Design of an APS CMOS image sensor for low light level applications using standard CMOS technology

CMOS image sensors (or APS: Active pixel sensors) are now the technology of choice for most imaging applications, such as digital video cameras. Whereas their sensitivity doesn't reach the one of the best actual CCD's (whose fill factor is about 100%), they are now commonly used because of their multiple functionalities (windowing, on-chip signal processing) and their easy serial fabrication. In this paper, we present a study of different pixel photodiodes and architectures, in order to increase their sensitivity and reduce their spatial and temporal noise. These chips will be used in satellite star trackers, and should be hardened to radiation. Two different architectures are investigated. The first one uses the photodiode capacitance for signal integration, as it is usually done in literature [1-3]. This capacitance should be as lower as possible, to increase conversion factor (the gain of the pixel) and reduce reset noise, and that's why different standard CMOS photodiodes have been studied and quantified. The second architecture uses a low-value poly1/poly2 capacitor inside each pixel for signal integration, thus resulting in increasing the gain but degrading the fill factor.