Investigation of waveguide modes in EUV mask absorbers
Background: Explaining imaging phenomena in EUV lithography requires more than a single point of view. Traditionally, the diffraction characteristics of EUV masks are analyzed in terms of the amplitude and phase of diffraction orders that are generated by the absorber pattern. Aim: We propose a complementary perspective to view the EUV mask absorber openings as waveguides. Approach: Comparisons between RCWA simulations and analytical solutions to waveguide equations are performed to prove that EUV mask absorbers behave as a waveguide. Results: This perspective can explain phenomena left unexplained by conventional analysis of far-field diffraction orders. Conclusions: The waveguiding effect in EUV mask absorbers explains the need for low refractive index and high extinction materials. The waveguide perspective explains why attenuated phase shift masks behave differently for EUV than our traditional understanding would suggest.