Effect of Ambients and Contact Area on the Asymmetric Conduction of Anodic Tantalum Oxide Films

Abstract

The effect of moisture and metallic contact area has been investigated on the asymmetry properties of anodic tantalum oxide films, approximately 2400Aå thick. Moisture has been reversibly desorbed and adsorbed by vacuum baking followed by exposure at 50–85% RH. The cathodic currents are not affected significantly by this cycle, while the anodic leakage and charging current can be changed by factors of 106 and 102, respectively. The contact area experiment confirm previous results that the asymmetry is due to local inhomogeneities in the oxide film, since nonpolar characteristics, in the absence or presence of moisture, can be obtained with a contact area less than 0.01 mm2. This result indicates that the moisture effect is associated with defects in the oxide film. The oxide film can thus be considered as a normal dielectric with high symmetrical anodic and cathodic breakdown voltages. Distributed on the average 0.1 mm apart throughout the film are heterogeneities of an unknown nature which cause a symmetrical but lower breakdown than the rest of the oxide. The asymmetry results from an increase in the anodic breakdown at these distinct sites due to the presence of water.

Kinetics of Tantalum Corrosion in Aqueous Electrolytes, II

“…[15] is satisfied by F = F~ ----const, so that Eq. [14] can be transformed into E = V -5 dFi [16] E-values derived in this manner were E ~ 0.gv for the H2SO4 and NaCl-electrolytes, and 1.6v for the NaOH-electrolyte. The differences between E1 by Eq.…”

“…Image force potentials extend through most, if not all, of the oxide films encountered in spontaneous corrosion of A1 and Ta in air, and the discrete nature of charges at the outer oxide surface leads to large field variations in the oxide along the metal-oxide interface. Last, but not least, the oxide growth may be governed by charge transport at certain structural faults (12)(13)(14)(15)(16), particularly, when means for lateral charge transport along the outer oxide surface exist, as in the case of corrosion in an electrolyte.…”

Asymmetric Conduction in Thin Film Tantalum/Tantalum Oxide/Metal Structures: Interstitial and Substitutional Impurity Effects and Direct Detection of Flaw Breakdown

“…3 for two different slides prepared under ostensibly identical conditions (2). The reduction in the area of the capacitor sample results in a marked increase in cathodic breakdown voltage.…”

“…The results of measurements on the breakdown voltage as a function of the capacitance (i.e., of the calculated area using 26.5 as the dielectric constant and 16.5 A/v for the anodization constant) for a constant oxide film thickness are shown in Fig. 3 for two different slides prepared under ostensibly identical conditions (2). The reduction in the area of the capacitor sample results in a marked increase in cathodic breakdown voltage.…”

“…Since the current flow is so time dependent at low moisture content, it is possible that an electronic mechanism is operative which depends markedly on the absence or presence of water, perhaps affecting some trap levels in the conducting oxide (2).…”