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A patent-pending process using single-crystal silicon NASA Goddard Space Flight Center invites companies to license its new, patent-pending process that helps significantly reduce the risk, time, and costs associated with producing lightweight mirrors for demanding instrument applications. The method employs a solid disc of single-crystal silicon (SCS) and calls for most of the polishing to be completed before lightweighting. Due to the extraordinary homogeneity of SCS, the distortion caused by traditional lightweighting processes is significantly reduced.
Goddard’s technology is ideal for use in environments in which lightweight, cryogenic operation, or high heat dissipation is required. The SCS technology provides a cost-effective solution for applications including:
How it works SCS lightweight mirrors typically weigh about one-fourth that of a solid quartz blank of the same size, making them useful for a variety of instruments where weight is a concern. Each mirror is a monolithic structure consisting of a face sheet with a highly polished front optical surface and a lightweight support structure. In Goddard’s process, the optical surface is formed in a solid SCS blank either by conventional grinding and polishing or by diamond turning. An element is temporarily attached to protect the optical surface during the lightweighting process. The blank is then lightweighted using abrasive machining, ultrasonic machining, or a combination of both. The temporary protector is then removed, and for most applications, the mirror is then ready for use. For critical applications, post-lightweighting polishing can be performed to further improve the optical surface. Due to the very small amount of material removed during this step, it produces no quilting or print-through of the lightweight support structure. At several points during the process, the mirror is heated to near its melting point to remove small crystalline defects caused by the fabrication process.
Each resulting SCS mirror is a monolithic structure formed from a single crystal of silicon, giving it a homogeneous composition free of internal stress. These parameters inhibit distortion when cooling the mirror to cryogenic temperatures. Under such conditions, the mirrors maintain their optical figure to 1/50th wave root mean square (RMS) or better. At room temperature, SCS has a thermal conductivity about the same as aluminum and a thermal coefficient of expansion about equal to Pyrex® glass. So SCS mirrors are extremely resistant to thermal shock and ideal for applications where high heat dissipation is required. Goddard has produced 11 SCS mirrors so far, ranging in size from 10cm (4") to 25cm (10"). The process can produce quality mirrors up to 40cm (16") in diameter, and the ultimate size, limited by the diameter of high-quality SCS boules, is about 50cm (20"). Why It Is Better Virtually all conventional lightweight mirrors are made by optically grinding and polishing an already lightweighted blank. Mirrors made this way always risk print-through to the optical surface from the underlying support structure. In some cases this can be removed by a zero-pressure process, such as ion-beam polishing, although these processes tend to be slow and costly. Lightweighting after optical polishing is not an option for conventional materials as their inhomogeneous qualities and internal stresses cause the lightweighting to distort the optical surface. By contrast, in Goddard's process for SCS mirrors, optical grinding and polishing is done before lightweighting, eliminating the possibility of print-through. This is due to the extreme homogeneity and absence of stress possible in a monolithic structure from a single crystal. This results in a simple and cost-effective process that is capable of producing mirrors of exceptional quality.
This technology is part of NASA’s Innovative Partnerships Program Office, which seeks to transfer technology into and out of NASA to benefit the space program and U.S. industry. NASA invites companies to consider licensing the Lightweight Optical Mirrors Formed in Single-Crystal Silicon technology (GSC-14393-1) for commercial applications. For information and forms related to the technology licensing and partnering process, please visit the Licensing and Partnering page. (Link opens new browser window) If you are interested in more information or want to pursue transfer of this technology (GSC-14393-1), please contact: Innovative Partnerships Program Office
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