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   Kelvin Probes - Iain Baikie

19 October 03
Wick Firm K P Technology Wins A Smart Scotland Awards
Iain Baikie's new firm K P Technology Ltd has won one of the awards in Smart Scotland

Enterprise Minister Jim Wallace announced the second round of winning companies from the 2003 SMART:SCOTLAND awards scheme.

A further �2.6 million is available to take projects to pre-production stage.

The SMART awards are designed to help new and existing businesses with less than 50 employees gain a competitive edge in the market by funding the development of innovative and commercially viable products and processes.

Mr Wallace said:  "I am delighted to be able to announce today the latest round of SMART:SCOTLAND winners. They really do epitomise what can be achieved in terms of turning ideas into market leading projects, and include significant technological advancements in such areas as biotechnology, environmental technology, electronics and fish vaccination.

"The Executive is committed to helping grow the economy by investing in the skills of our workforce and getting ideas and innovations out of our laboratories and into global markets.

"Today's awards demonstrates the Executive's continuing commitment to the creation of a pipeline of support to transform our brightest innovations into successful business opportunities.

"We are determined to realise our vision of a Scotland where there is a greater self confidence, an enterprising spirit and less fear of failure. Today's winners are real proof of what we can achieve, and I hope that other Scottish firms will follow their lead."

Kelvin Probes Reveal Surface Properties
Kelvin probes do not appear in the standard suite of surface analysis tools, yet they are highly sensitive to changes in the electrical, optical and mechanical properties of materials, Iain Baikie explains why.

Just over a century ago - in a talk at the British Royal Institution - the renowned Scottish Physicist Sir William Thomson later Lord Kelvin, used two large metal plates and a gold-leaf electroscope to demonstrate �The Contact Electricity of Metals�. Kelvin showed that a potential is generated between the surfaces of two conductors when they are brought into electrical contact. This experiment forms the basis of the Kelvin probe, a highly sensitive tool for analysing the surface properties of materials.

The Kelvin probe is an extremely versatile analytical tool. It measures changes in the contact potential between reference material and a sample, which in turn depends on changes in the work function of the material being studied. The work function - the amount of energy needed to release electrons from a material�s surface - is a barometer of the sample�s optical, electrical and mechanical properties.

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.

Although not as well known as some other surface analysis techniques, the 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, with prices ranging from �8 - 18,000. Compatibility with existing vacuum systems is ensured by mounting the equipment on standard flanges coupled with integrated retraction bellows and automated stand-alone operation.

Improvements have also been made to the spatial resolution technique. Once limited to around 100um, new designs based on an atomic force microscope can map surface properties with resolutions in the 50 nm range. Several spectroscopic variants have also been developed for the analysis of semi-conductor surfaces and thin films.

How does a Kelvin probe work?
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.

The Kelvin method is based upon detecting the surface charges that appear when two different conductors are brought into electrical contact.  Electrons flow from the material with the lower work function (green) to the one with a higher work function (red).  If the two conductors are made into a parallel-plate capacitor, equal and opposite surface charges appear.  the voltage developed over the Kelvin capacitor is called the contact potential.  If an external "backing" potential is introduced into the external circuit, the surface charges disappear when the backing potential is equal to the contact potential.
 If these materials are made into a parallel plate capacitor, equal and opposite surface charges form. Measuring the contact potential is then exquisitely simple: an external potential is applied to the capacitor until the surface charges disappear, and at this point the external potential equals the contact potential.

In 1932 William Zisman of Harvard University introduced a new method to measure 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 the external voltage. Changes in contact potential can then be detected by determining the external potential that yields a minimum or �null�, output voltage. This technique led to the development of systems that automatically track shifts in contact potential due to changes in the work function of the sample.

A major asset of this method is that 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. Biasing the sample rather than the tip should reduce noise, while the use of a signal amplifier sensitive to current rather than voltage minimizes interference from other vacuum components and the walls of the vacuum chamber.

It is important to recognise that the Kelvin probe is a relative technique. The work function of the tip must therefore be known to obtain an absolute work function of the sample with monochromatic ultraviolet light, and then measuring the energy at which current starts to flow. This technique can detect changes in the absolute work function with an accuracy of 50 meV.

Hardware Considerations
Several ingenious mechanisms have been used to achieve the required variation in spacing between the tip and sample. For vacuum applications piezo-electric and voice-coil drivers are the most convenient. A voice-coil driving system must be placed about 250mm from the tip to avoid interference between the actuator signal and the input voltage. Piezo-electric drivers are normally place much closer to the sample, typically less than 20mm, but my experience indicates that these drivers can generate noise due to capacitive coupling. In such cases it can be difficult to distinguish real changes in the contact potential from stray capacity effects.

A typical probe designed 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 1 mm at a frequency of 30-300Hz, and its mean position is kept constant to within 50nm. The suspension system can be easily adapted to include several tips, and provides facilities for both high and low spatial resolution on one head stage.

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 capture experimental data.

Boosting the signal
One major problem with the null method is the low signal-to-noise ratio at the balance point. Together with co-workers at Twente University in the Netherlands, I have addressed this problem by developing an �off-null� approach. In this approach the peak-to-peak voltage is determined at a range of external potentials around the balance point. The contact potential can then be found by extrapolating the data back to the point where the peak-to-peak voltage is zero. This allows changes in the contact potential to be detected with sub-mV resolution.
This off-null method ensures that measurements are performed on high signal levels. The variation in the peak-to-peak voltage is also sensitive to changes in the mean spacing between the tip ans surface, which means that it can be used to keep the mean spacing constant during an experiment. This is extremely important, since components within the vacuum system can capacitively interact with both the sample surface and the tip of the Kelvin probe. Changes in the mean spacing can therefore influence the measured contact potential.

Keeping the mean spacing constant allows the Kelvin probe to map out changes in the surface potential at the same time as revealing surface topography at the sub-micron scale. An image of an operational transistor, for example, clearly shows the changes in bias potential above a single p-n junction, as well as the associated topographic image

A scanning Kelvin probe scan of a 300x400�m2 section of a transistor shows (top) the surface potential and (bottom) a topographic image.  the surface potential image clearly shows a 1.2 V base-emitter potential through the native oxide layer.

Applications of Kelvin probes
Traditionally Kelvin probes have been used to investigate fundamental processes at the surfaces of metals and semiconductors, such as absorbed or evaporated layers, surface roughness, oxide layer imperfections and catalysis. But their use is now being extended into the realms of display technologies, novel materials for fuels cells and ion sources for new forms of mass spectroscopies. This is because the work function is directly implicated in certain surface processes such as surface ionisation and electron emission. Kelvin probes can also be used for measurements on negative electron affinity devices, defect analysis on semiconductor wafers, and fundamental studies into atomic wear and corrosion.

Kelvin probes are also being exploited to investigate a greater range of materials because they can measure surface charge distributions on thin films of non-conductors or dielectrics. This has opened up new research avenues on such divers materials as microporous PTFE and human skin.

The multi-parameter nature of the work function is both a major asset and a potential liability. It is often quite feasible to detect changes in work function but not able to ascribe them to a particular event or process. For this reason another surface analysis method is often needed to provide extra information on the local atomic structure or chemical composition of the surface.
A study of oxidation of silicon surfaces illustrates the sensitivity of the method. In this case the induced surface potential for monolayer coverage is quite large - upwards of 1V. If the Kelvin probe achieves sub-mV work function accuracy, it can detect the formation of oxide at coverages down to one-thousandth of a monolayer (ML). Although this oxidation reaction can also be followed with scanning tunnelling microscopy, the topographic image quickly becomes confused at coverages less than 0.1ML. Auger electron spectroscopy can also be used, but again it is difficult to obtain accurate chemical information below 0.3ML. Meanwhile, the probe data are continuous throughout this region and offer extra information on properties such as the atomic structure and density of surface states.

Similar measurements have made it possible to follow the high temperature oxidation characteristics of polycrystalline metals such as rhenium and tungsten. The resulting surfaces, which have work functions greater than 7eV, are destined for use as positive ion sources. It seems now that the emphasis has shifted from measuring the work function to engineering this crucial surface property to control the stability of novel surfaces.

Although the work function of an element is not unique, Kelvin probes do offer opportunities for chemical analysis. For example, certain regions of a polycrystalline aluminium were coated with a gold layer just a few nanometres thick. The gold layer is invisible to the naked eye, but the probe clearly reveals the difference in work function between these two metals.

A Kelvin probe can perform surface chemical analysis by detecting the difference in work function between aluminium (yellow) and a gold layer just a few nanometres thick

The Kelvin probe offers unsurpassed resolution for detecting changes in the work function, which is a key property of surfaces and thin films. The method can now be applied in both ambient and vacuum environments, and offers a versatile measurement technique that is a valuable complement to other surface analysis methods.

Iain Baikie is head of the scanning Kelvin probe group at the Robert Gordon University in Aberdeen, and the Kelvin Research Centre, ERI, Thurso, Scotland, UK

Other Sources
KP Technology Scientific Instrumentation experts, specialising in surface analysis methods, providing measurement services, consultancy and equipment.
Kelvin Probe Info Facts and figures about the Kelvin Probe, a vibrating capacitor measuring work function of materials.
Vacuum Solutions Web Site
LSST
University of Alberta 
McAllister Technical Services
Signal Simulation
Edinburgh Engineering Virtual Library