Ion-beam deposition of Nb and Ta refractory superconducting films

A Kaufman ion-beam source has been used to sputter deposit high quality superconducting Nb (Tc=9.1 K) and Ta (Tc=4.3 K) films. Superconducting and electrical properties were studied as a function of deposition conditions and an optimum set of conditions was found. Nb films always had the bulk bcc crystal structure. The Nb film deposition rates ranged from 1 to 5 Å/s with a Xe or Ar sputtering gas pressure of ∼1 to 4×10−4 Torr and a beam current density of 5–20 mA/cm2. Values of Tc above 8.3 K for Nb films were obtained with the use of Xe rather than Ar gas. In the case of Ta films, the deposition of a thin (≥3 Å) Nb underlayer was required for the nucleation and growth of Ta in the bulk bcc crystal structure. Ta films deposited without a Nb underlayer always had high resistivity, ∼150 μΩ cm, and a tetragonal β-Ta crystal structure. The Nb and Ta targets and Si substrates all remained below 70 °C during deposition and the films were easily patterned by standard photoresist liftoff.