Nonlinear Effects in Ultrasonic Transmission in Atomic Force Microscope Contacts
W.Arnold, S.Hirsekorn, M.Kopycinska, U.Rabe, M.Reinstadtler (Fraunhofer Institute for Nondestructive Testing (IZFP), Saarbruecken, Germany)
In Atomic Force Microscopy (AFM), the forces between a sharp tip and a surface of a sample determine the contrast observed in imaging. Atomic Force Acoustic Microscopy (AFAM) is a near-field technique which combines the ability of ultrasonics to image elastic properties with the high lateral resolution of scanning probe microscopes. In AFAM flexural vibrations are excited in a cantilever of an AFM. The eigenfrequencies of these modes depend on the forces acting between the tip and the sample surface. In case of a contacting tip a situation can be achieved where the Hertzian contact stiffness is prevailing. At sufficiently low amplitudes with cantilever tip displacements of a few Angstroms or less the contact stiffness can be measured exploiting the contact resonance frequencies. At higher ultrasonic amplitudes distinct non-linearities are observed in the contact resonance curves. Whereas in silicon and other materials a softening behavior is common, i.e. a decrease of the resonance frequency, the situation is more complex in piezoelectric ceramics. Theoretical approaches exist to analyze ultrasonic AFM data relating them to the non-linearities of the material properties and to the adhesion between the tip and the sample surface. Nonlinearities can also be observed when transverse surface displacements are induced in the sample leading to non-linear torsional resonances. These effects are related to frictional properties between the tip and the sample surface. We think that the nonlinear transmission of ultrasound in AFM "single" point contacts is related to the nonlinear transmission of a large ensemble of contacts in bonding interfaces and cracks as discussed in other contributions at this conference.
Section : 3