Copper Line Resistance Degradation Caused by Unidirectional and Bidirectional Pulsed Currents

Electromigration (EM) induced degradation of the resistance of metal lines by time-varying currents is analyzed with Multiphysics Finite Element based simulation and theoretical modeling. Studies indicate that hydrostatic stress within metal lines embedded in rigid confinement exhibits reversible evolution kinetics in response to time-dependent electrical stressing. The stress gradient formed during unidirectional stressing controls stress relaxation kinetics after the electrical load is removed or acts with the EM force if the load polarity changes. The variation in self-heating during on-and off-periods of unipolar pulse current had minimal impact on stress relaxation. The observed mean time to failure (MTF) decreases with stress current frequency due to reduced atomic diffusion activation energy from electron scattering. The energy transferred from the scattered electron to the atom jumping to the neighboring vacant lattice site depends on the electric field frequency. The controversial observations of the line resistance responses on the symmetrical bipolar stressing is explained by the thermoelectric effects activated in test-lines containing the preexisted voids generated at the previous thermal processing steps. It can lead to misinterpretation of measurement results by dealing with the apparent resistance variations caused by varying thermo-electromotive force.