Atomic Force Microscopy: Optical beam deflection

An AFM has a force sensor called a cantilever to masure the forces between a sharp tip and the surface [23]. Unlike the optical microscope that relies on two dimensional images, the images acquired with the AFM are obtained in three dimensions: the horizontal xy-plane and the vertical z-direction. As shown in figure 4, the tip at the end of the cantilever is brought in close proximity to the sample mounted on a piezoelectric element. The AFM can be compared to a record player such as an old stylus-based instrument [1]. It combines the principles of a scanning tunneling microscope (STM) and the stylus profiler. However, as the probe forces in the AFM are much smaller than those (~10 -5 N) achieved with a stylus profiler.

The AFM belongs to the family of scanning probe microscopes (SPMs). Like all other SPMs, the AFM uses an extremely sharp tip that moves over the sample surface with a raster scan (like the movement of the electron beam on the TV screen). In the first AFM, the bending of the cantilever was detected using an STM but now a sensitive and simple optical method is used in most AFMs [24]. As shown in figure 4, a laser beam is reflected off the cantilever onto a two-panel photodiode. As the cantilever bends, the position of the reflected laser light changes. Measurements are obtained as a result of the differences in the signal between the two segments of this photo-detector.

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