Characterization of a 3-D stacked CMOS SPAD depth sensor in 45/22-nm technology

In this article, the characterization of a 3-D stacked direct time-of-flight (dToF) depth sensor consisting of 256 × 128 single-photon avalanche diodes (SPADs) is examined. The sensor is fabricated in 45nm/22nm CMOS technology and its target use is in light detection and ranging (LiDAR) applications. The sensor is composed of identical digitally synthesized modules. Each module consists of 2 × (16 × 8) pixels with SPAD front-end circuitry and coincidence detection tree, time to digital converter, 4-bit photon counters, and readout. The functionality of the sensor has been demonstrated using telemetry measurements and flash 3-D imaging, and the SPAD characterization of the entire array has also been examined. The sensor’s range-finding capability has been demonstrated up to 100 meters under 10 klux sunlight. By utilizing progressive gating, a 31.7 dB improvement in signal-to-noise ratio (SNR) is achieved. Coincidence photons up to rank 3 are detected, resulting in a 17 dB improvement in SNR. The sensor’s photon-counting ability is also demonstrated in real-time at 25 FPS under 20 klux daylight.