# Ni-Al Alloys as Alternative EUV Mask Absorber
## 2 Materials and Methods  
Novel EUV absorber materials are primarily chosen for their optical performance in the EUV wavelength range, which is described by the complex refractive index. The imaginary part \(\kappa\)  
Figure 1: Cross-section of an extreme ultraviolet (EUV) binary intensity mask (BIM). The oblique incident EUV light, with chief ray angle of 6\({}^{\circ}\) from normal, is reflected by the multilayer (ML) mirror or absorbed by the absorber layer. The mask topography has an impact on EUV reflection, resulting in mask 3D (M3D) effects. Angles and thickness are for illustrative purposes only, and are not to scale. or extinction coefficient determines the attenuation, while the real part \(n\) or refraction coefficient determines the phase velocity. To reduce mask absorber height-dependent M3D effects, a material that absorbs more EUV and has higher \(\kappa\) than Ta is necessary [2]. Best focus shifts through pitch are caused by phase distortion due to a mismatch in refraction coefficient \(n\) at the vacuum and absorber interface, hence a material with \(n\) close to unity is preferred [13]. In this paper, the focus will be on material selection for BIM, i.e., high \(\kappa\) and \(n\) close to unity. Figure 2 shows the relationship between \(n\) and \(\kappa\) at 13.5 nm EUV wavelength of elements from Z = 11 to Z = 83, excluding elements from the lanthanide series and noble gases [14]. Nickel (Ni) is one of the highest EUV absorbing elements, but it can easily form crystal grains [8; 9; 15]. Alloying is an option to reduce Ni crystallinity [16]. Aluminum (Al) has non-negligible EUV absorption and \(n\) larger than unity. Based on rigorous imaging simulations, Al is expected to reduce M3D effects [17; 18], but imaging contrast will deteriorate due to Al's lower \(\kappa\) compared to Ta. By combining both elements into a Ni\({}_{x}\)Al\({}_{y}\) alloy, we target a material that reduces EUV phase distortion, while maintaining good contrast for imaging at reduced absorber thickness. Based on the thermodynamic phase diagram of Ni-Al system in Figure 3[19], three stable Ni-Al compositions were investigated: Ni\({}_{3}\)Al, NiAl and Ni\({}_{2}\)Al\({}_{3}\). The films were deposited with a nominal film thickness of 25 nm by physical vapor deposition (PVD) through co-depositing Ni and Al targets. We used 6\" Si-wafer substrates on which 30 nm SiO\({}_{2}\) was grown through wet oxidation. The wafers were mounted on a rotating wafer drum, sequentially passing Ni and Al direct current (DC) plasma's at Ar pressures of 5.10\({}^{-3}\) mbar resulting in alternating Ni and Al monolayers deposited and instantaneous intermixing. Furthermore, when selecting new EUV absorber materials, they need to satisfy a set of specifications summarized in Table 1[8; 17]. We present detailed results of the characterization with respect to some of these properties. Determining the optical constants of the absorber material with EUV reflectance requires an accurate absorber stack model, for which additional metrology is needed to assess the chemical composition and density profile of the absorber film. Figure 2: Extinction coefficient \(\kappa\) versus refraction coefficient \(n\) at 13.5 nm EUV wavelength for elements with atomic number from Z = 11 (Na) to Z = 83 (Bi), excluding the lanthanide series and noble gases. Data from [PERSON] et al. [14].