## 4 Discussion
Let us make rough estimates of the ion energies. For simplicity, only Ar\({}^{+}\) ions will be considered. For the strict review, the ions formed as a result of the decomposition of SF\({}_{6}\) should also be taken into account. According to [37], for W\({}_{\rm ICP}\) = 500 W, V\({}_{\rm bias}\) = 100 V and p = 0.03 mbar, the Ar\({}^{+}\) ion energies were distributed in the range from 20 to 80 eV. The form of the IED was bimodal and the average energy was equal to 40 eV. The increase in the bias voltage from 100 V to 190 V (similar to the value of V\({}_{\rm bias}\) in recipe #13) resulted in an increase in the average ion energy to \(\sim\)90 eV. Ion energy spread increased significantly too. When the pressure decreased from 0.03 mbar (in recipe #13) to 0.007 mbar (in recipe #14) and the V\({}_{\rm bias}\) decreased to 140 V, the average ion energy decreased to \(\sim\)30 eV. We assumed that at a certain energy value (transition point) in the ion energy range from 30 to 90 eV, the transition from plasma-chemical etching to RIE occurred. Note that the energy range given above was a rough estimation and requires clarification. To present the model explaining the kinetics of the RIE of LiNbO\({}_{3}\), let us turn to [PERSON]'s model described above. The proposed model of the RIE process was based on [PERSON]'s model, where the bombardment of the substrate surface with accelerated ions was also taken into consideration. [PERSON] et al. discovered numerous swellings of the LN subsurface layer (cavities with gaseous reaction products) at the induction etching stage. At the stage of intense etching, the swellings exploded with the following porous LiF film formation. It was assumed that the diffusion of gaseous reaction products was hindered due to the formation of a thin film of LiF in the near-surface layer. This was observed by [PERSON] et al. during the radical chemical reactions at relatively high process pressures (from 0.1 to 1 mbar) and low ion energies. A similar effect was observed in our experiments during the etching with recipe #14. In the case of RIE (with recipe #13), the average Ar\({}^{+}\) ion energy was close to 100 eV. This energy was sufficient for the formation of vacancies in the LiF layer [38]. Thus, an increase in the number of radiation-induced defects in the near-surface layer was observed. Additionally, [PERSON] et al. [39] observed the growth of dislocation density in the near-surface layer of the X-cut LN after the Ar\({}^{+}\) ion bombardment (note that the Ar\({}^{+}\) ion energies in [39] were lower than in recipe #13). The increase in the number of defects in the near-surface layer (vacancies or dislocations) led to an increase in the diffusion coefficient of the reaction products (the validity of these conclusions was confirmed by [40]). As a result, niobium fluorides and oxyfluorides did not form any swellings and were able to diffuse onto the surface of the LiNbO\({}_{3}\) through the LiF layer. In addition, high-energy ions enhanced the desorption of the reaction products from the surface. As for the LiF layer that formed on the LN surface, we supposed that the Ar\({}^{+}\) ion energy during the etching with recipe #13 was sufficient for the dissociation of Li and F, and, as a consequence, the surface in the near-waveguide region was smooth due to the effect of surface sputtering. At the moment, the model explaining the kinetics of RIE of LiNbO\({}_{3}\) made it possible to explain the results of performed experiments, but the model still required rigorous calculations and additional detailed experimental data.