We are developing new X-ray optical components which are used to improve existing or to establish new X-ray imaging techniques. The components are made either from X-ray stable polymer or metal and are fabricated using lithography methods (X-ray, electron beam, laser) and subsequent electroforming available through KNMF. New imaging modalities are developed together with different collaboration partners. The R&D activities are divided in two main parts:
Polymer X-ray lenses for beam shaping and X-ray microscopy
Under this topic we are developing and fabricating Compound Refractive X-ray Lenses (CRLs) which are used for sample illumination with focal spot size down to 100 nm and to build up full-field X-ray microscopes. X-ray Prism Lenses (XPLs) are realized for shaping the X-ray beams and for homogenous sample illumination. All activities are application oriented. Although the main focus is on synchrotron use, we also target for applications at X-ray tubes and at X-FELs.
The refractive index n for X-rays with photon energies of 10 keV – 50 keV is in the range of n = 1 - Δn ≈ 1 - 10-5 to 1 - 10-7 resulting in a small refractive power of the lens material. As n is below one, the lens elements of a focusing lens have to be biconcave, in contrary to lenses for visible light. In the case of X-rays, short focal lengths can only be achieved by positioning a large number of strongly curved lenses in a row. Using deep X-ray lithography available through KNMF, rows of such lens structures are fabricated in X-ray-stable polymer (for energies in the range of 8 – 60 keV) or in nickel (for 60 – 500 keV) on substrates in one step without time-consuming individual-lens alignment. Line focus lenses are fabricated within one exposure step. Point focus lenses are obtained by combining two line focus lenses tilted by 90° around the optical axis.
Microscopy resolutions below 200 nm (line and space) in a field of view of 70 µm x 70 µm have been obtained so far by means of IMT CRLs. The apertures of parabolic lenses are in the range of 50 µm up to 1.5 mm. The focal distances can be in the few centimeters range. XPL illumination optics are made with apertures of up to 2 mm and prism sizes down to 15 µm. Special beam shaping optics have been realized to transform for example a narrow 1 mm X-ray beam with Gaussian intensity distribution from a high brilliance synchrotron source into a wide 6 mm beam with a top-hat intensity distribution. In this way larger samples can be imaged.
Current research activities are dedicated to
- Improving resolution and increasing the field of view in full-field microscopy
- Developing X-ray microscopes with optimized condenser and objectives
- Development of low absorption, large aperture prism lenses
- Demonstrate the use of the X-ray lenses at X-ray tubes
- Optimization of beam shaping optics
- Development of X-ray zoom lenses with variable focal length
- Development of CRLs with liquids as lens material
Schematic view of an X-ray full-field microscope
|Parabolic line focus lens||Point focus lens||X-ray prism lens|
Grating based interferometry
Grating based interferometry relies strongly on high quality, high aspect ratio X-ray gratings. With our activities we are following the four demanding requests for high performance gratings which need to be fulfilled to bring this imaging technique to commercial application in medical diagnostics and materials analysis: larger (in terms of the grating area), higher (in view of structural height), better (in terms of quality and homogeneity) and smaller (in terms of grating periods). This needs process development, process modification, structural characterization and understanding of effects arising during processing. Using the capabilities to shape the gratings geometry we are following the need for high resolution imaging.
|1 µm period gratings
(left: “bridge design” absorption gratings; gold thickness: 20 µm;
right: long lamellae phase gratings; nickel thickness: 3 µm)
|X-ray LIGA KIT logo in gold in front of a 10 µm period 300 µm high grating
with sun-rays (before electroplating) (courtesy of JulesMarketing).
We continuously improve the gratings quality with focus on the following issues:
- repeatability of the process,
- downsizing the period and increasing the resist/metal thickness by modifying the photoresist sensitivity and contrast,
- increasing the mechanical stability by finding the best combination of applied dose and post exposure bake (PEB), by using freeze drying and room temperature electroplating,
- minimizing the harmful effects of secondary radiation by using a low Z electroplating layer/substrate, optimizing the design and the gratings’ geometry to obtain the minimum possible deviation from an ideal binary grating,
The characterization is done using:
- scanning electron microscopy (DC measurement),
- white light interferometry (G1 measurement thickness)
- radiography, visibility measurement using standard grating setups by our partners (CT-Lab at KARA, TUM, FAU).
Emerging X-ray optics
For broadening the X-ray imaging modalities for biomedical and material science applications at synchrotron facilities and at X-ray laboratories novel optical elements with two- and three dimensions are developed. They are used for multi-modal information acquisition based on small angle scattering, absorption and phase contrast.
The current activities concentrate on process and methodology development to pattern and characterize two-dimensional gratings for investigation of dynamical processes. Furthermore, considerable research is devoted to enhance the synthesis of the polymer used in the lithography process to obtain better physical-chemical properties and to use the optimized formulation in the fabrication of HAR grating structures. The novel structures will be employed as the main optical elements in Grating-based interferometry and Single-Shot imaging modalities to investigate weak X-ray absorbing samples and composite material with similar absorption cross-sections by phase and dark field Imaging.