Ionizing Radiation Effects on Hole Collection Backside-Illuminated p-Type Deep-Trench-Pinned Photo-MOS Pixels Under Image Acquisition

Dark-current degradation, origins, and annealing behavior after X-ray irradiation are studied in a p-type, hole collecting, backside-illuminated (BSI) image sensor currently being developed at STMicroelectronics and based on deep-trenched photo-metal–oxide–semiconductor (MOS) pixels. Different biasing conditions during irradiation, i.e., grounded or biased and sequenced, are compared. The dark-current increase with total ionizing dose (TID) and the dark-current annealing behavior seem to be driven by the backside interface between the P-epitaxy of the pixels and the oxide–nitride–oxide (ONO) stack. Despite still being under development, this pixel architecture already exhibits both very good electro-optical performance and a better radiation hardness than pinned photodiode (PPD)-based CMOS image sensors (CISs) that benefit from the same advanced CIS processing technologies. At high total dose range, the photogate challenges custom radiation-hardened-by-design photodiodes by exhibiting a comparable radiation tolerance while bringing new features, such as high resolution or correlated double sampling (CDS).