**2. Materials and Methods**

#### *2.1. Experimental Setup*

The experiment was conducted at the 9D (X-ray nano/micromachining [XNMM]) beamline [21] of the Pohang Light Source (PLS) using a polychromatic X-ray beam (Figure 1) emitted from an electron storage ring. Unlike conventional UV lithography, it is possible to impart dimensional changes to the X-ray mask to create gaps between the mask and the sample, thus improving technical performance.

**Figure 1.** Schematic of synchrotron X-ray lithography. MR, mirror; Be, beryllium.

The energy of the linearly accelerated electrons is 3 GeV, and the current moving around the storage ring is 400 mA (Table 1). The 9D beamline uses a bending magnet to direct the X-rays into the lithography chamber. The horizontal divergence of the beam source from the bending magnet is about 8 mrad (Table 1). As the distance from the magnet to the sample is about 20 m, the beam width is about 16 cm, which is sufficient for a four-inch wafer process. X-rays emitted from the magnet pass through two mirrors and the Be windows before finally reaching the lithography chamber. The mirrors are inserted in the beam path to remove high-energy emissions [22]. The Be windows isolate the vacuum and experimental zones, and are surrounded by He (an inert, non-toxic noble gas). The He gas prevents Be window oxidation. The experimental chamber is also filled with He because the gas exhibits good X-ray transmittance, and flux degradation is minimal. Another effect of He atmosphere is that He atmosphere promotes heat dissipation by convection in the chamber, since He has high heat capacity. The sample stage of the experimental chamber moves vertically to provide homogeneous X-ray exposure distribution. The beam width covers the entire sample in the horizontal direction, but the vertical beam dimension is only about 1 cm; thus, the sample stage must move vertically to cover the entire sample (Figure S1).


**Table 1.** Specifications of the Pohang Light Source 9D nano/micromachining beamline.

<sup>1</sup> http://pal.postech.ac.kr.
