Contrast mechanism of ultrasonic atomic force microscopy

In this paper, ultrasonic atomic force microscopy (U-AFM) was examined both experimentally and theoretically as a source of a high definition visualization of the boundary between materials. Specifically, a sample of diffusion bonded steel-Cu-Si₃N₄ was vibrated at its first resonant frequency, f=133.43 kHz, and the boundaries were examined. The experimental results show that the contrast between materials can be measured accurately by the change in the elastic properties of the materials. Moreover, it was found that this method is superior to the standard atomic force microscope (AFM) topological measurements. Numerical simulation based on the finite element method (FEM) was also carried out and compared with the experiment results. In general, the theoretical results fit the experimental findings and strengthen the interpretation that the nature of the contrast between materials is highly dependant on the elasticity of the material. Furthermore, the amplitude images were shown to produce higher contrast than phase images.