CMOS BASED SINGLE PHOTON AVALANCHE DIODE AND TIME-TO-DIGITAL CONVERTER TOWARDS PET IMAGING APPLICATIONS

Abstract

Time-of-flight (ToF) positron emission tomography (PET) is a non-invasive, functional biomedical imaging modality. It can be used to determine the metabolic activity difference between lesion and normal cells, thus, making it possible to detect tumors at their early stages. In such a system, photomultiplier tubes (PMTs) are typically employed as the photon sensors. However, PMTs are of large size, have fragile package, consume large power, require high bias voltage and are costly. To pursue the benefits of ToF PET system, there is a growing need for research on new types of photodetectors and photo-detection systems. This work focuses on studying and building a compact, low-cost time-of-flight photo-sensing system. To achieve this goal, we choose a standard digital CMOS technology to design and fabricate the photodetectors and associated electronics. A CMOS single photon avalanche diode (SPAD) is selected because of its low-cost, ultra-high light sensitivity and fast speed. Being implemented in IBM’s 130nm CMOS process, the impacts of silicide layer on the overall performances of the SPAD are investigated. By eliminating silicide on the active area of SPAD, a fivefold improvement in both dark noise and photon detection efficiency are demonstrated. Then, a SPAD comprehensive analytical model is proposed and implemented in Verilog-A hardware description language. This model includes all the noise sources and will provide useful guidance in optimizing SPAD and the associated circuits. A time-to-digital converter (TDC) is proposed and designed in the same 130nm CMOS process. The TDC is capable to digitize time intervals with 7.3ps resolution and covers up to 9ns dynamic range. The proposed TDC achieves state-of-the-art low power consumption. It will be used to extract the time-of-flight information, and to improve the image’s single-to-noise ratio (SNR) in a ToF PET imaging system. Finally, a time-of-flight sensing system prototype is built by integrating the CMOS SPADs and TDC on a printed circuit board. Based on the preliminary measurement results, this system achieves 440ps coincidence timing resolution. Factors of 2.5 and 6.1 improvements in image SNR and effective sensitivity, respectively, are expected with this prototype in ToF PET imaging applications.