Review of Back-Side Illuminated 3-D-Stacked SPADs for Time-of-Flight and Single-Photon Imaging

Single photon avalanche diodes have proven their usability for multiple applications such as time of flight (ToF), fluorescence lifetime imaging microscopy (FLIM), or quantum communication. In recent years, a number of devices utilizing 3-D-stacking with top-tier wafers fully optimized for optimum sensing performance and advanced digital nodes in the bottom tier for control processing have been published. They significantly outperform the early generation of stacked back-side illuminated (BSI) devices, especially in terms of photon detection efficiency (PDE) [peak and in the near-infrared (NIR)] and dark count rate (DCR). Currently, PDE approaches 90% (with fill factor included) in the visible range and the reported DCR is often well below 1 cps/ $\mu \text { m}^{{2}}$ . single photon avalanche diode (SPAD) pixels have been also scaled-down compared to the older devices, with the smallest reported pitch below $3~\mu $ m. A variety of manufacturing techniques, utilizing drift field engineering, deep trench isolation (DTI), and sensitivity enhancement features, are used to achieve this. In this review, we provide an overview of the evolution of SPAD technology and important diode characteristics and compare the performance of the recent generation of 3-D-stacked BSI devices.