Any method for the deposition of thin films can be used for the fabrication of coatings for the XUV region. Compared to coatings for visible light, the thicknesses and the permitted thickness errors are about a factor of 100 smaller, and the number of layers is typically a factor of 10 higher. Therefore, tighter control of the deposition process is required. For many coatings, the quality of the boundaries is the most important parameter for the performance of a coating. Boundaries have to be sharp within 1â10 of the multilayer period [see Eq. (7.17)], and the search for deposition processes which produce sharp boundaries has been a major effort in the development of coatings for the x-ray region.
The limits on the quality of the boundaries is of course determined by the size of the atoms, and some of the best boundaries reported in the literature (Ï â¤ 3 Ã
) are close to this limit. Drastic improvements over such a value could only be expected with crystalline multilayer structures, where a perfect crystalline structure is maintained throughout the entire stack. Perfect boundaries would require that the growth of one atomic plane starts only when the previous plane is completely filled, and that one would be able to switch materials exactly at this moment.
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