Anodic oxide growth on tantalum

• Main Author: Yang, Ta-Ming
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/6181

In the growth of anodic oxide films on tantalum, the ionic conduction at a given current density depends on the electrical history of the film. In this thesis, this history effect has been investigated by examining the frequency response of the high field ionic conduction process involved in the growth of anodic oxidation on tantalum. The admittivity, defined as the complex ratio of the ac current density to the ac field, was measured. Ideally, constant dc and ac components of field would be applied. This was approximated by applying a ramp voltage plus an ac voltage, both increasing with the average film thickness. The measured current density includes displacement current.. A series capacitance effect is caused by the ac component of the rate of change in film thickness. After correction for these effects, plots of the real vs. imaginary parts of the admittivity, which yielded arcs of circles with centers above the real axis, were fitted to a Cole-Cole type of expression: A+B/[1+(jωtm)1-θ], where 1 / t m is the angular frequency (o)) at the maximum magnitude imaginary part, π(l-θ) is the central angle subtended by the arc. This result was explained as being due to a distribution of relaxation times being involved in the history effect. This is consistent with the history effect being caused by changes in the structure of the semi-amorphous oxide film depending on formation conditions. tm was found to be close to inversely proportional to the dc ionic current density and almost independent of temperature. This is consistent with a process driven by current density. In anodic oxidation, there is the possibility of incorporation in the film of species from the electrolyte. Experiments were carried out to see if the curvature of the steady-state logJ vs. E relation and the dependence of the stepped-field line slope on the steady-state field can be caused by this incorporation. The steady-state log (J) vs. field (E) relation has been studied for films grown in 0.1M sulphuric acid and 0.1M oxalic acid. The refractive index and thickness of the films were measured using ellipsometry. Films grown in oxalic acid showed 1-2% higher field. Measurement of the oxide capacitance at different film thickness showed the oxide permittivity to be higher for the films grown in sulphuric acid. An overshoot transient in the field (E), with a peak field (Ep ), was observed when the ionic current was stepped from the steady state to a higher value (J2). The field as a function of time, E(t), was derived from the measured voltage divided by the thickness which was equivalent to the integration of ionic charge passed at each time t. Measurement of the initial slope of dE/dt showed the oxide permittivity agreed with the results from the capacitance measurement. The relation of logJ2 vs. Ep at a steady-state current density can approximately describe the stepped-field condition with the history effect held constant during the transient The relation of logJ2 vs. Ep was shown to be linear and the slope of dlogJ2/dEp was shown to be dependent on the initial field for films grown in both electrolyte solutions. Further work is necessary before a definite conclusion can be reached about the contribution of electrolyte incorporation to curvature of the Tafel plot.