Optimum Design of Quenching Capacitor Integrated Silicon Photomultipliers for TOF-PET Application

Abstract

The prototype SiPM was designed and fabricated for MRI compatible PET using the customized CMOS process at National Nanofab Center in KAIST. The SiPM was designed to have a size of 3x3 mm2 composed of micro-cells of 65x65 μm2 with a fill factor of 68%. The size of a micro-cell was determined by optimization between the photon detection efficiency (PDE) and the dynamic range for the photons of 511 keV from LYSO crystal. In the micro-cell structure, a specially designed quenching capacitor (QC) is added parallel to quenching resistor using the Metal-Insulator-Metal (MIM) process. This QC integrated SiPMs (QC-SiPM) was devised to realize rapid response of output pulses and to enhance the timing resolution of SiPM. Coincidence timing resolution of PET detectors depends on the output pulse shapes which are the convolution of the intrinsic pulse shape of scintillation crystals and the single photon pulse shape at the micro-cell in a SiPM. A quenching capacitor parallel to a quenching resistor provides a fast current path at the beginning stage of avalanche process, than reduces rising time of single photon pulse shape. In this study the rise time of the QC-SiPM signal was analyzed to be 22.5 ns while that for the regular SiPM was 34.3 ns.