Research on ultra-precise figuring for CaF2 single crystal

Abstract

In order to machine ultra-precise and extremely smooth calcium fluoride (CaF2) surface for optical system in ultra violet or deep ultra violet applications, a novel polishing method combining advantages of chemically mechanical polishing (CMP) and ion beam figuring (IBF) is put forward. CMP is aimed at extremely smooth surfaces, while ion beam figuring is aimed at ultra-precise figures without destroying the extremely smooth surface. The evolution of surface roughness and surface shape with material removal depth in CMP process are researched. Scratches produced in mechanically polishing process are removed by CMP and CaF2 plane with surface roughness of 0.268 nm RMS (0.94 mm×0.7 mm) is obtained. Evolutions of surface roughness in dependence of sputtering depth and ion incidence angle are studied. The surface tends to roughen after ion sputtering, but roughen slightly with optimum parameters such as incidence angle of 40°. So, Polished by combined method of CMP and IBF with incidence angle of 40°, CaF2 single crystal plane with 13.14 nm PV, 1.06 nm RMS in surface contour errors and 0.281 nm RMS in surface roughness is obtained. © 2013 Journal of Mechanical Engineering.