Does chemistry really matter in the chemical vapor deposition of titanium dioxide? Precursor and kinetic effects on the microstructure of polycrystalline films

A side-by-side comparison of the TiO₂ deposition kinetics and the corresponding microstructures was studied. The two precursors were titanium(IV) isopropoxide and anhydrous titanium(IV) nitrate, and all depositions were conducted at low pressures (<10^-4 Torr) in an ultrahigh vacuum chemical vapor deposition reactor. For both precursors deposition kinetics were qualitatively similar and exhibited three distinct regimes as a function of temperature. At the lowest temperatures, growth was limited by the rate of precursor reaction on the substrate surface. At intermediate temperatures flux-limited growth was obtained, and at the highest temperatures the growth rates decreased with increasing temperatures. The overall behavior was modeled quantitatively for each precursor using a two- step mechanism involving reversible adsorption followed by irreversible reaction. Titanium(IV) nitrate exhibited a lower activation energy of reaction (E(r) = 98 kJ/mol) which allowed deposition at lower temperatures compared to titanium(IV) isopropoxide (E(r) = 135 kJ/mol). The film microstructures were examined using transmission and scanning electron microscopy and X-ray diffraction. Comparison of the microstructures of films deposited at similar temperatures revealed significant differences in the reaction rate-limited regime. As the growth rates of the two precursors converged in the flux-limited regime, the respective microstructures became indistinguishable. The importance of precursor surface coverage and diffusion on this effect is described.