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 degrees and 90 degrees, with 15 degrees 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 theta 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 degrees angle with the real axis. The surface roughness of the Nb deposits, calculated from their double layer capacity, is low and constant at low theta, significantly increases at theta=45 degrees, goes through a maximum in the range 60-75 degrees and drops at theta=90 degrees. 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 theta have been made by performing the depositions under pulsed conditions or by heating the substrates at 400-600 degrees C. Heating at the higher temperature was a fairly effective method for decreasing the roughness of deposits formed at large theta. (c) 2005 Elsevier Ltd. All rights reserved.