Localized electrochemical oxidation of thin Nb Films in microscopic and nanoscopic dimensions

The mechanism and kinetics of localized anodic oxidation of thin Nb films are investigated by measurements in an electrochemical microcell and in the so-called nanocell, which is formed by water condensation between an AFM-tip and the Nb-substrate in humid air. In both, the microscopic and nanoscopic oxidation, the thickness of generated oxide structures increases linearly with the applied potential in accordance with the so-called high field growth model. The oxide growth factor depends on the polarization time and reaches in both cases for long times a value of about 2.8 nm/V. In the case of AFM tip-induced oxidation at constant voltage the oxide growth rate decreases rapidly with the polarization time, which is in good agreement with the proposed models including a rapid build-up of space charge within the oxide in the initial oxidation stages. The increase of the oxide thickness is limited by the thickness of the thin Nb layer. An experimental procedure for checking the complete local nanooxidation of thin Nb films is proposed and the possibility for preparation of lateral metal-insulator-metal (MIM) structures is demonstrated.