40 nm/40 nm 3D-Stacked SPAD Pixels With Sub-10 μm Pitch and 2 ns Dead Time

We present a comprehensive comparative study of passive quenching active recharge (PQAR) and active quenching active recharge (AQAR) analog front-ends (AFEs) designed for 64×64 pixel arrays of single-photon avalanche diode (SPAD) pixels. These AFEs were fabricated using a back-side illuminated (BSI) 3D-stacked 40 nm/40 nm CMOS image sensor process. While configured as an array, initial verification was conducted at the pixel level, comparing both AFE types. To enable a truly fair and direct performance evaluation, which was often difficult in prior independent studies due to the use of different devices, processes, architectures, or assumptions, both AFE types were fabricated under identical process, wafer, and layout conditions on the same die. The SPADs exhibit a breakdown voltage of 22.5 V, a low dark count rate (5.1 cps/μm2at 2 V excess voltage), and a high photon detection probability (25.1% at 850 nm at 2 V excess voltage). The AQAR pixel occupies a larger area (88.56 μm²) compared to the PQAR pixel (55.75 μm²), reflecting an area-performance trade-off. We analyzed methodologies for defining SPAD dead time, adopting inter-arrival time histogram analysis under intensive light. Experimental results demonstrate precise dead time control for both AFEs, achieving a minimum dead time of 2 ns and confirming maximum count rates up to near 500 Mcps. Moreover, this study experimentally verifies the effectiveness of active quenching in afterpulse suppression, leading to reduced noise and consequently contributing to an improved signal-to-noise ratio. The results of this study provide useful information on the design trade-offs and performance of PQAR and AQAR AFEs for various high-performance single-photon detection applications.