Measuring Euv Mask 3d Effects With Hyperspectral Zernike Phase Contrast

Achieving the ultimate resolution limit of EUV lithography is greatly impeded by the 3D photomask geometry, including an absorber whose thickness is comparable to the minimum lateral dimensions of the pattern, and a reflection plane a similar depth beneath the surface of the multilayer mirror. Rigorous simulations have shown that these effects can in theory be mitigated by adopting a thinner absorber and a multilayer with a reflection plane closer to the surface. But regardless of how rigorously the design is optimized, there is clearly a need to experimentally confirm that the as-built photomask conforms to the simulation's predicted complex electric field. This experimental confirmation is difficult because only the field's intensity is directly observable. One promising approach to unambiguously make this measurement is Zernike phase contrast imaging, which determines the complex electric field from intensity images acquired from a single illumination condition with different phase shifts on the 0 order. In this work we present an extension to a hyperspectral version of the technique. By varying the wavelength, we are able to empirically observe the complicated interaction between absorber, multilayer, pattern, and illumination. We performed an experimental demonstration of the technique on a patterned EUV mask with 60nm TaN absorber using specially fabricated zone plates on the SHARP IAN microscope at the Center for X-Ray Optics. Our results demonstrate the sensitivity of hZPC to both the Fresnel reflectance as well as more subtle 3D effects also observed in rigorous simulations.