High precision processing CaF2 application research based on the magnetorheological finishing technology

Abstract

Single crystal calcium fluoride (CaF2) is the excellent transparent optical substance that has extremely good permeability and refractive index from 120nm wavelength ultraviolet range to 12μm wavelength infrared range and it has widely used in the applications of various advanced optical instrument, such as infrared optical systems (IR), short wavelength optical lithography systems (DUV), as well as high power UV laser systems. Nevertheless, the characteristics of CaF2material, including low fracture toughness, low hardness, low thermal conductivity and high thermal expansion coefficient, result in that the conventional pitch polishing techniques usually expose to lots of problems, such as subsurface damage, scratches, digs and so on. Single point diamond turning (SPDT) is a prospective technology for manufacture the brittle material, but the residual surface textures or artifacts of SPDT will cause great scattering losses. Meanwhile, the roughness also falls far short from the requirement in the short wavelength optical systems. So, the advanced processing technologies for obtaining the shape accuracy, roughness, surface flaw at the same time need to put forward. In this paper, the authors investigate the Magnetorheological Finishing (MRF) technology for the high precision processing of CaF2material. We finish the surface accuracy RMS λ/150 and roughness Rq 0.3nm on the concave aspheric from originate shape error 0.7λ and roughness 17nm by the SPDT. The studying of the MRF techniques makes a great effort to the processing level of CaF2material for the state-of-the-art DUV lithography systems applications. Copyright © 2017 SPIE.