| The objective of this master's thesis is to design and implement a CMOS high-speed photon counting interface to be integrated within an Avalanche Photodiode (APD) operating in both linear and Geiger modes. Improving the main performance metrics of the structure including maximizing photon counting rate and minimizing both power consumption and system noise are among the objectives. Also, real-time monitoring of APD characteristics in terms of bias voltage, temperature, signal-to-noise ratio, gain, and extending control of thermal effects on APD performance are considered.;Since linear mode APD has low achievable current gain and therefore is not sensitive enough in detecting single photons, Geiger mode APD was selected for photon counting systems. Furthermore, since Geiger-mode operation requires an external stop, a Quench-Reset circuit was employed to detect photon arrival, count the pulse generated at photon arrival, and reset the APD in order to make it ready for the subsequent photon detection.;A high-speed quench-reset circuit was designed, implemented with 0.18 microm CMOS process, laid out and simulated and then fabricated by TSMC through CMC Microsystems. Wire bonding was done by LASEM facilities affiliated to Polystim laboratory. The control scheme of the whole system was implemented in an off-chip FPGA platform. For the sake of test and measurement, a PCB was made and test bench for digital part was implemented in FPGA. The scheme was responsible for controlling the effect of temperature on APD performance by monitoring the gain in both linear and Geiger modes of operation. The thermal effect was controlled through changing the bias voltage of APD. The code was tested in Modelsim and implemented to Igloo FPGA (Actel Co.). The chip was then tested using equipment available to Polystim and was characterized in terms of the performance metrics in question.;The quench time for the designed circuit was in the range (1 -- 4)ns while the reset time was in the range (1 -- 3)ns. The most appropriate value for hold-off time was unique for each system and it depended on the nature, density and lifetime of the traps of each specific APD; therefore the system was designed to have a controllable hold-off time in the range of 4ns - 2micros. The temperature-independent gain control system was capable of controlling the gain for both linear-mode and Geiger-mode APD in 10% variation.;This work is part of the multidisciplinary Imaginc research team to develop a clinical imaging system based on a real-time, noninvasive and portable NIRS/EEG (Near-infrared spectroscopy / Electroencephalography) signal acquisition system, which communicates wirelessly with a computer and images the whole brain cortex while improving comfort in long duration scanning. The presented work is part of a collaboration between Polystim research group, CIHR (Canadian Institutes of Health Research) and Heart and Stroke Foundation. |
