Anodically formed oxide films on niobium: Microstructural and electrical properties

doi:10.1016/j.jeurceramsoc.2008.10.019

Journal of the European Ceramic Society

Volume 29, Issue 9, June 2009, Pages 1743-1753

https://doi.org/10.1016/j.jeurceramsoc.2008.10.019 Get rights and content

Abstract

The electrical and structural properties of nanoscale niobium pentoxide (Nb₂O₅) dielectric layers in niobium-based solid electrolyte capacitors were studied. The Nb₂O₅ layers are formed by anodic oxidation of Nb-powder compacts. Capacitance measurements show a strong bias-voltage dependence of the capacitance after anodization. Heat treatments at temperatures up to 320 °C, which are applied in the capacitor-production process, lead to an increase of the capacitance and a reduction of the bias dependence. Based on the electrical and structural properties, which are characterized by electron microscopic techniques, a model is presented which explains the behavior of the specific capacitance after the various processing steps.

Introduction

Surface mounted device (SMD) solid electrolyte tantalum capacitors play a major role in the passive components industry due to their high reliability, high volumetric efficiency and low equivalent series resistance.1, 2, 3, 4, 5 Applications comprise for example buffer and smoothing capacitors in the power supplies of plug-in modules in computers. Solid electrolyte tantalum capacitors are fabricated on the basis of the tantalum/tantalum pentoxide (Ta₂O₅) system, where Ta₂O₅ dielectric layers are formed on porous metal powder compacts by anodic oxidation. The capacitance in these capacitor structures is given by C = ɛɛ₀(A/d) with the permittivity of the dielectric ɛ, the vacuum permittivity ɛ₀, the overall area A and the thickness d of the dielectric layer. The proportionality of capacitance and area motivates efforts to increase the overall area A within a specific capacitor form, which makes it an objective to reduce the grain sizes of the powders for capacitor fabrication. Alternatively, higher specific capacitances can be also achieved by substitution of tantalum with ɛ_Ta2O5 = 27 by a material system with allows the formation of a dielectric with higher permittivity. One logical substitute has always been the niobium/niobium pentoxide (Nb₂O₅) system with ɛ_Nb2O5 = 416, 7, 8 which offers in addition the advantage of higher abundance and hence, lower raw material price, as well as similar behavior under anodization. Although the emergence of solid electrolyte niobium capacitors already started in the late sixties of the last century in the former Soviet Union, the lack of sophisticated niobium powders prevented a broader use of niobium capacitors on the world market at that time. However, in order to meet further demands, leading capacitor powder producing companies developed new processes in the meantime which now allow manufacture of capacitor-grade niobium and niobium-based metal powders which meet all requirements for capacitor fabrication.9, 10, 11, 12, 13, 14 As a consequence, major capacitor manufacturers launched production of niobium-based capacitors in 2001 which have the potential to replace tantalum in certain applications.15, 16, 17, 18, 19

One drawback of Nb-based capacitors which is often referred to in the literature is the pronounced temperature sensitivity of the Nb₂O₅ film.20, 21, 22, 23 Since the fabrication process of solid electrolyte capacitors with MnO₂ cathode comprises several production steps at elevated temperatures, the comprehensive understanding of the influence of heat treatments on the electrical properties of nanoscale Nb₂O₅ layers on niobium is essential. One possibility to overcome the thermally driven degradation of the dielectric oxide is doping the niobium powders.²⁴ For example, nitrogen or oxygen was added to the niobium powders in order to modify the oxygen diffusion kinetics, since oxygen diffusion from the dielectric layer into the metal substrate is thought to be one major reason for final capacitor failure. But in spite of these efforts in the fabrication of Nb-based solid electrolyte capacitors, a comprehensive understanding of the relations between processing parameters, microstructure development and resulting electrical properties is still not achieved. It is therefore the goal of the present work to clarify the correlation between heat treatment, microstructure development and electrical performance which is essential for the manufacture of niobium capacitors with stable electrical parameters. We present the results of a detailed characterization of anodically grown nanoscale Nb₂O₅ layers on niobium after anodization and subsequent thermal treatment. Impedance spectroscopy was applied for electrical characterization. Thermogravimetry yields information on the uptake of oxygen during annealing. Detailed microstructural characterization was performed using scanning (SEM) and transmission electron microscopy (TEM). Energy-filtered selected area electron diffraction (SAED) allows the evaluation of nearest neighbor distances within the amorphous dielectric oxide layer. A model will be presented which explains the behavior of the specific capacitance after anodization, annealing treatments at various temperatures and after a second anodization step.

Section snippets

Sample preparation

For sample preparation, pure niobium powders (H.C. Starck Inc.) as described in detail by Schnitter et al.¹⁰ were used. Sample preparation followed the commercial capacitor fabrication process which involves uniaxial die pressing and sintering of capacitor-grade metal powder. During pressing, a tantalum wire was embedded into the green powder compact to ensure electrical contact. After vacuum sintering (p = 10⁻⁴ Pa, 1260 °C), the metal powder compacts had a size of approximately 3 × 1.5 × 4 mm³ and

Bias-dependence of mass-specific charge Q/m_Anode

Measurements of Q/m_Anode as a function of bias voltage (−0.5–15 V) were performed after (i) first anodization, (ii) annealing at 260 °C/1 h or 320 °C/1 h and after (iii) second anodization. Fig. 2 shows the values of specific capacitance of niobium anodes after these processing steps, determined at a fixed frequency of 120 Hz, as a function of applied DC bias voltage.

The electrical measurements reveal a large bias dependence of Q/m_Anode after the first anodization. The values of Q/m_Anode decrease

Conclusion

The influence of thermal annealing on the electrical properties and microstructure of oxidized Nb-anodes has been studied using impedance spectroscopy as well as scanning (SEM) and transmission electron microscopy (TEM). The results indicate, that anodically formed Nb₂O₅ layers on niobium contain oxygen vacancies which act as donors resulting in a n-type semiconducting behavior. The capacitance after anodization is strongly bias-voltage dependent which can be explained by a Schottky diode

Acknowledgments

The authors thank the Bundesministerium für Bildung und Forschung (BMBF) for financial support throughout the work. We are grateful to Dr. Stenzel and Dr. Zillgen, EPCOS AG, Heidenheim for fruitful discussion and for providing some of the samples investigated in this study.

References (47)

B.T. Boiko et al.
Comparison of the degradation modes in sandwich structures including amorphous oxides of niobium and tantalum
Thin Solid Films
(1993)
K.E. Heusler et al.
Electron-transfer reactions at semiconducting anodic niobium oxide films
Electrochimica Acta
(1975)
J.J. Randall et al.
A radiotracer study of the composition and properties of anodic oxide films on tantalum and niobium
Electrochimica Acta
(1965)
W. Lohwasser et al.
Processing and testing of high-cap tantalum capacitors
A. Kobayashi et al.
The history and future development of tantalum capacitors
D. Edson et al.
A new low ESR fused solid tantalum capacitor
R. Hahn et al.
Improved SMT performance of tantalum conductive polymer capacitors with very low ESR
D. Dias et al.
ESR improvement of Ta capacitors by optimisation of cathode formation
A. Shtasel et al.
An investigation of columbium as an electrolytic capacitor metal
J. Electrochem. Soc.
(1961)
N. Schwartz et al.
Niobium solid electrolytic capacitors
J. Electrochem. Soc.
(1961)

H.W. Ling et al.

Niobium solid electrolyte capacitors

J. Electrochem. Soc.

(1962)

Y. Qiu et al.

A novel substrate for solid electrolytic capacitors

C. Schnitter et al.

New niobium based materials for solid electrolyte capacitors

K. Beck et al.

Tuning the properties of tantalum- and niobium oxides; A key to tailor-made materials for passive components

W.A. Serjak et al.

A new dielectric material for capacitors: niobium

Fife, J. A., et al., U.S. Patent No. 6,165,623,...

Shekter, L. N., et al., U.S. Patent No. 6,171,363,...

H. Zillgen et al.

New niobium capacitors with stable electrical parameters

Active & Passive Electronic Components

(2002)

T. Zednicek et al.

Tantalum and niobium technology overview

T. Zednicek et al.

Niobium oxide technology roadmap

S. Zednicek et al.

Extended range NbO capacitors through technology and materials enhancements

S. Zednicek et al.

Low ESR and low profile technology on niobium oxide

D.M. Smyth et al.

The heat-treatment of anodic oxide films on niobium

J. Electrochem. Soc.

(1966)

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Anodically formed oxide films on niobium: Microstructural and electrical properties

Abstract

Introduction

Section snippets

Sample preparation

Bias-dependence of mass-specific charge Q/mAnode

Conclusion

Acknowledgments

Thin Solid Films

Electrochimica Acta

Electrochimica Acta

Processing and testing of high-cap tantalum capacitors

The history and future development of tantalum capacitors

A new low ESR fused solid tantalum capacitor

Improved SMT performance of tantalum conductive polymer capacitors with very low ESR

ESR improvement of Ta capacitors by optimisation of cathode formation

An investigation of columbium as an electrolytic capacitor metal

J. Electrochem. Soc.

Niobium solid electrolytic capacitors

J. Electrochem. Soc.

Niobium solid electrolyte capacitors

J. Electrochem. Soc.

A novel substrate for solid electrolytic capacitors

New niobium based materials for solid electrolyte capacitors

Tuning the properties of tantalum- and niobium oxides; A key to tailor-made materials for passive components

A new dielectric material for capacitors: niobium

New niobium capacitors with stable electrical parameters

Active & Passive Electronic Components

Tantalum and niobium technology overview

Niobium oxide technology roadmap

Extended range NbO capacitors through technology and materials enhancements

Low ESR and low profile technology on niobium oxide

The heat-treatment of anodic oxide films on niobium

J. Electrochem. Soc.

Bias-dependence of mass-specific charge Q/m_Anode