**6.3 At wavelength metrology**

Traditional optical metrology generally employs visible light as the illumination source, which is convenient and cost-effective. Recent developments in optics for extreme ultraviolet (EUV) lithography, x-ray synchrotron radiation optics, and free electron lasers operated at EUV and x-ray wavelengths place extraordinary requirements on the visible light metrology systems. Speer and collaborators(Harris et al., 1982; Speer et al., 1980) employed the point diffraction interferometer to perform at-wavelength measurements on grazing incidence optics into the far ultraviolet region. Sommargren, Goldberg and collaborators (Medecki et al., 1996; Tejnil et al., 1997; Sommargren 1996; Goldberg et al., 1995) extended the technique into the EUV region to test zone plate wave-fronts and Schwarzschild optics quality in the 13nm wavelength region. This work was driven by the need to produce multilayer-coated normal incidence optics that were diffraction-limited at wavelengths to be used in soft x-ray projection lithography. A 0.5nm wave-front error is much more easily seen with a 13nm source, where it is /26, than with a 633nm HeNe source, where it is more than /1000.

The point diffraction interferometer requires a system that produces a point focus somewhere along the optical axis. A more general method for testing x-ray wave-fronts was developed by Weitkamp and collaborators (Weitkamp et al., 2005) utilizing Talbot effect Moire fringes generated by phase and absorption gratings. The interferometer can be placed anywhere downstream of an optical element where it is convenient, making it a very versatile technique. Wave-front distortions in the range of /100 can be measured, where = 0.1 nm, and surface slope errors can be detected with an accuracy of better than 100 nrad over spatial periods from 1mm to 1 m.

Another versatile at-wavelength technique has been developed by Souvorov (Souvorov et al., 2002) and by Yumoto(Yumoto et al., 2006) and their coworkers, based upon the phase retrieval algorithms of Fienup (Fienup, 1982). By measuring the intensity variations in propagating x-ray beams downstream from a reflection or transmission element, the phase of the wave-front can be computed and projected back to the surface of the optical element. Yumoto has shown that surface figure errors measured by visible light interferometric means and by the phase retrieval methods agree to better than 1.5 nm on an 80mm long mirror. This method has the advantage over visible light interferometry in that it is sensitive to errors in the multilayer coatings on the optics, resulting in a measurement of the actual performance of the optic rather than just the profile of the top-most surface layer.
