A contact electromigration study of Al-Si-Cu alloys

With the advent of VLSI device density, new materials and technologies are being developed to ensure that interconnects and interlevel contacts are reliable. In addition to the use of Si-doped Al, barrier metallization has been introduced to prevent electromigration-induced junction spiking. To permit increased current densities, Cu has also been added to the Al alloy. It could be argued that with the use of a barrier metal, silicon in the Al alloy can be removed altogether. The experiments reported were designed to ascertain the validity of this argument. Three different aluminum alloys were investigated: Al-1%Si, Al-2%Cu, and Al-1%Si-0.5%Cu. A sample of 90 devices were tested in each case. The failure criteria chosen were a 50% increase in resistance of 50 mu A of leakage to substrate. The major failure mechanism for all three alloys was found to be resistance increase at the contact where electrons flow from the silicon into the aluminum, due to electromigration of aluminum away from the contact area. The significantly higher values of median time to failure for the two alloys containing copper reflect the higher resistance to electromigration of these alloys.<
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