ELECTROMIGRATION FAILURE OF INTEGRATED-CIRCUIT METALLIZATIONS SUBJECTED TO HIGH-FREQUENCY PULSED CURRENTS

Electromigration life tests were performed on copper-alloyed aluminum test structures that were representative of modern integrated circuit metallization schemes. A total of 18 electrical stress treatments were investigated. One was a steady de current, and the others were pulsed de currents varied according to duty cycle and frequency. The duty cy cle was varied from 33.3% to 80%, and the frequency was varied such th at three distinct orders of magnitude were roughly represented-100 kHz , 1 MHz, and 100 MHz. The median time to failure, t(50), was used as t he primary basis of comparison between test groups of six-nine samples . There was no discernible dependence of t(50) on the pulse frequency. In contrast, the duty cycle, d, played a strong role. Duty cycles gre ater than 50% produced median lifetimes that varied as 1/d(2), while d uty cycles equal to and less than 50% produced a shift toward (but not reaching) a lid dependence. This shift was most pronounced at the sma llest duty cycle of 33.3%, for which lifetimes were midway between the predictions of the 1/d(2) and 1/d relationships. Post-test optical mi crographs were obtained for each test stripe, and these suggested that the location of electromigration damage was influenced by the pulse d uty cycle. Most damage occurred near the cathode contact in all instan ces, but there was an increased incidence of damage farther downwind w ith decreasing duty cycle. The results are explained in terms of a Ble ch length effect that may operate as a result of the bamboo test strip e structure. (C) 1998 American Institute of Physics. [S0021-8979(98)02 111-2].