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 vacuum.
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 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.