ACOUSTIC PHASE-VELOCITY MEASUREMENTS WITH NANOMETER RESOLUTION BY SCANNING ACOUSTIC FORCE MICROSCOPY

With the increasing interest in nanostructures and thin films, the nee d for a quantitative measuring method for elastic constants on the nan ometer scale has become more evident. The fundamental physical quantit y characterizing the elastic constants is the acoustic phase velocity. Due to the strong localization of surface acoustic waves (SAWs) in th e near-surface region, SAWs are particularly favored for such investig ations. The velocity measurement is commonly performed by time delay a nd acoustic far-field methods. Therefore the lateral resolution of the velocity measurement is restricted by the wavelength involved to some tens of microns. Recently, we introduced the scanning acoustic force microscope (SAFM) for the measurement of SAW amplitude distributions w ith nanometer lateral resolution. The key to detecting high-frequency surface oscillations by the slowly responding force microscope cantile ver is the nonlinear force curve. This nonlinearity can be exploited i n a heterodyne-type setup for high-frequency wave mixing of a probe an d a reference wave, revealing the phase of the probe wave. The differe nce frequency can be chosen to be as low as 1 kHz. We present measurem ents of the phase velocity over a lateral distance of 19.9 nm. The pha se velocity dispersion due to Au layers on a quartz substrate was meas ured over distances as small as 200 nm and compared with calculations.