Acoustic resonant imaging technique for visualizing the acoustic properties and thickness of polymer film on substrate

This paper proposes an acoustic resonant imaging technique for visualizing the acoustic properties and thickness of a polymer film on a substrate. When ultrasound passes through a thin layer, transmission and reflection coefficients of sound pressure attain their extreme values at the resonant frequency. By obtaining the area information of the extreme value and resonant frequency and matching them with a theoretical model, the acoustic properties and thicknesses of a polymer film on a substrate can be visualized. Herein, this technique was applied to a photoresist film coated on a Si wafer, and in addition to visualizing fluctuations in film thickness, the differences in the film hardness that may have occurred during the curing process were successfully detected as the differences in the acoustic impedance of the film. The acoustic resonant imaging technique was successfully used to determine the frequency dependence of both the transmission and reflection coefficients.