Thickness measurement of polymer thin films with high frequency ultrasonic transducers

Abstract

In this paper we present a method for characterizing the thickness, and more interestingly, the variation of thickness in polymer thin films (<1 μm). The technique utilizes an optoacoustic transducer atop the polymer film; as the transducer's acoustic waves resonate within the polymer layer, we are able to optically monitor the interference of the waves, whose patterns are characteristic of the local polymer thickness. An optoacoustic model was used to simulate the interference of the transducer resonance and the polymer cavity modes, allowing a comparison with experiment in order to reverse engineer the polymer cavity length at each position across an area of the sample. This method yielded a thickness map which closely resembles the variation in experimental acoustic frequency, and indicated a thickness gradient of ≈200-500nm across the large 10×10mm sample. By utilizing Gigahertz frequency ultrasound, this technique provides access to nanometric features of polymer films and could be applied to film thickness monitoring applications within the field of polymer science.