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.