1

Experimental Project Report

on

Characterization of Silicon Photo

Multipliers

Submitted by

Saurabh Sandilya

Under the guidance of

Prof. Shashi Dugad

Tata Institute of Fundamental Research

Mumbai 400 005

[17th January, 2011]

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Characterization of Silicon Photo Multiplier

Abstract

This report describes the different tests carried out with Silicon Photomultiplier (SiPM) Detectors. We have

studied the SiPMs, which were made by Hamamatsu having a surface area of 3mm X 3mm. We have carried out

the Current-Voltage (I-V) Characteristics and measured the temperature coefficient of breakdown voltage. The

Pedestal Response, Low Intensity LED Response and High Intensity LED Response of SiPMare also studied at

different bias voltages.

Motivation

Today several experiments in High Energy Physics and Astrophysics require detectors efficient for low level

light detection. Most of the systems used for the detection of very low intensity light are based on vacuum photo

detectors, photomultiplier tubes (PMT), micro-channel plate photomultiplier tubes (MCP-PMT) or hybrid photo

detector (HPD). The advantages of such devices are high internal gain (106 - 10

7), very good timing resolution

and good single photon resolution. But, these devices have low quantum efficiency (limited by the photocathode

materials), high operating voltages, they are sensitive to magnetic fields and they have bulky shapes. Recently, a

new type of detectors, based on silicon diodes working in avalanche region has been developed and proved to be

the candidate to replace the existing vacuum based system. Some of the advantages offered by these solid state

devices are: higher quantum efficiency, lower operating voltage, insensitivity to the magnetic field,

compactness, robustness, and longer life span. [1] [2]

Introduction

A diode can be operated in two ways near the breakdown voltage for photon detection. If the bias voltage is

slightly below the operating voltage, then the device is called Avalanche Photodiode (APD). In this case, each

photon absorbed creates a finite number of electron-hole pair, so, the number of collected carriers is

proportional to the number of absorbed photons. In the second case, the bias voltage is set slightly above the

breakdown voltage, where the device is called Geiger mode APD (GM-APD). In this case, the electric field is so

high that a single charge carrier can trigger an avalanche, the carrier initiating the discharge can be either

thermally generated (noise) or photo generated (signal). An APD has a gain of few hundreds and that for a GM-

APD is 106 - 10

7 (gain comparable with PMTs, and this is the main advantage of working in Geiger Mode). The

problem in a single diode working as GM-APD is that the output signal is same, regardless of the number of

photons interacting.[3]

The GM-APD produces a standard signal when any of the cells fired. The amplitude Ai is

proportional to the capacitance of the cell (C) times the overvoltage,

… (1)

where, the overvoltage is defined as the difference between the operating bias voltage V and the breakdown

voltage Vb.

Silicon PhotoMultiplier (SiPM)

A SiPM is an array of multiple Avalanche Photodiodes, which are operated in the Geiger mode. These GM-

APDs are segmented into tiny microcells connected parallel to a single output. Each element, when activated by

a photon, gives the same current response, so the output signal is proportional to the number of cells hit by a

photon. The dynamic range is limited by the number of elements comprising the device, and the probability that

two or more photons hit the same microcell (pixel) depends on the size of pixel itself. This structure is called

Silicon PhotoMultiplier (SiPM). So, one can imagine each GM-APD pixel is being operated in binary mode,

which gives fixed signal charge, regardless of detected number of photons. The output signal from a SiPM is the

total charge of all signals coming from pixels, which is proportional to the number of detected photons [3] [4]

.So,

when many cells are fired at the same time, the output is the sum of the individual pulses

∑ … (2)

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However, the output signal is proportional to the number of fired pixels as long as the number of photons in a

pulse (Nphoton) times the photon detection efficiency (PDE) is significantly smaller than the number of cells

Ntotal.[5]

(

) … (3)

Operational Principle of SiPM

As mentioned the SiPM is a matrix of GM-APDs connected in parallel. A schematic representation is shown in

Figure 1.

Figure 1: Schematic diagram of the SiPM

A SiPM consists of an array of GM-APDs with integrated quenching resistors coupled to a common output. The

photon detection efficiency (PDE) of a SiPM is product of three parameters, fill factor (F), and quantum

efficiency [ԑ(λ)] and avalanche initialisation probability [ρ(V)].

… (3)

The fill factor is the fraction of total area of the array which is active (It is determined by size and numbers of

pixels). Quantum efficiency is a function of wavelength and Avalanche initialization probability is a function of

the bias voltage.

Reverse Current Voltage Characteristics:

We have studied the Reverse Current Voltage (I-V) characteristics of the SiPM (Hamamatsu MPPC, Type No.

S10931-050P(X), Sample no. 38, Vop=71.41V). The voltage is applied by using Keithley 6487 (voltage

source/picoammeter) in the voltage range of 500V (accuracy of the instrument in ±500V range is 0.15% +

40mV), and the current is also measured from the same instrument (as simultaneously serves as a picoammeter)

in the current range 20 μA (accuracy 0.1% + 1nA). The block diagram for the measurement of the I-V

characteristic is as shown in Figure 2.

Figure 2: Block diagram for the measurement of reverse current voltage characteristics

The Keithley 6487 is controlled by a LabVIEW program. We apply voltage from 0V to 73V in the steps of 0.1

V and the current measured by Keithely 6487 is also read by the same LabVIEW program simultaneously and

written to a text file. The current limit is given by LabVIEW program (15 μA). The program stops when the bias

voltage 73V is reached or the current limit exceeds.

The SiPM was placed in a metal box and properly wrapped inside a cloth for reducing the probability of external

light entering inside the box, and the box was placed inside an incubator (Model: Newtronics PT100) for

temperature variation. The thermometer (EL-USB-2-LCD, RH/Temp Data Logger) is placed inside the box with

SiPM to get temperature just around the SiPM.

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Figure 3: Reverse I-V Characteristics at temperature (23.5 0C, room temperature). Current in μA in Y-axis is plotted

in the logarithmic (base 10) scale

We varied the temperature inside the incubator (21.5 0C to 45

0C) and taken I-V characteristic at different

temperatures.

Figure 3 (a) Figure 3 (b)

Figure 3 (c) Figure 3 (d)

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Figure 3 (e) Figure 3 (f)

Figure 3 (g) Figure 3 (h)

Figure 3 (i) Figure 3 (j)

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Figure 3 (k) Figure 3 (l)

In Figures 3(a) to (l), we have plotted reverse IV characteristics of the SiPM at the different temperatures, and

hence found breakdown voltage of the SiPM for the corresponding temperature.

In Figure 4, shown below we have plotted the Breakdown Voltage vs. Temperature, and we have found a

positive temperature coefficient of value 50 ± 3mV per 0C.

Figure 4: Temperature vs. Breakdown Voltage plot

The temperature co-efficient of the breakdown voltage is given as the percentage change in the breakdown

voltage per degree centigrade change in the diode temperature. A positive co-efficient of temperature of

breakdown voltage is characteristic of Avalanche breakdown. However, when a true Zener breakdown is

involved the temperature co-efficient is negative.

A qualitative explanation of the sign (positive or negative) of the temperature coefficient:

A junction having a narrow depletion width (heavily doped), and hence high field intensity, will breakdown by

the Zener mechanism. An increase in temperature increases the energies of the valence electrons, and hence

makes it easier for electrons to escape from the covalent bonds. Less applied voltage is therefore needed to pull

these electrons to escape from their positions in the crystal lattice and convert them into conduction electrons.

Thus the Zener breakdown voltage decreases with the temperature.

A junction with a broad depletion layer width (lightly doped), and therefore a low field intensity, will

breakdown by the avalanche mechanism. In this case we rely on intrinsic carriers to collide with valence

electrons and to create avalanche multiplication. As the temperature increases, the vibrational displacement of

atoms in the crystal grows, which increases the probability of collisions with the lattice atoms of the intrinsic

particles as they cross the depletion width. The intrinsic holes and electrons thus have a less opportunity to gain

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sufficient energy between collisions to start the avalanche process. Therefore the value of the avalanche voltage

must increase with increased temperature.

Pedestal Response:

SiPM detectors are calibrated by measuring their photo-electron spectrum, which is a histogram of the charge

signal measured. When light (or thermally generated carrier) impinges on the detector, some SiPM pixels get

fired and the integrated charge is recorded. This measurement is recorded several times, and finally, a histogram

in well-defined peaks corresponding to number of pixel fired results. On occasion, when no pixel is fired the

integrated charge is small and is due to noise of the system i.e. leakage current shot noise, amplifier noise etc.

This charge forms the first peak of the spectrum and is referred to as the pedestal.

For getting the pedestal response of the SiPM (Hamamatsu MPPC, Type no. S10931-050P(x), Sample no. 35,

Vop=71.3), we appied bias voltage to SiPM by using Keithley 6487 (voltage source/picoammeter) in the voltage

range of 500V (accuracy of the instrument in ±500V range is 0.15% + 40mV), the current limit is fixed to 20

μA. The amplifier (25 X) was connected to SiPM and the voltage supply (of ±6V, 0V) is given by a regulated

power supply and the amplified signal is fed to QDC (CAEN, model-V792, 32channel QDC). From the

Arbitrary/Function Generator (Tektronics AFG 3252), square wave form of 1 KHz ( 1Vpp, offset 0V) is

generated and fed to Discriminator (LeCroy 623B), having threshold voltage = -30mV, which gives output of a

NIM pulse (-800mV and width of 100ns). The output of discriminator goes to a Timing Unit (CAEN N93B),

where we can tune our pulse width; the out coming pulse from the timing unit serves as a gate for the QDC.

Figure 5: Block diagram for taking pedestal response of the SiPM.

We have taken 106 events and plotted histogram for different bias voltages. The gain of the QDC is 100fC/count

and resolution is 12 bit. We have varied bias voltage from 70.9V to 71.9 V and the histograms are obtained in

Figure 6 (a) to (h) shown.

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Figure 6 (a) Figure 6 (b)

Figure 6 (c) Figure 6 (d)

Figure 6 (e) Figure 6 (f)

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Figure 6 (g) Figure 6 (h)

Figure 6: QDC spectra of SiPM for pedestal response

In Figure 6, pedestal peak (left most) and one photoelectron peak (corresponding to one pixel fired) are clearly

visible and distinguishable. Sometimes peak corresponding to two photoelectron (two pixel fired) is also visible

in the careful observation of figures 6 (d), (e), and, (f). In Table 1 below, we summarize our observation from

Figures 6 (a) to 6 (h).

Table 1

Figure VBIAS

(V)

Current

(μA) Pedestal Peak

Mean (μ)

1st P.E. Peak

Mean (μ)

Difference

∆μ

6 (a) 70.9 0.5 126.19 ± 0.04 139.1 ± 0.6 12.9 ± 0.6

6 (b) 71.1 0.8 122.81 ± 0.05 138.3 ± 0.5 15.5 ± 0.5

6 (c) 71.2 0.9 120.51 ± 0.07 137.7 ± 0.2 17.2 ± 0.2

6 (d) 71.3 1.1 118.20 ± 0.04 136.6 ± 0.2 18.4 ± 0.2

6 (e) 71.4 1.4 114.80 ± 0.04 135.2 ± 0.4 20.4 ± 0.4

6 (f) 71.5 1.6 111.17 ± 0.08 132.2 ± 0.2 21.0 ± 0.2

6 (g) 71.7 2.4 101.8 ± 0.1 124.1 ± 0.3 22.3 ± 0.3

6 (h) 71.9 3.5 89.3 ± 0.1 114.6 ± 0.8 25.3 ± 0.8

We can see clearly, distance between the pedestal peak and the 1st photoelectron peak increasing with increasing

bias voltage. We have looked into CRO also and found that pulse height increases with increasing bias voltage.

So, when the pulse is getting integrated by the QDC, it falls in the larger counts as it contains more charge.

Low Intensity LED response:

For getting the low intensity LED response of the SiPM (Hamamatsu MPPC, Type no. S10931-050P(x), Sample

no. 35, Vop=71.3), we apply bias voltage to SiPM by using Keithley 6487 (voltage source/picoammeter) in the

voltage range of 500V (accuracy of the instrument in ±500V range is 0.15% + 40mV), the current limit is fixed

to 20 μA. The amplifier (25 X) is connected to SiPM and the voltage supply (of ±6V, 0V) is given by a

regulated power supply and the amplified signal is fed to QDC (CAEN, model-V792, 32channel QDC). From

the Arbitrary/Function Generator (Agilent 33220A), Pulse wave form of 1 KHz (-3.430V, Pulse width 20ns ,

offset 0V) is applied to a LED (emitting blue light) and synchronised output (1KHz) from the Agilent 33220A

(TTL-output) fed to TTL to NIM convertor and then to Discriminator (LeCroy 623B), having threshold voltage

= -30mV, which gives output of a NIM pulse (-800mV and width of 100ns). The output of discriminator goes to

a Timing Unit (CAEN N93B), where we can tune our pulse width; the outcoming pulse from the timing unit

serves as a gate for the QDC.

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Figure 8: Block diagram for taking low intensity LED response of the SiPM.

We have taken 106 events and plotted histogram for different bias voltages. The gain of the QDC is 100fC/count

and resolution was 12 bit. We have varied bias voltage from 70.9V to 71.9 V and the histograms obtained are

shown in the Figures 9 (a) to (h) shown below. And observation made from these figures has been summarized

in Table 1.

Figure 9 (a) Figure 9 (b)

Figure 9 (c) Figure 9 (d)

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Figure 9 (e) Figure 9 (f)

Figure 9 (g) Figure 9 (h)

Table 2: Observations made on figure 9 (a) to (h)

Figure Voltage

(V)

Current

(μA)

Mean Avg. Diff. in

mean of

Consecutive

P.E.s

0 P.E. 1st P.E. 2

nd P.E. 3

rd P.E. 4

th P.E.

9(a) 70.9 0.49 127.4±0.1 143.0±0.1 158.2±0.1 173.2±0.4 - 15.3±0.4

9(b) 71.1 0.75 123.4±0.1 142.7±0.1 160.8±0.1 178.5±0.2 - 18.4±0.2

9(c) 71.2 0.91 120.7±0.2 141.4±0.1 161.5±0.1 180.6±0.3 199±1 20±1

9(d) 71.3 1.11 118.5±0.2 140.2±0.1 161.6±0.2 182.2±0.3 203.1±0.5 21.1±0.5

9(e) 71.4 1.33 115.4±0.2 139.0±0.1 161.4±0.2 183.2±0.3 203.3±0.8 22±0.8

9(f) 71.5 1.59 112.2±0.3 136.2±0.2 160.7±0.2 184.7±0.3 - 24.2±0.3

9(g) 71.7 2.28 102.8±0.4 129.5±0.3 156.6±0.4 183.2±0.7 210±1 27±1

9(h) 71.9 3.28 88±2 122±1 151±1 32±2

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As we can conclude from the observations made in the Table 2, the average separation between the P.E. peaks

increases linearly, with increasing the bias voltage applied across the SiPM, which is plotted in the Figure 13.

Figure 13: Linear variation of voltage gain of SiPM. Y-axis is the average distance between the photon peaks.

Estimating number of photons:

For estimating the number of photons in the high intensity LED with the SiPM (Hamamatsu MPPC, Type no.

S10931-050P(x), Sample no. 35, Vop=71.3), we applied bias voltage to SiPM by using Keithley 6487 (voltage

source/picoammeter) in the voltage range of 500V (accuracy of the instrument in ±500V range is 0.15% +

40mV), the current limit is fixed to 20 μA. The amplifier (25 X) is connected to SiPM and the voltage supply (of

±6V, 0V) is given by a regulated power supply and the amplified signal is fed to QDC (CAEN, model-V792,

32channel QDC). From the Arbitrary/Function Generator (Agilent 33220A), Pulse wave form of 1 KHz ( -

3.550V, Pulse width 20ns , offset 0V) is applied to a LED (emitting blue light) and synchronised output (1KHz)

from the Agilent 33220A (TTL-output) fed to TTL to NIM convertor and then to Discriminator (LeCroy 623B),

having threshold voltage = -30mV, which gives output of a NIM pulse (-800mV and width of 100ns). We want

to get the position of pedestal peak also, for this we applied an additional gate for pedestal of 100Hz frequency

(square wave, 1Vpp= 1.00V, offset = 0V) by the Arbitrary/Function Generator (Tektronix AFG 3252), output of

which goes to the same Discriminator (used for generating LED gate). The pulse coming out is OR gated with

the pulse for LED gate and fed to a Timing Unit (CAEN N93B), where we can tune our pulse width (in our case

it is 100ns); the outcoming pulse from the timing unit serves as a gate for the QDC.

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Figure 10: Block diagram for estimating number of photons from given LED pulse

We have looked into the CRO to the signal coming from the SiPM (amplified 25X by amplifier), which is

shown in figure 11 below.

Figure 11(a): CRO snapshot, LED (3.88mV/20ns)

Light given to the SiPM (Bias Voltage = 71.3V)

(Note : In the above figure the pulse was triggered at 110mV, Voltage scale (vertical) is 100mV/div and the time

scale (horizontal) is 50ns. ).

The Bias Voltage across the SiPM was fixed to 71.3V and below (in Figure 12,) is the histogram we have got

from (high intensity) LED (along with the pedestal distribution.)

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Figure 12(a): SiPM response on (high intensity) LED

In Figure 12, pedestal mean value is 117.1 ± 0.1 and mean of the photon distribution peak is 385.2 ± 0.4, So, the

separation between the pedestal peak and photon distribution peak is 268.1 ± 0.4, and from table 2, we have

obtained the average separation between each photo-electron peak (for 71.3V) is 21.1±0.5. So, estimated

number of photo-electrons can be obtained (12.71 ± 0.03) or, 13 photons per pulse.

Results and Discussion

We have found positive temperature coefficient (of value 50 ± 3mV per 0C) for the SiPM (Hamamatsu

MPPC, Type No. S10931-050P(X), Sample no. 38, Vop=71.41V), this confirms that breakdown in the

SiPM is by Avalanche Mechanism.

Gain of the SiPM varies linearly with the voltage applied. In table 1 (pedestal response) and table 2

(low intensity LED response), we can see this linear variation in the difference between the (mean of)

photon peaks.

We have also used SiPM as a photon counting device.

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Appendix

(A) To build an interactive user interface for instruments used in our experiment such as, Function

Generator (Tektronix AFG 3252), Voltage Source/Picoammeter (Keithley 6487), CAEN VME 8011 in

LabVIEW

Introduction: LabVIEW uses a graphical programming language “G”, to create programs in block diagram

form. In LabVIEW, there are libraries of functions designed specifically for instrument control and data

acquisition. LabVIEW programs are called Virtual Instruments (VIs).

VI contains an interactive user interface, called front panel. It simulates the panel of a physical

instrument. Figure A.1 shows a typical front panel of a VI.

The source code of a VI is in Block Diagram, in a pictorial form. A typical block diagram of a VI looks

like as shown in figure A.2.

VI uses a hierarchical and modular structure. We can use a VI (which is called SubVI) in a top level

VI. SubVI is analogous to subroutine in C.[1]

Figure A.1: Front Panel of my VI

My Assignment: We wanted an interactive user interface for characterising Silicon Photomultiplier (SiPM). We

were interested in designing VIs for instrumental control and data acquisition for getting Current-Voltage (I-V)

Characteristic, pedestal response and LED response of SiPM.

My task was to build a VI, which can generate two pulse waveforms at different channels of arbitrary function

generator (Tektronix AFG 3252), one for LED and other for TTL. And, to integrate my VI as a subVI to the top

level VI.

A brief description of my VI: we left a very few chance for a user to commit mistake and spoil the instrument

(SiPM or LED). The VI is interactive also; it pops the warnings and messages when user gives any wrong entry

or if some connection is not properly done.

When the VI is operated and we put ON initialize button (please see figure A.1), it checks the connection of

GPIB cable, and if the connection is proper, then only it allow user to input some data.

The VI allows user to input data in specified range only. Allowed range for Frequency is 10Hz to 1000Hz, for

LED pulse width is 10ns to 100ns, for TTL pulse width is 10ns to 1000ns; LED pulse amplitude is 0.01V to 3V

and pulse height of TTL pulse was made constant at 5.00V.

When user gives input and presses the button “Data in”, the VI check all the input, whether it is in the range or

not. The instrument Tektronix AFG 3252 cannot generate pulses of duty cycle lesser than 0.001%, so, VI checks

for the duty cycle of the pulse. If the input is in the specified range and also it has duty cycle greater than

0.001% then only VI allows input data to Arbitrary Function generator (Tektronix AFG 3252).

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Acknowledgement

I am highly indebted to Prof. Shashi Dugad for his valuable suggestions. He was always available for

discussion on the physics part of our project. I thank Mr. K. C. Ravindran for introducing the

experiment, for his kind support, and for providing me study materials. He kept me motivated

throughout the work and guided me in the best possible manner at the GRAPES-3 experimental site,

in Ooty.

I owe my sincerest gratitude to Prof. S. K. Gupta for providing me a golden opportunity to work at the

GRAPES-3 experimental site and to Dr. Gagan B. Mohanty for discussing the experiment and going

through the project report and correcting it.

Last, but not the least, I thank Mr. Sanket Kamthe for helping me in LabVIEW and whole GRAPES -

3 Group for making my Ooty-tour comfortable.

Reference:

1. “The Silicon Photomultiplier- A new device for high energy physics, Astropartice Physics,

Industrial and Medical Application” , SNIC symposium, Stanford, California, April 2006

2. “Development of the first prototype of Silicon photomultiplier at ITC-irst”, N Dinu et al.,

Nuclear Instruments and Methods in Physics Research (A) 572 (2007) 422-426.

3. “A new Silicon Photomultiplier Structure for blue light detection”, C. Piemonte, Nuclear

Instruments and Methods in Physics Research (A) 568 (2006) 224-232.

4. “SiPM development and application for astroparticle physics experiments”, H. Miyamoto,

Proceedings of the 31st ICRC, Lodz,2009

5. “Geiger-mode avalanche photodiodes, history, properties and problems”, D. Renker,

Nuclear Instruments and Methods in Physics Research (A) 567 (2006) 48-56.

6. “Integrated Electronics: Analog and digital circuits and system ”, J. Millman and C Halkias,

International student edition, Mc Graw-Hill

7. LabVIEW Tutorial Manual, January 1996 Edition.