Kavli Affiliate: Austin J. Minnich
| First 5 Authors: Ivy I. Chen, Jennifer Solgaard, Ryoto Sekine, Azmain A. Hossain, Anthony Ardizzi
| Summary:
Lithium niobate (LiNbO$_3$, 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, but no ALE process has been
reported for LN. Here, we report an isotropic ALE process for $x$-cut MgO-doped
LN using sequential exposures of H$_2$ and SF$_6$/Ar plasmas. We observe an
etch rate of $1.59 pm 0.02$ nm/cycle with a synergy of $96.9$%. We also
demonstrate ALE can be achieved with SF$_6$/O$_2$ or Cl$_2$/BCl$_3$ plasma
exposures in place of the SF$_6$/Ar plasma step with synergies of $99.5$% and
$91.5$% respectively. The process is found to decrease the sidewall surface
roughness of TFLN waveguides etched by physical Ar$^+$ milling by 30% without
additional wet processing. 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.
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