Nanofabrication of High-resolution, Nanofocusing X-ray Optics Based on Silicon and Diamond: Obstacles and Progress
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Nanofabrication of High-resolution, Nanofocusing X-ray Optics Based on Silicon and Diamond: Obstacles and Progress A. F. Isakovic1, K. Evans-Lutterodt1, A. Stein2, J. B. Warren3, S. Narayanan4, A. R. Sandy4 1
Brookhaven National Laboratory, National Synchrotron Light Source, Upton, NY, 11973 Brookhaven National Laboratory, Center for Functional Nanomaterials, Upton, NY, 11973 3 Brookhaven National Laboratory, Instrumentation Division, Upton, NY, 11973 4 Argonne National Laboratory, Advanced Photon Source, Argonne, IL, 60439 2
ABSTRACT Nanofocusing, high-resolution X-ray optics demand good surface quality, the absence of tapered sidewalls, and a depth reaching into tens, sometimes hundreds of microns, all requirements that must be satisfied over large areas. In this report, we discuss our motivation for choosing group IV materials (predominantly Si, and C in its diamond form) for nanofocusing and high resolution in the hard X-ray portion of the spectrum. We elaborate on the design and nanofabrication procedures, and detail the etching parameters that offer a path for overcoming obstacles in making better optics. We briefly review tests for the assessing the quality of the optics. INTRODUCTION AND THEORY New synchrotron hard X-ray sources, like the upcoming NSLS-II1, need to satisfy several criteria. Two such criteria are (1) nanoscale focus (preferably below 10 nm), and, (2) high resolution (below 1 meV). One choice of material for such optics is from group IV materials, like silicon and carbon (in diamond form). They are close to ideal candidates, mostly due to their relatively low atomic number Z, and the possibility that planar nanofabrication techniques developed for device physics can be modified for fabricating X-ray optics instrumentation, such as X-ray lenses, X-ray resonators, and X-ray prisms. We describe how to address issues facing the control of the parameters of a large-scale nanofabrication, like the surface roughness, the etch rate, etch selectivity and the undercut since they all affect the performance of X-ray optical devices. For silicon, an rms surface roughness of about 2 nm can be obtained for etches that go as deep as 80-100 microns2, while the roughness for diamond is 5 nm for an etch depth of 15 microns3. Currently, we work with the etch rates of 2 microns/min for Si, and 80 nm/min for diamond. This report discusses nanofabrication of refractive X-ray optics, variations of which include compound refractive optics4, 5, 6 and kinoform optics7. The latter are numerically generated phase-optics that deliver an image of a mathematically designed object8. A good starting point in designing kinoform X-ray optics is calculating the optimal phase-profile via Fermat’s theorem on path lengths and we previously showed that an ellipse is the optimal shape for the lens7. One design feature specific for large-scale nanofabrication is the removal (via deep reactive ion etching) of a significant portion of the lens material, a process that is accomplished in integer multiples of λ/δ (along the
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