Copper has drawn much attention as an interconnect material in advanced ultra large scale integration circuits due to its low electrical resistivity and superior resistance to electromigration when compared with Al. However, copper diffuses easily into Si and SiO₂ at relatively low temperatures (~200 °C) to form Cu-silicides, leading to the failure of integrated circuits. Therefore, in order to use Cu interconnects, a diffusion barrier need to be inserted between Cu and Si to prevent the interdiffusion or reaction between Cu and adjacent materials.

Various transition metals like Ta, W, Mo, Ti, and Nb have been examined as diffusion barrier for copper metallization. Single layer barrier structures are often found to be unstable at high annealing temperatures due to the presence of grain boundaries. These grain boundaries are the effective pathways for diffusion of Cu and/or substrate atoms leading to the failure of diffusion barriers. Furthermore, the breakdown of single layer diffusion barrier takes place due to the presence bulk defects or loss of barrier integrity. Thus it is necessary to use multilayer structures as diffusion barrier in order to restrict the diffusion through the grain boundaries. Also multilayer barrier structures have higher thermal stability as compared with single layer structures.

In this study, we investigate the diffusion barrier properties of physical vapor deposited (PVD) Ta/W multilayer structure in copper metallization. Ta barrier is deposited using e-beam evaporation at a base pressure of 5x10^-7 torr using a Ta source (99.99%). W layer is sputtered deposited using RF magnetron sputtering. W target (99.99%) is sputtered under a 4.5 mtorr total sputtering pressure in an Ar atmosphere. In order to test the thermal stability of the barrier structure, a 120 nm-thick Cu film is deposited using e-beam evaporation on top of the multilayer structure. The barrier performance is tested using four probe measurements for sheet resistance, X-ray diffraction analysis for phase identification and scanning electron microscopy for surface morphology, after annealing in rapid thermal processing furnace for 5 min in presence of N₂ at various temperatures starting at 500 °C. The formation of crystallites takes place on the surface of the copper layer at the barrier failure temperature. The quantitative analysis of these crystallites is done using energy-dispersive spectroscopy. The interaction of different layers due to high temperature annealing is evaluated by depth profiling using X-ray photoelectron spectroscopy. Results of these studies will be discussed in the context of Cu metallization technologies.