Deep Etching of LiNbO3 Using Inductively Coupled Plasma in SF6-Based Gas Mixture

This work is an extensive study of the plasma chemical etching (PCE) process of single-crystalline lithium niobate (LiNbO3) in the SF6/O2 based inductively coupled plasma (ICP). The influence of the main technological parameters of the LiNbO3 PCE process, including the distance between the sample and the lower edge of the discharge chamber, as well as the temperature of the substrate holder on the etching process rate, has been studied. It was shown that changing the temperature of the substrate holder in the range from 100 to 200°C leads to a gradual rise of the etching rate from 127 to 282 nm/min. A further increase in the temperature to 250°C results in a sharp increase in the etching rate to 711 nm/min. The maximum achieved etching rate in experimental series which were aimed at determining the dependence of the LiNbO3 etching rate on the temperature of the substrate holder was 812 nm/min at a substrate holder temperature of 325°C. With the help of X-ray photoelectron spectroscopy (XPS) technique was found and shown that during the etching process in fluorinated plasma the nonvolatile LiF compound is formed on the surface of the treated LiNbO3. On the basis of the obtained results, the optimal process of deep ( $> 80~\mu \text{m}$ ) LiNbO3 PCE was developed, with an etched wall inclination angle of ≈110°, with selectivity ratio to Cr mask of ≈ 20, and an etching rate of about 300 nm/min. [2020-0317]