New materials for multilayer soft x-ray optics for wavelengths below 124 angstrom by sputtering and molecular beam epitaxy.

Abstract

My goal in this work is to improve the performance of multilayer x-ray mirrors at wavelengths below 124 Å. My work includes three main tasks: identifying the materials to use, attempting epitaxy and studying interfaces by molecular beam epitaxy, and producing mirrors by sputtering. First, I identified material pairs which would likely form high-performance mirrors at wavelengths spanning 10-124 Å. Material pairs containing well behaved elements were considered in detail, while pairs containing compounds are also included. Attempts to grow epitaxial boron for use in superlattice mirrors were frustrated by the tendency of the boron to form a nearly amorphous structure even at the upper limit of substrate temperatures possible with our system. I studied the prospects for producing high performance mirrors from boron layered with silicon, palladium, and silver, by producing multilayers and performing XPS growth studies. Within our limits of detection, I saw no mixing or reactions at the silicon boron interface. Si/B multilayers, therefore, should produce narrow bandpass mirrors with reasonable reflectance while offering better long wavelength rejection than Mo/Si mirrors. Silver/boron mirrors have rough interfaces probably caused by the island growth mode of silver on boron. Without some way of reducing the interfacial roughness inherent in the system, silver/boron multilayers show little promise as x-ray mirrors. Palladium/boron mirrors should have a layered structure containing an alloy layer consisting of approximately 13 atomic percent boron and relatively sharp interfaces. Calculations indicate that multilayers containing boron layered with the alloy could produce high performance mirrors. Palladium/B₄C sputtered multilayers were deposited, and provided a near normal soft x-ray reflectance of 11.5% at 89 Å. Work continues on Si/B₄C multilayers. Sputtered Mo/Y multilayers achieved near normal reflectances of 19.4% at approximately 115 Å. The measured reflectance was reduced by oxygen contamination. Our collaborators have since been able to produce mirrors with near normal reflectances of 34% at 115 Å and 22% at 81 Å. The 34% normal incidence reflectance provided by Mo/Y surpasses any near normal incidence reflectance value I have seen in the literature for wavelengths from 10-124 Å.