Ultrasonic Force Microscopy: Nanometer Scale Mechanical Contrast
The mechanical sensitivity of acoustic microscopy and the spatial resolution of atomic force microscopy are combined in Ultrasonic Force Microscopy (UFM). To demonstrate the spatial resolution of this technique, UFM has been applied to map the mechanical compliance of thin films of silicon-germanium sandwiched between 50 nanometer (nm) layers of silicon. SiGe layers only 2.5 nm across have been identified for this system where the difference in the reduced modulii is less than 20 GPa. To interpret experimental results, extensive simulations of UFM measurements have also been performed for a wide variety of sample types. The contrast measured during UFM with a lock-in-amplifier is calculated and compared to experimental results. It is shown that UFM may be sensitive to the average mechanical properties within a 20 nm radius hemisphere that terminates at the surface.
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