Elastic-Constant Measurement in Oxide and Semiconductor Thin Films by Brillouin Oscillations Excited by Picosecond Ultrasound

Abstract

In this study, an elastic-stiffness evaluation in transparent or translucent thin films using Brillouin oscillations detected by picosecond ultrasound is conducted. An ultrahigh-frequency (≥50 GHz) strain pulse is generated using femtosecond light pulse in specimens and propagates in the film-thickness direction. The time-delayed probe light pulse enters the specimen, which is diffracted by the strain pulse, causing oscillations in the reflectivity change of the probe light pulse. The oscillation frequency gives the elastic modulus with ellipsometry for refractive index. The theoretical calculation predicts the accuracy of stiffness measurement. The methodology is applied to the study of amorphous silica, amorphous tantalum oxide, diamond thin films, and silicon wafers.