Atomic layer etching of SiO₂ using sequential exposures of Al(CH₃)₃ and H₂/SF₆ plasma

On-chip photonic devices based on $SiO 2$ are of interest for applications such as microresonator gyroscopes and microwave sources. Although $SiO 2$ microdisk resonators have achieved quality factors exceeding one billion, this value remains an order of magnitude less than the intrinsic limit due to surface roughness scattering. Atomic layer etching (ALE) has potential to mitigate this scattering because of its ability to smooth surfaces to sub-nanometer length scales. While isotropic ALE processes for $SiO 2$ have been reported, they are not generally compatible with commercial reactors, and the effect on surface roughness has not been studied. Here, we report an ALE process for $SiO 2$ using sequential exposures of Al(CH₃)₃ (trimethylaluminum) and Ar/H₂/SF₆ plasma. We find that each process step is self-limiting, and that the overall process exhibits perfect synergy, with neither isolated half-cycle resulting in etching. We observe etch rates up to 0.58 Å per cycle for thermally grown $SiO 2$ and higher rates for ALD, plasma enhanced chemical vapor deposition, and sputtered $SiO 2$ up to 2.38 Å per cycle. Furthermore, we observe a decrease in surface roughness by 62% on a roughened film. The residual concentration of Al and F is around 1%–2%, which can be further decreased by O₂ plasma treatment. This process could find applications in smoothing of $SiO 2$ optical devices and thereby enabling device quality factors to approach limits set by intrinsic dissipation.