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Quantitative contact spectroscopy by atomic-force acoustic microscopy

: Rabe, U.; Kester, E.; Scherer, V.; Arnold, W.

Lee, H.:
Acoustical Imaging
New York: Kluwer, 2000 (Acoustical imaging 24)
ISBN: 0-306-46518-3
International Symposium on Acoustical Imaging <25, 2000, Bristol>
Fraunhofer IZFP ()
Atomic Force Acoustic Microscopy; cantilever; spectroscopy

In Atomic Force Microscopy (AFM) deflection of a microfabricated elastic beam with a sensor tip at its end is used to generate high-resolution images of surfaces. Dynamic modes, where the cantilever is vibrated while the sample surface is scanned, belong to the standard equipment of most commercial instruments. With a variety of these techniques images can be obtained which depend on the elasticity of the sample surface. However, quantitative determination of Young's modulus of a sample surface with AFM is still a challenge especially when stiff materials such as hard metals or ceramics are encountered. In this contribution the evaluation of the cantilever vibration spectra at ultrasonic frequencies is presented in order to discern local elastic data quantitatively. In a first step the resonances of the AFM cantilever in free vibration without contact to a sample surface are determined. When the tip comes in contact with the surface, the resonance frequencies jump to higher values. From the position of the contact resonances relative to the free resonance frequencies, the contact stiffness is calculated. The contact stiffness is a measure of how much the boundary condition of the previously free end was changed. Stiffer surfaces and higher tip radii cause higher contact stiffness. Tip-sample damping and lateral forces also shift the resonances. From the contact stiffness, Young's modulus of the surface is calculated, using Hertz contact theory and taking into account the force applied by the cantilever and the adhesion force. Tip radii are measured with a special test sample. Quantitative results on different materials are presented and the limits of Hertz model and the linear force approximation are discussed.