This paper describes POCO's new capability to rapidly produce large silicon carbide mirror substrates by conversion joining segments of silicon carbide during the process of converting graphite to silicon carbide. Mirror segments and structures are machined from a special graphite and subsequently joined together during the conversion process with the end result being a high purity beta silicon carbide structure. Interface boundaries are removed by the crystal growth across boundaries as the graphite crystal structure is converted to the larger crystal size of silicon carbide. Results of conversion joining development, design guidelines and limitations of the conversion joining process will be presented.
Silicon carbide (SiC) has long been recognized as an attractive mirror material due to its superior mechanical and thermal properties when compared to conventional optical materials. However, the material properties of SiC which make the material attractive from a design standpoint have often precluded its use when low cost and rapid delivery of an optical mirror were required. This paper describes an approach that was developed by Zygo Corporation, working in conjunction with POCO Graphite Inc. for the rapid fabrication of lightweight silicon carbide optics. This approach utilizes a SiC substrate produced by POCO Graphite Inc. using a non-traditional process coupled with deterministic finishing techniques developed by Zygo. Using this approach, we are now capable of producing high quality prototype components (λ/10 PV figure, 10 Å rms. micro roughness) of SiC within a few weeks, rather than the traditional period of months. Applications include lightweight scan mirrors, stage mirrors for lithography positioning tools, as well as similar applications where high stiffness and low weight are significant performance parameters. MRF polishing results comparing SiC to conventional optical materials are also presented.
Silicon carbide may well be the best known material for the manufacture of high performance optical components. A combination of extremely high specific stiffness (r/E), high thermal conductivity and outstanding dimensional stability make silicon carbide superior overall to beryllium and low-expansion glass ceramics. A major impediment to wide use of a ceramic material such as silicon carbide is the ability to create a design that fully utilizes these properties while addressing limitations such as brittleness. This paper discusses an approach for design that will optimize the use of properties while considering the manufacturability of the component. POCO has developed a manufacturing process that lends itself to efficient design of complex optical components. The manufacturing process described here-in begins by machining the component from a special type of graphite. This graphite is easily machined with multi-axis CNC machine tools to any level of complexity and lightweighting required. The graphite is then converted completely to silicon carbide with very small and very predictable dimensional change. After conversion to silicon carbide the optical surface is coated with very fine grain silicon carbide which is easily polished to extreme smoothness using conventional optical polishing techniques. The design approach presented requires an iterative process between the 'system' designer and the materials and component designer. Both he fabrication process and the design approach are described in this paper.
Silicon carbide may well be the best known material for the manufacture of high performance optical components. This material offers many advantages over glasses and metals that have historically been used in high performance optical systems. A combination of extremely high specific stiffness (E/r), high thermal conductivity and outstanding dimensional stability make silicon carbide superior overall to beryllium and low-expansion glass ceramics. A major impediment to wide use of silicon carbide in optical systems has been the cost associated with preliminary shaping and final finishing of silicon carbide. Because silicon carbide is an extremely hard and strong material, precision machining can only be done with expensive diamond tooling on very stiff high quality machine tools. Near-net-shape slip casting of silicon carbide can greatly reduce the cost of silicon carbide mirror substrates but this process still requires significant diamond grinding of the cast components. The process described here begins by machining the component from all special type of graphite. This graphite can rapidly be machined with conventional multi-axis CNC machine tools to achieve any level of complexity and lightweighting required. The graphite is then directly converted completely to silicon carbide with very small and very predictable dimensional change. After conversion to silicon carbide the optical surface is coated with very fine grain CVD silicon carbide which is easily polished to extreme smoothness. Details of the fabrication process are described and photos and performance specifications of an eight-inch elliptical demonstration mirror are provided.