Properties of Superconducting Niobium Films Made by Asymmetric ac Sputtering

Superconducting films of niobium can be made by asymmetrical ac sputtering. This modification of the conventional sputtering process uses a reverse sputtering current to bombard the film as it deposits, thus dislodging gaseous impurities to obtain pure films. The deposition conditions to yield the purest films have been established using cylindrical geometry, that is, Vycor tubing substrates which are heated internally and surrounded by a large niobium cathode. At substrate temperatures of 500°C and with a sputtering to reverse sputtering current density ratio of 4:1, films with β=R300°K/R10°K as large as 4.7 have been obtained from bulk material of β=14. Films so made have critical temperatures between 8.9° and 9.2°K; the bulk material critical temperature is 9.25°K. The critical temperature of the films is a strong function of β, falling below 4.2°K for β<1.25.