Study On Fixed-abrasive Lapping Process Of Single Crystal Sapphire Substrates

Sapphire (single-crystal Al2O3) is a typical material which is difficult to machine with a high hardness (next to diamond) and great brittleness. Loose abrasive lapping followed by polishing are typically process for manufacturing of sapphire wafers. Yet, the lapping efficiency and cost of the lapping process are far from satisfaction because of the loose abrasive lapping itself. Furthermore, the loose abrasive lapping will introduce deep surface and subsurface damage and thus will cause more processing capacity for the polishing process. According to the disadvantages of the majority of processes to manufacture sapphire wafers, the main content and conclusion of this paper are shown as follows:(1) Different lapping means for manufacturing of sapphire wafers were compared and the mechanism of fixed abrasive lapping (FAL), the motion trace between the fixed abrasive lapping plate (FALP) and sapphire wafer were studied. The motion trace between the FALP and sapphire wafer was simulated according to the experiment setup using MATLAB program. To achieve the goal of uniform abrasion of lapping tool, the result of simulation was analyzed and the ratio of sapphire wafer speed against FALP speed was chosen as 0.6.(2) In order to solve the difficulty of FALP dressing, the loose abrasive lapping was applied in FALP dressing. The slurry with 4wt% of W12 Al2O3 abrasive was used for the dressing process. The results showed that the dressing method performed well in dressing FALP, and the lapping performance was stable. In this way, due to the hardness of the abrasive is no higher than sapphire substrates, the loose abrasive would not affect the further process.(3) The material removal rate, surface roughness Ra, self-sharpening ability and surface/subsurface damage induced by diamond FALP with different types of bond and different grain size were investigated. The results showed that the diamond FALP with vitrified-resin composite bond exhibited better grinding performance than another two diamond FALP with vitrified and resin bonds. As the decreased in grain size of the diamond lapping plate, the removal mode of ductile regime exhibited more than brittle fracture. The surface showed more scratches than brittle mode with a pitted surface.(4) The process parameter was optimized using FALP with different grain sizes of W40 and W7. The MMR and surface roughness Ra were compared with the different grain sizes of W40 and W7 FALP. The results showed that FALP with larger grain size was more sensible to lapping pressure according to less contact area induced by the low consistency of grain height. And FALP with fine grain exhibited a good manufacturing stability and less sensible to lapping pressure according to its high consistency of grain height. And this makes it possible to work in a high speed manufacturing condition.According to all the results, a new high efficiency and low damage lapping process of sapphire substrates was proposed by stages using W40 FALP to remove material quickly, W20 FALP to remove the subsurface damage caused by W40 FALP and reduce the allowance, W7 to obtain a low surface roughness less than 10nm, and W2.5 FALP to obtain a low subsurface damage and super smooth surface with Ra<2.5nm.