Subsurface contrast due to friction in heterodyne force microscopy

Abstract

The nondestructive imaging of subsurface structures on the nanometer scale has been a long-standing desire in both science and industry. A few impressive images were published so far that demonstrate the general feasibility by combining ultrasound with an atomic force microscope. From different excitation schemes, heterodyne force microscopy seems to be the most promising candidate delivering the highest contrast and resolution. However, the physical contrast mechanism is unknown, thereby preventing any quantitative analysis of samples. Here we show that friction at material boundaries within the sample is responsible for the contrast formation. This result is obtained by performing a full quantitative analysis, in which we compare our experimentally observed contrasts with simulations and calculations. Surprisingly, we can rule out all other generally believed responsible mechanisms, like Rayleigh scattering, sample (visco)elasticity, damping of the ultrasonic tip motion, and ultrasound attenuation. Our analytical description paves the way for quantitative subsurface-AFM imaging.

Subsurface phase imaging of tapping‐mode atomic force microscopy at phase resonance

“…[21][22][23] In the phase image, when the phase shift is above 90 • , the force between the probe and the sample is mainly attractive, while when the phase shift is below 90 • , the force is mainly repulsive. 24 When the probe vibration was driven by the frequency, the probe had the high amplitude sensitivity, [25][26][27] but the phase sensitivity was relatively low. As shown in Figure 1, in this work, the frequency at the phase resonance peak of probe was selected for driving the probe vibration to measure the sample of spin coated photoresist 3740 (Manufactured by ALLRESIST, Germany) etched with FIB (FEI Helios G4 DualBeam), which improved the image resolution.…”

Scattering contrast in GHz frequency ultrasound subsurface atomic force microscopy for detection of deeply buried features

“…In fact, imaging buried structures with the nanometer scale resolution of scanning probe microscopy has a wide range of applications running from metrology in the semiconductor industry [7,8], to investigating processes in live cells [9]. While there has been quite some debate over the contrast mechanism(s) which enable subsurface imaging [4,6,10], elasticity (or more properly, visco-elasticity) is generally cited as the main physical cause of the contrast [2,7,[11][12][13]. Such elasticity-based subsurface AFM has shown few nanometer resolution on buried structures [14], but the depth sensitivity of this method is limited by the amount of stress that can be applied without damaging the sample.…”

Nonlinear Phase Imaging of Gold Nanoparticles Embedded in Organic Thin Films

“…Many research fields require a simple technique for nondestructive subsurface imaging of inorganic nanoparticles. − In this respect, several advanced techniques based on ultrasound − or heterodyne − force detection demonstrated exceptional lateral resolution at the nanometer scale. , These techniques, implemented in some custom-built atomic force microscopes, represent significant advancements in subsurface imaging and tomography, but their use by the broader community unfortunately is hindered by added/intrinsic complexity . On the other hand, the possibility of measuring the phase of the cantilever oscillation with respect to the excitation phase at a fixed driving frequency has been implemented very early − in all atomic force microscopes working in dynamic mode.…”