EUV mask cleans comparison of frontside and dual-sided concurrent cleaning

The cleaning requirements for EUV masks are more complex than optical masks due to the absence of available EUVcompatible pellicles. EUV masks must therefore be capable of undergoing more than 100 cleaning cycles with minimum impact to lithographic performance. EUV masks are created on substrates with 40 multilayers of silicon and molybdenum to form a Bragg reflector, capped with a 2.5nm-thick ruthenium layer and a tantalum-based absorber; during usage, both ruthenium and absorber are exposed to the cleaning process. The CrN layer on the backside is used to enable electrostatic clamping. This clamp side must also be free of particles that could impact printing and overlay, and particles could also potentially migrate to the frontside and create defects. Thus, the cleaning process must provide decent particle removal efficiencies on both front- and backside while maintaining reflectivity with minimal surface roughness change. In this paper, we report progress developing a concurrent patterned-side and clamped-side cleaning process that achieves minimal reflectivity change over 120 cleaning cycles, with XPS and EDS indicating the presence of ruthenium after 125 cleaning cycles. The change in surface roughness over 100 cleaning cycles is within the noise (0.0086nm) on a mask blank, and SEM inspection of 100nm and 200nm features on patterned masks after undergoing 100 cleaning cycles show no indications of ruthenium pitting or significant surface damage. This process was used on test masks to remove particles from both sides that would otherwise inhibit these masks from being used in the scanner.