Understanding the Reaction Chemistry of 1,1,3,3-Tetramethyldisilazane as a Precursor Gas in a Catalytic Chemical Vapor Deposition Process

The reaction chemistry of 1,1,3,3-tetramethyldisilazane (TMDSZ) in catalytic chemical vapor deposition (Cat-CVD), including its primary decomposition on a heated W filament and secondary gas-phase reactions in a Cat-CVD reactor, was studied using 10.5 eV vacuum ultraviolet single-photon ionization and/or laser-induced electron ionization in tandem with time-of-flight mass spectrometry. It has been demonstrated that TMDSZ initially breaks down to form various species, including methyl radical (•CH3), ammonia (NH3), and 1,1-dimethylsilanimine (DMSA). The activation energies (Ea) for the formation of •CH3 and NH3 were determined to be 61.2 ± 1.0 and 42.1 ± 0.9 kJ mol-1, respectively, in the temperature range of 1400-2000 and 900-2400 °C. It was found that the formation of DMSA may have two different contributing routes, i.e., a concerted one (Ea = 33.6 ± 2.3 kJ mol-1) at lower temperatures of 900-1500 °C and a stepwise one (Ea = 155.0 ± 7.8 kJ mol-1) at higher temperatures of 2100-2400 °C. The secondary gas-phase reactions occurring in the Cat-CVD reactor environment were found to stem from two competing processes. The first one, free-radical short-chain reactions initiated by •CH3 formation and propagated by H abstraction reactions, is the dominating chemical process, producing many high-mass stable alkyl-substituted or silyl-substituted disilazane or trisilazane products via radical recombination reactions. Head-to-tail cycloaddition of unstable DMSA is the second contributing chemical process, which forms cyclodisilazane species. In addition, evidence was found for the conversion of NH3 into H2 and N2 in the Cat-CVD reactor.

Low energy Si+, SiCH5+, or C+ beam injections to silicon substrates during chemical vapor deposition with dimethylsilane

We found that the atomic-concentration-ratio of carbon to silicon (C/Si ratio) in silicon carbide (SiC) films formed by thermal chemical vapor deposition (CVD) was much greater than 1 when the source gas for CVD was dimethylsilane (DMS). Thus, we tried to change carbon-inclusion levels in the film by injecting some ion beams into a depositing SiC film during the CVD process with DMS. Three ion beams, i.e., Si+, SiCH5+, or C+ ions were injected to depositing SiC films. The energy of Si+, SiCH5+, and C+ ions was 110 eV. The temperature of the substrate was 800 °C. X-ray diffraction of the deposited films showed that 3C-SiC was included in all three samples. X-ray photoelectron spectroscopy (XPS) showed that the C/Si ratio of the obtained SiC film increased significantly following the Si+ or C+ ion beam irradiations. The XPS measurements also showed that the C/Si ratio of the SiC film obtained by injecting SiCH5+ beam during thermal CVD with DMS was lower than that of the SiC film formed by thermal CVD with DMS alone.

A Mechanistic Study of Thermal Decomposition of 1,1,2,2-Tetramethyldisilane Using Vacuum Ultraviolet Photoionization Time-of-Flight Mass Spectrometry

Thermal decomposition of 1,1,2,2-tetramethyldisilane was performed by flash pyrolysis in a SiC microreactor in the temperature range from 295 to 1340 K, followed by molecular beam sampling and vacuum ultraviolet photoionization mass spectrometry analysis. Density functional theory investigations on the energetics of reactants, intermediates, and products were carried out to support the experimental observations. Energetics for 1,1,2,2-tetramethyldisilane initiation decomposition reactions and important secondary reactions were calculated. Dimethylsilane, dimethylsilyl radicals, dimethylsilylene, trimethylsilane, and tetramethyldisilene were determined as the primary reaction products in the initiation thermal decompositions of 1,1,2,2-tetramethyldisilane. Further decomposition reactions of tetramethyldisilene, such as production of dimethylsilene (m/z = 72) and eventually SiC₃H₄ (m/z = 68) fragments, were examined. Other products from secondary reactions of dimethylsilane and dimethylsilylene such as SiC₂H_2-6 and SiCH_0-4 were also observed. The comprehensive pyrolysis mechanism of 1,1,2,2-tetramethyldisilane was proposed.

Theoretical Study of Decomposition Kinetics and Thermochemistry of Bis(dimethylamino)silane-Formation of Methyleneimine and Silanimine Species

The gas-phase decomposition kinetics and thermochemistry of bis(dimethylamino)silane (BDMAS), a potential precursor in the chemical vapor deposition of silicon nitride and silicon carbonitride thin films, were systematically studied using ab initio calculations at the CCSD(T)/6-311 + G(2d,p)//B3LYP/6-311 + + G(d,p) level of theory. The reaction routes were mapped out, exploring both the concerted and stepwise reactions in three different zones with the initial cleavage of Si-N, N-Me (Me = CH₃), and Si-H, respectively. It was found that the energy needed to break N-Me at 80.6 kcal·mol^-1 is lower than the ones for Si-N and Si-H, both at 87.4 kcal·mol^-1. When compared with tris(dimethylamino)silane (TrDMAS), it has been shown that the three bonds of N-Me, Si-N, and Si-H in BDMAS can be ruptured more easily, suggesting that BDMAS could be a more efficient precursor gas than TrDMAS. Upon decomposition, BDMAS tends to form methyleneimine and silanimine species, where four methyleneimine species and three silanimine species were produced. From the investigation of the effect of temperature on the kinetic and thermodynamic competition of different decomposition pathways, it has been demonstrated that the concerted formation of N-dimethylaminosilyl methyleneimine (H₂C═N-SiH₂NMe₂) by the elimination of CH₄ from BDMAS is the most kinetically and thermodynamically favored pathway in the whole temperature range from 0 K (ΔH₀^‡ = 62.7 kcal·mol^-1 and ΔH₀ = 7.7 kcal·mol^-1) up to 2673 K. In addition, lower temperatures favor the production of N-methyl methyleneimime (H₂C═NMe), whereas high temperatures promote the formation of N-methylsilanimine (H₂Si = NMe) and 1-dimethylamino-N-methylsilanimine (Me₂NSi = NMe).

Mechanistic Study of Thermal Decomposition of Hexamethyldisilane by Flash Pyrolysis Vacuum Ultraviolet Photoionization Time-of-Flight Mass Spectrometry and Density Functional Theory

Thermal decomposition of hexamethyldisilane (HMDS) was studied from room temperature to 1310 K using flash pyrolysis vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS). Decomposition pathways of HMDS and initial reaction intermediates were also investigated using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level. Unimolecular decomposition reactions of HMDS involving Si-Si and Si-C bond cleavage, as well as decomposition producing Me₄Si and :SiMe₂ via a three-centered elimination, were determined as the initiation reactions. Me₃SiSi(Me)₂^•, Me₄Si, Me₃Si^•, and :SiMe₂ were major products of the initiation reactions. These initial products were apt to decompose by homolytic reactions. Me₂Si═CH₂, :SiMe₂, and other silene/silylene intermediates preferred decomposing through molecular eliminations. Both homolytic and molecular elimination reactions are important in the pyrolysis of HMDS.

Catalytic dissociation of tris(dimethylamino)silane on hot tungsten and tantalum filament surfaces

The dissociation of tris(dimethylamino)silane (TrDMAS) on hot tungsten and tantalum surfaces was studied under collision-free conditions. The products from the hot-wire decomposition of TrDMAS were monitored using a 10.5 eV vacuum ultraviolet laser single-photon ionization in tandem with time-of-flight mass spectrometry. Formation of a methyl radical and N-methyl methyleneimine (NMMI) was detected. A transition from a surface reaction rate-limiting regime at filament temperatures lower than 1800-2000 °C to mass transport regime at higher temperatures (>1800-2000 °C) was observed for the formation of both products. In the surface reaction regime, the Arrhenius behavior was followed in two separate temperature regions with different activation energies. It was found that low temperatures (900-1300 °C) favor the production of the methyl radical and high temperatures (1400-2000 °C) favor the production of NMMI with lower activation energies. A theoretical investigation using ab initio calculations of the concerted and stepwise formation of NMMI along with the homolytic cleavages of N-CH₃ and Si-H in the gas phase has shown that the concerted pathway to form NMMI is the most energetically favorable one of all four routes with an activation barrier of 328 kJ mol^-1. The lower activation energy values determined experimentally for the formation of NMMI and ˙CH₃ as compared to those obtained from theoretical calculations indicate that the dissociation of TrDMAS, an N-containing organosilicon molecule, on the W and Ta surfaces is a catalytic cracking process.

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 (E_a) 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 E_a 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 E_a 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 E_a values compared with those from the theoretical calculations clearly suggest that the tungsten filament in the HWCVD reactor catalyzed the decomposition.

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.

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.

Effect of pressure on the gas-phase chemistry when using monomethylsilane and dimethylsilane in hot-wire chemical vapor deposition

The effect of source gas pressure on the gas-phase reaction chemistry of dimethylsilane (DMS) and monomethylsilane (MMS) in the hot-wire chemical vapor deposition process has been studied by examining the secondary gas-phase reaction products in a reactor using a soft laser ionization source coupled with mass spectrometry. For DMS, the increase in sample pressure has resulted in the formation of small hydrocarbons, including ethene, acetylene, propene, and propyne. This leads to a switch from silylene dominant chemistry to a free radical dominant one with the pressure increase at low filament temperatures of 1200 and 1300 °C. At the lower pressure of 0.12 Torr, the formation of 1,1,2,2-tetramethyldisilane by dimethylsilylene insertion reaction into the Si–H bond in DMS is favored over trimethylsilane produced from a free radical recombination reaction for a short reaction time. However, when the pressure is increased by 10 times, the gas-phase chemistry becomes dominated by the formation of trimethylsilane. We have demonstrated that trapping of the corresponding active intermediates by the small hydrocarbons produced in situ is responsible for the observed switch. In the study with MMS, the gas-phase chemistry is dominated by the formation of 1,2-dimethyldisilane and 1,3-disilacyclobutane at both pressures of 0.48 and 1.2 Torr. Unlike DMS, the gas-phase reaction chemistry with MMS does not involve free radicals, which are the precursors to produce small hydrocarbons. The absence of small hydrocarbons formed in situ with MMS explains the preservation in chemistry upon the increase in pressure when MMS is used as a source gas.