Dependence of etching resistance properties on different crystal orientation of sapphire in fluorine-based environment

Alumina have been widely applied as the first generation etching resistance ceramics materials, but their etching behavior and related mechanism were not investigated at atomic level in detail. In this paper, the etching-resistance performance of α-Al₂O₃ single crystal on the (11 $\overline{2}$ 0), (0001), and (11 $\overline{0}$ 2) planes (refer to A, C, R plane) were explored theoretically and experimentally. The first principle calculation was adapted to evaluate the adsorption energy of F and CF₃ radical on the three crystal planes with different orientations respectively. The results indicated that the C-plane oriented adsorption structure was the most stable. Then, the reaction energy barriers required to dissociate CF₃ radicals from the surface to form Al-F bonds were calculated to be 0.78, 0.71 and 0.82 eV, respectively, which further indicated that the C-plane was more prone to fluorinate to form AlF_xO_y on the surface, and the energy required to remove AlF₃ achieved the highest value of 6.15 eV, which relatively prevented the erosion to continue further. Meanwhile, the polished sapphire surfaces with (11 $\overline{2}$ 0), (0001) and (11 $\overline{0}$ 2) orientations were etched in SF₆ atmosphere to examine the etching-resistance performance. It was demonstrated that the C-plane exhibited the best surface quality and the lowest etching rate after fluorine-based plasma etching, while the R-plane exhibited significantly worse plasma etching resistance than that of the C-plane and A-plane. The experimental results were in good agreement with the DFT calculation.