David L. Windt

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

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 σ=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 ∼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., σ=0.35–0.4 nm) for structures grown at higher Ar pressures, higher background pressures, or w...

Microstructure of thin tantalum films sputtered onto inclined substrates: Experiments and atomistic simulations

We have combined experiments and atomistic modeling in order to better understand the growth and structure of metal films deposited onto sidewalls of trenches and vias. Using x-ray reflectance, atomic force microscopy, and high-resolution transmission electron microscopy to characterize the microstructure and morphology of Ta films grown by magnetron sputtering onto inclined substrates, we find that films deposited at larger incidence angles tend towards columnar microstructure with high roughness and low density. We have used a three-dimensional Monte Carlo model (ADEPT) to simulate the growth process, under conditions close to those investigated experimentally. A binary collision model is included in the Monte Carlo deposition procedure to describe the interaction of energetic particles with the surface. Examination of the film microstructure and morphology resulting from the simulations indicates that the energetic impinging particles are necessary to produce film densities comparable to those found ex...

Development of the High-Energy Focusing Telescope (HEFT) balloon experiment

The High Energy Focusing Telescope (HEFT) is a balloon-borne experiment employing focusing optics in the hard X-ray/soft gamma-ray band (20 - 100 keV) for sensitive observations of astrophysical sources. The primary scientific objectives include imaging and spectroscopy of 44Ti emission in young supernova remnants, sensitive hard X-ray observations of obscured Active Galactic Nuclei, and spectroscopic observations of accreting high-magnetic field pulsars. Over the last four years, we have developed grazing-incidence depth-graded multilayer optics and high spectral resolution solid stat Cadmium Zinc Telluride pixel detectors in order to assemble a balloon-borne experiment with sensitivity and imaging capability superior to previous satellite missions operating in this band. In this paper, we describe the instrument design, and present recent laboratory demonstrations of the optics and detector technologies.

EUV multilayers for solar physics

We present an overview of currently available EUV multilayer coatings that can be used for the construction of solar physics instrumentation utilizing normal-incidence optics. We describe the performance of a variety of Si-based multilayers, including Si/B4C and new Si/SiC films that provide improved performance in the wavelength range from 25 n 35 nm, as well as traditional Si/Mo multilayers, including broad-band coatings recently developed for the Solar-B/EIS instrument. We also outline prospects for operation at both longer and shorter EUV wavelengths, and also the potential of ultra-short-period multilayers that work near normal incidence in the soft X-ray region.

Optimization of graded multilayer designs for astronomical x-ray telescopes

We developed a systematic method for optimizing the design of depth-graded multilayers for astronomical hard-x-ray and soft-gamma-ray telescopes based on the instruments bandpass and the field of view. We apply these methods to the design of the conical-approximation Wolter I optics employed by the balloon-borne High Energy Focusing Telescope, using W/Si as the multilayer materials. In addition, we present optimized performance calculations of mirrors, using other material pairs that are capable of extending performance to photon energies above the W K-absorption edge (69.5 keV), including Pt/C, Ni/C, Cu/Si, and Mo/Si.

Reduction of stress and roughness by reactive sputtering in W/B4C x-ray multilayer films

We have investigated the stress and roughness in W/B4C X-ray multilayer films grown by reactive DC magnetron sputtering in a nitrogen-argon gas mixture. We have also studied the properties of single-layer W and B4C films, in order to understand specifically how the stress, roughness, chemical composition and microstructure in these materials depends on the sputter gas composition. We find that the stress and roughness in both single-layer and multilayer films deposited reactively is reduced substantially; we find a corresponding improvement in X-ray performance in the case of multilayer films designed to operate at X-ray energies for which the incorporation of nitrogen will not adversely affect the optical properties of these coatings. Furthermore, the observed reduction in film stress in these coatings will mitigate stress-driven adhesion failures. In the case of single-layer W films, the observed reduction in stress and roughness is correlated with a change in microstructure: the W film is amorphous when deposited reactively (and contains ~12-25% incorporated N, depending on the sputter gas composition), versus polycrystalline when deposited in pure argon gas. Finally, we find extremely low surface roughness in reactively-sputtered films of amorphous B4CNx; thus, in addition to their use in X-ray multilayer reflective coatings, these films can be used as smoothing layers to reduce the surface roughness of X-ray mirror substrates, thereby leading to reduced scattering and higher specular reflectance.

Development and testing of EUV multilayer coatings for the atmospheric imaging assembly instrument aboard the Solar Dynamics Observatory

We present experimental results on the development and testing of the extreme ultraviolet (EUV) reflective multilayer coatings that will be used in the Atmospheric Imaging Assembly (AIA) instrument. The AIA, comprising four normal incidence telescopes, is one of three instruments aboard the Solar Dynamics Observatory mission, part of NASAs Living with a Star program, currently scheduled for launch in 2008. Seven different multilayer coatings will be used, covering the wavelength region from 93.9 to 335.4 Å.

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

We have developed a new depth-graded multilayer system comprising W and SiC layers, suitable for use as hard x-ray reflective coatings operating in the energy range 100-200 keV. Grazing-incidence x-ray reflectance at E = 8 keV was used to characterize the interface widths, as well as the temporal and thermal stability in both periodic and depth-graded W/SiC structures, whereas synchrotron radiation was used to measure the hard x-ray reflectance of a depth-graded multilayer designed specifically for use in the range E approximately 150-170 keV. We have modeled the hard x-ray reflectance using newly derived optical constants, which we determined from reflectance versus incidence angle measurements also made using synchrotron radiation, in the range E = 120-180 keV. We describe our experimental investigation in detail compare the new W/SiC multilayers with both W/Si and W/B4C films that have been studied previously, and discuss the significance of these results with regard to the eventual development of a hard x-ray nuclear line telescope.

Development and production of hard X-ray multilayer optics for HEFT

The High Energy Focusing Telescope (HEFT) will observe a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters at energies between 20 and 70 keV. Large collecting areas are achieved by tightly nesting layers of grazing incidence mirrors in a conic approximation Wolter-I design. The segmented mirrors that form these layers are made of thermally formed glass substrates coated with depth-graded multilayer films for enhanced reflectivity. The mirrors are assembled using an over-constraint method that forces the overall shape of the nominally cylindrical substrates to the appropriate conic form. We will present performance data on the HEFT optics and report the current status of the assembly production.

Coatings for the NuSTAR mission

The NuSTAR mission will be the first mission to carry a hard X-ray(5-80 keV) focusing telescope to orbit. The optics are based on the use of multilayer coated thin slumped glass. Two different material combinations were used for the flight optics, namely W/Si and Pt/C. In this paper we describe the entire coating effort including the final coating design that was used for the two flight optics. We also present data on the performance verification of the coatings both on Si witness samples as well as on individual flight mirrors.