Exploitation of Brillouin spectroscopy for the characterization of a silica film

Measurement of the elastic properties of micrometric layers is important for MEMS design and manufacturing. Brillouin scattering, the scattering of light by ultrasonic waves, offers the possibility to probe the propagation of acoustic excitations of sub-micrometric wavelength, by contact-less and local measurements. From the acoustic properties the elastic ones can be derived. In the case of transparent supported films bulk waves can be measured in two different scattering geometries, together with surface acoustic waves. The smallness of the probed wavelengths allows a spatial resolution not achievable by other ultrasonic techniques. Brillouin scattering has been exploited to characterize a silica layer of micrometric thickness, thermally grown on a silicon substrate. The observation of various acoustic modes allows to derive the values of all the elastic moduli and of Poisson's ratio of the silica film, together with its refractive index.