Novel x-ray mask distortion measurement technique employing holographic gratings and phase-shifting interferometry

Abstract

This article presents a new optical metrology technique that has been developed at CXrL to investigate potential distortions induced in x-ray masks during their fabrication, mounting, and x-ray irradiation. The technique employs a Zygo phase-shifting Fizeau interferometer to acquire the in-plane distortion (IPD) and out-of-plane distortion (OPD) present in the masks. The interferometer measures the OPD directly, while IPD measurements are obtained with the aid of holographic gratings that have been printed onto the masks. When positioned at the Littrow angle of the gratings, the interferometer can acquire a retrodiffracted wavefront that contains the IPD information. Subtraction of the pre- and postdistortion interferograms yields the induced distortions. Computer models to simulate x-ray mask distortions have been developed and implemented using ansys, a commercially available finite element analysis software package. Numerical results have shown excellent agreement with interferometric data. The authors believe that these new interferometric techniques and models should prove invaluable as metrologic and predictive tools for the measurement, characterization, and design of x-ray masks in the 0.25 μm regime.