Scanning Probe Microscopy of Thin Films
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anning Probe Microscopy of Thin Films Steven M. Hues, Richard J. Colton, Ernst Meyer, and Hans-Joachim Giintherodt Introduction Atomic force microscopy (AFM) was invented in 1986 by Binnig, Quate, and Gerber' as "a new type of microscope capable of investigating surfaces of insulators on an atomic scale." Stemming from developments in scanning tunneling microscopy (STM), it became possible to image insulators, organic and biological molecules, salts, glasses, and metal oxides —some under a variety of conditions, e.g., ambient pressure, in aqueous or cryogenic liquids, etc. In 1987, Mate and co-workers2 introduced a new application for AFM where atomic-scale frictional forces could be measured. Likewise, in 1989, Burnham and Colton3 used the AFM to measure the surface forces and nanomechanical properties of materials. Today, there are many examples of using AFM as
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a high-resolution profilometer, surface force probe, and nanoindentor. Several new imaging techniques have been introduced; each depending on the type of force measured, e.g., magnetic, electrostatic, and capacitative. Because of the diverse nature of the field and instrumentation, the names "scanned probe microscopy"4 and "XFM" (where X stands for the force being measured, e.g., MFM is magnetic force microscopy) have been adopted. A force microscope consists of a sensor that responds to a force and a detector which measures the sensor's response. The sensor, a cantilever beam with an effective spring constant k, moves in response to the forces acting on its tip. The detector measures the cantilever's displacement which can then be converted to
Force Microscopy and Imaging Control of Interaction One of the main purposes of force microscopy is to image surfaces in real space. The most important aspect of this technique is to measure accurately the interaction between probing tip and sample. While scanning the tip relative to the sample, a feedback loop keeps the interaction constant. It can be measured either with the static displacement of the cantilever,1 Figure la, which is proportional to the force, or with the shift of the resonance of the cantilever oscillation.5 The change of the resonance frequency, av,, is due to the force gradient, F' and is roughly given by
where k is the spring constant. Therefore, keeping the resonance at a fixed position while the tip scans along the sample, corresponds to profiles of constant gradient. There are two main techniques to measure the change of the resonance curve (cf. Figures lb and lc):
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Figure 1. Schematic diagrams of modes of operation, (a) Equiforcc mode: the static displacement of the cantilever is kept constant while the tip scans over the surface, (b) AC mode performed with a lock-in detector: the amplitude of the cantilever oscillation is measured by lock-in technique. Under the assumption of constant Q-factor, profiles of constant force gradient are detected, (c) AC mode performed by
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