Electric Force (EFM) Imaging
Surface Potential Detection—Theory
Rev. B
MultiMode SPM Instruction Manual
261
14.5 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
fi
rst 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
fi
rst
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
oscillating electrostatic force at the frequency
ω
on the cantilever. The oscillating force has the
following amplitude:
where
is the vertical derivative of the tip/sample capacitance.
, the dc voltage difference between the tip and the sample,
and
is the amplitude of the oscillating voltage applied to the cantilever tip.
V
ac
ω
t
cos
F
C
d
z
d
-------
V
dc
V
ac
=
C
d
z
d
-------
V
dc
V
tip
V
sample
–
=
V
ac
