INTERACTION BETWEEN ELECTROMIGRATION AND MECHANICAL-STRESS-INDUCED MIGRATION - NEW INSIGHTS BY A SIMPLE, WAFER-LEVEL RESISTOMETRIC TECHNIQUE

The unavoidable presence of mechanical stress in VLSI interconnect lines has a nonnegligible influence on the characterization of electromigration (EM). In this work a wafer level resistometric technique is applied as an indirect way to detect the combined effect of mechanical stress migration and EM, the direct measurement of the former being difficult in narrow lines. In order to perform reliable high-resolution resistance measurements, a technique has been developed in which the compensation of small thermal instabilities is achieved by means of an additional measurement on a reference device. EM tests performed at constant temperature and current on Al-1%Si stripes exhibit an initial nonlinear resistance versus time behavior, probably due to the simultaneous action of the accumulated mechanical stress and the high current density, followed by linear behavior. An activation energy is extracted, using an original statistical analysis of the experimental data, and its meaning is discussed, taking into account temperature- and time-dependent mechanical stress influence. It is concluded that the kinetics of stress relaxation should be known more deeply in order to perform reliable operating temperature extrapolations from the calculated activation energies.