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 correspon ding microstructures was studied. The two precursors were titanium(IV) isop ropoxide and anhydrous titanium(IV) nitrate, and all depositions were condu cted at low pressures (<10(-4) Torr) in an ultrahigh vacuum chemical vapor deposition reactor. For both precursors deposition kinetics were qualitativ ely similar and exhibited three distinct regimes as a function of temperatu re. At the lowest temperatures, growth was limited by the rate of precursor reaction on the substrate surface. At intermediate temperatures flux-limit ed growth was obtained, and at the highest temperatures the growth rates de creased with increasing temperatures. The overall behavior was modeled quan titatively for each precursor using a two-step mechanism involving reversib le adsorption followed by irreversible reaction. Titanium(IV) nitrate exhib ited 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 a nd scanning electron microscopy and X-ray diffraction. Comparison of the mi crostructures of films deposited at similar temperatures revealed significa nt differences in the reaction rate-limited regime. As the growth rates of the two precursors converged in the flux-limited regime, the respective mic rostructures became indistinguishable. The importance of precursor surface coverage and diffusion on this effect is described.