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Effect of Nitrogen Incorporation on Dielectric Properties of Anodic Oxide Films Formed on Niobium

Anodic oxide films formed on niobium have attracted considerable attention as an alternative dielectric material to tantalum oxide due to its high abundant and high permittivity. Previously, we reported that the capacitance of anodic niobium oxide films formed in alkaline electrolyte adjusted by addition of ammonium hydroxide was significantly higher than that of the film formed in an acidic electrolyte. Furthermore, it was confirmed that the films formed in alkaline electrolyte, which have high capacitance, are composed of two layers: the outer oxide layer containing nitrogen (N) and the inner layer consisted of pure niobium oxide. In the present study, the effect of N content in the films formed in various electrolytes on the capacitance was investigated. To clarify the mechanism of nitrogen incorporation into the anodic film, two-step anodization by using acid and alkaline electrolytes at different formation voltages was also performed. High purity (99.93%) niobium foils were anodized at constant current density of 10Am up to 60V in various 0.1 mol dm electrolytes such as phosphoric acid, boric acid, nitric acid, succinic acid, sulfuric acid, oxalic acid and citric acid at 20 C after adjusting pH to 10 by the addition of ammonium hydroxide. The subsequent voltage holding was carried out for 30 min. To investigate the distribution of N in the films, the two-step anodization was conducted. Namely, anodization was first carried out in an alkaline mixture of phosphoric acid and NH4OH at constant current density of 10Am up to 30V, followed by second anodization in phosphoric acid up to 60V. Capacitance of the film was evaluated by impedance measurement in 150 g l ammonium adipate at ac amplitude of 50 mV. Relative N content in the films was estimated from the integrated elemental intensity measured by glow discharge optical emission spectroscopy (GDOES). Figure 1 shows change in capacitance with N content incorporated in the films formed in phosphoric acid electrolytes of different pH, as well as the films formed in various types of alkaline electrolytes adjusted to pH10. Capacitance increased linearly with increasing N content when the pH of electrolyte was in alkaline region. In the case of electrolytes of pH 10, capacitance increased in the following order: Boric < Nitric < Phosphoric < Succinic < Sulfuric < Oxalic < Citric acid. This order was coincident with the order of acidity, in other words, amount of added NH4OH, which was required to adjust the pH to 10. These results indicate that change in capacitance of the film was highly dependent on N content rather than electrolyte species. To investigate the incorporation behavior of N, depth profiles of constituent elements of the film formed by two-step anodization were analyzed by GDOES. In the case of the film formed by the first anodization at 30 V in alkaline electrolyte (pH10), GDOES elemental profiles indicated N penetration into the approximately 65 % depth of the film thickness (Fig. 2a). Figure 2b shows the elemental profiles after the second anodization up to 60 V in phosphoric acid (pH1.6). The penetration depth of phosphorous and N into the film were ~20 % and 35~70 %, respectively. Figure 3 shows schematic models of the cross section structure of the films shown in Fig. 2. Considering the transport number of Nb (0.25), the oxide grew at a rate of 25% at electrolyte/film interface and 75% at film/substrate interface during the second anodization. However, N distributed layer was detected in the middle part of the film. These results indicate that N containing species could migrate into inward of the film during anodization under the electric field.