Anodic Oxide Films on Niobium: Thickness, Dielectric Constant, Dispersion, Reflection Minima, Formation Field Strength, and Surface Area

Abstract

The wavelengths of minimum specular reflectivity (at 11' incidence) due to interference were determined using a spectrophotometer for a series of films formed on chemically polished niobium. With a value of the refractive index n = 2 . 4 6 f 1% a t 4358 A wavelength by the Abelhs method (reported elsewhere), the spectrophotometric data give the refractive index as a function of the wavelength X, 11 = 2.26+0.398/(X/103 A-2.56)l.~. T o analyze the spectrophotometric results, an auxiliary measure of thickness was required (though, with the chart given below, the thickness of a film may be determined directly from spectrophoto~netric measurements alone). A combination of measurements of the a-c. capacity and of the charge required to form the films gives a suitable measure of thickness (in terms of p /~, where r = dielectric constant and p = density) which is not dependent on a knowledge of the true surface area. The spectrophotometric data provide a calibration of this measure of thickness and thus give E /~. With p = 4.36 g cm+ (reported for the bullc amorphous oxide) this gives e about 41 (compared with about 27.6 for TazOs). The effective surface area of the chemically polished metal was then found to be about 7% greater than the apparent area. At the ionic current density used to form the films (10 ma cm-=), the field strength in the oxide was estimated as 4.96X106 v cm-1 within a few per cent uncertainty. Because the field to produce a given ionic current is l o t k than with Ta20s films, the capacity of films formed to a given voltage a t a given current density and temperature is not so much greater for niobium than for tantalum as the dielectric constants might lead one to expect. I t is suggested that there may be a correlatio~l between dielectric constant and ionic conductivity. The NbzOa films recrystallize like Taros films under an applied field but more readily, a t least with the purity of metal now available. INTRODUCTIONAs might be expected from their close chemical similarity, niobium and tantalum behave very similarly when anodically polarized. The present investigation of some of the properties of the film material followed the same method as was applied recently to tantalum (1). The aim was to obtain further data towards the coillparative study of the various inetals (Al, Nb, T a , Ti, Zr, W, etc.) which show similar anodic film forming properties. Niobium is of particular interest because of its possible future use as an electrolytic capacitor anode. The kinetics of forn~ation and solne other topics have been discussed elsewhere (2, 3, 1). E X P E R I M E N T A L METHODSThe electrodes (2XO.5 till with a tab) were cut (using a die to obtain a reproducible size) fro111 Fansteel h~Ietallurgica1 Corporation 0.005-in. sheet of greater than 99.9% purity. They were stopped off and chemically polished as previously described (2). T o obtain uniformity in the surface finish they were prepared in batches. After being stored dry, they were dipped in a dichromate-sulphuric acid cleaning mixture bef...

Conduction Properties of Valve Metal-Oxide Systems

“…Manuscript received Nov. 29,1964. This paper was presented in part at the Pittsburgh Meeting, April 15-18, 1963.…”

Interface Stoichiometry and Structure in Anodic Niobium Pentoxide

“…While these values are 30–70% higher than most cited in literature (Young, 1960; Schwartz et al, 1961; Kovacs et al, 2003; Stormer et al, 2006), it has been reported previously and noted by Chiou (1971) that the anodization constant can vary with anodization conditions, including hold times, temperature, and electrolyte composition. It should be noted that, using dilute ammonium pentaborate, Habazaki et al (2006) recently achieved anodized films ∼340 nm thick at a potential of 100 V.…”

“…Niobium metal is much more abundant and is chemically similar to tantalum (Nb, Z = 41 and Ta, Z = 73, both are group V transition metals). Both being valve metals (Dignam, 1981), when anodized they form an amorphous pentoxide layer where the thickness is governed by the anodization constant (Nb, ∼0.5–3.0 nm/V (Young, 1960; Kovacs et al, 2003) and Ta, ∼1.6 nm/V (Young, 1960)). Since capacitance is inversely proportional to the thickness of the dielectric layer, the capacitance of a formed anode can be tailored with the anodization voltage.…”

“…This process occurs during low-temperature oxidation as O atoms adsorb onto the metal surface and equilibrium with the metal is reached as electrons quickly tunnel from the metal through the oxide (Cabrera & Mott, 1948). After subsequent oxidation, whether through anodization or thermal treatment, the outer layer is always identified as Nb 2 O 5 (Young, 1960; Hahn & Halama, 1976; Perriere et al, 1978; Grundner & Halbritter, 1980, 1984; Oechsner et al, 1985; Darlinski & Halbritter, 1987 a , 1987 b ; Halbritter & Darlinski, 1987; Dalkaine et al, 1993; Dacca et al, 1998 b ), which is typically amorphous when formed below 500°C (Dalkaine et al, 1993).…”

Electrical Properties of Evaporated Aluminum Oxide Films

“…In recent years anodic oxidation of metal surfaces has been a popular method of preparing thin film capacitors, and the techniques of anodizing various metals have been extensively investigated (1). The oxide dielectric films formed by these methods are generally in the thickness range of 200-500A and appear to have comparatively high breakdown voltages.…”