Mechanism of oxygen desorption and outgassing of the niobium sputtering systems

The sputtering of niobium films is accompanied by desorption of oxygen-containing gases stimulated by interaction of the sputtering discharge with the oxide layers on all metal surfaces. Oxygen released into residual atmosphere impairs the superconducting characteristics of films. The sputter-coating process results in a drastic reduction of the residual resistance ratio: from 150-200 in the niobium cathode to 10-20 in deposited films. The effective pumping speed of the vacuum system is negligible compared to the gettering speed of the growing niobium film. Only the short initial stage of sputtering, when the polycrystalline structure of a film is not yet formed and the gettering speed is small, can be used for outgassing the system before sputtering. Theoretical calculations show that, in a normal sputtering process using argon, the duration of this stage is 0.5-1 s and the amount of oxygen removed is less than 2% of the total content in the oxide. These figures can be increased considerably in a helium discharge where the yield of niobium is as small as 5-6% of that in Ar. Experimental results confirm this reasoning.