Modeling And Suppression Of Incomplete Charge Transfer In CIS Pixels Caused By Si/SiO₂ Interface State Traps In The TG Channel Under Low Illumination

In recent years,CMOS image sensors（CISs）have been widely used in various imaging fields due to their high integration,low cost,and low noise.The charge transfer characteristic is a key performance parameter of CISs.Only when the photogenerated charges collected by pinned photodiodes（PPDs）are fully transferred through the transfer gate（TG）to the floating diffusion node（FD）can CIS read the complete signal value and achieve high-quality imaging.However,existing CMOS manufacturing processes usually introduce non-idealities such as potential barriers and interface state traps into this transfer path,making it difficult for CIS to achieve complete charge transfer.Therefore,it is necessary to establish an accurate physical model for the phenomenon of incomplete charge transfer in order to fully understand the restriction mechanism of these non-idealities on charge transfer.The existing researches have only established the incomplete charge transfer model that depends on potential barriers,potential pockets,and spill back effect under high illumination condition,ignoring the effect of Si/SiO₂ interface state traps on charge transfer in the TG channel,particularly under low illumination.However,in the actual manufacturing process,problems such as crystal defects or contamination often generate traps at the Si/SiO₂ interface,and under low illumination,the number of photogenerated charges that PPD can collect decreases,making the degradation of charge transfer characteristics caused by interface state traps more severe.Therefore,in order to overcome the shortcomings of the existing researches,a physical model for quantifying the incomplete charge transfer caused by the Si/SiO₂ interface state traps in the TG channel under low illumination is established,and the pixel structure capable of suppressing the influence of interface state traps on charge transfer is proposed.The main contributions are as follows:Firstly,in response to the problem that the existing models do not consider the influence of Si/SiO₂ interface state traps on charge transfer,a physical model for quantifying the incomplete charge transfer caused by Si/SiO₂ interface state traps in the TG channel under low illumination is established.The emission time constants of different trap energy levels are calculated based on Shockley-Read-Hall theory,and compared with the time of the TG transition to the off state to determine the value of the boundary trap energy level.Based on the small injection theory,the quasi-Fermi level is approximated to the Fermi level,and the relationship between the probability of electrons occupying the trap energy level and the Fermi level is established based on the Fermi-Dirac statistical distribution.Next,based on the trap energy level distribution,an explicit two-dimensional expression for the number of untransferred charges associated with the trap-state density and trap energy level is established,and the variation rules of the number of untransferred charges and charge transfer efficiency are given when the trap energy level follows the Level,Gaussian,and Exponential distributions under low illumination.The model has been simulated and validated on the Sentaurus Tcad 2018 platform.The results show that the simulation values are in greet agreement with the theoretical values.In addition,in order to verify the stability of the built model,the effects of different physical factors on the pixel incomplete charge transfer model have been simulated,the results show that the built model is always stable.Secondly,to address the problem that the existing pixel structure cannot effectively suppress the effect of Si/SiO₂ interface state traps on charge transfer,a pixel structure based on buried channel TG is proposed.Compared to traditional pixel structures,this structure injects a layer of N-type impurities under the TG,transforming the TG from an enhanced transistor to a buried channel transistor.When the buried channel TG operates in the surface depletion mode,the charge transfer path will move from the semiconductor surface to the inside of the semiconductor,avoiding the contact of moving carriers with the Si/SiO₂ interface.When the buried channel TG operates in the pinch mode,the TG can be cut off,allowing the pixel to enter the exposure stage.The proposed structure has been simulated and verified on the Sentaurus Tcad 2018platform.The results show that the structure can reduce the number of incomplete charge transfers caused by Si/SiO₂ interface state traps from 35 to 0,effectively suppressing the effect of Si/SiO₂ interface state traps on charge transfer,and optimizing the charge transfer characteristics of pixels.