Sputtered Nbn-Films For Rf Applications

Thin films of niobium nitride (NbN) have been deposited on copper substrates, using UHV DC-bias sputtering equipment with argon as sputter- and nitrogen as reactive-gas. The influence of nitrogen partial pressure and bias voltage (U(bias)) on the transition temperature (T(c)) and the lattice parameter (a) have been investigated. A marked dependence of the surface-morphology on the bias voltage applied has been observed. Under optimum conditions, critical temperatures of T(c) = 16.1 K and a transition width of 0.1 K (inductively measured; error of temperature determination is less than transition width) have been achieved. X-band (9.1 GHz) copper-cavities have been coated and the RF-properties of the superconducting (SC) NbN-films studied. Although the surface-resistance achieved (R(s)) of 11.3 +/- 1.2-mu-OMEGA at 4.2 K is lower than the value of high thermal conductivity niobium (20-mu-OMEGA/4.2 K/residual resistance ratio = RRR = R(s 300 K)/R(s 20 K) = 40) [1] the penetration depth change in these films is 4-5-mu-m.