Influence of Transfer Gate Design and Bias on the Radiation Hardness of Pinned Photodiode CMOS Image Sensors

The effects of Cobalt 60 gamma-ray irradiation on pinned photodiode (PPD) CMOS image sensors (CIS) are investigated by comparing the total ionizing dose (TID) response of several transfer gate (TG) and PPD designs manufactured using a 180 nm CIS process. The TID induced variations of charge transfer efficiency (CTE), pinning voltage, equilibrium full well capacity (EFWC), full well capacity (FWC) and dark current measured on the different pixel designs lead to the conclusion that only three degradation sources are responsible for all the observed radiation effects: the pre-metal dielectric (PMD) positive trapped charge, the TG sidewall spacer positive trapped charge and, with less influence, the TG channel shallow trench isolation (STI) trapped charge. The different FWC evolutions with TID presented here are in very good agreement with a recently proposed analytical model. This work also demonstrates that the peripheral STI is not responsible for the observed degradations and thus that the enclosed layout TG design does not improve the radiation hardness of PPD CIS. The results of this study also lead to the conclusion that the TG OFF voltage bias during irradiation has no influence on the radiation effects. Alternative design and process solutions to improve the radiation hardness of PPD CIS are discussed.

通过比较采用180纳米互补金属氧化物半导体图像传感器（CIS）工艺制造的几种传输门（TG）和钉扎光电二极管（PPD）设计的总电离剂量（TID）响应，研究了钴 - 60伽马射线辐照对钉扎光电二极管CMOS图像传感器的影响。在不同像素设计上测量的总电离剂量引起的电荷转移效率（CTE）、钉扎电压、平衡满阱容量（EFWC）、满阱容量（FWC）和暗电流的变化得出结论：只有三个退化源对所有观察到的辐射效应负责：金属前介质（PMD）正陷阱电荷、传输门侧壁间隔正陷阱电荷以及影响较小的传输门沟道浅沟槽隔离（STI）陷阱电荷。此处呈现的满阱容量随总电离剂量的不同变化与最近提出的一个分析模型非常吻合。这项工作还表明，周边浅沟槽隔离对观察到的退化没有责任，因此封闭布局的传输门设计不会提高钉扎光电二极管CMOS图像传感器的辐射硬度。这项研究的结果还得出结论：辐照期间传输门的关断电压偏置对辐射效应没有影响。讨论了提高钉扎光电二极管CMOS图像传感器辐射硬度的替代设计和工艺解决方案。