# Ni-Al Alloys as Alternative EUV Mask Absorber
## 2 Materials and Methods
The low temperature during PVD, causes the material deposition to be mainly kinetically controlled. Most atoms will lack the energy to arrange themselves into the most efficient close-packed lattice. Hence the crystal density as reported for the thermodynamically stable crystalline compound will likely not be obtained. Anisotropic etch of the absorber material must be capable of forming well defined patterns with straight sidewalls. Reactive ion etch (RIE) ensures these conditions, but requires the absorber material to chemically react with the etchant gas and to form volatile compounds below 150 \({}^{\circ}\)C. Materials with small CD difference before and after etch are preferred [32; 33]. Furthermore, the absorber material must have good adhesion to the Ru capping layer, even during cleaning, and with good etch selectivity to Ru. Etch development of alternative metals is an important challenge to solve. Despite good imaging performance and durability, Ni remains difficult to pattern. Physical sputtering has limited etch selectivity and cannot obtain the required pattern quality, due to uncontrolled redeposition of non-volatile Ni particles, resulting in sloped sidewalls and Ni residues on the mirror [9]. However, Ni can form volatile organo-metallic compounds at low temperature [34; 35], and emerging etch technology, such as atomic layer etch (ALE), can be promising for enabling transitional metal plasma etch [36]. Alternatively, a damascene-like additive patterning scheme can be adopted to circumvent the direct Ni etching [37]. This solution requires the ability to remove the template pattern selectively afterwards. Furthermore, the ability to form volatile Ni compounds induced by electron beam is still necessary for absorber defect repair of such Ni patterns. The absorber must remain inert in the EUV scanner ambient, and at the elevated temperatures during EUV exposure. It is not allowed to transform into a different solid phase to avoid volumetric changes. A phase transition of Ni\({}_{\mathrm{x}}\)Al\({}_{\mathrm{y}}\) alloy is unlikely below 150 \({}^{\circ}\)C though (cf. Figure 3), as both elemental Ni and Al have melting points much higher than the thermal budget for EUV photomasks. H\({}_{2}\) is used inside the projection optics (PO) box in EUV scanners [38]. Although the photomask is physically outside of the PO box, concerns remain on their capability to react with hydrogen under EUV exposure. A first estimation for the hydrogen durability of an element, is by assessing the volatility or melting point of its hydrogen compound. This assessment however, does not give information on kinetics or ease of formation of such compounds under scanner conditions. Hydrogen resistance can be additionally improved by doping with reduction resistant elements [39]. We have tested hydrogen compatibility by exposing the alloy samples to H\({}^{*}\) radicals for 110 h. Composition changes after the durability tests were measured with RBS for Ni detection and elastic recoil detection analysis (ERDA) for Al detection. For mask defect inspection in the DUV wavelength range, an oxide ARC is required to create contrast with the mirror where the absorber material has been etched away. For actinic inspection, the ARC layer is not necessary [40]. Another function of the ARC layer is to limit the native oxide growth of the absorber material. For defect repair the absorber material must be able to form volatile compounds by electron beam-induced chemical reaction [41]. Alternatively, nanomachining repair can remove non-volatile materials, but is less selective [42]. The absorber must remain stable under cleaning conditions, preferably basic solution to prevent the Ru capping layer from oxidizing and peeling [25]. The introduction of pellicles in EUV lithography will likely reduce the number of cleaning cycles necessary during the lifetime of an EUV mask. In this paper, we have tested cleaning compatibility of Ni\({}_{x}\)Al\({}_{y}\) alloy samples by 24 h submersion tests in DIW, and in NH\({}_{4}\)OH.