Tungsten Films Research Articles

A Low-Temperature Fabrication Method for WSe2 Films Grown from Nanocrystalline Precursors

WSe2 films have been fabricated using a low-temperature, two-step method involving the reaction of W(CO)6 and elemental selenium in refluxing (110 °C) toluene to form a nanocrystalline precursor consisting of amorphous tungsten and trigonal crystalline selenium. Drop cast or airbrush-deposited films of this precursor were annealed in an argon atmosphere using a two-step temperature ramp (250 °C for 15 min followed by 550 °C for 30 min). Raman and x-ray diffraction (XRD) characterization as well as the measured bandgaps of the resulting films are consistent with (002)-oriented WSe2 and are compared to the characterization of films produced via selenization of sputtered tungsten films in closed quartz tubes at 875 °C.

Chemical etching of Tungsten thin films for high-temperature surface acoustic wave-based sensor devices

Surface acoustic wave devices are widely used as wireless sensors in different application fields. Recent developments aimed to utilize those devices as temperature sensors even in the high temperature range (T>300°C) and in harsh environmental conditions. Therefore, conventional materials, which are used for the substrate and for the interdigital transducer finger electrodes such as multilayers or alloys based on Al or Cu have to be exchanged by materials, which fulfill some important criteria regarding temperature related effects. Electron beam evaporation as a standard fabrication method is not well applicable for depositing high temperature stable electrode materials because of their very high melting points. Magnetron sputtering is an alternative deposition process but is also not applicable for lift-off structuring without any further improvement of the structuring process. Due to a relatively high Ar gas pressure of about 10−1Pa, the sidewalls of the photoresist line structures are also covered by the metallization, which subsequently prevents a successful lift-off process.In this study, we investigate the chemical etching of thin tungsten films as an intermediate step between magnetron sputtering deposition of thin tungsten finger electrodes and the lift-off process to remove sidewall covering for a successful patterning process of interdigital transducers.

Measurement of thermophysical property of plasma forming tungsten nanofiber layer

Thermophysical property of a nanostructured tungsten layer formed on a tungsten film was investigated. A 1-µm-thick tungsten film deposited on a quartz glass substrate was irradiated with a high density helium plasma at the surface temperature of 1500 K. The plasma irradiation led to the formation of highly porous fiberform-nanostructured tungsten layer with a thickness of 3.5 µm. Impulse heating was applied at the interface of the film/substrate, and transient heat diffusion was observed using a pulsed light heating thermoreflectance apparatus. The thermoreflectance signals clearly differed between the nanostructure existing and mechanically removed regions; the difference can be attributed to thermal effusivity of the nanostructured tungsten layer. The estimated thermal conductivity of the nanostructured tungsten decreases to ∼2% of that of bulk when the density of the nanostructure is assumed to be ∼6% of the bulk value.

Large area synthesis, characterization, and anisotropic etching of two dimensional tungsten disulfide films

Emergent properties of tungsten disulfide at the quantum confinement limit hold promise for electronic and optoelectronic applications. Here we report on the large area synthesis of atomically thin tungsten disulfide films with strong photoluminescence properties via sulfurization of the pre-deposited tungsten films. Detailed characterization of the pre-deposited tungsten films and tungsten disulfide films are performed using microscopy and spectroscopy methods. By directly heating tungsten disulfide films in air, we have shown that the films tend to be etched into a series of triangular shaped pits with the same orientations, revealing the anisotropic etching behavior of tungsten disulfide edges. Moreover, the dimensions of the triangular pits increase with the number of layers, suggesting a thickness dependent behavior of etching in tungsten disulfide films. This method offers a promising new avenue for engineering the edge structures of tungsten disulfide films.

Synthesis of the cactus-like silicon nanowires/tungsten oxide nanowires composite for room-temperature NO2 gas sensor

In the present work, the tungsten oxide (WO3) nanowires functionalized silicon nanowires (SiNWs) with cactus-like structure has been successfully synthesized for room-temperature NO2 detection. The novel nanocomposite was fabricated by metal-assisted chemical etching (MACE) and thermal annealing of tungsten film. The WO3 nanowires were evenly distributed from the upper to the lower part of the SiNWs, indicating excellent uniformity which is conducive to adsorption and desorption of gas molecules. The gas-sensing properties have been examined by measuring the resistance change towards 0.25–5 ppm NO2 gas. At room temperature, which is the optimum working temperature, the SiNWs/WO3 nanowires composite showed two-times higher NO2 response than that of the bare SiNWs at 2 ppm NO2. On the contrary, the responses of composite sensors to high concentrations of other reducing gases were very low, indicating excellent selectivity. Simultaneously, the composite sensors exhibited good sensing repeatability and stability. The enhancement in gas sensing properties may be attributed to the change in width of the space charge region, which is similar to the behavior of p-n junctions under forward bias, in the high-density p-n heterojunction structure formed between SiNWs and WO3 nanowires.

Metal Oxide Thin Film Characterization for New Generation Chemical Mechanical Planarization Development

This study targeted to create a basis for the new generation semiconductor industry that deals with atomic scale devices and also expected to be utilized in many other fields such as advanced coatings, interfacial adhesion, biological systems i.e. corrosion prevention and enhanced biocompatibility of bio-implants and nano/bio interfaces. We focused on the accomplishments on the microelectronics applications of the CMP process as it is used for the current and future semiconductor applications, such as metal CMP applications of the tungsten T-gate transistors, high speed shallow trench isolation transistors with germanium and furthermore isolating thin films for advanced microelectronics applications such as encapsulation of the PZT arrays of the ferroelectric memory applications. The second part focused on the applications of the CMP induced metal oxide thin films on biomaterials and conventional metals such as steel that is used for heating elements and aluminum that is being used for airplane bodies to improve the corrosion resistance. First of all, a fundamental understanding on the growth of nano-scale protective oxide thin films was studied to determine the effect of oxidizer type and proper oxidation time on the changes in surface properties of the thin films. The preliminary model was developed on the very well established tungsten, which is being utilized as a gate dielectric for the novel T-gate transistors currently. Thin film analyses were conducted through advanced characterization techniques and also compared to the theoretical calculations for the modeling simulations. Atomic Force Microscope (AFM) was used to measure the surface roughness of the samples conditioned in the oxidizer environment before and after the CMP was conducted. The affect of surface roughness on wettability of the surfaces studied through contact angle measurements on the treated tungsten films. Fourier Transform Infrared Spectroscopy with Attenuated Total Reflectance FTIR/ATR technique in combination with the X-Ray Reflectivity (XRR) was utilized to determine the thicknesses of the oxidized nano films on the tungsten wafers. The results were evaluated through the comparison of the Pilling-Bedworth ratios of the oxidized nano films to determine the ability of the created oxide films as a self-protective oxide [1]. Furthermore, a new modeling approach was introduced to CMP process optimization by means of topographic evaluation of the metal oxide thin films. Cahn Hilliard Equation (CHE) was utilized as an alternative to classical nucleation theory in terms of analyzing the topographic nature of the protective metal oxide nano films and modeling their growth, which was observed to affect the CMP performance. It was concluded that the material removal rate mechanisms and the consequent planarization performance depend on the nature of nucleation of the metal oxide films, which is tailored by the oxidizer concentration. The basic knowledge defined on the chemically modified thin films were also expanded to the germanium CMP applications. Particularly, formation and selective removal of chemically modified germanium/silica thin films in the presence of cationic and anionic surfactants were evaluated through AFM wear tests as well as CMP and surface wettability responses. It was determined that while the self-assembled surfactant structures help improve slurry stability, they may retard the material removal rates by inhibiting the particle surface interactions. The results of this study have shown that in the presence of hydrogen peroxide in the slurry, removal rates were mainly affected by the oxidizers surface activity resulting in the formation of germanium oxide film. However, surface quality and the selectivity of the Ge/SiO2 systems were tuned through adjusting the concentration of the oxidizer and surfactant type/chain length in the system to optimize the planarization performance [2]. References Karagoz, A., Craciun, V., Basim, G.B., “Characterization of Nano-Scale Protective Oxide Films_ Application on Metal Chemical Mechanical Planarization”. ECS Journal of Solid State Science and Technology, 4 (2) P1- P8 (2015)Karagoz, A., and Basim, G.B. “Controlling Germanium CMP Selectivity through Slurry Mediation by Surface Active Agents”, ECS Journal of Solid State Science and Technology, CMP Special Issue, 4 (11) P5097-P5104 (2015).

The effect of thermal annealing in a hydrogen atmosphere on tungsten deposited on 6H[sbnd]SiC

Tungsten (W) film was deposited on a bulk single crystalline 6HSiC substrate and annealed in H2 ambient at temperatures of 700 °C, 800 °C and 1000 °C for 1 h. The resulting solid-state reactions, phase composition and surface morphology were investigated by Rutherford backscattering spectrometry (RBS), grazing incidence X-ray diffraction (GIXRD) and scanning electron microscopy (SEM) analysis techniques. These results are compared with the vacuum annealed results reported in our earlier work. As-deposited RBS results indicated the presence of W and O2 in the deposited thin film, the GIXRD showed the presence of W, WO3, W5Si3 and WC. RBS results indicated the interaction between W and SiC was accompanied by the removal of oxygen at 700 °C. The GIXRD analysis indicated the presence of W5Si3 and WC in the samples annealed at 700 °C. At temperatures of 800 °C and 1000 °C, W annealed in a H2 ambient further reacted with the SiC substrate and formed a mixed layer containing silicide phases and carbide phases, i.e.W5Si3, WSi2, WC and W2C. The SEM micrographs of the as-deposited samples indicated the W thin film had a uniform surface with small grains. Annealing at 800 °C led to the agglomeration of W grains into clusters making the surface rough.

Microtribological Mechanisms of Tungsten and Aluminum Nitride Films

Microtribology experiments were carried out on the W1−x Al x N films, deposited by radio frequency magnetron reactive sputtering on 304 stainless steel substrates and Si(100). Film wear mechanisms were investigated from the evolution of the friction coefficient and scanning electron microscopy observations. The results show that the WAlN films consist of a mixture of face-centered cubic W(Al)N and hexagonal wurtzite structure AlN phases and the preferred orientation changes from (111) to (200). The film damage after sliding test is mainly attributed to the composition and microstructure of the films. The amount of debris generated by friction is linked to the crack resistance. The better tribological properties for W1−x Al x N films (x < 0.4) are mainly determined by the higher toughness.

Comparison of different substrates for laser-induced electron transfer desorption/ionization of metal complexes

Four different substrates, namely, graphite, tungsten, amorphous silicon (α-Si) and titanium dioxide (TiO2) films, were compared in view of the laser-induced electron transfer desorption/ionization (LETDI) of metal coordination complexes. A rhenium complex with 8-mercaptoquinoline, a copper complex with diphenylthiocarbazone and chlorophyll A were studied as the test analytes. The dependencies of the ion yield and the surface temperature on the incident radiation fluence were investigated experimentally and theoretically. The temperature was estimated using the numerical solution of a one-dimensional heat conduction problem with a heat source distributed in time and space. It was found that at the same temperature, the ion yield from the different substrates varies in the range of three orders of magnitude. The direct comparison of all studied substrates revealed that LETDI from the TiO2 and α-Si films offer a better choice for producing molecular ions of metal coordination complexes.

Observation of Thermal Beaming from Tungsten and Molybdenum Bull’s Eyes

Although losses in plasmonic films can be detrimental for optoelectronics, they can be exploited to create novel thermal emitters. Surface plasmon polaritons that are thermally excited on a heated metal surface can be converted to photons with useful properties. We demonstrate highly tailored thermal emission from tungsten and molybdenum films patterned with a series of circular concentric grooves (i.e., a bull’s eye). At 900 °C our structures emit an infrared beam normal to the film that is spectrally narrow (tens of nanometers) and highly directional (∼2° angular divergence). The peak wavelength (3.5 μm) can be tuned with groove periodicity. To enhance the thermal stability of the structures, we add a thin layer of HfO2. Such devices, with their simple design and low thermal mass, provide interesting incandescent light sources for various applications.

Enhanced spin-orbit torques by oxygen incorporation in tungsten films.

The origin of spin–orbit torques, which are generated by the conversion of charge-to-spin currents in non-magnetic materials, is of considerable debate. One of the most interesting materials is tungsten, for which large spin–orbit torques have been found in thin films that are stabilized in the A15 (β-phase) structure. Here we report large spin Hall angles of up to approximately –0.5 by incorporating oxygen into tungsten. While the incorporation of oxygen into the tungsten films leads to significant changes in their microstructure and electrical resistivity, the large spin Hall angles measured are found to be remarkably insensitive to the oxygen-doping level (12–44%). The invariance of the spin Hall angle for higher oxygen concentrations with the bulk properties of the films suggests that the spin–orbit torques in this system may originate dominantly from the interface rather than from the interior of the films.

ZnO Nanowire-Based Corona Discharge Devices Operated Under Hundreds of Volts.

Minimizing the voltage of corona discharges, especially when using nanomaterials, has been of great interest in the past decade or so. In this paper, we report a new corona discharge device by using ZnO nanowires operated in atmospheric air to realize continuous corona discharge excited by hundreds of volts. ZnO nanowires were synthesized on microelectrodes using electric-field-assisted wet chemical method, and a thin tungsten film was deposited on the microchip to enhance discharging performance. The testing results showed that the corona inception voltages were minimized greatly by using nanowires compared to conventional dischargers as a result of the local field enhancement of nanowires. The corona could be continuously generated and self-sustaining. It was proved that the law of corona inception voltage obeyed the conventional Peek’s breakdown criterion. An optimal thickness of tungsten film coated over ZnO nanowires was figured out to obtain the lowest corona inception voltage. The ion concentration of the nanowire-based discharger attained 1017/m3 orders of magnitude, which is practicable for most discharging applications.

Tungsten as a Chemically-Stable Electrode Material on Ga-Containing Piezoelectric Substrates Langasite and Catangasite for High-Temperature SAW Devices.

Thin films of tungsten on piezoelectric substrates La3Ga5SiO14 (LGS) and Ca3TaGa3Si2O14 (CTGS) have been investigated as a potential new electrode material for interdigital transducers for surface acoustic wave-based sensor devices operating at high temperatures up to 800 °C under vacuum conditions. Although LGS is considered to be suitable for high-temperature applications, it undergoes chemical and structural transformation upon vacuum annealing due to diffusion of gallium and oxygen. This can alter the device properties depending on the electrode nature, the annealing temperature, and the duration of the application. Our studies present evidence for the chemical stability of W on these substrates against the diffusion of Ga/O from the substrate into the film, even upon annealing up to 800 °C under vacuum conditions using Auger electron spectroscopy and energy-dispersive X-ray spectroscopy, along with local studies using transmission electron microscopy. Additionally, the use of CTGS as a more stable substrate for such applications is indicated.

Ultra-broad band absorber made by tungsten and aluminium

A broadband absorber comprising tungsten cubic arrays, a alumina layer and a tungsten film, is numerically and experimentally investigated, which exhibits near-unity absorption of visible and near-infrared light from 400 nm to 1150 nm. Benefiting from high melting points of tungsten and alumina, this device has great application potential in solar cells and thermal emission.

Impact of Film Morphology on Chemical Mechanical Polishing of Tungsten

This study investigated the effects of tungsten (W) film morphology on the chemical mechanical polishing (CMP) of W. Chemical vapor deposited (CVD) W films with two distinctively different grain sizes were used for comparison. During polishing, W film thickness and optical reflectance, friction, and pad temperature were monitored in-situ. It was found that larger-grained W film took longer to pass the initial low removal rate stage. By correlating four different sensor signals, comparing friction dependency on film morphology, in slurry vs. DIW, it was concluded that W CMP comprises three main stages. First is the low rate initiation stage: grain is being partially planarized, reflectance increases, friction decreases. Second is the transition stage: rate is ramping, grain becomes fully planarized, optical reflectance reaches maximum, and friction becomes minimal followed by a significant rise caused by formation of tungsten oxide passivation layer on the planarized W surface. Third is the high and constant rate stage: passivation and removal occur in a repetitive cycle, friction is high and stable, optical reflectance changes as polishing reaches different film depths. In all three stages, pad temperature increases continuously as friction-induced heat dissipates, with the rate of temperature increase following that of friction magnitude.

Comparison of tungsten films grown by CVD and hot-wire assisted atomic layer deposition in a cold-wall reactor

In this work, the authors developed hot-wire assisted atomic layer deposition (HWALD) to deposit tungsten (W) with a tungsten filament heated up to 1700–2000 °C. Atomic hydrogen (at-H) was generated by dissociation of molecular hydrogen (H2), which reacted with WF6 at the substrate to deposit W. The growth behavior was monitored in real time by an in situ spectroscopic ellipsometer. In this work, the authors compare samples with tungsten grown by either HWALD or chemical vapor deposition (CVD) in terms of growth kinetics and properties. For CVD, the samples were made in a mixture of WF6 and molecular or atomic hydrogen. Resistivity of the WF6-H2 CVD layers was 20 μΩ·cm, whereas for the WF6-at-H-CVD layers, it was 28 μΩ·cm. Interestingly, the resistivity was as high as 100 μΩ·cm for the HWALD films, although the tungsten films were 99% pure according to x-ray photoelectron spectroscopy. X-ray diffraction reveals that the HWALD W was crystallized as β-W, whereas both CVD films were in the α-W phase.

Growth and characterization of α and β-phase tungsten films on various substrates

The growth conditions of tungsten thin films were investigated using various substrates including Si, Si/SiO2, GaAs, MgO, and Al2O3, and recipes were discovered for the optimal growth conditions of thick metastable β-phase tungsten films on Si, GaAs, and Al2O3 substrates, which is an important material in spin orbit torque studies. For the Si/SiO2 substrate, the crystal phase of the tungsten films was different depending upon the tungsten film thickness, and the transport properties were found to dramatically change with the thickness owing to a change in phase from the α + β phase to the α-phase. It is shown that the crystal phase changes are associated with residual stress in the tungsten films and that the resistivity is closely related to the grain sizes.

Tungsten oxide nanowires grown on amorphous-like tungsten films

Tungsten oxide nanowires have been synthesized by vacuum annealing in the range 500–710 °C from amorphous-like tungsten films, deposited on a Si(100) substrate by pulsed laser deposition (PLD) in the presence of a He background pressure. The oxygen required for the nanowires formation is already adsorbed in the W matrix before annealing, its amount depending on deposition parameters. Nanowire crystalline phase and stoichiometry depend on annealing temperature, ranging from W18O49-Magneli phase to monoclinic WO3. Sufficiently long annealing induces the formation of micrometer-long nanowires, up to 3.6 μm with an aspect ratio up to 90. Oxide nanowire growth appears to be triggered by the crystallization of the underlying amorphous W film, promoting their synthesis at low temperatures.

Infrared imaging diagnostics for intense pulsed electron beam.

Infrared imaging diagnostic method for two-dimensional calorimetric diagnostics has been developed for intense pulsed electron beam (IPEB). By using a 100-μm-thick tungsten film as the infrared heat sink for IPEB, the emitting uniformity of the electron source can be analyzed to evaluate the efficiency and stability of the diode system. Two-dimensional axisymmetric finite element method heat transfer simulation, combined with Monte Carlo calculation, was performed for error estimation and optimization of the method. The test of the method was finished with IPEB generated by explosive emission electron diode with pulse duration (FWHM) of 80 ns, electron energy up to 450 keV, and a total beam current of over 1 kA. The results showed that it is possible to measure the cross-sectional energy density distribution of IPEB with energy sensitivity of 0.1 J/cm(2) and spatial resolution of 1 mm. The technical details, such as irradiation protection of bremsstrahlung γ photons and the functional extensibility of the method were discussed in this work.

Deposition of Tungsten Thin Films on Flexible Polymer Substrates by Direct-Current Magnetron Sputtering

We have investigated thin tungsten films deposited on polymer substrates by direct-current magnetron sputtering under different conditions. Unlike tungsten films deposited on rigid substrates, films on polymer substrates grew at appropriate sputtering power, low sputtering pressure, and low substrate temperature. High sputtering power results in tungsten films with good crystal orientation, compact microstructure, and low electrical resistivity. However, high-power sputtering damages the polymer substrates. Enhancing sputtering pressure substantially degrades tungsten orientation and increases electrical resistivity. Furthermore, a slight increase in substrate temperature results in tungsten films with good crystal orientation, a dense microstructure, and low electrical resistivity. Nonetheless, a high substrate temperature results in soft and deformed polymer substrates; this degrades tungsten crystal orientation and substantially roughens tungsten films. On the basis of this study, compact and flat tungsten films with low electrical resistivity can be obtained at a sputtering power of 69 W, a sputtering pressure of 1 Pa, a substrate temperature of 100°C, and a distance between target and substrate of 60 mm.