Schematic of a front-coupling x-ray waveguide setup. The waveguide is positioned in the focus of a high gain optical system, e.g., KB, FZP, or CRL. A numerical simulation of the field propagation in the waveguide is shown below in logarithmic gray scales.
Design of an e-beam lithography pattern comprising of (A) broad one dimensionally confining waveguides (1DWGs), (B) smaller 1DWGs, (C) waveguide gratings (WGGs), and (D) two dimensionally confining waveguides (2DWGs), all on the same silicon substrate.
Schematic of the channel waveguide and coupling geometry.
Since KOH etches Si anisotropically, the etching results in V-shaped or U-shaped grooves, depending on the crystal orientation of the Si wafer.
(a) Layer system used for V-groove fabrication. PMMA acts as positive resist in EBL. After (b) the development of the pattern, it is (c) transferred into the evaporated Si by reactive ion etching in atmosphere. Residual PMMA is washed away with acetone. (d) The oxide layer is etched isotropically with HF, the remaining Si above the oxide is taken away (e) in a subsequent KOH bath which at the same time etches the V-groove into the (100)-oriented Si wafer. (f) The oxide is removed by another HF bath. (g) The groove is covered by bonding a second Si wafer on top of the structured one.
Sketch of the dependence of the final polymer layer thickness on the polymer concentration (PMMA, amount in g) in its solvent ( 2-methoxy-ethyl-acetate (MEA)) and on the rotational speed during the spincoating process. Lines are best fits to the data.
SEM micrographs taken during the preparation of single PMMA stripes for direct 2DWG fabrication on a Si (100) wafer with PMMA after e-beam lithography and development.
SEM micrographs of a Si wafer with PMMA spincoated on top after e-beam lithography of a grating and development.
Micrograph of the front side of a polymer core waveguide grating in Si after direct fabrication. The scale bar is .
SEM images of two different gratings (top: periodicity ; bottom: ) during V-groove fabrication process.
V-groove as seen by SEM, both in top view and at an angle of 45°.
U-groove grating (scale ) etched into a (110)-oriented Si wafer observed by SEM after removal of the oxide layer. After bonding, gratings can be used to facilitate the alignment of 2DWGs. Gratings also enable the testing of waveguide performance in unfocused beams, i.e., at flux densities which are too low for the examination of a single 2DWG.
Bonded V-grooves after annealing at . The periodicity of the waveguide grating is .
Measured far-field pattern (symbols) of the U-groove grating in Fig. 12. The inset specifies the angles used to describe the WG far-field measurement. Lines are fits to the data. The envelope in the first graph represents the far field of a single 2DWG of width .
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