基于4晶体管像素结构的互补金属氧化物半导体图像传感器总剂量辐射效应研究

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

对基于4晶体管像素结构互补金属氧化物半导体图像传感器的电离总剂量效应进行了研究,着重分析了器件的满阱容量和暗电流随总剂量退化的物理机理.实验的总剂量为200 krad(Si),测试点分别为30 krad(Si),100 krad(Si),150 krad(Si)和200 krad(Si),剂量率为50 rad(Si)/s.实验结果发现随着辐照总剂量的增加,器件的满阱容量下降并且暗电流显著增加.其中辐照使得传输门沟道掺杂分布发生改变是满阱容量下降的主要原因,而暗电流退化则主要来自于浅槽隔离界面缺陷产生电流和传输门-光电二极管交叠区产生电流.实验还表明样品器件的转换增益在辐照前后未发生明显变化,并且与3晶体管像素结构不同,4晶体管像素结构的互补金属氧化物半导体图像传感器没有显著的总剂量辐照偏置效应.

Radiation effects on four-transistor （4T） active pixel sensor complementary metal-oxide-semiconductor （CMOS） image sensor induced by γ-ray are presented. The samples are 4 megapixels resolution CMOS image sensor using 11 μm pitch high dynamic 4T pixels. They are manufactured with 0.18 μm specialized CMOS image sensortechnology. Three samples have been exposed to 200 krad（Si） 60Co γ-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 4T CMOS image sensor is the current from STI interface and TG-PD overlap region. Experimental result also shows that different from 3T CMOS image sensor, there is no biasing effect in 4T CMOS image sensor. This is because for the 4T 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.