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Both bcc and β‐Ta thin films (50 nm) have been deposited onto (100) Si by dc magnetron sputtering at pressures between 4 and 7×10-2 Pa. Ta nucleates in the bcc structure when the substrate is at ground potential. However, when a negative bias voltage is applied, a mixture of bcc and β phases is formed. Below -100 V bias, only the β structure is observed. We conclude that ion bombardment during the deposition process plays an important role in the growth of the Ta films. We also compare the effectiveness of thin bcc and β‐Ta layers as diffusion barriers to copper penetration into Si. Ta films (bcc and β) were sputtered onto Si wafers (100) and overcoated with 150 nm of copper. Diffusion and reaction were monitored as function of temperature and time (up to 60 min) by four point resistance measurements and by Rutherford backscattering spectrometry. It has been observed that bcc Ta films exhibited no copper–silicon interdiffusion up to 650 °C, indicating that this Ta structure is a potential candidate for use as diffusion barrier. We also point out the dramatic effect of the microstructure of the Ta barrier on the breakdown temperature.