This article introduces a modular, direct time-of-flight (TOF) depth sensor. Each module is digitally synthesized and features a 2 × (8 × 8) single-photon avalanche diode (SPAD) pixel array, an edge-sensitive decision tree, a shared time-to-digital converter (TDC), 21-bit per-pixel memory, and in-locus data processing. Each module operates autonomously, by internal data acquisition, management, and storage, being periodically read out by an external access. The prototype was fabricated in a TSMC 3-D-stacked 45/65-nm CMOS technology, featuring backside illumination (BSI) SPAD detectors on the top tier, and readout circuit on the bottom tier. The sensor was characterized by single-point measurements, in two different modes of resolution and range. In low-resolution mode, a maximum of 300-m and 80-cm accuracy was recorded; on the other hand, in high-resolution mode, the maximum range and accuracy were 150 m and 7 cm, respectively. The module was also used in a flexible scanning light detection and ranging (LiDAR) system, where a 256 × 256 depth map, with millimeter precision, was obtained. A laser signature based on pulse-position modulation (PPM) is also proposed, achieving a maximum of 28-dB interference reduction.

We present a high-performance back-illuminated three-dimensional stacked single-photon avalanche diode (SPAD), which is implemented in 45-nm CMOS technology for the first time. The SPAD is based on a P⁺/Deep N-well junction with a circular shape, for which N-well is intentionally excluded to achieve a wide depletion region, thus enabling lower tunneling noise and better timing jitter as well as a higher photon detection efficiency and a wider spectrum. In order to prevent premature edge breakdown, a P-type guard ring is formed at the edge of the junction, and it is optimized to achieve a wider photon-sensitive area. In addition, metal-1 is used as a light reflector to improve the detection efficiency further in backside illumination. With the optimized 3-D stacked 45-nm CMOS technology for back-illuminated image sensors, the proposed SPAD achieves a dark count rate of 55.4 cps/μm² and a photon detection probability of 31.8% at 600 nm and over 5% in the 420-920 nm wavelength range. The jitter is 107.7 ps full width at half-maximum with negligible exponential diffusion tail at 2.5 V excess bias voltage at room temperature. To the best of our knowledge, these are the best results ever reported for any back-illuminated 3-D stacked SPAD technologies.