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Alumina layers were grown from trimethylaluminum (TMA) and water, ozone as well as an oxygen plasma as co-reactants in low temperature spatial atomic layer deposition (ALD). The influence of the amount of precursor, the precursor exposure duration, and substrate temperature were investigated with respect to the growth rate while employing different oxygen sources. The TMA/water process provided alumina (AlO_x) films with superb film quality as shown by infrared measurements. Ozone-based processes allowed lower substrate temperatures. Nevertheless, carbon residuals in different binding states were found within the bulk material. However, the carbon impurities have no impact on the barrier performance, since 50 nm AlO_x grown by TMA either with water or ozone exhibited a water vapor transition rate in the range of 10⁻⁶ g/m²/day. However, when our home-built microwave plasma source was applied in a remote configuration, the water vapor transition rate was one order of magnitude higher due to a reduction in film quality. Furthermore, a TMA utilization of ∼50% demonstrated the highly cost-effective spatial ALD concept as a deposition technique which is very suitable for industrial deposition applications.