SiF4 Research Articles

Microcrystalline silicon by plasma enhanced chemical vapor deposition from silicon tetrafluoride

The amorphous and microcrystalline silicon growth from SiF4–H2–He plasmas has been studied by exploring various parameters, such as He addition, rf power, H2 dilution, and frequency. The dilution of SiF4–H2 plasmas with He gas has been found to promote the crystalline phase and entirely microcrystalline silicon films have been obtained at low temperature (120 °C) under optimized experimental conditions. All the obtained results are discussed on the basis of etching/growth competition mechanism, in which the etching process by F atoms is selective for the amorphous phase. As such, all the experimental conditions enhancing the etching channel let the microcrystalline growth prevail. The net deposition rate, monitored by the laser reflectance interferometry, results from the difference between growth rate, rG, and etching rate, rE. Optical emission spectroscopy is a powerful in situ diagnostic tool to monitor the crystallinity degree in the material, since the [F*]⋅[He*]/[SiF2*]⋅[Hα] ratio, obtained from the peak intensities and called the Γ ratio, has been found to be proportional to the rE/rG ratio.

Nanocrystalline Silicon TFTs With 50 nm Thick Deposited Channel Layer, 10 cm2/Vs Electron Mobility and 108 On/Off Current Ratio

ABSTRACTThin film transistors were made using 50 nm thick directly deposited nanocrystalline silicon channel layers. The transistors have coplanar top gate structure. The nanocrystalline silicon was deposited from discharges in silane, hydrogen and silicon tetrafluoride. The transistors combine a high electron field effect mobility of ∼ 10 cm2/Vs with a low “off” current of ∼ 10−14 A per µm of channel length, and an “on”/“off” current ratio of ∼ 108. This result shows that directly deposited silicon can combine high mobility with low “off” currents.

The crystallisation of glasses from the ternary CaF2-CaAl2Si2O8-P2O5 system

A study of glasses from the ternary system CaF2-CaAl2Si2O8-P2O5 has been carried out. It has been shown that glasses with low phosphorus contents and high fluorite contents crystallise to fluorite. Fluorine reduces the glass transition temperature and is also required for the formation of fluorapatite (FAP). In the absence of fluorine in the glass no apatite phase is formed. Bulk nucleation of FAP is favoured for glasses with Ca:P ratios close to the apatite stoichiometry of 1.67 and with low crosslink densities. Thermal gravimetric analysis showed significant weight losses attributable to the formation of volatile silicon tetrafluoride to occur on crystallisation of the aluminium containing phases, anorthite and mullite, which supports the view that silicon tetrafluoride formation is hindered by fluorine bonding to the aluminium atoms of the glass network. Anorthite crystallisation always occurred by a surface nucleation mechanism and appeared to be favoured by the higher silicon to aluminium ratio in these glasses compared to previously studied glass compositions.

NMR Spectra of Guest–Host Complexes of Boron and Silicon Fluorides with Crown Ethers in Nonaqueous Solutions

19F, 11B, and 29Si NMR spectroscopy was used to examine the behavior of the guest–host complexes (BF3· H2O)2· 18-crown-6 · 2H2O, (BF3· H2O)2· DCH-6B, and (DCH-6A · H3O)SiF5in acetone (DCH-6B and DCH-6A are the cis-anti-cis- and cis-syn-cis-isomers of dicyclohexano-18-crown-6, respectively). It was shown that molecular boron fluoride complexes undergo partial solvolysis in acetone to yield BF3· acetone as the main product; the ionic pentafluorosilicate complex does not experience significant solvolysis transformations.

Binary molecular complexes of silicon tetrafluoride with water, methanol, and dimethyl ether. Quantum-chemical study

The molecular structures, the energies of complex formation, and the vibrational spectra of the binary molecular complexes of SiF4 with water, methanol, and dimethyl ether were calculated by the ab initio MP2 method with the basis sets up to 6-311++G(2d,2p). In the complexes, which have been detected previously by IR spectroscopy in low-temperature (12—15 K) inert matrices, the five-coordinate Si atom is in a distorted trigonal-bipyramidal environment, which is formed through the donor-acceptor interaction of the O atom with the Si atom and is additionally stabilized by the H...F hydrogen bonds.

Low Temperature Plasmaless Etching of Silicon Dioxide Film Using Chlorine Trifluoride Gas with Water Vapor

The etching characteristics of silicon dioxide films using chlorine trifluoride gas with water vapor, without using the gas discharge method, have been studied. When the sample was cooled to −50°C, a high etching rate of 400 nm/min was obtained. On the basis of in situ Fourier transform infrared spectroscopic analysis for the vapor phase and for the sample surface, the etching mechanism of the high etching rate was proposed. The mechanism consists of (i) adsorption of onto the sample surface at low temperatures, (ii) hydrogen fluoride ion formation by the reaction of hydrogen fluoride (HF) with , in which the HF is formed by the hydrolysis of , (iii) silicon tetrafluoride formation by the reaction of with Si in networks, and (iv) desorption of as a gas from the surface. It has been confirmed that low temperature plasmaless etching using a gas mixture is effective for the removal of films. In addition, the etch rate for polycrystalline silicon (poly‐Si) can be controlled by changing the substrate temperature from 25 to −50°C. This property allows for the sequential etching of native oxide and poly‐Si films. © 2000 The Electrochemical Society. All rights reserved.

Water absorption characteristics of fluorinated silicon oxide films deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition using SiH 4, SiF 4 and O 2

Fluorinated silicon oxide (SiOF) film having low dielectric constant is hygroscopic, a characteristic which leads to the increase in the dielectric constant by the formation of highly polar –OH bonds in the film. We prepared SiOF films by electron cyclotron resonance plasma enhanced chemical vapor deposition with precursors of SiH 4, SiF 4 and O 2, and investigated the water absorption characteristics of the films and the effect of hydrogen on the film stability. In addition to the unstable silicon fluoride bonds, the Si–O bonds around the free volumes were found to be the significant factor in absorbing moisture by observing the variations of the shoulder peaks in the Si–O stretching vibration bands. The SiOF films deposited with additional SiH 4 exhibit a higher resistance to water absorption than the films not treated with additional SiH 4. The reactive hydrogen atoms dissociated from SiH 4 yield the films having stable silicon fluoride bonds by scavenging fluorines. Hydration of Si–O–Si bonds around free volumes is also suppressed since the generation of free volume is reduced as the excess fluorines are removed by hydrogen.

Fractional distillation of acid contaminants from sevoflurane.

On two occasions, sevoflurane distributed for clinical practice has been found to be contaminated with compounds thought to include hydrogen fluoride (HF) and silicon tetrafluoride (SiF(4)). Both compounds can produce pulmonary injury. However, injury would require fractional distillation of the compounds during the course of sevoflurane vaporization. We hypothesized that such distillation would occur and that the compounds would vaporize more rapidly than would sevoflurane. Thus, we tested whether fractional distillation occurs during vaporization of sevoflurane containing HF or SiF(4), or from sevoflurane containing HF converted to other compounds by contact with glass. Vaporization of < 10% of the sevoflurane distilled 65%-99% of these compounds, SiF(4) distilling most rapidly, HF (converted to other acidic compounds, including SiF(4)) distilling nearly as rapidly, and HF slowest. Nuclear magnetic resonance studies indicated that HF interaction with glass changed all HF to three other compounds, one being SiF(4) and the others being unknown. HF and SiF4 distill from sevoflurane more rapidly than sevoflurane is vaporized. Measurement of acidity after sevoflurane administration may not reveal a previous presence of such contaminants.

Surface analysis of the electropolishing layer on Si(111) in ammonium fluoride solution

The influence of potential and pH on the thickness and composition of the anodic films formed on n-Si(111) in dilute ammonium fluoride solutions is investigated by photoelectron spectroscopy. A combined electrochemistry/surface analysis system with very low contamination levels is used for the investigation of the oxidic layers. XPS data show that the film thicknesses increase with decreasing pH reaching 25 Å at pH 3. A potential dependence measured at lower coverage at a pH of 4.9 reveals changes in thickness and composition by increasing the potential from flatband to +5 V. Fluorine is incorporated into these films in substantial amounts which otherwise consist predominantly of silicon oxide. Silicon fluoride and silicon oxyfluoride are found. We define a current transmissivity, T c, of the layers which shows strong variations with potential and pH. For a pH of 3, T c is about 5×10 2 larger than for pH 4.9. The potential dependence shows a variation of T c by a factor of three and an increase for thicknesses exceeding 5 Å. These observations are explained by a structural transition from a high density pseudomorphic interfacial layer to a more open structure at larger thicknesses (up to 25 Å). The analysis of the UP-spectra shows only little changes of the electron affinity χ with coverage. We obtain χ≅4.15±0.1 eV. A distinct band bending is not detected. From HeII spectra, we determine the valence band offset Δ E v between Si and the oxidic layer to be 5.2 eV.

Purely theoretical electron-impact ionization cross-sections of silicon hydrides and silicon fluorides obtained from explicitly correlated methods

Electron impact total ionization cross-sections of small silicon hydrides, SiHn(n=1–4), and fluorides, SiFn(n=1–3), have been calculated by the application of a recently developed theoretical model. The binary-encounter-Bethe (BEB) model has a simple structure and requires information from calculations on the parent ground-state molecule only (binding energies, orbital kinetic energies, and occupation numbers). Previous applications of the BEB theory to the silicon hydrides and fluorides have employed a combination of experimental and Koopman’s theorem binding energies. In the current work binding energies have been calculated using the explicitly correlated multiconfigurational spin tensor electron propagator (MCSTEP) method which gives highly accurate ionization potentials for closed- and open-shell systems. Calculations have been performed using cc-pVDZ and cc-pVTZ basis sets with multiconfigurational self-consistent field (MCSCF) reference wave functions. Comparisons are made between our MCSCF/MCSTEP and previous Hartree–Fock (HF)/Koopman’s theorem results and available experimental data. The use of improved theoretical data does not have a significant effect on the resultant cross-sections; however, our new technique is a viable method for calculating electron impact ionization cross-sections for systems where Koopman’s theorem is known to be unreliable or no experimental data is available.

Investigation of the surface morphology of thermally evaporated thin gold films on mica, glass, silicon and calcium fluoride substrates by scanning tunneling microscopy

The surface morphologies of thin gold films thermally evaporated on glass, mica, CaF 2 and Si substrates were investigated by scanning tunneling microscopy (STM) and compared to each other. The surface roughness of the gold films and average area of the grains observed was investigated as a function of the temperature and length of time of prebake of the substrates. The dependence of the gold surface roughness on substrate temperature is discussed. For substrates held at room temperature the ionic interaction between gold particles and substrate surface determines the size of the grains. Large flat areas of dimensions of the order of 200×200 nm 2 are obtained for 80 nm thick films grown on glass heated at 300°C for 6 h and areas flat over a macroscopic distance greater than 500 nm are obtained on mica heated at 400°C for the same period of time. For CaF 2 gold epitaxial growth starts to occur above 200°C. Epitaxial growth of fcc metals on alkaline halides is discussed.

Inorganic–organic hybrids of silicon fluorides of high crystallinity

An easy and simple synthetic route is reported to obtain highly crystalline lamellar silicon fluorides by using amorphous silica gel and neutral N-dialkylamines, H 2N(CH 2) n NH 2 ( n = 8, 10 and 12) as precursors. The inorganic–organic hybrids formed present a degree of crystallinity much higher than that observed for lamellar silica synthesized through the sol–gel process. This new procedure can open an important route of synthesis applied to materials science.

A comparative Raman spectroscopy study on silicon surface in HF, HF/H 2O 2 and HF/NH 4F aqueous solutions

The nature of Si(100) surfaces during immersion in dilute hydrofluoric acids (DHF), HF/H 2O 2/H 2O mixture and buffered hydrofluoric acids (BHF) has been comparatively investigated using confocal Raman spectroscopy. In DHF solution, silicon surfaces are covered mainly with silicon trihydrides (SiH 3) at the beginning of etching. As the etching goes on, silicon dihydrides (SiH 2) become main surface bonds, and silicon monohydride (SiH) signal appears clearly. In HF/H 2O 2/H 2O solution, silicon surfaces are terminated with hydrides, oxides and hydrogen-associated silicon fluorides. In BHF solution, silicon surfaces are covered with hydrides and hydrogen-associated silicon fluorides.

Regioselective Ring-opening Fluorination of Oxetanes with Silicon Tetrafluoride

Regioselective Ring-opening Fluorination of Oxetanes with Silicon Tetrafluoride

Scanning tunneling microscopy of the Si(111) surface interacting with LiF adsorbates

Submonolayers of LiF were thermally deposited on the Si(111)-7×7 surface. Scanning tunneling microscopy studies showed that there are no preferential sites in the 7×7 unit cell for adsorption of LiF. At coverages higher than 0.4 monolayer (ML), the 7×7 structure disappeared. After annealing the LiF-covered surface, most of the corner Si adatoms reappear at a temperature of 325°C, which essentially exhibits a 7×7 geometry, and the surface returned to 7×7 structure with a few atomic-level defects at 800°C. The adsorption of LiF on silicon can be understood via the interaction between the directional dangling bonds and the active dipole of the adsorbed LiF molecule or F pieces formed by dissociation of LiF. The annealing behavior of the covered surface can be understood by desorption of silicon fluorides and silicides formed following dissociation of LiF on the surface, as well as diffusion of silicon clusters.

Thermal analysis of topaz synthesis from kaolinite

The present work represents a thermal analysis study of topaz synthesis from kaolinite by its sintering with aluminium fluoride using a derivatograph. Also, it includes the study of the influence of aluminium fluoride on the thermal behaviour of kaolinite. The products of sintering were identified microscopically and by using a Siemens Crystalloflex diffractometer. The DTA and X-ray powder diffraction data show the beginning of the appearance of both topaz and mullite at 600°C and indicate that the reaction of kaolinite and aluminium fluoride of different amounts takes place in two distinct steps. The first is marked by a medium endothermic peak at 750–790°C, representing the intensive formation of topaz, and the second by a large and sharp or wide endothermic peak at 930–950°C, representing its subsequent dissociation with the formation of either mullite or (corundum and mullite) or corundum, depending on the amount of aluminium fluoride. This process is accompanied by a sharp decrease in weight, (TG curve), due to the volatilization of silicon tetrafluoride, water vapour and other gases. The synthesized topaz is colourless in thin sections and crystallizes in orthorhombic system in the form of elongated prismatic crystals, with perfect basal (0 0 1) cleavage; it is optically positive.

ChemInform Abstract: Utility of Silicon Tetrafluoride as a Catalyst of Reactions with Xenon Difluoride: Fluorination of Phenyl Alkenes and Benzaldehydes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Dominant Overall Chemical Reaction in a Chlorine Trifluoride–Silicon–Nitrogen System at Atmospheric Pressure

This study evaluates the overall chemical reaction in a chlorine trifluoride–silicon–nitrogen system at atmospheric pressure, based on the observation of the dominant chemical species in the gas phase using a quadrupole mass spectra analyzer coupled with a horizontal cold-wall single-wafer epitaxial reactor. Chlorine trifluoride gas etches the silicon surface, producing two major products, silicon tetrafluoride gas and chlorine gas, at room temperature and 530 K. The production of chlorosilanes was not observed in this study. The results obtained in this study indicate that the dominant overall chemical reaction in a chlorine trifluoride–silicon–nitrogen system is 3Si + 4ClF3 →3SiF4 ↑+ 2Cl2 ↑.

Ring-opening fluorination of epoxides using hydrofluoric acid and additives

Epoxides underwent ring-opening fluorination reaction with hydrofluoric acid in the presence of silicon fluorides and additives to give fluorohydrines in good yields.

ChemInform Abstract: Stereocontrolled Halofluorination of Glycals with Silicon Tetrafluoride, Leading to a Facile Synthesis of Glycosyl Fluorides.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.