Effect of ion irradiation on α and β phase evolution of sputtered tantalum thin films

Tantalum thin films were deposited by RF (13.56 MHz) magnetron sputtering on silicon and aluminum substrates with ion irradiation (∼ 0.3 mA/cm ²) controlled by applying negative DC bias voltages (0-300 V) to the substrate. The presence of two main crystallographic phases of Ta in deposited films was investigated. Results showed that only the tetragonal β-phase formed in thin Ta films on both Si 〈100〉 and Al substrates at zero bias voltage (ion energy ∼10 eV), and only bcc α-phase of Ta formed at the ion irradiation energy of 150 eV. Both phases were present as the ion energy of 100 eV. With increase of the ion energy to 250 eV, or higher, β-phase Ta dominates the film structure on Si 〈100〉 but not on Al substrate. Ta films were deposited in compressive stress with the lowest stress measured for α-phase Ta films deposited with ion energy of 150 eV. A new set of deposition parameters, significantly different than those previously reported, for low temperature growth of Ta films with bcc crystallographic structure, desired for most applications, was found. An advantage of the new deposition parameters is relatively simplicity of the required equipment, which can be easily scaled up for an industrial process.