Cavity Etching Behavior on the M‐Plane of Sapphire Crystal

Wet etching has been a fundamental process for the fabrication of sapphire crystal substrates with critical position in the current semiconductor devices industry. The present work reveals the etching behaviors of cavities on the M‐plane {11¯${{\bar 1}}$00} of sapphire crystal by the hot KOH etchant. It is shown that the boundary of cavities changes from smooth circle to octagon‐like and the edges evolve sharper as the etching temperature increases. Carefully analyzing the structure of the etched cavities suggests they are partially framed by low index planes (21¯1¯${{\bar 1\bar 1}}$0) (1¯${{\bar 1}}$21¯${{\bar 1}}$0), (101¯${{\bar 1}}$0), (011¯${{\bar 1}}$0), and (112¯${{\bar 2}}$0). The appearance of the cavity on the M‐plane of sapphire is revealed to evolve into a nearly octagonal three dimensional structure with wet etching. Comparing the theory angles with the measured angles between the M‐plane and side walls of the cavity, the crystallographic planes in low‐index are determined.