Abstract
Imaging of subsurface features down to the nanometer scale is of great importance in various fields such as microelectronics, materials science, nanobiology, and nanomedicine. Since their invention 25 years ago, ultrasonic-based atomic force microscopy (AFM) techniques have attracted vast attention for their mechanical surface and subsurface sensing capability. In this Perspective article, we review the research on ultrasonic AFMs for subsurface imaging. We first describe the instrumentation setups and different detection schemes of ultrasonic AFMs. Then, attention is paid to the studies of the physical contrast mechanism, the evaluation of the detection capabilities, in particular, the detection depth limits, and the optimization approaches to enhance the contrast and to improve the detection depth. After that we present typical applications of using ultrasonic AFMs for detecting subsurface defects including dislocations, voids, and interfaces in functional materials and devices; visualizing embedded inclusions in composites; and imaging subcellular structures in biological materials. We conclude with an outlook of the challenges faced by ultrasonic AFMs toward fast, high resolution, and quantitative subsurface imaging.
Highlights
Subsurface imaging techniques with nanoscale resolution are becoming increasingly important
It is still not clear why nanoparticles in such deep depths influence the local stiffness and damping on the surface center, the experimental studies make us confident that the contact stiffness differences due to elasticity and damping variations relative to the host material induced by subsurface features, are the key contrast mechanism for subsurface imaging in ultrasonic atomic force microscopy (AFM) using mm wavelengths
The calculated interference pattern agrees well with the observed pattern. This corroborates the contribution of ultrasonic scattering to the subsurface imaging contrast in GHz ultrasonic AFM. This mechanism was further verified by van Es et al.[99] on silicon samples with grooves etched into the surface which were covered by a 5 μm thick poly(methyl methacrylate) (PMMA) polymer
Summary
Subsurface imaging techniques with nanoscale resolution are becoming increasingly important. AFAM and UAFM were operated at the resonance frequencies of the cantilever in contact with a sample They are referred to as contact-resonance AFM (CR-AFM).[25,26] The use of much higher ultrasonic frequencies than the cutoff frequency of the detection photodiodes in AFMs became possible by employing heterodyne or amplitude demodulation techniques.[14,27,28] The heterodyne force microscopy (HFM) is called scanning near-field ultrasound holography (SNFUH).[29]. We conclude with a discussion of the challenges and potential application opportunities facing ultrasonic AFMs
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