Non-thickness-limited growth of anodic oxide films on tantalum

The non-thickness-limited (NTL) growth phenomenon reported by Melody et al. in which anodic oxide films on tantalum continue to grow in dry glycerol solutions of dibasic potassium phosphate to large thicknesses (tens of micrometers) at low applied voltages (tens of volts) was investigated using mostly constant current growth. The field strengths in the oxide for growth of anodic oxide films at 180 degreesC with constant current density in wet glycerol solutions (1 vol % water) were in the MV/cm range and close to those calculated from the equations fitted to data from growth in aqueous solutions at lower temperatures. At constant current density and 180 degreesC in dry glycerol solutions, the field fell with time and eventually the voltage decreased as the NTL state developed. The current efficiency for the growth of oxide was estimated by comparing thickness increase measured by ellipsometry with that calculated from the charge passed. It was lower in the NTL state. This is consistent with increased electronic conductivity in the oxide. The dielectric losses of the NTL films were much higher than those of films grown in wet glycerol solutions. Those grown in wet glycerol showed the normal, almost frequency independent, tan delta as found for ordinary anodic oxide growth on tantalum. Those grown in dry glycerol solutions showed increasing tan delta as the frequency decreased, consistent with increased electronic conductance through the oxide. It is suggested that the increased electronic and ionic conductivity in the NTL state may be caused by nonstoichiometry induced by the change in hydroxyl ion availability to the oxide suggested by Melody et al. when dry glycerol solutions are used. (C) 2001 The Electrochemical Society.