Tantalum Diffusion Research Articles

Improvement of microstructure and acquirement of excellent mechanical properties of high-Nb Ti2AlC/TiAl alloy by Ta under ultrasonic action in solid-liquid zone

To promote element diffusion in the solid-liquid two-phase region, optimize solidification microstructure, and improve the matching degree of strength and toughness, Ti–46Al–8Nb-2.6C-xTa (at. %) alloys were prepared by arc melting under the action of ultrasound. The lamellar colony, precipitate phase, elements distribution, and related mechanisms were analyzed. Results show that the relative content of the γ phase decreases, while the relative content of the α2 phase and Ti2AlC phase increases as the addition of Ta increases. The coarse long rod-shaped Ti2AlC transforms into short rod-shaped particles. The lamellar colony size reduces from 32.1 to 18.6 μm, the lamellar spacing reduces from 1.4 to 0.7 μm, and the aspect ratio of Ti2AlC reduces from 5.8 to 2.5 as the addition of Ta increases from 0 to 1.0 at. %. The acoustic streaming effect of ultrasound promotes the diffusion of tantalum in the solid-liquid two-phase region for a certain ultrasonic processing time. Tantalum is a heterogeneous nucleation particle of Ti2AlC particles and causes composition supercooling at the forefront of the solidification interface, which is the main reason for refining the microstructure. Compressive strength increases from 1996 to 2596 MPa and the strain increases from 25.4 to 29.3% as the addition of Ta increases from 0 to 0.6 at. %. The main reasons for improving the properties of alloy are the refinement of the microstructure and the formation of a large number of dislocations.

Sintered Fe-Co-Si alloy with excellent magnetic and mechanical properties by coherent precipitation of submicrometer-sized carbide particles

A novel sintered Fe-Co-Si alloy with coherent precipitation of submicrometer-sized carbide particles of Ta3Co3C containing iron and silicon is fabricated by a unique synthesis route through evaporation of an Fe-Co-Si matrix and diffusion of tantalum and carbon by sintering in high vacuum at a high temperature. The carbide particles precipitate homogeneously on the surface, in the grain boundaries, and inside the grains of the matrix. In particular, many particles precipitate homogeneously along the grain boundaries, which can contribute to improving mechanical strength by preventing intergranular fracture in spite of very small volume fraction (0.8 vol%) of the precipitated particles. The strength improvement can be explained by the particle shearing mechanism by the coherent precipitation of carbide particles. The orientation relationships between the carbide particles and Fe-Co-Si matrix are (4̅04)particle//(2̅02)matrix, (04̅0)particle//(02̅0)matrix, and (4̅00)particle//(2̅00)matrix with a very small lattice mismatch (<2%). As a result of the coherent precipitation, the magnetic properties such as low magnetic loss and high magnetic permeability does not deteriorate, and are rather improved mainly by a promoted densification with very few pores and grain growth by sintering in high vacuum at a high temperature. It is also demonstrated that the fabricated alloy has excellent magnetic and mechanical properties compared to conventional sintered or compressed powder materials.

ВЛИЯНИЕ ЭЛЕКТРОХИМИЧЕСКОЙ ПОЛИРОВКИ ТАНТАЛА НА ДИЭЛЕКТРИЧЕСКИЕ ПАРАМЕТРЫ АНОДНОГО ОКСИДА

Изучено анодное поведение Та-электрода в 0.1 M водном растворе H3PO4 после стадии электрохимической полировки поверхности металла. Установлены оптимальные условия проведения процесса электрохимической полировки в электролите HF:C3H7OH:H2SO4. На основе анализа циклических вольтамперных кривых сделано заключение, что эффективность процесса формирования анодного оксида тантала близка к 100 %. Данные импедансной спектроскопии подтверждают образование оксидной пленки с высокими диэлектрическими характеристиками. Проведен расчет диэлектрической проницаемости сформированного оксида, дана оценка фактора шероховатости поверхности тантала после ее электрохимической полировки.

Mechanism of instability of carbides in Fe–TaC alloy under high energy electron irradiation at 673 K

Reduced activation ferritic/martensitic steels (RAFMs), such as F82H steel, are designed to enhance the high-temperature strength by formation of MX-type nanometer-scale precipitates, mainly TaC. However, their instability under irradiation was recently reported. The purpose of this work, therefore, is to clarify the mechanism employing simultaneous observations under electron irradiation at elevated temperature in a high voltage electron microscope. In this work, Fe-0.2wt.% TaC was fabricated as a model alloy of F82H steel. The instability of the precipitates was observed under electron irradiation at 1MeV or above. The remarkable shrinkage and disappearance were clearly observed under irradiation with 1.5MeV and above. On the contrary, the precipitates were mostly stable below 0.75MeV. Two kinds of mechanism of the irradiation-induced instability were deduced from the electron-energy dependence. One is the dissolution and diffusion of tantalum from precipitates in ferrite matrix. The other is the displacements of tantalum in precipitates that introduce dissolution of Ta into matrix.

Thermal stability of Ti, Pt, and Ru interfacial layers between seedless copper and a tantalum diffusion barrier

The thermal stability of 7 nm Ti, Pt, and Ru interfacial adhesion layers between Cu film (10 nm) and a Ta barrier layer (4 nm) has been investigated. The barrier properties and interfacial stability have been evaluated by Rutherford backscattering spectrometry (RBS). Atomic force microscopy was used to measure the surfaces before and after annealing, and all the surfaces are relatively smooth which excludes islanding or dewetting phenomena as a cause of the instability. The RBS showed no discernible diffusion across the adhesion layer/Ta and Ta/Si interfaces which provides a stable underlying layer. For a Ti interfacial layer, RBS indicates that during 400 °C annealing, Ti interdiffuses through the Cu film and accumulates at the surface. For the Pt/Cu system, Pt interdiffusion is detected which is less evident than Ti. Among the three adhesion layer candidates, Ru shows negligible diffusion into the Cu film indicating thermal stability at 400 °C.

Synthesis in molten salts and formation reaction kinetics of tantalum carbide coatings on various carbon fibers

Tantalum carbide (TaC) coatings were prepared on polyacrylonitrile (PAN)-based, graphitized PAN-based and pitch-based carbon fibers by synthesis in molten salts. The characteristics of the TaC coating were studied in relation to the structure of the carbon fiber, the molten salt composition, reaction temperature and reaction time. Thicker TaC coatings with larger grain sizes tended to form on carbon fibers with a lower degree of crystallinity. Molten salt mixtures with a relatively low melting point are appropriate for use as the reaction medium for synthesis of TaC coatings on carbon fibers. However, with increasing reaction temperature and reaction time, the thickness and grain size of the TaC coating increase obviously. Internal diffusion control and external diffusion control models of TaC coating growth were established on the basis of mass transfer and reaction dynamics theory. The result indicates that the TaC coating growth is controlled by the internal diffusion of tantalum in the TaC coating.

Mass transfer modeling on the separation of tantalum and niobium from dilute hydrofluoric media through a hollow fiber supported liquid membrane

The separation of a mixture of tantalum and niobium in dilute hydrofluoric media via hollow fiber supported liquid membrane (HFSLM) was examined. Quaternary ammonium salt (Aliquat336) diluted in kerosene was used as a carrier. The various effects on the transport and separation of tantalum and niobium were studied: concentration of hydrofluoric acid in the feed solution, concentration of the carrier (Aliquat336) in the membrane phase, types of stripping solutions (NaClO 4, thiourea and HCl) and their concentration. The extraction of tantalum in the membrane phase from 0.3 M hydrofluoric acid (HF) by 3% (v/v) Aliquat336 was achieved by leaving niobium in the feed solution. Quantitative recovery of tantalum was achieved by 0.2 M NaClO 4. Furthermore, a mathematical model focusing on the extraction side of the liquid membrane system was presented in order to predict the concentration of tantalum at different times. The mass transfer coefficients of the aqueous feed ( k i ) and the organic membrane phase ( k m ) were estimated as 1.19 × 10 −5 and 1.39 × 10 −7 cm/s, respectively. Therefore, the mass transfer limiting step is the diffusion of tantalum–Aliquat336 through the liquid membrane. Moreover, mass transfer modeling was performed and the validity of the developed model evaluated. Experimental data and theoretical values were found to be in good agreement when the concentration of Aliquat336 in the membrane phase was below 4% (v/v).

Sintering and Mechanical Properties of ZrB2–TaSi2and HfB2–TaSi2Ceramic Composites

Fully dense fine‐grained ZrB2‐ and HfB2‐based composites contaning 15 vol% TaSi2were produced by hot pressing at 1850°–1900°C. Gas formation and mass loss, which occurred during sintering in both systems, were in agreement with thermodynamic predictions. In both composites, the presence of a solid solution formed by the diffusion of tantalum into the boride matrix was observed. The HfB2‐based composite was harder (22 GPa), stiffer (528 GPa), and tougher (4.1M Pa·m1/2) than the ZrB2‐based composite. Although the room‐temperature flexural strength of the ZrB2‐based composite (830 MPa) was higher than that of the HfB2‐based composite (700 MPa), the opposite was true at 1200° and 1500°C. Contrary to the significant strength decrease observed for the ZrB2‐based materials at elevated temperature, the HfB2composite retained ∼86% of its room temperature strength up to 1500°C (∼600 MPa).

A comparative molecular dynamics study of copper trench fill properties between Ta and Ti barrier layers

The copper atoms deposition on tantalum diffusion barrier layer in a damascene process was studied using molecular dynamics simulation with the embedded atom method (EAM) as interaction potential for the present alloy metal system which is based on invariance-preserving alloy model. The present results are discussed in terms of void formation, coverage percentage and alloy fraction. The effects of different process parameters on the trench-filling morphologies and microstructures including incident energies of depositing atoms and substrate temperatures were investigated. Comparing with Ti diffusion barrier under the same process parameters, it is found that due to better thermal stability that significant improvement in coverage percentage can be obtained using the tantalum barrier layer, especially at high incident energies and high substrate temperatures.

Low temperature deposition of tantalum diffusion barrier by filtered cathodic vacuum arc

Tantalum (Ta) diffusion barrier films were deposited on un-patterned and patterned silicon substrates at ambient temperature and without substrate bias by filtered cathodic vacuum arc (FCVA). The films were characterized by atomic force microscopy, scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, four-point resistivity probe and surface profilometer. It was found that the Ta film was 750 Å thick and free of C and O except for surface contamination. The film morphology was smooth and uniform with root-mean-square roughness of ∼0.82 Å. The Ta film was polycrystalline β phase with a mean grain size of ∼3 nm and possessed a dense microstructure, which are ascribed to the high energy of the condensing species in FCVA. It was shown that the Ta filling of the trenches (0.33 μm wide, 1 : 1 aspect ratio) was very conformal and quite uniform. Also, it was preliminarily found that at the Ta film was effective against diffusion of Cu into Si at 600°C.

Tantalum diffusion barrier grown by inorganic plasma-promoted chemical vapor deposition: Performance in copper metallization

As-deposited and annealed tantalum films, grown by plasma-promoted chemical vapor deposition (PPCVD) using pentabromotantalum and hydrogen as coreactants, were evaluated as diffusion barriers in copper metallization. Stacks consisting of 500-nm-thick sputtered Cu/55-nm-thick untreated PPCVD Ta/Si were annealed in argon in the range 450 to 650 °C, in 50 °C intervals, along with sputtered Cu/preannealed PPCVD Ta/Si and sputtered Cu/sputtered Ta/Si stacks of identical thickness. Pre- and postannealed stacks were characterized by x-ray photoelectron spectroscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry, hydrogen profiling, x-ray diffraction, atomic force microscopy, sheet resistance measurements, and Secco chemical treatment and etch-pit observation by scanning electron microscopy. The sputtered and preannealed PPCVD Ta films acted as viable diffusion barriers up to 550 °C, while the as-deposited PPCVD Ta films failed above 500 °C. In all cases, breakdown occurred through the migration of Cu into Si, rather than an interfacial reaction between Ta and Si, in agreement with previously reported results for sputtered Ta films. The accelerated barrier failure for as-deposited PPCVD Ta might have been caused by the presence of approximately 20 at.% hydrogen in the as-deposited PPCVD Ta, an observation which was supported by the enhanced performance of the same PPCVD Ta films after annealing-induced hydrogen removal.

Tantalum diffusion barrier grown by inorganic plasma-promoted chemical vapor deposition: Performance in copper metallization

Tantalum diffusion barrier grown by inorganic plasma-promoted chemical vapor deposition: Performance in copper metallization

Organic Solution Deposition of Copper Seed Layers onto Barrier Metals

AbstractSpontaneous deposition of copper seed layers from metal bearing organic based solutions onto sputter deposited titanium, titanium nitride, and tantalum diffusion barrier thin films has been demonstrated. Based on electrochemically driven cementation exchange reactions, the process was used to produce adherent, selectively deposited copper metal particulate films on blanket and patterned barrier metal thin films on silicon substrates. The organic solution deposited copper films were capable of acting as seed layers for subsequent electrolytic and electroless copper deposition processes using standard plating baths. Electroless and electrolytic copper films from 0.1µm to 1.0µm thick were produced on a variety of samples on which the organic solution copper acted as the initial catalytic seed layer. The feasibility of using organic solution deposited palladium as a seed layer followed by electroless copper deposition has also been demonstrated. In addition, experiments conducted on patterned barrier metal samples with exposed areas of dielectric such as polyimide indicated that no organic solution copper or palladium deposition occurred on the insulating materials.

Effect of diffusionally accommodated interfacial sliding on the stress and deformation in copper/polyimide high density interconnects

The deformation and thermal stresses that develop in copper/polyimide high density interconnects subjected to thermal loading are investigated numerically. The finite element method is used to analyze the response of a single layer test structure composed of a thin film mounted on top of a silicon substrate. The film is made up of parallel copper lines surrounded by tantalum diffusion barriers and a polyimide dielectric. The structure is assumed to be stress free at an elevated temperature, and is then cooled at various rates. Based on the experimental observations by Zhmurkin et al. (Zhmurkin, D.V., Gross, T.S., BuchWalter L.P., Kaufman, F.B., 1996. J. Electronic Materials, 25, 976), we allow for diffusionally accommodated sliding at the copper/tantalum interfaces by assuming a linear relationship between the shear traction and the conjugate sliding rate. In the paper, the effects of interfacial roughness, cooling rate and tantalum liner thickness on the stress and deformation of a two-dimensional representative cell are investigated and the results are discussed.

Properties of interface states at Ta2O5/n-Si interfaces

Properties of interface states at Ta2O5/n-Si interfaces are studied by capacitance–voltage and deep-level transient spectroscopy (DLTS) measurements. The results show that the “slow” states at Ta2O5/n-Si interfaces are not significantly affected by annealing, and their density is about 1.4–1.9×1011 cm−2. The energy distribution of the “fast” interfacial states in the band gap of silicon is obtained, and some high concentration interfacial states are found. DLTS spectra reveal that the interfacial defects at Ta2O5/n-Si interfaces are all donor type. The high concentration interfacial defects are located at about 0.40 eV below the conduction-band minimum of silicon. Their density decreases with increasing annealing temperature. The origin is suggested to be related to the diffusion of tantalum into the Si substrate.

Tantalum-microcrystalline CeO2 diffusion barrier for copper metallization

A tantalum diffusion barrier incorporating microcrystalline CeO2 is proposed for Cu metallization and investigated using Auger electron spectroscopy, x-ray diffraction, optical microscopy, transmission electron microscopy, and sheet resistance measurements. The Cu/Ta+CeO2/Si contact system retained its structure up to 800 °C without an increase in resistivity. The cerium dioxide (CeO2) was stuffed along the grain boundaries during the deposition of Ta layer. Because of its heavier atomic weight than O2- or N2-stuffed elements, it inhibited an interdiffusion of Cu and Si through grain boundaries which can act as fast diffusion paths. It also resulted in preventing the outdiffusion of Ta into the overlayer Cu as well as suppressing the formation of Ta silicide up to 800 °C. It appears that the barrier properties of the tantalum incorporating microcrystalline CeO2 are superior to polycrystalline transition metal barriers, polycrystalline nitride barriers, ternary amorphous compound barriers, and N2- and O2-stuffed barriers.

Interdiffusion in the body cubic centered β-phase of Ta–Ti alloys

Interdiffusion in the β-phase of Ta–Ti alloys is studied in the temperature range 1000–1900°C. Interdiffusion coefficients are calculated with Den Broeder’s method from concentration profiles determined by EPMA. These coefficients depend strongly on the composition: the interdiffusion coefficient for the lower tantalum concentrations is approximately 1000 times greater than the one for the higher concentrations. The activation energy (150–240 kJ/mol) is characteristic of a monovacancy diffusion mechanism. Except for the impurity diffusion of tantalum in titanium, determined by the Vignes and Birchenall method, anomalous behaviour of the diffusion in the β-phase of titanium is not observed. The diffusion in these alloys is characterized by an important Kirkendall effect, materialized by a marker’s shift towards the titanium rich part of the sample. The intrinsic diffusion coefficients have been calculated between 1000°C and 1600°C, at the composition of the Kirkendall plane, by Heumann’s method: the intrinsic diffusion coefficient of titanium is approximately five times greater than that of tantalum.

The fabrication and properties of Nb/sub 3/Sn superconductors by the internal tin process

This paper describes recent progress in the development of Nb/sub 3/Sn wires in Switzerland using the internal tin method. The work was carried out in close collaboration between the Fusion Technology Division of CRPP and Swissmetal, with the emphasis on conductors suitable for fusion magnets. The manufacturing process is based on the tubular extrusion of a niobium-copper composite, followed by the cold working of the composite containing a tin core. 19 Nb-Cu-Sn subelements were assembled inside a tantalum diffusion barrier and a copper can and subsequently reduced to wire size by swaging and drawing. The intention was to obtain non-reacted filament sizes of typically 3 /spl mu/m in 0.8-1 mm wires. The evolution of the design and the manufacturing process are discussed. Experience gained with model conductors has been used to guide the design. The strands have been evaluated with respect to electrical and mechanical properties. In particular, critical current densities in the non-copper area exceeding 750 A/mm/sup 2/ at 12 T and 4.2 K have been achieved.

Comparison of high vacuum and ultra-high-vacuum tantalum diffusion barrier performance against copper penetration

We demonstrate that depositing Ta diffusion barriers under ultra-high vacuum conditions without in situ oxygen dosing allows for variations both in microstructure and in the concentration of chemical impurities that severely degrade barrier performance. The effects of deposition pressure, in situ oxygen dosing at interfaces, hydrogen and oxygen contamination, and microstructure on diffusion barrier performance to Cu diffusion for electron-beam deposited Ta are presented. 20 nm of Ta diffusion barrier followed by a 150 nm Cu conductor were deposited under ultra-high vacuum (UHV, deposition pressure of 1×10−9 to 5 ×10−8 Torr) and high vacuum (HV, deposition pressure of 1×10−7 to 5×10−6 Torr) conditions onto 〈100〉 Si. In situ resistance furnace measurements, Auger compositional depth profiling, secondary ion mass spectrometry, and forward recoil detection along with scanning and transmission electron microscopy were used to determine the electrical, chemical, and structural changes that occurred in thin-film Ta diffusion barriers upon annealing. Undosed HV deposited Ta barriers failed from 560 to 630 °C, while undosed UHV barriers failed from 310 to 630 °C. For UHV Ta barriers, in situ oxygen dosing during deposition at the Cu/Ta interface increased the failure temperatures by 30–250 °C and decreased the range of failure temperatures to 570–630 °C. Undosed UHV Ta barriers have no systematic relationship between failure temperature and deposition pressure, although correlations between breakdown temperature, oxygen and hydrogen concentrations, and microstructural variations were measured.

A stabilized tantalum diffusion barrier for the gold metallization system

A stabilized tantalum-gold thin film system on quartz substrates has been investigated as a metallization system for microwave power transistors. The stabilized tantalum was used as the barrier metal in order to minimize interdiffusion and eutectic formation between the gold and silicon. In order to stabilize the tantalum against rapid diffusion of gold at tantalum grain boundaries, a 200–300A film of platinum was deposited between 1000A layers of tantalum. The resulting structure was then overlaid with 3000A of sputtered gold. The present investigation has shown that the Au/Ta-Pt metallization is (1) metallurgically stable with no significant degradation of resistivity below 500°C, (2) resists electrochemical corrosion and (3) is readily processed with existing technology.