Sputtered Tantalum Oxide Films Research Articles

Preparation of aluminium doped Ta3N5 films via nitridation of sputtered tantalum oxide films

A multi-step synthetic method is presented for the synthesis of novel aluminium doped Ta3N5 films. Two mechanisms for the incorporation of aluminium into Ta3N5 are proposed and the effect of aluminium doping on the as-deposited tantalum oxides and annealed Ta3N5 films is investigated with XRD, EDS, SEM and SIMS analysis. These results are contrasted with previous work detailing the incorporation of Cr into Ta3N5 and it is observed that aluminium does not reach a solubility limit in Ta3N5 under the conditions explored. The preferred processing regimes for synthesizing Al–Ta3N5 films as p-type Ta3N5 photo-electrode candidates are outlined.

Chromium doping of Ta3N5 thin films via thermal nitridation of sputtered tantalum oxide films

The effects of chromium doping on magnetron sputtered and thermally annealed Ta3N5 thin films have been investigated for the first time. The structural and compositional evolution of these films were closely investigated using XRD, SEM and SIMS. A simple mechanism for the incorporation of chromium into the Ta3N5 lattice was described based on kinetic experiments and SIMS analysis. Some basic recommendations can be made regarding the optimal method of preparing Cr–Ta3N5, however there is still much work to be done in characterising Cr–Ta3N5 with appreciable properties. In particular, this investigation indicates poor Cr solubility in Ta3N5 in spite of the favourable ionic radius.

Surface chemistry and cytotoxicity of reactively sputtered tantalum oxide films on NiTi plates

NiTi, an equiatomic alloy containing nickel and titanium, exhibits unique properties such as shape memory effect and superelasticity. NiTi also forms a spontaneous protective titanium dioxide (TiO2) layer that allows its use in biomedical applications. Despite the widely perceived biocompatibility there remain some concerns about the sustainability of the alloy's biocompatibility due to the defects in the TiO2 protective layer and the presence of high amount of sub-surface Ni, which can give allergic reactions. Many surface treatments have been investigated to try to improve both the corrosion resistance and biocompatibility of this layer. For such purposes, we have sputter deposited tantalum (Ta) oxide thin films onto the surface of the NiTi alloy. Despite being one of the promising metals for biomedical applications, Ta, and its various oxides and their interactions with cells have received relatively less attention. The oxidation chemistry, crystal structure, morphology and biocompatibility of these films have been investigated. In general, reactive sputtering especially in the presence of a low oxygen mixture yields a thicker film with better control of the film quality. The sputtering power influenced the surface oxidation states of Ta. Both microscopic and quantitative cytotoxicity measurements show that Ta films on NiTi are biocompatible with little to no variation in cytotoxic response when the surface oxidation state of Ta changes.

High-acoustic-impedance tantalum oxide layers for insulating acoustic reflectors

This work describes the assessment of the acoustic properties of sputtered tantalum oxide films intended for use as high-impedance films of acoustic reflectors for solidly mounted resonators operating in the gigahertz frequency range. The films are grown by sputtering a metallic tantalum target under different oxygen and argon gas mixtures, total pressures, pulsed dc powers, and substrate biases. The structural properties of the films are assessed through infrared absorption spectroscopy and X-ray diffraction measurements. Their acoustic impedance is assessed by deriving the mass density from X-ray reflectometry measurements and the acoustic velocity from picosecond acoustic spectroscopy and the analysis of the frequency response of the test resonators.

X-ray analyses of thermally grown and reactively sputtered tantalum oxide films on NiTi alloy

Sputter deposition of tantalum (Ta) on the surface of NiTi alloy is expected to improve the alloy’s corrosion resistance and biocompatibility. Tantalum is a well-known biomaterial which is not affected by body fluids and is not irritating to human tissue. Here we compare the oxidation chemistry crystal structure evolution of tantalum oxide films grown on NiTi by reactive O2 sputtering and by thermal oxidation of sputter deposited Ta films. The effect of sputtering parameters and post-sputtering treatments on the morphology, oxidation state and crystal structure of the tantalum oxide layer have been investigated by field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The study has found that it may be better to avoid oxidation at and above 600°C. The study establishes that reactive sputtering in presence of low oxygen mixture yields thicker film with better control of the film quality except that the surface oxidation state of Ta is slightly lower.

Structural and electrical properties of radio frequency magnetron sputtered tantalum oxide films: Influence of post-deposition annealing

The as-deposited and annealed radio frequency reactive magnetron sputtered tantalum oxide (Ta 2O 5) films were characterized by studying the chemical binding configuration, structural and electrical properties. X-ray photoelectron spectroscopy and X-ray diffraction analysis of the films elucidate that the film annealed at 673 K was stoichiometric with orthorhombic β-phase Ta 2O 5. The dielectric constant values of the tantalum oxide capacitors with the sandwich structure of Al/Ta 2O 5/Si were in the range from 14 to 26 depending on the post-deposition annealing temperature. The leakage current density was <20 nA cm −2 at the gate bias voltage of 0.04 MV/cm for the annealed films. The electrical conduction mechanism observed in the films was Poole–Frenkel.

Effect of substrate temperature on the structural, optical and electrical properties of dc magnetron sputtered tantalum oxide films

dc reactive magnetron sputtering technique was employed for deposition of tantalum oxide films on quartz and silicon substrates by sputtering of pure tantalum target in the presence of oxygen and argon gases under various substrate temperatures in the range 303–973 K. The variation of cathode potential with the oxygen partial pressure was systematically studied. The influence of substrate temperature on the chemical binding configuration, crystal structure and optical properties was investigated. X-ray photoelectron spectroscopic studies indicated that the films formed at oxygen partial pressures ≥1 × 10 −4 mbar were stoichiometric. The Fourier transform infrared spectroscopic studies revealed that the films formed up to substrate temperatures <673 K showed a broad absorption band at 750–1000 cm −1 and a sharp band at 630 cm −1 indicated the presence of amorphous phase while at higher substrate temperatures the appearance of bands at about 810 and 510 cm −1 revealed the polycrystalline nature. The effect of substrate temperature on the electrical characteristics of Al/Ta 2O 5/Si structure was investigated. The dielectric constant values were in the range 17–29 in the substrate temperature range of 303–973 K. The current–voltage characteristics showed modified Poole–Frenkel conduction mechanism with a tendency for reduction of the compensation level. The optical band gap of the films decreased from 4.44 to 4.25 eV and the refractive index increased from 1.89 to 2.25 with the increase of substrate temperature from 303 to 973 K.

Heat treatment induced structural and optical properties of rf magnetron sputtered tantalum oxide films

AbstractRf magnetron sputtering technique was employed for preparation of tantalum oxide films on quartz and crystalline silicon (111) substrates held at room temperature by sputtering of tantalum in an oxygen partial pressure of 1x10‐4 mbar. The films were annealed in air for an hour in the temperature range 573 – 993 K. The effect of annealing on the chemical binding configuration, structure and optical absorption of tantalum oxide films was systematically studied. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effects of substrate temperature on properties of pulsed dc reactively sputtered tantalum oxide films

The effects of substrate heating on the stoichiometry and the electrical properties of pulsed dc reactively sputtered tantalum oxide films over a range of film thickness (0.14to5.4μm) are discussed. The film stoichiometry, and hence the electrical properties, of tantalum oxide films; e.g., breakdown field, leakage current density, dielectric constant, and dielectric loss are compared for two different cases: (a) when no intentional substrate/film cooling is provided, and (b) when the substrate is water cooled during deposition. All other operating conditions are the same, and the film thickness is directly related to deposition time. The tantalum oxide films deposited on the water-cooled substrates are stoichiometric, and exhibit excellent electrical properties over the entire range of film thickness. “Noncooled” tantalum oxide films are stoichiometric up to ∼1μm film thickness, beyond that the deposited oxide is increasingly nonstoichiometric. The presence of partially oxidized Ta in thicker (&amp;gt;∼1μm) noncooled tantalum oxide films causes a lower breakdown field, higher leakage current density, higher apparent dielectric constant, and dielectric loss. The growth of nonstoichiometric tantalum oxide in thicker noncooled films is attributed to decreased surface oxygen concentration due to oxygen recombination and desorption at higher film temperatures (&amp;gt;∼100°C). The quantitative results presented reflect experience with a specific piece of equipment; however, the procedures presented can be used to characterize deposition processes in which film stoichiometry can change.

Electrical characterisation of RF sputtered tantalum oxide films rapid thermal annealed with Ar, N 2, O 2 and N 2O

The effects of annealing ambient (Ar, N 2, O 2 and N 2O) and durations (60 and 200 s) of rapid thermal annealing (RTA) on the insulating property of RF sputtered tantalum pentoxide films were examined. It was found that films sputtered at 200 W were more leaky than those sputtered at 100 W. It was also discovered that RTA in O 2 and N 2O and at a longer duration (200 s) improved the insulating property of the films. This improvement was suggested to be due to a layer of SiO 2 formed at the Si-Ta 2O 5 interface when annealed. The conduction mechanism for the O 2 and N 2O annealed films was found to be of Poole-Frenkel type.

Rapid thermal annealing of reactive sputtered tantalum oxide films on GaAs in N2O atmosphere

The electrical properties of composite oxide (anodized oxide and tantalum oxide deposited by reactive sputtering) on GaAs annealed in N2O were studied. The leakage current was decreased and at moderate electric field strengths it is governed by Schottky emission. The breakdown electric field of the samples and the dielectric constant of tantalum oxide were increased after annealing.

Ejection angle dependence of electrical properties of ion beam sputtered tantalum oxide films

Ion beam sputtering was used to deposit tantalum oxide films on silicon substrates at different positions surrounding a Ta2O5 target. dc conductance of the films shows dependence on the ejection angle of sputtered particles and primary ion energy. Capacitance-voltage characteristics also vary by changing the primary ion energy. These variations of electrical properties are discussed by considering the effect of anisotropic ejection of the sputtered energetic particles.

Charge trapping/generation and reliability for high-performance tantalum oxide capacitors

Charge trapping/generation behavior and reliability of recently developed high-performance sputtered/anodized tantalum oxide films in structures of Al–tantalum-oxide–Ta metal-insulator-metal (MIM) capacitors under constant current stress are studied by observing the voltage-time and time-to-breakdown characteristics in comparison with those of the usual tantalum oxide MIM capacitors prepared by either anodization or reactive sputtering. The dominant charge trapping/generation in the anodized tantalum oxide films is either slow electron trapping under current stress on the Ta bottom electrodes or positive charge generation under current stress on the Al top electrodes. Positive charge generation due to oxygen vacancies after fast electron trapping is dominant in the sputtered tantalum oxide films which show defect-related breakdown phenomena. The sputtered/anodized tantalum oxide demonstrates much superior reliability because of its less charge trapping/generation and better enduring property under charge injection compared with the anodized or the sputtered tantalum oxide films.

Correlation of sputtering conditions with electronic conduction in Ta2O5 films

Sputtered tantalum oxide films usually exhibit very high dc leakage currents and are not suitable for making thin‐film capacitors. In this paper, a process is described in which rf‐sputtered tantalum oxide films have low dc leakage currents approaching those of liquid‐phase‐anodized films. The conduction characteristics of the sputtered films can be described by a modified Simmons type model. The sputtering parameters which control the conduction characteristics of these films are discussed.

Photoconduction and trapping in sputtered tantalum oxide films : F. G. Ullman, J. Phys. Chem. Solids28 (1967), p. 279

Photoconduction and trapping in sputtered tantalum oxide films : F. G. Ullman, J. Phys. Chem. Solids28 (1967), p. 279

Photoconduction and trapping in sputtered tantalum oxide films

Electron trapping in tantalum oxide films, prepared by reactive sputtering onto polished, fused-silica substrates, has been studied by measurements of photocurrents and thermallystimulated-currents. The results indicate the presence of a single set of traps situated about 0.25 eV from the nearest band edge. The initial trap concentration depends on the tantalum sputtering rate and the oxygen partial pressure during film preparation. Heating in a vacuum increases the trap concentration; heating in air or several days exposure to air at room temperature reduces it. Increasing the trap concentration increases both the steady-state current during optical excitation, and the time to rise to and decay from the steady state on initiation and removal of excitation, respectively. A model is proposed in which the empty traps are oxygen vacancies occupied by one electron. Once filled by optical excitation, the traps are proposed to behave like donors in quasi-thermal equilibrium with the conduction band. The rate of conductivity decay, on removal of excitation, is thus determined by the relative rates of retrapping and recombination which, in turn, depend on the relative concentrations and cross-sections of traps and recombination centers.