Research On Charge Transfer Potential Barrier And Process Optimization Of CMOS Active Pixel

CMOS image sensors(CISs)have been used in biometrics,consumer electronics,depth sensing and ranging,and other fields due to their low power consumption,high quantum efficiency,and compatibility with standard CMOS processes.As the photosensitive element of a CMOS image sensor,the performance of the pixels has a crucial influence on the image quality.The research on the characteristics of the charge transfer potential barrier(CTPB)in the pixel and the influence of different design parameters on the height of the CTPB is of great significance for analyzing the charge transfer limiting mechanism,optimizing process parameters,and pixel design.In this paper,a CTPB model is established to analyze the effect of different process parameters and pixel design on the potential barrier height and optimize the process of an active pixel with a four-tube structure.Firstly,the paper introduces the structure and working principle of active pixels in CMOS image sensors.Then,the origin of the CTPB in the pixel is analyzed,and the CTPB model is established under the assumptions of full-depletion and abrupt transition.Then,the effects of the transfer gate voltage,channel doping,and non-ideal effects due to pixel design defects on the potential barrier height are analyzed based on the CTPB model.Finally,the process design is optimized for an active pixel with a four-tube structure.Based on the CTPB model,a charge transfer path with a gradient potential distribution is established by using a non-uniformly doped transfer channel and adding N-type ion implantation at the interface between the pinned photodiode and the transfer channel,which improves charge transfer efficiency.The accuracy of the CTPB model was evaluated based on the TCAD simulation and measurement results.The maximum relative errors under different transfer gate voltages were 1.23% and 4.49%,respectively;the maximum relative errors under different channel doping were 0.93% and 5.19%,respectively.At the same time,the TCAD simulation results show that the four-tube active pixel optimized based on the model has no potential barrier on the charge transfer path,and the charge transfer efficiency reaches 99.9999%.In summary,the CTPB model proposed in this paper can be used to guide pixel design,improve charge transfer efficiency,and effectively eliminate the effect of image lag.