KEYSIGHT Technologies

The Atomic Force Microscopy Resource Library


Contact Mode or Quasi-static Atomic Force Microscopy (AFM)

In contact mode AFM, the sharp tip at the end of the microfabricated AFM cantilever is in perpetual contact with the sample surface. This can be done with the cantilever bent up or bent down from its free equilibrium position, the latter due only to attractive forces between the tip and the sample. The spring constant k (typically 0.001 - 5 nN/nm for Contact Mode) of the AFM cantilever and the deflection z (typically few nm, up to several 10’s of nm‘s for Contact Mode) of the cantilever from its free equilibrium position define the minimum value of the force Fc between the tip and the surface in contact mode AFM when the deflection is up:

The total force is larger than this minimum, because upon contact between the tip and the sample surface, attractive forces tend to hold the tip and the sample together even if the cantilever were pulled away from the sample so that the cantilever bends down past its equilibrium position.

These attractive forces include capillary and adhesive forces, as well as Van der Waals forces. They result in part from a thin layer of molecules adsorbed on the sample and the tip. The composition of this layer depends on the environmental conditions of the laboratory and also on the tip and the sample material. Hydrocarbons and water molecules are major contributors to this layer in ambient air and also in low to intermediate vacuum. After bringing the tip and the sample into contact, one can retract the two and record the deflection of the cantilever down, below its free equilibrium position, in order to get an estimate of the attractive forces.

The signal from the detector is the almost-vertical deflection of the AFM cantilever (deflection signal), measured usually at or near the free end of the cantilever using a reflected laser beam. This signal subtracted from the AFM feedback system’s setpoint value is what we call the error signal and is usually recorded in what is known as the Deflection Image - clearly a confusing misnomer, which has unfortunately taken root in the SPM jargon. If there is no feedback on this error signal, then, with proper calibration, and for small cantilever deflections, typically under 500nm, the deflection image can serve as a topography image on samples with hard surfaces (so that the AFM tip does not significantly indent into the surface).

Usually, the error signal is the input to a feedback system, the output of which (after significant amplification) serves to drive the AFM’s Z-actuator, which moves either the sample or the probe in the Z direction. Then, this feedback output signal, which is almost universally called the Z-signal, is the one that is mapped to create the Topography Image.

In this case, it is the Z-actuator whose movement is calibrated. The Z-actuator is typically a piezoelectric tube, or a flexure, or a hybrid of the two. Often, the Deflection Image and the Z-actuator Topography Image are collected simultaneously, with the Deflection Image highlighting the edges of features in the topography image. The Deflection Image on a softer sample often reveals subsurface structure more clearly than the Topography Image. If the feedback is optimized, then the deflection image is the first derivative of the topography image.

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