## 1 Introduction  
### Deep X-ray Lithography (DXRL)  
Deep X-ray lithography (DXRL), which is the first step of the LIGA (a German acronym for \"Lithographie, Galvanoformung, Abforming\") process, is used to manufacture high aspect ratio (HAR) micrometer and sub-micrometer structures out of polymer [1]. Depending on the structural requirements and other boundary conditions (e.g., tolerable exposure time), either a negative or a positive tone photoresist is used. Polymethylmethacrylate (PMMA) is the most suitable positive tone photoresist, delivering the highest structural quality, but it suffers from low sensitivity, resulting in prolonged exposure time especially for several hundred micrometer high structures [2; 3]. Chemically amplified negative tone photoresists are much more sensitive but show some structural deficiencies due to shrinkage during polymerization after exposure. It is evident that the quality of the products fabricated by the LIGA process is highly dependent on the raw materials used for patterning, as well as on their chemical behavior during processing. Hence, it is necessary to have strict control of the process parameters [4; 5; 6; 7; 8; 9; 10; 11; 12]. The photoresist layer to be patterned is fabricated by spin coating a photoresist to the desired thickness in a range from 5.0 \(\upmu\)m to several millimeters, depending on the application. The photoresist mixture contains at least a polymer/oligomer with photosensitivity, a catalyst, a solvent, and occasionally some additives [13; 14]. Negative photoresist solutions are usually based on epoxy resins, because this resin has mainly been used for microstructure fabrication (MEMS), optical sensors, micro-pieces of watches, radiation sensors, optical fiber components, conductive microstructures, photonic crystals, etc. [15; 16; 17]. Compared to other materials, this resin is relatively cheap; the microstructures exhibit excellent properties, such as high mechanical resistance, thermal and chemical stability, and excellent transparency [14; 17; 18; 19].  
Despite many advances in microstructure fabrication, unknown factors affecting the quality and reproducibility of the HAR microstructures still exist; some of these defects are shown in Figure 1 (all images were made after the development step).  
Figure 1a shows, in the region protected from the X-rays (the black area), a crosslinked material abnormality (\"cobweb\"). This cobweb effect can occur due to the high sensitivity of the photoresist to secondary irradiation (e.g., photo electrons). The lamellae deformation shown in Figure 1b is a direct result of the shrinkage/swelling properties of the photoresist during polymerization. Depending on the mechanical properties of the crosslinked photoresist, this may even result in cracks (Figure 1d). A thermal mismatch of the crosslinked photoresist with the substrate and the lower mechanical stability or capillary forces in the drying process may result in structural collapse (Figure 1c). Bubbles caused during the baking and/or exposure and/or post-exposure bake steps conglomerate in prominent clusters. The residual photoresist will then create a thin layer on top of the structures, which remains solid after the development process, affecting the mechanical stability of the HAR microstructures and  
Figure 1: Defects during the microfabrication process using LIGA (“Lithographie, Galvanoformung, Abformung”) technology: (**a**) unwanted cross-links; (**b**) lamellae deformation; (**c**) high aspect ratio (HAR) structure collapse; (**d**) cracks; (**e**) bubbles; and (**f**) bad adhesion (identified by non-homogeneity in the focusing imaging at a microscopic level, showing parts of the image blurry).  
blocking the electroforming process (Figure 1e). Figure 1f indicates problems in the adhesion between the photoresist and the substrate.