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

摘要

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 TaRh_x 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 H_2/95 N_2 gas for 30 min at various temperatures. Amorphous TaRh_x in contact with the Si substrate was stable up to 700 °C, whereupon TaRh_x decomposed and reacted to form TaSi_2 and RhSi. Si/amorphousTaRh_x (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/TaRh_x interface to form Cu_3Si particles. The formation of Cu_3Si was observed to trigger further silicidation of the barrier to form a discontinuous TaSi_2 layer.