In a method and apparatus for measuring a thickness of a metal layer formed on a semiconductor substrate first, second, and third light pulses are successively irradiated onto a top surface of the metal layer to generate respective first, second, and third second sonic waves in the metal layer. Interference between the first and second sonic waves alters a detected reflectivity of the third light pulse off the metal layer. Maximum interference of the sonic waves occurs where the first sonic wave travels to a bottom surface of the metal layer and back to the top surface in the same time that it takes for the second light pulse to arrive at the surface of the metal layer. Accordingly, the velocity of the first sonic wave and a time lag between the first and second light pulses are used to determine the thickness of the metal layer.