# Ni-Al Alloys as Alternative EUV Mask Absorber
## 5 EUV Imaging Simulations  
In this section, we utilize the optical constants, as measured by EUVR and tabulated in Table 5, to predict M3D impact of Ni\({}_{x}\)Al\({}_{y}\) mask absorbers, compared to current TaBN absorber and high \(\kappa\) single metal Ni absorber. We used Synopsys Sentaurus Lithography software to assess intensity threshold-to-size, shadowing effects, best focus shift through pitch, pattern shift through focus, contrast and process window (PW) variation of dense-to-isolated trenches at a fixed CD of 16 nm, which is a relevant structure for the 7 nm technology node. The simulated illumination source shapes are DipoleY with 90\({}^{\circ}\) opening angle and Quasar with 45\({}^{\circ}\) opening angle at numerical aperture (NA) of 0.33. The imaging impact of the Ni\({}_{x}\)Al\({}_{y}\) absorbers is compared to the reference 60 nm TaBN absorber, and a 32 nm high \(\kappa\) Ni absorber [9]. The EUV reflective mirror is a calibrated model of an experimentally validated Mo/Si ML mirror capped with Ru [52]. The Ni\({}_{x}\)Al\({}_{y}\) absorber models contains two layers: a surface oxide layer with fixed 3 nm thickness and a bulk metal absorber layer. The optimal thickness is selected at an EUV reflectance minimum below 2%, as depicted in Figure 13. The optical constants are tabulated in Table 5.  
The exposure dose needed to print the patterns on target, is estimated by the intensity threshold-to-size, with high threshold corresponding to low dose. Figure 14a predicts higher threshold-to-size for thin Ni\({}_{x}\)Al\({}_{y}\) absorbers, and for high Al-content absorbers. As expected, thinner absorbers and Al low \(\kappa\) result in less EUV dose required to print trenches on target. Impact on shadowing effect under the Quasar illumination is depicted in Figure 14b as the mask CD difference between horizontal (H) versus vertical (V) patterns, which are biased to print on target at wafer level. All Ni\({}_{x}\)Al\({}_{y}\) absorbers have smaller HV mask bias compared to reference TaBN and to the thin high \(\kappa\) Ni absorber, despite being physically thicker. Higher Al content further reduces HV mask bias. This signifies that shadowing effect is not purely a thickness-induced M3D effect, but is also impacted by phase deformation. Figure 14: Comparison of (**a**) threshold-to-size, (**b**) shadowing effect, (**c**) best focus shift through pitch, (**d**) pattern shift through focus, and (**e**) contrast change normalized to 32 nm Ni absorber at 32 nm pitch for the following absorber stack: 60 nm TaBN, 32 nm Ni, 32 nm Ni\({}_{3}\)Al, 31 nm NiAl, 31 nm Ni\({}_{2}\)Al\({}_{3}\), 39 nm Ni\({}_{3}\)Al, 38 nm NiAl, and 38 nm Ni\({}_{2}\)Al\({}_{3}\), at 0.33 NA for (**f**) Quasar (blue) and DipoleY (orange) illumination. Figure 13: Simulated EUV reflectance at 6\({}^{\circ}\) chief-ray incidence angle over a range of absorber thickness, for Ni\({}_{3}\)Al, NiAl, and Ni\({}_{2}\)Al\({}_{3}\) absorbers. The dashed line represents the 2% EUV reflectance spec. The marks show the absorber thicknesses chosen for the different absorber models. The best focus range from 32 nm to 100 nm trench pitch is shown in Figure 14c. The smaller best focus shift between different pitches is better for overlay. Most Ni\({}_{x}\)Al\({}_{y}\) absorbers exhibit slight reduction in best focus range compared to thin high \(\kappa\) Ni absorber, with more reduction at higher Al-content. More Al leads to smaller phase deformation as the refractive coefficient \(n\) is closer to unity. The thicker Ni\({}_{x}\)Al\({}_{y}\) absorbers have smaller best focus range, as the impact of phase deformation on best focus shift is reduced for darker absorbers with lower reflectance [2]. Figure 14d illustrates the range of pattern shifts through focus for pitches from 32 nm to 100 nm, expressed in milliradians (mrad). 10 mrad correspond to 1 nm pattern shift per 100 nm defocus.