Tungsten Films Research Articles

Intrinsic stress analysis of tungsten-lined open TSVs

The effects of silicon etching and subsequent metallization during the fabrication of tungsten-lined open TSVs are examined using a combination of measurements and simulations. The total stress through a tungsten film deposited on a flat wafer is measured and finite element simulations are performed in order to identify the intrinsic and thermal stress components in the film. The data is then used to observe and model the stress through a TSV structure, which is etched using the DRIE process, resulting in scalloped inner sidewalls through the TSV opening. The scalloped structure is then compared to the ideal flat alternative with regard to the stress through the metal film and the TSV's electrical parameters, including resistance, capacitance, and inductance. It is found that the stress around the scallop varies significantly while the average stress through the tungsten in the flat TSV is only slightly higher than the stress observed through the scalloped structure. The resistance, capacitance, and inductance are all found to increase in the presence of scallops.

Enhanced X-ray Shielding Ability of Polymer–Nonleaded Metal Composites by Multilayer Structuring

Polymer–nonleaded metal (tungsten, Sendust) composites were prepared to study the effects of multilayer structure on X-ray shielding ability. Though the metals were nonleaded, the metal contents were low, and the composite films were thin, the composite films showed favorable X-ray shielding ability due to the uniform dispersion of metal particles in the polymer matrix and the flake alignment (Sendust). This means that multi- layered particles can effectively absorb the penetrating photons and they fill the pin holes in the composite. More uniformly aligned Sendust shows better shielding effect than randomly oriented Sendust. The lamination order of tungsten film and Sendust composite film affects the final performance of the laminated composites. This work demonstrates that nonleaded metal–polymer composite can be applied for X-ray shielding as a reliable substitute material for lead.

Hot-wire assisted ALD of tungsten films:In-situstudy of the interplay between CVD, etching, and ALD modes

In this work, we investigated an approach of hot-wire assisted ALD (HWALD), utilizing a hot (up to 2000 °C) tungsten (W) wire. Tungsten films were deposited by this method using alternating pulses of WF6 gas and atomic hydrogen (at-H). The latter was generated by catalytic dissociation of molecular hydrogen (H2) upon the hot-wire. The W films were grown on a 100-nm thick thermal SiO2. The growth process was monitored in real time by an in-situ spectroscopic ellipsometer (SE). The real-time SE monitoring revealed the coexistence of three processes: CVD, etching, and ALD of the W film. WF6 could back-stream diffuse to the hot-wire, resulting in WF6 decomposition and generation of a flux of fluorine (F). The latter caused etching of the grown W film and the filament, and provided extra tungsten supply, which might cause CVD. Higher pressure and higher carrier gas flow rate were found to largely suppress the back-stream diffusion of WF6, which efficiently limited CVD. By controlling the dose of WF6 and process pressure, the etching had also been minimized. X-ray photoelectron spectroscopy of optimized HWALD grown W revealed 99 at% of W; concentrations of oxygen and fluorine were lower than 1%, below the detection limit.

Spin polarized low-energy positron source

This paper presents an investigation of spin polarization of positrons from a source based on the decay of 22Na isotopes. Positrons are moderated by transmission through a tungsten film and electrostatically focussed and transported through a 90 deg deflector to produce a slow positron beam with polarization vector normal to the linear momentum. The polarization of the beam was determined to be about 10% by comparison with polarized electron scattering asymmetries from a thin Fe film on W(110) at 10-10 Torr. Low energy electron emission from Fe layer on W(100) surfaces under positron impact is explored. It is shown that the intensity asymmetry of the electron emission as a function of the incident positron energy can be used to estimate the polarization of the positron beam. Also several materials with long mean free paths for spin relaxation are considered as possible moderators with increased polarization of the emergent positrons.

Thermal conductivity of nitride films of Ti, Cr, and W deposited by reactive magnetron sputtering

Nitride films of Ti, Cr, and W were deposited using reactive magnetron sputtering from metal targets in argon and nitrogen plasma. TiN films with (200) orientation were achieved on silicon (100) at the substrate temperature of 500 and 600 °C. The films were polycrystalline at lower temperature. An amorphous interface layer was observed between the TiN film and Si wafer deposited at 600 °C. TiN film deposited at 600 °C showed the nitrogen to Ti ratio to be near unity, but films deposited at lower temperature were nitrogen deficient. CrN film with (200) orientation and good stoichiometry was achieved at 600 °C on Si(111) wafer but the film deposited at 500 °C showed cubic CrN and hexagonal Cr2N phases with smaller grain size and amorphous back ground in the x-ray diffraction pattern. An amorphous interface layer was not observed in the cubic CrN film on Si(111) deposited at 600 °C. Nitride film of tungsten deposited at 600 °C on Si(100) wafer was nitrogen deficient, contained both cubic W2N and hexagonal WN phases with smaller grain size. Nitride films of tungsten deposited at 500 °C were nonstoichiometric and contained cubic W2N and unreacted W phases. There was no amorphous phase formed along the interface for the tungsten nitride film deposited at 600 °C on the Si wafer. Thermal conductivity and interface thermal conductance of all the nitride films of Ti, Cr, and W were determined by transient thermoreflectance technique. The thermal conductivity of the films as function of deposition temperature, microstructure, nitrogen stoichiometry and amorphous interaction layer at the interface was determined. Tungsten nitride film containing both cubic and hexagonal phases was found to exhibit much higher thermal conductivity and interface thermal conductance. The amorphous interface layer was found to reduce effective thermal conductivity of TiN and CrN films.

Quantum theory of the effect of grain boundaries on the electrical conductivity of thin films and wires

We calculate the electrical conductivity of a metallic sample under the effects of distributed impurities and a random distribution of grain boundaries by means of a quantum mechanical procedure based on Kubo formula. Grain boundaries are represented either by a one-dimensional regular array of Dirac delta potentials (Mayadas and Shatzkes model) or by its three-dimensional extension (Szczyrbowski and Schmalzbauer model). We give formulas expressing the conductivity of bulk samples, thin films and thin wires of rectangular cross-sections in the case when the samples are bounded by perfectly flat surfaces. We find that, even in the absence of surface roughness, the conductivity in thin samples is reduced from its bulk value. If there are too many grain boundaries per unit length, or their scattering strength is high enough, there is a critical value Rc of the reflectivity R of an individual boundary such that the electrical conductivity vanishes for R>Rc. Also, the conductivity of thin wires shows a stepwise dependence on R. The effect of weak random variations in the strength or separation of the grain boundaries is computed by means of the method of correlation length. Finally, the resistivity of nanometric polycrystalline tungsten films reported in Choi et al. J. Appl. Phys. (2014) 115 104308 is tentatively analyzed by means of the present formalism.

Using atomic layer deposited tungsten to increase thermal conductivity of a packed bed

This study investigated the effective thermal conductivity (keff) of packed-beds that contained porous particles with nanoscale tungsten (W) films of different thicknesses formed by atomic layer deposition (ALD). A continuous film on the particles is vital towards increasing keff of the packed beds. For example, the keff of an alumina packed bed was increased by three times after an ∼8-nm continuous W film with 20 cycles of W ALD, whereas keff was decreased on a polymer packed bed with discontinuous, evenly dispersed W-islands due to nanoparticle scattering of phonons. For catalysts, understanding the thermal properties of these packed beds is essential for developing thermally conductive supports as alternatives to structured supports.

Optical and infrared properties of glancing angle-deposited nanostructured tungsten films.

Nanotextured tungsten thin films were obtained on a stainless steel (SS) substrate using the glancing-angle-deposition (GLAD) method. It was found that the optical absorption and thermal emittance of the SS substrate can be controlled by varying the parameters used during deposition. Finite-difference time-domain (FDTD) simulations were used to predict the optical absorption and infrared (IR) reflectance spectra of the fabricated samples, and good agreement was found between simulated and measured data. FDTD simulations were also used to predict the effect of changes in the height and periodicity of the nanotextures. These simulations show that good control over the absorption can be achieved by altering the height and periodicity of the nanostructure. These nanostructures were shown to be temperature stable up to 500°C with the addition of a protective HfO2 layer. Applications for this structure are explored, including a promising application for solar thermal energy systems.

Nanostructures of Tungsten Trioxide, Nickel Oxide and Niobium Oxide for Chemical Sensing Applications

Tungsten trioxide nanowires, nickel oxide nanowires and niobium oxide nanostructures weresynthesized by thermal oxidation, vapour-liquid-solid (VLS) technique and hydrothermal method,respectively. Alumina (2 · 2 mm2) substrates were used for growth of the different metal oxidesnanostructures. Thin films of tungsten or niobium are used as source material for WO3 and Nb2O5. Anultrathin film of gold was deposited on alumina as catalyst for NiO. All these materials were depositedby RF magnetron sputtering. The morphology was investigated by scanning electron microscope andthe functional properties of these structures were tested towards several gaseous species at differentworking temperatures. The results obtained show that these sensing materials have similarperformances considering the different n- and p-type semiconductors behaviour. Tungstentrioxide (WO3) nanowires, nickel oxide (NiO) nanowires and niobium oxide (Nb2O5) nanostructures were synthesized by thermal oxidation, vapor-liquid-solid (VLS) technique and hydrothermal method, respectively. The morphology was investigated by scanning electron microscope and the functional properties of these structures as gas sensors were tested for several gaseous species at different working temperatures. In this work a comparison between the sensitivity of these devices towards acetone is shown.

Niobium and Tungsten Oxide Nanowires for Chemical Sensor

Thin films of tungsten and niobium (WNb) were the starting point for the growth of metal oxide nanowires. RF Magnetron sputtering was used to deposit a layer of 100nm on 2 · 2 mm2 alumina substrates. Prepared samples were thermally oxidized to obtain the growth of nanostructures whose morphology was investigated by a scanning electron microscope (SEM). Structural properties of these nanostructures were investigated by RAMAN spectroscopy. Functional characterization was carried out using a homemade test chamber. Interdigitated contacts and heating elements were deposited by DC Magnetron sputtering, and samples were bonded to TO packages thanks to gold wires. The gas sensing properties of samples were analyzed in a wide range of working temperatures for different target gases in order to verify the best working condition.

The diffusion behavior of carbon in sputtered tungsten film and sintered tungsten block and its effect on diamond nucleation and growth

Diamond was done on sintered tungsten block with or without sputtered tungsten films. The effects of various depositing conditions, including methane concentration, temperature, pressure, the diamond seeding step and reaction time, on diamond growth were investigated in detail. The results show that the sputtered tungsten film has a dual effect on diamond growth. Firstly, after ultrasonication with diamond slurry, the tungsten film will adsorb a large number of diamond nanoparticles. Therefore, the nucleation density of diamond will be substantially improved. Secondly, the film will be carbonized during the deposition process and the carbon on the surface of the film will decrease. Methane concentration generally does not affect the carbonization level of the tungsten film but higher temperature will lead to a higher level of carbonization. The carbonization process of sputtered tungsten films during deposition is made up of two steps. Also, the nucleation surface of diamond was revealed. The nucleation surface was a layer of ultrasmooth and seamless nanocrystalline diamond film with high-quality and special surface architecture (tiny peaks arrays), which is potential to be applied in MEMS and field-emission devices. A potential method to prepare ultrasmooth nanocrystalline diamond films is proposed.

Coexistence of colossal stress and texture gradients in sputter deposited nanocrystalline ultra-thin metal films

This paper demonstrates experimentally that ultra-thin, nanocrystalline films can exhibit coexisting colossal stress and texture depth gradients. Their quantitative determination is possible by X-ray diffraction experiments. Whereas a uniform texture by itself is known to generally cause curvature in so-called sin2ψ plots, it is shown that the combined action of texture and stress gradients provides a separate source of curvature in sin2ψ plots (i.e., even in cases where a uniform texture does not induce such curvature). On this basis, the texture and stress depth profiles of a nanocrystalline, ultra-thin (50 nm) tungsten film could be determined.

Fabrication of Nano-Columnar Tungsten Films and Their Deuterium and Helium Ion Irradiation Effects

We fabricate nano-columnar tungsten (W) films on polycrystalline silicon substrates by magnetron sputtering using a grazing angle deposition technique. The deposition process is performed at a base pressure of 5×10−4 Pa. The intersection angle between the direction of the incident beam and the normal direction of the substrate is set as 85°. Separate as well as synergetic irradiations of 30/50 keV deuterium ions and 60 keV helium ions are carried out for the nano-columnar W. Samples of normal structure W are also irradiated under the same conditions as a comparison. Scanning electron microscopy and x-ray diffraction are used to characterize the structure of the as-prepared as well as the irradiated samples. The experimental results show that blisters are difficult to form on the surface of nano-columnar W under the irradiation conditions in this work, which could be due to the particular structure.

Deuterium retention and surface modifications of nanocrystalline tungsten films exposed to high-flux plasma

Deuterium retention studies are presented for nanostructured tungsten films exposed to high-flux deuterium plasmas. Thin tungsten films of ∼1μm thickness were deposited with pulsed laser deposition (PLD) on bulk tungsten. Surface modifications were studied with scanning electron microscopy and deuterium retention with thermal desorption spectroscopy. Three types of PLD films with different densities and crystallinity were studied after exposure to deuterium plasmas. The surface temperature ranged from about 460K at the periphery to about 520K in the centre of the targets. The films withstand the intense plasma exposure well and maintain their overall integrity. An increase of deuterium retention is observed with decreasing tungsten density and crystallite size. We found that the filling of these thin films with deuterium is significantly faster than for pre-damaged polycrystalline tungsten. We observed formation of micrometer-sized blisters as well as structures on the nanometer scale, both depending on the layer type.

Effect of annealing with pressure on tungsten film properties fabricated by atmospheric plasma spray

In this study, the effects of various annealing conditions on tungsten film fabricated by atmospheric plasma spraying (APS) were studied to analyze the APS tungsten film and an adequate strategy for the improvement of the mechanical properties of the tungsten film. In general, the use of the APS method to create tungsten films results in splat boundaries in an oxidized state not unlike semi-cracks. We therefore conducted vacuum annealing of the tungsten film at 800 °C and 950 °C under less than 0.1 Torr pressure to strengthen the binding force between splats. Additionally, we conducted press annealing in order to achieve a more dramatic improvement. The micro-hardness of the as-sprayed tungsten film, the vacuum-annealed tungsten film, and the press-annealed tungsten film were 1.55, 1.52, and 2.19 GPa respectively. By utilizing analysis methods such as electron back-scatter diffraction, scanning electron microscopy, Raman spectroscopy, and nano-indentation, we found that filling the gap between the splats, while eliminating the oxides, played a critical role in enhancing the mechanical properties of the tungsten film fabricated by means of the APS method.

Fabrication of 5-20 nm thick β-W films

A technique to fabricate 5 to 20 nm thick sputter deposited β W films on SiO2 and Si substrates is presented. This is achieved by growing tungsten on a 5 nm SiO2 layer or in an oxygen controlled environment by flowing 2 sccm of O2 during deposition. Resistivity, X-ray photoelectron spectroscopy, X-ray diffraction and reflectivity studies were performed to determine the phase and thickness of tungsten films. These results demonstrate a technique to grow this film on bare Si or a SiO2 substrate, which can enable growth on the bottom of a write unit in a non-volatile spin logic device.

Characterization of tungsten films and their hydrogen permeability

Prediction of tritium migration and its retention within fusion reactors is uncertain due to a significant role of the structural disorder that is formed on the surface layer after plasma exposure. Tungsten films deposited by any of the suitable methods are always disordered and contain a high density of hydrogen traps. Experiments on such films with hydrogen isotopes present a suitable complementary method, which improves the picture of the hydrogen interaction with fusion relevant materials. The authors report on the morphology, composition, and structure of tungsten films deposited by the thermionic vacuum arc method on highly permeable Eurofer substrates. Subsequently, hydrogen permeation studies through these films were carried out in a wide pressure range from 20 to 1000 mbars at 400 °C. The final value of the permeation coefficient for four samples after 24 h at 400 °C was between P = 3.2 × 10−14 mol H2/(m s Pa0.5) and P = 1.1 × 10−15 mol H2/(m s Pa0.5). From the time evolution of the permeation flux, it was shown that diffusivity was responsible for the difference in the steady fluxes, as solubility was roughly the same. This is confirmed by XRD data taken on these samples.

Determination of Modulus of Metal Films Using Thermoreflectance

The acoustic displacements in a film of indium pressed on to a sapphire wafer were generated by incidence of a Q-switched Nd-YAG laser beam. Acoustic displacements from tungsten film deposited on silicon substrate were also measured by the same technique. Thermoreflectance from a continuous red laser incident on the surface was used simultaneously to measure the acoustic oscillations. The acoustic oscillations were simulated using the solution to the modified wave equation. The acoustic velocity and the modulus of the metal film were determined from the simulation of the experimental results. It is shown that this technique provides a nondestructive method of determination of acoustic velocity and modulus in the metal films.

Deuterium retention in tungsten films after different heat treatments

Tungsten films deposited by magnetron sputtering on polycrystalline tungsten substrates were used as a model system to study the influence of the film microstructure on deuterium retention behavior. Different microstructures were produced by annealing the films up to recrystallization temperature and the corresponding structural changes were investigated by scanning electron microscopy combined with focused ion beam (FIB) cross sectioning. The influence of the induced structural changes on D retention was investigated by both nuclear reaction analysis and temperature-programmed desorption. D concentration in the investigated W films is higher than in polycrystalline bulk tungsten by a factor of 3. D retention in the films decreases as a function of annealing temperature. After annealing at 2000K, FIB cross-section images reveal that cavities appeared at the grain boundaries within the film and at the initial interface between the W film and W substrate. This new microstructure strongly affects the D depth profile and leads to the increase of D retention. Although a further increase of the holding time at 2000K or an increase of the annealing temperature to 2150K lead to the reduction of the retained D amount, the D concentration in the recrystallized W films cannot be reduced to a level as low as that of bulk W recrystallized at 2000K for 30min.

Deuterium-ion implantation into composite structures with tungsten coatings

The results of studying the accumulation and thermal desorption of ion-implanted deuterium from tungsten films deposited on various composite substrates are presented. The influence of the tungsten-film thickness and irradiation dose with D+ ions of intermediate energy on the amount of trapped deuterium and the shape of the spectra of the thermal desorption of deuterium in vacuum are studied. Possible mechanisms of these processes are discussed.