2.3.3. Development

After PEB, the PR/PI film was immersed in SU-8 developer for about 15 min. During development, the area exposed to the X-ray remains, and the area that is not exposed is removed (Figure 4e). As the substrate is flexible, it can be flattened, and the structures then exhibit various tilt angles depending on their distances from the center (Figure 4f).

#### **3. Results**

#### *3.1. Fabricated Polymer Microstructures of Various Shapes on a Flexible Substrate.*

Representative examples of the tilted microstructures are shown in Figure 5. Figure 5e–h are optical images acquired in top–down view; the shapes include lines, circles, stars, and squares. As the microstructural material is a polymer, the microstructures are transparent to visible light, allowing the morphology of the bottom substrate to be observed (red arrows in Figure 5a–d). The bottoms of the microstructures are dark (green arrows in Figure 5a–d). The four microstructures are shown from the side via scanning electron microscopy (SEM) in Figure 5e–h. The width of the line pattern microstructure is 50 μm; the length is 5 mm, the height is about 200 μm, the hypotenuse is about 560 μm, and the tilt angle is about 69◦. The aspect ratio of the tilted line structure is about 1:11. As the pattern is narrow, long, and highly tilted, the structure is vulnerable to external forces. Especially, during development, the developer solution must be circulated to remove non-crosslinked SU-8 PR. If stirring is too strong, the structure will collapse. If stirring is too weak, some polymer will remain

near the bottom, i.e., under the tilted structure (blue arrow in Figure 5e). In Figure 5a, residual polymer is present in the dark area (blue arrow). The radius of the smaller circular microstructure is 100 μm; the height is about 240 μm, the hypotenuse is 840 μm, and the tilt angle is about 73◦. We also fabricated square-patterned microstructures with a tilting angle of 54◦. The height of the structure was about 300 μm, the length of a side was about 200 μm, and the hypotenuse was 510 μm. Interestingly, we observed that star-patterned microstructures had the highest tilting angles, as shown in Figure 5h. The area of the star pattern was about 45 μm2, the height of the microstructure was about 100 μm, the hypotenuse was 510 μm, and the tilting angle was about 79◦.

**Figure 5.** Fabricated polymer microstructures of various shapes on a flexible substrate. Optical microscopic images of various patterns (from the top) including (**a**) lines, (**b**) circles, (**c**) stars, (**d**) squares. Scanning electron microscopy (SEM) images of various patterns (side views): (**e**) lines, (**f**) circles, (**g**) stars, (**h**) squares. Scale bars, (**a**–**d**) 400 μm, (**e**–**h**) 200 μm.
