Effect of Trench Isolation Structure in the Multiplication Layer Resolution in EBCMOS

Electron Bombarded Complementary Metal-Oxide-Semiconductor （EBCMOS） is an advanced imaging device capable of capturing digital images under extremely low-light conditions. Its operating principle involves the conversion of incident photons into photoelectrons via a photocathode， followed by the acceleration of these photoelectrons through a high-voltage electric field to bombard the BSB-CMOS electron multiplication layer. This process amplifies the photoelectrons and facilitates the collection of secondary electrons within the BSB-CMOS layer. During the collection of secondary electrons， the electrons undergo diffusion due to concentration gradients. Some of them cross pixel boundaries and are collected by adjacent pixels. This phenomenon leads to edge blurring and a reduction in spatial resolution.To address this limitation， this study proposes the introduction of trench-isolation structures within the multiplication layer to constrain lateral electron diffusion. The influence of trench-isolation parameters on diffusion behavior and spatial resolution is systematically investigated， offering a novel structural approach for enhancing the resolution performance of EBCMOS devices. We develop a theoretical model for spatial resolution in trench-isolation electron multiplication layers based on carrier transport theory and Monte Carlo simulations. This model was used to evaluate the impact of variations in the height， width， and spacing of the trench-isolation on the spatial resolution of the EBCMOS multiplication layer.Under a base doping concentration of 1014 cm10， the trench-isolation width was 1 μm and the trench-isolation spacing was 20 μm， while the trench-isolation height was set to 1μm，1.5μm and 2 μm， respectively. Fig. 4（a） presents the Line Spread Function （LSF） curves for different trench heights. An increase in trench height reduces the LSF width， indicating sharper image edges. The peak value of the LSF remains stable， suggesting that image contrast is preserved. Fig. 4（b） displays the corresponding Modulation Transfer Function （MTF） curves. At a trench-isolation height of 2 μm， the spatial resolution of the device reaches 43 lp/mm， resulting in a 16.2% enhancement compared to pixels without trench-isolation structures； With a base doping concentration of 1014 cm-3， a fixed trench height of 2 μm， and a trench-isolation width of 1 μm， the trench spacing was varied among 10 μm，15 μm and 20 μm. As shown in Fig. 6（a）， decreasing the spacing reduces the LSF width， thereby enhancing edge sharpness. However， a reduction in LSF peak value was also observed， indicating a slight loss in image contrast. As evidenced by the MTF curves in Fig. 6（b）， a trench-isolation spacing of 10 μm enables the device to achieve a spatial resolution of 60 lp/mm. This represents a 62.2% improvement compared to pixels without trench isolation structures； Under the same doping concentration， with trench height and spacing the same 2 μm and 10 μm respectively， and the trench isolation width was varied between 1， 2， and 3 μm. Fig. 8（a） presents the LSF curves for various trench widths. As trench isolation width increases， the LSF width narrows， improving spatial resolution. However， a corresponding decrease in the LSF peak value is observed， which suggests reduced signal contrast. Fig. 8（b） presents the MTF curves for different trench widths. The results indicate that reducing the trench isolation width will enhance spatial resolution， but leads to a significant reduction in image contrast. Furthermore， an increase in the trench-isolation width reduces the effective area of the electron multiplication region， consequently diminishing the signal level. Thus， the trench width should be minimized to the greatest extent possible.Under uniform doping conditions in the electron multiplication layer， increasing the trench height and reducing the trench spacing effectively suppresses lateral electron diffusion， thereby improving the spatial resolution. Although increasing the trench width from 1 μm to 3 μm helps improve resolution， it also leads to image degradation and a reduction in the effective area of the electron multiplication layer. Therefore， excessive trench width should be avoided. After optimization， the device achieves a limiting resolution of 60 lp/mm under the following parameters： a doping concentration of 1014 cm-3 in the electron multiplication layer substrate， a substrate thickness of 10 μm， and isolation trench dimensions of 2 μm in height， 10 μm in spacing， and 1 μm in width. This research proposes a new structure for improving the resolution performance of EBCMOS multiplication layers.