Detailed Kinetic Modeling of Silicon Nanoparticle Formation Chemistry via Automated Mechanism Generation (original) (raw)
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Aerosol Science and Technology, 2009
In this work, a two-dimensional model was developed for silicon nanoparticle synthesis by silane thermal decomposition in a sixway cross laser-driven aerosol reactor. This two-dimensional model incorporates fluid dynamics, laser heating, gas phase and surface phase chemical reactions, and aerosol dynamics, with particle transport and evolution by convection, diffusion, thermophoresis, nucleation, surface growth, coagulation, and coalescence processes. Because of the complexity of the problem at hand, the simulation was carried out via several sub-models. First, the chemically reacting flow inside the reactor was simulated in three dimensions in full geometric detail, but with no aerosol dynamics and with highly simplified chemistry. Second, the reaction zone was simulated using an axisymmetric two-dimensional CFD model, whose boundary conditions were obtained from the first step. Last, a twodimensional aerosol dynamics model was used to study the silicon nanoparticle formation using more complete silane decomposition chemistry, together with the temperature and velocities extracted from the reaction zone CFD simulation. A bivariate model was used to describe the evolution of particle size and morphology. The aggregates were modeled by a moment method, assuming a lognormal distribution in particle volume. This was augmented by a single balance equation for primary particles that assumed locally equal number of primary particles per aggregate and fractal dimension. The model predicted the position and size at which the primary particle size is frozen in, and showed that increasing the peak temperature was a more effective means of improving particle yield than increasing silane concentration or flowrate.
The Journal of Physical Chemistry B, 1999
Product contamination by particles nucleated within the processing environment often limits the deposition rate during chemical vapor deposition processes. A fundamental understanding of how these particles nucleate could allow higher growth rates while minimizing particle contamination. Here we present an extensive chemical kinetic mechanism for silicon hydride cluster formation during silane pyrolysis. This mechanism includes detailed chemical information about the relative stability and reactivity of different possible silicon hydride clusters. It provides a means of calculating a particle nucleation rate that can be used as the nucleation source term in aerosol dynamics models that predict particle formation, growth, and transport. A group additivity method was developed to estimate thermochemical properties of the silicon hydride clusters. Reactivity rules for the silicon hydride clusters were proposed based on the group additivity estimates for the reaction thermochemistry and the analogous reactions of smaller silicon hydrides. These rules were used to generate a reaction mechanism consisting of reversible reactions among silicon hydrides containing up to 10 silicon atoms and irreversible formation of silicon hydrides containing 11-20 silicon atoms. The resulting mechanism was used in kinetic simulations of clustering during silane pyrolysis in the absence of any surface reactions. Results of those simulations are presented, along with reaction path analyses in which key reaction paths and rate-limiting steps are identified and discussed.
Journal of Aerosol Science, 2004
A numerical model has been developed to predict gas-phase nucleation, growth, and coagulation of silicon nanoparticles formed during thermal decomposition of silane. A detailed chemical kinetic model of particle nucleation was coupled to an aerosol dynamics model that includes particle growth by surface reactions, coagulation with instantaneous coalescence, and convective transport. Solution of the aerosol general dynamic equation was handled by three approaches: (1) the e cient and reasonably accurate method of moments;
Process control strategies for the gas phase synthesis of silicon nanoparticles
Chemical Engineering Science, 2012
In this contribution the identification of new reaction conditions for the production of nearly monodisperse silicon nanoparticles via the pyrolysis of monosilane in a hot wall reactor is considered. For this purpose a full finite volume model has been combined with a state-of-the-art trust-region optimisation algorithm for process control. Verified against experimental data, specific process conditions are determined accomplishing a versatile range of prescribed product properties. The main achievement of the optimisation is the possibility to control the different mechanisms in the particle formation process by mainly adjusting the temperature profile. Due to a successful separation of the nucleation and growth process, significantly narrower particle size distributions are obtained. Moreover, the presented optimisation framework establishes rate constants based on measured data.
Journal of The Electrochemical Society, 2000
The growth of silicon films via chemical vapor deposition (CVD) is of considerable importance in the microelectronics and photovoltaics industries. This process often involves the thermal decomposition of silane, which is achieved by heating the wafer or rod to be coated to a suitable temperature. A wide range of geometries and conditions are employed. For example, low-pressure chemical vapor deposition (LPCVD), at pressures around 1 Torr (133 Pa), is used with a stagnation-point flow geometry to deposit thin films of silicon in the fabrication of integrated circuits, while cylindrical polysilicon rods are grown on a heated filament using atmospheric-pressure CVD (APCVD).
Journal of Physical Chemistry C, 2008
Si nanocrystals 1-10 nm in size highly resistant to oxidation were prepared by thermal (680°C) or goldinduced (450-600°C) decomposition of tetramethylsilane and tetraethylsilane using trioctylamine as an initial solvent. Transmission electron microscopy analysis of samples obtained in the presence of gold showed that Si nanocrystals form via solid-phase epitaxial attachment of Si to the gold crystal lattice. The results of computational modeling performed using first principles density functional theory calculations show that the enhanced stability of nanocrystals to oxidation is due to the presence of N or N-containing groups on the surface of nanocrystals.
Multi-scale modelling of silicon nanocrystal synthesis by Low Pressure Chemical Vapor Deposition
Thin Solid Films, 2011
A multi-scale model has been developed in order to represent the nucleation and growth phenomena taking place during silicon nanocrystal (NC) synthesis on SiO 2 substrates by Low Pressure Chemical Vapor Deposition from pure silane SiH 4. Intrinsic sticking coefficients and H 2 desorption kinetic parameters were established by ab initio modelling for the first three stages of silicon chemisorption on SiO 2 sites, i.e. silanol Si-OH bonds and siloxane Si-O-Si bridges. This ab initio study has revealed that silane cannot directly chemisorb on SiO 2 sites, the first silicon chemisorption proceeds from homogeneously born unsaturated species like silylene SiH 2. These kinetic data were implemented into the Computational Fluid Dynamics Fluent code at the industrial reactor scale, by activating its system of surface site control in transient conditions. NC area densities and radii deduced from Fluent calculations were validated by comparison with experimental data. Information about the deposition mechanisms was then obtained. In particular, hydrogen desorption has been identified as the main limiting step of NC nucleation and growth, and the NC growth rate highly increases with run duration due to the autocatalytic nature of deposition.
Thermal processing and native oxidation of silicon nanoparticles
Journal of Nanoparticle Research, 2011
In this study, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS) were used to investigate in-air oxidation of silicon nanoparticles ca. 11 nm in diameter. Particle samples were prepared first by extracting them from an RF plasma synthesis reactor, and then heating them in an inert carrier gas stream. The resulting particles had varying surface hydrogen coverages and relative amounts of SiH x (x = 1, 2, and 3), depending on the temperature to which they had been heated. The particles were allowed to oxidize in-air for several weeks. FTIR, XPS, and EELS analyses that were performed during this period clearly establish that adsorbed hydrogen retards oxidation, although in complex ways. In particular, particles that have been heated to intermediate hydrogen coverages oxidize more slowly in air than do freshly generated particles that have a much higher hydrogen content. In addition, the loss of surface hydride species at high processing temperatures results in fast initial oxidation and the formation of a self-limiting oxide layer. Analogous measurements made on deuterium-covered particles show broadly similar behavior; i.e., that oxidation is the slowest at some intermediate coverage of adsorbed deuterium.
Formation energies of silicon nanocrystals: role of dimension and passivation
Physica Status Solidi (c), 2005
The structural properties of small silicon nanoclusters as a function of dimension and surface passivation are studied from ab initio technique. The formation energies are calculated and the relative stability of the considered clusters is predicted and discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Journal of Aerosol Science, 2003
This paper discusses an experimental and numerical study of the nucleation and growth of particles during low-pressure (∼1:0 Torr) thermal decomposition of silane (SiH 4 ). A Particle Beam Mass Spectrometer was used to measure particle size distributions in a parallel-plate showerhead-type semiconductor reactor. An aerosol dynamics moment-type formulation coupled with a chemically reacting uid ow model was used to predict particle concentration, size, and transport in the reactor. Particle nucleation kinetics via a sequence of chemical clustering reactions among silicon hydride molecular clusters, growth by heterogeneous chemical reactions on particle surfaces and coagulation, and transport by convection, di usion, and thermophoresis were included in the model. The e ect of pressure, temperature, ow residence time, carrier gas, and silane concentration were examined under conditions typically used for low-pressure (∼1 Torr) thermal chemical vapor deposition of polysilicon. The numerical simulations predict that several pathways involving linear and polycyclic silicon hydride molecules result in formation of particle "nuclei," which subsequently grow by heterogeneous reactions on the particle surfaces. The model is in good agreement with observations for the pressure and temperature at which particle formation begins, particle sizes and growth rates, and relative particle concentrations at various process conditions. A simpliÿed, computationally inexpensive, quasi-coupled modeling approach is suggested as an engineering tool for process equipment design and contamination control during low-pressure thermal silicon deposition. ?