Chemical Mechanical Polishing for Extreme Ultraviolet Lithography Mask Substrates

Abstract

We describe the effectiveness of depositing an amorphous silicon thin film to cover the existing defects on the surface of extreme ultraviolet (EUV) lithography mask substrates and then polishing the film using abrasive-free liquids. Preliminary investigations on a-silicon coated Si wafers showed that the surface roughness and removal rates (RRs) are largely dependent on the polishing pressure. Higher polishing pressures led to higher removal rates and surface roughness while a lower polishing pressure of 0.5 psi led to negligible RRs but achieved a surface roughness of ~0.1 nm RMS (root-mean square). A two-step CMP process, consisting of polishing at 1 psi followed by polishing at 0.5 psi on an IC-1000 pad gave acceptable surface roughness (~0.1 nm) as well as adequate material removal. The same polishing conditions when transferred to a-silicon films deposited on 6”x6”x0.25” quartz EUV substrates also resulted in a RMS surface roughness of ~0.1 nm.