Electric Techniques
Surface Potential Detection—Theory
296
Dimension 3100 Manual
Rev. D
16.3.6 Optimize Tune in Interleave
Find optimal lift height. Use
View
>
Sweep
>
Cantilever Tune
to open the tune window while
engaged. Choose a
Tip Offset
equal to the
Lift Height
. Make any changes to the
Drive frequency
and
Drive phase
to adjust the main tune (tip #3). In the
Sweep Controls
panel, select the
Interleave
Controls
button. Activate (green button) the
Drive frequency
and
Phase
, and
Analog
2
. With
Analog 2
set to operating voltage, set the frequency to the center of the peak and zero the
phase.
16.3.7 If Voltage is Needed, Use Analog 2 When Possible
It is often very difficult to get a high quality TappingMode image with an electric field present.
Analog 2 has an important advantage over an external power supply – it can be used to create an
E field only in interleave and be set to 0V in the main line (> v4.31). Its range is limited to +/- 12V.
16.3.8 Try Uncoated Si Tip
EFM has been successful with a variety of tips, including standard Si tips with no metal coating
(fesp, tesp, ltesp). The Si is highly n-doped and is often conductive enough for EFM measurements.
Metal coatings make the tip less sharp, decreasing lateral resolution. The standard tips also seem to
give cleaner images over longer periods of time. There is likely a reduction in sensitivity to small
E fields with the decreased conductivity.
16.4 Surface Potential Detection—Theory
Note:
Surface potential detection EFM is only possible using the one of the extender
modules or the NanoScope IV controller. This section does not apply to
microscopes which are
not
equipped
with
the Basic or Quadrex Modules, or the
NanoScope IV controller.
•
The Basic Extender Module allows measurement of local sample surface potential. This
is similar to techniques called Scanning Maxwell Stress Microscopy and Kelvin Probe
Microscopy. Surface potential detection is a two-pass system where the surface
topography is obtained in the first pass and the surface potential is measured on the
second pass. The two measurements are interleaved, that is, they are each measured one
line at a time with both images displayed on the screen simultaneously.
A block diagram of the surface potential measurement system is shown in
. On the first
pass, the sample topography is measured by standard TappingMode. In TappingMode the cantilever
is physically vibrated near its resonant frequency by a small piezoelectric element. On the second
pass, the piezo that normally vibrates the cantilever is turned off. Instead, to measure the surface
potential, an oscillating voltage
is applied directly to the cantilever tip. This creates an
V
ac
ω
t
cos
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