Tungsten–rhenium alloys as diffusion barriers between aluminum and silicon

Abstract

Tungsten–rhenium alloys have been investigated as possible diffusion barriers between aluminum and silicon for high-temperature metallization of microelectronic devices. Al/W–Re/Si structures with the alloy compositions W–30 wt. % Re, W–61 wt. % Re, and W–72 wt. % Re were prepared by electron beam evaporation of W–Re and Al on Si wafers. Thin W–Re films, without Al, were also deposited on Si3N4 windows. The samples were annealed at different temperatures and characterized using Rutherford backscattering spectroscopy, x-ray diffraction, and transmission, and scanning electron microscopy (TEM and SEM). The W–30% Re sample was crystalline with a β-W structure after deposition, while the W–61% Re and the W–72% Re samples were amorphous and crystallized into the α-Mn structure (χ phase) at a temperature of 700 and 600 °C, respectively. All Al/W–Re/Si structures were stable against Al penetration up to 500 °C. At an annealing temperature of 550 °C signs of a reaction leading to the formation of (W,Re)–Al12 could be observed. At 600 °C the barrier layers failed completely due to a reaction between W, Re, Al, and the Si substrate. SEM analyses of the bare Si substrate after the metal layer had been chemically removed showed that etch pits were formed after annealing the diffusion barrier structure at 550 °C for 30 min. The failure of the diffusion barriers is most likely due to local penetration of Al through defects in the films.