The investigation of the LiF redeposition process during reactive ion etching of thin-film lithium niobate with fluorine plasma

Abstract

This paper investigates LiF redeposition during reactive ion etching (RIE) of X-cut thin-film lithium niobate (TFLN) with Ar/SF6 ICP/CCP plasma. RIE of TFLN is a topochemical reaction (TCR), described by the Kolmogorov-Avrami-Erofeev equation, during which there occurs LiF redeposition from the supersaturated vapor phase onto the sample surface. It is shown that the redeposition process is characterized by an induction period, which depends on the etch rate and the TFLN crystal structure imperfections. To study the LiF redeposition process, we used spectroscopic reflectometry, energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The dislocation density of TFLN was investigated with the help of etch pit method and X-ray diffraction. This study is the first to show that the duration of the induction period of TCR on the TFLN surface is inversely related to the etch rate. The RIE process is characterized by critical etch rates, above which a thin LiF film grows according to the two-dimensional Volmer-Weber mode. It was experimentally shown that the critical etch rates decrease with increasing the process pressure. For the first time, the dislocation density of TFLN was experimentally evaluated by means of the etch pit method. An experimental comparison of the induction periods of TCR on the surface of bulk and TFLN was carried out. It has been revealed that the increased dislocation density of TFLN significantly reduces the duration of the induction period. The impact of the modification of the TFLN structure with proton exchange on the kinetics of RIE was studied. It was demonstrated that the replacement of Li+ with H+ leads to decrease in the degree of LiF vapor saturation above the TFLN surface and, as a consequence, to a significant increase in the induction period of TCR and an increase in the etch rates by 5–9 times compared to unmodified TFLN.