Alternative smoothing techniques to mitigate EUV substrate defectivity

The majority of extreme ultraviolet (EUV) lithography mask blank defects originate from chemical mechanical polishing (CMP) of the substrate. The fact that CMP has not yet been able to yield EUV substrates with low defect counts highlights the challenges of polishing doped fused silica surfaces. Here we investigate alternative techniques based on processing either the substrate or coatings of amorphous silicon thin films and inorganic metal oxides. In particular, we evaluate a novel polymer-based non-abrasive a-Si CMP process, a photo-induced dry etching of substrate protrusions, a smoothing coat of spin-on or capillary coated Inpria metal oxide solution, CO2 laser polishing of the substrate surface, and annealing an a-Si thin film surface in reducing atmospheres. Although CMP still remains the best process with respect to overall process integration, these techniques have the potential to support CMP in solving the substrate defectivity issue and help pave the way to commercializing EUV mask blanks.