Evaluation of elastic properties of submicrometer thin films using slanted finger interdigital transducers

Abstract

Accurate evaluation of film properties with thickness in the submicrometer range is very important throughout microelectronic industry as well as nano/microelectromechanical system industry. In this study, we developed a nondestructive evaluation technique, which is not only suitable for dielectric films but also valid for metallic films, to measure the elastic properties of submicrometer thin films. Firstly, we established the forward solution of the inverse evaluation and analyzed the dispersion of surface acoustic wave (SAW) in a SiO2∕YZ–LiNbO3 coated solid based on the effective permittivity approach. To measure the dispersion of SAW in the coated solid, a 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 obtain the optimal frequency response. SiO2 films with submicrometer thickness were deposited on the piezoelectric YZ–LiNbO3 substrate via the plasma-enhanced chemical-vapor deposition process. Pairs of the SFITs were also fabricated on the substrate. The measured frequency responses were then processed using the spectral analysis of surface wave to obtain the SAW dispersion in the coated solid. Finally, based on the forward solution and measured dispersion, we determined inversely the elastic properties of the SiO2 film through the use of the Simplex algorithm. The inversion result shows that the elastic properties of the submicrometer SiO2 film were measured successfully. We note that the results of this study provide an important basis for developing a SAW sensor which can be adopted to measure in situ film properties.