The fabrication of thick optical films by spinning from solution on silicon substrates is an important technique for integrated optics applications. In particular, several authors have studied the conditions under which the thickness of set-gel silica films deposited on silicon wafers from solutions of water, tetraethoxysilane (TEOS) and ethanol can be maximized. The influence of processing parameters, such as composition, ageing period of the solution and spinning rate, have been studied. The effect of the wettability of the silicon substrate on the film thickness was investigated. The wetting characteristics of the silicon surface may be changed by adequate chemical cleaning methods. The hydrophilic wafers obtained by controlled oxidation of the silicon were found to have greater affinity to the film forming solution and to lead to thicker films than hydrophobic wafers obtained by etching the silicon surface with HF solution.