EVALUATION OF PROPERTIES OF SUB-MICROMETER THIN FILMS USING HIGH FREQUENCY ULTRASONIC WAVES

Accurate monitoring of the thickness and elastic properties of thin films with thickness in the sub-micrometer range is very important in the area of N/MEMS fabrication. In this paper, we utilize a RF surface acoustic wave (SAW) device to measure the dispersion of SAW in a thin film deposited layered substrate first, and then, determine the elastic properties of the thin film inversely through an optimization algorithm. Firstly, we analyze the dispersion of SAW in a SiO2/YZ-LiNbO3 layered specimen and serve as the forward solution of the inverse evaluation. Secondly, a novel design of interdigital transducer pairs was proposed to measure the dispersion of SAW in such a layered specimen. To increase the bandwidth of the SAW device, slanted finger interdigital transducer (SFIT) was employed to generate wide band SAW signals. The SFIT was designed by using the coupling of modes method to ensure the best frequency response. Sub-micrometer thickness SiO2 thin films were deposited on the piezoelectric YZ-LiNbO3 substrate via the PECVD process. Pairs of the SFITs were then fabricated on the substrate. A network analyzer was used to measure the frequency response of the SFIT. The frequency responses were then processed using the spectral analysis to obtain the dispersion of SAW in such a layered specimen. With the forward solution and measured dispersion of the thin film deposited layered specimen, the elastic properties of the SiO2 layer can be reconstructed through the using of the simplex algorithm. Result of the inversion shows that the elastic properties of the sub-micrometer thin SiO2 film can be determined successfully. It is worth noting that results of this study can be employed to design an in-situ thin film thickness monitoring SAW sensor