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2006
Book Article
Titel
Atomic force acoustic microscopy
Abstract
According to Abbe's limit, the lateral resolution of microscopes is restricted to approximately one wavelength of the involved radiation, here the acoustic wave. Using near-field microscopes the lateral resolution can fall below Abbe's value. An Atomic Force Microscope (AFM) which senses the ultrasonic vibrations in a sample can be considered as a near-field acoustic microscope. Different types of near-field acoustic microscopes derived from AFM were proposed. The combination of ultrasound with atomic force microscopy (AFM) opens the high lateral resolution of scanning probe techniques to ultrasonics. Atomic force acoustic microscopy (AFAM) and ultrasonic friction force microscopy are techniques which use the vibration modes of AFM cantilevers. The resonance frequencies of commercial cantilevers of a few 100 µm length are predominantly in the ultrasonic frequency range between 20 kHz and several MHz. In the AFAM-mode the cantilever is vibrating in one of its flexural resonance frequencies while the sensor tip is continuously in contact with the sample surface. With this technique images can be obtained the contrast of which depend on the elasticity of the sample surface. The radius of the tip-sample contact area ranges between several nm's and several 10 nm's. Polycrystalline materials which appear mechanically isotropic on a macroscopic scale are therefore anisotropic on the length scale which is probed by an AFM and the elastic constant which is measured by the tip is the indentation modulus. When ferroelectric ceramics are imaged, the acoustic image reveals a substructure within the grains indicating variations in contact stiffness. In ferroelectrics an important contribution to the contact stiffness results from the electrical polarization of the individual domains. Analytical models to describe flexural cantilever vibration for quantitative measurement of elastic constants are summarized in this article. Examples of application by different research groups are given.