Experimental study and modeling of the effect of mixed size abrasive grits on surface topology and removal rate in wafer lapping

The semiconductor industry has continued to increase the diameter of wafers in recent years, which poses a challenge in the lapping of prime wafers as the processing time is proportional to the square of the diameter, and the surface quality is a function of the features generated in the lapping process. As a free abrasive machining (FAM) process, where the abrasive grits act as third-body particles, lapping is influenced by abrasive size distributions; however, past studies focus on a single abrasive or size distribution, where the effects of mixed size abrasive distributions on surface feature generation are still unknown. In this study, lapping experiments are conducted on silicon by mixing two SiC abrasive grits, with different mean sizes and at various ratios, under two normal loadings. Lapped surfaces are examined by optical microscopy, where the number and size of critical surface feature types are characterized quantitatively with image processing. The results are correlated with the material removal rate (MRR) by modeling a lapping quality index (LQI) to evaluate different mixed abrasive ratios, where it is shown that lapping performance can be improved by mixing abrasives at high loadings.