## 1 Introduction  
Lithium niobate on insulator (LNOI) or thin-film lithium niobate is a promising substrate for the production of high-frequency (up to 1 THz and more [1]) miniature photonic integrated circuits (PICs). The formation of two European platforms based on the application of LNOIs for PICs was started in 2022 (LOLIPOP (in combination with silicon-nitride (TriPleX) technology) [2] and ELENA [3]). This fact and the increasing number of science publications (for example [4, 5]) demonstrate the growing interest in LNOI for integrated optics applications around the world over the past few years. Using LNOI makes it possible to form high-contrast optical waveguides with a comparatively small curvature radius (100 \(\upmu\)m or less) and, consequentially, fabricate PICs with a high integration density (50-100 basic elements on less than 1 cm\({}^{2}\)). In the field of radio frequency (RF) applications, the use of LNOI provides an opportunity to adjust the propagation velocity of RF electric and optical waves and, as a result, to build miniature RF modulators [6] and high-frequency PICs based on it. Currently, the authors of this paper are focusing on developing the technology of PIC fabrication on LNOI substrates. The main basic step in this process is the search for technological regimes for producing waveguides with a minimal roughness of the sidewalls and material surface in the near-waveguide region. Typically, when producing integrated optical waveguides, a dry plasma etching process is used. In this case, significant efforts are being made to obtain the technological regimes for producing optical waveguides by multiple research groups applying a wide range of plasma sources and plasma-forming gases. As a consequence, the variety of regimes presenting a different quality and morphology of resulting surfaces were revealed (Table 1). In this study, the \"quality\" was measured using the following characteristics: the slope angle of optical waveguide sidewalls, aspect ratio, roughness of the sidewalls and material surface in the near-waveguide region. These characteristics significantly affect the light propagation in a waveguide and the waveguide parameters (such as backscattering, radiation losses and polarization maintenance). As shown in Table 1, ICP and ICP/RIE are the most commonly used types of plasma sources for the dry etching of LiNbO\({}_{3}\) (LN), and the majority of processes were carried out in a mixture of fluorine-containing gases and argon. Such a gas mixture makes it possible to form a profile of an optical waveguide with nearly vertical sidewalls [7; 10; 11; 12; 13; 14], but the formed structures have a high surface roughness [7; 8; 10; 14; 16]. Relatively high-quality structures were obtained in [9], but \(Z\)-cut LiNbO\({}_{3}\) was used as a substrate (\(Z\)-cut has a diverse X-cut etching mechanism that is complicated to use for RF PICs fabrication on LNOI since driving electrodes should be located above and below the optical waveguide). LiNbO\({}_{3}\) is well-known for being difficult to dry etch [17]. To overcome this in studies [11; 12; 13; 15], proton-exchanged (PE) LiNbO\({}_{3}\) was exposed to plasma. After the PE process, the concentration of Li ions in the near-surface layer of LiNbO\({}_{3}\) decreases, which subsequently has a positive effect on the roughness and aspect ratio of the plasma-etched surface and microstructures. At another point, the PE process of LiNbO\({}_{3}\) leads to the formation of a near-surface layer with structural disorder and the degradation of electro-optical and nonlinear prop