Tungsten Films Research Articles

Exact solution of the phase problem in in situ x-ray reflectometry of a growing layered film

The recent development of an exact solution of the phase retrieval problem is applied to the case of in situ x-ray reflectometry measurements performed during the growth of a tungsten film. The measurement of the phase variation during deposition provides information about the depth distribution of the material density, even though the grazing angle of the probe beam is fixed. A comparison of two different approaches for reconstructing the material density profile is presented. The effect of roughness on the phase retrieval is analyzed.

Ohmic contacts to single-crystalline 3C-SiC films for extreme-environment MEMS applications

This paper describes the ohmic contacts to single-crystalline 3C-SiC thin films heteroepitaxially grown on Si (001) wafers. In this work, a TiW (titanium–tungsten) film was deposited as a contact material by RF magnetron sputter and annealed through the vacuum and rapid thermal anneal (RTA) process. Contact resistivity between the TiW film and the n-type 3C-SiC substrate was measured by the circular transmission line model (C-TLM) method. The contact phases and interface of the TiW/3C-SiC were evaluated with X-ray diffraction (XRD), scanning electron microscope (SEM) and Auger electron spectroscopy (AES) depth-profiles. The TiW film annealed at 1000°C for 45s with the RTA plays an important role in the formation of ohmic contact with the 3C-SiC film and the contact resistance is less than 4.62×10−4Ωcm2. Moreover, the inter-diffusion at the TiW/3C-SiC interface was not generated during, before and after annealing, and was kept in a stable state. Therefore, the ohmic contact formation technology of single-crystalline 3C-SiC films by using the TiW film is very suitable for high-temperature micro-electro-mechanical system (MEMS) applications.

Frictional Characterization of Chemical–Mechanical Polishing Pad Surface and Diamond Conditioner Wear

We evaluated a contact metrology instrument used in chemical–mechanical polishing (CMP) systems for high-volume manufacture and examined in situ coefficient of friction (COF) monitoring to identify the tribology of CMP, and subsequently to determine the useful lifespan of consumables. The results showed that the direct measurement of the wear of the pad allowed for an accurate determination of both pad thickness and the ideal time to replace the pad and conditioner disk based on pad wear rate. We also presented a clear correlation between the working grid area of the conditioner disk and the tribological behavior of the pad break-in procedure, leading to the result showing that the variation in tungsten film removal rate decreased as the working grid density of the conditioner disk increased. This study has proven the effectiveness of measuring friction force for better CMP control.

Thermal Stresses and Microstructure of Tungsten Films on Copper

Tungsten films on polycrystalline copper are considered as possible material systems for high heat flux components in fusion reactors like the International Thermonuclear Experimental Reactor (ITER). Since high thermal loads will occur at the reactor wall it is necessary to analyse temperature-induced stress evolution and possible microstructural changes. In this study the thermal stresses in tungsten films with different thicknesses are characterized by advanced in-situ X-ray diffraction techniques including synchrotron scattering. The microstructure and stress behaviour are critically discussed.

Investigation of Static Corrosion Between W Metals and TiNx Barriers in a W Chemical-Mechanical-Polishing Slurry

In order to control the polishing qualities of tungsten (W) and titanium nitride films in W chemical-mechanical-polishing (WCMP) processes, the electrochemical behavior between the W and the films deposited at various flow rates was examined in this study. Metrologies, including X-ray diffractrometry, Auger electron spectrometry, and scanning electron microscopy, were used to verify the physical properties of the films, while electrochemical analyses, including electrochemical impedance spectroscopy, potential dynamic curves, and potential difference measurements, were used to characterize the mechanism of galvanic corrosion between the W and the films deposited at various flow rates. The results show that the N content of the films influences not only the physical properties of the films but also the chemical activity in the WCMP slurries. The equivalent circuit, including the charge-transfer resistance and the titanium-oxide resistance associated with tantalum-oxide capacitance, was built to characterize the mechanism of the galvanic corrosion between the W and the metals.

Synthesis and characterization of tungsten oxide nanorods from chemical vapor deposition-grown tungsten film by low-temperature thermal annealing

A simple thermal annealing was performed to prepare tungsten oxide nanorods directly from tungsten (W) film. The W film was deposited on Si(100) substrate by chemical vapor deposition (CVD) at 450 °C using W(CO)6. A high density of tungsten oxide nanorods was produced by heating of the W film at 600–700 °C. The morphology, structure, composition, and chemical binding states of the prepared nanorods were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) analysis. XRD and TEM results showed that the grown nanorods were single-crystalline W18O49. According to XPS analysis, the W18O49 nanorods contained ∼55.69% W6+, ∼32.28% W5+, and ∼12.03% W4+. The growth mechanism based on thermodynamics is discussed for the growth of tungsten oxide nanorods from W film.

Plasma erosion of tungsten films with different carbon content

The erosion of W coatings with different content of C in Ar and reactive Ar + H 2 plasmas under negative 300 V bias was investigated. The erosion rate was deduced from the sample's weight change. A strong dependence of the erosion rate on the content of C in W is observed. For the erosion in Ar plasma, the experimental dependences are in agreement with the model assuming that the ejection rates of W and C atoms depend on surface composition. Enhanced erosion of W–C coatings in Ar + H 2 plasma was registered for coatings containing 40 at.% of C and more, which might be owing to chemical erosion of carbon. The surface erosion kinetics was analyzed together with the studies of the coatings microstructure, phase composition and Vicker hardness properties. The registered changes of structural and mechanical properties did not correlate the plasma erosion properties. It was concluded ion was made that surface dynamic processes induced by ion irradiation diminish the role of the bulk structural properties and the registered changes of surface erosion kinetics might be adapted to the variations of coating surface composition in case of argon plasma and to the surface chemistry in case of reactive plasma.

Effects of Tungsten Metal Coatings on Enhancing the Characteristics of Ultrananocrystalline Diamond Films

Nucleation of diamonds on Si substrates was pronouncedly improved by pre-coating a thin tungsten film on the substrates, which is attributed to the enhancement on the formation of the W−C phase in the tungsten silicon interface. The fact that the W coating can enhance the generation of the W−C phase for the W/Si substrate is ascribed to the fast accumulation of carbon species in the W layer, which, in turn, is consequence of the limit volume for carbon inward diffusion. On the contrary, the carbon species can diffuse into the bulk W materials continuously, resulting in a markedly slower rate of accumulation for carbon species and thus delaying the formation of diamond nuclei.

Residual Stress Estimation of Tungsten Film by GIXRD

Subsurface residual stresses of tungsten films induced by Chemical Mechanical Polishing (CMP) processes were investigated by the Grazing Incident X-Ray Diffraction (GIXRD). Basis of the GIXRD measurement was introduced and the experiments were conducted for residual stress of tungsten film measurements. Experimental procedures of the GIXRD measurements were presented. The obtained residual stresses value of tungsten films from 800 nm to 400 nm varies from 1086.1 ± 105.2 MPa to 1670.6±103.4 MPa prior and after CMP process.

Enhancement of the Schottky Barrier Height using a Nitrogen-Rich Tungsten Nitride Thin Film for the Schottky Contacts on AlGaN/GaN Heterostructures

Tungsten, stoichiometric W2N, and nitrogen-rich W2N films were used as Schottky contacts on AlGaN/GaN heterostructures. The nitrogen content in the film was controlled by varying the nitrogen-to-argon gas flow ratio during the reactive sputter deposition. The diode with the nitrogen-rich film exhibited a higher Schottky barrier height and the leakage current was comparable to that of the Ni/Au Schottky contact. Analysis suggested that this was due to the increase of the tungsten nitride work function as the result of higher nitrogen incorporation. Furthermore, after 600°C thermal annealing, the diode was stable and showed no change in the leakage current.

Tungsten coatings by chemical solution deposition for ceramic electrodes in fluorescent tubes

In the lighting industry, the concept of ceramic cold cathodes provided with an inside tungsten coating is under consideration to be applied in mercury-free fluorescent lamps for advertising purposes. In this regard, the applicability of chemical solution deposition to fabricate metallic films on the inner surface of alumina tubules was established. The technique involves the preparation of tungsten oxide layers from a peroxotungstic acid precursor solution and their subsequent reduction to tungsten in the presence of hydrogen. Diverse characterisation of the precursor and the intermediates was carried out in order to determine the influence of their properties on the quality of the final coatings. By acquiring the optimum process parameters, highly-conducting tungsten films were obtained. The temperatures allowing for conversion of the oxide to the metal as well as for sintering of the derived tungsten films were observed to be substantially lower than in bulk processes. Electrode prototypes of the described type were accordingly realised and operated successfully in a fluorescent tube test setup, thus proving the feasibility of the hitherto theoretical idea.

Effect of substrate constraint on stress-induced cracking of sputtered tungsten thin film

A comparative investigation was performed on the stress-induced surface cracking of tungsten films sputter deposited on polyimide (PI) and Si substrates. Microscopic observations suggest that with the same film thickness， two different types of cracking behaviors were observed. Wedge-shaped cracks are formed on the surface of film deposited on the Si substrate. However， the cracks of film deposited on the polyimide consist of aligned grains. The localized plastic deformation in the nanocrystalline W film on polyimide is mediated by the alignment of grains， which results from the grain rotation along the in-plane and out-of-plane directions. However， on the Si substrates the wedge-shaped cracks originate from the in-plane rotation of grains. The analysis implies that the different deformation behaviors are associated with the evolution of stress in the film and the substrate constraint.

Beam distortion detection technique for picosecond ultrasonics

Beam distortion detection (BDD) technique was successfully used for the detection of picosecond longitudinal and shear acoustic pulses in micrometric metallic layers. It is optimised for the measurement of the displacement pulse shape. In this communication the application of BDD for the evaluation of displacement acoustic echoes in 250 nm tungsten film is presented. The reflectivity term has been removed from photoacoustic signal by the introduction of reference channel in the differential photodetection. The result of test experiment with W film has shown promising ability of BDD technique for the surface displacement detection without interferometry and deflectometry.

Effect of growth conditions on the structure and properties of tungsten films prepared using a thermal evaporation process

We report the fabrication and characterization of tungsten films obtained using a thermal evaporation method. Single phase α-tungsten films were obtained at higher substrate temperatures (520–680°C) than the single phase β-tungsten films (180–220°C). X-ray spectra show that both the α-tungsten films and the β-tungsten films are polycrystalline having unusual preferred (200) and (321) orientations, respectively. Poor adhesion was observed when the films were deposited on bare silicon substrate. The adhesion is greatly improved by the introduction of a carbon layer between the film and the substrate. The field emission properties of the α-tungsten films and the β-tungsten films were also examined. The α-tungsten films and β-tungsten films exhibit turn-on fields at 1.3 and 2.8V∕μm, respectively. Current densities in the order of 10−1mA∕cm2 were observed. These field emission properties are believed to be better or much better than that of both tungsten nanowires and nanorods. The relation between the field emission properties and the film structures is discussed.

Use of diamond-like carbon with tungsten (W-DLC) films as biocompatible material

Diamond-like carbon (DLC), also known as amorphous hydrogenated carbon (a-C:H), are a class of materials with excellent mechanical, tribological and biological properties. When the DLC films are enhanced with other elements, all of these properties can be changed within a certain range. In this work, reactive magnetron sputtering was used to deposit W-DLC (hydrogenated tungsten carbide) films on Ti6Al4V (implant material). Many films were made using pure tungsten (99.99%) target and different plasmas processes, with different ratio among argon and methane. It was possible to change the films composition (from pure amorphous carbon to carbon enhanced with tungsten) according to ratio of argon and methane plasma. Between all films processed, the carbon films enhanced with tungsten showed good results in the “in vitro” cytotoxicity testing. Raman spectroscopy was used to analyze the chemical bonds kinds and the chemical bonds quantities. The Rutherford Back Scattering (RBS) was used to analyze the films compositions. The chemical inertness was analyzed by scanning voltametry. W-DLC thin films obtained in these processes have low roughness, high chemical resistance, good adhesion and show a high biocompatibility, when compared with common DLC thin films. Hence we have concluded that the tungsten concentrations in the DLC films make an important role to improve the properties of the DLC layers.

Electrodeposition of Refractory Metals from Some ZnCl2 Based Molten Salts at 150-250{degree sign}C

The solubility of WO3 in a eutectic NaCl-KCl-ZnCl2 (20:20:60 mol%) melt at 250 {degree sign}C was increased by addition of KF, which enabled the electrodeposition of metallic tungsten. A smooth and dense tungsten film having 0.7 μm thickness was obtained on a nickel substrate by potentiostatic electrolysis at 0.06 V vs. Zn(II)/Zn for 3 h in the NaCl-KCl-ZnCl2-KF(4 mol% added)-WO3(0.54 mol% added) melt at 250 {degree sign}C. The surface of the nickel contact probe pin for semiconductor testing, which had been manufactured by the conventional LIGA process, was successfully coated with tungsten by using this molten salt. To lower the electrodeposition temperature, a new molten salt system, EMPyrCl-ZnCl2, was developed. An equimolar EMPyrCl-ZnCl2 system was a stable liquid at 40-200 {degree sign}C. A metallic molybdenum film was electrodeposited on a nickel substrate in the melt containing KF and MoCl5 at 150 {degree sign}C.

IR Radiometry Optical System View Factor and Its Application to Emissivity Investigations of Solid and Liquid Phases

An optical system for fast IR radiometry designed for investigations of thin film thermal properties and pulsed laser melting was analyzed in this work. A methodology for determination of the view factor from calibration measurements was developed. The view factor (0.0255) of the optical system containing two paraboloid mirrors was determined experimentally from calibration measurements on pure metals and metallic alloys. The knowledge of the view factor was then applied to normal emissivity investigations at IR wavelengths. The emissivity of tungsten films prepared by magnetron sputtering was determined for different deposition conditions, varying between 0.036 and 0.071. Liquid phase emissivities of Cu, Mo, Ni, Si, Sn, Ti, and steel were also determined and were found to be higher than solid-state emissivities as predicted from the literature. A knowledge of the liquid-state emissivity of silicon enabled recalculation of the IR signal evolution to the temperature evolution, during and after a nanosecond laser pulse. This was not possible by use of the usual calibration because of silicon’s semi-transparent behavior in the IR range (1–10 μm) in the solid-state phase.

Effects of Oxidation during Synthesis on Structure and Field-Emission Property of Tungsten Oxide Nanowires

We investigated the structure and field-emission property of tungsten oxide nanowires synthesized under different oxidation degrees. We annealed a sputtered tungsten film at 800 °C, controlling the ratio of hydrogen to oxygen (RHO) using Ar/H2 (97/3%) gas in a vacuum furnace. The resulting differences in shape, number density, length, and width of the nanowires were observed by scanning electron microscopy. In the RHO range of 0 to 0.4, beltlike structures were synthesized. In the RHO range of 0.8 to 4, only thin nanowires were synthesized. In this range, length and width did not differ with RHO, but the number density decreased as RHO increased. The sample with a nanowire density of 2 µm-2, annealed with an RHO of 4, showed the highest field-emission property, i.e., a current of 1 mA/cm2 for an electric field of 22 V/µm. We demonstrated their emission by fluorescence imaging and showed that the nanowires are promising candidates for field emitters.

High mobility W-doped In 2O 3 thin films: Effect of growth temperature and oxygen pressure on structural, electrical and optical properties

Highly conducting and transparent thin films of tungsten (W)-doped indium oxide were obtained using pulsed laser deposition to study the effect of growth temperature and oxygen pressure on structural, optical and electrical properties. The transparency of the films is seen to largely depend on the growth temperature. The electrical properties, however, are found to depend strongly on both the growth temperature and the oxygen pressure. High mobility (up to 358 cm 2 V −1 s −1), low resistivity (1.1 × 10 −4 Ω cm), and relatively high transmittance (∼90%) tungsten-doped indium oxide films have been prepared at a growth temperature of 500 °C and an oxygen pressure of 1 × 10 −6 bar.

Dynamic scaling of roughness at the early stage of tungsten film growth

The roughening of sputter-deposited tungsten films is studied in situ and in real time using grazing incidence x-ray scattering. It is shown that the power spectral density functions characterizing the external film surface roughness and the film-substrate roughness conformity can be uniquely extracted from a single scattering diagram when measured at a grazing incidence angle exceeding the critical angle of total external reflection. The temporal evolution of the roughness spectrum is demonstrated to obey a universal scaling form leading to a roughness exponent $\ensuremath{\alpha}=0.18\ifmmode\pm\else\textpm\fi{}0.02$ and to an extremely small growth exponent $\ensuremath{\beta}=0.06\ifmmode\pm\else\textpm\fi{}0.01$. In accordance with the scaling theory, these values of the scaling exponents provide a collapse into a single master curve of the ``renormalized'' power spectral density functions for film thicknesses ranging between 2 and $25\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.