Measuring EUV mask 3D effects with hyperspectral Zernike phase contrast imaging

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. While these effects can be mitigated by adopting a thinner absorber and a multilayer with a reflection plane closer to the surface, nonetheless it will be necessary to precisely characterize and compensate 3D scattering effects to create a high-quality image at the minimum feature size. Extending our past work on Zernike phase contrast imaging for measuring the phase and amplitude of patterned EUV masks, in this work we present a hyperspectral version of the technique. By varying the wavelength, we are able to empirically measure 3D phase information, directly probing the interaction between absorber, multilayer, pattern, and illumination. We present an experimental demonstration of the technique on a patterned EUV mask with 60nm TaN, performed on the SHARP EUV microscope at the Center for X-Ray Optics.