Characterization of electromigration under bidirectional (BC) and pulsed unidirectional (PDC) currents

The electromigration of Al-1%-Si interconnects under bidirectional (BC) and pulsed unidirectional (PDC) signals is discussed. The effect of signal asymmetry, duty cycle and frequency on the damage generation rate has been tested. The data suggests that the equivalent DC current of a non-DC signal is given by the average of the signal current for frequencies 1 kHz and higher. A transition region from an average DC equivalent circuit model to an ON-time equivalent type model has been identified around 1 Hz for PDC signals at 3.3E6 A/cm² and 200°C. The refilling of microvoids under current reversal prevents the observation of that transition region for symmetrical bidirectional currents. PDC signals are shown to behave as particular cases of a generalized bidirectional signal. A physical model of the kinetics of material accumulation/depletion is described to show that a high-frequency operation approximation naturally leads to the average DC equivalent-current model