The concentration and size of oxygen precipitations in Si wafers which called bulk microdefects (BMD) affect the electrical and mechanical properties of devices. Recent device miniaturization requires the evaluation of BMDs with a nondestructive manner. In this study, we propose to inspect those nano-defects from the stiffness change at local regions of Si wafers measured by ultrahigh-frequency acoustic waves. Brillouin oscillations provide the stiffness showing high sensitivity to thin defects such as BMDs. Application of an ultrafast light pulse to the specimen generates the acoustic phonon pulse. We then irradiate the specimen with the time-delayed light pulse. which is diffracted by the phonon pulse, and its reflectivity change shows oscillations. This oscillation frequency gives the longitudinal-wave velocity, from which the elastic stiffness is obtained. Our results indicate that the nano-defects decrease the out-of-plane elastic constant of the Si wafer, confirming that this methodology can be a powerful tool for quantitative evaluation of BMDs.
