Ion Beam Milling As Part of a Deterministic Approach to Optical Fabrication

Abstract

Ion beam milling is a deterministic optical fabrication process which allows precise control over the removal of material from the surface of an optical component. A deterministic fabrication process is a well characterized and controllable process capable of directly achieving final figure with minimum number of metrology-fabrication iterations. Examples of more deterministic processes are diamond turning and ductile grinding; polishing is considered less deterministic due to the frequent polishing-metrology iterations that are required during polishing. Several investigators have developed and demonstrated ion milling as a deterministic process for glass and glass-ceramic optical components.1,2 While ion milling is more deterministic, it must be used in conjunction with other processes, typically as the final fabrication step, because of its relatively low material removal rates and inability to reduce surface roughness. Early studies of ion milling relied on polishing processes to prepare the optical surface prior to the final deterministic ion milling operation. These demonstrations of ion milling were also limited to amorphous, glass-like optical materials. A complete manufacturing process based on this approach would therefore be limited by the less-deterministic polishing operation. In this paper we present the results of experiments which relied completely on the deterministic processes of single point turning and ion milling to fabricate gold coated, aluminum mirrors. These experiments produced approximately a factor of two improvement in optical figure for both flat and spherical aluminum mirrors in a single ion milling iteration. Besides demonstrating a deterministic approach to optical fabrication involving ion milling, these experiments also indicate that it is possible to ion mill metallic mirrors provided the mirror is overcoated with a material which can be ion milled. The integration of ion milling with other deterministic fabrication processes, combined with recent ion milling-material studies,3 suggest similar highly deterministic manufacturing processes can be developed for many important optical materials including: electroless nickel, silicon carbide, and several IR transmissive optical materials.