Diffusion kinetics in aluminium-gold bond contacts from first-principles density functional calculations
A common joining method in microelectronics is thermosonic bonding of gold wires to aluminium pads deposited on the integrated circuit. In the interface between the wire and the pad a number of intermetallic compounds Al xAuy can develop, which significantly affect the mechanical properties and corrosion resistance of the bonds. Based on Onsager's extremal principle of irreversible thermodynamics, the present paper describes the evolution of the intermetallic phases. This macroscopic model contains several thermodynamic and kinetic parameters, some of which are not available from databases. As an alternative to often cumbersome experiments, density functional theory is applied to calculate the formation energies of the phases and of atomic vacancies, as well as the vacancy migration energies in AlAu 4. To derive tracer diffusion coefficients from the atomistic vacancy jump rates, one must take peculiarities of the AlAu4 lattice into account: the vacancy migration energy between certain gold sites is found to be rather low, but these low-energy jumps are arranged in closed triangles and, therefore, do not provide paths for long-range diffusion of gold atoms. However, together with two other types of jumps with still moderate migration energies, a contiguous network of diffusion paths for gold becomes possible. Since the diffusion of aluminium in AlAu4 requires the generation of high- energy anti-site defects (at least temporarily), aluminium is expected to move very slowly, but no final solution is provided.