<title>Impact of the EUV mask phase response on the asymmetry of Bossung curves as predicted by rigorous EUV mask simulations</title>

Abstract

EUV masks generally mandate rigorous scattering models as the thickness to wavelength ratio is typically on the order of 30:1. In addition, boundary conditions at the absorber/air interface lead to non-zero electric fields even in the absorber region while the phase itself generally experiences a complex cross mask behavior that is pattern type as well as size dependent. Using two different types of rigorous simulators that are based on the differential method (LithoLand) and the time-domain finite-element method (EMFLEX) we explore and quantify the subtleties associated with the EUV mask when compared to a thin mask obeying the Kirchoff approximation. Both rigorous simulators predict that 30 nm isolated pattern, especially for NA >0.20 experience a focus shift of about 50 nm relative to best focus for the thin mask case. This effect occurs even when the illumination is normally incident to the mask. Furthermore, the Bossung curve for isolated patterns show an asymmetry through focus that is absent for the thin mask case and is sensitive to the partial coherence condition of the illuminator. Nested features seem to be far more immune to this unexpected anomaly. The origin of the predicated focus shift and asymmetry is explained through the complex phase behavior for the EUV mask which is fundamentally different from the thin mask case.