Structure and x-ray optical properties of MBE-grown multilayers (Invited Paper)

The structure, and therefore the performance, of multilayer x-ray mirrors is directly related to the growth mode of the constituent materials. We review some of our recent results from using molecular beam epitaxy (MBE) for the growth and study of materials for x-ray optics. This work involves the study of surfaces and interfaces important to improving conventional multilayer mirrors and development of superlattice mirrors. Recently, we have successfully grown hcp (0001) Be epitaxially on (0001) (alpha) -Al2O3 substrates. We find the orientation and morphology are strongly influenced by the substrate temperature during deposition. The discovery of a technique to grow Be epitaxially, coupled with the favorable optical constants of this material, gives Be-based multilayers great potential for the development of superlattice x-ray optics. In our preliminary studies of Be/Ge multilayers we found them to have a layered structure, making Be/Ge a potentially useful combination for soft x-ray mirrors. In another series of studies, we performed investigations of film growth and interface formation using a variety of surface analysis techniques for Mo on Si and for X on B (where X equals Pd, Ag, Si). These studies were carried out by depositing onto clean surfaces in ultra high vacuum, followed by characterization with in situ reflection high-energy electron diffraction (RHEED), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), and ex situ by scanning tunneling microscopy (STM) and x-ray diffraction. Continuous growth of multiple coverages on a single substrate was accomplished with a technique involving a moveable sample shutter. For Mo-Si, our data are consistent with a composition profile that has an atomically abrupt transition between Si and amorphous MoSix, where x equals 2 for the first 4 angstroms. The fraction of Si then decreases, with the composition approaching pure Mo after 15 - 20 angstroms depending upon the growth temperature. This silicide interlayer causes only a small reduction in the reflectivity of Mo/Si soft x-ray mirrors with (Lambda) > 60 angstroms, but has severe effects for smaller period structures. For Ag-B, the XPS, AES, and STM data are all characteristic of an island (three-dimensional) growth mode. At an Ag coverage 15 angstroms, the islands range in height from 100 to 200 angstroms and are 2000 to 3000 angstroms across. Although the interface between Ag and B may be sharp, the island growth mode causes too much roughness for the multilayers to be useful for short-wavelength optics. For Pd-B, the Pd reacts with the B to form an amorphous, Pd-rich boride. Although the absorber layer is an alloy, our calculations indicate that the ideal normal-incidence reflectivity of the reacted multilayers could be as high as 51% at 80 angstroms. For Si on B, the interface is quite sharp and the amorphous Si forms a smooth continuous layer. Si/B multilayers are useful as narrow bandpass mirrors for (lambda) > 125 angstroms.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.