Growth, structure, and performance of depth-graded W/Si multilayers for hard x-ray optics
- Columbia Astrophysics Laboratory, 550 West 120th Street, New York, New York 10027 (United States)
- Danish Space Research Institute, Copenhagen, (Denmark)
- California Institute of Technology, Pasadena, California 91125 (United States)
- Oak Ridge National Labs, Oak Ridge, Tennessee 37831 (United States)
We describe the development of depth-graded W/Si multilayer films prepared by magnetron sputtering for use as broad-band reflective coatings for hard x-ray optics. We have used specular and nonspecular x-ray reflectance analysis to characterize the interface imperfections in both periodic and depth-graded W/Si multilayer structures, and high-resolution transmission electron microscopy (TEM) and selected area electron diffraction (SAED) to characterize the interface structure and layer morphology as a function of depth in an optimized depth-graded multilayer. >From x-ray analysis we find interface widths in the range {sigma}=0.275-0.35 nm for films deposited at low argon pressure (with a slight increase in interface width for multilayers having periods greater than {approx}20 nm, possibly due to the transition from amorphous to polycrystalline metal layers identified by TEM and SAED), and somewhat larger interface widths (i.e., {sigma}=0.35-0.4 nm) for structures grown at higher Ar pressures, higher background pressures, or with larger target-to-substrate distances. We find no variation in interface widths with magnetron power. Nonspecular x-ray reflectance analysis and TEM suggest that the dominant interface imperfection in these films is interfacial diffuseness; interfacial roughness is minimal ({sigma}{sub r}{approx}0.175 nm) in structures prepared under optimal conditions, but can increase under conditions in which the beneficial effects of energetic bombardment during growth are compromised. X-ray reflectance analysis was also used to measure the variation in the W and Si deposition rates with bilayer thickness: we find that the W and Si layer thicknesses are nonlinear with the deposition times, and we discuss possible mechanisms responsible for this nonlinearity. Finally, hard x-ray reflectance measurements made with synchrotron radiation were used to quantify the performance of optimized depth-graded W/Si structures over the photon energy range from 18 to 212 keV. We find good agreement between the synchrotron measurements and calculations made using either 0.3 nm interface widths, or with a depth-graded distribution of interface widths in the range {sigma}=0.275-0.35 nm (as suggested by 8 keV x-ray and TEM analyses) for a structure containing 150 bilayers, and designed for high reflectance over the range 20 keV<E<70 keV. We also find for this structure good agreement between reflectance measurements and calculations made for energies up to 170 keV, as well as for another graded W/Si structure containing 800 bilayers, designed for use above 100 keV, where the peak reflectance was measured at E=212 keV to be R=76.5{+-}4% at a graze angle of {theta}=0.08 degree sign . (c) 2000 American Institute of Physics.
- OSTI ID:
- 20216840
- Journal Information:
- Journal of Applied Physics, Vol. 88, Issue 1; Other Information: PBD: 1 Jul 2000; ISSN 0021-8979
- Country of Publication:
- United States
- Language:
- English
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