P(VDF-TrFE) Transducer with a Concave Annular Structure for the Layer Thickness Measurement

Abstract

The present authors established a method¹ of the layer thickness measurement, in which a plane wave with oblique incidence was applied to a specimen with an overlay, and a Fourier spectrum of a reflected wave was analyzed(Fig. 1). The frequency dependence of the spectrum showed a dip when a surface wave was excited, and the layer thickness was deduced from the dip frequency. The frequency and the depth of the dip varied when the incident angle was changed, because the surface wave on a layered structure had a velocity dispersion. Therefore, an appropriate incident angle should be chosen in order to obtain a deep dip for the accurate measurement. However, a measurement at a small spot on the specimen was difficult because plane waves with beam width over 0.5mm were used. In order to measure the layer thickness on a small spot by this method, it is necessary to focus ultrasonic waves on the spot, while it is desirable to maintain the incident angle of the ultrasonic waves near the appropriate value. To fulfill this somewhat contradictory requirement, a new type of a transducer using a piezo-electric co-polymer film P(VDF-TrFE) (Vinylidene fluoride and trifluoroethylene) with concave annular structure is developed². To what extent does the transducer focus the ultrasonic waves? Is the layer thickness measurement possible? If so, in what way does the beam focusing affect the measurement? To investigate these questions, this paper presents a theoretical analysis using an angular spectrum theory, and describes experimental evaluation of the transducer.