Citation: Aryal, A.; Stricklin, I.; Behzadirad, M.; Branch, D.W.; Siddiqui, A.; Busani, T. High-Quality Dry Etching of LiNbO 3 Assisted by Proton Substitution through H 2 -Plasma Surface Treatment. Nanomaterials 2022, 12, 2836. https:// doi.org/10.3390/nano12162836 Academic Editor: Tzi-yi Wu Received: 12 July 2022 Accepted: 11 August 2022 Published: 18 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article High-Quality Dry Etching of LiNbO 3 Assisted by Proton Substitution through H 2 -Plasma Surface Treatment Arjun Aryal 1 , Isaac Stricklin 1,2 , Mahmoud Behzadirad 1 , Darren W. Branch 3 , Aleem Siddiqui 3 and Tito Busani 1,2, * 1 Center for High Technology Materials (CHTM), University of New Mexico, MSC01 04-2710, 1313 Godard St. SE, Albuquerque, NM 87106-4343, USA 2 Electrical and Computer Engineering (ECE), University of New Mexico, MSC01 11001, Albuquerque, NM 87131-0001, USA 3 Sandia National Laboratories, 1515 Eubank Blvd SE, Albuquerque, NM 87123, USA * Correspondence: busanit@unm.edu Abstract: The exceptional material properties of Lithium Niobate (LiNbO 3 ) make it an excellent material platform for a wide range of RF, MEMS, phononic and photonic applications; however, nano-micro scale device concepts require high fidelity processing of LN films. Here, we reported a highly optimized processing methodology that achieves a deep etch with nearly vertical and smooth sidewalls. We demonstrated that Ti/Al/Cr stack works perfectly as a hard mask material during long plasma dry etching, where periodically pausing the etching and chemical cleaning between cycles were leveraged to avoid thermal effects and byproduct redeposition. To improve mask quality on X- and Y-cut substrates, a H 2 -plasma treatment was implemented to relieve surface tension by modifying the top surface atoms. Structures with etch depths as deep as 3.4 μm were obtained in our process across a range of crystallographic orientations with a smooth sidewall and perfect verticality on several crystallographic facets. Keywords: thin films; Lithium Niobate; microstructures; fabrication; plasma etching; silicon integration 1. Introduction Lithium Niobate (LiNbO 3 or LN) has proven to be the material of choice for a wide range of applications due to its exceptional piezoelectric, electro-acoustical, electro-optical, and non-linear optical properties [1]. The different crystallographic orientations of LN are heavily utilized for applications in surface-acoustic-wave (SAW) resonators [2], optical filters [3], optical sensors [4], modulators [5–7], transducers [8,9], optical waveguides [10,11], Q-switch lasers [12,13], oscillators [14], etc. For example, high temperature (1000 ◦ C) annealing Ti-doped X cut LN can be utilized as an optical waveguide [15]. At the nano- and micro-scale, device performance is often constrained by the fabrication quality of processed LN films [16]. However, unlike many semiconductors and dielectric materials, LN substrates are complex for processing and are notoriously difficult to etch [16]. Typically, a long plasma dry-etch is required to obtain high-aspect ratio or deep etching profiles [17] in LN substrates, and, hence, a successful LN etching process needs to manage factors including substrate heating, redeposition of the etching byproducts and mask materials, and durability of the mask materials over the etching process. Dry etching of LN substrates was studied by different research groups [18–26] and different plasma etch conditions have been investigated to optimize the quality of the resulting structures. Fluorine (F 2 ) based plasma sources have most commonly been used to etch LN [24]; however, the use of this source is accompanied by the redeposition of LiF and its byproducts, which reduce the etching rate and create unfavorable features due to local micro-masking effects. The byproducts from fluorine-based dry-etch evaporate at only 800 ◦ C, thus remaining on the surfaces and the sidewalls of the structures. This Nanomaterials 2022, 12, 2836. https://doi.org/10.3390/nano12162836 https://www.mdpi.com/journal/nanomaterials