Stress relaxation in Cu thin films

Abstract

Contributions from various deformation mechanisms to relax stresses in Cu thin films are of technological interest for interconnect reliability. Stress-induced voiding in Cu interconnects for example is one effective process to relax thermally induced stresses which occur during processing and testing at elevated temperatures. Voiding can result in considerable resistance changes in integrated circuits and is hence a reliability concern (1,2). Concurrent to voiding, stresses can be relaxed by diffusional mass transport as previously reported (3), with the Cu/cap interface of particular importance. The use of CoWP capping layers instead of SiCN was shown to be effective in reducing both, stress relaxation rates (3) and electromigration failure rates (5). Depending on temperature, other deformation mechanisms may supplement the diffusion process or even dominate the relaxation rate. In this paper, we examined stress relaxation in Cu films with a Cu/CVD SiCN cap interface over a temperature range between 100°-300°C. Additionally, SEM based in-situ observations of voids were made. Experimentally, electroplated Cu films were used. During temperature cycling, the specimens were first heated to peak temperatures between 200°-400°C for 1hr, followed by cooling to the relaxation temperature. The effects of cooling rate, peak temperature, and specimen history need to be considered to assess the interfacial mass transport kinetics. With respect to the temperature dependence the use of a single activation energy is found to be inadequate, suggesting a mechanism change at approximately 150°C. Provided that stressinduced voiding was minimized, at higher temperatures an activation energy of 0.9eV was found, consistent with interface diffusion coupled with grain boundary diffusion. © 2009 American Institute of Physics.