About Park Systems
Unique Enhanced EFM
Capabilities Provided Only by the XE-series
Why Enhanced EFM?
Systems is the Atomic Force Microscope (AFM) technology leader, providing
products that address the requirements of all research and industrial nanoscale
applications. With a unique scanner design that allows for the True Non-Contact
imaging in liquid and air environments, all systems are fully compatible with a
lengthy list of innovative and powerful options. All systems are designed with
ease-of-use, accuracy and durability in mind, and provide your customers with
the ultimate resources for meetiong all present and future needs.
Boasting the longest history in the AFM
industry, Park Systems' comprehensive portfolio of products, software,
services and expertise is matched only by our commitment to our customers.
For EFM, the sample surface properties would be electrical properties and the
interaction force will be the electrostatic force between the biased tip and
sample. 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.
Three extra EFM modes are supported by the enhanced EFM option of the XE-series.
They are DC-EFM (DC-EFM is patented by Park
Systems US Patent 6,185,991), Piezoelectric Force Microscopy (PFM, same as
DC-EFM), and Scanning Kelvin Probe Microscopy (SKPM), also known as Surface
In the enhanced EFM of the XE-series
whose schematic diagram is shown in Figure 1, an external Lock-in Amplifier is
connected to the XE-series AFM for two purposes. One purpose is to apply AC
bias of frequency ω, in addition to the DC bas applied by the XE controller, to
the tip. The other purpose is to separate the frequency ω component from the
output signal. This unique capability offered by the XE-series
enhanced EFM is what excels in performance when compared to the Standard EFM.
Figure 1. Schematic diagram of the enhanced EFM of the
In the enhanced EFM, the voltage between the tip and the sample can be
expressed by the following equation:
VDC is the DC offset potential, and VS is the surface
potential on the sample, and VAC and ω are the amplitude and
frequency of the applied AC voltage signal, respectively.
The electrostatic force applied to the cantilever can be expressed by
Equation (2), which uses the two-parallel-plate capacitor model to describe the
electrostatic interaction between the cantilever and the sample.
Here, F is electrostatic force applied to the tip, q is the charge, E is the
electric field, V is the electric potential difference, C is the capacitance,
and d is the tip-sample distance. Note that since both AC and DC bias are
applied between the tip and the sample, three terms arise in the expression for
the force between the tip and the sample. These terms can be referred to as the
DC term (a), the ω term (b), and the 2 ω term (c), respectively.
The total cantilever deflection signal, which represents the force between
the tip and sample, can be analyzed in terms of its separate parts: DC part, AC
part with a frequency of ? and AC part with a frequency of 2ω.
The DC cantilever deflection signal can be read directly from the signal
channels accessible using XEP Data Acquisition software. The AC parts of the
cantilever deflection signal can be read by sending the signal to a Lock-in
amplifier, which can read either the part of the signal with a frequency of ω,
or the part of the signal with a frequency of 2ω. Together, the three signals
can be used to gain information about the electrical properties of the sample.
For example, the capacitance appears in the equation as the ratio of capacitance
to tip-to-sample spacing, C/d. If the tip-to-sample distance is kept constant by
the z feedback loop, then C/d is proportional to the capacitance. The ω signal,
which is the coefficient of the term labeled (b) in Equation (2) above, contains
contributions from both C/d and the surface potential, Vs. Assuming
VDC and VAC are known, you still cannot separate the
contributions of the capacitance and the surface potential to the measured ω
signal. However, the 2ω signal, which is the coefficient of the term labeled (c)
above, only includes a contribution from the capacitance. Thus, the 2ω signal
can be used to normalize the ω signal, isolating the contribution of the surface
Images can be generated from any of the above-mentioned signals. Analysis of
an image involves understanding the contributions to the signal used to generate
Conventional EFM is operated by unnecessary and inefficient double-pass scan,
prohibitively limiting the spatial resolution of surface potential map. The
Enhanced EFM by the XE-series is designed to provide efficient one-pass scan to
measure both topography and surface potential simultaneously without losing
spatial resolution (Figure 2). Moreover, this allows the two key innovations of
the Enhanced EFM: High frequency EFM signal measurement in,
- Surface charge distribution and potential imaging
- Failure analysis in micro electronics circuitry
- Mechanical hardness measurement (DC-EFM)
- Charge densitometry for ferroelectric domain
- Voltage drop on micro resistors
- Work function of a semiconductor
Figure 2. Conventional EFM vs. Enhanced EFM by the
DC-EFM (Dynamic Contact EFM) mode is the Enhanced EFM that operates in
Contact mode that provides the much improved spatial resolution and more
sensitive detection (See Figure 3).
Figure 3. Schematic diagram of the Dynamic Contact EFM
(DC-EFM) of the XE-series. The unique EFM capabilities are patented and provided
only by Park Systems.
Figure 4 makes the comparison of topography and surface charge images of TGS
single crystal acquired by DC-EFM (upper) and conventional EFM (lower),
respectively. The EFM image taken by conventional EFM shows strong coupling of
the topography signal to the image while the image taken by DC-EFM shows
complete separation of the topography. The key advantages of DC-EFM are as
- No need of special sample treatment
- High spatial resolution and noninvasive probing.
- Simultaneous topography and domain imaging (Figure 5)
- Real-time imaging of domain dynamics
- Nanoscale control and visualization of domains (Figure 6)
- Detailed local information rather than integral effect
Figure 4. (Left) (a) Topography and (b) surface charge
image of TGS single crystal by DC-EFM and (c) topography and (d) surface charge
image by conventional EFM.
Figure 5. (a) Topography and (b) EFM phase image of the
PZT film by DC-EFM.
Figure 6. (Right) (a) Domain switching behavior in
ferroelectric materials. Creation of small domains of TGS by (b) positive
applied voltage of 10 V, and (c) negative applied voltage of 10 V.
Source: Park Systems
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