The Brillouin light scattering technique has been exploited in order to study the elastic properties of dielectric filing used in microelectronics, such as silicon oxide, silicon oxinitride and silicon nitride films deposited on (100)-Si by different techniques. In the case of films as thick as approximately 0.4 mum, a number of Rayleigh-Sezawa modes, polarised in the sagittal plane were detected, as well as Love modes which have a shear horizontal polarisation. In addition, the longitudinal resonance corresponding to a longitudinal wave travelling parallel to the free surface was revealed. In thicker films (0.8-0.9 mum) we could also measure the longitudinal bulk mode, travelling at an angle with respect to the film normal. The mass density of the films was accurately measured by X-ray reflectivity. A best fit procedure of the calculated acoustic phase velocities on the measured ones enabled us to determine the elastic properties of the films and to put in evidence their dependence on the particular deposition technique and on the annealing treatment. The obtained values of the elastic constants were used to reproduce, using finite element calculations, the measured stress vs. temperature, achieving an estimation of both the thermal expansion coefficient and the viscosity of the films.