About Park Systems
Imaging of Electrostatic
Principle of EFM
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Electrostatic Force Microscopy (EFM) of the XE-series
maps electric properties on a sample surface by measuring the electrostatic
force between the surface and a biased AFM
cantilever. EFM applies a voltage between the tip and the sample while the
cantilever hovers above the surface, not touching it. The cantilever deflects
when it scans over static charges, as depicted in Figure 1.
Figure 1. EFM maps locally charged domains on the sample
EFM images contain information about electric properties such as the surface
potential and charge distribution of a sample surface. EFM maps locally charged
domains on the sample surface, similar to how MFM plots the magnetic domains of
the sample surface. The magnitude of the deflection, proportional to the charge
density, can be measured with the standard beam-bounce system. Thus, EFM can be
used to study the spatial variation of surface charge carrier. For instance, EFM
can map the electrostatic fields of an electronic circuit as the device is
turned on and off. This technique is known as “voltage probing” and is a
valuable tool for testing live microprocessor chips at the sub-micron scale.
Four different EFM modes, distinguished by the method which the surface
electrical information is obtained, are provided by XE-series
AFM. These are Standard EFM, Park
Systems’s own patented Dynamic-Contact EFM (DC-EFM), Piezoelectric Force
Microscopy (PFM), and Scanning Kelvin Probe Microscope (SKPM).
Most of the material properties investigated by AFM are
acquired by processing the deflection signal of the cantilever as depicted in
Figure 2, which is applied to the EFM measurements as well.
For EFM, the sample surface properties would be electrical properties and the
interaction force will be the electrostatic force between the biased tip and the
Figure 2. Schematic diagram of the surface property
measurement by the advanced XE modes.
However, in addition to the electrostatic force, the van der Waals forces
between the tip and the sample surface are always present. The magnitude of
these van der Waals forces change according to the tip-sample distance, and are
therefore used to measure the surface topography.
Hence, the obtained signal contains both information of surface topography
(called ‘Topo signal’) and information of surface electrical property (called
‘EFM signal’) generated by the van der Waals and electrostatic forces,
respectively. The key to successful EFM imaging lies in the separation of the
EFM signal from the entire signal. EFM modes can be classified according to the
method used to separate the EFM signal.
The standard EFM of the XE-series is based on the two facts. One fact is that van der
Waals forces and electrostatic forces have different dominant regimes. van der
Waals forces are proportional to 1/r6, while electrostatic forces are
proportional to 1/r2. Thus, when the tip is close to the sample, van
der Waals forces are dominant. Whereas the van der Waals forces rapidly decrease
and the electrostatic forces become dominant, as the tip is moved away from the
sample. The other fact is that the topography line is acquired by keeping the
tip-sample distance constant, which equals the line of constant van der Waals
force. In the Force Range technique, the first scan is performed by scanning the
tip in the region where the van der Waals force is dominant for topography
image. Then, the tip-sample distance is varied to place the tip in the region
where the electrostatic force is dominant and scanned for EFM image as shown in
Figure 3. The schematics of (a) Force Range technique and
(b) two pass technique.
In the Two Pass technique, the first scan is performed to obtain the
topography by scanning the tip near the surface as it is done in NC-AFM, in
the region where the van der Waals forces are dominant. In the second scan,
system lifts the tip and increases the tip-sample distance in order to place the
tip in the region where electrostatic forces are dominant. The tip is then
biased and scanned without feedback, parallel to the topography line obtained
from the first scan as shown in Figure 3 (b), therefore maintaining constant
Since the topography line is the line of constant van der Waals force, the
van der Waals forces applied to the tip during the second scan are constant.
Thus, the only source of the signal change will be the change of the
electrostatic force. So, from the second scan, a topography free EFM signal can
Figure 4. Standard sample is made of two micro comb
shaped electrodes with one’s teeth lying between the other’s (a). Topography
image (b) shows that neighboring teeth are of same height but EFM Phase image
(c) shows that neighboring teeth of same height differ in surface potential.
Source: Park Systems
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