Complementary metal oxide semiconductor (CMOS) image sensor is a semiconductor component that converts optical signals into electrical ones. With the progress in semiconductor technology, the performance of the CMOS image sensor has been significantly improved. Due to its many advantages, such as low power consumption, high integration, and strong radiation resistance, the CMOS image sensor has gradually replaced CCD image sensors in space optical communication, star sensors, astronomical observation, and space remote sensing, and played an important role in aerospace. However, in the space radiation environment, the CMOS image sensor will be affected by the radiation damage of space particles, resulting in the degradation of device performance parameters and imaging quality. High-energy protons are the main reason for the degraded performance of the CMOS image sensor in space environments. Therefore, it is important to study the damage effect and damage mechanism induced by proton irradiation of the CMOS image sensor for improving the reliability of its application in space radiation environments.

The proton irradiation experiments are carried out on Xi'an 200 MeV Proton Application Facility (XiPAF), which provides proton beams in the range of 0-200 MeV. It selects protons with 100 MeV energy and the fluences of 1×10¹⁰, 5×10¹⁰, and 1×10¹¹ p/cm². During proton irradiation, the device is in biased and unbiased states. The irradiation process ensures that the CMOS image sensor is in a dark environment. The test sample is a 0.18 μm CMOS image sensor and the total number of effective pixels is 2040×2048 with a pixel size of 11 μm×11 μm. It adopts a 4T pixel structure. In this study, the continuous gray images and dark signals collected by the radiation effect test system of the CMOS image sensor at different integration times are taken as the output signals, and the curve of the changing dark signal with the irradiation fluence is obtained. The data gray images are extracted and processed by image analysis software, and the change rules of the dark signal distribution, dark spikes, and random telegraph signal are obtained. The typical characteristics of CMOS image sensor single-particle transient response under bias voltage are obtained.

In this study, the single-particle transient response images with different shapes under bias voltage are obtained by conducting 100 MeV high-energy proton irradiation experiments, the change rules of dark signals and dark signal spikes under different fluences, and the changes of dark signals with irradiation fluence under different bias conditions are also obtained. The secondary particles generated by the interaction of high-energy protons and lattice atoms ionize on the transmission path to produce electron hole pairs, and the transient ionized charges collected in several adjacent pixel units will form transient bright spots or bright lines (Fig. 2). Since the N-type metal oxide semiconductor (NMOS) in the pixel unit is sensitive to the bias voltage during irradiation, the interaction between high-energy protons and the CMOS image sensor generates a large number of oxide defect charges and interface state charges, leading to a more significant change in the dark signal than that without bias voltage (Fig. 6). The volume defects generated by the interaction between protons and silicon atoms cause increasing dark signals and dark signal spikes. The increase in irradiation fluence results in rising volume defects, dark signals, and dark signal spikes (Figs. 5, 7, and 9). As proton irradiation damage mainly includes ionization damage and displacement damage, the dark signal distribution curve induced by proton irradiation is obtained by convolution of the Gaussian distribution curve induced by ionization damage and the gamma distribution curve induced by displacement damage. With the continuous increase in irradiation dose, the number of affected pixel units rises and the dark signal distribution curve shifts to the right (Fig. 8). The CMOS image sensor under proton irradiation will induce the generation of two-level and multi-level RTS, which is related to the density and distribution of bulk defects in the space charge region (Figs. 10 and 11).

The experiments of high energy proton irradiation with 100 MeV carried out on XiPAF are introduced and the experimental law of CMOS image sensor performance degradation induced by proton irradiation is analyzed in this study. The typical characteristics of single-particle transient responses are mainly a series of transient bright spots and bright lines, which are formed by the electron-hole pairs generated by high-energy protons on the trajectory and are collected by several pixel units. Proton-induced cumulative effects (ionization effect and displacement effect) lead to increasing dark signals which rise with the growing irradiation dose. Under the same amount of proton injection, the dark signal increases by nearly 50% under the condition with bias voltage than that without bias voltage. This is mainly because of the ionization effect induced by proton irradiation, which leads to the generation of a large number of electron-hole pairs in the pixel unit of the CMOS image sensor. Under the action of the electric field, a large number of electrons move to the gate of the NMOS, and the holes are trapped by defects and impurities at the Si-SiO₂interface to form an oxide trap charge. Dark signal spikes are typical features of displacement damage and increase with the rising irradiation fluence. Two-level and multi-level RTSs induced by proton radiation are associated with bulk defects in SCR. This experiment helps designers understand the radiation damage of the CMOS image sensor and improve its radiation resistance through radiation reinforcement design. More high-energy proton irradiation experiments will be carried out to further study the degradation mechanism of single-particle transient responses and sensitive parameters in the CMOS image sensor.