Showing papers on "Tungsten published in 1993"

Simple source of atomic hydrogen for ultrahigh vacuum applications

Abstract: A simple doser for atomic hydrogen is described. It basically consists of a tungsten capillary which on the outlet is heated by electron bombardment to 1800–2000 K. This temperature is sufficient to achieve nearly total H2 dissociation under relevant working pressures. The performance of the atomic hydrogen source was tested by H adsorption studies on Cu(110). The saturation coverage on this surface could be achieved with a nominal exposure which was smaller by a factor of ∼10−3 as compared to conventional methods of dissociation on a hot filament. The higher efficiency of the present H source greatly reduces the risk of sample contamination.

Microstructure and hydroabrasive wear behaviour of high velocity oxy-fuel thermally sprayed WCCo(Cr) coatings

Abstract: Sand erosion tests were performed on WC-Co and WC-CoCr coatings deposited by the high velocity oxy-fuel spraying method. Several analytical techniques, including X-ray diffraction, Auger electron spectroscopy and energy-dispersive spectroscopy in a transmission electron microscope were used to characterize the microstructures formed during powder processing and spraying. It was found that a substantial fraction of WC decomposed into W2C or reacted with the cobalt matrix to form ternary carbides such as Co3W3C and other mixed compounds. In both cases the binder phase had a nanocrystalline structure of size 4-8 nm containing tungsten, cobalt, carbon and chromium elements. The addition of chromium inhibits to a large extent the decomposition of WC and avoids the formation of metallic tungsten. In addition, chromium improved the erosion resistance by several times compared with the WC-Co coating. Scanning electron microscopy showed that the CoCr matrix binds carbides better than the cobalt matrix, thereby inhibiting carbide loss at the spray particle boundaries. The hydroabrasive wear behaviour of coatings and the mechanisms for material removal are discussed with respect to the microstructures formed during spraying.

Stress, strain, and microstructure in thin tungsten films deposited by dc magnetron sputtering

Abstract: Tungsten thin films were deposited on glass substrates by direct‐current planar magnetron sputtering. The induced thickness‐averaged film stress within the plane of the film was determined with the bending‐beam technique and changed from compressive to tensile on increasing working‐gas pressure. The microstructure of these films was investigated by cross‐sectional transmission electron microscopy. Compressively stressed films consisted of tightly packed columnar grains, whereas in films with a maximum value for the tensile stress the onset of a void network surrounding the columnar grains was observed. High‐pressure conditions resulted in dendritic‐like film growth, which brought about complete relaxation of internal stresses. The α phase was predominantly found in films under compression, while an increasing amount of β‐W coincided with the transition to the tensile stress regime. Special attention was focused on stress‐depth dependence and the development of two overlapping line profiles in x‐ray diffra...

Reactions of pulsed‐laser evaporated uranium atoms with molecular oxygen: Infrared spectra of UO, UO2, UO3, UO2+, UO22+, and UO3–O2 in solid argon

Abstract: Uranium atoms from the Nd:YAG laser ablation of a uranium target were codeposited with molecular oxygen and excess argon at 12 K. Infrared spectra following the U+O2 reaction revealed a wide range of reaction products. The 776.0 cm−1 band due to UO2 was the strongest product absorption, strong UO3 bands were observed at 852.5 and 745.5 cm−1, and a weak UO absorption appeared at 819.8 cm−1. These product absorptions are in agreement with earlier work, which evaporated UO2 from a tungsten Knudsen cell at 2000 °C. The 16O2/18O2 reaction gave only U 16O2 and U 18O2, which verified an insertion mechanism. New product absorptions were observed at 952.3, 892.3, and 842.4 cm−1. The 842.4 cm−1 absorption due to the UO3–O2 complex and the 892.3 cm−1 band assigned to the charge‐transfer complex (UO2+)(O2−) grew markedly at the expense of the other uranium oxides during annealing the matrix to allow diffusion and reaction of O2. With 25% 16O2, 50% 16O18O, and 25% 18O2 samples, the 952.3 cm−1 band became a sharp tripl...

Preparation of W-C:H coatings by reactive magnetron sputtering

Abstract: The reactive d.c.-magnetron sputter technique in conventional and unbalanced modes was used to prepare tungsten-containing hydrodcarbon (W-C:H) coatings which have low friction coefficients and high wear resistances. Reproducible deposition processes stable over a long time without poisoning could be realized using a plasma emission monitor (PEM) control unit. From investgigation of light emission spectra of the magnetron discharge the intensive tungsten line at 401 nm was selected as most suitable for process control. The dependences of characteristic process parameters such as target voltage, optical emission intensity or deposition rate on the reactive gas flow were quite different from those known for the reactive sputter deposition of TiN or other nitrides and oxides. For both tungsten and WC targets the PEM intensity decreased monotonically with increasing acetylene flow. However, the deposition rates on substrates with floating potential increased. An additional r.f. excitation of the substrate electrode caused only slight changes in rate and metal content in the coatings. It can be concluded that the sputter process is dominant for W-C:H growth. The contribution of direct plasma polymetrization at the substrate is small. From-X-ray diffraction investigations it was concluded that the W-C:H coatings contain microcrystallites of the metastable cubic tungsten carbide WC1 − x. The Vickers hardness and Young's modulus depend on the atomic ratio of tungsten to carbon.

Formation of solid solutions of new tungsten bronze-type microwave dielectric compounds Ba6-3xR8+2xTi18O54 (R=Nd and Sm, 0≤x≤1)

Abstract: The formation of solid solutions of microwave dielectric compounds having compositions in the vicinity of Ba3.75-R9.5Ti18O54 and/or BaOR2O34TiO2 (R=rare earth) has been investigated. 3BaO2R2O39TiO2 (Ba6R8Ti18O54) solid solutions with the ideal formulae of Ba6-3xR8+2xTi18O54 (0.0≤x≤1.0) have been derived on the tie line with the ratio Ti:O=1:3 between perovskite and R2Ti3O9 based on the new tungsten bronze-type crystal structure with 2×2 perovskitelike unit cells. The type of solid solutions is substitutional with following the relation 3Ba2+\rightleftarrows2R3++\Box. Lattice parameters as a function of composition have been determined by the WPPD (whole-powder-pattern decomposition) method.17) The formation range is located at 0.3≤x≤0.7 compositions in the case of R=Sm and 0.0≤x≤0.7 in R=Nd. In the latter case, two kinds of substitutions of the solid solutions are considered.

Lead-free bullett

Abstract: A composite lead-free bullet is disclosed comprising a heavy constituents selected from the group of tungsten, tungsten carbide, carballoy, and ferro-tungsten and a second binder constituent consisting of either a metal alloy or a plastic blend.

Phase equilibria effects on the enhanced liquid phase sintering of tungsten-copper

Abstract: The sintering behavior and mechanical properties of W-Cu are improved by the addition of elements that have solubility for W,e.g., Co, Ni, Fe, and Pd. The degree of enhancement with small concentrations of additive is dependent on specific phase diagram features, and the ranking of effectiveness does not follow the trend observed for the activated solid-state sintering of W. These observations are explained through a combination of liquid phase sintering and activated sintering theories that considers the combined W, Cu, and activator phase equilibria effects. In small concentrations, Ni and Pd have little effect on densification because they go into solution with Cu, resulting in only a slight increase in the solubility of W in the liquid phase. In this case, the sintered density, strength, and hardness increase with increasing additive concentration due to enhanced densification through solution-reprecipitation. Cobalt and Fe are the most ef-fective activators due to their limited solubility in Cu and the formation of a stable intermetallic phase with W at the sintering temperature. This promotes the formation of a high-diffusivity interboundary layer which enhances solid-state sintering of the tungsten grains at temperatures at which a liquid phase is present. With Co and Fe additions, the sintered density, strength, and hardness peak with activator concentrations of 0.35 to 0.5 wt pct. An evaluation of models for activated solid-state sintering and liquid phase sintering indicates a substantial solid-state contribution to densification when a high-diffusivity interboundary layer is present and the sol-ubility of W in the liquid phase is small.

Thermal stability of vanadia–titania catalysts

Abstract: The thermal behaviour of seven different ‘pure’ TiO2(anatase) preparations, as well as of materials obtained by doping them with potassium sulfate and carbonate, silica, and the oxides of tungsten, molybdenum and vanadium has been investigated by TG-DTA, XRD, FTIR and surface-area measurements. Vanadia–titania catalysts prepared by impregnation of these supports have also been investigated by the same techniques. The temperature at which anatase sintering and phase transformation to rutile occur strongly depends on the morphology of the TiO2 preparation. The anatase phase is much less stable in high-area, highly porous materials than in low-area powders. Vanadium oxide speeds up the anatase-to-rutile transition. However, common catalyst additives like silica, tungsten oxide and alkali-metal carbonates and sulfates strongly slow down both anatase sintering and its transformation to rutile. These phenomena influence the formulation of additives for vanadia–titania (anatase) catalysts for both selective oxidation and reduction of NOx.

Ceramic-metal composite structure and process of producing same

Abstract: A composite substrate 30 is constituted by an alumina substrate 16, a metallic layer 34, and a copper sheet 26 bonded to the alumina substrate 16 via the metallic layer 34. The metallic layer 34 is constituted by a tungsten sub-layer 34a having a low coefficient of thermal expansion, a tungsten/silver-copper alloy mixture sub-layer 34b, and a silver-copper alloy sub-layer 34c having a high coefficient of thermal expansion. In the mixture sub-layer 34b, the ratio of the percentage content of the silver-copper alloy to that of the tungsten increases with distances from the alumina substrate 16.

Refractory metal capped low resistivity metal conductor lines and vias formed using PVD and CVD

Abstract: Capping a low resistivity metal conductor line or via with a refractory metal allows for effectively using chemical-mechanical polishing techniques because the hard, reduced wear, properties of the refractory metal do not scratch, corrode, or smear during chemical-mechanical polishing. Superior conductive lines and vias are created using a combination of both physical vapor deposition (e.g., evaporation or collimated sputtering) of a low resistivity metal or alloy followed by chemical vapor deposition (CVD) of a refractory metal and subsequent planarization. Altering a ratio of SiH₄ to WF₆ during application of the refractory metal cap by CVD allows for controlled incorporation of silicon into the tungsten capping layer. Collimated sputtering allows for creating a refractory metal liner (15) in an opening (14) in a dielectric (11-13) which is suitable as a diffusion barrier to copper based metallizations (16) as well as CVD tungsten. Ideally, for faster diffusing metals like copper, liners are created by a two step collimated sputtering process wherein a first layer is deposited under relatively low vacuum pressure where directional deposition dominates (e.g., below 1mTorr) and a second layer is deposited under relatively high vacuum pressure where scattering deposition dominates (e.g., above 1mTorr). For refractory metals like CVD tungsten, the liner can be created in one step using collimated sputtering at higher vacuum pressures.

Tungsten and tungsten–carbon thin films deposited by magnetron sputtering

Abstract: Tungsten and tungsten–carbon thin films have been produced from a W target sputtered in argon and argon–methane mixtures, respectively. The deposition rate of W films was measured as a function of the sputtering power and argon pressure varying in the range of 0.3–3 Pa. The crystallographic structure and composition of W films deposited on silicon and carbon substrates were investigated by x‐ray diffraction and Rutherford backscattering spectroscopy. The electrical resistivity of the W films was minimum (12 μΩ cm) when the internal stresses in the films were negligible. The carbon concentration in the W–C films determined by nuclear reaction analyses and Rutherford backscattering spectroscopy was varied from 10 to 95 at. % with increasing CH4 content in the gas phase. The crystallographic structure of the W–C films was found to be dependent on the carbon concentration. Below 25 at. % of carbon, the structure of the W–C films was that of the cubic α‐W phase with a dilated lattice parameter. For higher carbon concentrations, the bcc α‐W phase disappeared and the structure was that of the nonstoichiometric cubic β‐WC1−x phase. The structure of W–C films with a carbon content greater than 65 at. % was nearly amorphous. Internal stresses and electrical resistivity of W–C films were determined as functions of the carbon concentration. The experimental parameters suitable to produce W and W–C films with low resistivities and reduced internal stress level are reported in this article.Tungsten and tungsten–carbon thin films have been produced from a W target sputtered in argon and argon–methane mixtures, respectively. The deposition rate of W films was measured as a function of the sputtering power and argon pressure varying in the range of 0.3–3 Pa. The crystallographic structure and composition of W films deposited on silicon and carbon substrates were investigated by x‐ray diffraction and Rutherford backscattering spectroscopy. The electrical resistivity of the W films was minimum (12 μΩ cm) when the internal stresses in the films were negligible. The carbon concentration in the W–C films determined by nuclear reaction analyses and Rutherford backscattering spectroscopy was varied from 10 to 95 at. % with increasing CH4 content in the gas phase. The crystallographic structure of the W–C films was found to be dependent on the carbon concentration. Below 25 at. % of carbon, the structure of the W–C films was that of the cubic α‐W phase with a dilated lattice parameter. For higher carb...

Method of nucleating tungsten on titanium nitride by cvd without silane.

Abstract: Nucleation of a refractory metal such as tungsten is initiated on a substrate of TiN without the use of silane by introducing hydrogen into a CVD reactor before the introduction of the reactant gas containing the metal, brought to reaction temperature and to reaction pressure. The process is most useful for CVD of tungsten onto patterned TiN coated silicon semiconductor wafers. Alternatively, hydrogen is introduced in a mixture with the metal containing gas, such as WF6, and maintained at subreaction pressure, of for example 100 mTorr, until the substrate is stabilized at a reaction temperature of approximately 400 °C or higher, to cause the dissociation of hydrogen on the wafer surface, then elevated to a relatively high reaction pressure of, for example, 60 Torr at which nucleation is achieved. Also, the reduction reaction that deposits the tungsten film proceeds without the need for a two step nucleation-deposition process.

Tungsten solution kinetics and amorphization of nickel in mechanically alloyed Ni-W alloys

Abstract: The kinetics of solution of W, and the subsequent amorphization of Ni, in mechanically alloyed Ni-W alloys has been investigated. As W is a highly abrasive material in the energy intensive devices used for mechanical alloying, we studied the above reactions in different mills. One used hardened steel balls as the grinding media, and the other Al2O3. Abrasion is common to both mills, but Fe wear debris from the hardened steel enters into solution in the Ni rich phases whereas Al2O3 debris is present as small dispersoids. The kinetics of W solution and those of subsequent amorphization do not appear strongly affected by the Fe in solution or the Al2O3 dispersoid. Tungsten dissolves in crystalline Ni in amounts in excess of the equilibrium solubility during alloying. Amorphization of the Ni phase occurs if the W content in this phase exceeds ca. 28 at. pct.

Tungsten solution kinetics and amorphization of nickel in mechanically alloyed Ni-W alloys. (Reannouncement with new availability information)

Abstract: The kinetics of solution of W, and the subsequent amorphization of Ni, in mechanically alloyed Ni W alloys has been investigated. As W is a highly abrasive material in the energy intensive devices used for mechanical alloying, we studied the above reactions in different mills. One used hardened steel balls as the grinding media, and the other Al2O3. Abrasion is common to both mills, but Fe wear debris from the hardened steel enters into solution in the Ni rich phases whereas Al2O3, debris is present as small dispersoids. The kinetics of W solution and those of subsequent amorphization do not appear strongly affected by the Fe in solution or the Al2O3 dispersoid. Tungsten dissolves in crystalline Ni in amounts in excess of the equilibrium solubility during alloying. Amorphization of the Ni phase occurs if the W content in this phase exceeds ca. 28 at.%. The kinetics of W solution in the crystalline Ni phase and of amorphization can be followed by X-ray analysis. The subsequent crystallization products, obtained by heating the amorphous phase to temperatures greater than about 1000 K, are primarily the intermetallic compounds Ni4W and NiW.

A convenient method for removing surface oxides from tungsten STM tips

Abstract: A convenient treatment that can be applied to tungsten scanning tunneling microscope tips prepared by electrochemical etching in aqueous base is described The treatment removes the majority of the tungsten oxide layer present after etching and produces a very stable tunnel junction and a high success rate in preparing tips for atomic‐resolution imaging

Tungsten and tungsten-carbon PVD multilayered structures as erosion-resistant coatings

Abstract: The aerospace industry is faced with a major erosive wear problem of engine components operating in dust environments. Various physical vapor deposition (PVD) or chemical vapor deposition (CVD) protective coatings have been used previously with limited success. The present paper deals with the elaboration and characterization of W/W-C multilayer coatings produced by magnetron sputtering from a W target in Ar and in an Ar+methane mixture respectively. This low temperature PVD process, compared with CVD processes, enables the deposition of protective coatings on Ti6A14V substrates at temperatures below 400°C. The microhardness of W-C films was found to be strongly dependent on the carbon concentration; two maximum hardness values of 26 000 MPa were obtained with W-C layers containing either 14–15 at.% or 40–45 at.%°C. This change in microhardness with the carbon content was correlated with the evolution of the crystallographic structure. Multilayer coatings of total thickness 60 μm composed of various W/W-C stacking arrangements have been elaborated with hard W-C films containing 14–15 at.% or 40–45 at.%C. These samples were subjected to erosive wear tests using a sand blast unit. As a comparison, Mo/Mo-C and Cr/Cr-C structures containing more than 10 at.%C were also tested. W/W-C coatings with an appropriate stacking arrangement and a C concentration of 14–15 at.% in W-C layers exhibited an erosion resistance improved by more than two orders of magnitude compared with that of uncoated Ti6A14V substrates. Complementary tests of the fatigue performance of Ti6A14V specimens coated with these very promising erosion-resistant coatings were also performed.

Synthesis and characterization of nanophase Group 6 metal (M) and metal carbide (M2C) powders by chemical reduction methods

Abstract: The group 6 metal chlorides CrCl[sub 3], MoCl[sub 3], MoCl[sub 4], and WCl[sub 4] were reduced in toluene solution with NaBEt[sub 3]H at room temperature to form the corresponding metal colloids in high yield. When the same metal chlorides were reduced in THF solution with LiBEt[sub 3]H and NaBEt[sub 3]H metal carbides (M[sub 2]C) were formed in approximately 95% yield. The metal and metal carbide colloids were shown to be comprised of 1-5-nm-sized particles by TEM. The metal and metal carbide powders derived from the colloids by centrifuging and washing with various solvents were characterized with SEM, TEM, EDS, and ED which showed that the powders were comprised of agglomerates of the primary 1-5-nm-sized crystallite. X-ray powder diffraction studies of these black powders exhibited broad peaks for the s-prepared powders at room temperature where the crystallite size estimated from the broadening analysis generally corresponded to the primary particle size as determined by TEM. Variable-temperature X-ray powder diffraction allowed unambiguous identification of the crystalline metal and metal carbide phases present. Carbon and hydrogen combustion elemental analysis was also used to identify the metal and metal carbide materials. Thermogravimetric analysis in air provided information on the oxidation behavior of these materials andmore » also enabled distinction between the formation of metals versus metal carbides, M[sub 2]C. Experiments to determine the exact origin of the carbide carbon were inconclusive. Treatment of nanocrystalline W with THF and THF/LiBet[sub 3]H solutions did not result in further reaction to form the metal carbide. 38 refs., 10 figs.« less

Isolated tungsten microelectromechanical structures

Abstract: A process for fabricating mechanically interconnected, released, electrically isolated metal microstructures, and circuit components and actuators produced by the process. A dielectric stack on a substrate is patterned and etched to produce trenches in which a metal such as tungsten is deposited. The dielectric is removed from selected regions of the metal to expose metal beams, and to form mechanically interconnecting, electrically insulating hinges supporting the beams. The beams and hinges are then released for relative motion. Electrical potentials may be established between adjacent beams to produce controlled motion.

Blanket tungsten etchback process using disposable spin-on-glass

Abstract: A new method of completing a tungsten contact is described. An insulator layer is formed over device structures in and on a semiconductor substrate. The insulator layer is flowed to planarize the layer. The insulator layer is covered with a spin-on-glass layer which is baked and cured. Contact openings are formed through the insulator and spin-on-glass layers to the device structures and to the substrate. A nucleation layer is formed over the spin-on-glass layer and within the contact openings. A layer of tungsten is deposited over the nucleation layer. The tungsten layer is etched back, thereby leaving the tungsten layer within the contact openings and leaving some of the tungsten layer as residue overlying the spin-on-glass layer. The spin-on-glass layer is removed, thereby removing any tungsten layer residue overlying the spin-on-glass layer. The contacts are completed by an aluminum metalization.

Effect of preparation conditions on the morphology and electrochromic properties of amorphous tungsten oxide films

Abstract: Electrochromic tungsten oxide thin films have been prepared by reactive dc‐magnetron sputtering under different deposition conditions. Through the use of electrochromic coloration and spectroscopic ellipsometry experiments, the effect of preparation conditions on film morphology and electrochromic properties has been studied. The results of this study are consistent with previous reports finding that amorphous tungsten oxide thin films are dominated by a columnar morphology and an intercolumnar void network that strongly influence the electrochromic properties. In addition real‐time spectroscopic ellipsometry (SE) experiments on the tungsten oxide electrochromic coloration process have been performed for the first time. In studies of very thin (∼15 nm) films by real‐time SE, the formation of tungsten bronze exhibits reaction‐limited kinetic behavior, and thus we are able to extract accurate dielectric function spectra for the film in different stages of the coloration process. These spectra can be understood in terms of a two‐phase physical mixture of tungsten oxide and tungsten bronze components with relative volume fractions determined in a regression analysis of the spectra.

Method for fabricating tungsten plug

Abstract: A method for fabricating a tungsten plug in a contact hole by depositing a tungsten film of a predetermined thickness several times to form a multilayer structure, thereby increasing the density of the tungsten plug. The method includes the steps of forming an insulating film over a conductive layer, removing a predetermined portion of the insulating film and thereby forming a contact hole through which the conductive layer is partially exposed, forming a glue layer over the entire exposed surface of the resulting structure including the contact hole and the exposed surface of the insulating film, depositing a blanket tungsten film to a small thickness over the glue layer, depositing a seed layer to a small thickness over the blanket tungsten film, sequentially depositing another blanket tungsten film and another seed layer over the seed layer and repeating the sequential deposition until the resulting structure fills the contact hole completely, and etching back the blanket tungsten films and seed layers disposed over the insulating film so that they remain only in the contact hole.