In situ diagnostics of the decomposition of silacyclobutane on a hot filament by vacuum ultraviolet laser ionization mass spectrometry

Hydrogen elimination from the dissociation of methyl-substituted silanes on tungsten and tantalum surfaces

The elimination of H2 from the dissociation of four methyl-substituted silane molecules, including monomethylsilane (MMS), dimethylsilane (DMS), trimethylsilane (TriMS), and tetramethylsilane (TMS), on a heated tungsten or tantalum filament surface has been studied using laser ionization mass spectrometry. Two complementary ionization methods, i.e., single photon ionization (SPI) using a vacuum ultraviolet wavelength at 118 nm (10.5 eV) and a dual ionization source incorporating both 10.5 eV SPI and laser-induced electron ionization, were employed to detect the production of H2. Examination of the intensity of the H2+ peak from the four molecules has shown that it increases with temperature until reaching a plateau at around 2000−2100 °C on both tungsten and tantalum filaments. These methyl-substituted silanes are dissociatively adsorbed on tungsten and tantalum surfaces by Si−H bond cleavage, and as the temperature is raised, by C−H bond rupture. Experiments with the isotopomers of MMS, DMS, and TriMS have shown that the formation of H2 follows the Langmuir−Hinshelwood mechanism where two adsorbed hydrogen atoms on metal surfaces recombine to produce H2. The determined activation energy (Ea) for H2 formation from MMS, DMS, and TriMS, in the range of 58.2−93.4 kJ mol−1, has been found to increase with the number of methyl substitutions in the precursor molecule. Comparison of these Ea values with the reported values of 51.1−78.8 kJ mol−1 for the methyl radical formation from the same three precursor molecules has led to the conclusion that the initial Si−H bond cleavage in the dissociative adsorption of MMS, DMS, and TriMS is the rate-limiting step for the formation of both H2 molecules and ·CH3 radicals.

Rapid Identification and Quantification of Linear Olefin Isomers by Online Ozonolysis-Single Photon Ionization Time-of-Flight Mass Spectrometry

The specific locations of the double bonds in linear olefins can facilitate olefin catalytic synthetic reactions to improve the quality of target olefin products. We developed a simple and efficient approach based on single photon ionization time-of-flight mass spectrometry (SPI-TOFMS) combined with online ozonolysis to identify and quantify the linear olefin double bond positional isomers. The online ozonolysis cleaved the olefins at the double bond positions that led to formation of corresponding characteristic aldehydes. The aldehydes were then detected by SPI-TOFMS to achieve unique spectrometric "fingerprints" for each linear olefin to successfully identify the isomeric ones. To accurately quantify the isomeric components in olefin mixtures, an algorithm was proposed to quantify three isomeric olefin mixtures based on characteristic ion intensities and their equivalent ionization coefficients. The relative concentration errors for the olefin components were lower than 2.5% while the total analysis time was less than 2 min. These results demonstrate that the online ozonolysis SPI-TOFMS has the potential for real-time monitoring of catalytic olefin synthetic reactions.

Hot wire chemical vapor deposition chemistry in the gas phase and on the catalyst surface with organosilicon compounds

CONSPECTUS: Hot wire chemical vapor deposition (HWCVD), also referred to as catalytic CVD (Cat-CVD), has been used to produce Si-containing thin films, nanomaterials, and functional polymer coatings that have found wide applications in microelectronic and photovoltaic devices, in automobiles, and in biotechnology. The success of HWCVD is largely due to its various advantages, including high deposition rate, low substrate temperatures, lack of plasma-induced damage, and large-area uniformity. Film growth in HWCVD is induced by reactive species generated from primary decomposition on the metal wire or from secondary reactions in the gas phase. In order to achieve a rational and efficient optimization of the process, it is essential to identify the reactive species and to understand the chemical kinetics that govern the production of these precursor species for film growth. In this Account, we report recent progress in unraveling the complex gas-phase reaction chemistry in the HWCVD growth of silicon carbide thin films using organosilicon compounds as single-source precursors. We have demonstrated that laser ionization mass spectrometry is a powerful diagnostic tool for studying the gas-phase reaction chemistry when combined with the methods of isotope labeling and chemical trapping. The four methyl-substituted silane molecules, belonging to open-chain alkylsilanes, dissociatively adsorb on W and Ta filaments to produce methyl radical and H2 molecule. Under the typical deposition pressures, with increasing number of methyl substitution, the dominant chemistry occurring in the gas phase switches from silylene/silene reactions to free-radical short chain reactions. This change in dominant reaction intermediates from silylene/silene to methyl radicals explains the observation from thin film deposition that silicon carbide films become more C-rich with a decreasing number of Si-H bonds in the four precursor molecules. In the case of cyclic monosilacyclobutanes, we have shown that ring-opening reactions play a vital role in characterizing the reaction chemistry. On the other hand, exocyclic Si-H(CH3) bond cleavages are more important in the less-puckered disilacyclobutane molecules. Metal filaments are essential in HWCVD since they serve as catalysts to decompose precursor gases to reactive species, which initiate gas-phase reaction chemistry and thin film growth. We discuss the structural changes in metal filaments when exposed to various precursor gases. Depending on the nature of the radical intermediates formed from the hot-wire decomposition and subsequent gas-phase reactions, metal silicides and carbides can be formed. Overall, study of the gas-phase reaction chemistry in HWCVD provides important knowledge of the chemical species produced prior to their deposition on a substrate surface. This helps in identifying the major contributor to alloy formation on the filament itself and the film growth, and consequently, in determining the properties of the deposited films. An integrated knowledge of the gas-phase reaction chemistry, filament alloy formation, and thin film deposition is required for an efficient deposition of high-quality thin films and nanomaterials.

Promotion of exocyclic bond cleavages in the decomposition of 1,3-disilacyclobutane in the presence of a metal filament

The primary decomposition of 1,3-disilacyclobutane (DSCB) on a tungsten filament and its secondary gas-phase reactions in a hot-wire chemical vapor deposition (CVD) reactor have been studied using laser ionization mass spectrometry. Under the collision-free conditions, DSCB decomposes on the W filament to produce H2 molecules with an activation energy of 43.6 ± 4.1 kJ·mol(-1). With the help of the isotope labeling and chemical trapping methods, the mechanistic details in the secondary gas-phase reactions important in the hot-wire CVD reactor setup have been examined. The dominant pathway has been demonstrated to be the insertion of the cyclic 1,3-disilacyclobut-1-ylidene, generated by exocyclic Si-H bond rupture, into the Si-H bond in DSCB to form 1,1'-bis(1,3-disilacyclobutane) (174 amu). The successful trapping of 1,3-disilacyclobut-1-ylidene by both 1,3-butadiene and trimethylsilane provides compelling evidence for the existence of this cyclic silylene species in the hot-wire CVD reactor with DSCB. Other reactions operating in the reactor include the DSCB cycloreversion to form silene and the ring opening of DSCB via 1,2-H shift to produce silene/methylsilylene and 1-methylsilene/silylene. The introduction of an additional Si atom in the four-membered ring monosilacyclobutane molecule has caused two major changes in the reaction chemistry assumed by DSCB: (1) The endocyclic cycloreversion reactions that dominate in the decomposition of monosilacyclobutane molecules only play a much less important role in the dissociation of DSCB; and (2) the exocyclic bond cleavages are promoted in DSCB due to the ring stabilization caused by the introduction of one additional Si atom.

Unraveling the complex chemistry using dimethylsilane as a precursor gas in hot wire chemical vapor deposition

At low filament temperatures and short reaction time, silylene chemistry dominates. The free-radical reactions become more important with increasing temperature and time.

Asymmetrically distorted structures of monosilacyclobutane and disilacyclobutane radical cations studied by ab initio and density functional theories

The geometrical and electronic structures of a series of six monosilacyclobutane and 1,3-disilacyclobutane radical cations were systematically studied using ab initio and density functional theories. It was shown that all six radical cations possess an asymmetrically distorted structure in their ground electronic states. In the asymmetrically distorted C1 structure of monosilacyclobutane cations, one Si–C bond was elongated and the other was shortened. For the disilacyclobutane cations, two ring bonds were elongated and the other two contracted. The asymmetrical distortion was enhanced by exocyclic methyl substitutions and weakened by endocyclic Si substitution. The unpaired electron was localized mainly in the elongated σ(Si–C) ring bond(s) in all six cations. Studies of the excited electronic states of the cations provided strong support that the asymmetrical distortion in the four-membered-ring cations originates from the second-order Jahn–Teller effect. It was found that the puckered ring structures in the monosilacyclobutane molecules were maintained upon ionization, whereas 1,3-disilacyclobutane cations changed to a planar ring structure. Examination of the potential energy surfaces of all six cations showed that the Si–C ring bond elongation is the main contributor to the significant difference in the geometry change between monosilacyclobutane and disilacyclobutane species upon ionization.

Gas-phase reaction chemistry of 1,1-dimethyl-1-silacyclobutane as a precursor gas in the hot-wire chemical vapor deposition process — Formation of tetramethylsilane and trimethylsilane

The secondary gas-phase reaction products of 1,1-dimethyl-1-silacyclobutane (DMSCB) and its isotopomer, 1,1-di(perdeuteratedmethyl)-1-silacyclobutane (DMSCB-d6), in a hot-wire chemical vapour deposition reactor were investigated using vacuum UV laser single photon ionization with time-of-flight mass spectrometry. Dimethylsilylene, one of the primary decomposition products, undergoes π-type addition across the double and triple C–C bond and an insertion reaction into the Si–H bond. A short-chain reaction mechanism, initiated by methyl radicals produced in the primary decomposition, is found to exist for both the source DMSCB molecule and its stable secondary products. The formation of tetramethylsilane and trimethylsilane via the reaction of 1,1-dimethylsilene with a methyl radical and an H2 molecule, respectively, has been demonstrated. These are two new reaction channels involving 1,1-dimethylsilene in secondary gas-phase reactions.

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.

Carburization of tungsten filaments in a hot-wire chemical vapor deposition process using 1,1,3,3-tetramethyl-1,3-disilacyclobutane

The alloying of tungsten filament when using 1,1,3,3-tetramethyl-1,3-disilacyclobutane (TMDSCB) in a hot-wire chemical vapor deposition reactor was systematically studied by scanning electron microscopy, Auger electron spectroscopy, analysis of the power consumed by the filament, and in situ mass spectrometric measurements of the gas-phase species produced in the process. Only carburization of the W filament was observed. The carburization is mainly caused by the interaction of methyl radicals with the filament. Graphite as well as both WC and W2C alloys can form on the filament surface, depending on the filament temperatures and source gas pressures. Both WC and graphite are converted to W2C with the diffusion of C into the filament. It is shown that filament carburization affects the consumption rate of the source gas and the intensities of gas-phase reaction products. Gas-phase reactions dominate at T < or = 1400 degrees C. The carburization rate increases with increasing filament temperatures and dominates at T > or = 1800 degrees C.

Two-photon resonant second harmonic generation in atomic xeon

Vacuum ultraviolet (VUV) radiation at 124.8 nm (9.93 eV) was produced from two-photon resonant second harmonic generation (SHG) in a xeon gaseous medium and used to probe molecular samples of acetone, furan, thiophene, ammonia, and methane. The mass spectra recorded from the species with ionization energies below 9.93 eV were dominated by the parent ions. The parent ions were only observed when the incident UV radiation was tuned to resonate with the two-photon transition 5p(5)((2)P(3/2) (0))6p[1/2](0)<--5p(6) (1)S(0) of Xe at 80 119.474 cm(-1). The pressure dependence and the resonant nature of the parent ions observed support the mechanism for SHG as the ionization-initiated electric field induced SHG via the third-order nonlinear susceptibility chi((3)), which is enhanced by the coupling between the 5p(5)((2)P(3/2) (0))6p[1/2](0) and the nearby 5p(5)((2)P(3/2) (0))5d[1/2](1) states of Xe atoms.

Application of laser induced electron impact ionization to the deposition chemistry in the hot-wire chemical vapor deposition process with SiH4-NH3 gas mixtures

The application of a laser-induced electron impact (LIEI) ionization source in studying the gas-phase chemistry of the SiH(4)/NH(3) hot-wire chemical vapor deposition (HWCVD) system has been investigated. The LIEI source is achieved by directing an unfocused laser beam containing both 118 nm (10.5 eV) vacuum ultraviolet (VUV) and 355 nm UV radiations to the repeller plate in a time-of-flight mass spectrometer. Comparison of the LIEI source with the conventional 118 nm VUV single-photon ionization (SPI) method has demonstrated that the intensities of the chemical species with ionization potentials (IP) above 10.5 eV, e.g., H(2), N(2) and He, have been significantly enhanced with the incorporation of the LIEI source. It is found that the SPI source due to the 118 nm VUV light coexists in the LIEI source. This allows simultaneous observations of parent ions with enhanced intensity from VUV SPI and their "fingerprint" fragmentation ions from LIEI. It is, therefore, an effective tool to diagnose the gas-phase chemical species involved with both NH(3) and SiH(4) in the HWCVD reactor. In using the LIEI source to SiH(4), NH(3) and their mixtures, it has been shown that the NH(3) decomposition is suppressed with the addition of SiH(4) molecules. Examination of the NH(3) decomposition percentage and the time to reach the N(2) and H(2) steady-state intensities for various NH(3)/SiH(4) mixtures suggests that the extent of the suppression is enhanced with more SiH(4) content in the mixture. With increasing filament temperatures, the negative effect of SiH(4) becomes less important.