Time-resolved Single-photon Counting IC for Flash Raman Imaging and Single-point Depth Imaging

Abstract The work presents the design and post-layout simulations of a 260 × 8 time-resolved single-photon counting (TRSPC) CMOS Single Photon Avalanche Diode (SPAD) sensor, currently under fabrication, with integrated 68 ps-resolution, 2.1 ns dynamic-range time-to-digital converters (TDC). The sensor supports multi-point flash Raman imaging and single-point depth imaging (SPDI). In multi-point mode, spatial information from ten sample locations is mapped into the time-domain using optical fibers of incrementally increasing lengths, each adding an additional 30 ns delay. By resolving these ten time windows on-chip, the sensor allows the entire SPAD array to be reused across all measurement points without sacrificing sensor area. In SPDI mode, a 68 ps time window is digitally selected and photon hits are accumulated within that window. The window is then shifted to the next 68 ps bin to perform digital depth sweeping. Delay elements are stabilized by a Delay Locked Loop (DLL) to maintain consistent timing over process, voltage, and temperature variations. Post-layout simulations validate SPDI functionality, characterize the timing performance of 68 ps and 1.4 ns delay elements, D flip-flop (DFF) behavior in the setup/hold violation region, and assess delay variation in the buffer-tree. The DFF analysis shows a 2.45 ps standard deviation in Cumulative Distribution Function (CDF) -derived timing error, while buffer-tree delay differences from middle to extreme channels range from 8.5–19.2 ps. The integrated TDCs enable digital time gating to reduce fluorescence and improve SNR. Overall, the sensor supports 10-point simultaneous measurement and digital depth sweeping with full-array reuse, while being scalable to larger arrays for real-time Raman imaging.