The stringent figure requirements for optics used in Extreme UV (EUV) projection systems pose a significant challenge to metrologists. The large asphericities combined with low RMS figure error tolerances require novel measurement techniques. Such a method to measure absolute optical path differences (OPD) with 0.1 nm accuracy over ranges of several millimeters is presented. The method combines two different measurement techniques: frequency modulation to determine the absolute OPD with an accuracy of half a wavelength and heterodyning to determine the OPD modulo one wavelength with an accuracy of 0.1 nm. By combining the results of the two methods, the absolute OPD is determined with the large range of the frequency modulation and the high accuracy of the heterodyning. Measurement results for the heterodyning method are presented. In addition, a method to dynamically characterize the frequency behavior of a sweeping tunable external cavity diode laser is shown.
A visible light interferometer to measure the figure of aspherical mirrors, as used in extreme-ultraviolet lithography, will be presented. Except for two fiber tips to generate the reference and object wavefronts, it contains no auxiliary optics. The phase difference is measured using frequency modulation and heterodyne techniques. The figure is computed from the phase difference. We determined the specifications for the two-dimensional detector array, with which the phase difference is measured, and computed the position accuracy of the optical components. The resolution of the frequency modulation system should be 0.3 micrometer and the accuracy of the heterodyne technique should be 0.1 nm.
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