*2.2. Fabrication of a Flexible X-Ray Mask*

Synchrotron XRL was used to create microstructures of various angles via simple dimensional transformations. Especially when using a curved substrate, the mask must be sufficiently flexible to adhere to the PR.

Figure 2 shows the fabrication of the flexible X-ray mask. The first step was the deposition of a gold layer on the polyimide (PI) film/silicon (Si) wafer (Figure 2a). The PI film was laminated onto the Si wafer using dry film resist (DFR) as adhesive. The PI film/Si wafer was chemically prepared and oxygen plasma-cleaned; the film was chemically resistant and thermostable. After cleaning and surface treatment, a 20-nm chrome layer and an 100-nm gold layer were then deposited. The chrome layer served to adhere the PI and the gold. A 100-nm gold seed layer for electroplating is then deposited. The second step in mask formation is shown in Figure 2b. The PR on the PI film is formed via UV photolithography. To create the PR, the PR (SU-8 3010; MicroChem, Westborough, MA, USA) was spin-coated onto the gold layer at 1,000 rpm for 30 s, and the sample was heated for 8 min at 95 ◦C (the soft bake). When all of the wrinkles had been removed, the PR was covered by the chrome mask and exposed to 15 mW of UV for 12 s (total energy: 175 mJ/cm2), followed by post-exposure baking (PEB) at 65 ◦C for 1 min and 95 ◦C for 23 min, rendering the latent PR pattern visible.

**Figure 2.** Fabrication of a flexible X-ray mask. (**a**) A gold layer on the polyimide (PI) film and silicon (Si) wafer; (**b**) Mask patterns on the gold layer after photolithography; (**c**) An electroplated gold (Au) layer between the mask patterns; (**d**) The completed flexible X-ray mask after separation from the Si wafer substrate. DFR, dry film resist.

The next step was the developmental process, after which only the pattern remains; other areas were chemically removed to expose the bottom gold surface, which acts as a seed layer for gold deposition during electroplating. Plasma ashing was used to remove residual PR. Electroplating (to form the masking pattern) was then performed (Figure 2c) using a customized electrochemical cell. The electrolyte solution was a gold preparation (SP Gold 2000; CNC Tech., Seoul, Korea). The gold substrate (the cathode) was covered with electroplating tape except in the patterned SU-8 area. DC current density of 1 mA/cm2 was delivered to an area of 2600 mm2 at 50 ◦C with mild agitation. The electroplating time was calculated using the experimental data, and was about 3 h at an electroplating rate of 4 μm/h, affording a final gold thickness of 12 μm. Then, the substrate was thoroughly rinsed with distilled water. Finally, the electroplated gold structures were detached (Figure 2d).

Optical microscopy confirmed that the SU-8 PR remained adherent to the substrate during mask fabrication. We created seven different patterns: four rows of lines with 20-μm, 50-μm, 100-μm, 200-μm, and 800-μm gaps between them; and rectangular, star, and circle patterns, as shown in Figure 3. Optical microscopy showed that the pattern edges were neatly formed, not only for relatively simple circles but also for (irregular) stars. We encountered no stress-induced gold peeling during electroplating.

**Figure 3.** Flexible X-ray masks. (**a**) Picture of fabricated X-ray mask with various patterns including. (**b**) lines, (**c**) stars, (**d**) circles, (**e**) squares.

#### *2.3. Fabrication of Microstructures of Various Angles*

In this research, PR SU-8, the material used for the microstructures, was micropatterned onto a flexible substrate, forming structures >100 μm in height. In fact, the PR exhibited excellent substrate adhesion, even when forming structures of height >3 mm via dry-chip casting [23]. Therefore, SU-8 is optimal for producing tall structures with various angles and shapes.

### 2.3.1. Spin-Coating

After fabricating the mask, the SU-8 was first spin-coated onto a flat structure. Although spraying and roller-coating, dip-coating, and extrusion-coating methods are available, spin-coating is most commonly used to form flat PR layers. The height of the final structure was about 75% of the thickness of the PR. The micropatterned structure was supported by a flexible PI film. The PI film (5 cm × 4 cm) was prepared via chemical and plasma cleaning, followed by surface treatment, as described for mask fabrication. PR (SU-8 2075; MicroChem) was spin-coated at 1000 rpm to a thickness of about 120 μm (Figure 4a), followed by heating (soft bake) to remove the solvent and cure the PR. The soft bake was performed at 65 ◦C for 8 min and 95 ◦C for 45 min. Wrinkles could have formed if the heating had been inadequate, resulting in surface distortion on contact with the mask, and thus the creation of dents on removal of the mask. In addition, the development time would have been extended. Then, the flexible X-ray mask was attached on top of the PR (Figure 4b) using an intermediate 30-μm PI film to prevent sticking.

**Figure 4.** *Cont*.

**Figure 4.** Fabrication of microstructures of various angles. (**a**) Spin-coating of photoresist (PR) onto a PI film. (**b**) The flexible PR X-ray mask. (**c**) The flexible structure wrapped on a curved substrate. (**d**) X-ray exposure of the curved structure. (**e**) Microstructures on a curved substrate. (**f**) Microstructures tilted at various angles.

### 2.3.2. X-Ray Exposure

The PR with the flexible X-ray mask was attached to a semicircular column by a lead sheet (Figure 4c), and then attached to a sample stage in the experimental chamber. Cooling lines were run under the sample stage. Heat control is important; heat generated by X-rays may deform the PR layer or create bubbles. After placing the sample in the stage, the chamber was evacuated and helium gas was injected. The X-ray beam dimensions were about 10 cm horizontally and 1 cm vertically. The stage had a reciprocating vertical motion to cover the entire field of view (FoV) of the sample (Figure 4d). The vertical scan speed was 2 cm/s and the total scan range was 7 cm. The dose delivered to the PR was about 90 J/cm3. After X-ray exposure, the PI/ PR construct was removed from the curved support, and PEB was performed at 65 ◦C for 5 min and 95 ◦C for 10 min. Irradiated, negative X-ray PRs form strong acids, and the affected areas become cross-linked during PEB. The brittleness and ductility of the photopolymer during exposure and PEB are influenced by the sensitivity of the PR to X-rays, X-ray intensity, exposure time, temperature, and PEB time; all must be controlled to ensure appropriate microstructural stiffness and adhesion to the flexible substrate.
