Gas assisted focused electron beam induced etching of alumina

This study investigates focused electron beam induced etching for the removal of alumina particles on patterned extreme ultra violet (EUV) mask using nitrosyl chloride (NOCl) as assist gas. As potential contaminant, particles of aluminum oxide (alumina, Al2O3) have been successfully removed, leaving the underlying layers undamaged. Particles were applied onto an EUV mask, consisting of a multilayer Bragg mirror capped with a thin ruthenium layer and a structured tantalum nitride (TaN/TaON) absorber/antireflective film. Alumina particles were selectively etched using the chlorine-based gas, NOCl. Neither the Ru nor the absorber was significantly etched during the process in spite of a square area scanned by the focused electron beam being larger than the particle. The process resolution is discussed based on Monte Carlo electron scattering simulations. Thermodynamic driving forces for the electron-induced reactions and its selectivity are discussed and a chemical rationale is proposed.