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Description
INTRODUCTION
Silicon Photomultiplier (SiPM) is an array of Single Photon Avalanche Diodes (SPADs), avalanche photodiodes operating in Geiger mode followed by quenching resistors, connected in parallel. Since SiPMs have short recovery time and superior time resolution with high energy resolution and are insusceptible to magnetic field, they are used as radiation detectors for diagnostic imaging equipment using nuclear medicine such as PET and SPECT in combination with a scintillator in substitution for photomultiplier tubes (PMTs). In recent years, with a background of improvement in time resolution and simplification of complex signal processing circuit, the integration of SiPM and readout circuit onto a single chip, such as digital-SiPM and Complementary Metal Oxide Semiconductor-Single Photon Avalanche Diode (CMOS-SPAD), have recently started getting attention [1]. The objective of this study is to develop SOI (Silicon on Insulator)-SiPM using SOI-CMOS process that enables the three-dimensional integration of sensors into circuits and also high-speed performance of transistors without mechanical bump bonding [2]. This time, focusing on the performance improvement of the sensor part, we designed and fabricated SiPM using SOI technologies, and evaluated the basic performance of both the sensor and readout circuit.
METHOD
The 36-channel SiPM array was designed using a 0.2 µm 5-metal SOI-CMOS process technology provided by LAPIS Semiconductor Co., Ltd. One 250 µm2 SiPM channel is constituted by 9×9 microcells, which are arranged in a 27.52 µm pitch.
SiPM amplifies and detects free carriers generated due to thermal processes, band-to-band tunneling, and diffusion in the absence of incident photons. This pulse is a dark count pulse, which causes a detection error. Also, its photon detection efficiency (PDE) depends on sensor’s active area, wavelength dependent quantum efficiency, and avalanche probability. In previous research, high dark count rate (DCR) and low PDE remain issues to be improved. As one of the causes, we considered that the inhomogeneous electric field derived from the shape of the edge of the microcell influenced the breakdown characteristics, dark count rate, and PDE of SiPM. Thus, we fabricated the circular cathode and anode to eliminate the edge.
A readout circuit consists of charge-sensitive preamplifier, shaping preamplifier, buffers, discriminators, and several bias circuits. The input signals, which are outputs of SPADs, are sensed and amplified by preamplifier, and finally digital output signal is generated. Each circuit part is followed by individual output pins; therefore, one can selectively look at the output of each circuit.
RESULTS
In order to evaluate sensor characteristics, IV characteristics are obtained by using the semiconductor parameter analyzer, and the peak histogram of incident photons by using laser pulses with a wavelength of 510 nm. As a result of modification, the breakdown voltage increases from 48.2 V to 50.7 V, the operating voltage range, which reflects the characteristics of gain and dark count rage, increases from 3.3 V to 3.8 V, DCR decreases from 245 MHz/mm2 to 218.8 MHz/mm2, and PDE increases from 0.47 % to 2.74 %. In circuit part, we measured analog circuit outputs of preamplifier and shaper by voltage pulse input from test terminal. The peaks of output waveforms linearly increase as test input voltage increases, from 0.25 V to 1.5 V with a step size of 0.25 V. Finally, we confirmed that the sensor and readout circuit properly operate as one system.
CONCLUSION and FUTURE PLAN
In this study, to develop a photodetector with the monolithic integration of sensor and readout circuit that can be applied to PET and SPECT, we designed and fabricated SiPM using SOI process. Measured results, such as reduction of DCR and increase of PDE, indicate that the structural change of the sensor from square to circle is effective. However, comparing to performances of commercially available SiPM, SOI-SiPM's DCR and PDE still need improvement. Also, a method of the integration of SOI-SiPMs and scintillators will be investigated further.
[1] DR.Schaart,DOI 10.1016/j.nima.2015.10.078, Nuclear Instruments & Methods in Physics
[2] Y.Arai et al, NIMA, Volume 636, Issue 1, Supplement, 21 April 2011, Pages S31-S36
