THE INFLUENCE OF TANTALUM ELECTROCHEMICAL POLISHING ON THE DIELECTRIC PARAMETERS OF ANODIC OXIDE

Abstract

This work involved the electrochemical polishing of a Ta-electrode in an electrolyte of HF:C₃H₇(OH) (propan-2-ol):H₂SO₄ with the relation between the components 1:1:3 (vol.) respectively. The following conditions are optimal for the process: current density – 2 - 3 A/cm², temperature – 23 - 25 °C, and time – under 60 sec.  The efficiency of electrochemical polishing treatment was proved by images which were obtained with the help of a scanning electron microscope. The anodic behaviour of a Ta-electrode in 0.1 M H₃PO₄ after electrochemical polishing was investigated by cyclic voltammetry. The cyclic voltammetry was performed from the open circuit potential (vs Ag, AgCl, 4 M KCl) to the upper anodic potential which was sequentially increasing with every cycle from 3.5 V to 9 V. Scan rate was 5 mV/sec. In agreement with the broadly accepted assumption, the limiting stage of the tantalum anodic oxidation was the diffusion of tantalum and oxygen ions through thin oxide layer which was indicated by current response in the cyclic voltammetry curves. It was shown that current efficiency for the formation of tantalum anodic oxide on the metal surface was about 100%. Impedance spectroscopy data confirmed the formation of anodic oxide with high dielectric properties on the tantalum surface during cyclic voltammetry. Afterwards, the dielectric constant of the tantalum oxide film was calculated from the slope of dependency of reciprocal capacitance on the thickness of the anodic oxide. The obtained value of 25.89 for the dielectric constant is close to the value which is broadly accepted in literature. This fact indicates that the electrochemical polishing of Ta in an electrolyte with an addition of C₃H₇OH (propan-2-ol) allows obtaining a tantalum surface with the lowest roughness factor (1.03 ± 0.02) in a reproducable manner.