Al3Sc thin films as alternative interconnect metallization

Abstract

New conductor materials are explored to replace Cu, W, or Co for interconnect metallization. Binary and ternary intermetallics are of particular interest to replace elemental metals in the latest research. Transition metal aluminides are amongst the intermetallics with potential as alternative metals in scaled interconnects, especially NiAl [1, 2], Al3Sc [3], or Al2Cu [4]. In this paper, we report on the resistivity of the Al3Sc compound and its complex dependence on thickness and composition. We explore routes to scale thickness below 10nm while maintaining low resistivity. AlxSc films have been deposited (with x between 20 to 30 at.% Al) and annealed up to 600oC in H2. A clear drop in resistivity is observed after annealing at 400oC in Figure 1. A resistivity minimum at 25 at.% Sc indicated the presence of order in the films, confirmed by XRD. The lowest resistivity was found to be as low as 12.5 Ωcm for stoichiometric 24 nm thick Al3Sc films (cf. bulk 7 Ωcm [5]) after a 500oC post deposition anneal (PDA) in H2. The film thickness dependence of the Al3Sc resistivity is shown in Figure 2, both as deposited and after annealing up to 600oC. Two regimes could be distinguished after annealing, analogous to the Ni-Al system [2]: for films thicker than 10 nm, the resistivity decreased strongly upon annealing, with little dependence on film thickness. However, for thinner films, the resistivity increased strongly, and the effect of annealing was weak. Compositional depth profiling showed a complex chemical profile of the surface oxide responsible for the resistivity increase. To avoid the formation of the surface oxide layer, in-situ annealing and TaN capping without air break were implemented successfully. For thin films (< 10nm), Al3Sc however formed islands (see Figure 3) during in-situ annealing, leading to large sheet resistances (as seen with TEM, EDS and AFM). To overcome, various in-situ deposited metals were explored as wetting layer for 7 nm of Al3Sc. Reactivity of the wetting layers with the Al3Sc film will be discussed since controlling the interface during PDA is identified as one of the key challenges to overcome.