Growth, structure, and performance of depth-graded W/Si multilayers for hard x-ray optics

Abstract

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 ͑TEM͒ and selected area electron diffraction ͑SAED͒ 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 ϭ0.275-0.35 nm for films deposited at low argon pressure ͑with a slight increase in interface width for multilayers having periods greater than ϳ20 nm, possibly due to the transition from amorphous to polycrystalline metal layers identified by TEM and SAED͒, and somewhat larger interface widths ͑i.e., ϭ0.35-0.4 nm͒ 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 ( r ϳ0.175 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 ϭ0.275-0.35 nm ͑as suggested by 8 keV x-ray and TEM analyses͒ for a structure containing 150 bilayers, and designed for high reflectance over the range 20 keV ϽEϽ70 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 Eϭ212 keV to be Rϭ76.5Ϯ4% at a graze angle of ϭ0.08°.

Development of thermally formed glass optics for astronomical hard X-ray telescopes

“…Our continuing series of measurements of 2-bounce optical performance for glass optics confirm that this factor is appropriate for optics constructed in the fashion we describe here. The reflectivity of the optics, typically above 70% throughout the 20-70 keV range the coatings were designed for, is consistent with expectations based on characterization of the W/Si multilayer coatings [5,9]. …”

“…The deposition technology for fabricating the depth-graded multilayer coatings (at least for the E<100 keV band) is now relatively mature [4,5]. In this paper, we describe the first demonstration of recently developed, thermally formed glass micro-sheet X-ray optics.…”

W/SiC X-ray multilayers optimized for use above 100 keV

“…͑In comparison, the interface widths for similar W͞Si films were found previously to be ϳ0.3 nm. 10 ͒ For structures having periods smaller than d ϭ 1.12 nm, the interface widths gradually increase to 0.34 nm.…”

<title>X-ray multilayer coatings for use at energies above 100 keV</title>

“…These new results are described in detail elsewhere [7]; we summarize here our most important fmdings with regard to the development of hard X-ray astronomical telescopes using these coatings. From X-ray analysis of periodic W/Si multilayers we find interface widths in the range cO.275 -0.35 nm for films deposited at low argon pressure (with a slight increase in interface width for multilayers having periods greater than -2O nm, possibly due to the transition from amorphous to polycrystalline metal layers identified by TEM and SAED), and somewhat larger interface widths (i.e., =O.35 -0.4 rim) for structures grown at higher Ar pressures, higher background pressures, or with larger target-to-substrate distances.…”