Tungsten Films Research Articles

Phase transformation in thin tungsten films during sputter deposition

For tungsten films sputter-deposited at room temperature, it has been commonly observed that the meta-stable A15 β-phase (β-W) nuclei are initially formed and then coalesced into continuous β-W layer during the early film deposition stage, which transforms into the equilibrium bcc α-phase (α-W) at a so-called critical thickness. Various previous studies, without experimental verifications, attributed the phase transformation to the increased film temperature (~100°C) resulting from the sputtering process. In this study, the substrate temperature was intentionally varied from 20 to 320°C in order to investigate the temperature dependence of the phase transformation, which reveals important characteristics to be contrasted by the prior reports. Based on the energy calculation using the elastic hard-sphere model, sputtered atoms and reflected Ar neutrals in case of W sputtering are shown to possess extraordinarily high energy (60–130eV). These energetic particles, by directly transferring kinetic energy to the lattice atoms on the deposit surface as well as inside the thin (~2.5nm) β-W layer, enhance atomic diffusion and thus drive the phase transformation in W films, which is supported by the conclusion of three dimensional monte Carlo simulation reported in literature. The results of this study indicate that processing efforts for the application of low-resistivity W interconnects should be directed at optimizing sputtering parameters such as power and pressure to control the energetic particles rather than focusing on the optimization of film temperature, which have been considered the dominant parameter for the phase transformation.

Fabrication of single crystal diamond microchannels for micro-electromechanical systems

Fabrication of single crystal diamond microchannels on HPHT diamond substrate has been carried out successfully. Firstly, tungsten film was patterned on diamond substrate surface by conventional photolithography and magnetron sputtering methods. Secondly, single crystal diamond epitaxial layer was grown on the surface of the patterned substrate by microwave plasma chemical vapor deposition system. Then the tungsten film was etched out by acid to form microchannel. Finally, different color liquid was introduced to check the hollow of microchannels. The morphology of the microchannels has been investigated by optical microscope, scanning electron microscope and atomic force microscope. The crystallinity was evaluated by X-ray diffraction systems and Raman spectroscope technology.

Design, fabrication and optical characterizations of large-area lithography-free ultrathin multilayer selective solar coatings with excellent thermal stability in air

A sub-micron-thick selective multilayer solar thermal absorber made of tungsten, SiO2 and Si3N4 multilayer thin films was theoretically designed and experimentally demonstrated in this work. The optical performance was optimized by the particle swarm optimization algorithm for this multilayer absorber, whose spectral selectivity is associated with the Fabry-Perot resonance and anti-reflection effects. The designed multilayer absorber was deposited by sputtering and chemical vapor deposition techniques on the wafer scale. Its spectral absorptance characterized by a Fourier Transform Infrared spectrometer (FTIR) was demonstrated to be greater than 0.95 in the solar spectrum and less than 0.1 emittance in the mid-infrared with angular insensitivity. Temperature dependent FTIR measurements with an optical fiber setup revealed stable optical performance up to 600°C in ambient, while thermal cycle testing showed its long-term thermal stability at 400°C. Theoretical analysis of solar to power efficiency for a Carnot heat engine driven by the Solar heat was performed, which clearly shows that the proposed ultrathin selective multilayer absorber with spectral selectivity, angular insensitivity as well as high temperature stability could significantly boost the conversion efficiency of solar thermal systems at mid to high temperatures.

Interface Energy Coupling between \u03b2-tungsten Nanofilm and Few-layered Graphene

We report the thermal conductance induced by few-layered graphene (G) sandwiched between β-phase tungsten (β-W) films of 15, 30 and 40 nm thickness. Our differential characterization is able to distinguish the thermal conductance of β-W film and β-W/G interface. The cross-plane thermal conductivity (k) of β-W films is determined at 1.69~2.41 Wm−1K−1 which is much smaller than that of α-phase tungsten (174 Wm−1K−1). This small value is consistent with the large electrical resistivity reported for β-W in literatures and in this work. The β-W/β-W and β-W/G interface thermal conductance (GW/W and GW/G) are characterized and compared using multilayered β-W films with and without sandwiched graphene layers. The average GW/W is found to be at 280 MW m−2K−1. GW/G features strong variation from sample to sample, and has a lower-limit of 84 MW m−2K−1, taking into consideration of the uncertainties. This is attributed to possible graphene structure damage and variation during graphene transfer and W sputtering. The difference between G2W/G and GW/W uncovers the finite thermal resistance induced by the graphene layer. Compared with up-to-date reported graphene interface thermal conductance, the β-W/G interface is at the high end in terms of local energy coupling.

Advanced characterizations of fluorine-free tungsten film and its application as low resistance liner for PCRAM

Using a metal-organic tungsten based precursor, a fluorine-free tungsten thin film has been obtained. The process deposition recipe includes a plasma-enhanced CVD (PECVD) step and atomic layer deposition (ALD) cycles. A set of physicochemical characterizations including X-ray reflectivity (XRR), in-plane X-ray diffraction (XRD), wavelength dispersive X-ray fluorescence (WDXRF), plasma profiling time of flight mass spectrometry (PPTOFMS) and microscope observations has been realized in order to study the W thin film structure and properties. The film is perfectly conformal whatever the structure size investigated (from tens of nanometers to micrometers wide). It was also highlighted that the F-free W film exhibits the lowest electrical resistivity phase (α-W) but is not pure. Indeed, in addition to a top surface oxidation, a layer located at the W film / substrate interface is present. This interface layer (IL) contains impurities, including carbon and oxygen, due to ligand decomposition. This IL might be deposited during the soak step or during the PECVD step.The W liner with thicknesses ranging from 3 to 4nm has been implemented on PCRAM structures in order to evaluate its impact on contact plug resistivity. First electrical results are promising and demonstrate the interest of using a F-free low resistance W liner. At the aspect ratio studied, the gain in terms of contact plug resistivity is about 20% compared to the process of reference using a TiN liner. Modeling shows that this benefit is mainly due to the reduction of interface resistances.

Sputter-deposited WOx and MoOx for hole selective contacts

Reactive sputter deposited tungsten and molybdenum oxide (WOx, MoOx) thin films are tested for their ability to form a hole selective contact for Si wafer based solar cells. A characterization approach based on analyzing the band bending induced in the c-Si absorber and the external and implied open-circuit voltage of test structures was used. It is shown that the oxygen partial pressure allows to tailor the selectivity to some extent and that a direct correlation between induced band bending and hole selectivity exists. Although the selectivity of the sputtered films is inferior to the reference films deposited by thermal evaporation, these results demonstrate a good starting point for further optimizations of sputtered WOx and MoOx towards higher work functions to improve the hole selectivity.

Effect of nanostructure on radiation tolerance and deuterium retention in tungsten

Understanding of radiation tolerance and hydrogen accumulation in nanomaterials is an urgent challenge since it may open new perspectives to design advanced materials for extreme conditions, for example, nuclear energy systems. In this work, intrinsic defects in nanostructured tungsten (W) films with different grain sizes were studied by decoration with deuterium (D). This method was also successfully applied to detect defects at the interface between the coating and the substrate, as well as radiation-induced defects. The build-up of D at the interface between the coating and the substrate was observed, which can be a concern for both un-irradiated and neutron-irradiated materials. It was found that the concentration of D in W materials drastically increases with decreasing mean grain size. However, the D concentration at radiation-induced defects produced by self-ion irradiation at room temperature to 3 displacements per atom is the same for all types of coatings, and it is the same as for polycrystalline W. This implies that the density of radiation-induced defects is the same for all types of W coatings, regardless of the crystalline structure of a W material. In this respect, a compromise in the development of new promising nanostructured tungsten films is necessary to ensure the radiation resistance, keeping the hydrogen concentration at an acceptable level and reducing/preventing high density of defects at the interface between the nanostructured coating and the substrate.

Simulation of residual stress and its impact on a poly-silicon channel for three-dimensional, stacked, vertical-NAND flash memories

In this work, we present the results of an investigation of the impact of the stress on a poly-silicon channel induced by the neighboring layers in three-dimensional vertical NAND (3D V-NAND) flash memories. Using 3D process simulations, we confirmed the distributions of the residual stress after each process step in the cross-section of a NAND flash unit cell. To investigate the impact of the stress on the poly-silicon channel, we also studied the residual stress after changing the intrinsic stresses of the oxide-nitride-oxide (ONO) layer and the tungsten layer used as a gate. We found that the amplitude of the residual stress in the applied layer became larger as the intrinsic stress increased. In addition, the intrinsic tensile/compressive stresses in the outer layers affected the residual stresses of the previously deposited layers in an opposite nature of the stresses. The cylindrical poly-silicon channel was influenced by the intrinsic stresses of the oxide layers adjacent to the nitride and the tungsten films, with the intrinsic stress of the tunnel oxide having the greater effect on the residual stress in the channel. Because such stresses affect the electrical properties of the devices, optimized deposition conditions are required to control them. Such conditions would aid in improving the performances of 3D NAND flash memories.

Hot‐Wire Assisted ALD: A Study Powered by In Situ Spectroscopic Ellipsometry

Hot‐wire assisted atomic layer deposition (HWALD) is a novel energy‐enhancement technique. HWALD enables formation of reactive species (radicals) at low substrate temperatures, without the generation of energetic ions and UV photons as by plasma. This approach employs a hot wire (tungsten filament) that is heated up to a temperature in the range of 1300–2000 °C to dissociate precursor molecules. HWALD has the potential to overcome certain limitations of plasma‐assisted processes. This work investigates the ability of a heated tungsten filament to catalytically crack molecular hydrogen or ammonia into atomic hydrogen and nitrogen‐containing radicals. The generation of these radicals and their successful delivery to the wafer (substrate) surface are experimentally confirmed by dedicated tellurium‐etching and silicon‐nitridation experiments. It further reports on deposition of low‐resistivity oxygen‐free tungsten films by using HWALD, as well as on the effect of hot‐wire‐generated nitrogen radicals and atomic hydrogen in deposition of aluminum nitride and boron nitride films. In parallel, this work provides important illustrative examples of using in situ real‐time monitoring of deposition and etching processes, together with extracting a variety of film properties, by spectroscopic ellipsometry technique.

Stability of tungsten oxide nanotubes film for improving photocatalytic oxidation reaction

ABSTRACTWell-aligned anodic tungsten oxide (WO3) nanotubes with lengths approaching 600 nm was successfully synthesised via electrochemical anodisation of tungsten (W) film at 40 V in a bath with electrolyte (pH 3) consisted of 1 M of sodium sulphate (Na2SO4) and 0.7 wt-% ammonium fluoride (NH4F) for 15 min. It was found that the production of dense compact oxide layer on pure W film could be explained with high concentration of H+ ions accelerated the hydrolysis ability on the W surface to form thick WO3 layer under acidic condition (pH: 3). The photocatalytic activity performance was increased by ≈15% for the dense WO3 nanostructures film as compared to the thin and irregular WO3 nanostructures film because of the high active surface area to absorb more photons from solar irradiation for triggering the charge carriers separation and then improvement of internal and external diffusion of the reactants.

Electron Microscopy of the Tin-oxide Nanolayer Formed on the Surface of Sn-Ag-Cu Alloys

Sn-Ag-Cu alloy used in the present study is commercial Sn-3.0Ag-0.5Cu solder ball alloys with a diameter of 400 and 300 μm which were long term atmospheric oxidized for about 6 years (specimen-1) and under high temperature/humidity at 85°C and relative humidity of 85% for 2140 h, respectively. Morphologies and nanostructure of the oxide nanolayers formed on the surface of Sn-Ag-Cu alloys were studied from the interface of the oxide film and the tin substrate by transmission electron microscopy (TEM) to verify the oxidation mechanism. Cross-sectional TEM specimens were prepared using a focused-ion-beam (FIB) micro-sampling technique. Before the FIB fabrication, the specimen surface was coated with carbon (C) and tungsten (W) films. Inhomogeneous thickness of tin-oxide nanolayer formed on specimen-1 and specimen-2 were fluctuated between 20-40 nm and 40-50 nm, respectively. The nanolayer on specimen-1, however, consists of polycrystalline SnO and SnO2, whereas the one on the specimen-2 comprises of polycrystalline SnO2. High resolution (HRTEM) image and fast Fourier transformation (FFT) spectra corresponding to the interface and the substrate areas have confirmed those results. The results verify that at very long atmospheric oxidation Sn was gradually oxidized to be SnO (Sn2+) and then SnO2 (Sn4+), in which SnO is present at the region closed to interface between Sn-substrate and the tin-oxide layer. At high temperature oxidation, however, Sn was completely oxidized to be SnO2.

Tungsten nitride coatings obtained by HiPIMS as plasma facing materials for fusion applications

In this work, tungsten nitride coatings with nitrogen content in the range of 19–50 at% were prepared by reactive multi-pulse high power impulse magnetron sputtering as a function of the argon and nitrogen mixture and further exposed to a deuterium plasma jet. The elemental composition, morphological properties and physical structure of the samples were investigated by Rutherford backscattering spectrometry, atomic force microscopy and X-ray diffraction. Deuterium implantation was performed using a deuterium plasma jet and its retention in nitrogen containing tungsten films was investigated using thermal desorption spectrometry. Deuterium retention and release behaviour strongly depend on the nitrogen content in the coatings and the films microstructure. All nitride coatings have a polycrystalline structure and retain a lower deuterium level than the pure tungsten sample. Nitrogen content in the films acts as a diffusion barrier for deuterium and leads to a higher desorption temperature, therefore to a higher binding energy.

Study of tungsten films deposited by DC sputtering dedicated to integrated heaters

In order to realize cost-effective semiconductor gas sensors, the authors have studied the feasibility of replacing platinum by tungsten for the metallic layer of heaters in a moderate temperature range (25–400 °C). Tungsten films were deposited on silicon substrates by direct current magnetron sputtering in argon gas. The deposition of tungsten films was investigated at various working gas pressures to modify the microstructure. The results have shown that low-stressed films showed a good adhesion to silicon substrates. Resistivity values as low as 27 μΩ cm were obtained for 600 nm films deposited at low argon pressure. After a thermal treatment at 500 °C for 30 min., no resistivity variation occurred for films deposited at low argon pressure. Finally, three different structures of tungsten heaters were elaborated by using an optical lithography technique and tested for 300 h at 400 °C.

An enhanced formulation to determine Young's and shear moduli of thin films by means of Impulse Excitation Technique

This work presents an enhanced approach to determine the Young's and shear moduli and Poisson's ratio of coatings by means of the Impulse Excitation Technique. The proposed relation takes into account the shift of the neutral axis after deposition, which makes it applicable for any film thickness. The proposed analytical expression was established thanks to the application of the Hamilton's principle allowing the determination of the warping function of the composite system and therefore its torsional resonant frequency. It was applied to determine the macroscopic elasticity constants of tungsten films deposited on glass substrates by conventional magnetron sputtering. Moreover, in order to discuss the values of shear and Young's moduli, the films structure and morphology were also analyzed by X-ray diffraction and scanning electron microscopy.

Development and study of a conceptual model of an X-ray source with a field emission cathode

A conceptual model of an X-ray source based on a field emission cathode and a transmission-type target combined with a silicon X-ray window is suggested. By numerical simulation, it is shown that the proposed structure can generate a substantial emission current. It is possible to obtain a small focal spot at the target and, therefore, a high resolution. The parameters of targets providing the maximum X-ray emission intensity are determined. It is shown that effective generation is reached in a 0.25-μm-thick tungsten film and a 0.13-μm-thick molybdenum film. The transparency of a 1–2-μm-thick silicon membrane to X-rays and sufficient mechanical strength of the membrane against a pressure drop on the order of 2 atm are demonstrated, which indicates the possibility of its application as an X-ray window.

Pulsed laser chemical vapor deposition of a mixture of W, WO2, and WO3 from W(CO)6 at atmospheric pressure

Pulsed laser irradiation at 355nm was used to deposit tungsten (W) films from tungsten hexacarbonyls (W(CO)6) on transparent glass substrates in air. The time dependence of W deposition revealed that the reaction proceeded via nucleation and growth; photolytic decomposition initiated W nuclei, which acted as laser absorbers and grew by direct deposition on the nuclei, driven mainly by a pyrolytic process. In addition, the laser power dependence showed that the thickness of W films linearly increases with power; however, the thickness decreased significantly at a sufficiently high power to allow the evaporation of tungsten oxide.Various analyses (X-ray diffraction (XRD), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS)) identified α-W, WO2, and WO3 in the deposited W films at 1.78–6.67W and at a scan rate of 4μm/s, and their compositional and microstructural changes according to laser power. The loss of carbon (C) is attributable to the background oxygen. An increase in laser power increased the oxygen content, the WO3 to WO2 ratio, and the size of W grains. The resistivity of W films was closely related to the oxygen concentration and microstructure of W. The minimum resistivity of ~80μΩ-cm was obtained at a power of from 3.56 to 4.0W, at which the effect of the laser-induced grain growth on resistivity is maximized, accompanied by the laser-enhanced oxidation of W.

Relative SHG measurements of metal thin films: Gold, silver, aluminum, cobalt, chromium, germanium, nickel, antimony, titanium, titanium nitride, tungsten, zinc, silicon and indium tin oxide

We have experimentally measured the surface second-harmonic generation (SHG) of sputtered gold, silver, aluminum, zinc, tungsten, copper, titanium, cobalt, nickel, chromium, germanium, antimony, titanium nitride, silicon and indium tin oxide thin films. The second-harmonic response was measured in reflection using a 150fs p-polarized laser pulse at 1561nm. We present a clear comparison of the SHG intensity of these films relative to each other. Our measured relative intensities compare favorably with the relative intensities of metals with published data. We also report for the first time to our knowledge the surface SHG intensity of tungsten and antimony relative to that of well known metallic thin films such as gold and silver.

Effects of Oxygen, Nitrogen and Fluorine on the Crystallinity of Tungsten by Hot-Wire Assisted ALD

A heated tungsten filament (wire) is well known to generate atomic hydrogen (at-H) by catalytically cracking molecular hydrogen (H2) upon contact. This mechanism is employed in our work on hot-wire (HW) assisted atomic layer deposition (HWALD), a novel energy-enhancement technique. HWALD has been successfully utilized to deposit tungsten (W) films using alternating pulses of WF6 and at-H. Depending on the conditions, either low-resistivity α- or higher-resistivity β-crystalline phases of W can be obtained. This work aims to clarify (i) which factors are decisive for the formed crystal phase and (ii) the role of the residual gases in the film growth mechanism. In this light, the effects of adding impurities (N2O, O2, NH3 and H2O) were investigated. Oxidizing species have a retarding effect on W growth but the process can be re-initiated after stopping their supply. In contrast, nitridizing species have a permanent inhibition effect. Further, the effects of WF6 overdose were studied. The surplus of WF6 appeared to be crucial for the process: in many cases this led to the formation of β-phase W instead of the α-phase, with a memory effect lasting for several deposition runs. Extra fluorine-containing species were thus identified as the likely cause of β-phase formation.

XPS Study of Tungsten and Barrier Film Transition at Various Stages of Chemical Mechanical Polishing Endpoint and of Surface Compositions Post-CMP Cleaning

The present study aims to determine wafer surface film compositions at various stages of optical end point during W CMP process. Wafers taken at reflectance peak time, peak plus time, slope time, end point time and overpolish time were analyzed by XPS at various locations of the wafer, with special focus on the compositional identification of the visual “residues”. Results showed that W and barrier film removal proceed in a non-uniform manner at XPS probe depth scale of ∼6 nm. At the optical reflectance peak time the wafer is substantially covered with W, and Ti barrier only appeared on the edge of the wafer due to center slow - edge fast removal profile of the polishing. Using peak to end point time to calculate Ti barrier removal rate may under-estimate the true barrier polishing time and led to artificially lower barrier rate. Ammonia-based post CMP cleaning process and its impact on W film surface composition is also discussed.

Transparent, flexible, and stretchable WS2 based humidity sensors for electronic skin.

Skin-mountable chemical sensors using flexible chemically sensitive nanomaterials are of great interest for electronic skin (e-skin) application. To build these sensors, the emerging atomically thin two-dimensional (2D) layered semiconductors could be a good material candidate. Herein, we show that a large-area WS2 film synthesized by sulfurization of a tungsten film exhibits high humidity sensing performance both in natural flat and high mechanical flexible states (bending curvature down to 5 mm). The conductivity of as-synthesized WS2 increases sensitively over a wide relative humidity range (up to 90%) with fast response and recovery times in a few seconds. By using graphene as electrodes and thin polydimethylsiloxane (PDMS) as substrate, a transparent, flexible, and stretchable humidity sensor was fabricated. This senor can be well laminated onto skin and shows stable water moisture sensing behaviors in the undeformed relaxed state as well as under compressive and tensile loadings. Furthermore, its high sensing performance enables real-time monitoring of human breath, indicating a potential mask-free breath monitoring for healthcare application. We believe that such a skin-activity compatible WS2 humidity sensor may shed light on developing low power consumption wearable chemical sensors based on 2D semiconductors.