Molecular Dynamics Analysis of Grain-Boundary Grooving in Thin-Film Interconnects for ULSIs

Grain-boundary grooving in an Al thin film was analyzed by molecular-dynamics simulation. The simulation result showed that the groove was formed by atom transport due to diffusion at the intersection of the grain-boundary and free surface. The effect of impurity on groove formation was analyzed in terms of atomic radius and bond energy. It was found that when the atomic radius of an impurity is smaller than that of Al and when Al/impurity bond energy is close to Al/Al bond energy, groove formation is prevented due to the suppression of the diffusion. This diffusion suppression is caused by large cohesive energy and small interatomic distance. 1. In t roduc t ion The problem of migration-induced open failures in thin-film interconnects is one of the major problems to be solved in order to improve product reliability of ULSIs. The migration is caused not only by an electrical field but also by the gradient of a mechanical-stress field. Grain-boundary grooving due to diffusion is a dominant factor in the migration [1], [2]. Conventionally, impurity doping, such as Cu doping into Al films [3], [4], is known to be an effective method for preventing the diffusion of the principal-element atoms of thin films (Al atoms, for example). However, such an effective impurity element has been found only by experimental trial and error. There has been no theoretical explanation of how the impurity reduces the diffusion that causes the transport of the principal-element atoms of thin films. The authors have therefore developed a molecular-dynamics simulation technique that analyzes grain-boundary grooving and diffusion in thin films. The effect of impurity on groove formation and diffusion was investigated using this simulation technique. 2. Ana lys i s Me thod The basic equations used in our simulation technique are Newton's well known equation of motion, given as m , d 2 r i / d ^ = 9 2 u ( | r , r j | ) / a r > , ( l ) where mi is the atomic mass of the i-th atom and w(|ri—rj|) is interatomic potential