Radiation Hardness Study 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 reversing biasing the substrate, and have been irradiated alongside reference devices in which the implant has been omitted. Event analysis under X-ray illumination has been used to study the charge collection and charge diffusion in the sensors. The results show no obvious charge collection degradation at 10 MeV equivalent proton fluence of up to 1011 p/cm2. Moreover, it has been shown that charge diffusion is largely suppressed in the fully depleted sensors. After exposure to a total dose of 100 krad(Si), both the fully depleted and the reference sensors show high sense node leakage current. This is believed to be due to an exposed edge of the shallow trench insulation (STI) oxide near the sense node. Dark signal distribution shows that hot pixels are dominant for both sensors after proton irradiation. An activation energy of ~0.62 eV derived from the temperature-dependent dark current result suggests that irradiation induced bulk defects is the main dark current source.