Influence of shield roughness on Mo/Si defect density for extreme ultraviolet lithography mask blanks

The authors investigated the influence of surface roughness of stainless-steel shields in an ion beam sputtering chamber on the particle defect density of deposited 50 pairs of Mo/Si bilayer films ([Mo/Si]50) used in extreme ultraviolet mask blanks. Shields with varying arithmetic average surface roughness (Ra range approximately 3 to 20 μm) were mounted close to the sputtering targets and the substrate, and along the vacuum chamber interior wall. Silicon-rich particles (Si and Si/Mo) with diameters in the range of several tens of nanometers or more were quantified within a 142 mm× 142 mm area of the prepared blank film using a mask blank inspection tool. Si-rich particle defect density was found to be proportional to the inverse square of the shield surface roughness, suggesting that Si-rich particles arise from the shield surface. The shields with roughness exceeding 8 μm effectively suppressed the accumulation of Si-rich particle defects on the mask blank film.