Review of Back-Side Illuminated 3D-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, fluorescence lifetime imaging microscopy or quantum communication. In recent years a number of devices utilizing 3D-stacking with top-tier wafer fully optimized for optimum sensing performance and advanced digital nodes in bottom tier for control processing has been published. They significantly outperform early generation of stacked back-side illuminated devices, especially in terms of photon detection efficiency (peak and in near infrared) and dark count rate. Currently, photon detection efficiency approaches 90% (with fill factor included) in the visible range and the reported dark count rate is often well below 1 cps/μm2. SPAD pixels have been also scaled down compared to the older devices, with the smallest reported pitch below 3 μm. A variety of manufacturing techniques, utilizing drift field engineering, deep trench isolation and sensitivity enhancement features, are used to achieve this. In this review we provide an overview of the evolution of SPAD technology, important diode characteristics and compare the performance of the recent generation of 3D-stacked back-side illuminated devices.