INTERPRETATION OF LASER-GENERATED LOW-ORDER LAMB WAVES FOR ELASTIC-MODULUS MEASUREMENTS IN THIN-FILMS

Variability in Lamb waveform structure with film thickness, material, source laser spot size and source laser power density was observed dur ing ultrasonic measurements for determination of elastic moduli of met allic films from 1 to 25-mu-m thick. To determine the effect of wavefo rm variability on ultrasonic velocity measurements (and, in turn, on e lastic modulus calculations), a series of experiments were performed i n which Lamb waves were excited in free standing films using a pulsed Nd:YAG laser and detected using heterodyne interferometry. It was show n that, regardless of waveform shape, the velocity of the first arriva l was that of the lowest order symmetric Lamb wave for a frequency-thi ckness product approaching zero.