Defect coverage profile and propagation of roughness of sputter-deposited Mo/Si multilayer coating for extreme ultraviolet projection lithography

Abstract

The presence of defects in multilayer reflective coatings to be used as mask blanks for extreme ultraviolet projection lithography can cause significant reduction in image intensity. The extent of the reduction depends on the size and position of the defects and also on the manner in which the deposition process covers the underlying defect. If the deposited film causes lateral propagation of defect geometry and does not reduce the step height, even a hundred angstrom-sized defect might cause an observable effect. However, if the deposition process can reduce the defects’ height and does not increase their effective size, the effect of defects on image would be less severe. Atomic force microscopy and transmission electron microscopy was used to study multilayer coating deposition profiles over programmed defects. It was found that the step height and edge slope is significantly reduced after film deposition with small lateral propagation of defect geometry. Roughness of the evaporated programmed defects was significantly reduced by the multilayer deposition process. A 200 nm Si buffer layer deposited before multilayer mirror deposition further smooth out the small features. However it also increases the roughness over the initially smooth wafer, resulting in lower achievable multilayer coating reflectivity.