We report the observation of surface acoustic waves (SAWs) launched from an interdigital transducer (IDT) at gigahertz frequencies, employing dynamic heterodyned electrostatic force microscopy (EFM). Dynamic heterodyned EFM is a relatively simple technique for the imaging of the polarization dynamics at the surface of the materials caused by complex interdigital transducer radiation patterns or any other excitation factor, with nanometer lateral resolution [1].

In this demonstration of the heterodyned EFM technique, the polarization at the substrate surface is induced by a passing surface acoustic wave. Amplitude modulation at the probe enables this method to be extended to frequencies well above the mechanical resonant frequency of the probe cantilever. This increases the bandwidth of the measurement and eliminates the need to an interference pattern in order to recover the high-frequency behavior of the SAW [2-4]. The electrostatic attraction between the probe and the piezoelectric material generated by the polarization dipoles in the surface layers of the material can be easily detected using techniques common in non-contact force microscopy. In earlier work using dynamic heterodyned EFM [1], the very small signals from the SAW were used to provide the dynamic equivalent of a grating to calibrate the technique.

Fig. 1. Test device and reflector configuration (left); a topographic AFM image of the main IDT structure (right). The average height and width of each electrode is 200 nm and 500 nm respectively.

Fig. 2. 10 μm × 10 μm image of the raw response of the EFM probe to the polarization due to the SAW propagation on the substrate surface. This image was obtained in lift mode 300 nm above the substrate, near the reflector area. It clearly shows the main wave (up-down) and reflected wave (left-right).

References:

[1] D. R. Oliver et al., Appl. Phys. Lett. 79p. 3729, [2001]

[2] T. Hesjedal et al., Appl. Phys. Lett. 78 p. 1948, [2001]

[3] G. Behme et al., J. Appl. Phys. 89 p. 4850, [2001]

[4] T. Hesjedal et al., Appl. Phys. Lett. 79 p. 1054, [2001]

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