Polymer X-ray lenses for beam shaping and X-ray microscopy
Optics with an easily adjustable focal length will serve X-ray microscopy a lot. An X-ray zoom lens renders the possibility to freely choose the magnification factor in full field microscopy. The distance between sample and image plane can be kept constant even when changing the photon energy. With such optics even X-ray spectroscopic measurements can profit, as in spectroscopy changing the energy is inherent in the method.
Principle of an X-ray zoom lens – single lens elements in and moved out of the beam
First measurement of a half lens with line focus in the beam at PETRA III, P05
|At IMT Compound Refractive X-ray Lenses (CRLs) have been developed. These lenses show good optical properties in the hard X-ray regime for photon energies above about 8 keV up to about 100 keV. Until now several customized lens layouts for applications are processed on one substrate. So one can select the most suitable lens, shift it into the beam, slightly readjust the system and use the lens. To achieve an adjustable focal length single lens elements have to be moved out and back into the beam. Therefore in this project we use piezo bender actuators to bend previous sawed substrate stripes with single lens elements at the end reversibly out of the optical path. This so called zoom lens is remote-controlled. An operator can easily give the required photon energy, sample and detector position and then the most suitable and low astigmatism lens setup will be chosen automatically.|
In comparison to a transfocator with a focal length in the meter range, a zoom lens can provide focal length in the centimeter to meter range. The zoom lens has a small overall size and can therefore easily be mounted in different beamlines. It allows for fast switching the focal length. Therefore we are always looking for new application and measurement methods where these new possibilities can be used.
First test of a full mounted zoom lens with point focus at ESRF, ID01
X-ray prism lenses (XPLs) and Fresnel lenses can be used for illumination purposes in the hard X-ray wavelength regime as they are used in visible light regime. In this project we develop polymer lenses for certain applications in full field X-ray microscopy and other imaging techniques.
The sample illumination in a full field microscope application should fulfil two requirements. The intensity in the sample plane shall be as homogenous as possible across the whole field of view (FoV). And the rays coming from the illumination optics shall be accepted by the objective lens. In this project we develop condensers fabricated using deep X-ray lithography (LIGA).
At high brilliance synchrotron sources the beam size is limited by the low divergence of the beam from the undulator source in vertical direction. This relatively small beam cross section limits the available field of view (FoV) for full field imaging. For many applications a larger FoV is desirable e.g. for medical, or biological samples or in materials science where the sample size is given by the manufacturing process. To cover long samples, several height steps are scanned and the tomograms are stacked, which is time consuming. Furthermore, the Gaussian shaped beam profile is not ideal for full field imaging where a more constant intensity over the sample would be advantageous.
To overcome these limitations we developed refractive beam shaping optics, produced via deep X-ray lithography using SU‑8, an epoxy based polymer. The designed optics consist of biconvex Fresnel-elements defocusing the beam. The local curvature is tailored to widen the incoming beam and at the same time change the incident Gaussian-like beam profile to a top-hat intensity distribution.
When testing the beam shaper at the P05 beamline of PETRA III (Hamburg, Germany) operated by HZG, we were able to widen up the original beam profile in vertical direction from 1.9 mm to 6 mm. In addition the beam was transformed from a Gaussian-like beam intensity profile to a more top-hat distribution. Due to the fact that we are free to shape the facet of the lens for a certain purpose, it is possible to generate any desired intensity distribution with a suitable designed beam shaper of this type.
Left: Image of beam profile at P05, with beam shaping optics widening the beam. Right: Beam profile changes from Gaussian like (upper graph) to more top-hat like distribution (lower graph, blue).
The field of view of a for example 2 m long X-ray full field microscope using refractive lenses normally is limited to roughly 100 µm, because the lenses aperture is about this size. To overcome this limit, an arrangement of n times m parallel lenses can be used. Such an array generates many images of different regions of the sample at the same time on the detector.
In another type of multi lens all lenses look at one point of the sample. Thus they capture light passing the sample under different directions. In this way more light from the sample is captured on the detector, providing several images taken under slightly different angles. This method could be used to reduce the exposure time in X-ray microscopes at tube sources.
Grating based interferometry
Grating based X-ray Differential Phase Contrast Imaging (DPCI) can enhance the common X-ray absorption imaging by two additional contrast modalities, dark field contrast and phase shift. Therewith materials and structures with similar absorption properties can be distinguished. For efficient imaging, the size of the Field of View (FoV) should be in the same range as the object. Currently the FoV is limited by the area of the microstructured X-ray gratings fabricated by the LIGA technology. To overcome this limit we are following different approaches to enlarge the grating area by keeping the image quality constant.
DPCI based on Talbot-Lau interferometry offers enhanced X-ray imaging contrast for weakly absorbing objects (e.g. soft tissues) and also allows analysing materials with small differences in absorption contrast (e.g. light-weight materials like fibre reinforced polymers). However, the possibilities of this method at lab sources are not popular yet due to existing gratings with periods >2.4µm. This can be realized by pushing the limits of grating periods, to build set-ups of less than 1m, for energies relevant to materials analysis. Additionally, the resolution of the acquired X-ray images depends strongly on the quality and aspect ratio of the X-ray gratings. Therefore, X-ray lithography is used to face the challenges of fabricating gratings with periods less than 2µm and heights of more than 50µm.
|Grating field with period 1.1µm and resist thickness ~10µm demonstrating resist framework for the fabrication of small period gratings.||Grating field with period 1.3µm and metal (Ni) electroformed structures ~11µm high, after stripping a ~20µm thick resist framework, demonstrating small period gratings.|
|Grating field with period 1.6µm demonstrating the possibility of combined electroforming with initial thin Ni layer (~1µm) at bottom, followed by Au deposition on top according to specifications (~6µm).|
The fabrication of sub-µm gratings is challenging and involves several parameters in the entire fabrication process. The fabrication process is based on X-ray lithography followed by electroforming (LIGA technology). The quality of such sub-µm gratings strongly depend on the stability of the resist template with continuous lamellas, which can suffer from shrinkage, bending and sticking during fabrication. This becomes more critical with decreasing structural widths. The exposure and development conditions can also generate variations in the outcome, and also physical effects on the nanometer level like photo-electron penetration or fluorescence radiation. To achieve the goals of developing and optimizing a process which allows to realize gratings with small periods, strongly below 2µm, the following measures are considered:
- Analysis of dose deposited during X-ray exposure as a function of exposure conditions, i.e. theoretical simulations.
- Study of resist behaviour, namely mechanical stiffness as a function of baking and exposure conditions.
- Modification of resist based on requirements and behavioural studies.
- Optimization of development and drying conditions to avoid sticking defects.
- New design concepts to mechanically stabilize the fragile structures.
- Testing of gratings in real applications with project partners.
ERATO (Exploratory Research for Advanced Technology) phase imaging project funded by JST (Japan Science and Technology Agency) aims the for development of a new type of X-ray Talbot interferometer by employing novel grating pattern. A simulator calculating the field strength map for an X-ray wave field (known as “Talbot carpet„) is currently in development for designing the phase grating; various grating shapes could be simulated. In parallel with the development of the simulator, gratings fabrication with small period on Silicon and/or Carbon wafer is also studied.
High quality absorption gratings, are fabricated routinely for energies up to 30 keV with heights of 50 µm to 70 µm using deep x-ray lithography2,3. Phase gratings are fabricated with continuous lamellas out of nickel or gold. Shifting to higher energies, most interesting in materials analysis, the fabrication of absorption gratings, in particular G2, becomes challenging: the absorbing grating lamellas must be high enough to fully absorb the high energy X-rays (e.g., to absorb 90% of 100 keV X-rays, the gold thickness has to be 230 µm). In addition, the period needs to be small (2-5 μm) to ensure high sensitivity of the system.
It is our goal to improve the gratings quality with focusing on the following issues:
- reproducibility 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 secondary radiation by using low Z electroplating layer/substrate like graphite,
- optimizing the design and the gratings geometry to obtain the minimum possible deviation from an ideal grating.
a) 4.8 µm period gratings 200 µm high after development
b) after electroplating and removing the resist
c) 10 µm period gratings 250 µm high after development, the +/- 30° pillar could be seen
d) 4.8 µm period 150 mm thick after electroplating and removing the resist, resist pillar leads to a hole in the final electroplated structure
1 µm period gratings
Emerging X-ray optics
High structure quality and good homogeneity of the gratings are key elements to achieve high sensitivity in grating based interferometry. To improve these, we continuously investigate the influence of numerous process parameters on the thermomechanical properties of the photoresist using a variety of methods such as tensile testing, gas pycnometry, differential scanning calorimetry or thermogravimetric analysis.
Even with laboratory X-ray sources, it is possible to achieve grating based differential phase contrast images with nanoradian angular sensitivity1. A necessary prerequisite for this performance is a high fringe visibility, which critically depends on the structure quality of the gratings. When it comes to high aspect ratios, the structure quality is determined by the properties of the photoresist2, therefore we investigate how these can be influenced by the process parameters.
The properties we measure include, but are not limited to Young’s modulus, maximum elongation and tensile strength, solvent content, as well as curing shrinkage and speed. The main parameters to influence them are processing times and temperatures, especially during softbake and crosslinking reaction, together with exposure dose and spectrum. Freeze drying after development provides a way to minimize forces acting on the microstructures and can therefore also increase quality at high aspect ratios3.
1 L. Birnbacher, M. Willner, A. Velroyen, M. Marschner, A. Hipp, J. Meiser, F. Koch, T. Schröter, D. Kunka, J. Mohr, F. Pfeiffer, and J. Herzen, Sci. Rep. 6, 24022 (2016).
2 D. Kunka, J. Mohr, V. Nazmov, J. Meiser, P. Meyer, F. Koch, M. Amberger, J. Schulz, M. Walter, T. Duttenhofer, A. Voigt, G. Ahrens, and G. Grützner, Microsyst. Technol. (2013).
3 F. Koch, F. Marschall, J. Meiser, O. Márkus, A. Faisal, T. Schröter, P. Meyer, D. Kunka, A. Last, and J. Mohr, J. Micromechanics Microengineering 25, (2015).
Several real-time applications require special optical elements with two and three dimensions for fast imaging in a simple and robust setup. This project aims to broaden the X-ray imaging technologies by supporting spatial harmonic imaging method with two-dimensional gratings in a single-shot imaging configuration. Two-dimensional gratings are patterned and characterized for investigation of dynamical processes.
Based on a single image it is possible to obtain multi-modal information by converting the raw image into spatial harmonic spectrum. The 0th harmonic of the spectrum is not affected by diffraction, thus provides the absorption contrast, while ratio between the 1st harmonic and the 0th gives the scattering image. Differential phase contrast information is obtained by the phase shift of the 1st harmonic image. In addition, due to the relation between the scattering length (SL) and autocorrelation of electron density distribution at specific distances it is possible to perform particle-size sensitive characterization.
Among our networking activities, collaboration with Universities of the South of Brazil for application of X-ray imaging to material characterization has been established (http://www.cct.udesc.br/?id=1966).
We look forward to new interferometric and non-interferometric applications where novel two-dimensional gratings are required.
This project is the part of the master thesis of Margarita Zakharova, intern from Tomsk Polytechnic University under my supervision.