High-Resolution Low-Light Electron-Bombarded Active Pixel Sensor With a Fully Frontside-Thinned Structure and Its Noise Characteristics

Electron-bombarded active pixel sensors (EBAPSs) possess outstanding imaging capability under low-light conditions owing to their low-noise electron-bombarded semiconductor (EBS) gain and high-sensitivity photocathode. To maximize photoelectron collection, EBAPS devices typically use backside-illuminated CMOS image sensors (BSI-CIS). However, the flat epitaxial layer of BSI-CIS makes it vulnerable to backscattered electrons and electrical crosstalk, which hinders EBAPS from achieving the intended resolution based on pixel size. This study introduces a fully frontside-thinned (FFST) image sensor-based EBAPS for higher-resolution imaging. The sensor utilizes economical etching processes to remove dielectric layers over each photodiode, providing its sensitivity to low-energy photoelectrons. The metal stacks retained at pixel edges reduce backscattered electrons by 30%. Although the dark noise of the sensor increased after etching, the established signal-to-noise ratio model indicated that the EBS gain and the high quantum efficiency of the GaAs photocathode could overcome the noise, resulting in satisfactory imaging quality under overcast starlight illumination. Additionally, an FFST sensor (12.5 μm pixel) and a reference BSI sensor (9.7 μm pixel) were separately integrated with a GaAs photocathode in proximity focus. The FFST-based EBAPS exhibited better modulation transfer function (MTF) and resolution performance compared to the BSI-CIS-based EBAPS. This study demonstrates that the noise of front-illuminated sensors can be greatly suppressed by the low-power EBS configuration, allowing them to provide advantages in cost, size, and resolution for low-light applications.