Comparison of electrical and microtensile evaluations of mechanical properties of an aluminum film

We compare two types of tests for measurement of mechanical properties of thin films and small scale structures: a microtensile test and a thermomechanical fatigue test induced by alternating current at low frequency and high current density. The microtensile test was used as a reference for evaluating the feasibility of using the electrical test for measurement of mechanical properties. Tests were performed on structures cofabricated from thin film Al deposited on Si to ensure comparable mechanical properties. The films had a grain diameter of 220 nm and a thickness of 1.9 mu m. The electrical test resulted in an estimated ultimate tensile strength of 230 +/- 40 MPa. This value was based on extrapolation of high-cycle fatigue data to one reversal through a modified Basquin equation while accounting for varying mean stress. An ultimate tensile strength of 239 +/- 4 M Pa was determined from the microtensile test. Differences between these values are explained in terms of the effects of substrate constraint on the strength of the thin film. We conclude that electrical testing methods offer a feasible means for measuring mechanical properties of individual patterned structures.