Copper Metallization Layers on Bismuth-Telluride Substrates: Effectiveness of Cr, Pt, and Ta₄₀Si₁₄ N₄₆ Thin Films as Diffusion Barriers

Abstract

For power-chip cooling devices based on Bi₂Te₃ thick films, copper is a desirable choice as metallic conductor, because it has high thermal conductivity and low electrical resistivity. On the other hand, Cu shows donor activity in Bi₂Te₃, and is known for its high diffusivity in bismuth-telluride and the rapid formation of Cu₂Te. Therefore, the deployment of a chemically inert, low resistivity diffusion barrier between the Cu metallization layer and the thermoelectric base material is indispensable. In this paper we report on the effectiveness of films of Cr, Pt, and Ta₄₀Si₁₄N₄₆ as diffusion barriers at temperatures up to 350°C for Cu overlayers on Bi₂Te₃ bulk substrates. The films, each about 100 nm thick, were magnetron-deposited on the Bi2 Te3 followed by about 300 nm thick copper overlayers without breaking vacuum. The samples were annealed in vacuum at temperatures of 200 and 350°C for up to 50 h. The performance of the metals and the tantalum-silicon-nitride films as diffusion barriers for Cu inward and Bi and Te toward diffusion was evaluated by 2.0 MeV⁴ He backscattering spectrometer and x-ray diffraction. While the Cr and Pt films fail already after hours of annealing at 200°C, the Ta₄₀Si₁₄N₄₆ film remains intact at this temperature after vacuum heat treatment for 50 h, and even at 350°C for 1 h as well. Ta-Si-N, and related ternary materials of this type, have already shown to be highly effective barriers in Cu/Si contacts. This study demonstrates that their field in application extends quite beyond Si-based technology.