Low temperature chemical vapor deposition of tungsten carbide for copper diffusion barriers

Abstract

Tungsten carbide films were grown by chemical vapor deposition using W(CO) 6 and C 2H 4 between 250 and 450°C. Pyrolysis studies indicate W(CO) 6 thermally decomposes over the 150–200°C temperature range with or without ethylene. Carbon incorporation increased from 13 to ∼33% when C 2H 4 was co-fed with W(CO) 6. The W/C carbon ratio, as established by X-ray photoelectron spectroscopy (XPS), remained approximately 2:1 regardless of the C 2H 4 to W(CO) 6 flow ratio for temperatures between 250° and 450°C. Films (50 nm thick) grown at 290°C have a resistivity of 250 μΩ-cm. Above 500°C, the W/C ratio decreased to ∼1:1.25 and the resistivity increased to 535 μΩ-cm. The tungsten and carbon are present in the tungsten carbide chemical state. X-Ray diffraction, XPS, selected area electron diffraction and transmission electron microscopy analysis reveal the films are a mixture of 5–6 nm W 2C crystallites in an amorphous matrix, whose stoichiometry is ∼2:1 W/C. Copper was not found to diffuse through 7-nm-thick tungsten carbide films in Cu/WC/SiO 2 stacks that were annealed at 400°C for 8–9 h.