra, NIM A 628(2010) 389.

[6] M. Sze, Physics of Semiconductor Devices, third ed. Simon.[7] E.J. Ryder, Physical Review 90 (1953) 766.[8] A. Lacaita, M. Mastrapasqua, M. Ghioni, S. Vanoli, Applied Physics Letters 57

(1990) 489.[9] K. Rajkanan, R. Singh, J. Shewchun, Solid-State Electronics 22 (1979) 793.

Single photoelectron timing resolution of SiPM as a function of the bias voltage, the wavelength and the temperatureIntroductionExperimentalDescription of the tested devices and experimental set-upTiming measurement methodSiPM SPTR measurementsSiPM SPTR as a function of the bias voltageSiPM SPTR as a function of the temperatureSiPM SPTR as a function of the wavelengthDelayed eventsConclusionsReferences