How Does AFM Work?

AFM (Atomic Force Microscopy) is possibly the most versatile and powerful type of microscopy technology available for studying samples at the nanoscale. It is versatile because a budget atomic force microscope can not only image in three-dimensional topography, but it also provides various types of surface measurements to the needs of engineers and scientists. It is powerful because an AFM can easily generate images at atomic resolution with angstrom scale resolution height information, with minimum sample preparation. 

AFM Principle

- Surface Sensing

An AFM uses a cantilever with a very sharp tip to scan over a sample surface. As the tip approaches the surface, the close-range, attractive force between the surface and the tip causes the cantilever to deflect towards the surface. However, as the cantilever is brought even closer to the surface, such that the tip makes contact with it, increasingly repulsive force takes over and causes the cantilever to deflect away from the surface.

- Detection Method

A laser beam is used to detect cantilever deflections towards or away from the surface. By reflecting an incident beam off the flat top of the cantilever, any cantilever deflection will cause slight changes in the direction of the reflected beam. A position-sensitive photodiode (PSPD) can be used to track these changes. Thus, if an AFM tip passes over a raised surface feature, the resulting cantilever deflection (and the subsequent change in direction of reflected beam) is recorded by the PSPD.

- Imaging

An AFM images the topography of a sample surface by scanning the cantilever over a region of interest. The raised and lowered features on the sample surface influence the deflection of the cantilever, which is monitored by the PSPD. By using a feedback loop to control the height of the tip above the surface—thus maintaining constant laser position—the AFM can generate an accurate topographic map of the surface features.

Standard Imaging

- Contact Mode

In this method, the cantilever scans across a sample surface. Because the cantilever is in contact with the surface, the strong repulsive force causes the cantilever to deflect as it passes over topographical features. 

- Non Contact Mode

In this technique, the cantilever oscillates just above the surface as it scans. A precise, high-speed feedback loop prevents the cantilever tip from crashing into the surface, keeping the tip sharp and leaving the surface untouched. As the tip approaches the sample surface, the oscillation amplitude of the cantilever decreases. By using the feedback loop to correct for these amplitude deviations, one can generate an image of the surface topography.

- Tapping Mode

In this alternative technique to non-contact mode, the cantilever again oscillates just above the surface, but at a much higher amplitude of oscillation. The bigger oscillation makes the deflection signal large enough for the control circuit, and hence an easier control for topography feedback. It produces modest AFM results but blunts the tip’s sharpness at a higher rate, ultimately speeding up the loss of its imaging resolution.