Nondestructive evaluation of elastic properties in porous silicon film on Si(100) by the phase velocity scanning of laser interference fringes

Investigating an attractive application of a porous silicon (PS) film as a micromachine jointing layer, we evaluated elastic properties of the PS film by the analysis of velocity dispersion of surface acoustic waves (SAW). We developed a novel laser ultrasonic method which utilized the laser interference fringes scanned at the phase velocity of the SAW and measured, for the first time, SAW velocity dispersion of the PS film in a dry condition. The phase velocity of the SAW of the PS film was found to decrease with an increase of the porosity. Curve fitting of the measured dispersion to the computed one gave extremely small elastic stiffness of the PS film compared to that of Si wafer. Morphology of pores in the PS film, estimated from a relationship between the porosity and elastic stiffness C₄₄, was found to be an ordered and less open structure