Temperature-dependent inelastic response of passivated copper films: Experiments, analyses, and implications

The temperature-dependent mechanical behavior of passivated copper films is studied. Stresses in copper films of thickness ranging from 40 to 1000 nm, deposited on quartz or silicon substrates and passivated with silicon oxide were measured by using the substrate curvature method. The thermal cycling spans a temperature range from –196 to 600 °C. It was observed that the passivated films do not exhibit a significant stress relaxation at high temperatures that is typically found in unpassivated films. The measured mechanical behavior was found to be rate insensitive within the heating/cooling rate range of 5–25 °C/min. Furthermore, a significant strain hardening during the course of thermal cycling was noted. Analyses employing continuum plasticity show that the experimentally measured stress–temperature response can only be rationalized with a kinematic hardening model. Analytical procedures for extracting the constitutive properties of the films that were developed on the basis of such model are presented. To emphasize the importance of the appropriate choice of constitutive model, results of finite element modeling for predicting thermal stresses in copper interconnects are presented. The modeling assumed parallel copper lines embedded within the combined low k/oxide dielectric materials. It was found that ignoring plastic strain hardening of copper can lead to significant errors in the stress and strain developed in the interconnect.