Toward Deterministic Material Removal and Surface Figure During Fused Silica Pad Polishing

Abstract

The material removal and surface figure after ceria pad polishing of fused silica glass have been measured and analyzed as a function of kinematics, loading conditions, and polishing time. Also, the friction at the workpiece/lap interface, the slope of the workpiece relative to the lap plane, and lap viscoelastic properties have been measured and correlated to material removal. The results show that the relative velocity between the workpiece and the lap (i.e., the kinematics) and the pressure distribution determine the spatial and temporal material removal, and hence the final surface figure of the workpiece. In cases where the applied loading and relative velocity distribution over the workpiece are spatially uniform, a significant nonuniformity in material removal, and thus surface figure, is observed. This is due to a nonuniform pressure distribution resulting from: (1) a moment caused by a pivot point and interface friction forces; (2) viscoelastic relaxation of the polyurethane lap; and (3) a physical workpiece/lap interface mismatch. Both the kinematics and these nonuniformities in the pressure distribution are quantitatively described, and have been combined to develop a spatial and temporal model, based on Preston's equation, called Surface Figure or SurF. The surface figure simulations are consistent with the experiment for a wide variety of polishing conditions. This study is an important step toward deterministic full‐aperture polishing, allowing optical glass fabrication to be performed in a more repeatable, less iterative, and hence more economical manner.