In previous work, we showed that laser ultrasonics (photo-acoustics) is a promising tool for the nondestructive and non-contact characterization of thin film structures. In particular it was shown that the modulus and residual stresses in two-layer freestanding Al/Silicon Nitride films could be measured using a narrow-band laser ultrasonic technique. In this technique, a microchip laser deposits pulsed laser energy as a spatially periodic source on the structure. The resulting narrowband ultrasonic modes are monitored using a broadband Michelson interferometer. By varying the geometry of the spatially periodic source, a wide range of wavenumbers can be probed. For the thin films investigated, which were less than a micron in thickness (300-900 nm), only the two lowest order modes were generated and these in turn can be related to sheet and flexural modes in plates. In this paper we present an extension of this approach. The sensitivity of the photo-acoustic system is examined by measuring variable residual stresses. Variable residual stresses are obtained in the thin films by two methods: by varying the growth rates for silicon nitride and by varying the membrane size.
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