Michael Richter, Thomas Beckenbach, Constantin Rauch, Stephan Schreiner, Marcus Zuber, Elias Hamann, Arndt Last, Martin Börner, Jan Korvink, Pascal Meyer
A problem with high aspect ratio x-ray gratings, fabricated by the deep x-ray LIGA process, is the collapse of the metallic structure when the resist is removed. A unique method that consists of positioning perpendicular metal bridges on top of the grating (roof bridges) is described and tested as a solution. First, a theoretical study is carried out on the transmission loss of such grids as a function of the thickness, their spacing, their materials (gold or nickel), and the x-ray energy. Different processes with their own advantages and disadvantages are possible and described. To further satisfy the requirement of curved gratings, two processes are tested in detail: structuring the x-ray grating with a laser and planarization followed by restructuring a second resist layer. In both cases, a second electroplating step is performed. Finally, a grating with a 12 cm bending radius and stabilization is fabricated. To assess the quality of the grids, two complementary methods are used: scanning electron microscopy and angular x-ray transmission. The latter one is an innovatively developed measurement process specially dedicated to x-ray gratings. The results for the fabrication processes are discussed and rated. The stability provided by the roof bridges works as intended, although the overall quality of the grating is slightly reduced.
X-ray phase imaging with grating interferometers, such as the Talbot interferometer, is widely performed even with a laboratory X-ray source. However, the achievable spatial resolution is normally limited by the period of gratings. In this work, two laboratory-based apparatuses are developed to overcome the constraint of the spatial resolution. One is the combination of a commercially available FZP-based X-ray imaging microscope and Lau interferometer optics. The two-step deconvolution approach is explained to attain phase tomography. The other is a sub-period super-resolution X-ray phase imaging, which is based on the sample-scanning scheme across the beamlet array formed by a triangular phase grating. A proof-of-concept result of the super-resolution approach is presented.
We describe the rationale for selecting graphite as a substrate material suitable for manufacturing curved high-aspect ratio metallic x-ray gratings and experimentally validate that its properties satisfy requirements relevant for clinical phase-contrast and dark-field x-ray imaging. Selection criteria applied to two candidate materials graphite and polyimide were compliance to bending, mechanical tenacity of the attachment of the lamellar grating structure to the substrate, the substrate material’s x-ray robustness, and the compatibility with the x-ray LIGA process used to manufacture the grating structures. In contrast to other standard materials such as silicon wafers with titanium layer, graphite wafers could be bent to smaller radii and are natively electrically conductive. While polyimide wafers allowed for even smaller bending radii, we found their high risk of grating structure detachment to be a strong detractor. Minimum achievable bending radii were 55 and 70 mm for pure graphite wafers and graphite wafers with mounted grating structure, respectively. Electron microscopy of graphite surface and cross-sections revealed a fine porous structure conducting to a very stable metal–wafer interface. Grating structures with heights of more than 200 μm were bonded to graphite wafers and their integrity confirmed in flat as well as in bent state using microfocus x-ray imaging. We conclude that graphite is a very well-suited substrate material for manufacturing curved x-ray gratings.
Simon Pinzek, Thomas Beckenbach, Manuel Viermetz, Pascal Meyer, Alex Gustschin, Jana Andrejewski, Nikolai Gustschin, Julia Herzen, Joachim Schulz, Franz Pfeiffer
Background: X-ray grating interferometry is an emerging imaging technique that strongly relies on fine grating structures. A common method to fabricate compatible gratings is deep x-ray lithography (DXRL).
Aim: To develop a method to fabricate grating structures by DXRL, which does not require a synchrotron source.
Approach: The synchrotron source is replaced by a conventional x-ray tube. The fabrication process is adapted for the divergent beam by cylindrically bending mask and substrate.
Results: A 10-μm period absorption grating with 80-μm-thick gold lamellae is successfully fabricated from an intermediate 110-μm high structured resist. This grating is characterized and implemented in a preclinical Talbot-Lau interferometer designed for medical thorax imaging.
Conclusion: This approach can overcome the strong dependence on synchrotron facilities for the fabrication of gratings for x-ray grating interferometry. As x-ray tubes are more widely available, this is a cost-efficient and scalable alternative suitable for industrial production.
Background: Deep x-ray lithography using synchrotron radiation is a prominent technique in the fabrication of high aspect ratio microstructures. The minimum lateral dimensions producible are limited by the primary dose distribution and secondary effects (Fresnel diffraction, secondary electrons scattering, etc.) during exposure. Aim: The influence of secondary radiation effects on the fabrication of high aspect ratio microstructures with submicrometer lateral dimension by deep x-ray lithography is characterized. Approach: The microstructures under investigation are one-dimensional gratings. The influence of secondary effects on structural dimension is simulated and compared to the experimental results. The quality criteria and possible defects arising in experiments highlight the importance of the mechanical stability of the photoresist. Results: From the simulation results, the minimum period of microstructures that can be produced is about 600 nm. Experimentally, microstructures with 1.2 μm minimum period (resist width of ∼700 nm) and height of ∼10 μm could be fabricated. Conclusions: Simulation results show the feasibility for fabricating gratings with a period less than 1 μm. To achieve these values also in experiment, it is necessary to increase the mechanical stability of the high aspect lamellae. The outcome of these results allows one to reduce the expensive and lengthy product development cycle.
X-ray differential phase-contrast imaging (DPCI) using a Talbot–Lau interferometer at a conventional tube source has continuously found applications since its first demonstration. It requires high aspect ratio grating structures with a feature size in the micrometer range that are fabricated using lithographie, galvanik und abformung technology. To overcome the current limitation in grating area, an exposure strategy—continuous exposure—has been developed. In this case, the mask is fixed in respect to the synchrotron beam and only the substrate is scanned. Thus, the grating area is given by the scanning length which is much larger than the actual mask size. The design, needs, and tolerances to adopt this process of dynamic exposure will be described. Furthermore, the first tests using this method will be presented. Gratings with a metal aspect ratio of 11 and a period of 10 μm were fabricated on an area of 165 mm×65 mm. First imaging results demonstrate the suitability of this method. No differences in the visibility or in x-ray image compared to gratings fabricated by the standard method could be found.
Deep proton writing (DPW) is a fabrication technology developed for the rapid prototyping of polymer microstructures. We use polymethylmethacrylate (PMMA) substrates, which act as a positive resist, for irradiation with a collimated 12-MeV energy proton beam. Using 12 MeV enables the irradiation of increasingly thick PMMA substrates with less conicity of the sidewalls compared to the lower energies used in previous work. A microhole of 47.7 μm diameter over a depth of 1 mm is achieved, leading to a maximum aspect ratio of 21∶1. The sidewalls of the irradiated structures show a slightly conical shape and their root-mean-square surface roughness is lower than 50 nm averaged over 72 measured areas of 56 μm×44 μm. This means that DPW components have optical surface quality sidewalls for wavelengths larger than 400 nm. Based on the trade-off among the sidewall roughness, conicity, and the development time, we determine that the optimal proton fluence for 12-MeV DPW in PMMA is 7.75×106 μm−2. Finally, we discuss some high aspect ratio microstructures with optical surface quality that were created with DPW to be used for a myriad of applications, such as micromirrors, microlenses, optofluidic devices, and high-precision alignment structures for single-mode optical fiber connectors.
X-ray phase contrast imaging has become a promising biomedical imaging technique for enhancing soft-tissue contrast. In addition to an absorption contrast image it provides two more types of image, a phase contrast and a small-angle scattering contrast image recorded at the same time. In biomedical imaging their combination allows for the conventional investigation of e.g. bone fractures on the one hand and for soft-tissue investigation like cancer detection on the other hand. Among the different methods of X-ray phase contrast imaging the grating based approach, the Talbot-Lau interferometry, has the highest potential for commercial use in biomedical imaging at the moment, because commercially available X-ray sources can be used in a compact setup. In Talbot-Lau interferometers, core elements are phase and absorption gratings with challenging specifications because of their high aspect ratios (structure height over width). For the long grating lamellas structural heights of more than 100 μm together with structural width in the micron range are requested. We are developing a fabrication process based on deep x-ray lithography and electroforming (LIGA) to fabricate these challenging structures. In case of LIGA gratings the structural area is currently limited to several centimeters by several centimeters which limit the field of view in grating based X-ray phase contrast imaging. In order to increase the grating area significantly we are developing a stitching method for gratings using a 625 μm thick silicon wafer as a carrier substrate. In this work we compare the silicon carrier with an alternative one, polyimide, for patient dose reduction and for the use at lower energies in terms of transmission and image reconstruction problems.
The LIGA process, which combines x-ray lithography with electroplating and modeling, is a world wide used technique for the fabrication of high aspect ratio microstructures. In the first step a resist layer, typically PMMA, which is applied to a metal coated substrate, is patterned by shadow printing through a x-ray mask with synchrotron radiation. The exposed parts are subsequently dissolved in an organic developer. The achievable quality of the microstructure is decisively defined by the development process. In order to define an effective development process and create a simulation tool, which allows to foretell the needed development parameters and the achievable quality already at a design stage, the development behavior and its influencing parameters need to be investigated. Much work has been done in this area. In these previous studies, the development rate was either studied using PMMA foils in which a homogeneous dose or a dose profile has been deposited, or using irradiated microstructures. In the first case, result obtained by ex-situ measurements show, that the development rate is a bare function of the dose. In case of irradiated microstructures, the experimentally obtained development rate was described as an empirical function of the dose value and depth of dose deposition. The aim of this work is to investigate the difference in the development behavior of a microstructure compared to a foil and to link the results. Therefore, using in-situ measurements, we have made experiments using foils and microstructures with crosslinked and non-crosslinked PMMA covering a wide dose range. The final purpose is to find a relation between dose and development rate to determine the necessary development time of a sample with a given dose profile, with high precision. Experiments, result and simulation of the development rate, for the two kinds of materials are presented and discussed.
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