Atomic Force Microscopy at Ultrasonic Frequencies

Atomic Force Microscopy (AFM) is a near-field technique to generate high-resolution images of surfaces. A micro-fabricated elastic beam with an integrated sharp sensor tip at its end is scanned over the sample surface. With various dynamic modes, leading to Force Modulation Microscopy [1], Ultrasonic Force Microscopy [2], Atomic Force Acoustic Microscopy (AFAM) [3–5], Microdeformation Microscopy [6], Scanning Local Acceleration Microscopy [7] or Pulsed Force Microscopy [8], images can be obtained in which the contrast depends on the elasticity of the sample surface. However, quantitative determination of Young’s modulus of a sample surface with AFM is a challenge, especially when stiff materials such as hard metals or ceramics are encountered. In this presentation the basic idea of AFAM, i.e. the evaluation of the cantilever vibration spectra at ultrasonic frequencies is discussed. The AFAM technique can be used for imaging as well as to discern local elastic data quantitatively. Nanocrystalline magnetic materials [9], multidomain piezoelectric materials and silicon [10], diamond-like carbon layers [11], polymeric materials and clay crystals [12] have been examined previously. The spatial resolution of the AFAM technique is approximately 10 nm. In an Atomic Force Acoustic Microscopy set-up [3-5] either the sample is insonified or the cantilever suspension is excited at ultrasonic frequencies. The vibration spectra of the cantilever depend on the local sample stiffness and hence