Tapping AFM

Tapping AFM also known as Intermittent Contact Mode is one of several different dynamic imaging modes used in atomic force microscopy (AFM). AFM is a type of scanning probe microscopy. These microscopes do not use light, as in conventional microscopes, but rather scan a sharp probe over a surface to image extremely small surface structures. With an AFM it is possible to image a surface at 0.1 nm resolution, (1.0 × 10⁻¹⁰ m) and generate a 3D map of the sample surface.

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The AFM probe sensor is known as a cantilever, generally a thin rectangular lever some hundreds of micrometers long and a few micrometers wide. This is generally referred to as a diving board cantilever because of its similar shape, but there are several different shapes and sizes available. One end of the cantilever is affixed to an inflexible base, usually called a "chip", attached to the AFM tip holder. The opposite side of the cantilever is completely free. On this free side a very fine-pointed tip is mounted perpendicular to the longitudinal axis of the cantilever. Generally the operating distance of the AFM tip is approached in two subsequent stages. In the first (coarse) stage the AFM tip is moved rapidly to the sample using precise stepping motors. After this movement a second (fine) approach is made by moving the sample with a piezoelectric actuator up toward the tip. At this stage, due to the extremely short distance between the tip and the sample surface, one can easily detect the effects of the repulsive/attractive atomic forces that give the name to this device.

In tapping mode the cantilever is driven to oscillate up and down at near its resonance frequency by a small piezoelectric element mounted in the AFM tip holder. The amplitude of this oscillation is greater than 10 nm, typically 100 to 200 nm. Due to the interaction of forces acting on the cantilever when the tip comes close to the surface, Van der Waals force or dipole-dipole interaction, electrostatic forces, etc cause the amplitude of this oscillation to decrease as the tip gets closer to the sample. An electronic servo uses the piezoelectric actuator to control the height of the cantilever above the sample. The servo adjusts the height to maintain a set cantilever oscillation amplitude as the cantilever is scanned over the sample. A Tapping AFM image is therefore produced by imaging the force of the oscillating contacts of the tip with the sample surface. This is an improvement on conventional contact AFM, in which the cantilever just drags across the surface at constant force and can result in surface damage.

References

US Patent Number 5,412,980

SPM - Scanning Probe Microscopy