Proton and Gamma Radiation Effects on a Fully Depleted Pinned Photodiode CMOS Image Sensor

The radiation hardness of a fully depleted pinned photodiode (PPD) CMOS image sensor (CIS) has been evaluated with gamma and proton irradiations. The sensors employ an additional n-type implant in pixel which allows full depletion to be achieved by reverse biasing the substrate and have been irradiated alongside reference devices in which the implant has been omitted. The results show no visible degradation of the sensor’s radiation hardness by the additional implant. The main degraded parameters caused by ionizing radiation, from both gamma and proton irradiation, are the image lag and the surface dark current. High dark current in the sense node (SN) is identified and is found to be caused by the shallow trench isolation (STI) around the SN. The dark current increase after proton irradiation has been shown to be dominated by displacement damage-induced bulk defects and the value is consistent with those from other PPD CISs in the literature. The charge collection efficiency is not significantly affected by the bulk traps even after 10-MeV-equivalent proton fluence of up to 1012 p/cm2. However, the ionizing damage from the highest proton fluence reached causes a considerable drop in the charge-to-voltage conversion factor.