CMOS image sensors (CIS) have rapidly become the sensor of choice for many imaging applications including consumer electronics, automotive electronics, machine vision, and medical imaging due to their significant performance improvements in the past two decades. However, specialized imaging devices such as intensified charge-coupled devices (ICCDs) or single-photon avalanche diodes (SPADs) are still chosen over CMOS image sensors in certain scientific applications, including fluorescence lifetime imaging microscopy (FLIM). FLIM is a widely-adopted imaging technique in biological research. The produced images are based on the differences in fluorophores' lifetimes. Fluorescence lifetime characterizes the exponential decay rate of fluorescence emission intensity, and the lifetime's sensitivity to the local environment of fluorophore provides a way of probing pH, oxygen concentration and other properties of living cells. The most important advantage of lifetime imaging over intensity imaging is its immunity to intensity-based artifacts such as non-uniform sample fluorophore concentration and variation in excitation source intensity.

A CMOS image sensor dedicated to a time-domain FLIM system is designed and fabricated in a standard 0.18 um CMOS image sensor process to demonstrate this type of sensor's capability of providing competitive performance in a compact and low-cost image system. The pixel design features an in-pixel pinned storage diode (PSD) for fast charge modulation. A pixel conversion gain of 121 uV/e- is achieved by creating a distal floating diffusion from transfer gate and reset gate without any process modification. 32-channel 10-bit on-chip column-shared pipelined ADCs with sampling rate up to 5MS/s are designed using area-efficient ring amplifiers for the sensor readout. The measured ADC DNL is -0.37/+0.42 LSB and INL is -1.11/+1.30 LSB. The ADC consumes 1.05mW/channel, and the total power consumption of the chip is 228mW. A total read noise of 2.53 e- rms is achieved with the on-chip ADCs. Fluorescence lifetime measurement with the new sensor is carried out on Alexa Fluor 647 dye and Alexa Fluor 680 dye, whose reported lifetime constants are 1.0 ns and 1.2 ns respectively [1].