Ta Thin Films Research Articles

A comprehensive study on the surface tribology of Ta thin film using molecular dynamics simulation: The effect of TaN interlayer, power and temperature

Molecular dynamics simulations (MD) was carried out to investigate the surface nano-tribology of grown Ta sputtered thin films on stainless steel with and without TaN interlayer. Ta and TaN films were deposited by magnetron sputtering technique at different temperatures. Argon and nitrogen mixture gas was used to create TaN interlayer. Using AFM technique, the surface of sputtered thin films was characterized. MD simulation results were validated by experimental results and a good consistency was observed the two techniques. The effects of TaN interlayer and process parameters were investigated. The results showed that the surface roughness can be remarkably improved by using a TaN seed layer. Higher sputtering powers at lower temperatures provide the best condition to produce smoother Ta films.

Highly anisotropic etching of Ta thin films using high density plasmas of halogen based gases

Inductively coupled plasma reactive ion etching of Ta thin films masked with photoresist was performed using C2F6/Ar, HBr/Ar and Cl2/Ar gases. The etch characteristics such as etch rate, etch selectivity and etch profile were investigated in different gas concentrations of each gas. The Cl2 chemistry showed high degree of anisotropy in etch profile as well as fastest etch rate and highest etch selectivity of Ta films among these gases. Optical emission spectroscopy revealed that the increase in chlorine radicals with increasing Cl2 concentration was responsible for the increase in the Ta etch rate. The etch rate of Ta films increased with increasing ICP rf power and dc-bias voltage, and decreasing process pressure. Good etch profiles were obtained at high rf power and dc-bias voltage, and low process pressure. X-ray photoelectron spectroscopy confirmed the existence of a chemical reaction of Ta with chlorine radicals by forming tantalum chlorides. These results suggested that Ta etching in Cl2/Ar follows the typical reactive ion etching mechanism. It was concluded that a high degree of anisotropy and high etch rate of Ta films were achieved using Cl2/Ar gas.

Phase tailoring of tantalum thin films deposited in deep oscillation magnetron sputtering mode

The effect of energetic ion bombardment on the properties of tantalum thin films was investigated. To achieve such energetic ion bombardment during the process the Ta thin films were deposited by deep oscillation magnetron sputtering (DOMS), an ionized physical vapor deposition technique related to high power impulse magnetron sputtering. The peak power was between 49 and 130kW and the substrate was silicon at room temperature and ground potential. The directionality and the energy of the depositing species was controlled by changing the ionization fraction of the Ta species arriving at the substrate at different peak powers. In this work, the surface morphology (AFM), microstructure (SEM), structure (XRD) and hardness and Young's modulus (nanoindentation) of the films were characterized. The ion energy distributions (IEDs) were measured using an electrostatic quadrupole ion energy and mass spectrometer (HIDEN EQP 300). The IEDs showed that the DOMS process applies a very energetic (up to 120eV) ion bombardment on the growing tantalum films. Therefore, with such conditions it was possible to deposit pure α-Ta (of 2μm of thickness) without the use of additional equipment, i.e., without substrate bias or substrate heating. Conditions are therefore significantly different than in previous works, offering a much simpler and cheaper solution to up-scale for industrial operation.

Analysis of the thermal and temporal stability of Ta and Ti thin films onto SAW–substrate materials (LiNbO3 and LiTaO3) using AR‐XPS

Modern surface‐acoustic‐wave devices are characterised by their trend to higher frequencies, power densities, and new applications. For this, a shrinking of the dimensions is necessary but hardly possible to achieve with standard Al‐based metallisation because of increased stress‐induced damaging (acoustomigration). In this context additional barrier interlayers between substrates and electrodes are necessary, especially for high‐temperature applications. For this, we present results of a detailed chemical interface analysis of deposited Ta and Ti thin‐film growth onto surface acoustic wave–substrate materials (LiNbO3 and LiTaO3) with respect to their interface layer structure and its temporal and thermal stability in vacuum. The analysis was mainly performed using non‐destructive and surface sensitive angle‐resolved X‐ray photoelectron spectroscopy. Besides a slight oxidation (oxygen adsorption) at the surface of these highly reactive metal thin films all interface sequences are stable up to 24 h. We also report higher thermal stability of Ta thin films up to 600 °C in contrast to Ti thin films (300 °C) because of presumed diffusion of Ti along grain boundaries. Copyright © 2016 John Wiley & Sons, Ltd.

Hanle Magnetoresistance in Thin Metal Films with Strong Spin-Orbit Coupling.

We report measurements of a new type of magnetoresistance in Pt and Ta thin films. The spin accumulation created at the surfaces of the film by the spin Hall effect decreases in a magnetic field because of the Hanle effect, resulting in an increase of the electrical resistance as predicted by Dyakonov [Phys. Rev. Lett. 99, 126601 (2007)]. The angular dependence of this magnetoresistance resembles the recently discovered spin Hall magnetoresistance in Pt/Y(3)Fe(5)O(12) bilayers, although the presence of a ferromagnetic insulator is not required. We show that this Hanle magnetoresistance is an alternative simple way to quantitatively study the coupling between charge and spin currents in metals with strong spin-orbit coupling.

Spectroscopic ellipsometry for compositionally induced bandgap tuning of combinatorial niobium–tantalum anodic oxides

Spectroscopic ellipsometry maps optical properties of anodic oxides grown on Nb–Ta thin film combinatorial library. The bandgap values deviate from the linear model based on mixed matter theory due to an in-depth compositional gradient of the oxides.

Oxynitrogenography: Controlled Synthesis of Single-Phase Tantalum Oxynitride Photoabsorbers

We propose an in situ UV–vis monitoring technique called “oxynitrogenography” as an approach toward the controlled and reproducible synthesis of thin films of different Ta–O–N phases, including the elusive β-TaON phase. The optical absorption changes are measured during annealing of the film at increasing NH3/H2O ratios, and can be directly correlated to the presence of different phases (Ta2O5, β-TaON, mixed TaON-Ta3N5, Ta3N5) due to the abrupt change in the absorption edge. After additional XRD analysis, the thermodynamic equilibrium conditions to obtain these various phases are determined, and a phase diagram is constructed. We observe that there is a very narrow range of parameters for the thermodynamic stability of the β-TaON phase. We observe that the carrier mobility increases with the nitrogen content in the sample, from 1 × 10–5 cm2/(V s) in Ta2O5, to 1 × 10–2 cm2/(V s) in β-TaON and the mixed TaON-Ta3N5, until 1 × 10–1 cm2/(V s) in Ta3N5. Although the carrier mobility of β-TaON and Ta3N5 is compa...

Synthesis of a ZnO nanorod/CVD graphene composite for simultaneous sensing of dihydroxybenzene isomers

A few-layered graphene nanosheets on a Ta thin film were synthesized by a direct current (DC) arc plasma jet chemical vapor deposition (CVD) process that does not critically depend on cooling rates. The thickness of the graphene layer is about 20 μm, and the graphene sheets superimposed transversely on the outermost layer become a blooming bouquet. This structure is especially suitable as an electrode sensor. The ZnO nanorods have excellent electrochemical performance, and thus we combined ZnO nanorods and CVD graphene for a ZnO/graphene/Ta sensing electrode. The electrochemical oxidation of hydroquinone (HQ), catechol (CC), and resorcinol (RC) were investigated using cyclic voltammetry at the ZnO/graphene/Ta electrode. The results showed that it was feasible to simultaneously measure HQ, CC, and RC. The linear response ranges for HQ, CC, and RC were 0–70, 0–80, and 0–700 μmol L−1. The detection limits (S/N = 3) were 0.1, 0.2, and 1 μmol L−1, respectively. The ZnO/graphene layer shows high electrocatalytic activity toward the redox reaction of HQ, CC, and RC. This activity had high selectivity, high sensitivity and low detection limits.

In vitro detection of small molecule metabolites excreted from cancer cells using a Tenax TA thin-film microextraction device.

We developed a new device for the in vitro extraction of small molecule metabolites excreted from cancer cells. The extraction device, which was biocompatible and incubated with cancer cells, consists of a thin Tenax TA film deposited on the surface of a cylindrical aluminum rod. The Tenax TA solid phase was utilized for the direct extraction and preconcentration of the small molecule metabolites from a cell culture sample. The device fabrication and the metabolite extraction were optimized, tested, and validated using HeLa cell cultures. Comparison of metabolic profiles with the control measurement from the culture medium enabled detection of metabolites that were consumed or produced by the cell culture. Tentative identification and semi-quantitative investigation of the excreted metabolites were performed by GC-MS analysis. The proposed approach can be a valuable tool for the characterization of low-volatile cancer cell metabolites that are not covered by use of conventional methods based on headspace solid phase microextraction.

On spinodal decomposition in Cu–34 at.% Ta thin films – An atom probe tomography and transmission electron microscopy study

In metals and alloys, phase separation takes place either by nucleation and growth or by spinodal decomposition. Here, transmission electron microscopy (TEM) and atom probe tomography (APT) are combined in an effort to study phase separation in an immiscible Cu–34at.% Ta thin film alloy on a nanometer scale. Upon annealing at 400–600°C the initially X-ray amorphous metastable alloy phase separates and crystallizes into a Cu-rich phase with fcc structure and a Ta-rich phase with β-Ta structure. While the fcc phase crystallizes already upon 10min annealing at 400°C, first indications of a crystalline β-Ta phase are observed after 30min annealing at 400°C, but this phase does not become fully crystalline at temperatures below 600°C. Proximity histograms of the APT data show that phase separation is taking place predominantly by diffusion of Cu, which is in accordance with the known relative diffusivities of the materials. An increase in the amplitude of the composition fluctuation and a decrease in the width of the interface between Cu-rich and Ta-rich regions are observed in the proximity histograms as phase separation progresses. These results suggest spinodal decomposition as the mechanism of phase separation.

Gilbert damping constants of Ta/CoFeB/MgO(Ta) thin films measured by optical detection of precessional magnetization dynamics

Gilbert damping constants of Ta/CoFeB/MgO(Ta) thin films measured by optical detection of precessional magnetization dynamics

Interplay between iridescent and non-iridescent coloration in bio-inspired electrically-tunable nanostructures

This work is inspired by biological nanostructured surfaces possessing structural color generation and wettability control. Nanostructured films consisting of a tantalum pentoxide (Ta2O5) nanopost array (nanograss) formed on top of a continuous Ta2O5 layer on a reflective Ta thin-film have been fabricated and investigated. A non-iridescent coloration typical for short-range order was produced by the nanograss while iridescent coloration was produced by the underlying continuous Ta2O5 layer. When the nanograss surface is wetted with organic liquids, a different non-iridescent blue color appears. Moreover, this blue color can be dynamically and reversibly switched on and off by applying electrical current to an indium-tin-oxide electrode.

Controlling the Structural and Optical Properties of Ta3N5 Films through Nitridation Temperature and the Nature of the Ta Metal

The development of a reliable synthetic route to produce high performance Ta3N5 photoanodes has been complicated by the large number of synthetic parameters, notably nitridation conditions. A systematic study of nitridation from 850 °C–1000 °C reveals that, contrary to common knowledge, nitridation temperature has little effect on the quality of the Ta3N5 produced. Rather, it is the nature of the tantalum starting material and substrate that play a key role. Ta3N5 films synthesized by thermal oxidation and subsequent nitridation of Ta thin films on inert fused silica substrates exhibit identical structural and optical properties, regardless of preparation temperature. The optical spectra collected on these samples reveal clear, distinct features that give insight into the electronic band structure. Films grown in the same manner on Ta foils, however, reveal that textured Ta2N is formed at the Ta3N5/Ta interface even at low temperature, as shown by grazing incidence X-ray scattering. Ta3N5 on Ta foils is c...

Measurements of the Optical Performance of Prototype TES Bolometers for SAFARI

We have measured the optical response of prototype detectors for SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI’s three bolometer arrays, coupled with a Fourier transform spectrometer, will provide images of a 2′×2′ field of view with spectral information over the wavelength range 34–210 μm. Each horn-coupled bolometer consists of a transition edge sensor (TES), with a transition temperature close to 100 mK, and a thin-film Ta absorber on a thermally-isolated silicon nitride membrane. SAFARI requires extremely sensitive detectors (NEP∼2×10−19 W/ $\sqrt{\mathrm{Hz}}$ ), with correspondingly low saturation powers (∼5 fW), to take advantage of SPICA’s cooled optics. To meet the challenge of testing such sensitive detectors we have constructed an ultra-low background test facility based on a cryogen-free high-capacity dilution refrigerator, paying careful attention to stray-light exclusion, shielding, and vibration isolation. For optical measurements the system contains internal cold (3–30 K) and hot (∼300 K) black-body calibration sources, as well as a light pipe for external illumination. We discuss our measurements of high optical efficiency in prototype SAFARI detectors and describe recent improvements to the test facility that will enable us to test the full SAFARI focal-plane arrays.

Control of stress in tantalum thin films for the fabrication of 3D MEMS structures

Sputtered tantalum (Ta) thin-films subjected to high compressive stress have been used for fabricating out-of-plane microelectromechanical systems (MEMS) clamped–clamped beams. The as-deposited film stress has been investigated as a function of sputtering conditions (pressure and power) in order to determine the optimal deposition parameters for achieving a compressive state. A variation of the as-deposited film stress has been observed as a function of time at atmospheric conditions and reduced by ∼85% using annealing in oxygen atmosphere at 100 °C. By surface-micromachining Ta thin-films deposited in compressive state (beyond the critical buckling load), buckled out-of-plane beams have been fabricated with lengths in the range of 500 μm−5.8 mm and central out-of-plane deflection up to ∼60 μm. A final stress of ∼−850 MPa has been calculated from the measured beams' deflection. The modal behavior of the buckled out-of-plane beams has been investigated with optical measurements and finite element simulations. The large curvature and high residual stress experienced by the beams have shown to affect the usual order of consecutive bending mode shapes. The devices presented can be utilized for implementing cutting-edge three-dimensional MEMS.

Pd/Ta2O5/SiC Schottky-diode hydrogen sensors formed by using rapid thermal oxidation of Ta thin films

Pd/Ta2O5/SiC Schottky-diode hydrogen sensors were fabricated, and their hydrogen gas sensing performance was investigated at 573 K and 773 K. Interfacial Ta2O5 films of 120 nm in thickness were formed by using rapid thermal oxidation (RTO) of the sputtered Ta films on SiC. The crystallinity of the Ta and the Ta2O5 films were characterized by using X-ray diffraction (XRD). As-sputtered Ta films on 4H-SiC are composed of α-Ta (body-centered-cubic) and β-Ta (tetragonal), and α-Ta (110) is the dominant orientation. After RTO at 573 K, the Ta films are converted to β-Ta2O5 (orthorhombic). The diode sensors show high sensitivity to H2 even at the low H2 concentration of 500 ppm, and the voltage change of the sensor upon H2 exposure is proportional to the H2 concentration in the range of 500 ∼ 2000 ppm at 573 K. The response voltage ΔV is shown to arise mostly from the change in the series resistance component of the sensor upon H2 exposure; the main origin of that change is believed to be the Ta2O5 interfacial layer. The response time t90 of the sensor at 573 K was estimated to be approximately 8 s.

In situ electrical resistance and activation energy of solid C60 under high pressure

The in situ electrical resistance and transport activation energies of solid C60 fullerene have been measured under high pressure up to 25 GPa in the temperature range of 300–423 K by using a designed diamond anvil cell. In the experiment, four parts of boron-doped diamond films fabricated on one anvil were used as electrical measurement probes and a W—Ta thin film thermocouple which was integrated on the other diamond anvil was used to measure the temperature. The current results indicate that the measured high-pressure resistances are bigger than those reported before at the same pressure and there is no pressure-independent resistance increase before 8 GPa. From the temperature dependence of the resistivity, the C60 behaviors as a semiconductor and the activation energies of the cubic C60 fullerene are 0.49, 0.43, and 0.36 eV at 13, 15, and 19 GPa, respectively.

Resistance Switching in Metal Oxide Thin Films and its Memory Application

We explore the unipolar resistance switching effect in sandwich structures based on Nb, Ta and Zr oxide thin films. The structures were fabricated by pulsed laser deposition and low temperature anodic oxidation methods. After electroforming process memory cells demonstrate reproducible switching between low and high resistance states with a resistance ratio around 102. Nonvolatile resistance storage was traced within 40 days. The low-temperature anodic oxidation of Nb was found to be suitable to fabricate flexible nonvolatile memory elements. The parameters of resistive switching are not degraded after 100000 flexing.

Effects of Magnetic Fields on Highly Sensitive TiAu TES Bolometers

In the process to develop the transition edge sensor (TES) bolometer technology for the SAFARI FIR Imaging Spectrometer on the SPICA telescope and for future FIR space telescopes, we have recently demonstrated a TiAu TES with a Ta thin film absorber on a thin SiN membrane, which has a dark NEP as low as 4.2 × 10-19 W/√Hz. It is known that superconductors are sensitive to magnetic fields. Therefore, it is important to know how these extremely sensitive TES detectors are affected by external magnetic fields. This is desirable from an application point of view since the design of the SAFARI instrument demands such knowledge and interesting from a fundamental point of view. In this paper, we will describe a detailed experimental study of the effects of magnetic fields, both parallel and perpendicular to the TiAu plane, on the performance of a TES. We investigated the effect on the current, responsivity, speed, and TES parameters of α (dependence of temperature) and β (dependence of current), around the operating points. In order to achieve this highly sensitive measurement, the temperature stability has been improved and a new coil assembly has been designed. The latter allows accurately applying a parallel magnetic field.

Effects of preparation conditions on the magnetocaloric properties of Gd thin films

The impact of the deposition temperature and chamber gettering on Ta(5 nm)/Gd(30 nm)/Ta(5 nm) thin films magnetocaloric effect properties was investigated. Increasing the deposition temperature generally improves the entropy peak (magnitude, full width at half max, and temperature of the peak) but also leads to significant oxidation. Gettering the chamber prior to deposition not only reduced this oxidation issue but also increased the relative cooling power of films grown at elevated temperatures by as much as 33% over ungettered samples.