Aluminum oxide was deposited by radical enhanced atomic layer deposition using trimethylaluminum (TMA) and oxygen radicals in the temperature range 25-300 degrees C. The radicals were produced by dissociating oxygen gas in a remote microwave plasma discharge. Oxygen was mixed with argon which was also used as the carrier and purge gas. Films were grown on silicon, glass, and indium tin oxide coated glass substrates. Additional growth experiments were conducted on heat-sensitive materials: polyethene, polypropene, and wool. The time to complete one deposition cycle was nearly independent of the deposition temperature, being around 10 s for all deposition temperatures. Growth rates were between 1.5 and 2.9 angstrom per cycle, which is higher than what has been obtained with the TMA-H2O process in similar reactor conditions. The films were amorphous according to X-ray diffraction. The films were also very smooth; the surface root-mean-square roughness was less than 0.8 nm for 180 nm thick films. The films had breakdown fields, defined as the field corresponding to the leakage current density of 1 mu A/cm(2), between 6 and 10 MV/cm, and dielectric constants between 6.5 and 8.1. The film impurity levels according to time-of-flight elastic recoil detection analysis were between 0.8 and 15 atom % for hydrogen and 0.2 and 4 atom % for carbon. The refractive indexes at 580 nm were between 1.60 and 1.64.