A new method for nondestructive testing of SiO2 film thickness using a portable one-port surface acoustic wave (SAW) resonator based on lithium niobate (LiNbO3) is proposed. First, the finite element method is used to simulate and analyze the relationship between the resonant frequency of SAW resonator and film thickness. Then, the vector network analyzer is used to nondestructively characterize the thickness of SiO2 film by SAW resonator. The relationship between the thickness and the corresponding resonant frequency in a certain range is obtained and given by a second order polynomial. The results show that the resonant frequency is negatively correlated with film thickness, where the resonant frequency changes from 339.27 to 318.40 MHz in the film thickness range of 100 to 2000 nm. To validate the prediction formula, when the film thicknesses are 201.20, 504.60, 842.10, and 1497.70 nm, the resonant frequency is used to verify the experimental fitting polynomial. The relative errors between the predicted thickness by SAW resonator and the actual film thickness are 1.60%, 0.34%, 0.67%, and 0.96%. The results show that SAW resonator has great potential in detecting thin film thickness with high sensitivity and accuracy.
The laser-generated surface acoustic wave (LSAW) nondestructive testing (NDT) technique is a promising method to characterize the mechanical properties of thin films. In this study, based on the thermoelastic mechanism, a finite element method (FEM) is put forward to simulate the LSAW in the film/substrate structure, and the effect of the temporal and spatial distribution of the Gaussian pulse laser on the Rayleigh-type SAW signals is revealed. For the SiO2 and low dielectric constant (low- k) dense Black Diamond™ (SiOC:H, BD) films with the thickness of 500 and 1000 nm, the typical displacement waveforms of SAW at a series of probing points along the propagation direction are obtained. By analyzing the full width at half maximum (FWHM) of the signal, the optimal NDT experimental conditions for laser are determined with the minimum possible pulse rising time and the linewidth less than 10 μm. Based on the FEM simulation result, the LSAW NDT experiment is carried out and the dispersion curve of SAW is calculated to characterize Young's modulus of the SiO2 and low- k samples. It is found that the experimental results are in good agreement with the simulation results. This study verifies the validity of FEM simulation of LSAW in layered structures containing thin film and that the laser parameters determined by FEM fit perfectly in characterizing the mechanical properties of thin films.
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