Low energy ion beam machining of ULE ® substrates: Evaluation of surface roughness

In order to apply ion beam figuring (IBF) to final shape correction of the substrates of projection optics for EUVL, ion beam machining characteristics such as high-spatial frequency roughness (HSFR) and mid-spatial frequency surface roughness (MSFR) of ULE ® substrate were investigated. Our previous research confirmed that the surface roughness of the ULE ® machined by Ar + ion beam with energy of 3–10 keV decreases with decreasing the ion beam energy. Therefore, we have conducted our research on ion beam machining of ULE ® substrate by Ar + ion beam with energy from 0.2 to 2 keV. The HSFR and MSFR of the mechanically pre-finished ULE ® substrate were 0.06 and 0.07 nm rms, respectively; whereas, the HSFR and MSFR of the substrate irradiated by Ar + ion beam at energy of 0.3 keV were less than 0.10 and 0.08 nm rms, respectively. The HSFR is the best result among our previous and other current research.