Lithography Support Technologies - Scattering of CaF2

The projection optics for 157nm lithography contains many calcium fluoride (CaF2) elements. An area of concern for lithography system designers is the extent to which surface and volume scattering from these elements will have a deleterious effect on the partially coherent imagery. Scattering due to surface figure errors in EUV projection optics is also a well-documented area of concern. The specific research we are proposing will address a critical lithography industry need for a precise understanding these effects. This will be accomplished through the development of new ray tracing algorithms that (1) model narrow and medium angle scattering of light due to residual surface figure errors, (2) model scattering of light due to scattering within the optical material due to inhomogeities, and (3) include the effects of both sources of scattering on the partially coherent image and point spread function. The resulting capability will enable the lithography equipment manufacturer to predict the degradation on image contrast due to scattering effects from both surface polishing errors and materials (CaF2) quality, which will drive future manufacturing improvements to extend the useful life of that technology node.