Abrasive-Free Polishing for Extreme Ultraviolet Lithography Mask Substrates

Abstract

In the development of photo-masks for extreme ultraviolet (EUV) lithography, it is a formidable task to achieve the stringent requirements of angstrom-scale surface roughness, sub-30 nm peak-valley flatness and defectivity in single digits. The majority of the defects present on the substrate arise from the polishing/cleaning processes. Here we describe the effectiveness of depositing an amorphous silicon (a-silicon) thin film to cover the existing defects (e.g. pits, scratches, bumps, embedded and adhered particles) on the surface of EUV substrates followed by polishing the a-silicon film using abrasive-free liquids to help meet the EUV substrate requirements of surface roughness and defectivity (ideally zero pits or bumps greater than 1 nm in depth or height). Chemical mechanical planarization (CMP) using an abrasive-free poly (ethyleneimine) solution showed that both the final surface roughness and removal rate are strongly dependent on polishing pressure. We developed a hybrid two-step CMP process consisting of polishing first at 1 psi to remove sufficient material to eliminate the underlying defects, followed by polishing at 0.5 psi to achieve low surface roughness. Under these polishing conditions, CMP of a-silicon films deposited on EUV mask substrates resulted in a root mean square (RMS) surface roughness of ∼0.09 nm.