| ABSTRACT: | QUANTITATIVE MEASUREMENT OF ELASTIC CONSTANTS OF ANISOTROPIC
MATERIALS BY ATOMIC FORCE ACOUSTIC MICROSCOPY
W. Arnold, S. Hirsekorn, M. Kopycinska-Müller, and U. Rabe
Fraunhofer-Institute for Non-Destructive Testing, Saarbrücken, Germany
In atomic force acoustic microscopy (AFAM) the cantilever is vibrating in one of its resonance frequencies
while the sensor tip remains in contact with the sample surface. The contact radius depends on the applied
static force, the tip radius, and the elastic constants of the tip and the surface. It ranges between several and
up to some tens of nm. Polycrystalline materials which appear elastically isotropic on a macroscopic scale
are therefore anisotropic on the scale which is probed by AFAM. When ferroelectric ceramics are imaged,
the acoustic images reveal the domains within the grains due to variations in contact stiffness. In
ferroelectrics an important contribution to the forces results from the electrical polarization of the individual
domains. We show that the elastic constant which is probed by a contacting AFM tip is the indentation
modulus. Furthermore, measurements on thin films of nanocrystalline piezo-ceramics revealing their piezo-
activity as a function of annealing temperature are discussed. These images show the orientation of the local
polarization as well, in particular when the ultrasonic displacements are excited by an ac voltage applied
between the tip and a counter-electrode placed below the sample. The ac frequency is adjusted so that it
corresponds to the contact resonance called by us the ultrasonic piezo-mode. The AFAM technique is
particularly suited to examine thin films used in MEMS.
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