Convergent Pad Polishing of Amorphous Silica

A new method of optical polishing termed Convergent Polishing is demonstrated where a workpiece, regardless of its initial surface figure, will converge to the lap shape in a single iteration. This method of polishing is accomplished by identifying the phenomena that contribute to non-uniform spatial material removal, and mitigating the non-uniformity for each phenomenon (except for the workpiece-lap mismatch due to the workpiece surface shape). The surface mismatch at the interface between the workpiece and lap causes a spatial and time varying pressure differential which decreases with removal, thus allowing the workpiece to converge to the shape of the lap. In this study, fused (amorphous) silica workpieces are polished using ceria slurry on various polyurethane pads. Polishing parameters were systematically controlled to prevent various sources of non-uniform material removal which include: (i) moment force, (ii) viscoelastic lap relaxation, (iii) kinematics, (iv) pad wear, and (v) workpiece bending. The last two are described herein. With these mitigations, removal uniformity has been demonstrated to within 1.0 mu m over the surface after 83 mu m of material removal corresponding to a within workpiece non-uniformity (WIWNU) of <1.2%. Also, convergence has been demonstrated down to 0.18 +/- 0.04 mu m peak-to-valley flatness on 100 mm-sized workpieces.