# XCT and DLW: Synergies of Two Techniques at Sub-Micrometer Resolution
## 1 Introduction
XCT and XRM techniques are both available as commercial laboratory devices in various configurations. However, the 3D imaging methods are not yet established in the same way as a standard light or electron microscope in research facilities. The general availability of commercial XCT devices is much larger than XRM machines. Compared to commercial laboratory devices, an advanced synchrotron facility offers a brilliant, monochromatic X-ray beam [22]. Therefore, the scan time for a sample is drastically reduced. Moreover, synchrotron facilities offer open and adjustable setups to achieve cutting edge results. Additional phase contrast or grating-based dark field imaging setups can be installed to achieve different contrast tuning for certain material classes [26; 27]. This is not available for commercial devices, and it is not necessarily needed, as the intrinsically X-ray absorption contrast for the two sample phases (polymer and air) analysed in the present work is expected to be sufficient. The synchrotron-based resolution limit depends on the installed X-ray setup (lenses, detector), and the limit is comparable to laboratory devices. The synchrotron facility beam time slots are limited and proposals need to be submitted. A beam time is not guaranteed. For this, synchrotron may offer advanced, fast recording and adjustable setups, but laboratory devices offer a higher availability. Note that the radiographic imaging methods can be accompanied by imaging artefacts, which may hinder a following measure of sample features [28]. For DLW products, a complete 3D view of the structure (and substrate) can be imaged by XCT/XRM. In addition, a sliced view of any written structure can be created to investigate the inner structure. [PERSON] et al. investigated the damage threshold of photoresists due to proximity effects via XRM [29] and has shown that with an additional share of iodine in the chemical formula of the photoresist the contrast in the XRM can be fine tuned [30]. However, this is not applied in the present work, as this is not a mandatory requirement for XCT/XRM imaging. Flaws or defects can occur at different times during the DLW process. They can occur in pre-DLW structuring, for example, in the bake phase of the photoresist,during the fabrication process itself or after the process while developing the sample to remove the unwanted photoresist. To understand the presented flaws, in this work, we will provide a short introduction to DLW fabrication and the used slicing software. During the DLW process, the in situ observation possibilities are limited due to the simple visible light camera setup used similar to the setup shown by [PERSON] [31]. Unintended defects can have multiple origins and require a complete understanding of the DLW process to trace their original source. Created flaws can be either human-related, or part of the whole DLW process. To obtain a physical DLW structure from a 3D model, the photoresist has to be polymerised in a way that allows to reproduce the outer and inner contours of the model. The actual beam path is determined by a slicing software that cuts the 3D model in slices perpendicular to the substrate and defines the beam scanning path along the slices. The distance between the slices is called the slicing distance. The distance between the parallel beam scanning paths within a slice is the hatching distance [32]. To shorten fabrication times in FDM (Fused Deposition Modelling), the option to use an infill is widely used [33]. When using an infill, only a certain percentage of the inner volume of an object is polymerised by the laser beam in a certain pattern. In the DLW technique, the remaining (liquid) polymer is trapped inside the pre-defined set of outer walls and can be polymerised in a second polymerisation step outside the fabrication chamber after finishing the laser writing process. Optimising the beam paths during this highly complex process is a challenging task and flaws can occur, e.g., additional fabricated lines when the model is not sliced correctly.