Two-dimensional X-ray waveguides: fabrication by wafer-bonding process and characterization

Abstract

The fabrication of two-dimensionally confining X-ray waveguides enables the generation of nanoscopic X-ray beams. First applications of such waveguides for lens-less holographic imaging have already been demonstrated, but were limited by the fabrication methods and the design. To overcome these limitations, we present here the fabrication process for a second generation of X-ray waveguide with air or vacuum as guiding channel, based on e-beam lithography, ion etching and subsequent wafer bonding. This is a first step towards waveguides fulfilling requirements of high transmission and high confinement, since the process can be scaled down to smaller channel dimensions from the present structures. We address the structuring method used and present results of first X-ray characterization at synchrotron beamlines, under two entirely different beam settings, corresponding to the coupling of a coherent beam and an incoherent beam. The use of X-rays for the investigation of condensed matter structure relies on the precise definition of X-ray beams, which are then used in scattering, imaging or spectroscopy experiments. Since high-resolution lenses in particular for hard X-rays are not available, most modern synchrotron beamlines and laboratory instruments use slits or pinholes to define the cross section of the X-ray beams. As the scattering volume or the sample size decreases in many applications to the nanometer range, beam definition at the nanoscale presents an entirely new challenge for X-ray science and technology. Already today, many imaging, spectroscopy and lithography experiments require the use of very small apertures (also called spatial filters in some microscopy techniques) with a cross section far in the sub-micron range for X-ray-beam definition. The spatial resolution of these techniques is limited by the cross-sectional dimensions of the available apertures; see for example lens-less X-ray Fourier transform holography [1] or proximity X-ray lithography [2]. In the limit of smaller and smaller cross sections, the beam-

Cone-beam x-ray phase contrast tomography of biological samples

“…For lower photon energies (here 7.9 keV) empty (air) channels are favourable. Recently, channel waveguides in silicon wafers with sub-100 nm lateral dimensions were realised[64,85,152] based on e-beam lithography and wafer-bonding[113,174].Fig. 4.5 c,d shows a sketch of the channel waveguide design along with a scanning electron micrograph of the exit surface of BWG 1 .…”

High aspect ratio x-ray waveguide channels fabricated by e-beam lithography and wafer bonding

“…23,24 Ideally, however, waveguide channels should be empty (air, vacuum), since a solid core material leads to unnecessary absorption, unwanted phase front modulations based on density variations, and in the case of polymer channels also drifts resulting from beam damage in the core material. To overcome these problems, a novel fabrication scheme based on dry etching of channels into silicon wafers has been reported previously, 38,39 using an etch mask defined by e-beam lithography, removal of the mask and subsequent capping by a second wafer bonded on top of the first. However, only lm-sized cross sections were achieved in both directions.…”

A dedicated endstation for waveguide-based x-ray imaging

“…Channel waveguides have been successfully fabricated with electron beam lithography in two variants [21,41]. In the first variant several waveguides with different channel width were exposed by electron beam lithography into a layer of resist.…”