Chemical vapour deposition of κ-Al2O3

The influence of experimental variables in combination with catalysis on the growth and microstructure of chemically vapour deposited κ-Al2O3 was investigated. The Al2O3 coatings were deposited in the temperature range of 800–1000°C and at pressures of 50–400 mbar. Hydrogen sulfide (H2S) was used as a doping/catalysing agent. General deposition characteristics of κ-Al2O3 as a function of temperature, pressure and H2S concentration are reported. In addition to scanning electron microscopy and X-ray diffraction, the microstructure and chemistry of the κ-Al2O3 layers were analysed using transmission electron microscopy. The growth rate of κ-Al2O3 could be strongly enhanced by applying H2S doping and reasonable deposition rates for κ-Al2O3 could be obtained at 800°C. Relatively very high growth rates could be obtained by increasing the total pressure simultaneously with H2S doping. Higher deposition pressure, however, increased the contribution of the homogenous gas phase reaction resulting in unacceptable thickness variations. In general, κ-Al2O3 deposited at different process conditions did not exhibit any pronounced microstructural or morphological differences, except at the deposition temperature of 800°C together with at higher doping levels (H2S>0.8%). Under these experimental conditions γ-Al2O3 was obtained. Enrichment of sulfur could be confirmed to occur in γ-Al2O3 while no sulfur was found in κ-Al2O3. Production-scale aspects concerning deposition of κ-Al2O3 and α-Al2O3 will be dealt with.