Dressed-photon nanopolishing for extreme ultraviolet mask substrate defect mitigation

Although the quality of extreme ultraviolet (EUV) mask substrates has improved by continuous refinement of the polishing processes, the yield of defect-free blanks is still very low. Dressed-photon nanopolishing (DPNP) is a novel vapor phase, photo-chemical, non-contact etching process that has been shown to locally smooth bumps and pits to below 1 nm in height/depth while not affecting the surface roughness. DPNP is based on the concept of a dressed photon, which is a quasi-particle in the optical near field of a surface that can couple with lattice phonons in nanometric regions (< 100 nm). When illuminated with light of a suitable wavelength, such coupled states are generated on a nanometrically rough material surface and impart sufficient energy to an etchant gas to enable its dissociation and etching in the rough regions only. DPNP can be the last polishing step on EUV substrates to eliminate any remnant pits and/or embedded particles on the surface to yield potentially defect-free substrates.