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Complementary metal oxide semiconductor (CMOS) image sensors have been increasingly widely used in the field of radiation environments due to their numerous advantages, and their radiation effects have also attracted much attention. Some experimental studies have shown that the saturation output of CMOS image sensors decreases after irradiation, while others have reported that it increases. In this work, the further in-depth research on the inconsistent results is conducted based on the experiments on proton irradiation and TCAD simulations, and the degradation mechanism in full well capacity (FWC), conversion factor (CVF), and saturation output of the 4T pinned photodiode (PPD) CMOS image sensors due to proton cumulative radiation effects are also analyzed. In experiments, the sensors are irradiated by 12 MeV and 60 MeV protons with a fluence up to 2×1012 cm–2. The sensors are unbiased during irradiation. The experimental results show that proton irradiation at 12 MeV and 60 MeV result in an increase of 8.2% and 7.3% in conversion gain, respectively, and a decrease of 7.3% and 3.8% in full well capacity, respectively. The saturation output shows no significant change trend under 12 MeV proton irradiation, but increases by 3% under 60 MeV proton irradiation. In the TCAD simulation, a three-dimensional 4T PPD pixel model is constructed. A simulation method that combines the trap and gamma radiation model in TCAD with the mathematical model of minority carrier lifetime is used to simulate global and local cumulative proton irradiation in order to analyze the degradation mechanism. It is proposed that the degradation of saturation output at the pixel level is determined by the FWC of PPD, the physical characteristics of the reset transistor and the capacitance of floating diffusion, but they have opposite effects. Proton irradiation leads to the accumulation of oxide-trapped positive charges in the shallow trench isolation on both sides of PPD, resulting in the formation of leakage current path in silicon, thereby reducing the full well capacity. A decrease in FWC leads to a decrease in saturation output. While, the radiation effect of the reset transistor causes the FD potential to increase during the FD reset phase, further leading to an increase in saturation output. The irradiation causes the capacitance of the floating diffusion to decrease, resulting in an increase in conversion factor and consequently increasing the saturation output. The difference in radiation sensitivity among the three influence factors at the pixel level may result in a decrease or increase in saturation output with proton fluence increasing. The above work comprehensively reveals and analyzes the mechanisms of degradation in FWC, CVF and saturation output after irradiation, and the research results have certain guiding significance for analyzing the radiation damage to CMOS image sensors.
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Keywords:
- CMOS image sensor /
- total ionizing dose /
- saturation output /
- full well capacity /
- conversion factor
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图 1 (a) CIS芯片及图像采集卡; (b) 测试系统的程控暗箱; (c) 质子辐照实验示意图
Figure 1. (a) CIS and image acquisition card; (b) the dark box of test system; (c) schematic diagram of CIS proton irradiation experiment setup.
图 2 系统增益随质子注量变化
Figure 2. Overall system gain versus the proton fluence.
图 3 (a) 平均输出信号; (b) 满阱容量随质子注量变化
Figure 3. (a) Mean signal level; (b) full well capacity versus proton fluence.
图 4 平均饱和输出随质子注量变化
Figure 4. Mean saturation outputs versus the proton fluence.
图 5 TCAD中建立的4T PPD像元模型 (a) 三维模型, STI和PMD略去以便于展示; (b) 沿(a)图X = 1.2 μm的横截面
Figure 5. The 4T PPD CIS pixel model built in TCAD: (a) 3D model, STI, and PMD removed for visualization; (b) 2D cross section taken along the X = 1.2 μm surface in (a).
图 6 仿真时序及FD电势输出
Figure 6. Simulation driving timings and the potential output of FD.
图 7 4T PPD像元输出电压随光照强度变化
Figure 7. Output variation of the 4T PPD CIS model with light intensity.
图 8 全局质子辐照前后氧化物中陷阱正电荷分布 (a) 辐照前; (b) 质子注量1.0×1012 cm–2
Figure 8. Distribution of trapped positive charges in the oxide: (a) Before irradiation; (b) proton fluence of 1.0×1012 cm–2.
图 9 12 MeV全局质子辐照 (a) PPD内电子数量; (b) 满阱容量
Figure 9. Global irradiation simulation by 12 MeV protons: (a) Total electron count within the PPD; (b) FWC.
图 10 辐照前后PPD两侧氧化物陷阱正电荷和电子浓度分布 (a) 辐照前陷阱正电荷; (b) 辐照后陷阱正电荷; (c) 辐照前电子浓度; (d) 辐照后电子浓度
Figure 10. Distribution of oxide-trapped positive charges in STI on both sides of the PPD during FD reset stage: (a) Positive charges before irradiation; (b) positive charges after irradiation; (c) electron concentration distribution before irradiation; (d) electron concentration distribution after irradiation.
图 11 12 MeV全局质子辐照 (a) FD电势; (b) 饱和输出
Figure 11. Global irradiation simulation by 12 MeV protons: (a) FD potential; (b) saturation output.
图 12 质子局部辐照(Y > 2.1 μm) (a) 12 MeV质子辐照PPD内电子数量; (b) 满阱容量
Figure 12. Local irradiation (Y > 2.1 μm): (a) Total electron count within the PPD irradiated by 12 MeV protons; (b) FWC.
图 13 质子局部辐照(Y > 2.1 μm) (a) 12 MeV质子辐照FD电势; (b) 饱和输出
Figure 13. Local irradiation (Y > 2.1 μm): (a) FD potential irradiated by 12 MeV protons; (b) saturation output.
图 14 质子局部辐照(Y < 2.1 μm) (a) 12 MeV质子辐照PPD内电子数量; (b) 满阱容量
Figure 14. Local irradiation (Y < 2.1 μm): (a) Total electron count within the PPD irradiated by 12 MeV protons; (b) FWC.
图 15 质子局部辐照(Y < 2.1 μm) (a) 12 MeV质子辐照FD电势变化; (b) 饱和输出变化
Figure 15. Local irradiation (Y < 2.1 μm): (a) FD potential irradiated by 12 MeV protons; (b) saturation output.
图 16 质子局部辐照(Y < 2.1 μm)前后浮置扩散区耗尽层厚度 (a) 辐照前; (b) 12 MeV质子辐照后; (c) 60 MeV质子辐照后
Figure 16. Depletion region of FD region before and after local proton irradiation simulation (Y < 2.1 μm): (a) Before irradiation; (b) after irradiation by 12 MeV protons; (b) after irradiation by 60 MeV protons.
图 17 60 MeV质子局部辐照(Y<2.1 μm)TG下方沟道和浮置扩散区的电子浓度
Figure 17. Electron density in the channel below TG and FD region after local proton irradiation simulation (Y < 2.1 μm).
表 1 辐照参数
Table 1. Irradiation parameters.
CIS编号 质子能量/MeV 最大质子注量/cm–2 DDD/(TeV·g–1) TID/krad(Si) CIS_1 12 2.0×1012 17638 962 CIS_2, 3, 4 60 2.0×1012 8286 275 
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表 2 12 MeV和60 MeV质子等效计算结果
Table 2. Equivalent results for 12 MeV and 60 MeV protons.
质子能量
/MeV质子注量
/(1011cm–2)等效中子
注量
/(1011 cm–2)等效TID
/krad(Si)12 1.0 3.0 48.1 4.0 12.0 192.3 7.0 21.0 336.6 10.0 30.0 480.8 60 1.0 1.0 13.7 4.0 4.0 55.1 7.0 7.0 96.4 10.0 10.0 137.4
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