Atomic Force Microscope

Saturday, July 9, 2011

Atomic force microscopy (AFM) is a method of measuring surface topography on a scale from angstroms to 100 microns, using an atomic force microscope. The technique involves imaging a sample through the use of a probe, or tip, with a radius of 20 nm. The tip is held several nanometers above the surface using a feedback mechanism that measures surface–tip interactions on the scale of nanoNewtons. Variations in tip height are recorded while the tip is scanned repeatedly across the sample, producing a topographic image of the surface.

The atomic force microscope is an instrument which can analyze and characterize samples at the microscope level. This means that we can look at surface characteristics with accurate resolution ranging from 100 microns to 1 micron. An atomic force microscope consists of a cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law. Depending on the situation, forces that are measured in AFM include mechanical contact force, van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces. If the tip was scanned at a constant height, a risk would exist that the tip collides with the surface, causing damage. Hence, in most cases a feedback mechanism is employed to adjust the tip-to-sample distance to maintain a constant force between the tip and the sample.