Focused-Ion-Beam Direct-Writing of Ultra-Thin Superconducting Tungsten Composite Films

Tungsten composite films of thickness as low as 19 nm have been deposited using a 30 keV Ga⁺ focused-ion-beam with tungsten carboxyl (W(CO)₆) as the gas precursor. Films of thickness 25 nm or more are superconducting with a transition temperature exceeding 5 K. Films in the thickness range 25 nm to 50 nm show an increasing T_c for a decreasing film thickness. This correlates well with the measured dependence of the normal state resistivity upon film thickness. We attribute this behavior to an increase in the BCS electron-phonon interaction potential resulting from a reduction in the electron mean-free-path as the film thickness is reduced. In the light of these data we discuss the applicability of FIB-deposited tungsten for devices requiring ultra-thin superconducting films, including photon detectors and phase-slip qubits.