Simulation Research On Irradiation Effect Of CMOS Image Sensor Pixel

With the continuous optimization of CMOS structure and the continuous progress of semiconductor technology,the performance of CMOS image sensor has made a huge leap.Thanks to the advantages of low cost,low power consumption,and high integration,CMOS image sensors have been used in more and more fields.In space and medical fields,CMOS image sensors also play an important role,and 4T pixel structure is one of the mainstream pixel structures of the current CMOS image sensor.Generally,CMOS image sensors in space environment are exposed to radiation from X-rays,gamma rays,high-energy electrons,protons,and other heavy ions.These radiations can damage the device and affect the performance of the device.Therefore,studying the radiation damage mechanism and performance degradation of CMOS image sensors is very important to improve the performance of CMOS in radiation environment.In this paper,the research work of device structure optimization and damage mechanism analysis on the damage effect of radiation on the pixels of 4T CMOS image sensor is carried out.Based on the standard CMOS process,TCAD numerical simulation software was used for process simulation,a 4T CMOS pixel structure model was constructed,and the characteristic simulation was verified.Carried out the research on the effect of X-ray total ionizing dose radiation on 4T CMOS image sensor pixels,focusing on the effect of the threshold adjustment layer on the total ionizing dose effect,the effect of total ionizing dose radiation on the charge transfer path,and the effect of the transfer gate bias voltage on the total ionizing dose effect influences.The results show that under X-ray total ionizing dose radiation,compared with the 4T CMOS image sensor pixels without the threshold adjustment layer,the negative shift of the threshold voltage of the transmission gate of the pixel with the threshold adjustment layer is reduced,and the leakage current is reduced,which can be significant reduce the effect of total dose radiation on 4T pixel devices.Under the total ionizing dose radiation,the electric potential on the charge transfer path increases,and the electric potential changes greatly in the PD-TG junction area,which easily forms a potential well.The transmission gate threshold voltage degradation is significant at 3.3V bias voltage,and the transmission gate threshold voltage degradation is slight at-0.5V bias voltage,indicating that a smaller bias voltage can suppress the effect of the total dose effect.An isolation layer was added between the STI and PPD regions,and the degradation difference of the pixel structure of the STI-PPD isolation layer with different isolation strengths subjected to total ionizing dose radiation was studied.The results show that the STI-PPD isolation layer can effectively reduce the dark signal caused by total ionizing dose radiation and slow down the degradation of the full well capacity.The effect of proton radiation displacement damage on 4T CMOS image sensor pixels was studied.The degradation mechanism of output signals and dark signals caused by proton radiation was analyzed.The structure parameters of the threshold adjustment layer in the sensor pixels and the proton radiation displacement damage were studied.Correlation,the effect of proton radiation displacement damage on the pixels of 4T CMOS image sensor with STI-PPD isolation layer was studied,and the effect of PPD length on displacement effect was studied.The research results show that proton radiation will reduce the output signal of the pixel device and increase the dark signal,and a longer threshold adjustment layer will cause a larger dark signal.Compared with pixels without STI-PPD isolation layer,the pixel dark signal of the STI-PPD isolation layer is smaller;the longer the PPD length,the greater the dark signal.