Numerical Modeling of Gas-Phase Nucleation and Particle Growth during Chemical Vapor Deposition of Silicon (original) (raw)
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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, 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. ?
The Journal of Physical Chemistry A, 1998
A mechanism for the homogeneous gas-phase decomposition of SiHCl 3 , SiH 2 Cl 2 , and SiH 3 Cl in hydrogen is derived from the results of ab initio molecular-orbital studies. It consists of 39 reversible elementary reactions among 25 species, including pressure-dependent unimolecular decomposition of the chlorinated silanes and secondary chemistry due to reactions of SiH 2 , SiHCl, and SiCl 2 with one another and with the chlorinated silanes. Rate parameters in the mechanism have been calculated based on results of ab initio studies using transition-state theory and unimolecular rate theories. This allows us to construct a reasonably complete mechanism that provides qualitative explanations for several features of dichlorosilane decomposition that have been presented in the literature, including observations on the presence and concentrations of SiCl 2 , SiHCl, and Si atoms. Several chain reactions in which the chain carriers are divalent silicon species have been identified. S1089-5639(97)03174-5 CCC: $15.00
Kinetics of homogeneous decomposition of silane
Journal of Crystal Growth, 1992
The kinetics of homogeneous decomposition of silane in a low-pressure reactor for polysilicon film deposition is investigated. The process is found to proceed as a first-order reaction and its rate depends exponentially on the residence time of silane in the system. A kinetic equation is derived and the activation energy is determined to be 43 kcal/mol. A mathematical model of the polysilicon film growth in a LPCVD reactor is developed, taking into account the simultaneous gas-phase and surface-catalytic pyrolysis of silane. Its analysis reveals that the relative share of the homogeneous reaction in the overall process grows with the increase in temperature and pressure in the system, leading to a substantial degradation of the layer thickness uniformity. The results obtained contribute to the more precise and fuller prediction and optimization of the process of thermal decomposition of silane.
Modelling of silicon hydride clustering in a low-pressure silane plasma
Journal of Physics D: Applied Physics, 2000
A new silicon hydride clustering model was developed to study the nucleation of particles in a low-temperature silane plasma. The model contains neutral silanes, silylenes, silenes and silyl radicals as well as silyl and silylene anions. Reaction rates were estimated from available data. Simulations were carried out for typical discharge parameters in a capacitive plasma. It was shown that the main pathway leading to silicon hydride clustering was governed by anion-neutral reactions. SiH 2 radical insertion was found to be important only in the initial stages of clustering, whereas electron-induced dissociations were seen to lead to dehydrogenation. Increased ion density (radiofrequency power density) leads to faster clustering due to increased formation of reactive radicals.
Theoretical study of the chemical vapor deposition of (100) silicon from silane
Physical Review B, 2001
We use density functional theory to investigate the chemical vapor deposition of ͑100͒ silicon from silane. The reaction proceeds through four sequential steps. The first step is activation of surface sites through hydrogen abstractions by atomic H or through H 2 desorption. We find that hydrogen abstraction barriers by atomic H are less than 1 kcal/mol while H 2 desorption proceeds through a two-step pathway with an overall barrier of 61.1 kcal/mol. Next, adsorption of SiH 4 onto bare dimer sites occurs. We calculate the B3LYP barrier to SiH 4 adsorption on a single dimer to be 7.4 kcal/mol while the barrier across two dimers is 14.3 kcal/mol. Then, adsorbed SiH 3 transforms to bridged SiH 2 (a) with a barrier of 5.7 kcal/mol relative to SiH 3 (a) for the mechanism requiring H(g) while the barrier for the mechanism requiring no H(g) is 32.9 kcal/mol, where ͑g͒ and ͑a͒ represent gas and adsorbed species, respectively. Finally, the dihydride surface transforms to the monohydride surface through two-sequential steps with an overall barrier of 47.0 kcal/mol, which agrees well with the TPD barrier of 43 kcal/mol. The B3LYP H 2 desorption barrier of 61.1 kcal/mol and SiH 4 adsorption barrier of 7.4 kcal/mol are in good agreement with the TPD values of 57.2 to 58 kcal/mol and 3.3 to 4.0 kcal/mol, respectively.
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;
Kinetic model of silicon — Hydrogen network formation
Journal of Non-Crystalline Solids, 1991
A model is proposed to explain the mechanism of a-Si:H network formation via gas-phase decomposition of silanes. I t is based on a set of thermally independent reactions of.SiH 3 and:SiH 2radicals with surfaceSi andSi-H bonds. Some steric l i m i t a t i o n s of the reactions at the interface gas/solid are taken in focus. I t is suggested that such l i m i t a t i o n s result i n a growth mechanism consisting of two p a r a l l e l processes: 1)propagation of dense Si network covered with a hydrogen enriched surface; 2)pulsed development of polyhydride chains. The competition between these processes results in a Si-H network containing (SiH2) n bubbles. I t is shown that density and dimensions of bubbles are k i n e t i c a l l y determined by the temperature of the solid phase and by the r a t i o of.SiH 3 and :SiH 2 concentrations at the growing surface.
Homogeneous gas-phase nucleation in silane pyrolysis
Journal of Aerosol Science, 1994
Dilute and particle free mixtures of silane in the range of 100 ppm to 10% by volume in different carrier gases were decomposed thermally in a tube reactor. The onset of homogeneous nucleation was determined as a function of temperature and silane concentration for each carrier gas using a CNC with a detection limit of ca 0.01/~m. The gaseous decomposition by-products, disilane, trisilane and hydrogen were measured simultaneously using gas-phase chromatography. The onset of gas-phase nucleation was found to be inversely proportional to the temperature and influenced by the nature of the carrier gas. In inert gases, no chemical reaction took place between the decomposition products of silane and the carrier gas. In hydrogen, equilibrium displacement with the primary decomposition product (Sill2) lead to a retardation of particle formation. Thus, the temperatures of onset of gas-phase nucleation was higher for mixtures in hydrogen than for mixtures in inert gases.
Models for thermal dissociation of silane on a polysilicon surface
Applied Physics A Solids and Surfaces, 1990
so,,,, Physics A "' Surfaces Abstract. Simple models for the thermally activated dissociation reaction of silane and silicon growth on a polycrystalline silicon surface are presented. The models are fitted to recent experimental molecular beam scattering data for the low-pressure reactive sticking coefficient. Thermally activated few-step models fit the data reasonably well, and thus, we are able to explain the temperature and pressure dependencies of the observed deposition rate.