Ultrasonic Modes in Atomic Force Microscopy

Abstract

The development of Atomic Force Microscopy (AFM) [1] enabled one to execute experiments on a nano-scale. Next to the operation modes allowing topography measurements e.g. contact mode and tapping mode [2], other techniques have been developed. With help of the AFM based techniques it is possible to image other properties like e.g. friction (Friction Force Microscopy) [3], elasticity (Force Modulation, Pulsed Force Mode, Ultrasonic Force Microscopy) [4–6], and magnetic (Magnetic Force Microscopy) [7] and ferroelectric domains (piezo-mode techniques) [8]. Atomic Force Acoustic Microscopy (AFAM) [9] and Lateral Atomic Force Acoustic Microscopy (Lateral AFAM) [10] are dynamic modes of AFM that combine the high resolution of AFM with the enhanced sensitivity of a vibrating cantilever to elastic properties of a sample surface. AFAM and lateral AFAM can be applied in the spectroscopy and in the imaging mode. In the spectroscopy mode one measures the bending and the torsional contact resonance frequencies from which one determines the vertical and lateral contact stiffness. In addition the adhesion and friction forces have a pronounced influence on the shape of measured contact-resonance spectra [10,11]. In the imaging mode the AFM cantilever vibrates at a frequency close to the contact resonance frequency and the amplitude of the vibrations changes with the local tipsample contact stiffness. Using the amplitude of the cantilever vibrations as contrast provides qualitative images of changes in the tip-sample contact stiffness. The bending and the torsional modes can be also excited in the so-called piezo-mode.