How an AFM Works
Analogous to how an STM works, a sharp tip is raster-scanned over a surface using a feedback loop to adjust parameters needed to image a surface. Unlike STM, the AFM does not need a conducting sample. Instead of using the quantum mechanical effect of tunneling, atomic forces are used to map the tip-sample interaction.
Often referred to as scanning probe microscopy (SPM), there are AFM techniques for almost any measurable force interaction - van der Waals, electrical, magnetic, thermal. For some of the more specialized techniques, modified tips and software adjustments are needed.
In addition to Angstrom-level positioning and feedback loop control, there are 2 components typically included in AFM: Deflection and Force Measurement.
AFM Probe Deflection
Traditionally, most AFMs use a laser beam deflection system where a laser is reflected from the back of the reflective AFM lever and onto a position-sensitive detector. AFM tips and cantilevers are typically microfabricated from Si or Si3N4. Typical tip radius is from a few to 10s of nm.
Laser beam deflection for atomic force microscopes
Because the AFM relies on the forces between the tip and sample, these forces impact AFM imaging. The force is not measured directly, but calculated by measuring the deflection of the lever, knowing the stiffness of the cantilever.
Hooke’s law gives
F = -kz
where F is the force, k is the stiffness of the lever, and z is the distance the lever is bent.
Force-distance curve for AFM
Feedback Loop for AFM
AFM has a feedback loop using the laser deflection to control the force and tip position. As shown, a laser is reflected from the back of a cantilever that includes the AFM tip. As the tip interacts with the surface, the laser position on the photodetector is used in the feedback loop to track the surface for imaging and measuring.
Schematic for contact mode AFM