LabAdviser/Technology Research/Microfabrication of Hard x-ray Lenses: Difference between revisions

=Microfabrication of Hard x-ray Lenses=

*'''Project type:''' Ph.d project

*'''Project responsible:''' Frederik Stöhr

==Project description (Summery of Ph.d. thesis)==

A Si-CRL comprises multiple bi-parabolic cylindrical cavities. The bi-parabolic patterns are defined lithographically and vertically transferred into the Si substrate using deep reactive ion etching (DRIE). Based on a theoretical framework for CRLs, stringent requirements on the pattern transfer are found. Most crucially, the sidewalls of the cavities must be strictly parallel. Already slight deviations from the ideal parabolic shapes result in non-uniform and broadened waists of focused x-ray beams.<br>

Two strategies are demonstrated, which guarantee shape fidelity, while the heights of etched lenses can be increased. Both are based on defining the bi-parabolic cavities at their perimeter by trenches of uniform width, where one trench wall is comprised of sacrificial material. The two strategies differ in the way the unwanted sacrificial material inside the cavities is removed subsequent to DRIE. While the first strategy utilizes etching of the trenches through the entire thickness of the wafer for releasing the sacrificial portions, the second strategy relies on thin sacrificial structures that can be completely oxidized and removed by selective etching. Both strategies have proven to be equally successful in achieving a substantial increase of the heights of Si-CRLs and to facilitate accurate sidewall profile control necessary for uniform x-ray focusing.<br>  

A precise manufacture in turn asks for highly precise metrology. Therefore, a mix of techniques including optical profilometry and atomic force microscopy (AFM) has been used to obtain reliable information about the detailed three-dimensional shapes of the lenses. Adequate sample preparation and measuring procedures have been developed. Inverse replica molding in PDMS of the CRLs was established as an effective way to circumvent the limitations AFM probes have when concave surfaces need to be characterized, e.g. due to the finite lengths of AFM probes.

Four different x-ray optical components have been designed, manufactured and characterized with respect to their shape. Their optical performances were tested at the European Synchrotron Radiation Facility (ESRF). Two 1D-focusing Si-CRLs suitable as condensers in hard-XRM were developed utilizing the aforementioned two different strategies. The first Si-condenser showed focusing of a 56 keV x-ray beam into a 310 μm wide line and a waist of 980 nm (FWHM, full width at half maximum) at a focal length of 1.3 m. The second Si-condenser allowed the focusing of 17 keV x-rays into a 180 μm-wide line with a waist of 430 nm (FWHM) at a focal length of 0.215 m. Both systems leave plenty of space for sample surroundings and ensure low-divergent and wide x-ray beams with narrow waists. Both results are substantial improvements to what was available at the start of this thesis work.<br>

===Sacrificial structures for deep reactive ion etching of high-aspect ratio kinoform silicon X-ray lenses===

Stöhr F, Michael-Lindhard J, Hübner J, Jensen F, Simons H, Jakobsen A C, Poulsen H F and Hansen O. 2015 Sacrificial structures for deep reactive ion etching of high-aspect ratio kinoform silicon X-ray lenses. Journal of Vacuum Science and Technology B 33(6), 062001 [http://dx.doi.org/10.1116/1.4931622 LINK]

===Three-dimensional nanometrology of microstructures by replica molding and large-range atomic force microscopy===