Tungsten Chemical Vapor Deposition Research Articles

Experimental characterization and crystal plasticity finite element simulation of the microstructural evolution in CVD tungsten during multi-step rolling

Atmospheric pressure chemical vapor deposition (CVD) is a promising technique for the preparation of tungsten products with high purity, high density and almost unlimited thickness. The tungsten prepared by using the CVD method (CVD-W) usually has a columnar coarse grain microstructure with strong preferred orientations. Multi-step rolling at gradually decreased operating temperatures is an effective method to lower the ductile-to-brittle transition temperature in pure tungsten. So far, the microstructural evolutions of CVD-W during the multi-step rolling processing is still unclear. In this study, the microstructure evolution and texture development of CVD-W during multi-step rolling are investigated by the coupling of electron backscattering diffraction technique and full-field crystal plasticity finite element simulations. After multi-step rolling, the sample exhibits through-thickness texture gradient. A layered structure with elongated 〈100〉 // ND and 〈111〉 // ND grains is observed in the central region and Goss orientation mainly appears in the surface region. Crystal plasticity finite element simulations show that {001}〈110〉 is a stable orientation under the plane strain compression, which is realized by the rotation of {100} grains around ND. Meanwhile, the shear flow promotes the rotation of {100} grains around TD, leading to the formation of 〈111〉 // ND orientation. 〈110〉 // ND orientation is observed near the shear bands, which is generated through the rotation of {100} grains around <100> // RD axis. The rolled CVD-W exhibits crystallographic orientation dependent recrystallization behavior, which is related to the orientation dependent distortion energy generated in the rolling processing. Our studies provide fundamental insights into the microstructure evolution and texture development of CVD-W under the multi-step rolling processing.

Surface blistering and deuterium retention in chemical vapor deposition tungsten exposed to deuterium plasma

Chemical vapor deposition tungsten (CVD-W) is a promising material for plasma-facing materials. This work evaluated CVD-W samples with a large grain size (CVD-L) and a small grain size (CVD-S) together with rolled W (ND-W) and recrystallized (Rec-W) samples for their performance under deuterium (D) plasma exposure with a flux of 1021 D m−2 s−1 and fluences up to 2 × 1025 D m−2 at a surface temperature of 500 K. Both CVD-L and CVD-S samples featured a columnar grain structure with a preferred orientation close to 〈001〉 || Z, with a more preferred orientation in CVD-L samples. Both CVD-L and CVD-S samples show good resistance to blistering and only few micron-diameter blisters were observed at fluences. In ND-W and Rec-W samples, the number and size of blisters increased with fluence. The D desorption spectra show two desorption peaks in both CVD-W samples at 520 K and 570–604 K. Whereas in ND-W and Rec-W samples, two peaks were at 520 K and 607–700 K. The total D retention in both CVD-L and CVD-S samples was less than 30 % of that in ND-W and Rec-W samples. The low D retention in CVD-W samples was attributed to the low desorption intensities with the peak at 570–604 K. It is suggested that the microstructure of the columnar grain and the 〈001〉 texture in CVD-W samples helps suppressing plasma-induced blistering, therefore reducing D retention. In terms of the stability of the surface integrity and hydrogen isotopes retention, CVD-W demonstrates a significant potential as a wall material for future fusion reactors.

Cyclic ring fiber textures in single tungsten fiber-reinforced tungsten composites

Tungsten fiber-reinforced tungsten composites are considered as plasma-facing material in future fusion reactors. Such composites are obtained by chemical vapor deposition of tungsten on potassium-doped, drawn tungsten wires. In model composites containing single fibers, particular texture types develop due to the cylindrical geometry of the deposition process. The vapor-deposited tungsten layers form a cyclic <100> ring fiber texture with one of the <100> directions pointing radially along the growth direction. The procedure for revealing this non-standard texture type from orientation data obtained by electron backscatter diffraction is presented. Identification of cyclic textures requires carefully chosen acquisition regions as well as a suitable coordinate system for their analysis. It is demonstrated that quite erroneous conclusions about the texture of the layer would be drawn if this is not accounted for properly.

Recent research and development of thick CVD tungsten coatings for fusion application

As the interface closest to the edge plasma of a fusion reactor reacting deuterium (D) and tritium (T), plasma-facing material (PFM) need to withstand extreme service conditions with the high particle flux, high heat load, and neutrons with energy up to 14.1 MeV. Tungsten (W) is the primary candidate of PFMs in future fusion reactors due to its high melting point, good thermal conductivity, excellent irradiation resistance, and low hydrogen/helium retention. So far, powder metallurgy is a leading route for the preparation of W-based PFM. An alternative approach could be the coating technique, which has advantages on fabricating W PFMs and plasma-facing component (PFC) simultaneously. In the past several years, inspiring results were achieved in the preparation process and performance evaluation of the W coating with high purity, excellent thermal conductivity, and thickness at the millimeter level by atmospheric pressure chemical vapor deposition (APCVD). No obvious grain growth and hardness decrease were observed when the annealing temperature was lower than 1500 °C, indicating its good thermal stability. The as-deposited coating exhibited a comparable thermal shock resistance with the conventional W bulk. While the polished sample showed a high crack threshold (0.33–0.44 GW·m−2) when exposed to edge localized mode like transient at room temperature, compared to the unpolished counterpart. Irradiation performance of the chemical vapor deposition (CVD)-W exposed to deuterium (D) plasma and fission neutron were also evaluated. Additionally, the practicality of preparation of large-scale W-based PFM by this technique is also demonstrated. This paper gives a short overview on the recent research and development status of the thick W coating prepared by APCVD at Xiamen Tungsten Co., Ltd and Southwestern Institute of Physics for using as PFM and PFC in fusion devices.

Dissimilar Welding and Joining of Cemented Carbides

Cemented carbides have been widely used in aerospace, biomedical/wearable sensor, automobile, microelectronic, and other manufacturing industries owing to their superior physical and chemical properties at elevated temperatures. These superior properties, however, make it difficult to process these materials using conventional manufacturing methods. In this article, an overview of the welding and joining processes of cemented carbide and steel is given, followed by a few examples of welding processes. Cemented carbides can be successfully joined by sinter-bonding, brazing and soldering, laser beam welding, tungsten inert gas (TIG) welding, diffusion welding, friction welding, electron-beam welding, and chemical vapor deposition. An overview of the benefits and drawbacks of brazing and soldering of cemented carbide and steel is presented, including reports on joint design, processes, and selection of brazing filler metals. The laser welding of cemented carbide and steel is addressed and reviewed, including reports on gap bridging ability, the inclusion/absence of filler metals, interlayers, and laser/TIG hybrid welding. Finally, a section is devoted to explaining the main issues remaining in the welding and joining of cemented carbide, corresponding solutions, and future work required.

Nanostructured CVD Tungsten Carbide Coating on Aircraft Actuators and Gearbox Shafts Reduces Oil Leakage and Improves Durability

Wear and corrosion of aircraft actuator parts, helicopter gearbox rotating shafts and wear of seals working against these parts can lead to oil leaks which require expensive maintenance and increase the risk of failure. The Hardide® nanostructured chemical vapor deposition (CVD) Tungsten Carbide coating was tested on metal parts working against seals as an approach to reduce oil leakage from gearboxes and actuators and increase maintenance intervals. The coating protects metal pistons and shafts from abrasion and corrosion, keeping their surface roughness parameters within optimum ranges for longer; this reduces seal wear and makes the whole unit more durable and reliable. The 50-100 micron-thick CVD Hardide coating can be applied uniformly on internal and external surfaces and has enhanced fatigue and anti-galling properties. The coating has high wear resistance outperforming Hard Chrome by 14 times and Thermal Spray WC-Co (12%) by three times. The fine-grain coating nanostructure wears uniformly so even worn Hardide coating shows no hard micro-grain asperities which are abrasive for seals. The coating is free from porosity and from Cobalt binder and is an effective barrier against corrosion. As a result the coating keeps the optimal seal-friendly surface finish for longer even in abrasive and corrosive environments. The coating was qualified by Airbus as an environmentally friendly replacement for Hard Chrome plating.

Impact of TiN Barrier Layer on Contact Resistance of Tungsten Filled Vias

In this paper, we directly compare the influence of sputtered and chemical vapor deposited TiN liners on the contact resistance of large tungsten filled vias with an aspect ratio of 3:1. Scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and time-of-flight secondary ion mass spectrometry studies revealed that during WF6-based tungsten chemical vapor deposition (CVD), fluorine diffuses through a poor physical vapor deposited (PVD) TiN barrier layer and reacts with the underlying titanium and aluminum to form high resistive compounds. Aside from depositing a thicker TiN PVD barrier, the replacement of the 40 nm PVD layer with a thinner TiN CVD liner has led to significantly reduced via resistances with very small deviations across the wafer and lot. We show that the CVD layer has a superior barrier performance against fluorine penetration, provides a conformal coverage and can be used at a reduced thickness compared to the PVD process.

Logic gates based on electrically driven nanoelectromechanical switches

This letter reports the design, fabrication, and evaluation of logic gates including NAND and NOR gates which are composed of electrically driven nanoelectromechanical (NEM) switches. These logic gates are formed by four NEM switches consisting of four cantilevers, two input ports, two source ports, and an output. Devices are successfully fabricated by a combination of electron beam lithography, deep reactive ion etching (RIE), and selective tungsten (W) chemical vapor deposition. Truth tables of NAND and NOR are examined and demonstrated. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley &amp; Sons, Inc.

Microstructure and growth mechanism of tungsten carbide coatings by atmospheric CVD

A tungsten carbide coating was prepared by atmospheric chemical vapor deposition (CVD) with tungsten hexafluoride (WF6), hydrogen, and dimethyl ether (DME) as the reaction gases, under a dimethyl ether partial pressure (PDME) of 3.2–21.0 kPa and a temperature of 550–600 °C. The phase compositions of the coatings are W2C + W and W3C + W, and it is found that when PDME is low, the coating has a lamellar structure, but at a high PDME, the lamellar structure and fibrous tissue are simultaneously present in the scanning electron microscope (SEM) image. In the latter case, no delamination was found, which was visible at a low PDME. In addition, as the PDME increases, the deposition rate of the coating decreases, and ultimately, the coating cannot continue to grow. The growth model of the tungsten carbide coating based on the kinetics of the chemical vapor deposition of tungsten by the hydrogen reduction of tungsten hexafluoride was proposed, and the influences of PDME and temperature on the coatings were discussed and the orientation relationship between W and W2C in high resolution transmission electron microscopy (HRTEM) was analyzed.

Influence of surface morphology and microstructure on performance of CVD tungsten coating under fusion transient thermal loads

Thick tungsten coatings have been deposited by chemical vapor deposition (CVD) at a rapid growth rate. A series of tungsten coatings with different thickness and surface morphology were prepared. The surface morphology, microstructure and preferred orientation of the CVD tungsten coatings were investigated. Thermal shock analyses were performed by using an electron beam facility to study the influence of the surface morphology and the microstructure on the thermal shock resistance of the CVD tungsten coatings. Repetitive (100 pulses) ELMs-like thermal shock loads were applied at various temperatures between room temperature and 600°C with pulse duration of 1ms and an absorbed power density of up to 1GW/m2. The results of the tests demonstrated that the specific surface morphology and columnar crystal structure of the CVD tungsten have significant influence on the surface cracking threshold and crack propagation of the materials. The CVD tungsten coatings with a polished surface show superior thermal shock resistance as compared with that of the as-deposited coatings with a rough surface.

Thermal stress analysis on chemical vapor deposition tungsten coating as plasma facing material for EAST

Chemical vapor deposition-tungsten (CVD-W) coating on CuCrZr surface forming a flat-type structure is likely to be used as the vacuum wall materials except the divertor areas in EAST during its up-grade phase. This paper describes the residual thermal stress distribution in CVD-W coating substrate system using finite element analysis method (ANSYS code). Especially, the influence of interlayers, i.e. oxygen-free Cu (OFC) and W/Cu functionally gradient materials (FGMs) interlayer have been investigated in view of stress alleviation. Both the OFC and W/Cu FGMs interlayer can reduce the residual thermal stress, in which the OFC shows better interface and surface stress distribution than W/Cu FGMs, and is a preferred interlayer for CVD-W coating processing.

Analysis of Residual Thermal Stress in CVD-W Coating as Plasma Facing Material

Chemical vapor deposition-tungsten (CVD-W) coating covering the surface of the plasma facing component (PFC) is an effective method to implement the tungsten material as plasma facing material (PFM) in fusion devices. Residual thermal stress in CVD-W coating due to thermal mismatch between coating and substrate was successfully simulated by using a finite element method (ANSYS 10.0 code). The deposition parametric effects, i.e., coating thickness and deposition temperature, and interlayer were investigated to get a description of the residual thermal stress in the CVD-W coating-substrate system. And the influence of the substrate materials on the generation of residual thermal stress in the CVD-W coating was analyzed with respect to the CVD-W coating application as PFM. This analysis is beneficial for the preparation and application of CVD-W coating.

The relationship between structural evolution and electrical percolation of the initial stages of tungsten chemical vapor deposition on polycrystalline TiN

This paper presents experimental results and a geometric model of the evolution of sheet resistance and surface morphology during the transition from nucleation to percolation of tungsten chemical vapor deposition over ultrathin polycrystalline titanium nitride (TiN). We observed two mechanisms of reduction in sheet resistance. At deposition temperatures higher than 310 °C, percolation effect is formed at ∼35% of surface coverage, θ, and characterized with a sharp drop in resistance. At temperature below 310 °C, a reduction in resistance occurs in two steps. The first step occurs when θ = 35% and the second step at θ = 85%. We suggest a geometric model in which the electrical percolation pass is modulated by the thickness threshold of the islands at the instant of collision.

Tungsten Contact and Line Resistance Reduction with Advanced Pulsed Nucleation Layer and Low Resistivity Tungsten Treatment

This paper describes electrical testing results of new tungsten chemical vapor deposition (CVD-W) process concepts that were developed to address the W contact and bitline scaling issues on 55 nm node devices. Contact resistance (Rc) measurements in complementary metal oxide semiconductor (CMOS) devices indicate that the new CVD-W process for sub-32 nm and beyond – consisting of an advanced pulsed nucleation layer (PNL) combined with low resistivity tungsten (LRW) initiation – produces a 20–30% drop in Rc for diffused NiSi contacts. From cross-sectional bright field and dark field transmission electron microscopy (TEM) analysis, such Rc improvement can be attributed to improved plugfill and larger in-feature W grain size with the advanced PNL+LRW process. More experiments that measured contact resistance for different feature sizes point to favorable Rc scaling with the advanced PNL+LRW process. Finally, 40% improvement in line resistance was observed with this process as tested on 55 nm embedded dynamic random access memory (DRAM) devices, confirming that the advanced PNL+LRW process can be an effective metallization solution for sub-32 nm devices.

Tungsten-dual polymetal technology for low resistive gate electrode

Abstract We developed ultra-low resistive tungsten dual polymetal (W/barrier metal/dual poly-Si) gate technology suitable for a high performance and high density dynamic random access memory (DRAM) device by using a Ti-based diffusion barrier and a unique tungsten chemical vapor deposition (CVD) process with a B2H6-based nucleation layer. The new low resistive CVD-W deposited on Ti/WN diffusion barrier of dual polymetal gate process not only reveals good oxide reliability comparable to the physical vapor deposition, PVD-W process, but also highly improved transistor performance with signal delay characteristics.

Minimum entropy based run-to-run control for semiconductor processes with uncertain metrology delay

A novel run-to-run control methodology for semiconductor processes with uncertain metrology delay which is developed by combining the minimum error entropy and the optimal control strategy is presented. In most semiconductor processes, the product quality data from the previous run are not often available before the start of the next run. Thus, the corrective step is often delayed by one batch or more, and the duration of the delay is uncertain with stochastic characteristics. Coupled with inaccurate process models, the delay may lead to significant variations of the process outputs even with the use of exponentially weighted moving average (EWMA) controllers. This paper proposes a new method of handling the uncertain metrology delay from a probability viewpoint. The fundamentals of the run-to-run control systems are first reexamined, and then an innovative performance index is given by incorporating the entropy (or information potential) and the mean value of tracking error with constraints on control input energy. The probability density function (PDF) based optimal control algorithm is proposed for processes where the disturbance and delay are non-Gaussian and the stability of the algorithm is analyzed. In addition, the methodology of the proposed control strategy is extended to include recursive PDF estimation and on-line real time implementation. The paper also includes a simulation example of minimum entropy control of a tungsten chemical-vapor deposition process to illustrate the methodology. Furthermore, comparisons between the conventional EWMA method and the proposed method are done to show the advantages of our newly proposed method.

An object-oriented framework for modular chemical process simulation with semiconductor processing applications

This paper discusses the development of a set of object-oriented modular simulation tools for solving lumped and spatially distributed models generated from chemical process design and simulation problems. Developed in the context of simulating semiconductor manufacture equipment, this framework reduces the software development cycle time associated with designing process systems and it improves the overall efficiency of the simulator development process. Modularized simulator components can be solved and analyzed individually, as well as formed into modular systems that can be solved using sequential or simultaneous approaches. A tungsten chemical vapor deposition simulation application is presented to illustrate the capability of the proposed tools in facilitating an evolutionary simulation approach.

Topography simulations for contact formation involving reactive ion etching, sputtering and chemical vapor deposition

With the shrinking design rule of semiconductor devices, the aspect ratios of contact holes that connect transistor electrodes to wirings have exceeded 10 at a design rule less than 0.15μm. The contact is formed through sequential processes of reactive ion etching (RIE), TiN sputtering, and tungsten chemical vapor deposition (W–CVD). In such a formation process, a contact hole with a large bottom diameter is required to reduce contact resistance. We developed a contact simulation method for optimizing contact formation. This contact simulation involves sequential simulations of RIE, TiN sputtering, and W–CVD processes, which adopt a particle model based on the Monte Carlo method. These topography simulations were calibrated using experimental results, and each simulation was combined in order to calculate these sequential simulations. We calculated the dependences of etching and W–CVD filling profiles on contact hole depth. The simulation profiles of etching and W–CVD filling were in agreement with the experimental results. The sequential simulations showed that W disconnection occurs at over 2.5μm contact depth with a aspect ratio of 19.2 and the contact resistance increases markedly.

On the structure, stress and optical properties of CVD tungsten oxide films

Tungsten oxide thin films were prepared by low temperature chemical vapor deposition (CVD) process from a metallorganic precursor at atmospheric pressure. The influence of the deposition temperature and oxygen flow-rate on the film structure, density and built-in stress were investigated in a comparative way employing different characterization techniques. The XRD structural analysis of the films showed co-existence of WO 3 and WO 2.9 phases. On the basis of the performed studies it was inferred that the film density decreases, the film stresses change and the film transmission increases at higher oxygen flow-rate values during the deposition. The growth window for preparation of tungsten oxide films with very low density, facilitating fast kinetics of the electrochromic effect, was established.

A simple assessment method to prevent silicon hydride explosion during tungsten chemical vapor deposition

Silicon hydride (SiH 4 ) is a toxic material and should be reduced to a low concentration to prevent explosions during tungsten chemical vapor deposition. A simple method is presented for the safety assessment in this paper. Empirical equations were deduced to generalize the calculation procedure. The method should help process engineers to estimate whether a recipe will meet minimum safety requirements before a process begins.