Dependence of Stress on Deposition Conditions for Sputtered Copper Films onto Flexible Polyimide Substrates

Abstract

We have investigated the dependence of internal stress on argon pressure and deposition rate for 0.25 μn copper films sputtered onto a 1-mil (25 μn) — thick polyimide substrate. A dc planar magnetron was used to deposit the copper onto a flexible substrate (Kapton) which was held flat by top and bottom edges. We observed two types of stress: tension, in which the film is trying to contract on the substrate and compression, in which the film is trying to expand on the substrate. The stress is primarily of an “intrinsic” nature, relating to the interfacial mismatch between the natural atomic structures of the growing film and the substrate surface. Using a formula due to Stoney, stress was estimated directly from the resulting radius of curvature of the relaxed film and substrate. At a fixed deposition rate of 2.0 Å/s, there is a stress transition from compression to tension at a pressure of about 2.5 mTorr. At a fixed pressure of 5.0 mTorr, there is a stress transition from tension to compression at a deposition rate of about 4.8 Å/s. Scanning electron micrographs (SEM) show columnar grains with a void network for films under tension and smooth, tightly packed surfaces for films under compression. The origin of the tensile morphology is a “shadowing” mechanism operative at higher argon pressures; at lower pressures, an “atomic peening” mechanism is responsible for the compressive morphology. The morphology data are consistent with the Movchan-Demchishin zone model diagram as extended by Thornton for the growth of sputtered films. Film resistivity and reflectivity were closest to their bulk values for compressive films deposited at low argon pressure and high deposition rate. At a deposition rate of 2 Å/s, the critical pressures at which resistivity starts to rise and reflectance begins to decrease are very close to the aforementioned stress transition pressure. In general, the data are consistent with the dependencies of physical properties on pressure and deposition rate observed for other metals. We are currently examining the effects of film thickness and residual gas pressure on stress. The ultimate goal of this research is to correlate stress with the adhesion between film and substrate.