Tutorials - Atomic Force Microscopy (AFM)

Introduction to Imaging modes

The detection system measures the cantilever response as the tip is moved over the surface by the scanning system. In most AFM systems there is also a feedback loop, which adjusts the position of the cantilever above the surface as it is scanned, to take account of the changes in surface height. A value of the cantilever deflection, for example, is selected and then the feedback system adjusts the height of the cantilever base to keep this deflection constant as the tip moves over the surface. This is known as contact mode imaging.

There are other ways of operating the system, however, using dynamic modes where the cantilever vibrates, and this oscillation of the cantilever is measured rather than the static deflection of the tip. There are different ways to excite the oscillations - the cantilever substrate can be shaken directly, or a magnetic field can be used to drive the cantilever itself if it is coated with a ferromagnetic layer. In aqueous conditions, the most common technique is to drive the cantilever acoustically through the liquid. In all these cases, however, the measurement of the cantilever oscillation and control systems are the same, and the cantilever is usually driven close to resonance.

In these dynamic modes, a setpoint amplitude is chosen, and the height adjusted to match this amplitude through the feedback system. In addition to the height and error signal information from this constant amplitude mode, the phase between the drive signal and the cantilever can also be measured. There are several different dynamic modes, depending on how much of the oscillation cycle the tip actually makes contact with the surface.

Intermittent contact mode, is widely used, and can give a combination of the benefits of the other modes. The cantilever oscillates and the tip makes repulsive contact with the surface of the sample at the lowest point of the oscillation.

The lateral forces can be much lower than contact mode, since the proportion of the time where the tip and sample are in contact is quite low. There may be a higher normal force between the tip and sample when they are in contact, however.

In non-contact mode the cantilever oscillates close to the sample surface, but without making contact with the surface. This mode is not so widely used, since the attractive force means that there is a possibility of the tip jumping into contact with the surface. The capillary force makes this particularly difficult to control in ambient conditions. Very stiff cantilevers are needed so that the attraction does not overcome the spring constant of the cantilever, but the lack of contact with the sample means that this mode should cause the least disruption.

Another mode is possible, where the tip does not leave the surface at all during the oscillation cycle. This is something like a dynamic form of contact mode, and is usually called force modulation mode.