Introduction to Atomic Force Microscopy

Atomic force microscopy (AFM) is a very high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. The technique proves its worth by giving scientists the ability to see and interact with samples at a level of detail much finer than is achievable with traditional optical microscopy techniques. This breakthrough has made AFM an indispensable measurement tool across many scientific disciplines including materials science, semiconductor physics, biophysics and many others.

Basic principle of operation

At the heart of an Atomic Force Microscope is a sharp tip, usually made of silicon or silicon nitride, attached to the free end of a cantilever that is typically 100-200 micrometers long and has a normal force constant of 0.1-10 N/m. By measuring the deflection of the cantilever with a simple laser/photodiode system as the tip is scanned over a sample surface, a "topographical" map of the surface is constructed.

The key to high resolution in an AFM relies on using very small interatomic forces to deform the sample surface rather than gross contact as in stylus profilers. Forces emanating between the tip and sample are typically less than 1 nN and result in cantilever deflections on the order of manometers. The laser spot is positioned to reflect off the top of the cantilever into the photodiode. As the tip passes over surface features, the cantilever bends up and down due to interactions between the tip and the surface. By carefully monitoring the position of the reflected laser spot on the photodiode, a computer records the vertical movement of the cantilever and constructs a topographical map of the sample surface.

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