This paper focuses on the conformality of the plasma-enhanced atomic layer deposition (PE-ALD) of Al2O3 using trimethylaluminum [Al(CH3)(3); (TMA)] as a precursor and O-2 plasma as an oxidant source. The conformality was quantified by measuring the deposited film thickness as a function of depth into macroscopic test structures with aspect ratios of similar to 5, 10, and 22. A comparison with the thermal TMA/H2O process indicates that the conformality of the plasma based process is more limited due to the surface recombination of radicals during the plasma step. The conformality can slightly be improved by raising the gas pressure or the radio-frequency power. Prolonging the plasma exposure time results in further improvement of the conformality. Furthermore, there are indications that the H2O produced during the plasma step in the PE-ALD process for Al2O3 contributes to the observed conformality through a secondary thermal ALD reaction. The conformality of Al2O3 is also compared to the conformality of AlN deposited by PE-ALD from TMA and NH3 plasma. For the same exposure, O-2 plasma results in better conformality compared to NH3 plasma, suggesting a faster recombination of the radicals in the NH3 plasma.