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X-ray nanoprobes with 10 nm or sub-10 nm spatial resolution are highly desirable for the next-generation synchrotron beamlines due to the nanoscale imaging capability with high sensitivity to elemental, chemical and structural variations they provide. Multilayer Laue lens (MLL), a type of volume diffractive optics, was shown in theory to be able to focus X-rays to well below 1 nm [1]. In addition to this very high spatial resolution, it can also achieve much higher efficiency than conventional Fresnel zone plate [1]. An MLL is fabricated by sectioning thousands of planar depth-graded layers with nanometer thickness cross-sectionally into several to tens micron thick slivers. The section thickness is critical to achieve the optimum efficiency at desired energy. The requirement of high-aspect-ratio structure of an MLL presents enormous challenges in the post-growth processing, especially as the aperture size (deposition thickness) keeps increasing and therefore the residual stress increases as well. Sectioning without damaging multilayers becomes more and more difficult. At National Synchrotron Light Source II (NSLS-II), we have demonstrated that high quality MLLs can be successfully fabricated by combining mechanical polishing and focused ion beam (FIB) milling. The former removes most of the unwanted material and the latter is used for the fine and final polishing process [2]. MLLs with aperture size of 53 microns made by this method demonstrate around 10 nm focusing capability [3], suggesting that no additional aberration is introduced in the post-growth processing. Wedged MLLs are the best option to achieve sub-10 nm X-ray focusing, because they can produce higher efficiency compared with flat MLLs with the same section thickness [4]. However, sectioning requirement for a wedged MLL is more stringent because best focusing of a wedged MLL (in terms of both focus size and efficiency) can only achieved at a specific section thickness for one optimized energy. In this presentation, I will introduce the process, advantages, and technical difficulties of using the combined mechanical polishing and FIB method to make MLLs. The strategies to tackle the problems and make site-specific wedged MLLs will also be presented.
[1] J. Maser, G.B. Stephenson, S. Vogt, W. Yun, A. T. Macrander, H. C. Kang, C. Liu, and R. Conley, “Multilayer Laue lenses as high-resolution x-ray optics”, in Design and Microfabrication of Novel X-Ray Optics II, edited by A. Snigirev, D. Mancini, Proc. SPIE 5539, 185-194, SPIE, Bellingham, WA, (2004)
[2] H. Yan, R. Conley, N. Bouet, and Y. S. Chu, “Hard x-ray nanofocusing by multilayer Laue lenses”, Journal of Physics D: Applied Physics 47, 263001 (2014)
[3] H. Yan, N. Bouet, J. Zhou, X. J. Huang, E. Nazaretski, W. H. Xu, A. P. Cocco, W. K. S. Chiu, K. S. Brinkman and Y. S. Chu, “Multimodal hard x-ray imaging with resolution approaching 10nm for studies in material science”, Nano Futures, 2, 011001 (2018)
[4] H. Yan, J. Maser, A. Macrander, Q. Shen, S. Vogt, G. Stephenson, and H. Kang, “Takagi-Taupin description of x-ray dynamical diffraction from diffractive optics with large numerical aperture”, Phys. Rev. B, 76, 115438 (2007).
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