Quantitative Contact Spectroscopy and Imaging by Atomic-Force Acoustic Microscopy
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Fraunhofer-Institute for Nondestructive Testing (IZFP) Bldg. 37, University, D-66123 Saarbrficken, Germany
([email protected]) ABSTRACT Atomic Force Acoustic Microscopy is a near-field technique which combines the ability in using ultrasonics to image elastic properties with the high lateral resolution of scanning probe microscopes. We present a technique to measure the contact stiffness and the Young's modulus of sample surfaces quantitatively with a resolution of approximately 20 rum exploiting the contact resonance frequencies of standard cantilevers used in Atomic Force Microscopy. The Young's modulus of nanocrystalline ferrite films have been measured as a function of oxidation temperature. Furthermore images showing the domain structure of piezoelectric lead zirconate titanate ceramics have been taken. INTRODUCTION
In Atomic Force Microscopy (AFM) the 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, are standard in commercial instruments. Various techniques, such as Force Modulation Microscopy [1, 2], Ultrasonic Force Microscopy [3, 4], Atomic Force Acoustic Microscopy [5], Microdeformation Microscopy [6], Scanning Local Acceleration Microscopy [7], or Pulsed Force Microscopy [8] enable one to obtain images 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 for stiff materials such as hard metals or ceramics. In this contribution the evaluation of the
cantilever vibration spectra at ultrasonic frequencies is presented in order to discern local elastic data quantitatively. Images are presented showing differences in contact stiffness, and hence elasticity. Resolution is given by the contact radius of the tip of the cantilever on the surface of the sample being examined. One type of AFM cantilevers are elastic beams of typically several 100 gm length, a width of several tens of gm and a few gm thickness. One end of the beam is fixed on a substrate and the free end holds the sensor tip. Having a rectangular cross section, the dominant acoustical vibrations of the beam are flexural, extensional and torsional modes [9]. When the sensor tip approaches a sample surface, the tip-sample interaction forces, such as Van-der-Waals forces, electrostatic forces, repulsive contact forces or damping forces, represent a nonlinear springdashpot system. The forces change the boundary conditions of the cantilever and, consequently, its resonance frequencies. Acoustic vibrations can be coupled from the vibrating sample surface into the cantilever via the spring and also from the vibrating cantilever into the sample. Resonance frequencies are shifted relatively to the free resonances when they are in force interaction with a surface. This fact can be exploited to determine the contact stiffness 183 Mat. Res. Soc. Symp. Proc. Vol. 5910 2000 Mater
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