A Mathematical Model for Chemical Vapor Deposition Processes Influenced by Surface Reaction Kinetics: Application to Low‐Pressure Deposition of Tungsten

Abstract

A model for the simultaneous reaction kinetics and transport processes in chemical vapor deposition (CVD) reactors has been extended to treat deposition of materials that have a broad range of surface characteristics, e.g., sites with multiple dangling bonds and adsorbates with multiple bonding configurations. The model uses the nature of the surface to determine the elementary processes that can take place during growth. Rate constants for these processes are calculated from first principles using statistical thermodynamics, transition state theory, and bond dissociation enthalpies. In this way, deposition rates are determined without either assuming the reaction mechanism or arbitrarily choosing any kinetic parameter values. The utility of the approach is illustrated by modeling low-pressure CVD of tungsten from tungsten hexafluoride and hydrogen. The treatment considers 14 species and eight reactions in the gas together with 21 species and 65 processes at the surface. The calculations indicate that, for the range of operating conditions considered, the process is controlled by surface reaction kinetics and that gas-phase reactions are unimportant. Deposition rates and surface fluxes predicted by the model show quantitative agreement with available experimental data. In addition, the major reaction pathways and rate-limiting steps are identified. This information is used to develop simplified rate expressions (still without using any fitted rate constants) that give reasonable predictions for the growth rate. Sensitivity studies are performed to assess the impact of uncertainties in species properties and rate constants on the theoretical results.

Development of a spatially controllable chemical vapor deposition reactor with combinatorial processing capabilities

“…The gas phase reactions associated with these deposition processes are negligible due to low reactor pressure during the process operation (Arora and Pollard 1991;Kleijn et al 1991), therefore, the deposition rate is determined only by surface reaction and chemical species transport rates in the gas/wafer surface interface and through the deposited film itself.…”

“…While W CVD deposition mechanisms and reactor systems have been studied extensively (e.g., Arora and Pollard 1991;Kleijn 2000;Kleijn et al 1991;Kleijn and Werner 1993), open process development issues remain. Even in the simplest case of blanket deposition using H 2 and WF 6 , the reactant/reducing gas ratio and WF 6 gas concentration result in two operational degrees of freedom that force a tradeoff between film conformality and deposition rate, where the latter is compounded by reactor design factors that determine where the transition from reaction rate to mass-transfer limited operation takes place.…”

Numerical Modelling of CVD Processes and Equipment

“…[2,3,41. Similar studies, but with a smaller number of species have been performed also for WF6 [23], TEOS chemistry [6], silicon nitride [7], and silicon carbide deposition [21].…”

“…The dependence of the deposition rate on WFs concentration enters only through the mass transport limitation, while the chemical reaction is taken as order zero in W.6. This formula was first proposed by Ulacia [9] and has been proven to reproduce experimental data over a wide range of WF6 partial pressures [14, 111. A deeper investigation of this process has been performed by Arora and Pollard [23], who identified the dominant reaction pathways and derived a more complicated formula for the reaction rate as a function of temperature and concentrations. In Figure 1 we show a schematic of the chemistry investigated in this model.…”

Kinetic modeling of tungsten silicide chemical vapor deposition from WF6 and Si2H6: Determination of the reaction scheme and the gas-phase reaction rates

“…Comprehensive investigations including chemical reactions, gas flow condition in deposition apparatus are required, however, only a limited attempts have been carried out to analyze the reaction mechanism. As to tungsten-CVD from WF 6 and H 2 , mathematical calculations of reaction kinetics were carried out, where plausible elementary steps and reaction species were considered and the rate constants for each step were determined by first principles using statistical mechanics, transition state theory, and bond dissociation enthalpies (Arora and Polland, 1991). Wang and Polland (1995) applied this methodology to CVD-tungsten (CVD-W) from WF 6 and SiH 4 and predicted the process parameters dependency, however, such procedures still contain many uncertainties.…”