EIS study of niobium films sputtered at different target–substrate angles

Nb films have been magnetron sputtered onto quartz sheets oriented with respect to the target at angles varying between 0° and 90°, with 15° steps. Impedance plots have been obtained by contacting these films with aqueous Na2SO4, either at the open circuit potential or at a potential where Nb is covered by an anodic passive Nb2O5 film. As the target–substrate angle θ increases, the shape of the impedance plots changes from that of a smooth electrode to that of a porous one, characterised in the high frequency range by a straight line forming a 45° angle with the real axis. The surface roughness of the Nb deposits, calculated from their double layer capacity, is low and constant at low θ, significantly increases at θ = 45°, goes through a maximum in the range 60–75° and drops at θ = 90°. AFM surface profiling confirms this trend, but estimates a lower surface roughness. Attempts to obtain Nb deposits with a surface roughness less strongly dependent on θ have been made by performing the depositions under pulsed conditions or by heating the substrates at 400–600 °C. Heating at the higher temperature was a fairly effective method for decreasing the roughness of deposits formed at large θ.