Absorber stack with transparent conductive oxide layer for extreme ultraviolet lithography

In this study, the authors propose transparent conductive oxide layer as a new absorber for extreme ultraviolet lithography mask. The optical constant of indium tin oxide (ITO) at 13.5 nm is calculated and the extinction coefficient of ITO is found to be 1.55 times higher than that of TaN, resulting in higher absorption in the absorbing stack. Using an ITO layer in the absorber stack enables to design the attenuated phase-shift masks with smaller height difference of only 32.1 nm than the conventional TaN absorber stack of about 80 nm between high reflecting and absorbing stacks in extreme ultraviolet lithography, indicating that the geometric shadow effect can be significantly reduced. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3043471]