The atomic force microscope

Figtire 3,17. Schematic diagriim of alamic fotce microscope (AFM). 

Therefore, Binnig, Quate and Gerber devised the atomic force microscope (AFM) shown schematically in Fig. 3.17. The probe was now much smaller and lighter so as to detect the molecular adhesion forces. These forces caused a slight movement of the probe which could be observed by several different sensor methods. However, laser deteetion, as shown in Fig. 3.17, turned out to be most convenient. Small silicon cantilever probes were made by etching a silicon wafer. Laser light was reflected from the top of the silicon cantilever probe, and entered a detector where any deflection could be registered. 

A typical adhesion experiment using the AFM was performed by raising the sample towards the probe while observing the cantilever deflection. When the probe was distant from the sample, nothing happened, but as the probe came within range of the adhesion forces, the probe was attracted and pulled down, as shown by the result of Fig. 3.18. 

As the sample is pushed nearer to the probe, the attractive force increases rapidly and the probe then jumps into contact with the sample. This appears as a sudden deflection of the cantilever. Then, once the probe is stuck to the sample surface, the movement of sample and cantilever are equal, as shown by the linear contact deflection in Fig. 3.18. 

Moving the sample in the opposite direction can then be used to pull the probe off the surface. This result is shown in Fig. 3.19. The deflection of the cantilever is reversed along the same linear path as the probe sticks to the sample at first. Then, at a certain deflection of the cantilever, the probe jumps off the 

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