# Ni-Al Alloys as Alternative EUV Mask Absorber
## 3 Film Characterization
### Composition and Density Profile  
The composition and density of the absorber material is needed to estimate its theoretical optical constants by providing the weight percentage \(w_{j}\) and alloy density \(\rho_{alloy}\) in Equation (1) respectively. Many metals form a native surface oxide layer as well, which will be determined with XPS and TEM-EDS. The composition and density of both surface oxide and bulk layer can be determined by fitting XRR measurements. XPS is an effective technique to determine the oxidation state on the surface, which provides information on whether the atom is bound in a compound, or whether it is in pure metallic state. Depth profile can be obtained in sputter-assisted mode. XPS measurement of Ni\({}_{2}\)Al\({}_{3}\) is presented in Figure 8. XPS data for Ni\({}_{3}\)Al and NiAl are available in the Supplementary Materials section Figure S1. Ni2p\({}_{3/2}\), and Ni2p\({}_{1/2}\) peaks are found at 852.7 eV, and 870.0 eV respectively. On the surface, there is a negligible amount of Ni present. The peak positions inside the bulk of the film correspond to metallic Ni. In case Ni was in a compound, the Ni2p\({}_{3/2}\) and Ni2p\({}_{1/2}\) peaks will shift slightly to higher binding energy around 854.4 eV and 871.9 eV respectively, possibly splitting in multiplet peaks and having more pronounced satellite peaks. Figure 7: Average composition comparison of Ni\({}_{3}\)Al reference, after 110 h H*, and after 24 h DIW. The areal atomic density is normalized to Ni signal. The error bars represent the areal atomic density standard deviation. Al2p peak is observable at 72.6 eV, 74.6 eV and 75.6 eV for metallic Al, Al\({}_{2}\)O\({}_{3}\) and AlOx-on-Al respectively. On the surface (0 s sputter time), a main Al\({}_{2}\)O\({}_{3}\) peak with a side metallic Al peak can be discerned. Just below the surface (100 s sputter time), the Al\({}_{2}\)O\({}_{3}\) peak shifts to higher binding energy, corresponding to Al suboxide-on-Al, while the metallic Al peak becomes more dominant. In the bulk (up to 1300 s sputter time), the main peak is metallic Al with a small sidelobe at higher binding energy, indicating the presence of some Al suboxide. TEM-EDS image of Ni\({}_{2}\)Al\({}_{3}\), illustrated in Figure 9, confirms that the native oxide is composed out of Al and O. Ni is confined within the bulk, which also contains Al and traces of O. The bulk composition therefore has changed compared to nominal. All elements within the bulk metal layer seem to be uniformly distributed. TEM-EDS line scan reveals preferential Al diffusion towards high oxygen concentration, such as at the surface and at the SiO\({}_{2}\) interface. This leaves a Ni-rich layer just below the native oxide and on top of SiO\({}_{2}\). The Al:O ratio is not stoichiometric 2:3 beneath the surface, indicating formation of sub-oxides where less than the maximum number of Al-O bonds were formed. Figure 10 presents the XRR spectra of Ni\({}_{2}\)Al\({}_{3}\). The measurements for the other samples can be found in the Supplementary Materials section Figure S2. A multilayered model was used to account for density gradients, with layer thickness, density and roughness as fitting parameters. The nominal line follows the crystalline density of Al\({}_{2}\)O\({}_{3}\) and Ni\({}_{2}\)Al\({}_{3}\) along the layer thickness measured from TEM images. The density of the largest bulk layer is considered the bulk density of the film, and is tabulated in Table 4. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Characterization** & **Sample** & **XRR** & **Literature/Nominal** \\ \hline \multirow{2}{*}{Bulk density (g/cm\({}^{3}\))} & Ni\({}_{3}\)Al & 6.95 \(\pm\) 0.05 & 7.44 [47] \\  & NiAl & 6.77 \(\pm\) 0.12 & 5.92 [48] \\  & Ni\({}_{2}\)Al\({}_{3}\) & 5.46 \(\pm\) 0.04 & 4.75 [49] \\ \hline \multirow{2}{*}{Bulk composition Ni/Al (at \%/at \%)} & Ni\({}_{3}\)Al & 76.7/23.3 & 75/25 \\  & NiAl & 74.3/25.7 & 50/50 \\  & Ni\({}_{2}\)Al\({}_{3}\) & 54.8/45.2 & 40/60 \\ \hline \hline \end{tabular}
\end{table}