ISSN:
1089-7550
Source:
AIP Digital Archive
Topics:
Physics
Notes:
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 dc current, and the others were pulsed dc currents varied according to duty cycle and frequency. The duty cycle was varied from 33.3% to 80%, and the frequency was varied such that three distinct orders of magnitude were roughly represented—100 kHz, 1 MHz, and 100 MHz. The median time to failure, t50, was used as the primary basis of comparison between test groups of six–nine samples. There was no discernible dependence of t50 on the pulse frequency. In contrast, the duty cycle, d, played a strong role. Duty cycles greater than 50% produced median lifetimes that varied as 1/d2, while duty cycles equal to and less than 50% produced a shift toward (but not reaching) a 1/d dependence. This shift was most pronounced at the smallest duty cycle of 33.3%, for which lifetimes were midway between the predictions of the 1/d2 and 1/d relationships. Post-test optical micrographs were obtained for each test stripe, and these suggested that the location of electromigration damage was influenced by the pulse duty cycle. Most damage occurred near the cathode contact in all instances, but there was an increased incidence of damage farther downwind with decreasing duty cycle. The results are explained in terms of a Blech length effect that may operate as a result of the bamboo test stripe structure. © 1998 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.367431
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