Deposition parameters, structure and electrical properties of evaporated tantalum films

Abstract

Over 100 tantalum films were prepared by evaporation at a deposition pressure between 1 × 10−8 and 2 × 10−8Torr in the thickness range 11.5–970 nm. Any substrate temperature from 350 to 770 K could be maintained during evaporation, and evaporation rates of between 5 and 45 nm min−1 were possible. The structures of samples could be characterized as (1) f.c.c. (for a group of films of less than about 20 nm thick), (2) b.c.c. (for thicknesses greater than about 25 nm) or (3) a mixture of f.c.c. and b.c.c. (even for thick films). Films in category (2) exhibited some degree of preferred orientation. The lattice parameters a0 for the f.c.c. and b.c.c. structures were found to be 0.450±0.001 nm and 0.331±0.001 nm respectively. Grain sizes for both f.c.c. and b.c.c. structures increased with increasing film thickness. For a given film thickness f.c.c. grains were found to be larger than b.c.c. grains.The electrical properties of over 80 of the films are also given. The presence of an f.c.c. structure in thick films reduced both the temperature coefficient of resistivity and the resistivity ratio. The resistivity was almost doubled if the percentage of f.c.c. structure present was increased from 0 to 50%. Matthiessen's rule was found to hold for all films with b.c.c. structure that were greater than about 40 nm in thickness. Films with near bulk resistivities were prepared, even at low evaporation rates, provided the substrate was held at a temperature greater than about 600 K.