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SPM Principles

AFAM

 

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The basic idea of Atomic Force Acoustic Microscopy (AFAM) is to excite the cantilever of an atomic force microscope into flexural vibrations when the tip is in contact with the sample [1, 2]. The frequencies of the eigen-modes of the cantilever depends, among with other parameters, on the stiffness of tip-sample contact and on the contact radius, which in turn both are a function of Young's modulus of the sample and the tip, the tip radius, the load exerted by the tip, and the geometry of the surface. Such a technique allows one to determine the Young's modulus from the contact stiffness with a resolution of a few tens of nanometers.

As is shown on the animation under AFAM measurements sample is coupled to the piezoelectric transducer. It emits acoustic waves into the sample which cause out-of-plane vibrations of the sample surface. The surface vibrations are transmitted into cantilever via the sensor tip. The cantilever vibrations are measured by 4-sectioned photo-diode and evaluated by lock in amplifier. This setup can be used to acquire acoustic images – maps of cantilever amplitude on the fixed excitation frequency near the resonance (AFAM imaging). The Constant Force mode topography image is acquired simultaneously with the acoustic one. AFAM images reveal the sample surface elasticity distribution. Also this setup can be used to take cantilever vibration spectra - AFAM Contact Resonance Spectroscopy (CRS). By usage of AFAM CRS one can determine Young's module [3, 4].  

References
  1. US Pat. 5675075.
  2. US Pat. 5852233. 
  3. Rev. Sci. Instrum 67, 3281 (1996).
  4. J. Appl. Phys. 82, 966 (1997).
   
 
 
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