Simulation study of illumination effects in high-NA EUV lithography

Abstract

Current EUV exposure systems employ a numerical aperture (NA) of 0.33. This relatively small NA is a consequence of geometrical design limitations of all reflective projection systems with a 4 demagnification in the orthogonal x- and y-directions of the image plane. Anamorphic imaging, which employs different demagnification in horizontal (y) and vertical (x) direction can increase the NA to a value of 0.55. The consequences of using anamorphic high-NA imaging system have to be studied by rigorous methods. Since the range of illumination angles of the anamorphic system is different in x and y directions, one way to understand the involved phenomena, is to investigate and compare the impact of illumination angles for both the high-NA 4 8 anamorphic system at 0.55NA and the lower-NA 4 4 system at 0.33NA. We employ fully coherent that is single source point illumination and imaging to study the impact of the illumination direction on the most relevant lithographic metrics. These metrics include the resulting feature size or critical dimension (CD), the feature position, a local contrast or the normalized image log slope (NILS) and the best-focus position of the projected images. In this study, aerial images from a uniformly-distributed grid of 230 illumination positions were computed and analyzed. The results of the simulation study confirmed that larger illumination angles cause more pronounced shadowing effects and significant variations of the position and feature size versus the illumination direction. The larger demagnification direction of the anamorphic system involves a smaller object-side angular spread of the illumination direction, resulting in less pronounced variation of CD and position versus the illumination direction compared to the isomorphic system. Both systems exhibit a drop of the NILS for more oblique angles. However, the larger image side angles of the high-NA system result in more pronounced polarization effects, which reduce the NILS values compared to that of the lower NA system. The high NA achieved by anamorphic imaging increases the importance of 3D mask effects in EUV lithography. It is not a priori known, which of these 3D mask effects can be attributed to the absorber or the multilayer part of the mask. A hybrid mask simulation approach addresses this question. In the second part of this study, simulations using an hybrid of real and ideal mask elements were performed in an attempt to understand their individual effects of the mask elements and which mask element contributes to which of the observed effects.