Refractive index, sound velocity and thickness of thin transparent films from multiple angles picosecond ultrasonics

Abstract

We present a method for refractive indices and longitudinal sound velocity measurements from picosecond ultrasonic experiments made at different probe incidence angles. For transparent or semitransparent materials such as dielectrics or semiconductors, picosecond ultrasonic experiments can lead to oscillations in the reflectivity curves whose frequency depends on the refractive indices, the sound velocity and the experiments angle. From these data we establish a simple method for the calculation of the refractive indices and verify it on a GaAs sample. We show on fluorinated silica glass and aluminum nitride practical applications of this method on thin films. From two experiments we measure the refraction index and the sound velocity of these materials, with no assumption on the materials properties or on the sample layers’ thicknesses. Here the materials are buried under a thin aluminum film. It illustrates the fact that the method can be applied to multilayers. From the same experiments we then derive the thickness of the layers. It shows that this method can render picosecond ultrasonic experiments independent from other characterization means.