Correction of full spatial frequency errors with ion beam figuring

Abstract

For Lens that will be used in photo lithography systems, not only Low Spatial Frequency (LSF) errors, but also Middle and High Spatial Frequency (M/HSF) errors should be corrected to very low level. Ion Beam Figuring (IBF) is believed to be one of the most effective ways that can fabricate lens to especially high surface accuracy; In fact, it has also certain effects on removing errors in higher spatial frequency range. Firstly, the principle of IBF was introduced; Then, Power Spectral Density (PSD) is introduced, which is used to evaluate IBF’s corrective effects on errors in different spatial frequency ranges; Finally, by the use of IBF machine, a convex lens with diameter 90mm was fabricated, and its surface error was improved from PVr value15nm to less than 10nm, measured by Fizeau phase shift interferometer precisely. Also, M/HSF errors were measured before and after IBF fabrication, by White Light Interferometer and Atom Force Microscope separately. The initial and final Power Spectrum Density (PSD) curves of the lens were generated and compared with each other, which shows that IBF technic has really good ability to remove LSF errors, and can also reduce M/HSF errors greatly in the meantime. The results help to evaluate the corrective effects of IBF technic on errors in the full spatial frequency range.