Dear
Vacuum / Surface Scientist,
Surface Analysis Technique
The Kelvin Probe is a non-contact, non-destructive
vibrating capacitor device used to measure the
work function difference,
or for non-metals, the surface potential,
between a conducting specimen and a vibrating tip.
Although not as well known as some other surface
analysis techniques, the Kelvin Probe has undergone
a dramatic renaissance over the last few years.
Advances in hardware design and signal processing
technology have improved the resolution of the
instrument while also ensuring that it can be used
in a vacuum.
The equipment is also reasonably affordable.
Improvements have also been made to the spatial
resolution of the technique, new designs can map
surface properties with resolution in the 50 nm
range. Several spectroscopic variants have also been
developed for the analysis of semiconductor surfaces
and thin films.
The Kelvin Probe is a non-invasive technique, yet it
is extremely sensitive to changes in the top-most
atomic layers, such as those caused by deposition,
absorption, corrosion and atomic displacement. In
some cases it can detect less than one-thousandth of
an absorbed layer.
Mode of Operation
When two materials with different work functions are
brought together, electrons in the material with the
higher work function flow to the one with the lower
work function. If these materials are made into a
parallel plate capacitor, equal and opposite surface
charges form. The voltage developed over this
capacitor is called the contact potential and
measuring it is done by applying an external backing
potential to the capacitor until the surface charges
disappear, at that point the backing potential will
equal the contact potential.
The traditional Kelvin Probe method consists of a
flat circular electrode (termed the reference
electrode) suspended above and parallel to a
stationary electrode (the specimen), thus creating a
simple capacitor. In 1932 William Zisman of Harvard
University introduced a new method to measure the
contact potential. He mounted a vibrating reference
surface, or tip, just above a sample electrode. The
output voltage varies periodically as the tip
vibrates, and the peak-to-peak voltage depends on
the difference between the contact potential and
external voltage.
This technique led to the development of systems
that automatically track shifts in the contact
potential due to changes in the work function of the
sample.
A major asset of this method is that the surfaces do
not need to touch each other. It also requires only
very weak electrical fields, which are not likely to
influence the electrical or chemical structure of
the material. Several ingenious mechanisms have been
used to achieve the required variation in spacing
between the tip and sample. For vacuum applications
piezoelectric and voice-coil drivers are most
convenient. A typical probe design for ultrahigh
vacuum is made from stainless steel, including the
suspension system that controls the tip movement.
The tip vibrates with an amplitude of 0.1 to 1mm at
a frequency of 30-300 Hz and its mean position is
kept constant to within 50 nm.
A complete scanning Kelvin Probe system includes a
digital oscillator to drive the tip movement, a tip
actuator, a signal amplifier and a scan controller.
A computer with a data acquisition system is used to
control the instrument and to capture measured data.
Click for
3 videos on Kelvin Probes |
Kelvin Probe 50micron
tip measuring a Single Electron Transistor Device
UHV Kelvin Probe |
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