| Popis: |
Photo-acoustic microscopy (laser ultrasonics) is a potentially powerful tool for nondestructive, in situ, MEMS device characterization. This paper discusses the use of narrowband photo-acoustics to characterize the properties of free-standing nanometer-sized thin films. Photo-acoustic generation is achieved by use of a micro-chip laser which deposits pulsed laser energy (10mJ in 300 picoseconds) in the form of a spatially periodic source on the structure. The resulting narrowband ultrasonic modes are monitored using a Michelson interferometer. By varying the geometry of the spatially-periodic source, a wide range of wavenumbers is probed. Experiments were conducted on two-layer Al/Si3N4 membranes (aluminum thickness: 300-500nm; silicon nitride thickness: 240-400nm. For such thin films, only the two lowest order modes are generated and these in turn can be related to sheet and flexural modes in plates. The mechanical properties and residual stress in the thin films are evaluated from the dispersion curves for these two lowest order modes. |
