Secondary Gas-phase Reactions Research Articles

Decomposition of hexamethyldisilane on a hot tungsten filament and gas-phase reactions in a hot-wire chemical vapor deposition reactor

To study the effect of an Si-Si bond on gas-phase reaction chemistry in the hot-wire chemical vapor deposition (HWCVD) process with a single source alkylsilane molecule, soft ionization with a vacuum ultraviolet wavelength of 118 nm was used with time-of-flight mass spectrometry to examine the products from the primary decomposition of hexamethyldisilane (HMDS) on a heated tungsten (W) filament and from secondary gas-phase reactions in a HWCVD reactor. It is found that both Si-Si and Si-C bonds break when HMDS decomposes on the W filament. The dominance of the breakage of Si-Si over Si-C bond has been demonstrated. In the reactor, the abstraction of methyl and H atom, respectively, from the abundant HMDS molecules by the dominant primary trimethylsilyl radicals produces tetramethylsilane (TMS) and trimethylsilane (TriMS). Along with TMS and TriMS, various other alkyl-substituted silanes (m/z = 160, 204, 262) and silyl-substituted alkanes (m/z = 218, 276, 290) are also formed from radical combination reactions. With HMDS, an increasing number of Si-Si bonds are found in the gas-phase reaction products aside from the Si-C bond which has been shown to be the major bond connection in the products when TMS is used in the same reactor. Three methyl-substituted 1,3-disilacyclobutane species (m/z = 116, 130, 144) are present in the reactor with HMDS, suggesting a more active involvement from the reactive silene intermediates.

Mass spectrometric study of gas-phase chemistry in a hot-wire chemical vapor deposition reactor with tetramethylsilane

To understand the gas-phase chemistry in the processes of hot-wire chemical vapor deposition (HWCVD) with tetramethylsilane (TMS), the gas-phase products from the decomposition of TMS on a hot tungsten filament and the secondary gas-phase reactions in a HWCVD reactor have been studied using vacuum ultraviolet laser single photon ionization time-of-flight mass spectrometry. It is found that TMS decomposes on the filament to methyl and trimethylsilyl radicals. Subsequent reactions between these two primary radicals and with the parent molecule have resulted in the formation of alkyl-substituted silanes ( m/ z = 102, 116, 146) and silyl-substituted alkanes ( m/ z = 160, 174, 188, 232, 246). Small alkenes (C n H 2 n , n ≤ 4) are produced at high filament temperatures ( T ≥ 1800 °C). The dominant pathways in the secondary gas-phase reactions are found to be the biradical combination reactions involving the alkylsilyl-substituted methyl radicals formed from the H abstraction reaction of either alkyl-substituted silane or silyl-substituted alkanes by methyl radicals. Silicon carbon (Si C) bond is found to be the major bond connection in the gas-phase reaction products in mass regions higher than the parent mass. 1,1,3,3-Tetramethyl-1,3-disilacyclobutane molecule ( m/ z = 144) is believed to be formed in the HWCVD reactor, implying the existence of the unsaturated silene intermediates.

Astrochemistry of dimethyl ether

Dimethyl ether (DME, CH 3 OCH 3 ) is one of the largest organic molecules detected in the interstellar gas and shows high abundances in star-forming regions, known as hot molecular cores. The observed DME might be formed on grains or by secondary gas phase reactions from a precursor molecule, which in turn was sublimed into the gas phase from the grain surface. Studies on the stability and degradation pathways of DME therefore provide important constraints on the evolutionary cycle of large organic molecules and chemical pathways in the interstellar medium. We studied the UV photodestruction rate of DME in a solid argon matrix. DME was destroyed with a half-life of 54 seconds under laboratory conditions, which corresponds to 8.3 × 10 5 years in a dense cloud. We discuss the UV photochemistry of DME in the context of two issues: its formation mechanism and its chemistry in hot cores. Chemical models of shielded hot core regions indicate that UV photodestruction is relatively unimportant for DME, even by the internally-generated radiation field. These models clearly show that gas phase processes are almost certainly responsible for the formation of interstellar DME.

High pressure regime of plasma enhanced deposition of microcrystalline silicon

An investigation of the effect of the total gas pressure on the deposition of microcrystalline thin films form highly diluted silane in hydrogen discharges was carried out at two different frequencies. The study was performed in conditions of constant power dissipation and constant silane partial pressure in the discharge while using a series of plasma diagnostics as electrical, optical, mass spectrometric, and in situ deposition rate measurements together with a simulator of the gas phase and the surface chemistry of SiH4∕H2 discharges. The results show that both the electron density and energy are affected by the change of the total pressure and the frequency. This in turn influences the rate of high energy electron–SiH4 dissociative processes and the total SiH4 consumption, which are favored by the frequency increase for most of the pressures. Furthermore, frequency was found to have the weakest effect on the deposition rate that was enhanced at 27.12MHz only for the lowest pressure of 1Torr. On the other hand, the increase of pressure from 1to10Torr has led to an optimum of the deposition rate recorded at 2.5Torr for both frequencies. This maximum is achieved when the rate of SiH4 dissociation to free radical is rather high; the flux of species is not significantly hindered by the increase of pressure and the secondary gas phase reactions of SiH4 act mainly as an additional source of film precursors.

Fundamental studies on hydrogasification of Taiheiyo coal

Rapid hydrogasification of a Japanese Taiheiyo coal was performed in a specially developed laboratory scale batch type reactor under conditions similar to an industrial gasifier to obtain fundamental data and a comprehensive understanding of the process. The experiments were carried out at 1073 K, 7.1 MPa atmosphere, with varying reaction times from 1 to 80 s and H 2/coal ratio in the range of 0.05–0.30 g/g. It was found that coal conversion was promoted at a higher H 2/coal ratio. The amount of methane was seen to increase with reaction time, whereas most of the other products showed dynamic characteristics with maximum formation during the observed reaction time. A comparison with results of pyrolysis in helium indicated that hydrogasification is mainly composed of two steps, initial primary pyrolysis followed by secondary gas phase reactions, including hydrocracking of intermediate hydrocarbons and methanation of carbon oxides.

Electron-impact silane dissociation and deposition rate relationship in the PECVD of microcrystalline silicon thin films

An investigation of the relation between silane electron impact dissociation and deposition rates of microcrystalline silicon thin films, has been performed in highly diluted SiH 4 in H 2 discharges, by applying a combination of experimental measurements and modeling of the process. A wide range of frequencies (13.56 MHz - 50 MHz), power densities (11 mW/cm 2 -162 mW/cm 2 ) and silane partial pressures (2% - 6%) at two total SiH 4 /H 2 pressures of 0.5 Torr and 1 Torr has been studied. In the lower pressure, independent of all other discharge parameters, SiH 4 primary dissociation has been found to be responsible for about 70% of the total silane consumption in the discharge, while a fraction of 12% of the initially produced silicon hydrides are incorporated into the growing film. The increase of pressure leads to a drop of the contribution of the SiH4 primary dissociation to the total silane consumption and to an increase of the deposition efficiency of the initially produced radicals to a value of 26%. This result is attributed to the production of additional, two silicon atom precursors via secondary gas-phase reactions.

1-8.石炭のコークス化に及ぼすタールの二次気相反応の影響(Session 1 石炭・重質油)

1-8.石炭のコークス化に及ぼすタールの二次気相反応の影響(Session 1 石炭・重質油)

Spectroscopic diagnostics of organic chemistry in the protostellar environment

A combination of astronomical observations, laboratory studies, and theoretical modelling is necessary to determine the organic chemistry of dense molecular clouds. We present spectroscopic evidence for the composition and evolution of organic molecules in protostellar environments. The principal reaction pathways to complex molecule formation by catalysis on dust grains and by reactions in the interstellar gas are described. Protostellar cores, where warming of dust has induced evaporation of icy grain mantles, are excellent sites in which to study the interaction between gas phase and grain-surface chemistries. We investigate the link between organics that are observed as direct products of grain surface reactions and those which are formed by secondary gas phase reactions of evaporated surface products. Theory predicts observable correlations between specific interstellar molecules, and also which new organics are viable for detection. We discuss recent infrared observations obtained with the Infrared Space Observatory, laboratory studies of organic molecules, theories of molecule formation, and summarise recent radioastronomical searches for various complex molecules such as ethers, azaheterocyclic compounds, and amino acids.

A Simulation Model for the Pyrolysis of Orimulsion

A simulation model for the pyrolysis of Orimulsion at 500-1200 °C was developed based on previously obtained experimental results. The pyrolysis could be divided into two steps: the first step was the primary reaction, and the second was the secondary gas phase reaction. The primary reaction was completed almost instantaneously and a large amount of tar was produced by the devolatilization reaction. The secondary reaction was much slower than the primary reaction, and it controlled product distributions and selectivity. Nine reaction paths taking into account the changes of 11 products of C 10 H 20 , C 10 H 8 , C 6 H 6 , C 4 H 8 , C 2 H 4 , CH 4 , CO, CO 2 , H 2 O, H 2 , and coke were formulated. Some of the kinetic parameters of the reaction path network were obtained from the literature, and the other kinetic parameters were determined by fitting the simulated curves with the experimental data. The simulation model developed using the information was successful in calculating the complicated product distribution.

Amorphous and microcrystalline silicon by hot wire chemical vapor deposition

Amorphous hydrogenated silicon (a-Si:H) was deposited by SiH4 decomposition on a hot tungsten filament. The substrate temperature was held at 400 °C for all samples, maintaining conditions where material combining a low defect density and a low hydrogen content is obtained. A systematic study of the effects of gas pressure, substrate-to-filament distance, and filament temperature on film properties is presented, allowing insight into the growth condition required for this material as well as the significance of secondary gas phase reactions. Material of good optoelectronic quality is obtained at high growth rates. The stability with respect to light degradation was compared to typical plasma deposited films. Conditions for the transition from amorphous to microcrystalline films, observed under gas phase dilution with hydrogen, were investigated. By in situ ellipsometry and atomic force microscopy the nucleation and film morphology were shown to be significantly different from those for plasma-chemical vapor deposition material.

オリノコタールならびにオリマルジョンの熱分解・ガス化に関する基礎的研究

The deposits of Orinoco tar in Venezuela are estimated to amount to 80 percent of the deposits of crude oil in the Middle East. To examine the possibility for utilizing the Orinoco tar as chemical feedstocks, pyrolysis behavior of Orinoco tar and Orinoco tar-water emulsion (Orimulsion) was examined using a Curie point pyro-lyzer (CPP) and an entrained bed reactor. The gasification reactivity of the coke formed during the pyrolysis was also measured. Heavy tar was produced in about 80% yield from the Orinoco tar through the primary pyrolysis reaction regardless of temperature and pressure. The heavy tar, however, could be converted to valuable chemicals such as hydrocarbon gases, and BTX (benzene, toluene, xylene) in high yield through secondary gas phase reaction. At 800°C the sum of the yields of methane, ethylene, BTX and naphthalene reached more than 50%. Two kinds of coke were found to be formed: one was produced through the primary reaction, and the other was produced through the secondary gas phase reaction from aromatic compounds such as naphthalene. The yield of the former one was little dependent on pyrolysis conditions, whereas the yield of the latter one increased with the increase in the temperature and/or the residence time. At 1200°C the total coke yield more than 60%. The gasification rate of the coke was only one-seventh of that of a coal char in a steam atmosphere even at 1000°C.

Characterization of a remote hydrogen plasma reactor with electron spin resonance

Gas phase chemical reactions, of importance in the deposition of amorphous semiconductors, were studied in a remote hydrogen plasma reactor with electron spin resonance (ESR). The following reactant pairs (and their observed room-temperature rate coefficients) were characterized: (1) H+SiH4(2.4×1011 cm3 mole−1s−1), (2) D+SiH4 (2.1×1011 cm3 mole−1 s−1), and (3) H+C2H2 (1.2×1010 cm3 mole−1 s−1). The interpretation of these coefficients in terms of primary and secondary gas phase reactions is discussed, and the values are compared where possible with previously published data. In addition, the paramagnetic centers of the silicon-based film that is deposited in situ during the ESR measurement can now be microscopically identified in light of recent studies.

Aluminum deposition by ultraviolet laser photofragmentation of trimethylaluminum on Al: Identification of photoproducts and desorption dynamics

Important new mechanistic details regarding aluminum deposition by excimer laser photodecomposition of trimethylaluminum adsorbed on aluminum covered SiO2/Si surfaces have been obtained using a time-of-flight quadrupole mass spectrometer. CH3 radicals and Al–(CH3)n(n=1,2,3) species desorb efficiently at 193 nm leaving the surface with up to 0.22 eV of translational energy. No evidence is found for desorption of Al from the surface indicating that Al is more tightly bound than methyl-aluminum fragments. A threshold behavior in the laser intensity dependence of the time-of-flight transient intensity for Al(CH3)2 and AlCH3 fragments suggests a dissociation mechanism involving stepwise removal of methyl fragments. Desorbed fragment translational energy depends upon surface coverage indicating a distribution of surface binding sites. Finally, no evidence of ethane or methane desorption from the sample surface is found implying that radical recombination and hydrogen abstraction are primarily secondary gas phase reactions which are not surface initiated.

Excimer Laser Photofragmentation of TMA on Aluminum: Identification of Photoproduct Desorption Dynamics

ABSTRACTNew mechanistic details regarding aluminum deposition by ArF excimer laser photodecomposition of trimethylaluminum (TMA) adsorbed on aluminum covered SiO 2/Si substrates have been obtained using a time-of-flight quadrupole mass spectrometer. CH3 radicals and Al-(CH3)n (n = 1,2,3) species are efficiently photoejected from the surface with up to 0.22 eV of translational energy. Experiments at various TMA dosing levels reveal differences in desorbed fragment translational energy presumably associated with variations in surface site binding energy. No direct evidence is found for desorption of A1 from the surface indicating that A1 is more tightly bound than methyl-aluminum fragments. By carefully monitoring changes in fragment translational energy as an A1 deposit forms on the clean SiO2/Si substrate, we examine how the surface influences the onset of A1 growth. No evidence of ethane or methane desorption from the sample surface is found implying that radical recombination and hydrogen abstraction are primarily secondary gas phase reactions which are not surface initiated.

Chemical processes involved in the etching of silicon by xenon difluoride

The spontaneous etching of silicon by xenon difluoride has been investigated under single collision conditions using multiphoton ionization mass spectrometry for the detection of radical products. SiF2 is the only gas phase radical observed with an apparent activation energy of 6.7±0.5 kcal/mol. Electron ionization techniques were used to determine an apparent activation energy for gas phase production of SiF4 of 5.6±0.8 kcal/mol. The identification of SiF2 as the sole reactive desorbing species during the steady-state etching process is useful for understanding many aspects of primary surface processes and secondary gas phase reactions in etching.

Mass spectrometric investigation of reaction between tungsten and bromine at high temperature

The reaction between tungsten and bromine has been studied in the temperature interval between 1000 and 1500 K by means of a Knudsen double cell connected to a mass spectrometer. At pressures of bromine between 6 and 50 torr in the reaction cell only WBr 4(g) is formed while at higher pressures formation of W 2Br 6(g) could be observed as the result of a secondary gas-phase reaction. From the measured intensities of different ions the pressure-temperature curve obtained for WBr 4(g) exhibits abnormalities which could be explained in terms of the reaction kinetics leading to equilibrium conditions above 1270 K. On the basis of these considerations ΔH 4 o (−36.7 ± 1.7 kcal/mole), ΔG T o (−8.6 ± 0.6 kcal/mole) and ΔS T o (−21.3 ± 1 e.u.) have been obtained and compared with some thermodynamic data calculated using the third-law method.

Hydrogen-atom initiated decomposition of monosilane

The H-atom initiated decomposition of SiH/sub 4/ at room temperature produces mainly H/sub 2/, Si/sub 2/H/sub 6/, and a solid deposit with trace amounts of Si/sub 3/H/sub 8/ and Si/sub 4/H/sub 10/ appearing later as secondary products. The initial reaction appears to be nearly completely described by a gas-phase conversion of SiH/sub 4/ to Si/sub 2/H/sub 6/ and H/sub 2/ but the secondary gas-phase reactions are strongly influenced by surface reactions involving silicon hydride polymer. Detailed studies of the H--D distribution in the disilane product of an equimolar mixture of SiH/sub 4/ and SiD/sub 4/ show conclusively that the disilane is formed by insertion reactions of SiH/sub 2/, SiHD, and SiD/sub 2/ radicals into SiH/sub 4/ and SiD/sub 4/ and that combination of SiH/sub 3/ and SiD/sub 3/ radicals play virtually no role.

Study of Secondary Reactions in Gas Phase During the Pyrolysis of Zinc Salts

Study of Secondary Reactions in Gas Phase During the Pyrolysis of Zinc Salts