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Lithium niobate (LiNbO$_3$, LN) is a ferroelectric crystal of interest forintegrated photonics owing to its large second-order optical nonlinearity andthe ability to impart periodic poling via an external electric field. However,on-chip device performance based on thin-film lithium niobate (TFLN) ispresently limited by optical loss arising from corrugations between poledregions and sidewall surface roughness. Atomic layer etching (ALE) couldpotentially smooth these features and thereby increase photonic performance,but no ALE process has been reported for LN. Here, we report a directional ALEprocess for $x$-cut MgO-doped LN using sequential exposures of H$_2$ andSF$_6$/Ar plasmas. We observe etch rates up to $1.01 \pm 0.05$ nm/cycle with asynergy of $94$%. We also demonstrate ALE can be achieved with SF$_6$/O$_2$ orCl$_2$/BCl$_3$ plasma exposures in place of the SF$_6$/Ar plasma step withsynergies above $90$%. When combined with a wet post-process to removeredeposited compounds, the process yields a 50% decrease in surface roughness.With additional optimization to reduce the quantity of redeposited compounds,these processes could be used to smoothen surfaces of TFLN waveguides etched byphysical Ar$^+$ milling, thereby increasing the performance of TFLNnanophotonic devices or enabling new integrated photonic capabilities.

Directional atomic layer etching of MgO-doped lithium niobate using sequential exposures of H$_2$ and SF$_6$ plasma