WSi2 Research Articles

Development of Si–W transient tolerant plasma facing material

Abstract Solid W is projected as the preferred plasma facing material. Unfortunately, W surfaces could suffer radiation damage under DT operation and will melt under Type-I edge localized modes and disruption events. A possible approach is the use of a low-Z sacrificial material, like Si deposited on the W-surface to withstand a few type-I ELMs and/or disruptions via the vapor shielding effect. Accordingly, sets of Si–W test buttons were fabricated and exposed in the DIII-D lower divertor. We found that when the Si–W buttons were exposed to a few DIII-D vertical displacement event disruptions, tungsten–silicide was formed which melts at 1414 °C. This clearly indicates that the Si–W combination cannot be used as a transient tolerance surface material, since the W surface can be damaged. Even when Si is used as a wall conditioning material the Si–W surface temperature should be operated at much lower than 1400 °C.

Effect of Ar Gas Pressure on Phase Separation of Tungsten Silicides Intermetallic Compound <i><i>In Situ</i></i> Self-Propagating High Temperature Synthesis-Casting Process

Tungsten silicides intermetallic compound (WSi2) was synthesized in-situ by self-propagating high temperature synthesis-casting of WO3-Si-Al system in Ar gas environment. It is proposed that the extent of phase separation between oxide ceramic and intermetallic compound depends on the reaction temperature that made a lower viscosity and longer lifetime of the melted. The effects of inert gas pressure on densification of the intemetallic product were investigated.

First principles investigation of the electronic, elastic and vibrational properties of tungsten disilicide

Interest in tungsten disilicide is growing due to its use as protective coatings and non-volatile memory devices but fundamental investigations on tungsten disilicide vibrational properties are lacking in literature. In particular, the phonon vibration modes have never been described. This paper presents a first-principles investigation of the vibrational properties of WSi2 crystals. The elastic and electronic properties are considered as well.First, the electron band structure is computed. Extra electronic levels for the valence electrons as compared to previously published results are found, highlighting the need for state-of-the-art DFT calculations. It is shown that the ionicity plays only a little role in the W–Si bonding. Instead, a strong degree of covalency is found.The elastic constants are computed in good agreement with the available experimental data. The complete phonon density of state as well as band structure are presented and all vibration modes are described. The phonon vibrations are also correlated to IR and Raman investigations available in the literature.

Multicomponent coating for enhanced oxidation resistance of tungsten

The development of robust high temperature oxidation resistant coatings for tungsten was evaluated for a Mo–Si–B coating system that was applied by a two step process. The synthesized coating shows a graded structure composed of various amounts of molybdenum and tungsten disilicide as well as an aluminoborosilica top layer. Oxidation tests conducted at 1100–1400°C demonstrate that the coating provides effective oxidation protection for tungsten. Furthermore, the Mo–Si–B coatings provide an enhanced oxidation resistance compared to that for W–Si–B-coatings or WSi2 coatings.

Tungsten silicide films for microwave kinetic inductance detectors

Microwave kinetic inductance detectors provide highly multiplexed arrays of detectors that can be configured to operate from the sub-millimeter to the x-ray regime. We have examined two tungsten silicide alloys (W5Si3 and WSi2), which are dense alloys that provide a critical temperature tunable with composition, large kinetic inductance fraction, and high normal-state resistivity. We have fabricated superconducting resonators and provide measurement data on critical temperature, surface resistance, quality factor, noise, and quasiparticles lifetime. Tungsten silicide appears to be promising for microwave kinetic inductance detectors.

Low Temperature Chemical Vapor Deposition of Silicon-rich Tungsten Silicide Films from Tungsten Hexafluoride - Disilane Pre-activated Mixtures

Abstract Kinetics of chemical vapor deposition (CVD) of silicon-rich tungsten silicide films from WF6/Si2H6 mixtures was investigated as a function of the configuration of a hot-wall tubular reactor. The step coverage within micron-sized trenches was analyzed to determine the overall sticking probability, which is the ratio between the film-forming species deposited on a surface and the flux of that species to the surface. The overall sticking probability was found in the range 0.01 - 0.30 as the deposition temperature was varied from 120 to 360°C. The activation energy was 25 kJ/mol. These sticking probabilities were lower than those for CVD of tungsten silicide film from WF6/SiH4 mixtures, which yielded better step-coverage profiles. The extinction temperature, Tex, below which no deposition was observed, ranged from 70 ‹ Tex ‹ 80°C. Tex = 70°C corresponded to an inner reactor diameter, d, of 22 mm, and Tex = 80°C corresponded to d = 11 mm. The dependence of Tex on d strongly suggests that radical chain reactions are involved in the CVD process. The reaction producing active intermediate species occurs in the gas phase and these active species are consumed on the surface of the reactor wall. Therefore, the volume to surface ratio of the reactor may be the key factor to control Tex, and an increase in d leads to a decrease in Tex. For Tex = 80°C, pre-heating of the gas phase can be used to induce the chain reaction and increase the silicon content in the tungsten silicide films.

Composition and structure of functional groups on the surface of tungsten disilicide powder

X-ray photoelectron spectroscopy and infrared spectroscopy are used to establish that functional groups of three types form in air in the presence of water traces on the surface of WSi2 powder: silanol groups and residues of H2WO4 tungsten acid and H4[Si(W3O10)4] tungstate–silicate heteropolyacid (with the dominating role of silanol groups present on the surface in the greatest amount). It is shown that the surface properties of WSi2 are similar to those of SiO2 and SiC.

A thermogravimetric study of the reactions of tungsten disilicide with anhydrous hydrogen fluoride and fluorine

The thermogravimetric results of a study into the dry fluorination of tungsten disilicide, WSi2, using hydrogen fluoride and dilute fluorine gas as fluorinating agents, are reported. The reaction between tungsten disilicide and fluorine follows the thermodynamically preferred route, viz. the formation of the volatile tungsten hexafluoride along with gaseous silicon tetrafluoride, with the reaction starting just below 200°C. The reaction with hydrogen fluoride yields solid tungsten metal and gaseous silicon tetrafluoride, similarly thermodynamically predicted, above 250°C. No reaction is observed at low temperatures where solid tungsten tetrafluoride is expected to form. The results of a kinetic analysis of the data for the reaction with hydrogen fluoride are reported. At temperatures below 450°C, the kinetics cannot be reliably determined. In the higher temperature region, mass transport of the reactive hydrogen fluoride through the stagnant gas film surrounding the particle, and penetrating the pitted areas, controls the mass loss rate.

Fluorine Effects on NMOS Characteristics and DRAM Refresh

We observed that in chemical vapor deposition (CVD) tungsten silicide (WSix) poly gate scheme, the gate oxide thickness decreases as gate length is reduced, and it intensifies the roll-off properties of transistor. This is because the fluorine diffuses laterally from WSix to the gate sidewall oxide in addition to its vertical diffusion to the gate oxide during gate re-oxidation process. When the channel length is very small, the gate oxide thickness is further reduced due to a relative increase of the lateral diffusion than the vertical diffusion. In DRAM cells where the channel length is extremely small, we found the thinned gate oxide is a main cause of poor retention time.

Observation of Peptide-Ion Generation by Laser-Induced Surface Heating from Tungsten Silicide Surfaces

We report observation of laser desorption/ionization (LDI) of peptides from flat surfaces of tungsten silicide (WSi2). In contrast to MALDI (matrix-assisted laser desorption/ionization) and SALDI (surface-assisted laser desorption/ionization)mass spectrometry, this study did not utilize any matrices and surface nanostructures. In this work, LDI on WSi2 surfaces is demonstratedto cover a mass range up to 1,600 Da (somatostatin; monoisotopic mass = 1637.9 Da). In addition, it exhibited a high sensitivity, which could detect peptides, which could detect peptides of low femtomole levels (20 fmol for angiotensin II). The observed LDI process was discussed to be largely thermal, more specifically, due to laser-induced surface heating that is mostlikely promoted by the low thermal diffusivity (κ) of WSi2 substrate.

Analysis of Peeling Mechanism in Annealed Tungsten Silicide Thin Films

Tungsten silicide (WSix) peeling is a noticeable issue from the manufacturing viewpoint, especially as WSix is widely applied in very large scale integrated circuit (VLSI) fabrication as a gate and interconnecting material, even on a 25-nm-node NAND flash device. In this study, we attempt to determine the margin of the Si/W atomic ratios and the mechanism of WSix film peeling after thermal annealing. The use of an as-deposited WSix > 2.0 film and 30 s rapid temperature annealing (RTP) at least 750 °C for a tungsten-rich WSi1.85 film are the minimal conditions for preventing peeling. Moreover, we found that silicon and phosphorus atoms diffuse upward on WSix films, driven out of the underlying doped poly-Si film, while they acquire sufficient thermal budgets based on energy dispersive X-ray (EDX) analysis and secondary-ion mass spectroscopy (SIMS). During this process, the strong mutual interaction and the rough interface formation between the tetragonal phase of WSix and polycrystalline silicon (poly-Si) enhance the adhesion of WSix films. A strong adhesion might reduce the risk of peeling in the subsequent processes.

Oxidation behaviour of silicon-free tungsten alloys for use as the first wall material

The use of self-passivating tungsten alloys as armour material of the first wall of a fusion power reactor may be advantageous concerning safety issues. In earlier studies good performance of the system W−Cr−Si was demonstrated. Thin films of such alloys showed a strongly reduced oxidation rate compared to pure tungsten. However, the formation of brittle tungsten silicides may be disadvantageous for the powder metallurgical production of bulk W−Cr−Si alloys if a good workability is needed. This paper shows the results of screening tests to identify suitable silicon-free alloys with distinguished self-passivation and a potentially good workability. Of all the tested systems W−Cr−Ti alloys showed the most promising results. The oxidation rate was even lower than the one of W−Cr−Si alloys, the reduction factor was about four orders of magnitude compared to pure tungsten. This performance could be conserved even if the content of alloying elements was reduced.

Analysis of Peeling Mechanism in Annealed Tungsten Silicide Thin Films

Tungsten silicide (WSix) peeling is a noticeable issue from the manufacturing viewpoint, especially as WSix is widely applied in very large scale integrated circuit (VLSI) fabrication as a gate and interconnecting material, even on a 25-nm-node NAND flash device. In this study, we attempt to determine the margin of the Si/W atomic ratios and the mechanism of WSix film peeling after thermal annealing. The use of an as-deposited WSix > 2.0 film and 30 s rapid temperature annealing (RTP) at least 750 °C for a tungsten-rich WSi1.85 film are the minimal conditions for preventing peeling. Moreover, we found that silicon and phosphorus atoms diffuse upward on WSix films, driven out of the underlying doped poly-Si film, while they acquire sufficient thermal budgets based on energy dispersive X-ray (EDX) analysis and secondary-ion mass spectroscopy (SIMS). During this process, the strong mutual interaction and the rough interface formation between the tetragonal phase of WSix and polycrystalline silicon (poly-Si) enhance the adhesion of WSix films. A strong adhesion might reduce the risk of peeling in the subsequent processes.

Microstructure and mechanical properties of SiC-nanowire-augmented tungsten composites

The effect of an addition of SiC nanowire on the microstructure and mechanical properties of tungsten-based composites is investigated in this study. SiC-nanowire-augmented tungsten composites were prepared by a spray-drying process and an in situ spark plasma sintering process. Three distinctive reaction phases, tungsten, tungsten carbide (W 2C) and rod-type tungsten silicide (W 5Si 3) were formed during the sintering process. The flexural strength was significantly increased from 706 MPa to 924 MPa in tungsten composites augmented with SiC nanowires, as was the formation of W 2C and W 5Si 3 phases. The rod-type W 5Si 3 bears significant stress by both sharing a portion of the load and providing a bridging mechanism. Furthermore, a high ablation resistance at an elevated temperature was observed for tungsten composites augmented with SiC nanowires.

Cast silicides of molybdenum, tungsten, and niobium by combustion synthesis

Cast silicides of molybdenum, tungsten, and niobium were prepared by combustion synthesis under nitrogen pressure from powder mixtures of respective metal oxides with Al and Si. Upon variation in green composition, cast MoSi2, WSi2, NbSi2, and their solid solutions with variable phase composition have been synthesized. NbSi2 was obtained by combustion in the presence of added heat-generating CaO2-Al powder mixture.

DC magnetron sputter‐deposited tungsten silicide films for microelectronic applications

PurposeThe purpose of this paper is to share valuable information about metallization in microelectronic industries by implementing tungsten silicide (WSi) thin film materials.Design/methodology/approachDirect current plasma magnetron sputtering technique was employed for the WSi film growth. Different sputtering parameters were investigated, and the WSi films were characterized using four‐point probe electrical measurement method.FindingsThe experimental results reveal that the sputtering parameters such as deposition pressure and substrate temperature exert significant influence on the electrical properties of the WSi films.Research limitations/implicationsBy tuning the sputtering parameters, the electrical properties of the WSi films can be optimized and the film resistivity can be reduced significantly.Practical implicationsThe investigation results presented in this paper are useful information for microelectronic industries in the area of microelectronic devices metallization.Originality/valueThe fabrication method described in this paper allows fabricating low‐resistivity WSi films by employing a lower deposition pressure and a lower substrate temperature.

Vapor–solid–solid Si nano-whiskers growth using pure hydrogen as the source gas

This paper describes the growth mechanism and structure of Si whiskers grown on a Si substrate in a tungsten hot filament chemical vapor deposition reactor with pure hydrogen as source gas using a two step process. In the first step, atomic hydrogen etched the silicon surface, forming silicon hydrides that react with tungsten from the filament. The resulting silicide particles deposited on the silicon surface forming a mesh-like pattern. The particles work as an etching mask against hydrogen radical etching of the silicon surface and inverted-pyramids or V-groove-shaped surface texture were obtained. In the second step the filament current was reduced and whiskers grew onto the substrate due to the interaction of the silicon hydrides with the particles and subsequent precipitation of saturated Si. The whiskers were found to have tungsten silicide particles on their tip, suggesting the whisker growth was through the Vapor–Solid–Solid (VSS) mechanism. A balance between the hydrogen radical etching effect and the supply of silicon hydride from the etching reaction on the silicon surface is crucial for the growth of dense silicon whiskers.

Effect of filament temperature and deposition time on the formation of tungsten silicide with silane

The effect of filament temperature and deposition time on the formation of tungsten silicide upon exposure to the SiH 4 gas in a hot wire chemical vapor deposition process was studied using the techniques of cross-sectional scanning electron microscopy and Auger electron spectroscopy. At a relatively low temperature of 1500 °C, the decomposition of WSi 2 phase and the diffusion of Si towards the silicide/W interface produce a thick W 5Si 3 layer. The diffusional nature leads to a parabolic rate law for silicide growth. An exponential decrease of silicide thickness with temperature between 1600 and 2000 °C illustrates the dominance of Si evaporation at higher temperatures (T ≥ 1600 °C) over the silicide formation.

Near-Infrared Polarizer with Tungsten Silicide Wire Grids

We fabricated a near-infrared wire-grid polarizer consisting of a 230-nm-pitch tungsten silicide (WSi) grating on a SiO2 substrate using two-beam interference lithography and dry etching. The transverse magnetic (TM) polarization transmittance of the fabricated polarizer exceeded 80% in the 1000–1600-nm wavelength range. The extinction ratio was higher than 20 dB in the 650–1500-nm wavelength range. We also measured the extinction coefficient κ of WSi and verified that WSi is a suitable polarizing material in the near-infrared range.

Characterization of condensed phase beryllium species in the presence of aluminium and silicon matrices during electrothermal heating on graphite and tungsten platforms

Condensed phase beryllium species occurring on graphite and tungsten platforms in the presence of aluminium and silicon matrices were characterised over a wide temperature range. The solid residue was viewed by scanning electron microscopy (SEM), while the chemical composition was probed by energy dispersive (ED) X-ray spectrometry (XRS), Fourier transform-infrared (FT-IR) spectrometry and Raman microanalysis. Beryllium oxide phases were found to be the predominant species over a wide temperature range, persisting up to about 1800 °C on the graphite platform. Beryllium metal species were also identified at high temperatures (1500 °C), but the transformation of beryllium oxide to beryllium was influenced by the amount and localised behaviour of concomitant species on the platform surface. For graphite platform atomisation, aluminium and silicon concomitants are present as metal oxides. Other silicon species, such as silicon carbide, were found mainly at temperatures above 900 °C. Little or no beryllium oxide was found on tungsten platforms up to 1800 °C, although there was evidence of some beryllium alloyed to tungsten. Tungsten from the platform supports some hydration forming different tungsten oxidation states (W6+, W5+, W4+). Also, at 900 °C, silicon was present as an oxide, but also as elemental silicon, silicon carbide, and silicon alloyed to tungsten forming tungsten disilicide at the surface interface. When tungsten platform atomisation was used for samples containing silicon, evidence of degradation of the graphite tube through formation of carbon clusters and nanostructures was more easily noticeable and evaluated by Raman spectrometry.