"Advances in hardware design and signal processing technology
have improved the resolution of the
while also ensuring that it can be
used in a vacuum"
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 Porbe has undergone a dramatic reneissance 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
The equipment is also reasonably affordable with prices starting from
£8000. Improvements have also been made to the spatial resolution of the
technique, new designs can map surface properties with resoltuion 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 is 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
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
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
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.