Molecular Dynamics Study on Grain Boundary Diffusion in Aluminum under Hydrostatic Stress

Fracture in an aluminum conductor of an LSI(large-scale integrated circuit) is often caused by stress-induced diffusion of atoms along a grain boundary, which is called"stress migration". As the conductor is subjected to tensile hydrostatic stress at a high temperature due to thermal mismatch, it is important to understand the effect of hydrostatic stress on the diffusion. However, this effect has not yet been examined since experiments under tensile hydrostatic stress are extremely difficult. In this study, the effect is investigated on the basis of a molecular dynamics simulation using the Nose-Hoover and the Parrinello-Rahman methods. The simulation enabled us to observe the motion of atoms under constant stress and temperature. The results obtained are summarized as follows. (1)The simulated coefficient of lattice diffusion under a stress-free condition agrees very well with the experimental ones. This indicates the validity of the present simulation. (2)The motion of atoms is accelerated by tensile hydrostatic stress in the region of about 5 atomic layers near the grain boundary. (3)Tensile hydrostatic stress accelerates the diffusion along the grain boundary, while compressive stress suppresses it. (4)The coefficient is proportional to exp(-(ΔE)/(k(T/Tm)))regardless of the magnitude of hydrostatic stress where T and Tm are the temperature and the melting temperature of the grain boundary under the corresponding hydrostatic stress, respectively.