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Temperature-compensated silicon photomultiplier
Nuclear Inst. and Methods in Physics Research, A (
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Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Temperature-compensated silicon photomultiplier Evgeny Kuznetsov * University of Alabama in Huntsville, Huntsville, AL 35899, USA
a r t i c l e
i n f o
Keywords: Silicon photomultipliers SiPM temperature dependence
a b s t r a c t Silicon PhotoMultipliers (SiPM) have significant temperature dependence of important parameters such as gain, photon detection efficiency and optical crosstalk. Special temperature-controlled power supplies and off-line correction are typically used to mitigate this temperature dependence, though these methods are still not optimal for multichannel photodetectors exposed to significant temperature variations. We report on a design concept for a custom SiPM chip with an integrated thermo-compensation circuit that automatically adjusts an applied bias voltage thus maintaining a stable SiPM overvoltage value and gain over a wide range of temperatures. A prototype of the thermo-compensated SiPM chip was built using commercially available discrete components and tested in the temperature range from −30 to +30 ◦ C. The measured temperature dependence of the thermo-compensated SiPM chip was reduced by factor 38 in comparison with the non-compensated device. © 2017 Elsevier B.V. All rights reserved.
1. Introduction In recent years photodetectors on SiPM sensors have been widely used in medical, aerospace, scientific and industrial devices [1–3]. One major disadvantage of SiPM sensors is their temperature dependence, which requires implementation of temperature stabilizing systems, offline data correction based on recorded temperature logs or use of special temperature-controlled power supplies that are unique for each system [4,5]. Reported values of SiPM breakdown voltage temperature dependence range from 18.3 mV/◦ C with Vbd = 28.5 V or 649 ppm/◦ C [6] to 70.2 mV/◦ C with Vbd = 34.5 V or 2035 ppm/◦ C [5]. The temperature dependence reported in [7] is 27.5 mV/◦ C with Vbd = 28.5 V measured at 20 ◦ C or 965 ppm/◦ C. There are several techniques used for building temperature-controlled power supplies for SiPMs. The most common approach is to use signals from discrete temperature sensors mounted at most critical locations near the SiPM sensors [8]. In some applications, averaged SiPM current is used as a temperature-sensing signal for the adjustment of a bias supply voltage [9]. Implementation of such a system allows the temperature dependence of the SiPM gain to be reduced to 0.78% per 10 ◦ C of the temperature change. A blind SiPM in a light-tight package was also proposed as a temperature sensor, providing input for a temperature-controlled SiPM power supply [10]. All these thermocompensating systems require additional resources and are not optimal for large aperture photodetectors consisting of SiPM chips with varying parameters such as breakdown voltage, leakage current and gain. In this paper, we propose a design concept for a custom SiPM sensor containing
a thermo-compensation circuit fabricated on the same die with the SiPM matrix. We describe two types of on-chip temperature sensors. One is based on a thermistor and the second is based on serially connected p–n junctions. Preliminary tests were done on prototypes built with discrete components and a Hamamatsu SiPM sensor, Model# S1036233-25C. The integrated thermo-compensation circuit automatically adjusts applied SiPM bias voltage, thus maintaining a stable overvoltage value and gain. This approach eliminates requirements for temperaturecontrolled power supplies. Residual temperature dependence of the custom chip can be easily adjusted after the device is manufactured. SiPM overvoltage values can also be precisely adjusted by application of a low-level DC voltage at the SiPM control input. This feature allows the use of SiPM chips with varying breakdown voltages in multichannel photodetectors while utilizing a common voltage supply for all SiPM sensors. The proposed custom SiPM chip is not limited to CMOS implementation. Components of the integrated thermo-compensation circuit (p–n junctions, resistors) can be manufactured using the same technological process as that of the SiPM micro-pixels. 2. Design concept Fig. 1 shows a basic concept of the SiPM sensor with on-chip thermocompensation circuit. The temperature sensor is manufactured on the same substrate as the SiPM matrix and provides a prompt response for temperature changes of the SiPM chip. The on-chip Ubias adjustment
* Correspondence to: 320 Sparkman Drive, NSSTC, ZP-12, Huntsville, AL, 35805, USA.
E-mail address: [email protected]. https://doi.org/10.1016/j.nima.2017.11.060 Received 20 September 2017; Received in revised form 11 November 2017; Accepted 17 November 2017 Available online xxxx 0168-9002/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: E. Kuznetsov, Temperature-compensated silicon photomultiplier, Nuclear Inst. and Methods in Physics Research, A (2017), https://doi.org/10.1016/j.nima.2017.11.060.
E. Kuznetsov
Nuclear Inst. and Methods in Physics Research, A (
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Fig. 1. Basic functional diagram of the thermo-compensated SiPM. Fig. 4. Circuit diagram of the thermistor-based steering circuit.
Fig. 2. Thermo-compensated SiPM with on-chip thermistor. Fig. 5. Temperature dependence characteristic of the thermistor-based steering circuit. Linear trend line defines its coefficient, −20.127 mV/◦ C.
The second type of the temperature sensor contains a series of diodes powered in direct polarity. Each silicon diode has a specific temperature dependence coefficient for the forward voltage equal to −2.3 ⋯ −2.5 mV/◦ C. The total temperature dependence of such a steering circuit can be adjusted by choosing the required number of diodes in the chain. A circuit diagram of the SiPM sensor with this type of thermocompensation circuit is shown in Fig. 3. The current source provides a stable current through the temperature sensor based on the diode chain. The subtracting circuit subtracts the total forward voltage drop over the chain of diodes from the bias voltage, Ubias.
Fig. 3. SiPM with thermo-compensation on serially connected p–n junctions.
3. Measurement of the temperature dependences circuit adjusts the voltage applied at the SiPM matrix in accordance with the signal from the temperature sensor. The temperature dependence coefficient of the temperature sensor is adjusted to match the temperature dependence of the SiPM breakdown voltage, thus maintaining a constant overvoltage value (Ubias U_SiPM_breakdown) with temperature change. Two types of temperature sensors were investigated in this work: (1) an on-chip thermistor manufactured using standard CMOS technology; (2) a chain of serially connected p–n junctions or diodes powered in the direct polarity. In the first case, the thermistor’s temperature dependence coefficient (TCR) should exceed the temperature dependence of the SiPM breakdown voltage. As noted above, the reported values of SiPM breakdown voltage temperature dependence range from 649 ppm/◦ C to 2035 ppm/◦ C. Standard N-well type CMOS resistors with TCR in the range of 2000 to 3000 ppm/◦ C meet these requirements. A circuit diagram of the steering circuit based on the on-chip thermistor, Rt, is shown in Fig. 2. The external low-TCR resistor, Rext, can be used to adjust the temperature dependence of this steering circuit to match temperature characteristics of the SiPM matrix.
All measurement results for preliminary evaluation of the proposed design concept presented in this paper were acquired with test prototypes built on discrete components and a Hamamatsu SiPM, Model # S10362-33-25C. Two types of the thermo-compensation circuits were evaluated in the temperature range from −30 to +30 ◦ C. Both circuits were initially designed to obtain temperature dependence equal to −21 mV/◦ C. Additional tests with the Hamamatsu SiPM were conducted using the thermocompensation circuit based on the serially connected p–n junctions. This circuit was adjusted to achieve a temperature dependence of −58 mV/◦ C to match characteristics of the SiPM sample. Fig. 4 shows the circuit diagram used to test the thermistorbased steering circuit. A standard 10 kΩ NTC thermistor (type — NTSS0603E3103FLT from Vishay) was used as a temperature sensor. The value of the parallel resistor was chosen to be 24 kΩ in order to achieve the required the temperature dependence. The U_adj versus temperature relationship was calculated for the circuit shown in Fig. 4 using the specified temperature dependence of the thermistor resistivity, Ubias = 30 V and with 86 μA of current through the resistor, R1. 2
Please cite this article in press as: E. Kuznetsov, Temperature-compensated silicon photomultiplier, Nuclear Inst. and Methods in Physics Research, A (2017), https://doi.org/10.1016/j.nima.2017.11.060.
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Nuclear Inst. and Methods in Physics Research, A (
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Fig. 6. Temperature dependence characteristic of the steering circuit using eight serially connected p–n junctions. Linear trend line defines its coefficient, −18.718 mV/◦ C.
Fig. 8. The test prototype of the SiPM sensor with the thermo-compensation circuit.
This type of SiPM has temperature dependence for the breakdown voltage of 58 mV/◦ C. To compensate for this temperature dependence, a chain of 22 serially connected n–p–n transistors, MMBT5088, with connected collector-base terminals was used. The plastic scintillator detector was exposed to a radioisotope 241 Am with a distinct 5.49 MeV alpha peak in order to establish a stable signal source. The SiPM response to the light produced in the plastic scintillator detector was acquired with a LeCroy oscilloscope. Two types of spectra were acquired for each temperature: (1) SiPM pulse amplitude and (2) pulse integrated over 600 ns. Spectra for the base line DC offset were measured as well for further subtraction. Fig. 9 shows the spectra of the noncompensated SiPM pulse area integrated over 600 ns, acquired at two settled temperatures. The temperature dependence of the test article was calculated using the temperature drift of the 241 Am alpha peak. The temperature dependence of the SiPM output signal measured using this technique was determined more precisely than by measuring the SiPM breakdown voltage temperature dependence. Another advantage of this approach is that it allows for the measurement of the complex temperature dependence of the SiPM output, including the temperature dependences of the gain, avalanche probability, optical crosstalk and afterpulsing. Two temperature sensors were used to measure temperatures of the SiPM ceramic package and the PC board with the electronic circuit. Data were acquired when temperature readings from both sensors reached equilibrium with 0.1 ◦ C precision.
Fig. 7. Temperature dependence of the compensation circuit for various currents through the diode chain (Is) and corresponding slope (S) of the U_SiPM control characteristic.
Fig. 5 shows the calculated values and measurement results of the test article in temperature. Measured results are in a good agreement with calculated values. The second type of steering circuit was built using eight serially connected p–n junctions. Type n–p–n transistors, MMBT5088, with shortened base-collector terminals, were used as single p–n junctions. Measurement results for the steering circuit of this type are presented in Fig. 6. The temperature drift of a single p–n junction forward voltage at 10 μA flowing current, was measured to be −2.34 mV/◦ C. The total temperature dependence of this steering circuit, including temperature dependence of the adjusting transistor, was determined to be −21.05 mV/◦ C. Further tests of SiPM sensors were conducted with this type of the thermo-compensation circuit, owing to its good linearity over the whole temperature range and the ease of adjustments of the temperature dependence by selecting the appropriate number of diodes in the chain. The forward voltage of the p–n junction also depends slightly on the current flowing through it. For the additional tests with a Hamamatsu SiPM sensor, a thermo-compensation circuit based on 22 p–n junctions was built. It was also characterized for temperature response at different current values from 14.15 to 73.7 μA through the diode chain (Is). The results of these measurements are shown in Fig. 7. A test prototype of the SiPM sensor with the thermo-compensating circuit was built using commercially available discrete components and tested in a high-precision temperature chamber. A Hamamatsu SiPM sensor, Model# S10362-33-25C, with a 3 × 3 mm active aperture was setup to view a plastic scintillator detector, as shown in Fig. 8, and mounted on a printed circuit board with a thermo-compensation circuit.
4. Measurement results The temperature dependence of the SiPM with a thermocompensation circuit based on 22 serially connected p–n junctions was improved from −2.2%/◦ C to 0.058%/◦ C for the SiPM pulse amplitude, compared to a non-compensated SiPM. Measurement results are presented in Fig. 10. 3
Please cite this article in press as: E. Kuznetsov, Temperature-compensated silicon photomultiplier, Nuclear Inst. and Methods in Physics Research, A (2017), https://doi.org/10.1016/j.nima.2017.11.060.
E. Kuznetsov
Nuclear Inst. and Methods in Physics Research, A (
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Fig. 9. 241 Am spectrum acquired at two different temperatures with the Hamamatsu SiPM, S10362-33-25C without temperature compensation. Trace C1 — SiPM response; F2 (magenta) — spectrum of the SiPM pulse integrated over 600 ns; F3 (blue) — base line integral over 600 ns prior to trigger. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The temperature stability of the SiPM pulse area integrated over 600 ns was measured at the same time. Temperature drift of the thermocompensated SiPM output was measured to be −0.20%/◦ C (See Fig. 11.). The less favorable temperature compensation can be explained by the temperature dependence of the quenching resistors that define the pulse shape at the SiPM output. In [6] the value of quenching resistors was shown to decrease with −0.66%/◦ C rate at −20 ◦ C and −0.83%/◦ C at +20 ◦ C. Another factor that contributes to the temperature dependence of the SiPM pulse integrated over 600 ns is the temperature dependence of the SiPM afterpulses that fall within this 600 ns integration time frame. For applications measuring integrated SiPM response, compensation of this residual temperature dependence (−0.20%/◦ C) can be improved by increasing the number of temperature sensing diodes in the chain from 22 to 23 or 24 diodes. An example of such application is a CsI(Tl) scintillator charged-particle detector which has a light pulse response with a decay time constant of more than 1 μs.
Fig. 10. SiPM pulse amplitude temperature stability as characterized by the temperature drift of the mean value of the 241 Am peak.
5. Adjustments of the SiPM parameters The SiPM sensor with integrated thermo-compensation circuit can also be constructed using a simple circuit that allows adjustments of the temperature compensation coefficient and the overvoltage value. Fig. 12 shows a circuit diagram of the thermo-compensated SiPM chip with adjustment capabilities. The temperature compensation coefficient can be adjusted by installing a wire jumper between any of two Dn terminals, thereby excluding diodes from the temperature sensing diode chain and reducing the temperature compensation factor. This allows the adjustment of the SiPM temperature dependence in accordance with a specific application — (1) to achieve higher temperature compensation factor for applications where SiPM pulse integration is required and (2) to lower temperature compensation for devices measuring SiPM pulse amplitudes.
Fig. 11. Temperature stability of the SiPM pulse area integrated over 600 ns as characterized by the temperature drift of the mean value of the 241 Am peak.
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flux on the order of 0.67 Hz per single pixel was measured at pixel currents 46 μA and chip temperature +60.8 ◦ C. When the direct current through the SiPM was set at 36 mA, corresponding to single pixel current of 40 μA, the sensor temperature increased to +53 ◦ C and the count rate increased to 350 Hz after 2 min. After 10 min, the temperature increased to +60.8 ◦ C and count rate increased to 600 Hz. The rise of the sensor temperature during tests at high direct currents is explained by heating of the 900 SiPM quenching resistors. 7. Conclusions A chain of serially connected p–n junctions, integrated on the same chip with a SiPM matrix, can be used to compensate for the temperature dependence of the SiPM breakdown voltage. The temperature correction coefficient can be easily adjusted to compensate for the temperature dependence of secondary SiPM parameters such as afterpulsing and pulse recovery time constant. The on-chip bias adjustment circuit can be used to tune varying breakdown voltages of SiPM sensors by the application of a low-voltage DC potential at the SiPM control terminals. This makes it possible to achieve similar channel gains while using a common bias supply in multichannel photodetectors. It was proven that the diode chain does not emit photons at direct DC currents up to 30 μA and temperatures up to +30 ◦ C.
Fig. 12. SiPM chip with thermo-compensation and adjustment capabilities.
Multichannel photodetectors might include SiPM sensors with varying breakdown voltage parameters. To achieve similar gain-noise parameters for different channels, the applied bias voltage must be adjusted individually for each SiPM sensor, according to its breakdown voltage, to obtain the same overvoltage value for all channels. The device shown in Fig. 12 has the capability of tuning the voltage applied at the SiPM matrix by the application of a certain control voltage at the Udc terminal. This control voltage can be used to set the current through the temperature sensor (Its = Udc/R2) and also to adjust the voltage applied at the silicon matrix U_SiPM. The adjustment voltage is a voltage drop over resistor, R1 (𝑈𝑅1 = 𝐼𝑡𝑠 ∗ 𝑅1). The temperature dependence of the resistors R1 and R2 does not change the temperature dependence of the adjustment voltage if values of these resistors are made equal.
Acknowledgments This work was supported by the University of Alabama in Huntsville Charger Innovation Fund [grant number ED16HDQ0200012]. The technology is patent pending. The author would like to thank James Douglas Tubbs for his help and John W. Watts for his comments to this manuscript.
6. Photon emission test of the thermo-compensation circuit
Appendix A. Supplementary data
To test the light-emitting feature of the p–n junctions powered in direct polarity a SiPM sensor, which in fact represents an array of p– n junctions, was used. A Hamamatsu SiPM sensor, Model#S10362-33100C with 900 pixels of 100 × 100 μm each was attached to the PMT entrance window. First, the SiPM sensor was tested for the ability to emit photons in a standard reversed-biasing polarity. The PMT response was measured using a LeCroy oscilloscope configured in pulse counting mode. A SiPM upward-emitted photon flux on the order of 10% of its specified dark count rate was measured using this technique. The SiPM sensor was then connected to the bias power supply in direct polarity, and the photon flux emitted by the SiPM was measured. Measurements were conducted for the current flowing through each of the 900 pixels from 0 to 40 μA. No upward-emitted photons were measured for pixel currents up to 30 μA and temperatures up to +30 ◦ C. A slight increase in the photon
Supplementary material related to this article can be found online at https://doi.org/10.1016/j.nima.2017.11.060. References [1] B. Seitz, et al., Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), IEEE, 2013. http://dx.doi.org/10.1109/NSSMIC.2013.6829685. [2] C.M. Whitney, et al., Solar Physics and Space Weather Instrumentation VI, 2015, p. 960405. http://dx.doi.org/10.1117/12.2188384. [3] R. Agishev, et al., Opt. Laser Technol. 49 (2013) 86–90. [4] R. Shukla, et al., IJERGS 2 (4) (2014) 599–603. [5] M. Petasecca, et al., IEEE Trans. Nucl. Sci. 55 (3) (2008) 1686–1690. [6] P.K. Lightfoot, JINST 3 (2008) P10001. [7] A. Ferri, et al., JINST 9 (2014) P06018. [8] A. Kaplan, et al., Nucl. Instrum. Methods Phys. Res. A 610 (1) (2009) 114–117. [9] F. Licciulli, C. Marzocca, IEEE Trans. Nucl. Sci. 62 (1) (2015) 228–235. [10] F. Licciulli, et al., IEEE Trans. Nucl. Sci. 60 (2) (2013) 606–611.
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Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Temperature-compensated silicon photomultiplier Evgeny Kuznetsov * University of Alabama in Huntsville, Huntsville, AL 35899, USA
a r t i c l e
i n f o
Keywords: Silicon photomultipliers SiPM temperature dependence
a b s t r a c t Silicon PhotoMultipliers (SiPM) have significant temperature dependence of important parameters such as gain, photon detection efficiency and optical crosstalk. Special temperature-controlled power supplies and off-line correction are typically used to mitigate this temperature dependence, though these methods are still not optimal for multichannel photodetectors exposed to significant temperature variations. We report on a design concept for a custom SiPM chip with an integrated thermo-compensation circuit that automatically adjusts an applied bias voltage thus maintaining a stable SiPM overvoltage value and gain over a wide range of temperatures. A prototype of the thermo-compensated SiPM chip was built using commercially available discrete components and tested in the temperature range from −30 to +30 ◦ C. The measured temperature dependence of the thermo-compensated SiPM chip was reduced by factor 38 in comparison with the non-compensated device. © 2017 Elsevier B.V. All rights reserved.
1. Introduction In recent years photodetectors on SiPM sensors have been widely used in medical, aerospace, scientific and industrial devices [1–3]. One major disadvantage of SiPM sensors is their temperature dependence, which requires implementation of temperature stabilizing systems, offline data correction based on recorded temperature logs or use of special temperature-controlled power supplies that are unique for each system [4,5]. Reported values of SiPM breakdown voltage temperature dependence range from 18.3 mV/◦ C with Vbd = 28.5 V or 649 ppm/◦ C [6] to 70.2 mV/◦ C with Vbd = 34.5 V or 2035 ppm/◦ C [5]. The temperature dependence reported in [7] is 27.5 mV/◦ C with Vbd = 28.5 V measured at 20 ◦ C or 965 ppm/◦ C. There are several techniques used for building temperature-controlled power supplies for SiPMs. The most common approach is to use signals from discrete temperature sensors mounted at most critical locations near the SiPM sensors [8]. In some applications, averaged SiPM current is used as a temperature-sensing signal for the adjustment of a bias supply voltage [9]. Implementation of such a system allows the temperature dependence of the SiPM gain to be reduced to 0.78% per 10 ◦ C of the temperature change. A blind SiPM in a light-tight package was also proposed as a temperature sensor, providing input for a temperature-controlled SiPM power supply [10]. All these thermocompensating systems require additional resources and are not optimal for large aperture photodetectors consisting of SiPM chips with varying parameters such as breakdown voltage, leakage current and gain. In this paper, we propose a design concept for a custom SiPM sensor containing
a thermo-compensation circuit fabricated on the same die with the SiPM matrix. We describe two types of on-chip temperature sensors. One is based on a thermistor and the second is based on serially connected p–n junctions. Preliminary tests were done on prototypes built with discrete components and a Hamamatsu SiPM sensor, Model# S1036233-25C. The integrated thermo-compensation circuit automatically adjusts applied SiPM bias voltage, thus maintaining a stable overvoltage value and gain. This approach eliminates requirements for temperaturecontrolled power supplies. Residual temperature dependence of the custom chip can be easily adjusted after the device is manufactured. SiPM overvoltage values can also be precisely adjusted by application of a low-level DC voltage at the SiPM control input. This feature allows the use of SiPM chips with varying breakdown voltages in multichannel photodetectors while utilizing a common voltage supply for all SiPM sensors. The proposed custom SiPM chip is not limited to CMOS implementation. Components of the integrated thermo-compensation circuit (p–n junctions, resistors) can be manufactured using the same technological process as that of the SiPM micro-pixels. 2. Design concept Fig. 1 shows a basic concept of the SiPM sensor with on-chip thermocompensation circuit. The temperature sensor is manufactured on the same substrate as the SiPM matrix and provides a prompt response for temperature changes of the SiPM chip. The on-chip Ubias adjustment
* Correspondence to: 320 Sparkman Drive, NSSTC, ZP-12, Huntsville, AL, 35805, USA.
E-mail address: [email protected]. https://doi.org/10.1016/j.nima.2017.11.060 Received 20 September 2017; Received in revised form 11 November 2017; Accepted 17 November 2017 Available online xxxx 0168-9002/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: E. Kuznetsov, Temperature-compensated silicon photomultiplier, Nuclear Inst. and Methods in Physics Research, A (2017), https://doi.org/10.1016/j.nima.2017.11.060.
E. Kuznetsov
Nuclear Inst. and Methods in Physics Research, A (
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Fig. 1. Basic functional diagram of the thermo-compensated SiPM. Fig. 4. Circuit diagram of the thermistor-based steering circuit.
Fig. 2. Thermo-compensated SiPM with on-chip thermistor. Fig. 5. Temperature dependence characteristic of the thermistor-based steering circuit. Linear trend line defines its coefficient, −20.127 mV/◦ C.
The second type of the temperature sensor contains a series of diodes powered in direct polarity. Each silicon diode has a specific temperature dependence coefficient for the forward voltage equal to −2.3 ⋯ −2.5 mV/◦ C. The total temperature dependence of such a steering circuit can be adjusted by choosing the required number of diodes in the chain. A circuit diagram of the SiPM sensor with this type of thermocompensation circuit is shown in Fig. 3. The current source provides a stable current through the temperature sensor based on the diode chain. The subtracting circuit subtracts the total forward voltage drop over the chain of diodes from the bias voltage, Ubias.
Fig. 3. SiPM with thermo-compensation on serially connected p–n junctions.
3. Measurement of the temperature dependences circuit adjusts the voltage applied at the SiPM matrix in accordance with the signal from the temperature sensor. The temperature dependence coefficient of the temperature sensor is adjusted to match the temperature dependence of the SiPM breakdown voltage, thus maintaining a constant overvoltage value (Ubias U_SiPM_breakdown) with temperature change. Two types of temperature sensors were investigated in this work: (1) an on-chip thermistor manufactured using standard CMOS technology; (2) a chain of serially connected p–n junctions or diodes powered in the direct polarity. In the first case, the thermistor’s temperature dependence coefficient (TCR) should exceed the temperature dependence of the SiPM breakdown voltage. As noted above, the reported values of SiPM breakdown voltage temperature dependence range from 649 ppm/◦ C to 2035 ppm/◦ C. Standard N-well type CMOS resistors with TCR in the range of 2000 to 3000 ppm/◦ C meet these requirements. A circuit diagram of the steering circuit based on the on-chip thermistor, Rt, is shown in Fig. 2. The external low-TCR resistor, Rext, can be used to adjust the temperature dependence of this steering circuit to match temperature characteristics of the SiPM matrix.
All measurement results for preliminary evaluation of the proposed design concept presented in this paper were acquired with test prototypes built on discrete components and a Hamamatsu SiPM, Model # S10362-33-25C. Two types of the thermo-compensation circuits were evaluated in the temperature range from −30 to +30 ◦ C. Both circuits were initially designed to obtain temperature dependence equal to −21 mV/◦ C. Additional tests with the Hamamatsu SiPM were conducted using the thermocompensation circuit based on the serially connected p–n junctions. This circuit was adjusted to achieve a temperature dependence of −58 mV/◦ C to match characteristics of the SiPM sample. Fig. 4 shows the circuit diagram used to test the thermistorbased steering circuit. A standard 10 kΩ NTC thermistor (type — NTSS0603E3103FLT from Vishay) was used as a temperature sensor. The value of the parallel resistor was chosen to be 24 kΩ in order to achieve the required the temperature dependence. The U_adj versus temperature relationship was calculated for the circuit shown in Fig. 4 using the specified temperature dependence of the thermistor resistivity, Ubias = 30 V and with 86 μA of current through the resistor, R1. 2
Please cite this article in press as: E. Kuznetsov, Temperature-compensated silicon photomultiplier, Nuclear Inst. and Methods in Physics Research, A (2017), https://doi.org/10.1016/j.nima.2017.11.060.
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Nuclear Inst. and Methods in Physics Research, A (
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Fig. 6. Temperature dependence characteristic of the steering circuit using eight serially connected p–n junctions. Linear trend line defines its coefficient, −18.718 mV/◦ C.
Fig. 8. The test prototype of the SiPM sensor with the thermo-compensation circuit.
This type of SiPM has temperature dependence for the breakdown voltage of 58 mV/◦ C. To compensate for this temperature dependence, a chain of 22 serially connected n–p–n transistors, MMBT5088, with connected collector-base terminals was used. The plastic scintillator detector was exposed to a radioisotope 241 Am with a distinct 5.49 MeV alpha peak in order to establish a stable signal source. The SiPM response to the light produced in the plastic scintillator detector was acquired with a LeCroy oscilloscope. Two types of spectra were acquired for each temperature: (1) SiPM pulse amplitude and (2) pulse integrated over 600 ns. Spectra for the base line DC offset were measured as well for further subtraction. Fig. 9 shows the spectra of the noncompensated SiPM pulse area integrated over 600 ns, acquired at two settled temperatures. The temperature dependence of the test article was calculated using the temperature drift of the 241 Am alpha peak. The temperature dependence of the SiPM output signal measured using this technique was determined more precisely than by measuring the SiPM breakdown voltage temperature dependence. Another advantage of this approach is that it allows for the measurement of the complex temperature dependence of the SiPM output, including the temperature dependences of the gain, avalanche probability, optical crosstalk and afterpulsing. Two temperature sensors were used to measure temperatures of the SiPM ceramic package and the PC board with the electronic circuit. Data were acquired when temperature readings from both sensors reached equilibrium with 0.1 ◦ C precision.
Fig. 7. Temperature dependence of the compensation circuit for various currents through the diode chain (Is) and corresponding slope (S) of the U_SiPM control characteristic.
Fig. 5 shows the calculated values and measurement results of the test article in temperature. Measured results are in a good agreement with calculated values. The second type of steering circuit was built using eight serially connected p–n junctions. Type n–p–n transistors, MMBT5088, with shortened base-collector terminals, were used as single p–n junctions. Measurement results for the steering circuit of this type are presented in Fig. 6. The temperature drift of a single p–n junction forward voltage at 10 μA flowing current, was measured to be −2.34 mV/◦ C. The total temperature dependence of this steering circuit, including temperature dependence of the adjusting transistor, was determined to be −21.05 mV/◦ C. Further tests of SiPM sensors were conducted with this type of the thermo-compensation circuit, owing to its good linearity over the whole temperature range and the ease of adjustments of the temperature dependence by selecting the appropriate number of diodes in the chain. The forward voltage of the p–n junction also depends slightly on the current flowing through it. For the additional tests with a Hamamatsu SiPM sensor, a thermo-compensation circuit based on 22 p–n junctions was built. It was also characterized for temperature response at different current values from 14.15 to 73.7 μA through the diode chain (Is). The results of these measurements are shown in Fig. 7. A test prototype of the SiPM sensor with the thermo-compensating circuit was built using commercially available discrete components and tested in a high-precision temperature chamber. A Hamamatsu SiPM sensor, Model# S10362-33-25C, with a 3 × 3 mm active aperture was setup to view a plastic scintillator detector, as shown in Fig. 8, and mounted on a printed circuit board with a thermo-compensation circuit.
4. Measurement results The temperature dependence of the SiPM with a thermocompensation circuit based on 22 serially connected p–n junctions was improved from −2.2%/◦ C to 0.058%/◦ C for the SiPM pulse amplitude, compared to a non-compensated SiPM. Measurement results are presented in Fig. 10. 3
Please cite this article in press as: E. Kuznetsov, Temperature-compensated silicon photomultiplier, Nuclear Inst. and Methods in Physics Research, A (2017), https://doi.org/10.1016/j.nima.2017.11.060.
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Nuclear Inst. and Methods in Physics Research, A (
)
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Fig. 9. 241 Am spectrum acquired at two different temperatures with the Hamamatsu SiPM, S10362-33-25C without temperature compensation. Trace C1 — SiPM response; F2 (magenta) — spectrum of the SiPM pulse integrated over 600 ns; F3 (blue) — base line integral over 600 ns prior to trigger. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The temperature stability of the SiPM pulse area integrated over 600 ns was measured at the same time. Temperature drift of the thermocompensated SiPM output was measured to be −0.20%/◦ C (See Fig. 11.). The less favorable temperature compensation can be explained by the temperature dependence of the quenching resistors that define the pulse shape at the SiPM output. In [6] the value of quenching resistors was shown to decrease with −0.66%/◦ C rate at −20 ◦ C and −0.83%/◦ C at +20 ◦ C. Another factor that contributes to the temperature dependence of the SiPM pulse integrated over 600 ns is the temperature dependence of the SiPM afterpulses that fall within this 600 ns integration time frame. For applications measuring integrated SiPM response, compensation of this residual temperature dependence (−0.20%/◦ C) can be improved by increasing the number of temperature sensing diodes in the chain from 22 to 23 or 24 diodes. An example of such application is a CsI(Tl) scintillator charged-particle detector which has a light pulse response with a decay time constant of more than 1 μs.
Fig. 10. SiPM pulse amplitude temperature stability as characterized by the temperature drift of the mean value of the 241 Am peak.
5. Adjustments of the SiPM parameters The SiPM sensor with integrated thermo-compensation circuit can also be constructed using a simple circuit that allows adjustments of the temperature compensation coefficient and the overvoltage value. Fig. 12 shows a circuit diagram of the thermo-compensated SiPM chip with adjustment capabilities. The temperature compensation coefficient can be adjusted by installing a wire jumper between any of two Dn terminals, thereby excluding diodes from the temperature sensing diode chain and reducing the temperature compensation factor. This allows the adjustment of the SiPM temperature dependence in accordance with a specific application — (1) to achieve higher temperature compensation factor for applications where SiPM pulse integration is required and (2) to lower temperature compensation for devices measuring SiPM pulse amplitudes.
Fig. 11. Temperature stability of the SiPM pulse area integrated over 600 ns as characterized by the temperature drift of the mean value of the 241 Am peak.
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Please cite this article in press as: E. Kuznetsov, Temperature-compensated silicon photomultiplier, Nuclear Inst. and Methods in Physics Research, A (2017), https://doi.org/10.1016/j.nima.2017.11.060.
E. Kuznetsov
Nuclear Inst. and Methods in Physics Research, A (
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flux on the order of 0.67 Hz per single pixel was measured at pixel currents 46 μA and chip temperature +60.8 ◦ C. When the direct current through the SiPM was set at 36 mA, corresponding to single pixel current of 40 μA, the sensor temperature increased to +53 ◦ C and the count rate increased to 350 Hz after 2 min. After 10 min, the temperature increased to +60.8 ◦ C and count rate increased to 600 Hz. The rise of the sensor temperature during tests at high direct currents is explained by heating of the 900 SiPM quenching resistors. 7. Conclusions A chain of serially connected p–n junctions, integrated on the same chip with a SiPM matrix, can be used to compensate for the temperature dependence of the SiPM breakdown voltage. The temperature correction coefficient can be easily adjusted to compensate for the temperature dependence of secondary SiPM parameters such as afterpulsing and pulse recovery time constant. The on-chip bias adjustment circuit can be used to tune varying breakdown voltages of SiPM sensors by the application of a low-voltage DC potential at the SiPM control terminals. This makes it possible to achieve similar channel gains while using a common bias supply in multichannel photodetectors. It was proven that the diode chain does not emit photons at direct DC currents up to 30 μA and temperatures up to +30 ◦ C.
Fig. 12. SiPM chip with thermo-compensation and adjustment capabilities.
Multichannel photodetectors might include SiPM sensors with varying breakdown voltage parameters. To achieve similar gain-noise parameters for different channels, the applied bias voltage must be adjusted individually for each SiPM sensor, according to its breakdown voltage, to obtain the same overvoltage value for all channels. The device shown in Fig. 12 has the capability of tuning the voltage applied at the SiPM matrix by the application of a certain control voltage at the Udc terminal. This control voltage can be used to set the current through the temperature sensor (Its = Udc/R2) and also to adjust the voltage applied at the silicon matrix U_SiPM. The adjustment voltage is a voltage drop over resistor, R1 (𝑈𝑅1 = 𝐼𝑡𝑠 ∗ 𝑅1). The temperature dependence of the resistors R1 and R2 does not change the temperature dependence of the adjustment voltage if values of these resistors are made equal.
Acknowledgments This work was supported by the University of Alabama in Huntsville Charger Innovation Fund [grant number ED16HDQ0200012]. The technology is patent pending. The author would like to thank James Douglas Tubbs for his help and John W. Watts for his comments to this manuscript.
6. Photon emission test of the thermo-compensation circuit
Appendix A. Supplementary data
To test the light-emitting feature of the p–n junctions powered in direct polarity a SiPM sensor, which in fact represents an array of p– n junctions, was used. A Hamamatsu SiPM sensor, Model#S10362-33100C with 900 pixels of 100 × 100 μm each was attached to the PMT entrance window. First, the SiPM sensor was tested for the ability to emit photons in a standard reversed-biasing polarity. The PMT response was measured using a LeCroy oscilloscope configured in pulse counting mode. A SiPM upward-emitted photon flux on the order of 10% of its specified dark count rate was measured using this technique. The SiPM sensor was then connected to the bias power supply in direct polarity, and the photon flux emitted by the SiPM was measured. Measurements were conducted for the current flowing through each of the 900 pixels from 0 to 40 μA. No upward-emitted photons were measured for pixel currents up to 30 μA and temperatures up to +30 ◦ C. A slight increase in the photon
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Please cite this article in press as: E. Kuznetsov, Temperature-compensated silicon photomultiplier, Nuclear Inst. and Methods in Physics Research, A (2017), https://doi.org/10.1016/j.nima.2017.11.060.