Acoustically excited probe microscopy
Acoustically excited probe microscopy
Valiant attempts were made to increase the resolution of acoustic microscopy by employing ever higher frequencies and ever less attenuating coupling fluids. But this did not prove the ultimate way to resolution, which was achieved instead by abandoning the concept of focusing to a diffraction‐limited spot, and instead adopting a scanning probe to give near‐field resolution. In the ultrasonic force microscope (UFM), excitation is fed through the sample to the contact with the tip of an atomic force microscope (AFM). Modulation of the ultrasonic excitation is detected through the non‐linear stiffness of the tip–sample contact, which acts as a mechanical diode. The technique can be used for a wide range of materials characterization, for both soft materials such as polymers and stiff materials such as semiconductor nanostructures. In each case UFM gives contrast from the mechanical properties and structure, with resolution of a few nanometres or better. Variations of the technique, such as the heterodyne force microscope (HFM), enable phase resolution on a nanosecond timescale to be combined with stiffness measurement with nanometre spatial resolution.
Keywords: ultrasonic force microscope, atomic force microscope, heterodyne force microscope, near‐field, mechanical diode, polymer, semiconductor, nanostructure
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