High-Resolution Materials Characterization by Conventional and Near-Field Acoustic Microscopy

Acoustic microscopy not only allows imaging of sample surfaces with high spatial resolution but also can be exploited to determine quantively surface properties. The scanning acoustic microscope (SAM) uses focused waves within a frequency range of 100 MHz-2GHz which yields a spatial resolution of a few µm limited by the wavelength of the imaging ultrasound. Quantitative determination of elastic surface properties can be done by evaluating measured material signatures of the acoustic microscope in reflection, so-called V(z) curves, and by calibrated measurements of the maximum amplitude of specular reflection. Both methods are based on theoretical calculations of ultrasonic reflection at sample surfaces in the acoustic microscope with special reference to the maximum of specular reflection. The applicability of these methods is shown experimentally. The fact that ultrasonic waves in a sample can be detected with the tip of an atomic force microscope (AFM) can be exploited for ultrasonic imaging with a lateral resolution given by the sensor tip diameter which is about 10 nm and much smaller than the wavelength. Quantitative evaluation of acoustic AFM images to determine elastic, anelastic and adhesive surface properties requires the theoretical description and calculation of the transfer of ultrasonic vibrations from an insonified sample to an AFM-cantilever its sensor tip is in contact with the sample surface, which together with some experimental results is discussed in this contribution.