Stress measurement in sputtered copper films on flexible polyimide substrates

Abstract

We have investigated the dependence of internal stress on argon pressure for 0.25-μm copper films sputtered onto a 1-mil(25-μm)-thick polyimide substrate. A dc planar magnetron was used to deposit the copper at a rate of 2 Å/s onto the flexible substrate which was held flat by top and bottom edges. The stress was estimated directly from the resulting radius of curvature of the relaxed film and substrate. The critical argon pressure at which a transition from compression to tension occurs was found to be about 2.5 mTorr. In addition, the dependencies of film morphology, resistivity, and optical reflectance on argon pressure were studied. The critical pressures at which resistivity starts to rise and reflectance begins to decrease are very close to the stress transition pressure. In general, the data are consistent with the dependencies of physical properties on pressure observed for other sputtered metals.