Effects of compound diamond slurry with graphene for lapping of sapphire wafers

Recently, sapphire has been used in a wide range of applications including optics, consumer electronics, and especially the rapidly growing LED wafer manufacturing. This paper investigates effects of a novel compound slurry composed of micron scaled diamond abrasives and various nano-platelets of graphene oxide (GO) for improving lapping performance of c-plane single-crystal aluminum oxide (Al2O3) sapphire wafers. Four kinds of graphenes including GO, reduced graphene oxide (RGO), ML-RGO (laminate), and MF-RGO (few-layer) are added into the diamond slurry to achieve compound slurries. Those compound slurries have been prepared with various graphene weight fractions in the range of 0.3 to 2.0 wt%, by LP (low power) or HP (high power) ultrasonication with different time spans in the range of 10 min to 48 h to adjust size of GOs. For the lapping process conducted by 10-min HP ultrasonicated GO (2.0 wt%)/diamond slurry, combination of medium down force pressure and the highest platen rotation speed of 45 kPa/80 rpm results in highest material removal rate (MRR) of 926.73 nm/min, which is 324% higher than MRR by pure diamond slurry. It is caused by high reactivity of GOs with sapphire wafer. As lapped at pressure/speed of 45 kPa/70 rpm by ultrasonicated GO (0.5 wt%)/diamond compound slurry, the average surface roughness R-a of wafer can reach 7.19 nm, which is 0.7% lower than 7.24 nm by pure diamond slurry. Experimental results demonstrate that the optimized planarization of sapphire wafer with high MRR and low surface roughness can be achieved using the GO compound slurry when the mechanical removal effect and the chemical reaction mechanism are in synergistic equilibrium. The XPS and EDS analyses clearly support the material removal model that a reactant aluminum carbonate layer and a residual carbon layer exist and both are efficiently removed by diamond abrasives. Results of this study can be applied to further explore high volume production of CMP of sapphire wafers.