Electron bombardment complementary metal-oxide-semiconductor (EBCMOS) is a new type of external photoelectric conversion image enhancement device that can realize digital imaging of targets under very low illumination. Unlike traditional low-light imaging devices, EBCMOS has the advantages of a small sensor size and weight, high sensitivity and dynamic range, fast response, and high contrast and resolution. The structure of an EBCMOS is mainly composed of a back-side bombarded complementary metal-oxide-semiconductor (BSB-CMOS), photocathode, and vacuum tube. The photogenerated electrons directly bombard the BSB-CMOS under the action of a strong electric field applied between the photocathode and BSB-CMOS anode. The photogenerated electrons then multiply, and the secondary electrons are collected in the BSB-CMOS. When incident electrons are accelerated to bombard the surface of a solid, some of these electrons are scattered back into the vacuum region between the photocathode and BSB-CMOS due to their interaction with atoms in the solid. Under the action of the near-focused electrostatic field, these scattered electrons re-incident onto the BSB-CMOS, resulting in backscatter noise, which affects the stability of the temporal and spatial distribution of the incident electrons and in turn affects the performance of the EBCMOS.

Based on the principle of interaction between high-energy electrons and solids and the Monte-Carlo simulation method, this study simulated and studied the characteristics of backscattered electrons near the surface of BSB-CMOS during electron bombardment. We mainly studied the angular distribution (θ_B and Φ_B), the ratio of the number of backscattered electrons to the number of incident electrons (R_BI), and the distance distribution between the backscattered electrons and incident electrons (D_BI). We then analyzed how these scattering characteristics are affected by the surface structure of the passivation layer, energy of the incident electrons, gating voltage, and diameter of the electron beam.

The use of a high-density passivation material composed of elements with lower atomic numbers at the BSB-CMOS surface is beneficial in reducing the average scattering step, depth of incidence, and elastic scattering radius of elastic collisions, which in turn reduces the R_BI (Fig. 2). Increasing the thickness of the passivation layer can reduce the emission range D_BI of the backscattered electrons such that the elastic scattering of electrons is concentrated on the surface of the passivation layer and the R_BI of the backscattering rate is reduced (Fig. 3). The backscattering rate of Si₃N₄ at different passivation layer thicknesses is very close to that of Al₂O₃ because of the atomic number factor (Table 1). The change in the diameter of the incident electron beam does not significantly affect the R_BI. However, the difference between the maximum D_BI and diameter of the incident electron beam is approximately 100 nm (Fig. 4). Choosing the energy of the incident electrons that matches the thickness of the passivation layer can yield a lower backscatter rate R_BI (Fig. 5). The angular distribution (θ_B and Φ_B) of the backscattered electron properties does not change with the aforementioned variables. Finally, when the passivation layer material, thickness, and incident electron energy are optimized and when only the influence of the passivation layer material on the R_BI is considered, Al₂O₃ can be selected as the passivation layer, the incident electron energy can be set to 4.5 keV, and the minimum R_BI can reach 19.0%. In addition, if the device gating voltage and backscatter characteristics are considered as a compromise, Si₃N₄ can be selected as the passivation layer material, the incident electron energy is set to 3.6 keV, and the minimum R_BI can reach 21.2% (Fig. 6).

Based on the electron-solid interaction principle and the Monte-Carlo simulation method, the characteristics of backscattered electrons near the surface of a BSB-CMOS in a complementary metal-oxide-semiconductor imaging device were studied. First, a calculation model for the characteristic parameters of the backscattered electrons was established by analyzing the trajectory of the incident electrons. The effects of different passivation layer materials, passivation layer thickness, incident electron beam diameter, incident electron energy, and gating voltage on the backscattered electron characteristics were then studied. The results show that when the surface structure of the BSB-CMOS without a passivation layer is modified with 50 nm passivation layer thickness of Al₂O₃ and the incident electron energy is optimized at 4.5 keV, the performance of the simulated optimized device is improved, and the backscattering rate is ultimately reduced from 33.0% to 19.0%. In addition, the performance of the simulated optimized device is improved when the surface structure of the BSB-CMOS without a passivation layer is modified with 50 nm passivation layer thicknes of Si₃N₄, and the incident electron energy is optimized at 3.6 keV. The backscatter rate is eventually reduced from 33.0% to 21.2%, which improves the resolution of EBCMOS and reduces the backscatter noise. This study thus provides a theoretical basis for studying EBCMOS backscatter noise.