Contrast Mechanism of Ultrasonic-based Atomic Force Microscopy for Subsurface Imaging

Imaging of buried defects on a nanoscale is of significant importance for semiconductor devices and intracellular biological structures. In the last two decades, ultrasonic-based atomic force microscopy (AFM) has attracted intense attention as a surface mechanical characterization tool and is now commercially available. In this type of AFM mode, the eigenmodes of the cantilever are excited. Its amplitude, phase and eigenfrequencies are used as imaging quantities. Since its first introduction, several derivatives have been developed based on different ultrasonic excitation/detection schemes. Many successful applications have been carried out to measure local elastic and viscoelastic surface properties. Meanwhile, subsurface imaging using ultrasonic-based AFM is also shown to be possible by various groups. However, the contrast mechanism of subsurface ultrasonic AFM is not yet fully understood. This does not only prevent the evaluation of its detection capabilities, but also prevents accurate data interpretation and quantitative reconstruction of the subsurface object. The present study aims at understanding the contrast mechanism of subsurface ultrasonic AFM. Our results show that the contact stiffness variation is the key contrast mechanism for both subsurface cavity structures and embedded heterogeneous features. Applications of using ultrasonic AFM for detecting subsurface defects in graphite are also demonstrated.