Improvement of Surface Figure in the Polyurethane Pad Continuous Polishing Process

Abstract

Primary factors determining surface figure of optics, including the velocity and pressure distributions, have been analyzed and improving methods provided accordingly for the pad continuous polishing (CP). With a septum on the exterior of the workpiece pre-pressing the pad, the inevitable depression on the edge of the workpiece incurred by the elastic response of the pad has been relieved obviously. By introducing the driving wheel system, the rotation speed of the septum & workpiece can be regulated and adjusted to that of the lap/pad, which produces a uniform velocity distribution on the polishing surface. Diamond conditioners with ring & disk patterns are employed to planarize the annular polyurethane pad and hence uniformise the pressure distribution on the workpiece/pad interface. Polishing experiments (Fused silica flat, 320 mm in diameter x 30 mm thick) conducted on the typical CP machine have demonstrated advantages of these methods.

Deterministic measurement and correction of the pad shape in full-aperture polishing processes

“…After the correction process, convexity of the pad shape is markedly decreased and hence the concave figure is obviously improved. With the corrected pad, the PV value is decreased from 2.38λ to 0.42λ (@ 632.8 nm), which is superior to most other polishing results [30,31]. For instance, Suratwala in Lawrence Livermore National Laboratory attained higher than 0.2λ PV flatness on 100 mm-sized workpieces by mitigating the non-uniform spatial material removal [31].…”