Ta–Rh binary alloys as a potential diffusion barrier between Cu and Si: Stability and failure mechanism of the Ta–Rh amorphous structures

Thermodynamic calculations were carried out to derive the Gibbs free energy diagram for the amorphous and crystalline phases in the Ta–Rh system. These calculations predicted that the compositional range for the amorphous Ta–Rh phase was within 37–66 at.% Ta, which was validated by X-ray diffraction (XRD) analysis, high-resolution transmission electron microscopy (HRTEM) observations and resistivity measurements of as-deposited films. The thermodynamic modeling provided a valuable guide for selecting an amorphous composition suitable for diffusion barrier applications. The stability and metallurgical failure mechanism for TaRhx diffusion barriers in contact with Cu and/or Si were investigated by resistivity measurements, XRD analysis and detailed electron microscopy on samples annealed in 5% H2/95% N2 gas for 30 min at various temperatures. Amorphous TaRhx in contact with the Si substrate was stable up to 700 °C, whereupon TaRhx decomposed and reacted to form TaSi2 and RhSi. Si/amorphousTaRhx (13 nm)/Cu stacks, on the other hand, were stable only up to 550 °C. Failure occurred by reaction of Rh with the Si substrate to form RhSi at the interface. The large density of defects formed in the barrier layer as a result of outward diffusion of Rh facilitated diffusion of Cu to the Si/TaRhx interface to form Cu3Si particles. The formation of Cu3Si was observed to trigger further silicidation of the barrier to form a discontinuous TaSi2 layer.