Aluminum oxide films were deposited on silicon wafers by plasma-enhanced chemical vapor deposition, using trimethylaluminum, N2O and He gases. The chemical composition, states of functional groups and microstructure of the aluminum oxide films were investigated using FTIR, XPS, AES and TEM. Etch rates were measured and related to the microstructure of the films. It was found that carbon and hydrogen atoms are incorporated less at higher deposition temperatures and are almost completely removed as gas phases, such as CO2 and H2O, by post-deposition heat treatment at 800 degrees C in an oxygen environment. Carbon atoms incorporated into the films are in the chemical form of AICH(3) or AlCOOH, and the atomic concentration varies from 2% at 300 degrees C to 5% at 120 degrees C. Hydrogen atoms are in the chemical form of Al-OH, and the atomic concentration estimated from the absorbance FTIR band of the O-H stretching mode varies from about 7% at 300 degrees C to about 28% at 120 degrees C. The aluminum oxide films deposited at 300 degrees C have a microcrystalline structure of hydrogen-stabilized gamma-Al2O3 with an O/Al ratio of 1.6, whereas those deposited at 120 degrees C have an amorphous structure. Etching properties of the films were related to the change in the microstructure.