A Compact Ultraviolet-Sensitive CMOS SPAD Pixel With In-Pixel AQR and 13-bit Counter

Designing compact ultraviolet (UV)-sensitive SPAD pixels is crucial for scientific, industrial and medical applications. This paper presents a 40 μm × 40 μm front-illuminated SPAD pixel fabricated in a 180 nm CMOS process, integrating an optimized active quenching circuit and 13-bit digital counter. A shallow junction and a passivation opening above the diode active region are introduced to enhance UV detection. Circuit simulations show nanosecond quenching and recharge and a tunable hold-off time via a current-starved inverter. The pixel achieves a peak photon detection probability (PDP) of 29% at 400 nm and a dark count rate (DCR) of 69 cps/μm2at 1.3 V excess bias. The photo detection rate (PDR) reaches at least 10 MHz. Measurements using UV LEDs (260 - 540 nm) and a 266 nm laser show deep UV responsivity and linear count-rate scaling with source repetition rate. The results of the current work show the feasibility of silicon-based UV-sensitive SPAD pixel array with fully integrated electronics for low-cost, large-format imaging applications.