Atomic layer etching of MgO-doped lithium niobate using sequential plasma exposures

Lithium niobate (LiNbO₃, LN) is a ferroelectric crystal of interest for integrated photonics owing to its large second-order optical nonlinearity and the ability to impart periodic poling via an external electric field. However, on-chip device performance based on thin-film lithium niobate (TFLN) is presently limited by propagation losses arising from surface roughness and corrugations. Atomic layer etching (ALE) could potentially smooth these features and thereby increase photonic performance. Here, we report an isotropic ALE process for x-cut MgO-doped LN using sequential exposures of H₂ and SF₆/Ar plasmas. We observe an etch rate of 1.59 ± 0.02 nm/cycle with a synergy of 96.9%. The process is found to decrease the sidewall surface roughness of TFLN waveguides etched by physical Ar⁺ milling by 30% without additional wet processing. We also discuss alternate ALE chemistries using chlorine or bromine, as well as results for a directional etch. Our ALE process could be used to smooth sidewall surfaces of TFLN waveguides as a post-processing treatment, thereby increasing the performance of TFLN nanophotonic devices and enabling new integrated photonic device capabilities.