An experimental and finite element investigation of thermally induced inelastic deformation of single-level damascene copper high density interconnect structures

Abstract

An atomic force microscope was used to investigate thermally induced deformation mechanisms in the areas of copper and polyimide vias in copper-polyimide interconnect structure, as a result of a thermal cycle to 350°C. The copper films exhibited evidence of copper grain boundary sliding, Coble creep, and voids formation. Cu-Ta interfacial sliding was observed in the Cu and polyimide via areas. The direction of the Cu-Ta sliding changes as the polyimide via size decreases. The polyimide experienced a residual deformation attributed to the Cu-Ta sliding and in-plane deformation of the copper vias. A finite element method was used to simulate the effect of Cu-Ta sliding on interfacial and liner plane stresses in Benzocyclobutane (BCB)-Cu and SiO2-Cu interconnect structures, heated to 400°C. A one rum thick element was used to produce a sliding effect at the Cu-Ta interface and at Cu grain boundary. The shear stresses in the SiO2-Cu system are completely relaxed by the Cu-Ta sliding. The Cu-Ta sliding increases the Ta liner plane stress in the BCB-Cu system to potentially damaging values, while the Ta stress in the SiO2-Cu system changes from tensile to compressive. Sliding at Cu grain boundary has a minor impact on Cu-Ta sliding and shear stress relaxation in the BCB-Cu system with large aspect ratio. A finite element technique was used to model the classical Nabarro-Herring creep in 1 mum square copper grain subjected to biaxial stresses of +/-10 MPa, at 800°C. A linear elastic mechanical analysis was carried out to simulate the mechanical loading and transient thermal analysis was utilized to simulate the diffusive vacancy flow process. The steady state flux components were used to predict the creep deformation of the grain and to estimate the creep strain rate at the boundaries. It was shown that the finite element procedure is capable of modeling the Nabarro-Herring creep, satisfactorily. The finite element result agrees with the analytical prediction within a factor of two