Freestanding thin-film filters are important transmissive optical elements for applications in extreme ultraviolet (EUV) bands. Silicon (Si) has the L_2,3 absorption edge at the wavelength of 13 nm, providing high transmission at λ=13.5 nm. Therefore, it has been employed as one of the filtering materials in EUV lithography. Previously, Si is mostly adopted as an interlayer to form a multilayer structure with metallic materials, or attached to a nickel mesh to form a grid-supporting structure. However, till now there has been no thorough investigation on self-supporting thin-film filters conducted by sputtering a single-component Si material. To promote the application of Si-based freestanding filters in the EUV field and bridge such a gap in domestic research, we designed and fabricated a 50 nm-thin freestanding Si filter with high transmission at 13.5 nm.

The Si thin film was deposited on soluble or quartz substrates by pulsed direct current (DC) magnetron sputtering, and upon fabrication and gluing for encapsulation, a 50 nm-thickness filter sample with a flat surface is shown in Fig. 1. Then, the film thickness and morphology were characterized by X-ray reflectivity (XRR) and field emission scanning electron microscopy (FE-SEM). The EUV transmission spectrum measurements were performed at the National Synchrotron Radiation Laboratory (NSRL). Furthermore, the difference between the theoretical and measured transmission values of the filter in the 12.5-20 nm band was further analyzed by X-ray photoelectron spectroscopy (XPS) and IMD software calculations.

According to the XRR fitting results shown in Table 1, the measured thickness of the thin film is 50.8 nm with a thin SiO₂ layer of 1.9 nm. Figure 2 (b) presents the cross-section SEM image of the Si filter, indicating the filter thickness around 50.26 nm is consistent with the XRR fitting results. Then, a sandwich model of "SiO₂/Si/SiO₂" was built in IMD to calculate the EUV transmission spectra. Figure 3 shows the measured transmission values and theoretical calculation ("cal.1") values in the 10-20 nm band for the filter sample, demonstrating that the measured transmission value reaches 86.02% at 13.5 nm, with an obvious difference (ΔT%) between the two curves in the 12.5-20 nm region. To explain this phenomenon, we examined the sample's composition by XPS, as shown in Fig. 4. A 5 nm-thin SiO_x is the majority compound at the surface, and there is a certain level of "bulk oxidation" according to the deep etching results in Fig. 4 (b). With such optimization of the sandwich model from "SiO₂/Si/SiO₂" to "SiO_x/SiO_y/SiO_x" based on these XPS results, in Table 3 and Fig. 5, the ΔT% is decreased from 2.62% to 0.18% and the two curves coincide much better.

To obtain a highly transmissive EUV filter at 13.5 nm, we prepared a freestanding Si filter (50 nm-thin) with its transmission as high as 86.02% at 13.5 nm, combined with decent suppression in the deep ultraviolet (DUV) range. Meanwhile, the XPS results and the optimized IMD calculation model show that both the surface and bulk oxidation levels of the filters exert a significant influence on its EUV transmission, which is a direction that needs further research efforts. Our results will substantially expand the further applications of such ultra-thin Si filters to areas such as EUV lithography and other large-scale scientific facilities in short wavelengths.