Aluminum Electromigration Under Pulsed D.C. Conditions

Abstract

Aluminum thin films were stressed under constant and pulsed DC conditions. The pulsed tests were performed at a frequency of 1kHz and duty cycles of 25%, 50%, and 75%. A peak current density of 2 × 106 A/cm2 was used. To compare conductor lifetimes under constant and pulsed conditions and to determine the effect of current pulsing on the activation energy for electromigration, tests were performed from 125°C to 300°C. Thermal cycling, resulting from current pulsing, was minimized by testing whole, patterned wafers directly on the heated stage of a probe station. Conductor lifetime is found to increase as duty cycle decreases. The amount of improvement is more than a correction for on-time would predict. The lifetime under pulsed DC conditions increases inversely with the square of the duty cycle. Pulse frequency has no effect, as shown by additional tests by 1OHz and 20kHz. A model is developed to explain this phenomena.