# Ni-Al Alloys as Alternative EUV Mask Absorber
## 6 Discussion and Outlook  
In this paper, we present an extensive experimental methodology flow to evaluate candidate absorber materials capable of reducing M3D effects. We have started characterizing alloys to a list of specifications, including film morphology, durability under cleaning conditions, and under hydrogen conditions. The optical specification for absorber materials for BIM require high extinction coefficient \(\kappa\) and refractive coefficient \(n\) close to unity. Before determining the optical constants, the film profile needs to be characterized first with sufficient depth resolution, especially in the case when more than a single layer is formed after deposition. Accurate measurement of the film profile is used to set up an absorber model with improved fitting convergence of the EUV reflectance data, and from which realistic optical constants can be precisely extracted. Three nominal compositions of the Ni-Al system have been investigated with various material and optical characterization techniques. Alloying Ni and Al was unable to remove crystallinity completely, but the Ni-Al crystal grain size has decreased compared to pure Ni crystal grains. The as-deposited Ni\({}_{2}\)Al\({}_{3}\) alloy shows nano-crystallinity morphology, but recrystallizes above 200 \({}^{\circ}\)C. As-deposited NiAl and Ni\({}_{3}\)Al exhibit poly-crystallinity and retain this morphology at higher temperatures. A native Al\({}_{2}\)O\({}_{3}\) layer forms naturally on the Ni\({}_{x}\)Al\({}_{y}\) alloy surface when Al is exposed to atmospheric oxygen. The surface oxide grows with increasing Al-content. A chemical stable capping layer can possibly protect Al from oxidizing. Alternatively, saturating all Al-O bonds resulting in a uniform Al\({}_{2}\)O\({}_{3}\) composition, can ensure the absorber composition does not change over time. EUVR verified higher \(\kappa\) with increased Ni content and \(n\) closer to unity with increased Al content. Measurement through wavelength detected Al L2 absorption edge shift towards 16.6 nm wavelength, resulting from Al oxidation in the Ni\({}_{x}\)Al\({}_{y}\) material. This near-edge anomalous scattering behavior causes the refractive coefficient \(n\) to be much closer to unity at 13.5 nm wavelength, which can be advantageous for BIM absorbers. Lithography imaging simulations predict a trade-off between better phase distortion reduction for high Al-content alloys, and higher contrast for high Ni-content alloys. Nevertheless, higher EUV absorption compared to 60 nm TaBN, allows the Ni\({}_{x}\)Al\({}_{y}\) absorbers to be thinner, resulting in lower dose-to-size, less shadowing effect, less pattern shift through focus, smaller best focus shift range, and larger overlapping process windows. Taking contrast loss into consideration, the best candidate out of the simulated absorber models is a 39 nm Ni\({}_{3}\)Al absorber stack. Although Ni etch is still challenging, promising mitigation strategies to overcome the issues of Ni in respect to etch and repair are under investigation. We have demonstrated that Ni-Al alloys are worthwhile candidates for mask absorber material with good imaging and mask life time expectations. The presented extensive evaluation of Ni-Al alloys is encouraging to increase the efforts in promising patterning strategies for magnetic metals. This project has received funding from the Electronic Component Systems for European Leadership undertaking under grant agreement number 662338. This Joint Undertaking receives support from the European Union's Horizon 2020 research and innovation program and Netherlands, France, Belgium, Germany, Czech Republic, Austria, Hungary, and Israel. The authors are grateful to [PERSON] and [PERSON] (IMEC) for their thin film expertise. For metrology support, we thank [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] (IMEC). We appreciate the support of [PERSON], and [PERSON] (IMEC). **Author Contributions:** We confirm that all authors contributed to this study. [PERSON] and [PERSON] did project management and gave initial ideas.