Quantitative i n s i t u characterization of x-ray mask distortions

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Abstract
The control of x-ray mask distortion is a central issue in x-ray lithography technology. Three causes of distortion have been examined: the interaction between a stressed absorber and substrate, the heating of the mask during x-ray exposure, and the radiation damage resulting from the cumulative absorption of the x rays. Interferometric techniques have been used to measure both in-plane distortion (IPD) and out-of-plane distortion (OPD). To measure OPD, wave fronts reflected from the mask surface are compared with a reference plane wave. For IPD it is necessary to have a diffraction grating on the mask. IPD causes changes in the grating pitch that result in the changes in the interference pattern. For thermal studies, an in situ shear interferometer was used to measure both OPD and IPD. Temperature increases were monitored using an infrared camera. The results have been modeled using the finite element program ansys. The distortion due to aborber substrate interaction was observed to occur at pattern edges. Thermal distortions were observed to cause IPD in regions with no absorber and both IPD and OPD for areas covered with absorber. Radiation damage was found to cause both IPD and OPD. The OPD was in the form of a plateau and can be modeled assuming the relaxation of the material is dependent on the absorbed dose and taking into account the depth dependence on absorption in the mask.

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