Constant Force mode
In Contact mode of operation the cantilever deflection under scanning reflects repulsive force acting upon the tip.
Repulsion force F acting upon the tip is related to the cantilever deflection value x under Hooke's law: F = -kx, where k is cantilever spring constant. The spring constant value for different cantilevers usually vary from 0.01 to several N/m.
In our units the vertical cantilever deflection value is measured by means of the optical registration system and converted into electrical signal DFL. In contact mode the DFL signal is used as a parameter characterizing the interaction force between the tip and the surface. There is a linear relationship between the DFL value and the force. In Constant Force mode of operation the deflection of the cantilever is maintained by the feedback circuitry on the preset value. So vertical displacement of the scanner under scanning reflects topography of sample under investigation.
Constant Force mode has some advantages and disadvantages.
Main advantage of Constant Force mode is possibility to measure with high resolution simultaneously with topography some other characteristics - Friction Forces, Spreading Resistance etc.
Constant Force mode has also some disadvantages. Speed of scanning is restricted by the response time of feedback system. When exploring soft samples (like polymers, biological samples, Langmuir-Blodgett films etc.) they can be destroyed by the scratching because the probe scanning tip is in direct contact with the surface. Thereunto under scanning soft unhomogeneous samples the local flexure of sample surface varies. As a result acquired topography of the sample can prove distorted. Possible existence of substantial capillary forces imposed by a liquid adsorption layer can decrease the resolution.
References
- Magonov, Sergei N. Surface Analysis with STM and AFM. Experimental and Theotetical Aspects of Image Analysis.VCH 1996.