THE INFLUENCE OF CU PRECIPITATION ON ELECTROMIGRATION FAILURE IN AL-CU-SI

This paper reports a study of the effect of Cu precipitation on electromigration failure in Al-2Cu-1Si thin-film conducting lines. The films were 0.5-mu-m in thickness, and patterned to widths of 1.3 and 4-mu-m, providing width-to-grain-size ratios (W/G) of approximately 0.5 and 2. The lines were aged for various times at 226-degrees-C, and were then tested to failure at a current density of 2.5 X 10(6) A/cm2. Scanning and transmission electron microscopy were used to study the Cu precipitate distribution, its evolution during aging and electromigration, and the microstructural failure mechanism. Aging produces a dense distribution of intragranular theta' (Al2Cu; coherent), with stable theta (Al2Cu; incoherent) in the grain boundaries. The theta' is replaced by theta as aging proceeds. In the wide lines (W/G almost-equal-to 2), the mean time to failure (MTF) increases slowly and monotonically with prior aging time. The failure happens through the growth and coalescence of intergranular voids. In the narrow lines (W/G almost-equal-to 0.5), both the MTF and the time to first failure increase by more than an order of magnitude when the line is aged for 24 h prior to testing, then decrease on further aging. The dominant failure mode is the "slit" failure mode previously observed in pure Al. However, the 24 h specimens fail by gradual, uniform thinning. Failure occurs in regions that have been swept free of intragranular precipitates. The failure time appears to be proportional to the intragranular density of theta precipitates, and related to the time required to sweep these from a critical length of line.