Depth-Sensing using AFM contact-resonance imaging and spectroscopy at the nanoscale

Abstract

Subsurface metrology techniques are of significant importance at the nanoscale, for instance, for imaging buried defects in semiconductor devices and in intracellular structures. Recently, ultrasonic-based atomic force microscopy has attracted intense attention also for subsurface imaging. Despite many applications for measuring the real and imaginary part of the local surface modulus, the physical mechanism for subsurface imaging is not fully understood. This prevents accurate data interpretation and quantitative reconstruction of subsurface features and hinders the development of an optimized experimental and engineering setup. In this paper, we present quantitative depth-sensing of subsurface cavity structures using contact-resonance atomic force microscopy (CR-AFM) imaging and spectroscopy. Our results indicate that for imaging subsurface cavity structures using CR-AFM, the induced contact stiffness variations are the key contrast mechanism. The developed algorithm based on this mechanism allows one to precisely simulate the experimental image contrasts and give an accurate prediction of the detection depth. The results allow a better understanding of the imaging mechanism of ultrasonic-based AFM and pave the way for quantitative subsurface reconstruction.