Atomic Force Microscopy

Atomic Force Microscopy (AFM) is a scanning probe method for imaging the surfaces of solid bodies in real space.

Measuring principle

AFMWith a fine tip located on a microscopic cantilever the sample is scanned at a short distance. The cantilever bends due to the interaction forces (in the order of nN) between tip and sample. The ideally mono-atomic tip is moved line by line over the sample using piezoceramic actuators. In contact mode 'the cantilever is guided with a preset force across the sample. In non-contact mode the cantilever is set to oscillation measuring the frequency shift when approaching the sample. At that the interaction variable can be kept constant by a control loop. The manipulated variable of the control loop as a function of x and y position of the probe represents a contour image of constant interacting forces. For homogeneous samples this represents in a good approximation the actual topography. In order to prevent a rapid contamination of the surface most of the measurements are carried out under vacuum.

The control loop

In order to keep the distance of the cantilever to the sample surface constant, the normal force FN is maintained constant by an electronic control loop. The control loop measures the instantaneous force FN (actual value) and compares it to the preset value (target value). If these differ, the controller attempts to eliminate the difference via its output (control value). The required changes are transmitted by piezoelectric elements which can move the cantilever in the three spatial directions. The controller settings depend on the entire system and therefore cannot be set arbitrarily high. Moreover, the control loop in itself represents an oscillatory system with a cutoff frequency above which natural oscillations occur. These natural oscillations can damage cantilever and sample. The scanning speed may greatly affect the quality of the AFM image. Too high a speed usually degrades the image. One must also consider that the control rate is closely coupled to the scanning speed.

The cantilever

The cantilevers used have according to electron micrographs radii of curvature of some 10 nm at the tip. Since atomic resolution on the AFM can be achieved this can be explained only by the presence of so-called mini tips. This means that almost all of the force effect is caused solely by the closest to the sample mini tip. The arrangement of these mini tips on the cantilever is by no means stable, but can change during the measurements. This can have both positive and negative effects on the resolution and the overall quality of the images. The cantilevers are usually made of silicon or the harder silicon nitride.

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