Method of Assessing Uniformity of Periodic Structures Based on the Decay Time of Laser-Generated Acoustic Vibrations

Quality control of a periodic structure is performed using the damping rate of acoustic waves generated in the periodic structure. In this technique, an excitation light beam illuminates the first layer in the periodic structure to excite an acoustic wave. Possible irregularities in the periodic structure can scatter the acoustic wave, thereby increasing the damping rate of the acoustic wave. A sequence of probe light beams illuminates the periodic structure to measure the acoustic wave as a function of time to generated a temporal signal representing the damping rate of the acoustic signal. The acquired damping rate is employed to evaluate the quality of the periodic structure.