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APL_paper - APPLIED PHYSICS 06 2005 A mechanical microscope...

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A mechanical microscope: High-speed atomic force microscopy A. D. L. Humphris, a ! M. J. Miles, and J. K. Hobbs b ! University of Bristol, H.H. Wills Physics Laboratory, Tyndall Avenue, Bristol, BS8 1TL, United Kingdom s Received 4 August 2004; accepted 14 December 2004; published online 14 January 2005 d An atomic force microscope capable of obtaining images in less than 20 ms is presented. By utilizing a microresonator as a scan stage, and through the implementation of a passive mechanical feedback loop with a bandwidth of more than 2 MHz, a 1000-fold increase in image acquisition rate relative to a conventional atomic force microscope is obtained. This has allowed images of soft crystalline and molten polymer surfaces to be collected in 14.3 ms, with a tip velocity of 22.4 cm s -1 while maintaining nanometer resolution. © 2005 American Institute of Physics . f DOI: 10.1063/1.1855407 g Since its invention in 1986 1 atomic force microscopy s AFM d has become the most widely used form of scanning probe microscope s SPM d with applications in surface, mate- rials, and biological sciences. 2,3 However, the inherent me- chanical nature of the AFM, requiring the serial collection of the image, limits the microscope’s maximum speed of opera- tion. A typical image is collected over a period of , 30 s, which is much slower than the millisecond time resolution required for the visualization of macromolecular process, and restricts nonimaging applications such as nanolithography 4,5 and data storage. 6 Integral to the AFM is a sharp stylus, which is mounted on the end of a microcantilever. By raster scanning the stylus across the surface of the sample and monitoring the deflec- tion of the microcantilever beam an interaction map and thus image is constructed. The image acquisition time of an AFM, and in fact any SPM, is limited by three factors: s i d the mea- surement bandwidth of the local interaction between the tip and sample, s ii d the rate at which the tip can scan the surface of the sample in an x , y plane, and s iii d how quickly the tip can follow the contours of the sample. Recently these limits have been addressed by miniaturizing the microcantilever and constructing small, lower mass and high stiffness scan- ners. This has achieved image acquisition rates of , 12 frames/s 7 but is limited to imaging small areas of , 250 3 250 nm with a maximum tip velocity of , 600 m m/s. 8 An alternative approach has been to incorpo- rate a piezo actuator into the AFM cantilever, increasing the frequency response of the feedback loop enabling a maxi- mum tip velocity of 5 mm s -1 . 9 Ultimately this approach of refining instrument design will reach a limit that is arguably not far from the capabilities already demonstrated. This letter introduces a physically different implementa- tion of an AFM that is not limited by the same constraints as the conventional approach, by solving two fundamental bar- riers. First, a micro-resonant scanner that has been previously demonstrated by Humphris et al.
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