Abstract

In recent years, Silicon Photomultipliers (SiPMs) have been increasingly used as photo-detectors in Positron Emission Tomography (PET) application, which is a nuclear imaging tech-nique that is used to accurately image biochemical processes inside the human body. A SiPM is composed by an array of parallel connected micro-cells of Single Photon Avalanche Diodes (SPADs), and can be classified mainly into two categories Digital-SiPMs (D-SiPMs) and Analog-SiPMs (A-SiPMs). In A-SiPMs, all the microcells share the same bias voltage and have a common readout line. Through custom manufacturing process, the performance of A-SiPMs can be exten-sively optimized. With respect to A-SiPMs, D-SiPMs are composed by many SPAD pixels, each one containing one SPAD and local front-end circuitry. The digital SiPM takes advantage of CMOS technology to perform a 1-b direct A/D conversion per SPAD thus providing a fully digi-tal implementation. On the other hand, SPADs fabricated in CMOS process typically suffer high noise since the critical SPAD implants can hardly be optimized by using the standard CMOS process flow. The main activities carried out within this PhD thesis have been focused on two critical as-pects relevant for the optimization of PET systems performance: (I) the improvement of the per-formance of SPAD in D-SiPMs and (II) the development of high-performance A-SiPM readout application specific integrated circuit (ASIC). Concerning the first point, novel SPADs have been fabricated in CMOS 150-nm technology targeting at low noise, high sensitivity and excellent timing jitter. Three structures with different shapes, geometries and diameters, have been implemented in three test chips. Measurement re-sults of one p+/n-well SPAD array show a 0.4Hz/µm2 dark count noise, 0.85% afterpulsing for a dead time of 150ns at 3V excess bias. The photon detection probability is about 31% at 450nm wavelength at 5V excess bias. The SPAD exhibits a timing jitter of 82ps (FWHM) under a blue laser, which is potentially suitable for D-SiPMs in PET application. The second objective of this PhD work was to develop A-SiPM readout ASIC for PET appli-cation. To utilize the high intrinsic time resolution of A-SiPMs, the development of specialized, highly integrated readout electronics is required. Therefore, two ASICs, first chip with test struc-tures and 16 channels and the second chip with 32 channels, have been developed in 150-nm CMOS technology, with the aim of developing a compact A-SiPM module. The performance of the second chip has been validated by using 3 × 3 × 5 mm3 LYSO crystals coupled to 4 × 4 mm2 SiPMs (FBK-NUV-HD). The measurements show an energy resolution of 14.7% FWHM for the detection of 511 keV photons and the coincidence time resolution is 433ps (FWHM). To improve the timing resolution, part of the PhD work was carried on Stanford University, focused on char-acterization of A-SiPMs and analysis of noise contribution.