Article pubs.acs.org/JPCA
Analysis of the Gas Phase Reactivity of Chlorosilanes Stefano Ravasio, Maurizio Masi, and Carlo Cavallotti* Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano via Mancinelli 7, 20131 Milano, Italy S Supporting Information *
ABSTRACT: Trichlorosilane is the most used precursor to deposit silicon for photovoltaic applications. Despite of this, its gas phase and surface kinetics have not yet been completely understood. In the present work, it is reported a systematic investigation aimed at determining what is the dominant gas phase chemistry active during the chemical vapor deposition of Si from trichlorosilane. The gas phase mechanism was developed calculating the rate constant of each reaction using conventional transition state theory in the rigid rotor−harmonic oscillator approximation. Torsional vibrations were described using a hindered rotor model. Structures and vibrational frequencies of reactants and transition states were determined at the B3LYP/6-31+G(d,p) level, while potential energy surfaces and activation energies were computed at the CCSD(T) level using aug-cc-pVDZ and aug-cc-pVTZ basis sets extrapolating to the complete basis set limit. As gas phase and surface reactivities are mutually interlinked, simulations were performed using a microkinetic surface mechanism. It was found that the gas phase reactivity follows two different routes. The disilane mechanism, in which the formation of disilanes as reaction intermediates favors the conversion between the most stable monosilane species, and the radical pathway, initiated by the decomposition of Si2HCl5 and followed by a series of fast propagation reactions. Though both mechanisms are active during deposition, the simulations revealed that above a certain temperature and conversion threshold the radical mechanism provides a faster route for the conversion of SiHCl3 into SiCl4, a reaction that favors the overall Si deposition process as it is associated with the consumption of HCl, a fast etchant of Si. Also, this study shows that the formation of disilanes as reactant intermediates promotes significantly the gas phase reactivity, as they contribute both to the initiation of radical chain mechanisms and provide a catalytic route for the conversion between the most stable monosilanes.
1. INTRODUCTION When talking about silicon it seems always the same old story. Silicon is so extensively present in our lives that it is hard to recognize its importance. Since more than 50 years, both the integrated circuit and the photovoltaic technologies still rely on its massive production at a very high quality and low price. All these applications have as central product, the hyper-pure polycrystalline silicon to be used in the casting processes for the photovoltaic industry or in the Czochralski pulling for microelectronic applications. Because of the dramatic rise of photovoltaic applications the today production capacity of poly silicon reached the 350 kton/year from the 26 kton/year of the beginning years of this century. In this production scenario the paramount parameters are the product purity and cost. Definitively, one of the dramatic parts of the overall process cost is inherent in the gaseous precursor reduction to solid silicon. This process relies on the use of silane or trichlorosilane as precursor gases to deposit Si from the gas phase, and it is performed either in the traditional Siemens reactors or in the emerging fluidized bed reactors.1−4 These two reactors present management issues of different nature, being the former a batch reactor and the latter continuous. Moreover, these reactors operate in conditions significantly different from those usually addressed in silicon chemical vapor deposition microelectronic processes, being the latter operated either at reduced pressure © 2013 American Chemical Society
or in diluted conditions, as summarized by the data reported in Table 1. In fact, these very large scale reactors operate at high Table 1. Summary of the Process Conditions Adopted to Deposit Polysilicon in the Most Important Industrial Processes precursor inlet mole fraction pressure temperature growth rate (μm/min)
film epiSi
film polySi
mass polySi
SiHCl3 0.05 1 atm 1200 °C 1.0−5.0
SiH4 1.00 1 Torr 800 °C 0.4−0.5
SiHCl3
