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

On-chip photonic devices based on SiO₂ are of interest for applications such as microresonator gyroscopes and microwave sources. Although SiO₂ 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 SiO₂ 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 SiO₂ using sequential exposures of Al(CH₃)₃ (trimethylaluminum, TMA) and Ar/H₂/SF₆ plasma. We find that each process step is self-limiting, and that the overall process exhibits a synergy of 100%. We observe etch rates up to 0.58 {A} per cycle for thermally-grown SiO₂ and higher rates for ALD, PECVD, and sputtered SiO₂ up to 2.38 {A} 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 SiO₂ optical devices and thereby enabling device quality factors to approach limits set by intrinsic dissipation.