A method to determine the origin of remaining particles after mask blank cleaning

Extreme ultraviolet lithography (EUVL) is a strong contender for the 32 nm generation and beyond. A defect-free mask substrate is an absolute necessity for manufacturing EUV mask blanks. The mask blank substrates are, therefore, cleaned with different cleaning processes to remove all defects down to 30 nm. However, cleaning suffers from the defects added by various sources such as the fab environment, chemicals, ultra pure water, and the cleaning process itself. The charge state of the substrate during and after cleaning also contributes to the number of adder defects on the substrate. The zeta potentials on the substrate surface and the defect particles generated during the cleaning process determine whether the particles get deposited on the surface. The zeta potential of particle or substrate surfaces depends on the pH of the cleaning fluids. Therefore, in this work, pH-zeta potential maps are generated for quartz substrates during the various steps of mask cleaning processes. The pH-zeta potential maps for defect particles commonly seen on mask substrates are measured separately. The zeta potential maps of substrate and contaminant particle surfaces are used to determine whether particles are attracted to or repulsed from the substrate. In practice, this technique is especially powerful for deriving information about the origin of particles added during a cleaning process. For example, for a known adder with a negative zeta potential, all cleaning steps with a positive zeta potential substrate could be the source of added particles.