Aluminum oxynitride (AION) films were prepared on silicon, glass and polymer substrates using electron cyclotron resonance plasma-enhanced chemical vapor deposition. The composition, structure, optical and mechanical properties were investigated. It was found that the composition of AION films varies from nearly pure AIN to Al2O3, controlled by O-2 flow rate. Films deposited at room temperature is amorphous with hardness of 9.1-9.4 GPa, while those deposited at high temperature is crystalline with grain size of 10-20 nm. The hardness of crystalline films accords with the mixture rule. All AION films are transparent and the transmittance increases with increasing O-2 concentration. (C) 2004 Elsevier B.V. All rights reserved.