Reactive Ion Figuring of Large Optical Membrane Components

A simpler and lower cost Reactive Ion Figuring (RIF) process was demonstrated, combining with flexible film photomasks and an algorithm of masking layers distribution for less iteration. As a novel alternative figuring technology, RIF with parallel removal mode and chemical removal mechanism has showed the actual potential for large segment class figuring, especially in mass production of lightweight optical membrane components. Although diffractive membrane optics has been expected to meet the requirement of large aperture space-based telescopes, the flexible membrane has so different properties from traditional materials that the desired geometrical form is hardly obtained by current ultra-precision surface manufacturing technologies. In this paper, a 300 mm aperture polyimide membrane substrate was figure-corrected by RIF from the initial figure error of 62 nm rms to the final figure error of similar to 20 nm rms in total effective figuring time of similar to 4.3 minutes. Film photomask lowered the cost of photomask fabrication down to less than two hundredths of that of quartz photomask fabrication. All distributions of protected regions and removed regions, i.e., the patterns to be fabricated onto film photomasks, were obtained from the transmitted wave-front map just at the start of the entire RIF process. After 2 iterations, it took similar to 4.3 min to figure this sample and the 20 nm RMS is obtained with a RMS convergence ratio of 3.1.