Abstract
This paper studies the elastic constant and microstructure of amorphous SiO_2 (a-SiO_2) thin films deposited on Si substrates. a-SiO_2 thin films were prepared by the reactive sputtering with a Si target in the argon/oxygen mixture gas. We measured the Brillouin-oscillation frequency using a femtosecond pump-probe method. We determined the longitudinal-wave elastic constant from the measured frequency and the refractive index measured by the thin-film ellipsometry. The results show that the elastic constant of the a-SiO_2 thin film highly depend on the microstructure and microcracks. The micromechanics model was developed to explain the decrease of the stiffness with thin pancake-shaped microcracks, which were randomly distributed in the a-SiO_2 matrix. This study demonstrates that the Brillouin oscillation technique is an effective tool for evaluation of not only acoustic properties but also reliability of transparent materials.
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