Thermal strain and stress in copper thin films

Abstract

Desired improvements in the performance and reliability of integrated circuit interconnects may necessitate a move from aluminum alloys to copper. Before copper is adopted, however, characterization of the thermal stress behavior of copper thin films is necessary in order to identify mechanical reliability concerns and to determine differences from aluminum behavior. In this study, the behavior of copper films is evaluated to determine effects of film texture, thickness, and the presence of a passivation layer. Mechanistic models based on bulk deformation maps and interface-controlled dislocation glide are compared with the measured behavior. A preferred [111] grain orientation is found to slightly increase the stress throughout a thermal cycle as compared with a film with random grain orientation. An inverse relationship between film thickness and strength, similar to that seen in aluminum, is quantified. The presence of a passivation layer significantly reduces stress relaxation at high temperatures, resulting in behavior that closely resembles that of unpassivated aluminum films. Neither model adequately predicts the thickness and passivation effects over the entire temperature and stress range, emphasizing the need for more characterization of the flow processes active in metallic thin films.