Veeco Dimension 3100, Manual

Page: 295 / 376

Electric Techniques
Electric Force Microscopy
Rev. D Dimension 3100 Manual 275
16.2 Electric Force Microscopy
16.2.1 Electric Force Microscopy Theory
Electric Force Microscopy is analogous to standard MFM, except that gradients being sensed are
due to electrostatic forces. In this method, the cantilever is vibrated by a small piezoelectric element
near its resonant frequency. The cantilever’s resonant frequency changes in response to any
additional force gradient. Attractive forces make the cantilever effectively “softer,” reducing the
cantilever resonant frequency. Conversely, repulsive forces make the cantilever effectively “stiffer,”
increasing the resonant frequency. A comparison of these force additives is shown in
Figure 16.2a .
Figure 16.2a Comparison of Attractive and Repulsive Forces
Changes in cantilever resonant frequency are detected in one of the following ways:
• Phase detection (with Extender Electronics Module only)
• Frequency modulation (with Extender Electronics Module only)
• Amplitude detection (not recommended due to artifacts)
All of the above methods rely on the change in resonant frequency of the cantilever due to vertical
force gradients from the sample.
Figure 16.2b shows a diagram of how the Extender electronics
module provides signal enhancement and feedback allowing gradient detection. The best
candidates for electric field gradient imaging are samples that have large contrasts in the electric
force gradient due to material differences or regions at substantially different potentials. For other
samples having rough surface topography or small voltage variations, this technique may be
undesirable because topographic features appear in the
LiftMode image.
Attractive gradient equivalent to additional spring in tension attached
to tip, reducing the cantilever resonance frequency.
Repulsive gradient equivalent to additional spring in compression attached
to tip, increasing the cantilever resonance frequency.
Frequency
Amplitude
∆
F
0
Frequency
∆
F
0
Amplitude