The multielement structures (zone plates and multilayer mirrors) that have been fabricated during the last two decades can be used as normal incidence imaging elements, and high-quality soft x-ray imaging has become one major application. The narrow bandwidth of the devices has also become useful for spectroscopy, often simultaneously with imaging. Applications cover many areas in science and technology such as astronomy [1, 2], microscopy [3â8], x-ray lithography , spectroscopy [10â12], microanalysis , plasma diagnostics [14â16], and soft x-ray laser research [17â19].
The earliest interference structures for x rays lost their reflectivities during aging [20, 21], and this fact has been developed into a technique to measure extremely small diffusion constants , and Greer  has reviewed this application. Data on the high-temperature stability of multilayer mirrors can be found in Refs. [24â32]. Multilayer structures are also fabricated for their electric, magnetic, mechanical, chemical, or superconducting properties. While the description of these properties is beyond the scope of this book, those structures are often characterized by the x-ray methods described in Chapter 10.
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