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) 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 SiO2 and higher rates for ALD, plasma enhanced chemical vapor deposition, and sputtered SiO2 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 SiO2 optical devices and thereby enabling device quality factors to approach limits set by intrinsic dissipation.