Aluminum oxide films are obtained in a remote-microwave-plasma-enhanced chemical vapour deposition (RMPECVD) reactor. In this technique, only oxygen gas plasma is generated while trimethylaluminum (TMA) carried by argon is fed near the substrate holder which can be RF biased and deposition occurs at the surface of a silicon wafer. Optical emission spectroscopy (OES) is used in order to gain information on the plasma excitation of various species. The presence of an inert gas (argon) allows an evaluation of the relative densities of reactive species by the actinometry technique. A comparison has been done with and without RF biasing or TMA injection at different microwave powers in the same conditions of Ar and O-2 flow rates. The RF biasing creates an additional plasma near the substrate holder but has a weak influence on the OES signal. The microwave plasma is the major mode in the present experimental set up. Less than 2% of TMA modifies the plasma chemistry by improving the atomic oxygen formation. In addition, the TMA. decomposition involves a cooling effect of the electron energy. Although this dual mode configuration of the plasma production induces a slight decrease of the deposition rate, it limits the incorporation of impurities (O, H) in the layer by improving the ion bombardment and etching of the growing layer.