Nanostructured alpha and beta tantalum formation—Relationship between plasma parameters and microstructure

Nanostructured tantalum films with an average grain width of 40–50 nm were synthesized using direct current (DC) magnetron sputtering. The relationship between sputtering conditions (power and pressure), plasma kinetics and impurities levels was investigated with respect to the phase formation. All samples processed at pressures ranging from 0.3 to 1.4 Pa led to the formation of beta phase (tetragonal) or mixed phase films, except at 0.7 Pa sputtering pressure which yielded alpha phase (body centered cubic) tantalum. The correlations between hardness, residual stress and texture development are presented for all conditions. It was found that the hardness values do not reflect the variation in texture or phase; however, for films having the same texture alignment the hardness value decreases as the residual stress becomes more tensile. Overall, the formation of alpha tantalum was correlated to a lower amount of impurities during deposition as well as to an increase in the ion bombardment energy.