Constant Height 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 Height mode of operation the scanner of the microscope maintains fixed end of cantilever on the constant height value. So deflection of the cantilever under scanning reflects topography of sample under investigation.
Constant Height mode has some advantages and disadvantages.
Main advantage of Constant Height mode is high scanning speeds. It is restricted only by resonant frequency of the cantilever.
Constant Height mode has also some disadvantages. Samples must be sufficiently smooth. 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 samples with relatively high relief the pressure upon the surface varies , simultaneously varies local flexure of sample surface. 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.
- Magonov, Sergei N. Surface Analysis with STM and AFM. Experimental and Theotetical Aspects of Image Analysis.VCH 1996.