A Reconfigurable 3-D-Stacked SPAD Imager With In-Pixel Histogramming for Flash LIDAR or High-Speed Time-of-Flight Imaging

A 256 × 256 single-photon avalanche diode (SPAD) sensor integrated into a 3-D-stacked 90-nm 1P4M/40-nm 1P8M process is reported for flash light detection and ranging (LIDAR) or high-speed direct time-of-flight (ToF) 3-D imaging. The sensor bottom tier is composed of a 64 × 64 matrix of 36.72-μm pitch modular photon processing units which operate from shared 4 × 4 SPADs at 9.18-μm pitch and 51% fill-factor. A 16 × 14 bit counter array integrates photon counts or events to compress data to 31.4 Mb/s at 30-frame/s readout over 8 I/O operating at 100 MHz. The pixel-parallel multi-event time-to-digital converter (TDC) approach employs a programmable internal or external clock for 0.56-560-ns time bin resolution. In conjunction with a per-pixel correlator, the power is reduced to less than 100 mW in practical daylight ranging scenarios. Examples of ranging and high-speed 3-D ToF applications are given.