Direct Sintered Silicon Carbide (SSiC) is a promising material to fabricate large (over 1 meter diameter) land and space
based mirror optics due to its low areal density, high stiffness and high thermal stability. To make large mirror optics for
visible wavelength applications, sub-nanometer surface roughness is required, which can be achieved by cladding a SSiC
substrate using SiC chemical vapor deposition (CVD). Limitations on available equipment to clad monolithic structures
of this size require that smaller segments need to be clad first and then joined prior to being optically finished. To
demonstrate the viability of this method of fabrication, a segmented &nullset;300mm visible quality lightweighted concave
mirror has been manufactured and characterized. The mirror's 6 radial segments, coated with a SiC CVD layer on the
SSiC substrate were joined by means of a silicon based braze, formulated so that its thermal expansion matched that of
the SSiC substrate and SiC CVD layer. After figuring and polishing to optical quality, the mirror's stability was
characterized under vacuum at three temperatures (120 K, 293 K, and 520 K) by measuring the wave front error (WFE).
Direct Sintered Silicon Carbide (SSiC) is a promising material for mirror optics due to its low density, high stiffness and high thermal stability. In order to make large mirror optics (over 1 meter diameter), processing limitations to create monolithic structures of this size class require that smaller segments need to be fabricated and then joined in a post sintering operation. Fabrication of segmented &nullset;300mm lightweighted concave mirrors to demonstrate different fabrication methods is presented here. The mirrors are comprised of 6 radial segments joined by means of silicon braze technology and are coated with a SSiC Chemical Vapor Deposition (CVD) layer for improved surface finish to reduce straylight scatter. Evaluation of conventional pitch lap polishing of brazed and coated optic surfaces has shown no degradation to surface figure and surface roughness.
The following is an overview on sintered silicon carbide (SSiC) material properties and processing requirements for the manufacturing of components for advanced technology optical systems. The overview will compare SSiC material properties to typical materials used for optics and optical structures. In addition, it will review manufacturing processes required to produce optical components in detail by process step. The process overview will illustrate current manufacturing process and concepts to expand the process size capability. The overview will include information on the substantial capital equipment employed in the manufacturing of SSIC.
This paper will also review common in-process inspection methodology and design rules. The design rules are used to improve production yield, minimize cost, and maximize the inherent benefits of SSiC for optical systems. Optimizing optical system designs for a SSiC manufacturing process will allow systems designers to utilize SSiC as a low risk, cost competitive, and fast cycle time technology for next generation optical systems.