Formation of diamond trenches with flat {111} sidewalls via thermochemical etching by Ni films in water vapor

• Diamond trench {111} flat sidewalls were achieved via thermochemical etching. • The key success factor was the sufficient etching time for the sidewalls. • This achievement is critical to realize ultimate low-loss diamond MOSFETs. Trench-type inversion-channel diamond metal-oxide-semiconductor field-effect transistors (MOSFETs) are considered the ultimate low-loss power devices due to their superior material properties and structural advantages. The formation of diamond trenches with {111} sidewalls is essential for realizing these devices. Our recently developed thermochemical etching technique using Ni films in water vapor enables the formation of such trenches, which is challenging when conventional methods are used. However, under the standard etching temperature conditions, macrosteps of submicron scale or larger form on the sidewalls, hindering the formation of high-quality MOS interfaces and inversion channels. In this study, we systematically investigated the relationship between the etching temperature and the sidewall flatness to identify the optimal conditions for suppressing macrostep formation. Our findings indicate that macrosteps form on the sidewalls at temperatures of 940 °C or higher. In contrast, nanoscale-flat {111} sidewalls without macrosteps detectable by SEM were successfully fabricated at lower temperatures. This improvement in sidewall flatness was primarily attributed to the extended etching time for the sidewalls. These findings provide critical insights and a fundamental technique for fabricating trench-type inversion-channel diamond MOSFETs, thereby contributing to the advancement of next-generation low-loss power devices.