# Ni-Al Alloys as Alternative EUV Mask Absorber
## 3 Film Characterization
### Durability  
The photomask material must remain stable under cleaning conditions and working scanner conditions during the mask lifetime. The initial cleaning durability test composes of submerging samples in two solutions with different acidity: DIW (slightly acidic pH 5.7 due to CO\({}_{2}\) diffusion from atmosphere) and NH\({}_{4}\)OH (basic pH 11.4). Hydrogen durability was tested by exposing samples to a flow of H\({}^{*}\) radicals over  
Figure 5: **(a)** X-ray diffractometry (XRD) spectra of Ni (black), Ni\({}_{3}\)Al (blue), NiAl (red), and Ni\({}_{2}\)Al\({}_{3}\) (green). Samples as-deposited and after thermal load up to 500 \({}^{\circ}\)C, are represented by diamonds and plus signs respectively; **(b)** In-situ XRD (IS-XRD) spectra of Ni\({}_{3}\)Al, NiAl, and Ni\({}_{2}\)Al\({}_{3}\), showing the evolution of diffracted intensity as function of the temperatures of measurement. The intensity scale is represented in a pseudo-color map, with white representing the lowest intensity, blue intermediate intensity and red highest intensity. Onset of Ni\({}_{2}\)Al\({}_{3}\) recrystallization is indicated by the red arrow. a period of 110 h. H* radicals were formed through cracking H2 with a hot tungsten filament using a EUV Tech hydrogen cleaner [24]. After the durability tests the samples, together with a reference, were measured with XRR and RBS/ERDA to assess thickness and composition changes respectively. The results are depicted in Figure 6a. The XRR spectra of Ni3 Al reference and after 110 h H* have the same oscillation frequency, indicating no thickness loss. There is a slight increase in roughness after H* test, which can be recognized as attenuation of the amplitude at higher incident angles. Attenuation is also observed in the samples after 24 h in DIW, and even more so in NH4 OH. Furthermore, a reduction in oscillation frequency indicates a film thickness loss for the sample in NH4 OH. The Pourbaix diagram shown in Figure 6b, helps understanding the behavior of Al in aqueous solution [44]. The thermodynamic stable phases of Al are shown in relation to the solution's electrochemical potential V\({}_{\text{SHE}}\), acidity pH, and effective concentration of the soluble species (Al3+ and AlO2-). The stability region of water lies between the dashed blue lines, which define the reduction and oxidation reaction of water. Under negative V\({}_{\text{SHE}}\) or highly reducing condition, metallic Al is immune to reaction in aqueous solution. Under more oxidizing condition, Al will react depending on the acidity. Highly acidic and highly basic condition will dissolve Al into soluble Al3+ and AlO2- ions respectively, causing corrosion of the surface Al. Therefore, a thickness loss and increased roughness are observed by XRR for the sample in NH4 OH. Around neutral pH, Al reacts by forming an Al2O3 layer that functions as a stable passivation layer against corrosion. Based on XRR measurement, the passivating Al2O3 layer causes increased roughness. The pH-range of the passivating region is also dependent on the concentration of Al3+ and AlO2- in the solution. The driving force for Al dissolution is larger in diluted solutions, resulting in a small passivation region between pH 4 and 8 (orange line). Solutions with high Al3+/ AlO2- concentration, can increase the passivation region even more towards basic conditions (green line). The average composition changes of Ni3 Al are compared between a reference sample, after 110 h H*, and after 24 h DIW. The basic cleaning condition with NH4 OH was disregarded, as Al was not stable under this condition. Ion beam characterization techniques, such as RBS and ERDA, are very sensitive to changes in elemental composition. These techniques were used to measure Ni and Al areal atomic density.