Low temperature wet etching to reveal sub-surface damage in sapphire substrates

Low temperature wet etching using various etchants was investigated to reveal sub-surface damage in sapphire wafer induced during lapping. Surface scratches during wafer preparation is conveniently observed using optical or atomic force microscopy whereas sub-surface damage to crystal requires techniques such as X-ray diffraction, Raman spectroscopy etc. In this study, sub-surface damage in sapphire was revealed as shallow scratches by etching in H2SO4 at temperatures ∼125 °C and 3:1 H2SO4–H3PO4 at temperatures ∼75 °C. These etching conditions showed no measurable etch rate of sapphire and also did not affect the pristine sapphire surface. The heavily damaged and sub-surface damaged layer was determined to be 1.6 ± 0.1 and 2.2 ± 0.1 μm deep by repeated chemical mechanical polishing and etching of sapphire wafer lapped with 1 μm diamond abrasive.