Exploring The Sensitivity Window In Subsurface Scanning Probe Microscopy Using Frequency Mixing Scheme

The subsurface contrast in Scanning Probe Microscopy(SPM) based techniques is usually a result of change in Contact Stiffness. In subsurface ultrasound imaging using a frequency mixing scheme, a high frequency excitation is used to increase the mechanical impedance and a modulation frequency sensitive to contact stiffness changes is used, simultaneously. The nonlinear nature of the tip-sample interaction, which depends on various probe and sample properties, causes the instantaneous contact stiffness to vary greatly. Adding the complexity of multi-frequency excitation makes it difficult to do quantifiable subsurface imaging. Subsurface ultrasound imaging can be used to measure subsurface structures through various different layers with hundreds of nanometers of depth sensitively. In this technique the tip is excited at mixed frequencies with a high carrier frequency which is either amplitude modulated or frequency modulated to a modulation frequency near the contact resonance. Thus, benefiting from the increasing mechanical impedance of the tip leading to an effective transfer of ultrasonic energy into the sample and the sensitivity to change in local stiffness changes of the material. This dynamic variation in stiffness is a culmination of depth of the nanofeatures, local (visco)-elastic properties, tip velocity and time. In this research, we thus study the influence of experimental parameters like the carrier signal, modulation signal, depth of modulation of the excitation and indentation force on the achievable sensitivity for the subsurface contrast.