The influence of Cu precipitation on electromigration failure in Al-Cu-Si

Abstract

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 μm in thickness, and patterned to widths of 1.3 and 4 μm, providing width-to-grain-size ratios (W/G) of approximately 0.5 and 2. The lines were aged for various times at 226 °C, and were then tested to failure at a current density of 2.5×106 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 θ′ (Al2Cu; coherent), with stable θ (Al2Cu; incoherent) in the grain boundaries. The θ′ is replaced by θ as aging proceeds. In the wide lines (W/G≊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≊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 θ precipitates, and related to the time required to sweep these from a critical length of line.