# Ni-Al Alloys as Alternative EUV Mask Absorber
## 2 Materials and Methods
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Characterization** & **Specification** & **Metrology Performed** \\ \hline \multirow{6}{*}{Film morphology} & & Transmission electron microscopy (TEM) \\  & Amorphous or nano-crystalline to reduce & X-ray diffraction (XRD); theta-2 theta \\  & line edge roughness [18; 19] & configuration to determine crystal orientation \\  & & parallel to surface [20]. In-situ XRD (IS-XRD) to \\  & & inspect crystallization across temperature range \\ \cline{2-3}  & Reference TaBN absorber has surface & - \\  & roughness \textless{}0.3 nm RMS [21] & - \\ \hline \multirow{3}{*}{Mechanical stress} & Residual film stress of full photomask stack & \\  & must be within \(\pm\)180 GPa to maintain & - \\  & photomask flatness [21] & \\ \hline \multirow{6}{*}{Mask processing} & Good adhesion on Ru capping layer, & - \\  & including during cleaning & - \\ \cline{2-3}  & Small critical dimension (CD) etch bias and & - \\ \cline{2-3}  & etch selectivity to the capping layer & - \\ \cline{2-3}  & Thermal budget \textless{}150 \({}^{\circ}\)C to protect the ML & - \\ \cline{2-3}  & mirror’s reflectance [22] & \\ \hline \multirow{6}{*}{Scanner compatibility} & Low vapor pressure to avoid volatile & \\  & formation at reduced scanner pressure, in & - \\ \cline{2-3}  & combination with elevated temperature & - \\ \cline{2-3}  & during exposure [23] & \\ \cline{2-3}  & No phase transformation between room & - \\ \cline{2-3}  & temperature and working temperature & \\ \cline{2-3}  & & 110 h of exposure to H\({}^{\star}\) radicals, generated by \\  & & EUV Tech hydrogen cleaner [24]; \\  & Hydrogen resistant & Composition change measured with Rutherford backscattering spectroscopy (RBS) and elastic \\  & & recoil detection analysis (ERDA) \\ \hline \multirow{2}{*}{Defect inspection} & Oxide anti-reflective coating (ARC) for & \\  & contrast in deep ultraviolet wavelength & - \\  & range (DUV) inspection & \\ \hline \multirow{2}{*}{Defect repair} & Capability to form volatiles to be & \\  & compatible with electron beam repair & - \\ \hline \multirow{2}{*}{Cleaning durability} & Stable in (preferably basic) cleaning & Beaker test by submerging alloys in solutions of \\  & solutions [25] & deionized water (DIW) and ammonium \\ \cline{2-3}  & & hydroxide NH\({}_{4}\)OH; \\ \cline{2-3}  & & Composition change measured with RBS/ERDA \\ \hline Optical constants & High x and \(n\) close to unity & EUVR for fitting optical constants [20; 26] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specifications for novel EUV absorbers and the performed metrology. Figure 3: Thermodynamic binary phase diagram of Ni-Al system. Stable phases are colored yellow. Red dashed lines represent the nominal Ni\({}_{x}\)Al\({}_{y}\) compositions. Reproduced from [PERSON] et al. [19]. The absorber film morphology is preferably nano-crystalline or amorphous, which is challenging to achieve for most single metals. Crystallinity however, is very likely to impair surface roughness and etch bias. Preferential etching along crystal grains can cause increased absorber line edge roughness (LER) and lead to critical dimension (CD) variations, which will be transferred to wafer [27; 28]. Additionally, absorber morphology affects mechanical film stress. Stress induced by absorber, and ML mirror must be controlled within a range of \(\pm\)180 GPa to ensure the flatness of the photomask [19] and to avoid distortion-induced pattern placement error [29]. Doping or alloying with additional elements, or a multilayered absorber structure are possible solutions to reduce crystallinity [15; 30]. We have measured alloy crystallinity using X-ray diffractometry (XRD) and transmission electron microscopy (TEM). During processing, temperature must be below 150 \({}^{\circ}\)C to avoid intermixing between the mirror's Mo/Si bilayers to maintain high EUV reflectance [22]. This constraint limits deposition of the absorber film to PVD, as most chemical deposition techniques require high temperature [31].