A lamb wave propagation behavior on a freestanding nanoscale membrane was investigated using a laser ultrasonic technique in the present study. A 110-nm thick aluminum (Al) layer was deposited on a rectangular 200-nm thick silicon nitride (SiN) membrane and the Lamb wave was launched using a pulsed laser. The transfer matrix technique was employed to obtain a theoretical dispersion curve so that material properties of the SiN membrane could be estimated through curve-fitting. In addition, picosecond ultrasonic measurement was used to characterize the Al film. Results showed that the dispersive behavior of Lamb wave in the fundamental antisymmetric mode could be clearly observed on the membrane. However, comparison of dispersion curves indicated that the effect of residual stress of the film became more influential at a low dimensional scale.

The elastic property of a copper (Cu) thin film was investigated using the surface acoustic wave (SAW) measurement technique. The Cu film was deposited on a quartz substrate using a direct current magnetron sputter and its surface morphology was inspected using atomic force microscopy. Time-domain waveforms of the SAW on the film were acquired at different propagation distances to estimate the Young’s modulus of Cu such that the experimentally-obtained dispersion curve can be compared to the analytical result calculated using the Transfer Matrix method for curve-fitting. Results showed that the film’s elastic property value decreased by 18.5% compared to that of the bulk state, and the scale effect was not significant in the thickness range of 150-300 nm, showing good agreement with those by the nanoindentation technique. The property, however, increased by 15.5% with the grain coarsening.

Picosecond ultrasonic evaluation on the Young’s modulus of a ceramic thin-film was performed in the present study. A 100nm thick silicon nitride thin-film was deposited on a silicon wafer using the plasma enhanced chemical vapor deposition technique and gigahertz-frequency longitudinal bulk waves were excited in the film using a femtosecond laser setup. A thermoelastic equation was numerically solved using the finite difference method and compared to the experimental data to estimate the elastic property of the film. Results show that the present measurement technique can effectively evaluate the film’s Young’s modulus and it is recognized that the modulus is 60-70% lower than that of its bulk status. This study is expected to provide a way to characterize nanoscale ceramics with very high spatial and temporal resolutions for the design and analysis of microelectromechanical systems and thin-film based devices.