Composite Single Crystal Silicon Scan Mirror Substrates

Single crystal silicon is a desirable mirror substrate for scan mirrors in space telescopes. As diameters of mirrors become larger, existing manufacturing capabilities of single crystal silicon (SCSi) cannot supply sizes that are larger than about 450 mm in diameter. Onyx Optics proposes to develop a process based on our established technique of Adhesive-Free Bonding (AFB®) of single crystals, optical ceramics and glasses that can produce sizes that are larger than commercially available SCSi. The precision composites, consisting of SCSi components, are expected to perform as well as a single crystal. Precision grinding and polishing of large composite SCSi without subsurface damage is a desirable feature for large scan mirror substrates. As part of the proposed manufacturing process, Onyx Optics proposes to develop a technique that is based on electrolytic in-process dressing (ELID) of grinding wheels of decreasing diamond grain sizes. The process is generally applicable to ductile grinding of hard materials such as glass, sapphire, silicon carbide, silicon nitride and SCSi and is known to result in low stress components. Composite SCSi interfaces will be charactized interferometrically at 1.55 micron by their transmitted wavefront. Heat transfer measurements across bonded interfaces of SCSi will be performed at room temperature and liquid nitrogen. Equi-biaxial fracture strength of composite disks will be determined and compared with control disks.

Technology continues to be developed at NASA GSFC to produce SCSi lightweight mirrors for cryogenic and other applications. This proposal aims to contribute to this effort by developing a process to supply mirrors of larger diameter than can readily be produced directly by crystal growth, i.e. larger than about 450 mm diameter. SCSi mirrors have important performance and cost advantages over ULE, SiC, Zerodur and beryllium for the whole spectrum of applications for scan mirrors and telescopes. NASA applications include GOES-R satellites and Lidar for planetary explorations and space-based astrophysics. The present proposal represents a solution to producing high-quality low-stress large SCSi mirrors and substrates at much reduced cost in comparison to any other technology.

Besides NASA, the excellent properties of SCSi mirror substrates make then desirable for Defense applications as well, such as scan mirrors for telescopes for imaging, surveillance and reconnaissance and for high energy air-borne laser systems. Customers for Onyx Optics will include Northrop Grumman and Lockheed Martin.

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