For integrated optical waveguides, based on a silicon substrate technology, it is necessary to insulate the dielectric optical medium from the substrate due to its light absorption characteristics. Silicon oxide is the most common material used as insulator layer (buffer), in addition, it is a MOS compatible material. The silicon oxide buffer layer, used in this work, was obtained by PECVD, using TEOS as silicon source, in a home made cluster tool system. This buffer layer has suitable optical properties and the PECVD deposition technique allows us to have an effective control of the refractive index. The buffer layer must be thick (similar to 3 mu m) to avoid optical leakage to the silicon substrate. Due to the high mechanical stress (causing film cracking) characteristic of the TEOS PECVD silicon oxide, deposited in our lab, it was not possible to obtain an oxide thickness higher than 0.7 mu m per deposition step. In order to achieve a silicon oxide thickness up to 4.2 mu m with controlled refractive index, we developed a multilayer process, which consists of successive deposition steps of a thin silicon oxide layer (0.5 mu m), followed by an RTA process after each deposition step. These films were analyzed by ellipsometry, FTIR, RES, stress measurements and GIXR techniques. The main results are: thickness about 4 mu m; constant refractive index (1.44 +/- 0.02) in all silicon oxide layers; reduction of O-H bond contamination; low surface roughness and uniform stoichiometry along the silicon oxide layers. The buffer layer was covered by an optical thin film, LPCVD silicon nitride, resulting in a reduction of the total residual stress.