| Many MEMS devices such as capacitive and metal contact switches use thin film membranes or micro-cantilever beams that are nanometers thick and microns wide. To characterize such MEMS devices, it is important to accurately and nondestructively measure elastic stiffnesses and residual stresses in freestanding thin films. Current methods for obtaining mechanical properties and residual stresses include the bulge test, resonance test, nanoindentation testing, and the Membrane Deflection Experiment. Unfortunately, these measurements are either destructive or depend critically on knowledge of the thin films geometries and support conditions. Laser ultrasonics (photo-acoustics) is a potentially powerful tool for nondestructive, in-situ, MEMS device characterization. This work discusses the use of narrowband photo-acoustics to characterize the properties of freestanding nanometer-sized thin films.; Photo-acoustic generation is achieved by use of a micro-chip laser which deposits pulsed laser energy (3--13muJ in 300--800 picoseconds pulse widths) in the form of a spatially periodic source on the structure. The resulting narrowband ultrasonic modes are monitored using a balanced 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. Specifically, it has been shown that up to three parameters (composite stiffness, composite flexural rigidity, and average residual stress) can be obtained photo-acoustically on the thin films. The effects of process parameters on the residual stress and membrane size on the measured properties were also investigated. |
