## 4 Discussion  
A detailed comparison of the surface state of the samples etched with recipes #13 and #14 was undertaken for an analysis of the obtained results and an understanding of the kinetics of the etching process. Magnified views of the samples' surfaces etched with recipes #13 and #14 are shown in Figure 6.  
The following should be noted:  
1. The mask selectivity was equal to 1:5.3 during the etching with recipe #13, and after the reduction in the pressure during the etching with recipe #14, the mask selectivity improved to 1:8. 2. The surface roughness in the near-waveguide region dramatically increased when recipe #14 was used. 3. During the etching with recipe #13, microtrenches were formed along the waveguide base. A schematic representation of the waveguide profile obtained with recipe #13 is shown in Figure 7.  
Figure 5: SEM images of the waveguide profiles formed with various etching recipes: (**a**) waveguide profile etched with recipe #13; (**b**) waveguide profile etched with recipe #14. Defects on the waveguide front edge were formed because of a cleavage procedure. Figure 6: SEM images (at magnification 47 k) of the waveguide and the near-waveguide region: (**a**) sample etched with recipe #13; (**b**) sample etched with recipe #14. Microtrenches may affect optical mode characteristics, such as the confinement factor; however, a detailed analysis was not performed during this work. It is shown in articles [32; 33; 34] that the microtrench formation process is a feature of RIE processes, where the physical component of the plasma has a significant impact. According to mentioned articles, the reason for the microtrench formation is as follows. Positive ions are accelerated to the wafer by the RF-biased electrode sheath. At the same time, plasma electrons tend to accumulate on the sidewalls of the feature (i.e., edges of the mask layer and any microstructures with relief, including the waveguide sidewalls). Therefore, ion trajectories tend to be deflected toward sidewalls, resulting in increasing the etching rate near the edges of the feature (including the base of the waveguide) and reducing the etching rate over a distance from features. In the case of a low etching pressure (<10 Pa or 0.1 mbar) and a relatively high biasing potential, the physical component of plasma, namely, accelerated ions, becomes significant and, as a consequence, microtrenches may be formed, which happened during the etching with recipe #13. During the RIE process with recipe #14, microtrenches were not formed. Regarding mask selectivity, chromium was not chemically etched in the used gas mixture, and the decrease in mask selectivity during the RIE with recipe #13 also indicated an increased influence of the physical component of the plasma. Let us consider the kinetics model of the LiNbO\({}_{3}\) etching process proposed by [PERSON] et al. in [23] to explain the results obtained in this work after the RIE with recipe #14. [PERSON] et al. considered the radical etching process of LiNbO\({}_{3}\) as a topochemical reaction. The proposed model was based on experimental data for a range of pressures from 10 to 1000 Pa. In the case of an absence of a significant bias voltage, the abovementioned pressure range determines the etching mode with a dominating chemical plasma component, where the energy of the physical plasma component is insufficient for the near-surface layer sputtering and defects creation. The authors [23] explained the kinetics of plasma etching as a two-stage process that includes the induction period and the stage of intense etching. The etching rate is low during the induction stage. This stage includes the following:  
1. Diffusion of the fluorine radicals from the plasma to the sample surface via a plasma sheath. 2. Adsorption of fluorine radicals on the surface of the LiNbO\({}_{3}\). 3. Diffusion of adsorbed atoms into the near-surface layer. 4.