Nanomechanical surface characterization by atomic force acoustic microscopy

We constructed an atomic force acoustic microscope that enables one to detect out-of-plane and in-plane sample surface vibrations in a wide frequency range from about 100 kHz to several MHz. From the point of view of acoustics, an AFM cantilever is a miniaturized elastic beam that can vibrate in different types of modes as, for example, flexural, torsional, and extensional. We present a method of how to calculate the vibrational behavior of a cantilever with its sensor tip in contact with or near a sample surface, and how to deduce surface properties like the Young's modulus from the vibrations induced in the cantilever. Further, we present single-point measurements in which we compare ultrasonic cantilever vibration spectra where the tip is in contact with surface areas with different stiffnesses to force calibration curves. Finally, we present ultrasonic images taken with our microscope. (C) 1997 American Vacuum Society.