Infrared spectroscopic ellipsometry for residual water detection in annealed sol–gel thin layers

Sol–gel silicon dioxide films have received a great deal of attention due to their numerous advantages, such as the low cost of the process and the possibility of incorporating rare earth elements in the layer. Film preparation is based on two fundamental steps, the gel deposition and a heat treatment that results in network densification together with the removal of water, solvent and organic residues. Since the optical properties are strongly influenced by the water content of the film, accurate control of this parameter is necessary. In the case of thin films, the poor sensitivity of classical FTIR spectrophotometry may be overcome by using the attenuated total reflection configuration combined with spectroscopic guided wave absorption, SIMS and infrared spectroscopic ellipsometry measurements. In this work, the water content in a 300-nm thick sol–gel silica film deposited on silicon is investigated by optical techniques after heat treatment. With the SIMS results as reference, it is shown that the most sensitive non-destructive technique is infrared spectroscopic ellipsometry. Indeed, while FTIR absorption analysis indicates that the heat treatment is effective, the ellipsometry still detects residual water in the film.