Diffusion barriers in thin films

Abstract

Some of the conditions which thin film diffusion barriers should satisfy are enumerated. Various ways to try to meet these conditions are illustrated by means of examples. It is shown that metal films chosen for their mutual immiscibility with the adjoining metals (passive barriers) usually fail as barriers ( i.e. are non-barriers) because extended structural defects in the metal film constitute fast diffusion paths. Single-crystal barriers of such metals are effective but not practical. Barriers which are thermodynamically partially stable (partially stable barriers) and those which are fully stable (stable barriers) are discussed. Metal compounds of particular interest for such barriers are suggested. The concept of the sacrificial barrier, which is based on an irreversible loss of barrier material by interfacial reactions with the adjoining metals, is introduced and successful applications are presented. The stabilizing effect of impurities on an otherwise unstable barrier (stuffed barriers), the importance of mechanical stress and the critical influence of the fabrication process of a thin film barrier on its actual performance are described by practical examples. Values of the electrical resistivity for borides, carbides, nitrides and silicides of the early transition metals and values of the coefficient of linear thermal expansion of silicides are compiled for reference purposes.