Total ionizing dose radiation effects in foue-transistor complementary metal oxide semiconductor image sensors

Radiation effects on four-transistor (4 T) active pixel sensor complementary metal-oxide-semiconductor (CMOS) image sensor induced by -ray are presented. The samples are 4 megapixels resolution CMOS image sensor using 11 upm pitch high dynamic 4 T pixels. They are manufactured with 0.18 upm specialized CMOS image sensortechnology. Three samples have been exposed to 200 krad(Si) 60Co -ray with different biasing condition (1# is static-biased, 2# dynamic-biased, and 3# is grounded during irradiation), and the dose rate is 50 rad(Si)/s. The influences of radiation on full well charge capacity, dark current, and conversion gain of the device are investigated. Experimental result shows that the conversion gain is not sensitive to the ionizing radiation, and it is mainly determined by the CMOS digital or analog circuits. It is known that the total ionizing dose for induced degradation in deep submicron MOSFET is negligible and so there is almost no radiation effect on the digital or analog circuits exposed to the ionizing radiation. Therefore, conversion gain does not have obvious degradation after irradiation. While full well charge capacity has a degradation after irradiation, which is due to the change of TG channel doping profile induced by the radiation. As the dose increases, dark current increases rapidly. The main source of dark current in 4 T CMOS image sensor is the current from STI interface and TG-PD overlap region. Experimental result also shows that different from 3 T CMOS image sensor, there is no biasing effect in 4 T CMOS image sensor. This is because for the 4 T CMOS image sensor most of the degradation come from STI interface and TG-PD overlap region, while biasing condition almost has no influence on both ofthem.