Removal of Surface Residual Stress in a Sapphire Substrate by Etching

An etching method is proposed for the removal of surface residual stress in sapphire substrates after lapping. In this method, a double-sided lapped sapphire substrate is etched with concentrated acid in a 3:1 ratio of sulfuric acid to phosphoric acid at 280 °C and then polished with single-sided chemical mechanical polishing (CMP) until the substrate damaged layer is removed. The bow and warp of the sapphire substrate are measured and compared before and after single-sided CMP. The changes of bow and warp can indirectly reflect the surface residual stress of the etched substrate. The sapphire substrate with a relatively small etching thickness shows a relatively large change in bow and warp after single-sided CMP, and vice versa, which indicates that a larger etching thickness results in a smaller surface residual stress in the substrate and a larger residual stress removed. However, the sapphire substrate doesn't show much change in the bow and warp after applying single-sided CMP at an etching thickness of 15 μm and beyond, which signifies the complete removal of the residual stress. This method demonstrates that the surface residual stress in a sapphire substrate can be completely removed by etching a suitable thickness.