Crystalline Selenium Layer Stacked Image Sensors With Pixel-Parallel A/D Converters Toward Photon Counting

We report crystalline selenium (c-Se) photo-conversion layer stacked image sensors with pixel-parallel A/D converters (ADCs) toward photon counting. The sensor separates photo-conversion and multiplication function from complementary metal–oxide–semiconductor (CMOS) circuits, which has advantages in terms of high sensitivity and high spatial resolution of the sensors. The sensor also has a wide dynamic range for input illuminance because the pixel-parallel 1-bit ADCs prevent saturation of the floating diffusion (FD). The circuits require no quenching operation, which is suitable for high-speed operation. The c-Se layer is directly stacked onto the CMOS sensor surface, which has been flattened by chemical mechanical polishing, and connected to each pixel with Au electrodes. A photo-generated electron charge can be multiplied in the c-Se layers, and introduced to 1-bit ADC, where the charges are converted into pulse signals to be counted by in-pixel counters. Linear output characteristics are confirmed with a wide dynamic range for illuminance of 96 dB. Video images of 128
$\times96$
pixels are successfully demonstrated including avalanche multiplication mode in the supply voltage of more than 10 V, which exhibits the feasibility of high spatial resolution and high-speed image sensing applicable to photon counting and wide dynamic range imaging.