Avalanche Multiplication Image Sensor Bonded With Crystalline Se Photoconversion Layer Using Se–Se Bonding Process

— We have developed a bonded CMOS image sensor using a gallium oxide (Ga₂O₃)/polycrystalline selenium (c-Se)/nickel oxide photodiode and a Se–Se bonding process. These photodiodes can amplify the signal by avalanche amplification when a high electric field is applied, thus realizing a high-sensitivity image sensor. The Se–Se bonding process has many advantages, including the ability to handle high-temperature processing, no sputtering damage to the p-n junction interface, and no need for high-precision alignment. In this study, we have shown that the application of a high-temperature process to crystallize Ga₂O₃ improves the orientation of the c-Se, which is the photosensitive layer, and reduces the dark current. In addition, the Se–Se bonding process reduces the sputtering damage at the Ga₂O₃/c-Se interface, improving film quality. By applying the developed bonding process, we successfully realized a high-sensitivity CMOS image sensor with a threefold increase in magnification.