SPAD Image Sensor With Analog Counting Pixel for Time-Resolved Fluorescence Detection

This paper presents a 32 × 32 pixel image sensor for time-gated fluorescence lifetime detection based on single-photon avalanche diodes. The sensor, fabricated in a high-voltage 0.35- μm CMOS technology, uses an analog counting approach to minimize the area occupation of pixel electronics while maintaining a nanosecond timing resolution and shot-noise-limited operation. The all nMOS pixel is formed by 12 transistors and features 25- μm pitch and 20.8% fill factor. The chip includes a phase-locked loop circuit for gating window generation, working at a maximum repetition frequency of 40 MHz, while the sensor can be gated at frequency up to 80 MHz using an external delay generator. Optical characterization with a picosecond-pulsed laser showed a minimum gating window width of 1.1 ns. Example images acquired in both continuous and time-gated mode are presented, together with a lifetime image obtained with the sensor mounted on a fluorescence microscope.