Method for examining a surface of a sample by means of ultrasound

Method for examining a surface of a sample by means of ultrasound, comprising:

• a step of irradiating the sample through a liquid medium with a focused ultrasound beam;
• a step of repeating detection of ultrasound reflected by said sample while varying the frequency of said focused ultrasound beam in a predetermined frequency range;
• a step of storing detection output of the ultrasound reflected by said sample in memory means;
• a step of repeating said irradiation, detection and storage while varying the distance between said sample and the source of said focused ultrasound beam;
• a step of reading out successively the detection output stored in said memory means and obtaining curves representing variations of said detection output with respect to variations of said distance for each of the frequencies;
• a step of obtaining a frequency dependence curve for the propagation velocity of surface acoustic wave in said sample by measuring the propagation velocity on the basis of the period in said curves representing variations of detection output for each of the frequencies;
• and
• a step of evaluating the thickness of a layer having surface defects of said sample by using said frequency dependence curve for the propagation velocity.