Θ CuAl₂ Precipitate Coarsening in Al-2% Cu Thin Films

Abstract

AbstractΘ phase CuAl2 precipitate size evolution during coarsening at 310°C in 0.5 μm thick Al-2% (wt) Cu thin films was characterized by transmission electron microscopy. Films were sputter deposited onto oxidized Si substrates by standard techniques. The coarsening process preferred the growth of blocky Θ morphologies at Al triple points. Coarsening was via solute Cu diffusion along Al grain boundaries during annealing. The average Θ size dependence on annealing time (t) is approximately (t)1/4 in general agreement with models for particle coarsening along grain boundaries. Concurrent Al grain growth was shown to initially enhance the Θ coarsening rate above (t)1/4 behavior. This boundary coarsening process leads to a grain size dependence of the coarsening rate which has been observed in related and other previous work in thin films. These results are shown to be relevant for effects produced during accelerated electromigration testing, such as previous ‘curious’ 0 morphologies at triple points observed by others, the enhanced flux of Cu during testing, and possible mechanisms affecting electromigration failure processes.

Grain Growth in Al-2% Cu Thin Films

“…More data taken within the first 5 minutes of annealing, or data at lower temperatures, would better describe the initial rapid but normal grain growth kinetic behavior. Results of grain growth at 310*C in this system in fact showed [ 12] a kinetic exponent 13= 0.5.…”

“…13values at 345"C for the 0.4 I.tm and 0.8 I.tm films were not significantly smaller than for the other conditions, lt is likely that grain growth at 345"C was not retarded by the small as-deposited 19 phases since they rapidly coarsen (and shrink) when impinged by migrating boundaries. Recent results [12] show such a process for these films. There it was found during annealing of AI-2% Cu films at 310"C that the rate of coarsening of small asdeposited O phases was enhanced by the concurrent motion of boundaries during grain growth, and that ® phase pinning of boundary motion was negligible due to the rapid coarsening of the small O phases in favor of large 19at triple points.…”

“…First, normal grain growth is not inhibited by the 2nd phases present, which could, under other circumstances, lead to abnormal grain growth. This due to the relative instability of the O second phases due to either complete dissolution at high enough temperatures or coarsening [12] and reduction of their "Zener drag" [10] at lower temperatures. Secondly, there exists no surface energetic or boundary mobility advantage for abnormal grain growth over normal growth [20].…”

“…Previously we have reported [11] on the O (CuAI2) reactions in these films during annealing between 345°C and 4720C. Recently, O coarsening and the effects of concurrent Al grain growth were studied [12] in more detail. We ' presently report the grain growth kinetics in the AI-2% Cu thin films for anneals between 345°C and 472°C.…”