Ion beam milling of silicon carbide optical components

ABSTRACT Silicon carbide (SiC) is emerging as an important ceramic material for optical applications requiring stiff,lightweight structures with good thermal conductivity. This report discusses the application of ion milling in thefabrication of SiC optical components. Ion beam milling combined with either ductile grinding or polishingprovides an excellent approach to deterministic fabrication of SiC optical components. Results of recent roughnessevolution studies for SIC samples prepared by several pre-ion milling fabrication processes suggest that ductilegrinding and some polishing processes can be used to produce low-subsurface-damage components suitable for ionmilling. Results are also presented of experiments in which these processes have been used in conjunction withion milling to figure SiC optical components. Typical improvements with optical figures after ion milling haveconvergences on the order of 2 or 3. Overall, these experiments indicate that ion milling combined with otherfabrication processes represents a viable, highly deterministic approach to producing high-precision SiC opticalcomponents.1. iNTRODUCTIONIon beam milling is an advanced optical fabrication process that offers a viable alternative to mechanical polishing.Mechanical polishing requires many iterations of polishing and measuring the surface to achieve figure accuracieson the order of hundreds of angstroms. Each iteration is time-consuming and costly. However, ion beam millingis a highly deterministic fabrication process. A predictable amount of material is removed; hence, as little as asingle iteration is needed to achieve the desired final surface accuracy, saving both time and money.Because ion beam milling applies no mechanical force to the optic being milled, fixturing the part is greatlysimplified. Unlike mechanical polishing, ion beam milling produces no edge roll-off or difficulty in corners and istherefore applicable to unusual component shapes. An additional benefit is that the process yields a final surfacethat is atomically clean, making it possible to coat in the same chamber used for milling. This capability makes theprocess even more economical.Because ion beam milling removes material in extremely small quantities, the optic must first be prepared bypolishing, diamond turning, or ductile grinding. If the ion beam milling process is used to remove larger amountsof material, it may uncover subsurface damage if present in the surface. The ion milling process itself does notcause subsurface damage in SiC; rather, it simply uncovers subsurface damage already present as a result of otherfabrication processes. When ion beam milling is used on prepolished chemical vapor deposited (CVD) SiC opticalcomponents, the process uncovers minor subsurface damage but is able to accurately figure optics.CVD SiC is emerging as an important ceramic material for optical applications requiring lightweight, stiffstructures with good thermal conductivity. CVD SiC thermal conductivity is 250 W/m•K, and other glass ceramicsare on the order of only 1—6 W/m•K. CVD SiC is well suited ftr many applications such as in the space andmicroelectronics industries. The Optics Manufacturing Operations Development and Integration Laboratory(MODIL) ion milling efforts have resulted in the successful figuring of CVD SiC flats and spheres.