Study of Ta as a Diffusion Barrier in Cu/SiO₂ Structure

Abstract

Tantalum (Ta) thin films of 35 nm thickness were investigated as diffusion barriers as well as adhesion-promoting layers between Cu and SiO₂ using X-ray diffractometry (XRD), Scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). After annealing at 600°C for 1h in vacuum, no evidence of interdiffusion was observed. However, XPS depth profiling indicates that elemental Si appears at the Ta/SiO₂ interface after annealing. In-situ XPS studies show that the Ta/SiO₂ interface was stable until 500°C, but about 32% of the interfacial SiO₂ was reduced to elemental Si at 600°C. Upon cooling to room temperature, some elemental Si recombined to form SiO₂ again, leaving only 6.5% elemental Si. Comparative studies on the interface chemical states of Cu/SiO₂ and Ta/SiO₂ indicate that the stability of the Cu/Ta/SiO₂/Si system may be ascribed to the strong bonding of Ta and SiO₂, due to the reduction of SiO₂ through Ta oxide formation.