Amorphous metallic alloys: a new advance in thin-film diffusion barriers for copper metallization

Copper, which has a lower electrical resistivity and a higher resistance to electromigration than aluminum, is currently being evaluated for ULSI applications as a replacement for aluminum. Drawbacks to the use of copper include its strong tendency to oxidation, a high mobility in metals and semiconductors, and a high reactivity with silicon at temperatures as low as 200°C. To overcome these problems, very effective diffusion barriers need to be developed. These barriers should have a low diffusivity for copper, a high thermal stability, and should lack a driving force for chemical reactions with Cu, silicon or silicides. Unlike aluminum, copper does not form stable intermetallic compounds with the transition metals of the V and Cr groups, and the mutual solid solubilities of these metals with Cu are low, so that these metals would seem th be a logical choice for barrier applications. It has long been known, however, that these arguments are misleading[1]. Previous studies have indeed shown Cu diffuses through grain boundaries and defects in a tantalum layer and inth silicon at a relatively low temperature (450°C) causing a failure of devices[2,3]. The effectiveness of non-reactive and insoluble tantalum barriers can be improved by adding impurities like oxygen or nitrogen th stuff grain boundaries of the material in order th suppress fast grain boundary diffusion[4]. It is difficult, however to reproducibly improve the effectiveness of barriers by adjusting the level of impurities. Since amorphous alloys lack grain boundaries that can act as fast diffusion paths, they should offer an improved alternative for effective barriers [5-71. In this paper we report on the properties and diffusion barrier performance of amorphous tantalum and tungsten silicides and tantalum-silicon-nitrogen ternary alloys [3,81 for Cu metallizations.