# Ni-Al Alloys as Alternative EUV Mask Absorber
## 1 Introduction  
Extreme ultraviolet lithography is being industrialized as the next candidate printing technique for high-volume manufacturing of scaled down microcircuit devices for logic and memory applications. Because of the short wavelength of 13.5 nm used in extreme ultraviolet (EUV) lithography, it is not possible to use refractive optics or transmission masks for image formation, and the optical elements and EUV photomask are reflective mirrors. The EUV mask is a reflective multilayer (ML) mirror, consisting of molybdenum/silicon (Mo/Si) bilayers having peak reflectivity at 13.5 nm, and is coated with a mask absorber where light has to be attenuated. The chief-ray incidence angle on mask is set at 6\({}^{\circ}\) from normal to avoid overlap of incident and reflected light.  
Figure 1 depicts the cross-section of an EUV photomask. The substrate consists of a low thermal expansion material (LTEM) glass plate with a Mo/Si ML mirror coating on front side, and a conductive backside coating for electrostatic chucking. A thin ruthenium (Ru) capping layer protects the mirroragainst oxidation of the top Si layer, and against the cleaning of organic contaminants by hydrogen reduction [1]. Currently, tantalum (Ta) is the main component of most common absorber materials developed for commercial EUV photomasks. The absorber layer is etched into a pattern to be printed on wafer, and is capped with an oxide anti-reflective coating (ARC) to create contrast between the absorber and the mirror for defect inspection in the deep ultraviolet wavelength range (DUV).  
The combination of EUV light at oblique incidence, absorber thickness, and non-uniform mirror reflectance through incidence angle, creates photomask-induced imaging aberrations, known as mask 3D (M3D) effects [2]. These effects are experimentally observable, as feature orientation-dependent shadowing effects [3], best focus variation through pitch [4], feature size-dependent pattern shift through focus [5; 6], and pattern asymmetry and contrast loss [7]. Selecting the correct mask absorber material and thickness helps in reducing M3D effects [2; 8; 9]. This approach is complementary to other M3D mitigation strategies, such as source optimization [10], sub-resolution assist feature placement [11], and transition to anamorphic high-NA EUV lithography [12].  
This paper will focus on absorber material as a mitigation strategy for M3D effects, and we verify whether mask absorber materials with high extinction coefficient \(\kappa\) and refractive coefficient \(n\) close to unity can be generated by combining Al and Ni in an alloy. The motivation for this particular material selection is explained in the next section.  
In Section 2, we review the material selection criteria, and methods to deposit and characterize the alloys. In Section 3 we elaborate on the film characterization results, such as film morphology and material durability. Composition and density profile are determined to calculate the optical constants, which will be experimentally verified with EUV reflectometry (EUVR). The comparison between the calculated and measured optical constants will be discussed in Section 4. In Section 5, the measured optical constants are incorporated in an accurate mask model for rigorous imaging simulations to determine M3D imaging impact. Finally, we summarize our findings and give an outlook in Section 6.