FORCES AFFECTING THE SUBSTRATE IN RESONANT TAPPING FORCE MICROSCOPY

We propose a simple model to describe the interaction of a forced cant ilever oscillation with a specimen in a tapping-mode scanning force mi croscope experiment in order to make a rough estimation of the forces affecting the surface with each touch down of the tip. Assuming weak d amping of the cantilever (quality factor of the cantilever between 100 and 1000) and of the surface, we can estimate the forces to be in the range of those in the contact mode. These forces can vary by orders o f magnitude, e.g. 10(-6) to 10(-11) N. To reduce the interaction force we suggest scanning on the low-frequency side of the resonance freque ncy of the non-contact cantilever oscillation. Increasing the differen ce of phase between the non-contact oscillation of the cantilever in a ir and the oscillation during contact introduces strong Variations of the force. The improvement in resolution which can be achieved for sof t samples by using the tapping-mode system results from the eliminatio n of shear forces and the possibility of minimizing the force on the s urface by varying the set-point of the scanning amplitude. Forces on t he substrate will be enhanced by a large substrate stiffness.