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Atomic Force Microscopy (AFM) appears as a very natural tool to work in the single molecule domain. This apparatus was originally designed to visualize surfaces with atomic scale resolution. Its working principle is to scan the surface of a sample with a very sensitive position detector and to record the modulation of the topological signal. The AFM relies on a very thin cantilever as a detector. Somewhat similar to the needle of a DJ�s record player, this micro-fabricated beam is typically 100 microns long, 10 microns wide and a fraction of a micron thick. It has an extremely sharp tip at its end (radius of curvature in the tens of nanometers). The position of this cantilever is measured by reflecting a collimated laser beam onto its surface and imaging the light spot on a two- or four- quadrant diode detector. The sample is scanned horizontally by a XY piezo stage, providing atomic resolution.

As we have stated, the AFM is primarly a visualization tool and its use as a single molecule micromanipulation device is a secondary feature. The very sensitive cantilever has motivated researchers to use this device to measure the force required to stretch a biopolymer or to break a molecular bond. To achive this goal, the operator stops the horizontal sample-scanning process and gently moves the cantilever vertically above the sample while recording its deflection. The AFM provides some natural features :

• The position of the cantilever can be adjusted with sub-nanometer resolution and the vertical scanning speed can be high.
• The deflection of the cantilever is also read with sub-nanometer resolution, and the response time is in the milli-seconde range.
• The AFM may be used to visualize the sample.
• Strong forces in the nanoNewton range may be applied to the sample.

The force sensitivity of the AFM is related to the cantilever�s size and stiffness. Different cantilevers provide a range of stiffness from 1 N/m to 10-3 N/m (or 1pN to 1 nN per nanometer) of cantilever deflection. A very common error consists in saying that the best sensor is the one with the smallest stiffness. We shall see that the best sensor (in term of signal to noise) in in fact the smallest device. Since the cantilever size is typically 100 microns they are not the best sensors in terms of noise. A strong effort is under way to reduce the size of the cantilever to improve their signal to noise. Typically the minimum force measured with the AFM is 5 pN.