Physical mechanisms of megahertz vibrations and nonlinear detection in ultrasonic force and related microscopies

Abstract

Use of high frequency (HF) vibrations at MHz frequencies in Atomic Force Microscopy (AFM) advanced nanoscale property mapping to video rates, allowed use of cantilever dynamics for mapping nanomechanical properties of stiff materials, sensing μs time scale phenomena in nanostructures, and enabled detection of subsurface features with nanoscale resolution. All of these methods critically depend on the generally poor characterized HF behaviour of AFM cantilevers in contact with a studied sample, spatial and frequency response of piezotransducers, and transfer of ultrasonic vibrations between the probe and a specimen. Focusing particularly on Ultrasonic Force Microscopy (UFM), this work is also applicable to waveguide UFM, heterodyne force microscopy, and near-field holographic microscopy, all methods that exploit nonlinear tip-surface force interactions at high frequencies. Leveraging automated multidimensional measurements, spectroscopic UFM (sUFM) is introduced to investigate a range of common experimental parameters, including piezotransducer excitation frequency, probed position, ultrasonic amplitude, cantilever geometry, spring constant, and normal force. Consistent with studies of influence of each of these factors, the data-rich sUFM signatures allow efficient optimization of ultrasonic-AFM based measurements, leading to best practices recommendations of using longer cantilevers with lower fundamental resonance, while at the same time increasing the central frequency of HF piezo-actuators, and only comparing results within areas on the order of few μm2 unless calibrated directly or compared with in-the-imaged area standards. Diverse materials such as Si, Cr, and photoresist are specifically investigated. This work thereby provides essential insight into the reliable use of MHz vibrations with AFM and provides direct evidence substantiating phenomena such as sensitivity to adhesion, diminished friction for certain ultrasonic conditions, and the particular benefit of UFM and related methods for nanoscale mapping of stiff materials.

Subsurface imaging of two-dimensional materials at the nanoscale

“…36 Therefore local variations in the ultrasonic amplitude cannot be responsible for the effects observed. The origin of the UFM subsurface imaging must be found elsewhere and specifically in its sensitivity to the indentation.…”

“…When the adhesion properties of the sample are homogenous, the contrast of an UFM image can be interpreted as follows: brighter colours indicate regions where the indentation is lower, corresponding to areas with larger Young's modulus. 36 For the UFM and TM-AFM measurements, our experimental setups are hybrid systems made of commercial and custom components. One is based on a Multimode-type head with a Nanoscope III controller (Bruker), the other on a SMENA-type head (NT-MDT) with home-built electronic controller.…”

Nanoscale Imaging and Characterisation of Amyloid-β

“…TM-AFM preformed in ambient conditions involves using an oscillating cantilever near its resonance frequency, usually in order of 100-300 kHz, at an amplitude of 20-100 nm by means of the piezo built into the cantilever holder driving the oscillation (Fig 1. 31 . This variation of contact AFM allows qualitative 32,33 and quantitative 34,35 measurements of the elastic behaviour of a sample by oscillating the sample at frequencies well above the cantilever resonant frequency 0 (typically at =2-10 MHz compared to 0 = 10 − 300 kHz). This out-of-plane vibration of the sample increases the effective cantilever's spring constant due to inertia 29 .…”

Nanomechanical morphology of amorphous, transition, and crystalline domains in phase change memory thin films

“…The crystalline line present in all samples studied also serves as a consistent reference for UFM measurements up to 10-20 µm on either side. This alleviates the need for independent calibration of the absolute ultrasonic amplitude (and hence UFM contrast) that is otherwise necessary, since it can vary on the scale of many hundreds of µm [30] due to the underlying transducer, resonant effects, or spatially dependent ultrasonic transmission. Since the m scale UFM images presented here are orders of magnitude smaller than the ultrasonic wavelength, and include an identical crystalline region in each case, they may thus be compared with confidence [31].…”

“…There are occasional similar features present within the amorphous region as well, which could result from the amorphous region of GST containing a small fraction of crystalline phase grains, likely due to localized temperature elevations during the initial sputtering process. To more accurately compare the relative stiffness of all three phase regions, which are necessarily obtained over different regions of the underlying piezotransducer, the RMS variation of the UFM signal was normalised to the average response over the entire imaged area as described elsewhere [30]. For GST, the UFM stiffness contrast in the thermally induced amorphous and crystalline regions varied by 7% and 11%, respectively.…”