Characterization of anisotropic wet etching of single-crystal sapphire

Abstract

Based on the use of a number of spherical samples of single-crystal sapphire, this paper reports the etch rate characteristics of this material as a function of orientation during anisotropic wet etching at high temperatures (202, 223, and 236 °C) in various solutions (H2SO4:H3PO4 = 1:1, 3:1, and 6:1 by volume). The results show that sapphire has a different three-dimensional anisotropy as compared to other trigonal crystals. In addition, having measured the etch rates at different temperatures, the apparent activation energy of the etch rate can be determined for any orientation. Furthermore, the side wall planes appearing on micro structures during the etching process can be well explained by the overall etch rate distribution. Finally, the use of the truncated spheres enables studying the surface morphology of any crystal plane. In particular, the hillock morphology observed on the c-plane can be easily described by a simple mask/maskless etching mechanism involving the onset/offset of micromasks by bubbles and/or solidified products randomly in space and time. The resulting etch rate database allows the numerical prediction of the etch front and helps understanding the etching mechanism of the sapphire crystal.