Directional atomic layer etching of MgO-doped lithium niobate using Br-based plasma

TL;DR

This paper introduces a directional atomic layer etching process for lithium niobate using Br-based plasma, enabling precise pattern transfer with reduced redeposition and no aspect ratio dependent etching, improving fabrication quality for photonics.

Contribution

The study develops a novel directional ALE process for lithium niobate with Br-based plasma, overcoming previous isotropic limitations and redeposition issues, advancing TFLN device fabrication.

Findings

HBr chemistry reduces redeposition compared to F- and Cl-based plasmas.

The process achieves aspect ratio independent etching down to 150 nm gaps.

Etch depth of 220 nm with no ARDE observed.

Abstract

Lithium niobate (LiNbO$_3$, LN) is a nonlinear optical material of high interest for integrated photonics with applications ranging from optical communications to quantum information processing. The performance of on-chip devices based on thin-film lithium niobate (TFLN) is presently limited by fabrication imperfections such as sidewall surface roughness and geometry inhomogeneities over the chip. Atomic layer etching (ALE) could potentially be used to overcome these difficulties. Although an isotropic ALE process for LN has been reported, performing LN fabrication completely with ALE faces several challenges, including the lack of a directional ALE process for pattern transfer and the redeposition of involatile compounds. Here, we report a directional ALE process for LN consisting of sequential exposures of HBr/BCl$_3$/Ar plasma for surface modification and Ar plasma for removal. The HBr chemistry is found to decrease redeposition compared to F- and Cl-based plasmas, which we attribute to the higher vapor pressures of Br-based products. A grating pattern etched entirely by the process (total etch depth of 220 nm) exhibits no aspect ratio dependent etching (ARDE) down to the smallest tested gap of 150 nm, in contrast to ion milling in which ARDE manifests even at 300 nm gaps for the same etch depth. The HBr plasma chemistry is also found to support an isotropic process consisting of sequential exposures of H$_2$ plasma and HBr/BCl$_3$/Ar plasma. These processes could be used together to perform the complete fabrication process for TFLN devices, eliminating imperfections arising from ion milling.