11.3 A 160×120-pixel analog-counting single-photon imager with Sub-ns time-gating and self-referenced column-parallel A/D conversion for fluorescence lifetime imaging

CMOS Single-Photon Avalanche Diodes (SPADs) offer the possibility to realize image sensors able to gather information about photon position, number and time distribution, enabling cost-effective devices for scientific imaging applications like fluorescence lifetime imaging microscopy (FLIM), Raman spectroscopy, time-resolved near-infrared spectroscopy, but also consumer applications like 3D imaging. Despite the digital behavior of SPADs, pixels using signal processing in the digital domain suffer from very low fill-factor (FF), on the order of 1% [1]. Alternatively, analog counting enables compact electronics with 20.8% FF and down to 1.1ns time gating capability in 2D arrays [2] or 530ps in test structures [3]. Smaller technology nodes allow up to 26.8% FF, but high non-uniformity reduces the dynamic range and makes extensive external processing necessary [4]. Architectures with large off-pixel circuitry have been investigated [5] but demand high processing power and suffer from poor scalability. Time-resolved scientific imaging like FLIM requires fast time-domain processing of low-rate events, with high FF and repeated measurements. This paper presents a single-photon imager having 7T + 1MOSCAP per-pixel with photon counting and 750ps minimum time gating. Column-parallel A/D conversion allows for fast readout, and stable internal waveform generation is guaranteed by an integrated delay-line loop (DLL).