Tetramethylsilane Research Articles

Effect of additional sample bias in Meshed Plasma Immersion Ion Deposition (MPIID) on microstructural, surface and mechanical properties of Si-DLC films

Meshed Plasma Immersion Ion Deposition (MPIID) using cage-like hollow cathode discharge is a modified process of conventional PIID, but it allows the deposition of thick diamond-like carbon (DLC) films (up to 50μm) at a high deposition rate (up to 6.5μm/h). To further improve the DLC film properties, a new approach to the MPIID process is proposed, in which the energy of ions incident to the sample surface can be independently controlled by an additional voltage applied between the samples and the metal meshed cage. In this study, the meshed cage was biased with a pulsed DC power supply at −1350V peak voltage for the plasma generation, while the samples inside the cage were biased with a DC voltage from 0V to −500V with respect to the cage to study its effect. Si-DLC films were synthesized with a mixture of Ar, C2H2 and tetramethylsilane (TMS). After the depositions, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectrons spectroscopy (XPS), Raman spectroscopy and nanoindentation were used to study the morphology, surface roughness, chemical bonding and structure, and the surface hardness as well as the modulus of elasticity of the Si-DLC films. It was observed that the intense ion bombardment significantly densified the films, reduced the surface roughness, reduced the H and Si contents, and increased the nanohardness (H) and modulus of elasticity (E), whereas the deposition rate decreased slightly. Using the H and E data, high values of H3/E2 and H/E were obtained on the biased films, indicating the potential excellent mechanical and tribological properties of the films. In this paper, the effects of the sample bias voltage on the film properties are discussed in detail and the optimal bias voltage is presented.

Kinetics of the thermal decomposition of tetramethylsilane behind the reflected shock waves between 1058 and 1194 K

Thermal decomposition of tetramethylsilane (TMS) diluted in argon was studied behind the reflected shock waves in a single pulse shock tube (SPST) in the temperature range of 1058–1194 K. The major products formed in the decomposition are methane (CH 4) and ethylene (C 2 H 4); whereas ethane and propylene were detected in lower concentrations. The decomposition of TMS seems to be initiated via Si-C bond scission by forming methyl radicals (CH 3) and trimethylsilyl radicals ((CH 3) 3Si). The total rate coefficients obtained for the decomposition of TMS were fit to Arrhenius equation in two different temperature regions 1058–1130 K and 1130–1194 K. The temperature dependent rate coefficients obtained are k total (1058–1130 K) = (4.61±0.70) ×1018 exp (−(79.9 kcal mol −1±3.5)/RT) s −1, k total (1130-1194 K) = (1.33 ± 0.19) ×106 exp (−(15.3 kcal mol −1±3.5)/RT) s −1. The rate coefficient for the formation of CH 4 is obtained to be k methane (1058–1194 K) = (4.36 ± 1.23) ×1014 exp (−(61.9 kcal mol −1±4.9)/RT) s −1. A kinetic scheme containing 21 species and 38 elementary reactions was proposed and simulations were carried out to explain the formation of all the products in the decomposition of tetramethylsilane.

Design of a high temperature chemical vapor deposition reactor in which the effect of the condensation of exhaust gas in the outlet is minimized using computational modeling

Tetramethylsilane (TMS) was recently proposed as a safe precursor for SiC single crystal growth through high temperature chemical vapor deposition (HTCVD). Because the C content of TMS is much higher than Si, the exhaust gas from the TMS-based HTCVD contains large amounts of C which is condensed in the outlet. Because the condensed C close to the crystal growth front will influence on the thermodynamic equilibrium in the crystal growth, an optimal reactor design was highly required to exclude the effect of the condensed carbon. In this study, we report on a mass/heat transfer analysis using the finite element method (FEM) in an attempt to design an effective reactor that will minimize the effect of carbon condensation in the outlet. By applying the proposed reactor design to actual growth experiments, single 6H–SiC crystals with diameters of 50mm were successfully grown from a 6H–SiC seed. This result confirms that the proposed reactor design can be used to effectively grow 6H–SiC crystals using TMS-based HTCVD.

Evaluation of adhesion promoters for Parylene C on gold metallization

AbstractDelamination of thin film polymeric coatings from metallization layers is a common cause of failure in biomedical implants. To address the problem, different adhesion promotion techniques can be applied which include surface pre-treatment with oxygen and argon plasma and the use of different adhesion promoters. In this paper the applicability of titanium (Ti), silicon oxide (SiOx), diamond-like carbon (DLC), tetramethylsilane (TMS) and aluminium oxide (AlOx) as adhesion promoters is evaluated. A cross cut, peel and scratch test are used to qualify and quantify the adhesion before and after storage in phosphate buffered saline (PBS) for 48 hours at a temperature of 37 °C. Promising results could be achieved by a combination of Ti and DLC as well as by AlOx.

Absolute value of the nuclear magnetic shielding of silicon and germanium atoms in Si/Ge(CH3)4

The reference value of NMR magnetic shieldings, σref(X), of a nucleus X is of highest importance when theoretical analysis of chemical shifts are envisaged. For light atoms the absolute scale can be obtained using an old relationship among spin–rotation constants and magnetic shieldings, though for heavy-atom-containing molecules such nonrelativistic relationship is not valid any longer. Then, for such molecules the search for σref needs new strategies to be followed. We present here new values of σref(Si) and σref(Ge) for tetramethyl silane and tetramethyl germanium that were obtained applying a simple procedure which mix accurate experimental chemical shifts and theoretically obtained magnetic shieldings in a set of selected molecular systems. We considered that in experiments one usually measures chemical shifts, (δ). We calculated σref(Si) and σref(Ge) from a set of seventeen and sixteen heavy-halogen-containing molecules, respectively. We found out that σref[Si; Si(CH3)4] in gas phase should be close to 410.49±6.77ppm and σref[Ge; Ge(CH3)4] should be close to 1705.29±19.51ppm. Such theoretical values were obtained by performing calculations within the relativistic polarization propagator method, RelPPA, at RPA level of approach. Based on those values of σref, we recalculated the chemical shifts of the whole set of molecular systems obtaining a good reproduction of known experimental results. We also obtained a highly correlated relationship among the absolute values of the nuclear magnetic shieldings of Si and Ge atoms in halogen substituted tetrahedral compounds.

Growth of β-SiC interlayers on WC–Co substrates with varying hydrogen/tetramethylsilane flow ratio for adhesion enhancement of diamond coatings

Cubic silicon carbide (β-SiC) thin films were synthesized on cemented carbide (WC–Co) substrates as an interlayer for modifying the adhesion of diamond coatings. The influence of varying the hydrogen (H2)/tetramethylsilane (TMS) flow ratios on the microstructure, phase composition and adhesion of the β-SiC films was investigated. It was found that with the increase of the H2/TMS flow ratios, the SiC crystallite size increases from 6.5nm to 22.2nm. When the flow ratio was 40:5, the film was formed of loose cauliflower-like agglomerates, containing SiC granular particles. With the flow ratio increased from 40:5 to 120:5, the films became more uniform and denser, resulting in adhesion enhancement. However, when the ratio increased from 160:5 to 200:5, clusters composed of faceted particles replaced the agglomerates, and the adhesion reduced. The β-SiC film deposited with a H2/TMS flow ratio of 120:5 possessed a more uniform and denser structure, as well as better adhesion than the others. After subsequent diamond deposition, homogeneous nanocrystalline diamond coatings were realized on the β-SiC interlayered substrates. Compared with the results on the well-known two-step chemically etched substrates, the diamond coatings deposited on the substrates with the β-SiC interlayer possess excellent adhesion. It was also validated in this research that the β-SiC interlayer deposited with a H2/TMS flow ratio of 120:5 was effective in enhancing the adhesion of diamond coatings prepared on WC–Co substrates.

Surface modification of polyester synthetic leather with tetramethylsilane by atmospheric pressure plasma

Much works have been done on synthetic materials but scarcely on synthetic leather owing to its surface structures in terms of porosity and roughness. This paper examines the use of atmospheric pressure plasma (APP) treatment for improving the surface performance of polyester synthetic leather by use of a precursor, tetramethylsilane (TMS). Plasma deposition is regarded as an effective, simple and single-step method with low pollution. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) confirm the deposition of organosilanes on the sample's surface. The results showed that under a particular combination of treatment parameters, a hydrophobic surface was achieved on the APP treated sample with sessile drop static contact angle of 138°. The hydrophobic surface is stable without hydrophilic recovery 30 days after plasma treatment.

Characterization of tetramethylsilane for liquid-filled ionization dosimeters: Ion mobilities, free-ion yield and general recombination

Liquid-filled ionization chambers (LICs) are interesting detectors for the dosimetry of radiotherapy beams due to their water-equivalent response and high spatial resolution. Isooctane is the liquid most often used as an active medium, but other hydrocarbons, particularly tetramethylsilane (TMS), can be suitable for dosimetry.In this work we present a characterization of TMS (Merck, NMR calibration grade, purity>99.7%) for its use in LICs. The characterization consisted of measuring ion mobilities, using low dose 6MV photon pulses from a medical linac, and free-ion yield, using a continuous cobalt-60 beam (the reference beam quality used in radiotherapy dosimetry). Those values were then used to model general recombination in a TMS-filled LIC.Measured ion mobilities, (2.6±0.3)×10−8 and (3.6±0.4)×10−8m2V−1s−1, are similar to mobilities in isooctane, and two- to three-fold lower than some values reported for TMS. Such discrepancy can probably be attributed to the presence of different impurities. On the other hand, free-ion yield values obtained are approximately two-fold higher than for isooctane, in agreement with published data. Such high free-ion yield values result in a higher signal-to-noise ratio and may allow even better spatial resolution to be obtained with TMS-filled LICs. However, it comes at the cost of higher recombination effects that can compromise the operation of the chamber. Such high recombination and the low boiling point of TMS (≃28°C) make isooctane-filled LICs preferable to TMS-filled LICS for radiotherapy applications.

Novel silica-based adsorbents with activated carbon structure

The preparation of silica adsorbances from chemical vapor infiltration of activated carbon with tetramethylsilane (TMS) is shown. The method bases on a two step process. In a first step, activated carbons are infiltrated at 943 K in a nitrogen-TMS-vapor atmosphere of 200 mbar. At these conditions, the carbon skeleton is covered with silicon. The resulting material is highly porous and well adapting the surface structure of the carbon templates. It is already partly oxidized at room temperature when coming into contact with air. In a second step, the infiltrated carbons are completely oxidized in air at 853 K. At these conditions, the supporting carbon skeleton is completely burned and the silicon becomes silicon oxide. The resulting materials are highly porous with extremely large surface areas around 835 m2/g. Overall, the novel material seems to well adapt the original macro- and microstructure of the activated carbon used.

Influence of the process parameters on the characteristics of silicon-incorporated a-C:H:SiOx coatings

Silicon is considered as a unique, the most effective and interesting doping element that makes the diamond-like carbon coatings promising material for a variety of technical and medical applications. For CVD processes, the most popular silicon precursors are silane and tetramethylsilane (TMS) in combinations with methane, benzene or acetylene. Considerably less attention is paid to the other precursors of silicon, namely, these containing oxygen, particularly hexamethyldisiloxane (HMDSO). Despite few reports in this subject, so far there is a lack of the comprehensive presentation of the influence of the working gas mixture and process parameters on the overall properties of obtained layers. This work is related to in-depth analysis of the topography, morphology, chemical structure and composition of a-C:H:SiOx layers, produced using wide range of process parameters, that is based on the scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy. Results of the analysis show that the morphology, chemical structure and phase composition of synthesized layers depend on the applied negative bias potential and working atmosphere, wherein this correlation is not always straightforward.

Condensation of vapor species at the outlets in high temperature chemical vapor deposition using tetramethylsilane as a precursor for SiC bulk growth

High temperature chemical vapor deposition (HTCVD), an alternate method for crystal growth of SiC, was recently deemed to be a safe method when tetramethylsilane (TMS) is used. In this study, we report on the characteristics of condensation of vapor species as a function of geometric location and temperature by analyzing outlet closing in TMS-based HTCVD under the conditions of 2000 °C for 1–2 hours with a Si/H ratio of 6.2 × 10−4. Thermodynamic estimation of the classified reaction zones was experimentally verified by micro-Raman spectroscopy and microscopic inspections.

Selective secondary nucleation controlled (001)-texture in boron doped diamond films by increasing the concentrations of tetramethylsilane and trimethylborane

Diamond shows anisotropic physical and chemical properties, depending on its crystallographic orientations. Owing to the lack of large-scale single crystalline diamond substrates, however, the growth of textured diamond film becomes a popular way in tailoring its optical, electrical, mechanical, and thermal properties for specific applications. In the present study, we show that the (001)-texture of boron doped diamond (BDD) thin film can be controlled by a selective secondary nucleation process. To achieve this, a low concentration of tetramethylsilane (TMS) was introduced to induce a high secondary nucleation rate on the non-{001} diamond facets, hindering their lateral growth. In contrast, the continuous growth and expansion of the {001} facets are kept, which leads to the formation of (001)-textured diamond film. In addition, the orientation of the film can be continuously improved by increasing the concentration of either TMS or trimethylborane (TMB) in the gas phase within a relatively large parameter window. This study not only presents a new mechanism in controlling the textured of diamond, but also sheds some light on the fundamental research regarding the diamond growth.

Effect of tetramethylsilane flow on the deposition and tribological behaviors of silicon doped diamond-like carbon rubbed against poly(oxymethylene)

In this study, silicon doped diamond-like carbon (Si-DLC) was deposited on stainless steel (JIS SUS304) by using surface wave-excited plasma (SWP). The effects of tetramethylsilane (TMS) flow on the composition, topography, mechanical properties and tribological behavior were investigated. Pin-on-disc tribo-meter was used to investigate the tribological behavior of the Si-DLC coating rubbed against poly(oxymethylene) (POM). The results show that the deposition rate, roughness of Si-DLC increased and the hardness of Si-DLC decreased with the increase of TMS flow rate from 2 to 4 sccm; the roughness increase therein led to the increase of ploughing term of friction. The increase of adhesion term was also seen with the increase of TMS flow rate, being attributed to the decrease of hydrogen concentration in the coating. It was considered that more POM transferred onto the Si-DLC deposited at higher TMS flow rate due to larger heat generation by friction.

High-Temperature Chemical Vapor Deposition for SiC Single Crystal Bulk Growth Using Tetramethylsilane as a Precursor

SiC single bulk crystals were grown using a high-temperature chemical vapor deposition (HTCVD) method, with SiH4 and hydrocarbons as the source materials. SiH4 is a pyrophoric gas, which frequently causes fatal accidents in experiments. In this study, therefore, we propose the use of a HTCVD method using tetramethylsilane (TMS), a cheap and safe precursor, for growing SiC bulk crystals. Although TMS contains C four times more than Si in its chemical formula, a stoichiometric SiC layer was successfully synthesized from TMS in the presence of a high concentration of H2 based on the thermodynamic process design. 6H-SiC single crystals were successfully grown on a 6H-SiC seed crystal using the same process conditions. The resulting single crystalline layer was evaluated by rocking curve analysis by X-ray diffraction, which showed that the crystal properties of the grown SiC layer were improved compared to the seed crystals. This suggests that the TMS-based HTCVD method is feasible for practical use in SiC bul...

Thermal decomposition of tetramethylsilane and tetramethylgermane by flash pyrolysis vacuum ultraviolet photoionization time-of-flight mass spectrometry

Thermal decomposition of tetramethylsilane (TMS) and tetramethylgermane (TMG) was studied on a short time scale of 20–100μs using flash pyrolysis vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS). Primary decomposition of TMS and TMG occurred via loss of a methyl radical to form Si(CH3)3 and Ge(CH3)3, respectively. Both the Si(CH3)3 and Ge(CH3)3 radicals underwent secondary loss of a second methyl radical to form :Si(CH3)2 and :Ge(CH3)2, respectively. A previously unobserved secondary decomposition process in TMS involving loss of H atom from Si(CH3)3 followed by elimination of H2 to form SiC3H8, SiC3H6, and SiC3H4 was also identified. Sequential loss of the third and fourth methyl radical with significant formation of Ge and Ge2 was observed in the TMG pyrolysis. Loss of a third methyl radical in the TMS pyrolysis was not significant, while Si and SiC products were possibly produced. Secondary reactions of methyl to form unsaturated CxHy species, particularly in the TMG decomposition, were also observed.

Chemical vapor infiltration of activated carbon with tetramethylsilane

Chemical vapor infiltration of activated carbon with tetramethylsilane (TMS) at 200hPa total pressure and a gas phase concentration of 15 (mol-)% TMS in nitrogen is studied. The influence of temperature on the infiltration process is discussed in detail. Up to 873K, the infiltration is performed in the kinetically controlled regime resulting in high loadings up to around 42 (wt.-)%. The modified materials show high values for BET-surface and pore volume indicating a sufficient adoption of the infiltrated silicon layer to the surface morphology of the carbon substrates. Low oxidation resistance of the infiltrated material and EDX measurements give rise to the assumption that the infiltrated material is silicon. At higher infiltration temperatures above 873K, particles are formed which have the shape of cylindrical nanostructures. EDX measurements reveal that silicon carbide is produced at these temperatures.

Pressure Tuning of Electron Attachment to Benzoquinones in Nonpolar Fluids: Continuous Adjustment of Free Energy Changes

Changing pressure from 1 to 2500 bar continuously tunes free energy changes for electron attachment to molecules in nonpolar liquids by nearly 0.3 eV. Rate constants for electron attachment to substituted benzoquinones were determined over an extended free energy range of nearly 1 eV by a combination of solute, pressure, temperature, and use of solvents with differing energies of the quasifree electron, V0: tetramethylsilane (TMS) and 2,2,4-trimethylpentane (TMP). The rates of attachment to both benzoquinone (BQ) and 2,5-dichlorobenzoquinone in TMS increase as the pressure increases to 2500 bar, while in TMP the rates are higher but change little with pressure; the rate of attachment to fluoranil in TMS is similarly high at 1 bar but decreases with increasing pressure. Together the observed rate constants can be qualitatively interpreted to yield a rate vs free energy relation having both normal and Marcus inverted region behavior. Because the electron attachment reactions yield excited states, quantitative interpretation of the free energy dependence requires knowledge of the excited state energies. The electron enters the second lowest π* orbital to form a π*-π* excited state, which quickly relaxes to the lower n-π* excited state. The rate of attachment to this excited state is low when the free energy of reaction, ΔG°, is positive and increases as ΔG° decreases until near -0.2 eV, after which the rate decreases. While excited state energies are uncertain, reasonable estimates are obtained from absorption, excitation, and fluorescence spectra of the product radical anions measured here. The results are modeled using Marcus theory with inclusion of a high frequency molecular vibration.

Growth mechanism and composition of ultrasmooth a-C:H:Si films grown from energetic ions for superlubricity

Growth mechanism and ion energy dependence of composition of ultrasmooth a-C:H:Si films grown from ionization of tetramethylsilane (TMS) and toluene mixture at a fixed gas ratio have been investigated by varying the applied bias voltage. The dynamic scaling theory is employed to evaluate the roughness evolution of a-C:H:Si films, and to extract roughness and growth exponents of α ∼ 0.51 and β ∼ 0, respectively. The atomically smooth surface of a-C:H:Si films with Ra ∼ 0.1 nm is thermally activated by the energetic ion-impact induced subsurface “polishing” process for ion dominated deposition. The ion energy (bias voltage) plays a paramount role in determining the hydrogen incorporation, bonding structure and final stoichiometry of a-C:H:Si films. The hydrogen content in the films measured by ERDA gradually decreases from 36.7 to 17.3 at. % with increasing the bias voltage from 0.25 to 3.5 kV, while the carbon content in the films increases correspondingly from 52.5 to 70.1 at. %. The Si content is kept almost constant at ∼9–10 at. %. Depending on the ion-surface interactions, the bonding structure of a-C:H:Si films grown in different ion energy regions evolves from chain-developed polymer-like to cross-linked diamond-like to sp2-bonded a–C as revealed by XPS, Raman, and FTIR analysis. Such a structural evolution is reflected in their measured nanomechanical properties such as hardness, modulus, and compressive stress. An enhanced viscoplastic behavior (i.e., viscoplastic exponent of ∼0.06) is observed for polymeric a-C:H:Si films. A hydrogen content threshold (H > 20 at. %) exists for the as-grown a-C:H:Si films to exhibit superlow friction in dry N2 atmosphere. An extremely low friction coefficient of ∼0.001 can be obtained for polymer-like a-C:H:Si film. These near-frictionless a-C:H:Si films are strongly promising for applications in industrial lubricating systems.

Silicon carbon nitride films as passivation and antireflective coatings for silicon solar cells

A new method to obtain antireflective and passivation layers for p-type silicon solar cells is presented. Hydrogenated silicon carbon nitride (SiCxNy:H) films are obtained by low frequency plasma enhanced chemical vapor deposition (PECVD), using ammonia (NH3) and tetramethylsilane (TMS) as precursors; the influence of the deposition temperature and gas ratio on the film composition and properties is studied. It can be observed that the chemical composition of the film as well as its optical and passivation properties vary strongly with the gas ratio and temperature used. The optimal conditions with respect to the refractive index, the absorption coefficient and the minority carrier lifetime after rapid thermal annealing are determined. These conditions lead to an approximate stoichiometry of SiC0.6N0.8:H thin film including nearly 5×1021cm−3 hydrogen bonds.

Synthesis of α-SiC from tetramethylsilane by chemical vapor deposition at high temperature

Tetramethylsilane (TMS) is a common liquid-phase precursor that is used in the synthesis of β-SiC by chemical vapor deposition (CVD). At temperatures above 1500 °C, however, it has been shown that C is also formed together with SiC. In this study, based on thermodynamic modeling, we report on the successful synthesis of single-phase SiC with no evidence of the inclusion of C from TMS through CVD at temperatures over 1900 °C. X-ray diffraction data showed that α-SiC phases are formed at temperatures over 2000 °C. On the basis of these results, TMS can be used as a precursor for α-SiC synthesis.