Investigation of the effects of Pulsed Direct Current at low frequencies on the Electromigration Lifetime : Student Paper

Accelerated electromigration (EM) testing generally utilizes a constant direct current (DC). However, in operation or “real life” the metal interconnect is commonly exposed to an alternating current (AC) or pulsed direct current (PDC). If at all, compensation for the use of PDC failure is measured by the `time-on' current stress and then modeled as a linear multiple of the duty cycle (percentage of time-on current) [1], [2]. EM life-time is determined by an electromigration driving force as well as a current-induced mechanical stress gradient driving force. In the event current was interrupted or turned off for a period, the stress gradient driving force would continue to act. In the event a void is nucleated, the stress gradient that once opposed EM, would work in the same direction.We studied the failure of copper interconnects using a low frequency pulsed direct current, 10 Hz. We investigated the effects of the duty cycle, and current on the EM failure behavior using PDC and compared the results to accelerated testing preformed with constant direct current. From the comparison of PDC and DC accelerated testing, it is shown that at low current densities, PDC stressed devices show extended lifetime of interconnects and at high current densities (above 8 MA/cm
²
), they showed a reduced lifetime of the interconnects.