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SPIE Professional July 2011

Getting to the point

U.S. scientists measure the resonant frequency of an AFM sensor tip to gauge wear and tear.

Scientists at the National Institute of Standards and Technology (NIST) have developed a way to measure the wear and degradation of the microscopic probes used to study nanoscale structures in situ and as it's happening.

Their technique, called contact resonance force microscopy, can both dramatically speed up and improve the accuracy of the most precise and delicate nanoscale measurements done with atomic force microscopy (AFM).

If you're trying to measure the contours of a surface with a ruler that's crumbling away as you work, then you at least need to know how fast and to what extent it is being worn away during the measurement.

 As an atomic force microscope's tip degrades...
As an atomic force microscope's tip degrades, the change in tip size and shape affects its resonant frequency and which can be used to accurately measure, in real time, the change in the tip's shape, thereby resulting in more accurate measurements and images at nanometer-size scales.

This has been the challenge for researchers and manufacturers trying to create images of the surfaces of nanomaterials and nanostructures. Taking a photo is impossible at such small scales, so researchers use atomic force microscopes to measure the peaks and valleys on the surface of nanostructures. But the AFM tips are so small that they tend to wear down as they traverse the surface being measured.

Today, most researchers stop the measurement to "take a picture" of the tip with an electron microscope, a time-consuming method prone to inaccuracies.

Now NIST materials engineer Jason Killgore has developed a method for measuring in real time the extent to which AFM tips wear down. Killgore measures the resonant frequency of the AFM sensor tip while the instrument is in use. Because changes to the size and shape of the tip affect its resonant frequency, he is able to measure the size of the AFM's tip as it works—in increments of a tenth of a nanometer, essentially atomic scale resolution.

The metrology technique was described in the journal Small in April.

Source: Killgore, J., et al. "Continuous measurement of AFM tip wear by contact resonance force microscopy," Small 7, 1018-1022 (2011).

Have a question or comment about this article? Write to us at spieprofessional@spie.org.

DOI: 10.1117/2.4201110.13

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