Super hard and brittle material removal mechanism in fixed abrasive lapping: Theory and modeling

Currently, the problems of inefficient fixed abrasive lapping (FAL) for super hard and brittle material wafers (such as sapphire) with fixed abrasive pads (FAPs), restrict the development of the semiconductor industry. Most importantly, the mechanisms of FAL processes are still unclear, often analyzed based on the theoretical models of chemical mechanical polishing (CMP). However, the material removal models of CMP are specific to loose abrasives, do not effectively apply to the lapping processes for super hard and brittle material wafers with a FAP. In this paper, the spherical crown model modified by elastic-plastic mechanics and brittle-plastic transformation mechanism was applied to super hard and brittle material removal mechanism in FAL processes. In consideration of the influences of the yielding abrasives embedded in the FAP matrix and the matrix surface topography on the wafer-pad contact, a material removal model suitable for super hard and brittle material wafers lapped by FAPs was obtained. The loads applied on the fixed abrasives and the matrix asperities during the elastic-plastic-brittle deformation processes of wafers were analyzed, respectively. The model indicates that the back-off of the abrasives embedded in the FAP matrix intensifies along with the decrease of the matrix hardness and the increase of the abrasive size, respectively. The penetrating depth of the fixed abrasive can be raised by increasing matrix hardness, applied pressure, abrasive size, and decreasing abrasive number, as same as the elastic recovery of the super hard and brittle material slid by the abrasive after unloading. The pad matrix surface dressed by loose grains greatly affects the MRR of wafers. The predicted MRR of sapphire wafers based on the FAL model is consistent with experimental data. Finally, the keys to improving the processing efficiency of super hard and brittle material wafers are clarified in detail.