Phase formation and microstructure changes in tantalum thin films induced by bias sputtering

The effects of argon-ion bombardment on the structure and properties of sputtered tantalum films have been studied. Applied substrate bias voltage was used to control the bombardment energy in a hollow-cathode-enhanced low-pressure magnetron sputtering system. The films were characterized by x-ray diffraction, electrical measurements, Rutherford backscattering spectrometry, and stress measurements. The findings concerning the effects of negative substrate bias on film resistivity and structure run counter to earlier work. In particular, as opposed to results found in many early studies, which primarily involved higher-pressure discharges, at zero bias voltage the films have low resistivity and contain the bcc phase. Increasing the bias to −100 V, increases the resistivity dramatically, and induces formation of β-Ta, with no significant change in film impurity levels. The difference from earlier work is attributed to the lower relative impurity flux, as well as more energetic substrate bombardment in low-pressure magnetron sputtering. Energetic substrate bombardment is clearly demonstrated by the high level of argon content in films deposited with no applied bias (2%). At very high bias voltage, argon incorporation increases dramatically and resistivity increases in the β phase. Additionally, a decrease in the compressive stress and change in preferred orientation occur. The results suggest that the formation of the β phase is not controlled by impurity effects, but by Ta forward scattering and related stress changes.