Use of Kirkpatrick-Baez multilayer optics for x-ray fluorescence imaging.

We discuss the possibilities for using Kirkpatrick-Baez (K-B) multilayer elements to directly image the fluorescence distribution from a specimen under x-ray illumination. X-ray fluorescence would be collected by K-B elements close to the specimen, with a magnified image formed at an area detector about 0.5 m away from the source. This is in contrast to the use of K-B optics to form a microprobe beam that is rastered across a surface while the x-ray fluorescence is recorded to form a fluorescent image. Tungsten-carbon multilayers on curved substrates have been fabricated at the Advanced Photon Source, and their focusing properties have been characterized by a laboratory x-ray source. Synchrotron applications would illuminate specimens with lithographic patterns, for example, with the image being formed at an x-ray CCD camera. The ability to form an x-ray fluorescent image, and therefore a map of the specimen's elemental distribution near the surface, could provide a useful analytical tool without the usual need for a microfocusing beam. Furthermore, there are interesting possiblities offered by combining fluorescence imaging with x-ray near edge absorption spectroscopy (XANES), extended x-ray absorption spectroscopy (EXAFS), and x-ray standing waves.