Influences of electrolyte, pre-thermal treatment and substrate composition have been examined to elucidate the mechanism of field crystallization of anodic niobia formed on magnetron-sputtered niobium. The field crystallization occurs during anodizing at 100 V in 0.1 mol dm -3 ammonium pentaborate electrolyte at 333 K, with the crystalline oxide growing more rapidly than the amorphous oxide, resulting in petal-like defects. The nucleation of crystalline oxide is accelerated by pre-thermal treatment of the niobium at 523 K in air, while vacuum treatment hinders nucleation. Notably field-crystallization is also absent in 0.1 mol dm -3 phosphoric acid electrolyte or when anodizing Nb-10at.% N and Nb-29at.% W alloys in the ammonium pentaborate electrolyte. The behaviour is explained by the role of the air-formed oxide in providing nucleation sites for field crystallization at about 25% of the thickness of the subsequently formed anodic film, the location being due to the growth mechanism of the anodic oxide and the nature of crystal nuclei. Incorporation of tungsten, nitrogen and phosphorus species to this depth suppresses the field crystallization. However, boron species occupy a relatively shallow layer and are unable to affect the nucleation sites.

No abstract

Thermal treatment of anodized niobium and oxygen-containing niobium has been carried out to elucidate the thermal degradation mechanism of niobium capacitors and to clarify the influence of oxygen dissolved in niobium on thermal degradation. The capacitance and leakage current of the anodized specimens increase with thermal treatment above 423 K in air, although the oxygen content in the substrate has no significant effect up to 523 K. At increased temperatures, the changes in capacitance and leakage current are suppressed with increasing oxygen content. The anodic film formed on the Nb-50 at% O substrate thickens significantly during thermal treatment at 623 K in air, while only a slight reduction of the thickness is evident for those on the niobium and Nb-20 at% O substrates. In contrast, vacuum thermal treatment at 623 K causes thinning of the anodic film on niobium, with evidently no change in the film thickness on the Nb-50 at% O substrate. These are interpreted in terms of oxygen diffusion from the anodic film to the substrate as well as thermal oxide growth.

The present work demonstrates effective inhibition of field crystallization of amorphous anodic niobium oxide by incorporation of silicon species from substrate. The field crystallization, detrimental for capacitor application of niobium, occurs during anodizing of magnetron sputtered niobium at 100 V in 0.1 mol dm -3 ammonium pentaborate electrolyte at 333 K, while amorphous structure of the anodic oxide is totally retained during anodizing of magnetron sputtered Nb-12 at% Si. Even after prior thermal treatment in air, which accelerates field crystallization of anodic oxide on niobium, no crystallization occurs on the Nb-12 at% Si. Through examination of the crystallization behaviours of anodic films formed on a thin Nb-12 at% Si layer superimposed on a niobium layer as well as on a thin niobium layer superimposed on an Nb-12 at% Si layer, it has been confirmed that air-formed oxide or thermal oxide becomes a nucleation site for crystallization. Modification of the air-formed or thermal oxide by incorporation of silicon species inhibits the nucleation of crystalline oxide.The modification, however, does not influence the growth of crystalline oxide. The growth is suppressed by continuous incorporation of silicon species into anodic film from the substrate during anodizing.

Solid-solution Nb-O films containing up to 50 atom % oxygen, prepared by magnetron sputtering, were used to investigate the influence of the oxygen on field crystallization during anodizing at 100 V in 0.1 mol dm −3 ammonium pentaborate electrolyte at 333 K. The findings reveal that field crystallization is hindered dramatically by addition of 20 atom % oxygen to the substrate, while no crystallization occurs for a Nb-50 atom % O substrate. Prior thermal treatment accelerates field crystallization of niobium, but not the Nb-50 atom % O substrate. The thermal treatment is considered to promote generation of precursor sites for crystal nucleation. However, sufficient oxygen in the substrate may restrict precursor development and/or reduce the compressive stresses in the amorphous anodic niobia that can facilitate crystal growth. Niobium is a potential alternative material to tantalum for electrolytic capacitors due to its many attractive properties 1 and its relatively high natural abundance. However, niobium capacitors are more susceptible to field crystallization during both growth of the anodic oxide dielectric and packaging of capacitors. Field crystallization in niobium and tantalum leads to degradation of the initial amorphous oxide, involving cracking and peeling due to growth of crystalline oxide, 2 which increases the leakage current. The effects are enhanced at increased anodizing temperatures and forming voltages.2-4 Inclusions 5,6 and roughness, especially convex surfaces, of the substrate assist crystal nucleation. 7,8 Recent studies using flat, inclusion-free, magnetron-sputtered niobium suggested nucleation is associated with precursor sites in the original, air-formed film, while incorporation of foreign species from the electrolyte or from the substrate at precursor regions can hinder nucleation.9 Other investigations indicate an enhanced susceptibility to field crystallization due to oxygen impurity in the substrate. 5,10,6,11 Oxide precipitates, possibly at grain boundaries, may be involved, although the detailed processes are not well understood. Here, the influence of oxygen in the substrate is examined on field crystallization of the anodic niobia. ExperimentalNiobium and Nb-͑20 and 50͒ atom % O films were deposited by sputtering 99.9% niobium in either argon or a mixture of argon and oxygen at ϳ0.1 Pa using magnetron sputtering enhanced with a radio frequency plasma source to increase the plasma density. Three types of flat substrate were employed: glass plates, silicon wafers, and electropolished and anodized aluminum sheets. Subsequent thermal treatment of the deposited films, either in air at 523-623 K for 1.8 ks or in vacuum ͑ϳ10−5 Pa͒ at 923 K, was employed selectively. Structures of the deposits were determined by X-ray diffraction ͑XRD͒ ͑Rigaku RINT 2000͒ using Cu K␣ radiation, with patterns obtained in an ␣−2 ͑␣ = 1°͒ mode.The films were anodized at 50 A m −2 to 100 V with current decay in stirred 0.1 mol dm −3 ammonium pentaborate electrolyte at 333 K. A platinum sheet was used as a...