Sub-Surface Crack Analysis for Diamond Slurry Lapping of Sapphire Wafers

Sapphire or mono-crystalline aluminum oxide wafers have been popularly adopted as the substrates of epi-layer process for the rapid growing demands of high-brightness light emitting diode (HB LED) industry. The HB LED devices for solid-state illumination have attracted immerse investigation of sub-surface crack problem induced in sapphire wafer processing. The diamond lapping has been recognized as a critical process for affecting the final polishing of sapphire wafers. This paper is to investigate the sub-surface damage layer induced by the diamond slurry lapping processes of sapphire wafers with two kinds of benchmark diamond slurries with different properties. All test wafers have been measured by dial gauge to observe the variation of thickness first and then the surface topography and subsurface properties have been inspected by coherence interferometer, CCI-Lite (Taylor Hobson, UK) instrument and then dual-focus focused ion beam, FIB (FEI Quanta 3D FEG) for sub-surface crack observation. The diamond slurry is composed with diamond abrasive grits and carrier for lapping with resin copper grooved plate. In this study, two slurries are noted as F and S both with the nominal diamond grit size 3 micron. The carriers are glycol based with viscosity ranged around 20 cP in room temperature and the viscosity-temperature plots have been measured for such slurry. Experimental results have shown that the 3D average surface roughness, S_a is obtained as 9.12 nm for sapphire wafer lapped by F slurry and as 6.63 nm lapped by S slurry. Thus the relationship of particle size distribution (PSD) of diamond grits and also the viscosity-temperature effect can be considered as key factors to MRR and surface quality of diamond lapping process of sapphire wafers. The sub-surface cracks can then be observed by FIB instrument and then quantified to evaluate the effects of diamond lapping process in order to shorten the further polishing time.