  Ultrasound Atomic Force Microscopy (US-AFM) has been used for subsurface
imaging of nanostructures. The contact stiffness variations have been suggested
as the origin of the image contrast. Therefore, to analyze the image contrast,
the local changes in the contact stiffness due to the presence of subsurface
features should be calculated. So far, only static simulations have been
conducted to analyze the local changes in the contact stiffness and,
consequently, the contrast in US-AFM. Such a static approach does not fully
represent the real US-AFM experiment, where an ultrasound wave is launched
either into the sample or at the tip, which modulates the contact stiffness.
This is a time-dependent nonlinear dynamic problem rather than a static and
stationary one. This letter presents dynamic 3D ultrasound analysis of contact
stiffness in US-AFM (in contrast to static analysis) to realistically predict
the changes in contact stiffness and thus the changes in the subsurface image
contrast. The modulation frequency also influences the contact stiffness
variations and, thus, the image contrast. The three-dimensional time-dependent
ultrasound analysis will greatly aid in the contrast optimization of subsurface
nanoimaging with US-AFM.
