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Why Enhanced EFM?
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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
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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
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
Principle of SKPM is similar to Enhanced EFM with DC bias feedback (Figure
3). DC bias is controlled by feedback loop to zero the ω term. The DC bias that
zeros the force is a measure of the surface potential. The difference is in the
way the signal obtained from the Lock-in Amplifier is processed. As presented in
previous section, the ω signal from Lock-in Amplifier can be expressed as
The ω signal can be used on its own to measure the surface potential. The
amplitude of the ω signal is zero when VDC = Vs, or when
the DC offset bias matches the surface potential of the sample. A feedback loop
can be added to the system and vary the DC offset bias such that the output of
the Lock-in Amplifier that measures the ω signal is zero. This value of the DC
offset bias that zeroes the ω signal is then a measure of the surface potential.
An image created from this variation in the DC offset bias is given as an image
representing the absolute value of the surface potential (Figure 4).
Figure 3. Schematic diagram of the Scanning Kelvin Probe
Microscopy (SKPM) of the XE-series.
Figure 4. Surface Potential distribution on an ASIC.
Source: Park Systems
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