Atomic Force Microscope

RECOMMENDED SAMPLE SIZE : Capable of 150 x 150 micron scanned area, maximum height variation of 14 microns.

Adelaide Microscopy has a TopoMetrix TM2000 Scanned Probe Microscope equipped with the Discoverer and Explorer heads for both materials and biological applications.

The atomic force microscope (AFM) probes the surface of a sample with a sharp tip, a few microns long and often less than 100Å in diameter. The tip is located at the free end of a cantilever that is 100 to 200µm long.

Forces between the tip and the sample surface cause the cantilever to bend, or deflect. A detector measures the cantilever deflection as the tip is scanned over the sample, or the sample is scanned under the tip. The measured cantilever deflections allow a computer to generate a map of surface topography.

AFMs can be used to study insulators, semiconductors and electrical conductors. The great advantage of an AFM is that it is able to operate in air but can also operate in vacuum or in a liquid.

Several forces typically contribute to the deflection of an AFM cantilever. The force most commonly associated with atomic force microscopy is an interatomic force called the van der Waals force.

Figure 1 click on the image for larger version

The dependence of the van der Waals force upon the distance between the tip and the sample is shown in Figure 1. Two distance regimes are labelled on Figure 1:

1. the contact regime; and
2. the non-contact regime.

In the contact regime, the cantilever is held less than a few angstroms from the sample surface, and the interatomic force between the cantilever and the sample is repulsive. In the non-contact regime, the cantilever is held on the order of tens to hundreds of angstroms from the sample surface, and the interatomic force between the cantilever and sample is attractive (largely a result of the long-range van der Waals interactions).

Contact AFM uses a micro-fabricated cantilever and tip to probe a sample surface. A deflection sensor detects cantilever bending in response to surface height variations while a feedback system regulates the force between the tip and sample. A piezoelectric scanner scans the tip over the sample under computer control while a 3-dimensional image of surface topography is recorded.

Contact AFM can record topographic, frictional and elasticity variations with close to atomic resolution, allowing surface measurements and images on a scale beyond the capabilities of conventional microscopes. Contact AFM images require little sample preparation, and the typical scan time is generally under a few minutes. Image processing software generates 3-d renditions of data and provides quantitative surface roughness and other measurements.

Figure 2 click on the image for larger version

Most AFMs currently on the market detect the position of the cantilever with optical techniques. In the most common scheme, shown in Figure 2, a laser beam bounces off the back of the cantilever onto a position-sensitive photo-detector (PSPD).

As the cantilever bends, the position of the laser beam on the detector shifts. The PSPD itself can measure displacements of light as small as 10Å. The ratio of the path length between the cantilever and the detector to the length of the cantilever itself produces a mechanical amplification. As a result, the system can detect sub-angstrom vertical movement of the cantilever tip. Other methods of detecting cantilever deflection rely on optical interference, or even a scanning tunnelling microscope tip to read the cantilever deflection. One particularly elegant technique is to fabricate the cantilever from a piezo-resistive material so that its deflection can be detected electrically. (In piezo-resistive materials, strain from mechanical deformation causes a change in the material's resistivity.) For piezo-resistive detection, a laser beam and a PSPD are not necessary.

Once the AFM has detected the cantilever deflection, it can generate the topographic data set by operating in one of two modes- constant-height or constant-force mode. In constant-height mode, the spatial variation of the cantilever deflection can be used directly to generate the topographic data set because the height of the scanner is fixed as it scans. In constant-force mode, the deflection of the cantilever can be used as input to a feedback circuit that moves the scanner up and down in z, responding to the topography by keeping the cantilever deflection constant. In this case, the image is generated from the scanner's motion. With the cantilever deflection held constant, the total force applied to the sample is constant.

In constant-force mode, the speed of scanning is limited by the response time of the feedback circuit, but the total force exerted on the sample by the tip is well controlled. Constant-force mode is generally preferred for most applications. Constant-height mode is often used for taking atomic-scale images of atomically flat surfaces, where the cantilever deflections and thus variations in applied force are small. Constant-height mode is also essential for recording real-time images of changing surfaces, where high scan speed is essential.

Figure 3 click on the image for larger version

Figure 3: Atomic Force Microscope image of contaminant on a Compact Disc surface.