In this thesis the relation between structure evolution, mechanical properties, and stability was investigated for Al2O3 thin films. The existence of different polymorphs of this material offers a range of properties for applications, but also meets the process control challenges when the formation of phase pure material is desired. The aim of this work is to affect the phase structure evolution and the stability of vapor deposited alumina thin films, especially for the case of PECVD. This goal is achieved by experimental investigations of the PECVD process which show that a-Al2O3 phase is obtained only at conditions with a low precursor ratio, a long pulse length and a high power density.