A Fully Digital Indirect Time-of-Flight Image Sensor with Multi-Frame Integration and Time-gated Single-Photon Counting Method

This paper presents an indirect time-of-flight (iToF) image sensor, based on a single-photon avalanche diode (SPAD) and a time-gated single-photon counting (TGSPC) method. The SPAD detects an incident photon and generates a voltage pulse through avalanche multiplication, enabling single-photon detection. The proposed SPAD utilizes a P-well and Deep N-well junction with a retrograde P-substrate guard-ring, designed to mitigate premature edge breakdown at the junction edges. Additionally, doping conditions of the SPAD, such as concentration, depth, and profile, are optimized to effectively suppress dark carriers. Also, the TGSPC method is employed, which measures the phase delay proportional to the distance between the sensor and the target. To achieve this, the proposed pixel incorporates two multiplexers and two 5-bit counters, along with an active-recharge circuit for asynchronous SPAD recharge and row-selection switches for digital readout. Thanks to this compact pixel design, a small pixel pitch of 35 μm can be achieved. To overcome the limited depth of the 5-bit counter, a multi-frame integration technique is applied, effectively extending the counter depth to 10-bit. The proposed sensor is fabricated in a 110 nm backside-illumination (BSI) CMOS image sensor process, optimized for light detection and ranging (LiDAR) applications. By incorporating these optimizations, a detection range of 40 m and 0.19% depth precision are achieved simultaneously, with a frame rate of 180 frame/s. These results demonstrate the potential of the proposed sensor for low-cost, high-precision LiDAR systems.