Vacuum Ultraviolet Laser Single-photon Research Articles

A kinetic study of the gas-phase reactions of 1-methylsilacyclobutane in hot wire chemical vapor deposition.

The reaction kinetics of the decomposition of 1-methylsilacyclobutane (MSCB) in a hot wire chemical vapor deposition (HWCVD) reactor was investigated. The stable reaction products were monitored using vacuum ultraviolet laser single photon ionization in tandem with time-of-flight mass spectrometry. Steady-state approximation was used to determine the rate constants of three individual decomposition pathways of MSCB, i.e., cycloreversion to form ethene and methylsilene (R1), ring opening to form propene and methylsilylene (R2), and exocyclic Si-CH3 bond cleavage to form ˙CH3 radicals (R3). The activation energies (Ea) for R2 and R3 in a HWCVD reactor were determined to be 86.6 kJ mol-1 and 106 kJ mol-1, respectively. The fact that these Ea values are close to those obtained for the MSCB decomposition on metal surfaces under collision-free conditions indicates that the heterogeneous reactions on the hot wire surface govern the gas-phase reaction kinetics in the HWCVD reactor. In addition, the Ea values obtained from a theoretical study of the decomposition kinetics using ab initio calculations at the CCSD(T)/6-311++G(3d,2p)//MP2/6-311++G(d,p) level were 62.9 kcal mol-1 (i.e., 263 kJ mol-1), 62.0 kcal mol-1 (i.e., 259 kJ mol-1), and 86.2 kcal mol-1 (i.e., 361 kJ mol-1) for R1, R2, and R3, respectively. The much lower experimental Ea values compared with those from the theoretical calculations clearly suggest that the tungsten filament in the HWCVD reactor catalyzed the decomposition.

Gas-phase reaction kinetics of 1,3-disilacyclobutane in a hot-wire chemical vapor deposition reactor

The reaction kinetics of the decomposition of 1,3-disilacyclobutane (DSCB) was investigated using a hot-wire chemical vapor deposition reactor. The reaction products were monitored using a vacuum ultraviolet laser single-photon ionization source coupled with a time-of-flight mass spectrometer. Steady-state approximation was used to determine the rate constants for three main decomposition pathways of DSCB: the exocyclic H2 elimination (k1), the cycloreversion (k2), and the ring-opening via 1,2-H shift (k3). Separate k2 and k3 were not obtained, but 2k2+k3 and k1 were determined. The activation energy (Ea) for the exocyclic H2 elimination reaction was determined to be 63.5kJ·mol−1. Compared to the Ea value of 43.6kJ·mol−1 previously obtained under collision-free conditions at much lower pressures, the value from this work is higher. This is attributed to the filament aging caused by the formation of silicon carbide (SiC) and tungsten sub-carbide (W2C) on the wire surface. The heterogeneous reactions on the metal surface also led to a much faster decay constant, koverall, for DSCB than those for each individual decomposition pathway, k1 and 2k2+k3.

Dominance of silylene chemistry in the decomposition of monomethylsilane in the presence of a heated metal filament.

The gas-phase reaction chemistry of the decomposition of monomethylsilane (MMS) has been studied in the presence of a heated metal filament in a hot-wire chemical vapor deposition (HWCVD) reactor. A 10.5 eV vacuum ultraviolet laser single-photon ionization time-of-flight mass spectrometry was employed in combination with isotope labeling and chemical trapping to examine the mechanistic details in the reaction chemistry. We have demonstrated the dominant involvement of the methylsilylene (HSiCH3) intermediate in the gas-phase reaction chemistry. Free radical and silene intermediates do not play a role. Major products are found to be H2, 1,2-dimethyldisilane (DMDS), and 1,3-disilacyclobutane (DSCB). The formation of DMDS proceeds by the insertion reaction of methylsilylene, whereas DSCB originates from the dimerization reaction of methylsilylene. Similar reaction chemistry has been observed when using the different filament materials of tungsten and tantalum in the HWCVD reactor. This indicates that changing the filament material from Ta to W does not affect the gas-phase reaction chemistry when using MMS in the HWCVD process. Finally, comparison of the reaction chemistry of MMS with those of dimethylsilane, trimethylsilane, and tetramethylsilane sheds light on the influence of increasing Si-H bonds. A switch in the dominated chemistry from free-radical short-chain reactions to silylene insertion/dimerization reactions occurs as the number of Si-H bonds increases in the four methyl-substituted silane molecules.

Decomposition of 1,1‐dimethyl‐1‐silacyclobutane on a tungsten filament—evidence of both ring CC and ring SiC bond cleavages

The decomposition of 1,1-dimethyl-1-silacyclobutane (DMSCB) on a heated tungsten filament has been studied using vacuum ultraviolet laser single photon ionization time-of-flight mass spectrometry. It is found that the decomposition of DMSCB on the W filament to form ethene and 1,1-dimethylsilene is a catalytic process. In addition, two other decomposition channels exist to produce methyl radicals via the Si-CH(3) bond cleavage and to form propene (or cyclopropane)/dimethylsilylene. It has been demonstrated that both the formation of ethene and that of propene are stepwise processes initiated by the cleavage of a ring C-C bond and a ring Si-C bond, respectively, to form diradical intermediates, followed by the breaking of the remaining central bonds in the diradicals. The formation of ethene via an initial cleavage of a ring C-C bond is dominant over that of propene via an initial cleavage of a ring Si-C bond. When the collision-free condition is voided, secondary reactions in the gas-phase produce various methyl-substituted 1,3-disilacyclobutane molecules. The dominant of all is found to be 1,1,3,3-tetramethyl-1,3-disilacyclobutane originated from the dimerization of 1,1-dimethylsilene.

A mechanistic study of gas-phase reactions with 1,1,3,3-tetramethyl-1,3-disilacyclobutane in the hot-wire chemical vapor deposition process

The gas-phase chemical species produced from both the direct decomposition of 1,1,3,3-tetramethyl-1,3-disilacyclobutane (TMDSCB) on a tungsten filament and the secondary reactions in the HWCVD reactor were identified by vacuum ultraviolet laser single-photon ionization coupled with time-of-flight mass spectrometry. TMDSCB decomposes on the filament to methyl and 1,1,3-trimethyl-1,3-disilacyclobutane-1-yl radicals. Subsequent hydrogen abstraction reactions from the parent molecule by methyl radicals and biradical combination reactions are dominant in the reactor. The formation of dimethylsilene ( m/ z = 72) through the ring Si–C bond cleavage is indirectly confirmed by the observation of the signal from 1,1,3,3,5,5-hexamethyl-1,3,5-trisilacyclohexane at m/ z = 216. Tetra- and tri-methylsilane are also found to be formed in the HWCVD reactor.

Study of tungsten filament aging in hot-wire chemical vapor deposition with silacyclobutane as a source gas and the H2 etching effect

The tungsten filament aging when using silacyclobutane (SCB) as a source gas in a hot-wire chemical vapor deposition reactor was systematically studied by the characterization of surface morphology using scanning electron microscopy and the chemical composition analysis of the filament surfaces using Auger electron spectroscopy. It is shown that filament aging involves the formation of silicides and under more severe conditions, a pure silicon deposit. At low pressures of SCB samples, e.g., 0.06 and 0.03Torr, only Si3W5 alloy was formed. Silicon-rich silicide, Si2W, was found when using a higher pressure of SCB at 0.12Torr. At the high SCB pressure of 0.12Torr and low temperatures, pure silicon was deposited on the W filament surface. It is also demonstrated that H2 can etch the aged filament at high temperatures above 1900°C. The etching products detected by the 10.5eV vacuum ultraviolet laser single photon ionization∕time-of-flight mass spectrometer include SiH4, SiCHx (x=2–5), and SiC2Hy (y=4–7).

Mass spectrometric study of gas-phase chemistry in the hot-wire CVD processes of SiH 4/NH 3 mixtures

The gas-phase reaction products of SiH 4, NH 3 and their mixtures from a hot-wire CVD chamber were investigated using laser ionization time-of-flight mass spectrometry. Both vacuum ultraviolet laser single photon ionization and laser-induced electron impact ionization were used. The main products observed from a 50% NH 3/He sample were H 2 and N 2. The study of an NH 3/SiH 4 mixture ( P NH 3 : P SiH 4 = 100:1) has shown that the NH 3 dissociation on the filament was suppressed by the presence of SiH 4 in the system. Signals from Si(NH 2) 4 and Si(NH 2) 3 species were identified as products from the 100:1 NH 3/SiH 4 mixture. The spectrum for a 1:1 NH 3/SiH 4 mixture was dominated by mass peaks characteristic of SiH 4 chemistry in the reactor, i.e. H 2, Si 2H 6, and Si 3H 8, at low temperatures. The extent to which the decomposition of NH 3 is suppressed is enhanced with more SiH 4 molecules in the system.

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.

A vacuum ultraviolet laser single-photon zero kinetic energy photoelectron spectroscopic study of the 2E3/2 ground electronic state of CH3Br+

The vacuum ultraviolet laser single-photon zero kinetic energy (ZEKE) photoelectron spectrum of the [Formula: see text]2E3/2 ground electronic state of the methyl bromide cation is reported. The spectrum is dominated by the origin band 000 of the transition [Formula: see text]2E3/2 ← [Formula: see text]1A1. In addition, the 210 band and the 311 hot band are observed. All observed bands show similar rotational contours. Simulation of the rotational contour of the origin band yields the first ionization energy of methyl bromide (85 031.2 ± 1.0 cm–1) and the rotational constants of the cation in its ground electronic state. Key words: methyl bromide, vacuum ultraviolet laser, single-photon excitation, zero kinetic energy photoelectron spectroscopy.

A spectroscopic and computer simulation study of butanol vapors

Clusters of butanol formed above neat liquid samples were entrained in a supersonic jet and probed using 10.5 eV vacuum ultraviolet laser single-photon ionization/time-of-flight mass spectrometry. The four different isomers of butanol (n-butanol, sec-butanol, iso-butanol, and tert-butanol) were studied separately to assess the influence of the structure of the alkyl chain on the formation and stability of the hydrogen bonded clusters. Most of the higher mass features observed in the mass spectra could be assigned to protonated alcohol clusters, H(ROH)n+, n⩽3; R=C4H9, that arise from facile proton-alkoxy radical/alkoxide anion dissociation. Signals due to protonated trimers were only evident in the spectra of tert- and sec-butanol. Empirical force fields, density functional theory and ab initio methods were used to identify the geometries of all clusters up to the pentamers for the different isomers. Monte Carlo simulations established vapor-phase cluster distributions, while molecular dynamics was used to assess the relative stability of the isomeric tetramers. Together, these experimental and theoretical results suggest that butanol tetramers are “magic-number” structures, and that the protonated ion signals of size n could be correlated with the neutral cluster of size n+1, provided the vapor pressures sampled in the supersonic jet exceeded equilibrium values.

A 118 nm vacuum ultraviolet laser/time-of-flight mass spectroscopic study of methanol and ethanol clusters in the vapor phase

Clusters of methanol and ethanol formed above neat liquid samples were entrained in a supersonic jet of helium and probed in the expansion using 118 nm vacuum ultraviolet laser single-photon ionization/time-of-flight (TOF) mass spectrometry. Almost every cluster ion observed in the TOF mass spectra could be represented by the formula H(ROH)n+, where R=CH3 or C2H5, and n=1–5. Formation of these species is attributed to a well-established ionization pathway where each protonated (n−1)-mer originates from its n-mer neutral parent. Signals in the TOF mass spectra due to the protonated trimers H(CH3OH)3+ and H(CH3CH2OH)3+ were found to be the most intense and provides direct evidence that these particular cluster ions are “magic-number” structures. The possible relationships between the observed ion data and the neutral cluster vapor phase distributions are discussed. In this context, methanol and ethanol vapor cluster distributions at 298.15 K and at several pressures⩾the equilibrium vapor pressure were computed using the grand canonical Monte Carlo and molecular dynamics techniques. Lastly, differences between these experiments and the results of bimolecular reaction studies are discussed.