# XCT and DLW: Synergies of Two Techniques at Sub-Micrometer Resolution
## 3 Results
### Evaluation of DLW Structures via XCT  
Due to the complex environment and setup of the DLW process, a multitude of flaws of different origin can occur. Such flaws may include cracks, pores, unintended polymerised regions, damaged regions, shape deviations such as binding or bulging or a lack of surface adhesion. Imperfections most probably stem from the difficult sample handling and preparation, but also fabrication process related issues may occur, e.g., in the fabrication or development phase. During the DLW process, the in situ observation possibilities are limited due to the simple visible light camera setup used. Unintended defects can have multiple origins and require a complete understanding of the DLW process to trace their original source. This includes software or hardware errors, as well. Therefore, non-destructive methods have to be used after the fabrication step in order to analyse the quality of the fabricated part and trace the origin of flaws. An SEM image and corresponding XCT images of a fabricated fractal pyramid (sample 1) are shown in Figure 7a and Figure 7c, respectively. The lateral dimensions and geometry of the fractal pyramid could be resolved by XCT. Pores and cracks in the solid polymer region are not detected, but there were unintended polymerised regions, and small misalignments. The pyramid is slightly tilted and pressed into the substrate, which highlights the difficulties of using a bendable substrate (PET foil) and alignment within the DLW setup. (Note the video in the Appendix A that illustrates the tilt). Unintended polymerised regions, e.g., additional fabricated lines between the actual structure, are presumably caused by a bug in the slicing software. This line cannot be observed via SEM as it is hidden. We assume that those lines stem from the slicing algorithm itself as Cura sliced samples were fabricated without additional lines. They occur in one direction exclusively, e.g., the X-direction. One of those flaws is highlighted on a horizontal slice of the fractal pyramide in Figure 7c. Such lines are caused due to an error in either the STL (3D model data format) or the slicer or bad command interpretation from the DLW device. If such a flaw is hidden inside of a complex structure, it cannot be detected from the outside (SEM). Nevertheless, they may cause fundamental issues in the application of the structure, e.g., hinder the laminar flow in microfluidic applications. XCT or XRM testing is advised to detect such hidden flaws. Figure 6: Different examined process possibilities to fabricate a structure via DLW, left to right: Through a glass substrate (standard route), through a thin polymer foil (tested, but no structures observed) and on top of polymer foil (applied route in this study). Radiographic testing methods become especially beneficial for DLW feedback of the produced parts quality when geometries with infill or intrinsic structures are produced. In the following, we want to investigate such a structure (sample 2) that was produced with a non-polymerised inner volume by using the Infill approach known from the FDM technique. Analogous to the FDM process, the inner volume of a structure does not have to be completely polymerised, but can be only partially converted by increasing the distance between the hatching lines. In Cura, a certain percentage of the inner volume can be left unchanged due to the reduction of written lines, and the total production time of structures is reduced significantly. Additional infill lines are then created by the slicer to strengthen the outer walls from collapse. The result is drastically reduced fabrication times. For instance, a bulk pyramid with a height of 100 \(\upmu\)m and a side length of 141 \(\upmu\)m and hatching and slicing distances of 200 nm has a fabrication time of 39 min 48 s. The same pyramid sliced with Cura and 8 outer walls and an infill line every 4 \(\upmu\)m needs only 7 min 15 s to fabricate with a similar layer height of 200 nm.