Pulsed Power Electromigration in Copper Metallization (10 Hz – 4 MHz)

Most of the electromigration testing in the industry is performed with DC currents to evaluate the electromigration reliability of metal interconnects. However, most of the real-world applications operate using a pulsed current rather than a constant direct current. During direct current testing electromigration is the dominant mass transport driving force. However, during pulsed power testing during the periods of no current stress, the stress gradient is the dominant driving force. When a void is nucleated, its surface becomes stress-free, and the location of highest stress becomes the point of lowest stress after nucleation. The stress gradient in the immediate vicinity of the void switches direction and works in the same direction as electromigration. This temporary switch in stress gradient due to the change in boundary conditions causes additional damage during the periods of no current stress.In this paper we discuss the results of pulsed power electromigration at high frequencies up to 4 MHz in Copper. Furthermore, this paper investigates the use of average current in pulsed power electromigration testing. Industry often uses the average current in design rules to estimate electromigration damage caused during a pulsed powering. It considers the use of average current passing through the interconnects over the duration of the test to calculate the lifetime of the interconnect. It is useful to see whether the use of average current remains relevant in light of the recent findings of additional damage incurred during the periods of no current stress.