Catalyzed precipitation in Al-Cu-Si
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NTRODUCTION
AL-CU and Al-Cu-Si alloys containing 0.5 to 2Cu and 0.5 to 1Si (wt pct) are the most common alloys used for thin-film metallization in microelectronics. Electromigration voiding and failure in these alloy films is affected by the precipitation of Al2Cu intermetallics in the form of either u phase (the equilibrium phase) or u 8 phase (the metastable phase). Recent data suggest that aging treatments that enhance the lifetime of Al-Cu-Si[1,2] thin films are much less effective for binary Al-Cu.[3,4] An obvious hypothesis is that Si affects the precipitation process in some favorable way.[5] However, while precipitation in binary Al-Cu has been studied so extensively that it is used as the “textbook example” of precipitation in the solid state,[6] there is very little published work on the influence of small additions of Si. The authors know of only two pieces of published work dealing with the effect of Si on the precipitation in binary Al-Cu.[7,8] Both are by Starink and Van Mourik and concern alloys that are relatively rich in Si. There is a similar lack of work on the influence of Cu on precipitation in Al-(0.5 to 1)Si. The only published study known to the authors is by Steward and Martin,[9] who found that additions of 0.45 wt pct Cu to Al-0.5 wt pct Si caused the Si precipitates to spheroidize and inhibited coarsening. They suggest that Cu segregates to the Al-Si interface, reducing the interfacial energy. D. MITLIN, Graduate Student, and J.W. MORRIS, Jr., Professor of Metallurgy, are with the Department of Materials Science and Engineering, University of California, Berkeley, and Center for Advanced Materials, Lawrence Berkeley Laboratory, Berkeley, CA 94720. V. RADMILOVIC, Staff Scientist, is with the National Center for Electron Microscopy, Lawrence Berkeley Laboratory. Manuscript submitted July 30, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
Given this background, the present work was undertaken to explore the influence of Si on precipitation from AlCu alloys. To do this, we compared precipitation in a bulk alloy of Al-2Cu-1Si (wt pct) to that in the binary alloys Al-2Cu and Al-1Si. As discussed subsequently, we found that Si precipitates preferentially from Al-2Cu-1Si in conventional aging treatments and provides heterogeneous nucleation sites for the direct precipitation of u 8 phase (Al2Cu). Si also influences the morphology of the u 8 phase. It inhibits the growth of u 8 phase, promoting a dense distribution of precipitates that, although plate-like, are somewhat less so (with a lower aspect ratio) than those in binary Al-2Cu. The shape, state, and density of Si precipitates are also different in the ternary Al-2Cu-1Si than in the binary Al-1Si. The possibility that third-component additions can produce heterogeneous nucleation sites in Al-Cu alloys is well documented. For example, Ringer et al.[10] found that Ag forms co-clusters with Mg, which act as heterogeneous sites for nucleation of the V phase (a distorted form of the equilibrium Al2Cu phase) in Al-Cu-Mg lightly alloyed with Ag.
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