Impact of mask absorber thickness on the focus shift effect in extreme ultraviolet lithography

A focus shift is a common phenomenon in extreme ultraviolet lithography. It depends on the pattern pitch and has a significant effect on the process window. This work proposes an approximate but analytical method to investigate the focus shift effect using a modified thin mask model and the Hopkins formula of imaging. The source of the focus shift is identified, and the impact of the absorber thickness is discussed. The focus shift effect can be reduced and the process window can be improved by choosing the appropriate absorber thickness. (C) 2012 American Vacuum Society. [http://dx.doi.org/10.1116/1.3697718]