# Ni-Al Alloys as Alternative EUV Mask Absorber
## 3 Film Characterization
### Film Morphology  
The degree of crystallinity of the as-deposited film provides an estimation for the susceptibility to surface roughness and LER. The absorber morphology is preferably nano-crystalline or amorphous, and can be determined using cross sectional TEM and XRD. Figure 4a displays the cross-section TEM images of Ni\({}_{3}\)Al, NiAl and Ni\({}_{2}\)Al\({}_{3}\). An amorphous native oxide layer is clearly visible on the sample surface, which grows thicker with increasing Al-ratio. All Ni\({}_{x}\)Al\({}_{y}\) compositions are polycrystalline, which can be observed as contrast and lattice orientation change in the Moire patterns. Figure 4b presents the same samples under different magnification and contrast to better visualize the crystal grains. Ni\({}_{3}\)Al and NiAl have similar crystallinity with columnar grains spanning the full film thickness. Ni\({}_{2}\)Al\({}_{3}\) exhibits smaller grains near the surface and substrate interface. \begin{table}
\begin{tabular}{c c} \hline \hline
**Characterization** & **Metrology** \\ \hline Surface chemical state & X-ray photoelectron spectroscopy (XPS) [26] \\ \hline Bulk elemental composition & \
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\begin{tab  
Crystallinity is further investigated with XRD scans in Bragg-Brentano geometry, which is mostly sensitive to lattice planes parallel to the film surface. The atomic planes of a crystal cause incidence X-rays to diffract in specific angles. The most intense diffraction peak of a reference Ni film of 25 nm is located at 44\({}^{\circ}\) 2 theta, which is caused by Ni (111) oriented lattice planes. The full width at half-maximum (FWHM) of the Ni (111) peak can be used to estimate the average crystal grain size \(\tau\) through the Scherrer's equation [43], with \(K\) the crystal shape-dependent Scherrer constant, \(\lambda\) the Cu K\(\alpha\) X-ray wavelength, and \(\theta\) the X-ray incident angle corresponding to the observed XRD peak:  
\[\tau=\frac{\mathrm{K}\,\lambda}{\mathrm{FWHM}\,\cos(\theta)} \tag{2}\]  
The XRD spectra of Ni\({}_{\mathrm{x}}\)Al\({}_{\mathrm{y}}\) films are compared with a reference 25 nm thick Ni film, and are monitored with in-situ XRD (IS-XRD) during thermal loading up to 500 \({}^{\circ}\)C. After cooling down to room temperature, the XRD spectra of the Ni\({}_{\mathrm{x}}\)Al\({}_{\mathrm{y}}\) films are measured again. Figure 5a compares the XRD spectra of the as-deposited films, and of the films after thermal loading. Peak fitting results and estimated average grain size are tabulated in Table 3. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Before/After Thermal** & **Peak Position** & \multirow{2}{*}{**FWHM (\({}^{\circ}\))**} & **Avg. Ni(-Al)** \\  & **Loading to 500 \({}^{\circ}\)C** & & **20 (\({}^{\circ}\))** & **Grain Size \(\tau\) (nm)** \\ \hline
**Ni** & **before** & 44.430 \(\pm\) 0.011 & 0.653 \(\pm\) 0.035 & 13.7 \(\pm\) 5.4\% \\ \hline
**Ni\({}_{\mathrm{3}}\)Al** & **before** & 44.607 \(\pm\) 0.013 & 1.894 \(\pm\) 0.052 & 4.7 \(\pm\) 2.7\% \\  & **after** & 44.457 \(\pm\) 0.009 & 1.851 \(\pm\) 0.034 & 4.8 \(\pm\) 2.0\% \\ \hline
**NiAl** & **before** & 44.618 \(\pm\) 0.006 & 0.794 \(\pm\) 0.018 & 11.3 \(\pm\) 2.2\% \\  & **after** & 44.688 \(\pm\) 0.004 & 0.676 \(\pm\) 0.011 & 13.3 \(\pm\) 1.7\% \\ \hline
**Ni\({}_{\mathrm{2}}\)Al\({}_{\mathrm{3}}\)** & **before** & 44.502 \(\pm\) 0.044 & 1.323 \(\pm\) 0.157 & 6.8 \(\pm\) 12\% \\  & **after** & 45.239 \(\pm\) 0.007 & 0.687 \(\pm\) 0.021 & 13.1 \(\pm\) 3.1\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Peak position, full width at half-maximum (FWHM) and estimated average Ni(-Al) grain size of Ni, Ni\({}_{\mathrm{3}}\)Al, NiAl, and Ni\({}_{\mathrm{2}}\)Al\({}_{\mathrm{3}}\) as-deposited and after thermal loading up to 500 \({}^{\circ}\)C by in-situ X-ray diffractometry (IS-XRD).