Acoustic waves generated by phase velocity scanning of laser interference fringes

Abstract

A method for noncontact and selective generation of surface acoustic waves (SAWs), and bulk acoustic waves (BAWs) is discussed. To generate SAWs, the thermoelastic sources of scanning interference fringes (SIFs) are scanned on an opaque solid surface with a velocity equal to the phase velocity of SAWs, and to generate BAWs the SIFs are scanned at a higher velocity. SAW velocity within the laser beam spot is measured as the ratio of observed frequency of SAWs and wave number of the SIF. The frequency measurement is precise because of the large number of generated carriers, amplitude enhancement of about 10 times, and suppression of other spurious modes (back-propagation SAWs and BAWs). This method was applied to evaluate thin films of SiO2, Si3N4, and porous silicon layers, with a precision around 0.1%. In addition, since a large-amplitude SAW (e.g., 1.4 nm) was realized at high frequencies (e.g., 110 MHz), a large acceleration (up to 109 m/s2) of the surface is achieved, with a moderate peak laser power of 710 kW/cm2. This feature may be attractive for applications in particle manipulation, such as removing dust particles from LSI wafers, in a dry process.