Anodic oxidation of tantalum in water and biological solutions using current limiting constant voltage method

A reliable method is developed for preparing tantalum pentoxide film targets in natural water and biological fluids (urine, blood plasma and serum) by the anodization of tantalum metal using a current limiting constant voltage method. Tantalum pentoxide film targets are successfully prepared at a current density of 10 mA cm⁻² at an anodic voltage ranging from 20 V to 100 V without any oxide breakdown. The results show that for the same applied voltage, more ionic concentration in biological solutions leads to a higher rate of oxide growth than in water and a darker interference color. The analysis shows that anodic oxidation is more likely to breakdown in a biological environment than in pure water for the same oxidation time and applied voltage. The oxide film capacitance is found to be only slightly dependent on pH and anodic voltage with higher capacitive films in biological solutions than for water.     

Anodization of the dental arch wires

Colorful dental arch wires are produced by anodizing the commercial beta-Ti and NiTi arch wires. Electrolytes of sulfuric acid, phosphoric acid, sodium sulfate, and trisodium phosphate are used. The anodization is conducted at a constant applied voltage, 10–60 V, and at a constant temperature 25 °C. The colors of the wires vary with the applied voltages, and wires of a wide spectrum of colors are obtained. The anodized wires are metallographically examined. The compositions and the thickness of the oxide layers are determined by using electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES). TiO₂ layers are found on both kinds of anodized wires, and no nickel element is detected on the surface of the anodized NiTi wire. The colors of the wires are caused by light interferences through the oxide layers. The thickness of the oxide layers increases with the increasing applied voltages. The layer thickness also increases with the anodization time in the beginning, and it reaches a plateau after a certain period of reaction time at a constant applied voltage. The oxide layer is uniformly formed on the anodized beta-Ti wires. However, the surface roughness of the NiTi wires increases after anodization.     

Electrical conduction phenomena in thin tetracene films

Thin film aluminium oxide capacitors using anodic Al₂O₃ as the dielectric are described. The dependence of oxide thickness and dielectric loss on anodization voltage was studied. Variation of capacitance with temperature and frequency was also investigated. The capacitors were used in conjunction with tantalum resistors to fabricate an astable multivibrator circuit and the waveforms were recorded. The combination of aluminium oxide capacitors and tantalum resistors has some advantages over all-tantalum RC networks.     

The nanoporous structure of anodic aluminum oxide fabricated on the Au/Nb/Si substrate

We have studied the anodization behavior of an Al film evaporated on the Au/Nb/Si substrate and demonstrated an effective approach to fabricate the through-hole anodic aluminum oxide (AAO) template on the conducting substrate. The smoothness of the initial metal films and an appropriate wet etching of the oxide film anodized in the first step were found to be critical factors for successfully anodizing the Al film on Au surface. The barrier layer of the obtained AAO structure presented a convex and thinner characteristic, and the underlying Au surface became porous after the anodization. This phenomenon was similar to the case of anodizing the Al film on an ITO glass substrate and could be explained reasonably by the effect of high pressure O₂ gas impelling and H⁺ etching at the interface of the barrier oxide and the Au layer.     

Substrate bias voltage influenced structural, electrical and optical properties of dc magnetron sputtered Ta2O5 films

Tantalum oxide (Ta₂O₅) films were formed on silicon (111) and quartz substrates by dc reactive magnetron sputtering of tantalum target in the presence of oxygen and argon gases mixture. The influence of substrate bias voltage on the chemical binding configuration, structural, electrical and optical properties was investigated. The unbiased films were amorphous in nature. As the substrate bias voltage increased to −50 V the films were transformed into polycrystalline. Further increase of substrate bias voltage to −200 V the crystallinity of the films increased. Electrical characteristics of Al/Ta₂O₅/Si structured films deposited at different substrate bias voltages in the range from 0 to −200 V were studied. The substrate bias voltage reduced the leakage current density and increased the dielectric constant. The optical transmittance of the films increased with the increase of substrate bias voltage. The unbiased films showed an optical band gap of 4.44 eV and the refractive index of 1.89. When the substrate bias voltage increased to −200 V the optical band gap and refractive index increased to 4.50 eV and 2.14, respectively due to the improvement in the crystallinity and packing density of the films. The crystallization due to the applied voltage was attributed to the interaction of the positive ions in plasma with the growing film.     

Nanoscale characterization of anodic oxide films on Ti-6Al-4V alloy

The paper analyses, at nanoscale levels, the chemical composition and mechanical properties of the anodic oxide films formed on Ti-6Al-4V alloy by galvanostatic polarization at maximum final voltages of 12-100 V. For the investigations Auger Electron Spectroscopy, Photoelectron Spectroscopy and nanoindentation measurements have been used. The results have shown that anodizing the Ti-6Al-4V alloy produces an oxide film whose thickness depends on the final voltage. The chemical composition is not significantly dependent on the thickness, the film consists of TiO₂ and Al₂O₃. However, the best insulating properties of the films, determined from the growth parameter nm/V, are achieved with a final voltage between 30 and 65 V. Nanohardness and Young's modulus measurements have shown that the anodic films formed by different voltages exhibit similar mechanical properties which is consistent with the results of the surface analysis.     

Growth of dielectric films on semiconductors and metals using a multipole plasma

A multipole plasma source (a hot electron emitter associated with magnetic confinement by permanent magnets) is very suitable for plasma deposition and anodization because it can create a high density (10¹⁰-10¹¹ cm^-3) homogeneous plasma which is free from energetic particles. The anodization kinetics of metals and semiconductors as well as technological applications of the oxide layers were investigated. Space charge effects in the oxide are shown to control the transport of negative oxygen ions and positive substrate ions during growth. Anodization through a thin CaO-stabilized ZrO₂ (CSZ) film results in strong enhancement in the anodization rates of aluminium, tantalum and silicon, probably because of an alteration in the surface chemistry between the plasma and the oxide. The applications of this process are very attractive: the room temperature plasma anodization of silicon resulting in good quality SiO₂, and the protective filter effect of the CSZ layer.A combination of a multipole source and an ultrahigh vacuum system is described and will be used to study the first steps in the interactions of a surface (mostly GaAs) with a plasma.     

Effects of temperature and voltage mode on nanoporous anodic aluminum oxide films by one-step anodization

Many conventional anodic aluminum oxide (AAO) templates were performed using two-step direct current anodization (DCA) at low temperature (0–5 °C) to avoid dissolution effects. This process is relatively complex. Pulse anodization (PA) by switching between high and low voltages has been used to improve wear resistance and corrosion resistance in barrier type anodic oxidation of aluminum or hard anodization for current nanotechnology. However, there are only few investigations of AAO by hybrid pulse anodization (HPA) with normal-positive and small-negative voltages, especially for the one-step anodization, to shorten the running time. In this article, the effects of temperature and voltage modes (DCA vs. HPA) on one-step anodization have been investigated. The porous AAO films were fabricated using one-step anodization in 0.5 M oxalic acid in different voltage modes including the HPA and DCA and the environment temperature were varied at 5–15 °C. The morphology, pore size and oxide thickness of AAO films were characterized by high resolution field emission scanning electron microscope. The pore size distribution and circularity of AAO films can be quantitatively analyzed by image processing of SEM. The pore distribution uniformity and circularity of AAO by HPA is much better than DCA due to its effective cooling at relatively high temperatures. On the other hand, increasing environment temperature can increase the growth rate and enlarge the pore size of AAO films. The results of one-step anodization by hybrid pulse could promote the AAO quality and provide a simple and convenient fabrication compared to DCA.     

Titanium oxide films produced on commercially pure titanium by anodic oxidation with different voltages

Titanium oxide films produced on commercially pure Ti by anodic oxidation with different voltages were analyzed. Anodic oxidation was carried out at room temperature using 1.4 M H₃PO₄ electrolyte and a platinum counter-electrode, in potentiostatic mode under the following conditions: 50 V, 100 V, 150 V, 200 V and 250 V. It was observed that porous titanium layers were formed at all voltage values but morphological differences were observed. Initially, the film was thin but with increasing voltage it broke down locally and porous regions became evident due to the dielectric breakdown. The porosity and the pore size increased with the increasing voltage. The surface morphology in samples formed with 200 V had substantially different porous structures than those formed with other voltage values. The anodic film surface displayed pores and craters formed on the relatively flat ground oxide surface. AFM images showed that higher voltages produced thicker titanium oxide films.     

Inherent ductility in amorphous Ta2O5 films

Thin films (30 to 80 nm) of refractory tantalum metal were successfully sputter-deposited on uniformly deformable fluoropolymer and polyimide substrates in stress free form. These films were later anodized into amorphous Ta₂O₅ which is a non-porous (barriertype) oxide with excellent corrosion resistant properties. X-ray photo-emission spectroscopy studies were carried out on tantalum and Ta₂O₅ to determine the chemical composition and oxidation states of elements. Thin tantalum and Ta₂O₅ films on fluoropolymer substrates contained fluorine as an impurity while similar films on polyimide substrate contained no fluorine and, in general, fewer impurities. Both thin tantalum films and the corresponding anodic oxides, when deformed in tension to 10% strain, exhibited the expected ductile behaviour of metals where slip bands were observed in the electron microscope. In some cases, minor cracks were observed in the deformed anodic films due to suspected local detachment of the film from the substrate.     

Structure of nanotubular titanium oxide templates prepared by electrochemical anodization in H2SO4/HF solutions

The electrochemical formation of nanotubular titanium oxide films was investigated in 1 M H₂SO₄ and 0.05-0.4 wt.% HF electrolytes. Depending on anodization condition, i.e. cell voltage, anodization time, HF concentration, TiO₂ porous films having different thickness (from 350 to 500 nm) and pore diameter (from 40 to 150 nm) were obtained. By varying the cell voltage from 10 V to 40 V it was possible to gradually change the crystal structure of titanium oxide from anatase to rutile. The effect of annealing temperature and duration on crystal structure was also considered.     

Properties of planar-magnetron-sputtered tantalum films

The effects of sputter conditions on the physical properties of tantalum films deposited from a planar magnetron cathode were investigated using voltages between 250 and 600 V. The films were deposited onto glass and Al₂O₃ ceramic substrates at a rate of 2000 Å min^?1 in argon-nitrogen mixtures, which resulted in a variation of the nitrogen content in the tantalum between zero and 34 at.%. In contrast to the results reported for films sputtered at 200 Å min^?1 and at several kilovolts without a magnetic field only two crystalline phases were observed. Films sputtered without nitrogen had the structure of β-tantalum and the crystallographic properties and temperature coefficient of resistance were found to depend on the sputter voltage and/or argon pressure. A change in film structure from β-tantalum to α-tantalum (body-centred cubic) was observed at about 5 at.% nitrogen. No further change in crystalline structure occurred as the nitrogen content increased to 34 at.% with increasing flow rate. It appears that the magnetron-sputtered tantalum films can accommodate much larger quantities of nitrogen interstitially that films sputtered by conventional high voltage techniques which form the hexagonal close-packed Ta₂N phase above 25 at.% nitrogen.     

The growth and electrical transport properties of self-organized metal/oxide nanostructures formed by anodizing Ta-Al thin-film bilayers

Anodizing of Ta-Al metal bilayers (Al on Ta) sputter-deposited onto SiO₂ substrates was performed in oxalic acid electrolytes at anode potentials of 53 to 21.5 V in order to form nanoporous alumina layers and sequentially oxidize the tantalum underlayers through the alumina pores. The films formed consist of arrays of tantalum oxide nanohillocks percolating through the residual tantalum layer down to the substrate, so that a self-organized network of tantalum nanowires forms between the substrate and the alumina film. The average width (25–<10 nm), length (70–35 nm), and population density (10⁹–10¹¹ cm^-2) of the nanowires are systematically defined by the initial tantalum thickness (8–22 nm) and the anodizing conditions. The mesh-like, nano-sized morphologies of the tantalum underlayers result in a remarkably wide range of potential-dependent, controlled electrical sheet resistances (10²–10⁷ _Ω/sq). The periodical, tunable, metal/insulator film structure, allowing an increased transition to hopping or tunneling conduction at elevated temperature, leads to negative temperature coefficients of resistance, ranging 300 to 5 ppm/K. Oscillations of the potential-dependent dc conductance registered in the films at room temperature are attributed to the quantum-size effects in the metal/oxide nanostructures. The films are of technological importance for fabrication of thin-film, planar, adjustable resistors with significantly improved performances.     

DLTS and conductance transient investigation on defects in anodic tantalum pentoxide thin films

In this work we report on deep level transient spectroscopy (DLTS) and conductance transient measurements (G-t) carried out on films of tantalum oxide fabricated by anodic oxidation of tantalum nitride and tantalum silicide with thickness ranging from 10 to 450 nm. These films exhibit greatly improved leakage currents, breakdown voltage and very low defect density, thus allowing the fabrication of large area capacitors. Leakage currents in the insulator under thermal stress have been carefully studied in order to determine the nature and physical origin of the dominant conduction mechanisms in the insulator. We have found noticeable differences in the dominant conduction mechanisms for thin and thick anodic tantalum pentoxide films. These differences are explained in terms of the thickness dependence of the insulator layer structure. We have characterized the physical nature of the conduction mechanisms in the dielectric films. The Poole–Frenkel effect and the modified Poole–Frenkel effect are suggested. No DLTS signals have been obtained, because transients do not change for temperatures ranging from 77 to 300 K. Conductance transients have important dependencies on voltage bias pulse amplitude and frequency that seem to be closely related to the physical nature of the anodic tantalum pentoxide.     

Preparation of nanoporous titanium oxide films by electrochemical anodic oxidation

We have investigated the conditions of the formation of tubular layers of nanoporous TiO₂ (NPTO) by the anodic oxidation of Ti in a 1% ammonium fluoride solution in ethylene glycol. The results demonstrate that increasing the anode current density and anodization time increases the nanotube diameter. A model has been proposed for the formation of tubular NPTO layers. The model builds on the concept of anisotropic Ti etching. The rate of the formation of the tubular structure of TiO₂ has been shown to be limited by the oxide film growth rate under the conditions of this study.     

Rutherford backscattering investigation of thermally oxidized tantalum on silicon

The Rutherford backscattering technique utilizing 2 MeV He⁺ ions was used for studying Ta and thermally grown Ta oxide films on Si substrates. Significant impurity effects were observed for the as-deposited Ta films and are attributed to gettering during deposition. Partially oxidized Ta films exhibit a surface Ta₂O₅ layer with substantial oxygen incorporation in the underlying Ta film. In contrast with anodic Ta₂O₅ films on tantalum, there is no sharp boundary between Ta and Ta₂O₅. Tantalum oxide films on silicon are, to a first approximation, stoichiometric. Their apparent density, as determined from the areal density of Ta atoms, increases with thickness (from 4.7 to 7.3 g cm^-3) as do their refractive indices. This supports the contention that incorporation of silicon is responsible for these effects and that they are not merely due to a change in stoichiometry.     

Tantalum pentoxide (Ta2O5) thin films for advanced dielectric applications

This paper reviews the present knowledge on tantalum pentoxide (Ta₂O₅) thin films and their applications in the field of microelectronics and integrated microtechnologies. Different methods used to produce tantalum oxide layers are described, emphazing elaboration mechanisms and key parameters for each technique. We also review recent advances in the deposition of Ta₂O₅ in the particular field of microelectronics where high quality layers are required from the structural and electrical points of view. The physical, structural, optical, chemical and electrical properties of tantalum oxide thin films on semiconductors are then presented and essential film parameters, such as optical index, film density or dielectric permittivity, are discussed. After a reminder of the basic mechanisms that control the bulk electrical conduction in insulating films, we carefully examine the origin of leakage currents in Ta₂O₅ and present the state-of-the-art concerning the insulating behaviour of tantalum oxide layers. Finally, applications of tantalum oxide thin films are presented in the last part of this paper. We show how Ta₂O₅ has been employed as an antireflection coating, insulating layer, gate oxide, corrosion resistant material, and sensitive layer in a wide variety of components, circuits and sensors.     

Anodic oxidation and stress corrosion cracking (SCC) of titanium alloys

Uniform and reproducible oxide films were formed on alloy Ti-6Al-6V-2.5Sn by anodic oxidation in aqueous 0.5% H₃BO₃, at 10 mA cm⁻² and voltages up to 110V; dielectric break down occurred above 120V. A parallelism was found between the effect of environmental factors on stress corrosion cracking (SCC), as reported in the literature and the anodic behaviour, as observed by ourselves: factors that increased susceptibility to SCC (increase in temperature or viscosity, alloying, introductions of CI⁻ or methanol, lowering of pH) reduced passivity under anodic polarization, while factors that inhibited SCC (thicker oxide films, high pH, phosphate ions) increased passivity. The passivity was associated both with the presence of the anodic oxide and with a transitory effect of the electric field across the oxide and the oxide-electrolyte interface.     

The effect of an interfacial tantalum oxide layer on interactions in the Ta/Sn system

Liquid Sn is a corrosive agent for tantalum at high temperatures. Oxidation of tantalum is often used to form a surface protective oxide layer in order to improve its chemical resistance in the liquid metal. In this study the stability of tantalum oxide layer, obtained by thermal oxidation and plasma anodization of tantalum, in contact with liquid Sn was examined in the 700–1000 °C temperature range. It was established that the stability of the oxide layer is controlled by its dissolution into the substrate, which takes place along with the formation of non-stable Ta sub-oxides. It was found that the thermal oxide layer may provide sufficient protection against tantalum corrosion by liquid Sn up to 700 °C, whereas the protective layer obtained by the plasma anodization gives an adequate corrosion resistance up to 1000 °C.     

Surface and optical characterization of the porous silicon textured surface

In the present studies, the structural and optical properties of the electrochemically etched PS layers are presented. The formation conditions under constant anodization current density was varied to get a variety of PS samples to analyze the structural and optical characteristics of the porous silicon layers and, then to correlate the resultant surface morphology with the etching process. The low-porosity PS layers thus formed on the silicon substrate have a refractive index value (n_ps = 1.9), which is an intermediate value between bulk silicon substrate (n_Si = 3.4) and air (n_air = 1.0). The results of diffused reflectance, surface morphology by atomic force microscopy (AFM), and Raman scattering measurements show that the resultant surface morphology of the PS layers consist of irregular and randomly distributed nanocrystalline Si structures. The reduction in reflection of the low porosity porous silicon layers is due to light scattering and light trapping of the incoming light by total randomization of the incoming light within the PS structure. The Fourier transform infrared (FTIR) measurements on the PS layer on Si substrate show that PS surface is characterized by chemical species like Si—H and Si—O etc., co-existing on the surface. The presence of hydrogen-related species on the PS layer can provide to some extent a surface passivation effect.