The final test of any coating is its performance for the intended use. In the extreme ultraviolet and soft x-ray region this final test often requires a substantial effort, like travel to a synchrotron light source or the launch of a space telescope.
For the fabrication of coatings it is important to have a short feedback loop; ideally data on the performance of the coating in one deposition run should be available before the next run, and the analysis of the data should yield sufficient information to give a guide in which direction the deposition run should be modified to obtain improved performance. This information cannot be obtained from a test of a mirror for the intended application alone; other tests are needed to characterize a structure more completely. Insufficient reflectivity can be caused by thickness errors, boundary roughness, mixing of the materials, contamination, or wrong composition, and it is desirable to use characterization methods that can separate these effects. Table 10.1 gives a listing of common methods to test structures, sorted roughly by the time it takes to obtain a result. The first methods average the properties of a structure over the size of the monitoring beam, which usually has a size in the millimeter range; they have angstrom resolution only in the z-direction perpendicular to the layers. The last two methods have high resolution in all three dimensions. A much more time consuming sample preparation is required, sample preparation can modify a structure, and a single picture might not be representative for the entire structure. Usually one has to take statistical averages from a large number of micrographs if one wants to correlate the data with those from the first methods.
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