KINETICS AND MULTIPLE STEADY-STATES IN THE CHEMICAL-VAPOR-DEPOSITION OF TITANIUM CARBIDE

Kinetics of the reaction between TiCl4 and C3H8 to form TiC(s) and HCl in a well-mixed flow reactor at feed pressures from 0.01 to 0.5 Torr have been studied by monitoring reaction rates with a differentially pumped mass spectrometer to monitor reactant and product partial pressures. A resistively heated tungsten foil was used as the deposition substrate to provide surface temperatures up to 1500-degrees-C. This technique provides rapid measurement of gas composition in a well-mixed flow reactor so that reaction steady states and transients could be examined. Measured film deposition rates agreed well with those determined by monitoring reaction products. Film quality was also monitored by scanning electron microscopy, x-ray photoelectron spectroscopy, scanning Auger microprobe, and energy dispersive x-ray analysis. Rates were found to vary from first order to negative order with respect to both P(TiCl4) and P(C3H8). A simple bimolecular Langmuir-Hinshelwood rate expression, r(R) = k(r)K(C3H8)K(TiCl4)P(C3H8)P(TiCl4)/(1 + K(C3H8)P(C3H8) + K(TiCl4)P(TiCl4))2, was found to describe the steady-state kinetics remarkably well. This equation suggests reactants and/or intermediates are competitively adsorbed. Temperature dependences of these parameters yielded heats of adsorption of reactants and reaction rate parameters. Under some conditions multiple steady states were observed vs. P(TiCl4) under nearly isothermal conditions and also vs. surface temperature for fixed reactant pressures. The bifurcation points were found to be analgous to ignition/extinction points observed in combustion systems although these deposition reactions are endothermic. The multiplicity behavior was found to be also consistent with the Langmuir-Hinshelwood model. However, we occasionally observe as many as three stable steady states at sufficiently high TiCl4 pressures, with only a single steady state at lower reactant pressures.