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

TL;DR

This work demonstrates a directional atomic layer etch (ALE) process for MgO-doped LiNbO3 using Br-based plasma chemistry (HBr/BCl3/Ar) followed by Ar removal, achieving nm-scale etch per cycle and high pattern-transfer fidelity with ARDE-free etching. The process reduces redeposition of nonvolatile products, particularly at 0 °C, while maintaining surface smoothness, whereas higher temperatures increase the HBr-assisted etch rate but reduce synergy due to spontaneous etching. The authors also show an isotropic Br-based ALE variant and apply the directional process to a TFLN grating, achieving complete etching down to 150 nm gaps with preserved top widths and reduced surface roughness. These results jointly indicate that Br-based directional ALE can enable complete, nm-precise fabrication flows for TFLN devices, potentially improving device performance by mitigating roughness and geometric imperfections. The work lays the groundwork for integrating directional ALE into TFLN manufacturing, offering a path toward high-fidelity pattern transfer and smoother device surfaces in photonic circuits.

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.

Figures (7)

• Figure 1: Schematic of the directional ALE process for LN. First, an HBr/BCl$_3$/Ar plasma exposure modifies the surface, followed by a purge. A low-power Ar plasma follows to sputter off the modified surface. A final purge completes the ALE cycle.
• Figure 2: Etch per cycle for the half cycles and full cycle of the directional ALE process at 0 and 200. The synergy at 0 (200) is 84.6% (56.8%).
• Figure 3: (a) EPC versus HBr/BCl$_3$/Ar plasma exposure time with Ar plasma exposure time fixed at 60 s. (b) EPC versus Ar plasma exposure time with HBr/BCl$_3$/Ar plasma exposure time fixed at 40 s. All experiments were performed at 0. Dashed lines are guides to the eye.
• Figure 4: Surface XPS spectra of (a) Nb3d, (b) O1s, and (c) Br3d, Nb4s, Li1s, and Mg2p. The spectra are (top) original (middle) directional-ALE etched LN over 50 cycles at 0 and (bottom) directional-ALE etched LN over 50 cycles at 200. (d) Atomic concentrations for original, 50 cycles directional ALE etched at 0, and 50 cycles directional ALE etched at 200. The atomic percentages are listed in \ref{['tab:xps_table']}.
• Figure 5: AFM scan showing height-maps of LN surface (a) before, (b) after 20 ALE cycles at 0, (c) after 20 ALE cycles at 200, and (d) 20 ALE cycles at 200 using a Cl$_2$/BCl$_3$ modification step.