David M. Broadway

Status of small d-spacing x-ray multilayers development at Osmic

The deposition and X-ray performance of multilayer structures with d-spacings ranging from 1.2nm to 3.5nm has been presented. Different pairs of materials such as W (Mo, Ni, Cr, and La)/B4C, Ni (Cr, Co, V)/C have been considered. X-ray reflectivity of the multilayers has been measured in the photon energy range from ~0.18keV to ~100keV. W/B4C structures with d~1.5nm showed reflectivity greater than 30% at Cu-Kα (E~8keV). Performance of a W/B4C structure with d~1.5nm has been compared with TIAP crystal performance in analysis of Si-Kα (1.74keV), Al-Kα (1.5keV), Mg-Kα (1.25keV) and Na-Kα (1.04keV) radiation. Results showed that small d-spacing multilayers can be considered for TIAP crystal replacement in Wavelength Dispersive X-Ray Fluorescence (WDXRF) spectrometers. The absolute reflectivity of near normal incidence structures at O-Kα (~293eV), C-Kα (~277eV) and B-Kα (~183eV) radiation lines has been measured to be ~1.5%, 14% and 43% respectively with spectral resolution of up to ~0.6%.

Development of grazing incidence multilayer mirrors for hard x-ray focusing telescopes

We are developing depth-graded, multilayer-coated mirrors for astrophysical hard x-ray focusing telescopes. In this paper, we discuss the primary technical challenges associated with the multilayer coatings, and report on progress to date. We have sputtered constant d-spacing and depth-graded W/Si multilayers onto 0.3 - 0.5 mm thick DURAN glass (AF45 and D263) and 0.4 mm thick epoxy replicated aluminum foils (ERAFs), both of which are potential mirror substrates. We have characterized the interfacial roughness, uniformity, and stress of the coatings. The average interfacial roughness of each multilayer was measured from specular reflectivity scans ((theta) i equals (theta) r) using Cu Kalpha x-rays. The thin film stress was calculated from the change in curvature induced by the coating on flat glass substrates. Thickness and roughness uniformity were measured by taking specular reflectivity scans of a multilayer deposited on the inside surface of a quarter cylinder section. We found that interfacial roughness ((sigma) ) in the multilayers was typically between 3.5 and 4.0 angstrom on DESAG glass, and between 4.5 and 5.0 angstrom on the ERAFs. Also, we found that coatings deposited on glass that has been thermally formed into a cylindrical shape performed as well as flat glass. The film stress, calculated from Stoneys equation, for a 200 layer graded multilayer was approximately 200 MPa. Our uniformity measurements show that with no baffles to alter the deposition profile on a curved optic, the layer thickness differs by approximately 20% between the center and the edge of the optic. Interfacial roughness, however, remained constant, around 3.6 angstrom, throughout the curved piece, even as the layer spacing dropped off.

Normal-incidence condenser mirror arrangement for compact water-window x-ray microscopy

We demonstrate a water-window condenser arrangement for transmission x-ray microscopy based on table-top sources. A spherical normal-incidence multilayer mirror is used to focus and monochromatize water-window x-ray emission from a high-brightness droplet-target laser-plasma source. The condenser arrangement is compact, has high collection efficiency, and is easy to align. The maximum normal- incidence reflectivity at the desired wavelength, (lambda) equals 3.37 nm, was determined to be up to 3 percent. The potential for compact water-window transmission x-ray microscopy using the condenser arrangement is discussed.

Achieving zero stress in iridium, chromium, and nickel thin films

We examine a method for achieving zero intrinsic stress in thin films of iridium, chromium, and nickel deposited by magnetron sputter deposition. The examination of the stress in these materials is motivated by efforts to advance the optical performance of light-weight x-ray space telescopes into the regime of sub-arc second resolution. A characteristic feature of the intrinsic stress behavior in chromium and nickel is their sensitivity to the magnitude and sign of the intrinsic stress with argon gas pressure, including the existence of a critical pressure that results in zero film stress. This critical pressure scales linearly with the film’s density. While the effect of stress reversal with argon pressure has been previously reported by Hoffman and others for nickel and chromium, we have discovered a similar behavior for the intrinsic stress in iridium films. Additionally, we have identified zero stress in iridium shortly after island coalescence in the high adatom mobility growth regime. This feature of film growth is used for achieving a total internal stress of -2.89 MPa for a 15.8 nm thick iridium film with a surface roughness of 5.0 ± 0.5Å based on x-ray reflectivity (XRR) measurement at CuKα. The surface topography was also examined using atomic force microscopy (AFM). The examination of the stress in these films has been performed with a novel in-situ measurement device. The methodology and sensitivity of the in-situ instrument is also described herein.

Deposition of x-ray multilayers on long-size substrates for synchrotron applications

W-B4C multilayers with single d-spacing period of 2.2 nm have been deposited on 330 long by 50 mm wide Si substrates to be used as monochromators for a computed tomography application. Using magnetron sputtering and a substrate masking technique, d-spacing uniformities of +/- 0.86% and +/- 1% were obtained over a 180 mm by 100 mm area for 2.2 nm and 4.2 nm d-spacings respectively. Two separate processes were used to coat the 330 mm long substrate, wherein half of the substrate was coated in each process. A similar process was used to deposit depth graded W-B4C supermirrors on Si and CVD SiC substrates for a beamline pre-mirror application. The 330 mm long by 50 mm wide Si and 300 mm long by 79 mm wide SiC substrates were coated with 20 bi-layer supermirrors with d-spacings ranging from 4.4 nm to 10.8 nm. For an angiography research application laterally graded W-B4C multilayers were deposited on 150 mm by 120 mm silicon substrates. A strong nonlinear d-spacing gradient, from 1.6 nm to 3.8 nm was achieved across the mirrors surface in an attempt to provide uniform intensity over the reflected area. The maximum and minimum d-spacing gradient was 0.06 nm/mm and 0.003 nm/mm, respectively. We measured and mapped the d-spacing gradient using a custom Cu-Ka diffraction system. The measured d-spacings were within +/- 1.5% of the intended d-spacings.

X-ray reflectivity and mechanical stress in W/Si multilayers deposited on thin substrates of glass, epoxy-replicated aluminum foil, and Si wafer

Reflectivity at (lambda) equals 0.154 nm and mechanical stress in the bulk thin films of tungsten and silicon and single d- spacing multilayers on their basis with d approximately equals 2.8 nm deposited by the magnetron sputtering technique on flat thin substrates of Si wafer (approximately 0.2 mm), glass (approximately 0.3 mm), and epoxy gold replicated aluminum foil (approximately 0.3 mm) have been studied. The interfacial roughness of the multilayers has been calculated from the x- ray reflectivity curves as the following: on Si wafer (sigma) approximately equals 0.31 nm, on glass (sigma) approximately equals 0.32 nm, and on foil (sigma) approximately equals 0.34 nm. There was not observed a significant dependence on the stress in the Si film with change in rf power, Ar gas pressure and biasing. For the W films an increase of dc power results in an increase of stress. A similar relationship is also evident for W films deposited by rf power, but this dependence is less pronounced. The influence of low temperature (up to 200 degrees Celsius) annealing on x-ray reflectivity and stress in the multilayers has been investigated. There was not found an appreciable changes in the absolute value of reflectivity or in d-spacing with annealing temperature. The stress in the coatings changes with annealing temperature from compressive to tensile. There was observed a temperature of annealing at which the stress is no longer present in the film. The absolute value of this temperature measured for W/Si multilayer is approximately 120 degrees Celsius.

Achieving desired thickness gradients on flat and curved substrates

A mathematical model has been developed for the calculation of the film thickness distribution on flat, spinning, and curved substrates deposited by the magnetron sputtering technique. With the use of the model it is possible to design shield or mask shapes to intercept material between source and substrate to achieve a particular gradient in film thickness. Such considerations have significant relevance in the deposition of multi-layer thin films for x- ray and neutron optics in which the allowable deviation in the measured thickness gradient from the desired is a few tenths of a percent. Examples of the procedure used to obtain uniform coatings on flat and curved substrates has been given. Further, the consequences of target wear on the film thickness distribution has been considered. Finally, the consequence of spinning the substrate through the deposition region to improve uniformity has also been considered. Good agreement between initial experimental result sand the theoretical calculations has been shown.

Progress in Differential Deposition for Improving the Figures of Full-Shell Astronomical Grazing Incidence X-Ray Optics

One of the developments at MSFC that is underway to meet the demand of high-resolution X-ray optics for future X-ray astronomy missions is the ‘differential deposition’ technique. This process corrects the axial figure profile of optics by selectively depositing material onto the mirror’s reflective surface. The process relies on accurate metrology achieved using a long trace profiler whose slope resolution is better than 1μrad. From these metrology data an error map is generated that shows the profile of material to be deposited to correct the optic’s figure. A computer-controlled, deposition system then applies this corrective coating. Simulations show that a substantial improvement in angular resolution is possible with this approach after multiple correction ‘cycles’. To assess this, custom coating systems have been developed and corrections of full-shell optics are underway. To date, a factor of < 2 improvement in the imaging quality of the optics has been demonstrated in x-ray tests after a single stage of correction.

X-Ray Optics at NASA Marshall Space Flight Center

NASAs Marshall Space Flight Center (MSFC) engages in research, development, design, fabrication, coating, assembly, and testing of grazing-incidence optics (primarily) for x-ray telescope systems. Over the past two decades, MSFC has refined processes for electroformed-nickel replication of grazing-incidence optics, in order to produce highstrength, thin-walled, full-cylinder x-ray mirrors. In recent years, MSFC has used this technology to fabricate numerous x-ray mirror assemblies for several flight (balloon, rocket, and satellite) programs. Additionally, MSFC has demonstrated the suitability of this technology for ground-based laboratory applications—namely, x-ray microscopes and cold-neutron microscopes and concentrators. This mature technology enables the production, at moderately low cost, of reasonably lightweight x-ray telescopes with good (15–30 arcsecond) angular resolution. However, achieving arcsecond imaging for a lightweight x-ray telescope likely requires development of other technologies. Accordingly, MSFC is conducting a multi-faceted research program toward enabling cost-effective production of lightweight high-resolution x-ray mirror assemblies. Relevant research topics currently under investigation include differential deposition for post-fabrication figure correction, in-situ monitoring and control of coating stress, and direct fabrication of thin-walled full-cylinder grazing-incidence mirrors.

Optimization and analysis of depth-graded multilayer structures

We introduce a method for the determination of the thickness variation through the stack of multilayer structures deposited by magnetron sputter deposition. The deposited structure is determined by minimizing a merit function based on the difference between actual x-ray reflectivity data and the theoretical calculation of the reflectivity from a multilayer structure. This method uses only four parameters and is independent of the total number of layers deposited. Further, this simple method provides a good initial guess if one wishes to increase the number of independent parameters in order to investigate finer detail of the structure. We illustrate the usefulness of this method through comparison of a desired and deposited Ni/C multilayer. The thickness distribution through the stack was designed in such a way as to maximize integrated reflectivity over some angular range. Finally, we determine the dependence of layer thickness with annealing temperature for the depth graded Ni/C multilayer by use of our method.