Silicon oxinitride films are of great industrial interest due to their singular electrical, optical and mechanical properties. The interest in obtaining these films from liquid sources is greatly increasing because of the lower harmfulness and of the new potential features that can be obtained from the precursor chemical structure. Thin films of silicon carbo-oxi-nitrides have been deposited by PECVD from a liquid source, hexamethyldisilazane, and different mixtures of oxygen and ammonia. We have investigated the effects of the variations in the composition of the reactant gases and in the applied power levels, on the visible and infrared optical and micromechanical properties. The refractive index can be varied continuously from 1.4 to 1.8, the IR absorption in the 8-13 mu m region can be tailored to the desired shape, and the hardness and Young's modulus modified, depending on the deposition conditions.