A CMOS 8×8 SPAD array for Time-of-Flight measurement and light-spot statistics

The design and simulation of a CMOS 8 × 8 single photon avalanche diode (SPAD) array is presented. The chip has been fabricated in a 0.18μm standard CMOS technology and implements a double functionality: measuring the Time-of-Flight with the help of a pulsed light source; or computing focal-plane statistics in biomedical imaging applications based on a concentrated light-spot. The incorporation of on-chip processing simplifies the interfacing of the array with the host system. The pixel pitch is 32μm, while the diameter of the quasi-circular active area of the SPADs is 12μm. The 113μm² active area is surrounded by a T-well guard ring. The resulting breakdown voltage is 10V with a maximum excess voltage of 1.8V. The pixel incorporates a novel active quenching/reset circuit. The array has been designed to operate with a laser pulsed at 20Mhz. The overall time resolution is 115ps. Focal-plane statistics are obtained in digital format. The maximum throughput of the digital output buffers is 200Mbps.