Kelvin Probe FAQ Page

• What is work function?

The work function of a material is the amount of energy necessary to remove an electron from the surface. As such, the exact value under any given set of circumstances is not only difficult to determine precisely, but it is also a point of considerable controversy.

 The Kelvin Method measures the contact potential difference (CPD) between the sample and reference electrode and not the absolute work function. It is, therefore, used to monitor the change in work function or CPD over time. Many factors can influence CPD, including temperature, stress, strain, adsorption or desorption of molecules (including moisture) and, depending on the sample material, even photo excitation.
 
 

• Is the KP6500 a complete system, e.g. can we perform a measurement without the need to buy additional equipment?

The vacuum system is not normally supplied since customers usually wish to simply add the Kelvin Probe (KP) to their existing ultra-high vacuum system. We have, however, designed and built several complete systems which included KP as well as other surface analysis techniques such as LEED, Auger, STM, XPS, UPS, HREELS, etc. Sample handling devices such as manipulators, sample holders and load locks are also available. It really depends on what the individual customer wants.

The same is true for the computer. Customers usually have their own PC and do not need an additional computer. We can, however, supply a quote for a computer but, since there are so many different permutations, the user will first need to supply detailed requirements for processor speed, memory, OS (WinXP, Vista), extra peripherals, etc. Alternatively, the user can purchase a computer directly from one of the on-line retailers , and have it shipped to McAllister Technical Services for configuration and software installation. We will then ship the pre-configured and tested computer along with the KP.



• What, exactly, is included in the KP6500 system?

KP data acquisition and control is via an IBM-compatible computer. As above, we usually do not provide the computer (since almost everyone already has one) but, otherwise, it is a complete system. Included are the KP Head, control electronics and preamplifier, software, data acquisition card (PCI or PCMCIA), all cables and documentation. Minimum computer requirements are Pentium-100 or higher, running Windows XP / Vista. A fully-functional Demo version of the software is posted on the KP Download Page. It contains all the features of the regular software but includes a real-time signal simulation, complete withrandom noise (just like the real world) that allows the user to completely familiarize themself with operation. The various settings have realistic effects on the resultant signal and the data can be stored and exported (but probably should not be published. :-)
 


• Which element is grounded, the probe tip (reference electrode) or the sample?

The MTS Kelvin Probe (KP) consists of both a sample and a vibrating reference electrode in a parallel-plate configuration. A typical KP configuration connects the probe tip to the preamplifier and the sample to ground, but the reverse is also possible and can be even desirable. This reversed configuration where the preamplifier is connected to the sample and the probe is connected to ground is called the 'vibrated plate earthed'.
 
 
• How does the Kelvin method work?

The plates are electrically connected via an external "backing" potential, VBACKING. When the VBACKING is zero, a contact potential difference, Vcpd, exists between the plates due to the difference in the work functions ( wf) of the inner plates surfaces. By definition,
e * Vcpd = wf where e is the electronic charge. Setting VBACKING to create a potential difference between the plates induces a charge on the surfaces of the plates proportional to the capacitance of the plates. Because one plate is being oscillated relative to the other, the value of the capacitance is also a function of time. The KP signal is the current that flows between the plates as charge moves from one plate to the other due to motion of the probe.

At the point where VBACKING = -Vcpd, the circuit is balanced, meaning the electric field between the plates vanishes, and the induced charge is zero so the current vanishes. This is the balance or null point. Note that this definition of the backing potential at the null point also defines Vcpd.

Strictly speaking, the Vcpd is defined as the work function of the plate connected to the preamp (through which VBACKING is applied), minus the work function of the other plate. By detecting changes in the balance condition, you can monitor work function changes induced by such processes as adsorption, film growth, strain or stress etc., to a very high accuracy. This null field mode of operation has the advantage that it does not run the risk of desorbing weakly bound adsorbates. This method provides a mean wf of the sample under study and is not biased toward low wf patches, as is the case in thermionic emission, photoemission and field emission.



• What is the KP tip material?

It is usually type 304 stainless steel but other materials can, and have, been used. The reason we usually use stainless is because the native oxide is very robust as well as chemically stable. One researcher found that a gold plated tip changed work function after simply wiping it with a paper towel, presumably due to scratching. Tips made of other materials can be supplied upon request.
 
 
• What is the work function of the stainless steel tip?

One researcher wrote, suggesting that it is approximately 4.10 eV. However, it is not necessary to know the work function in order to take relative measurements or to monitor the time scale of changes.

 But in order to monitor the time scale of changes on the sample or compare different measurements over time, it is important to be sure that the work function of the tip is stable over time. The experiment should, perhaps, be designed to either allow a periodic check against a known reference or to use a tip material that is unaffected by whatever influences are imposed upon the sample.

 It is important to remember that the Kelvin Method is a measurement and is not a spectroscopy. Therefore, it is up to the experimentalist to control the (many) variables that influence CPD changes.
 
 

• If the tip gets contaminated due to crash or other misfortune, how do you clean the tip?

The tip can be cleaned mechanically or chemically. It all depends on the tip material and the contaminating material. When the tips are made, they are polished the lathe, with very fine abrasive cloth or abrasive paper. If you do not have a lathe, the tip can be put in the chuck of a hand drill. Use either #400 or #600 grit paper. It is a bit coarser than the abrasive powder used to clean a sink.

The accuracy and precision of the measurement is dependent upon the accuracy and precision of the whole system, not just the accuracy of the reference electrode. A calibration does not just calibrate the reference potential. It must also calibrate the entire instrument (in effect, finding the "instrument transfer function"). There are numerous factors that affect this, including temperature variations of the data acquisition card, spurious electronic emfs from noise, circuit biases, unshielded charges in the vacuum chamber, etc. Additional factors that are often overlooked include the effects of stray capacitances, the R/C time constant of the tip-to-preamp cable, and other sources of noise. So you cannot avoid the need to characterize the instrument.

One key calibration task is to determine the effects of stray capacitance and to minimize them. The KP provides a very effective tool for this. The Probe Characterization sub-routine make the probe oscillate at a frequency which is then swept over a range of frequencies. The sample is removed during this procedure. The software helps the user pick a "quiet" frequency where the effect of the stray capacitance is minimized. Stray capacitances can lead to errors of hundreds of millivolts.

Also, note that the KP method is a differential method. It measures a difference in work functions, not absolute values. In order to measure absolute values it is necessary to have one electrode be at a well defined potential. One could, for instance, use a Ag/AgCl surface prepared in a standard manner. This is a well studied system, fairly stable in air (not light) with a well understood potential. Most KP6500 users measure change in contact potential difference over time, temperature or some other varible.

• Is Photo-voltage Spectroscopy possible with the KP-6500 system?

On photo-sensitive samples, Photo-voltage spectroscopy can be done by measuring the CPD of an otherwise stable sample while varying the frequency and/or intensity of photo excitation.
 

• What is the accuracy of the CPD measurement?

Minimum single CPD measurements are ~2.5 mV with a 12 bit A/D (±5V/4096) card (standard). This accuracy value ignores, however, the effects of stray or fringe fields, and other, random electrical interference. For best accuracy, a series of measurements should be made where the automatic signal averaging can eliminate random variations. Using signal averaging, accuracies of less than 1 mV are readily achieved.

A higher-resolution, 16-bit acquisition system has been developed and is available as and option. Regardless of the acquisition, the effects of stray capacitance and fringe fields remain and it is up to the user to compensate for their effects.

The KP is not normally used to measure height. In principle, however, it could, since the system can detect separation changes between the tip and sample of a few microns. Please contact MTS for more information.

Is it possible to perform an area scan, or only line scans?
The "standard" KP comes with the ability to measure at a single point. In order to scan a line or an area, either the KP head or the sample must be moved and addtitional data collected. The KP data file format is comma delimited ASCII text. Multiple datasets can be combined in a single spreadsheet to produce a graph. Spatial resolution is a function of tip size. Some researchers have used a software deconvolution routine to get lateral resolution far smaller than the actual tip size. We are dubious about the scientific merits of this scheme since it requires unproven assumptions to be made about the data. It does, however, produce very pretty images.

Also, a 3D scanning UHV Kelvin Probe with software has recenly been developed and shipped. The scan area is limited only by the range of motion available. As above, spatial resolution is a function of tip size.
 
 
• What is the maximum (and minimum) sample size?

The tip should be a millimeter or so smaller than the sample so that the sample appears to the tip as an infinite plane.  If the sample is smaller than the tip, fringe fields can cause spurious readings.  Three tips are provided with each KP as standard.  Sizes are user-selectable from 2 - 20 mm in diameter.  Different tips are easy to fabricate and exchange.
• What size tip should be used?

The data measured is an average CPD of the area under the tip. Therefore, select a tip size that corresponds to the area desired.
• What is the tip-to-sample spacing during measurement?

Typical tip-to-sample spacing is sub-millimeter. Be sure the setup of the mechanical support/mounting system is sufficiently stable to maintain a uniform tip-to-sample distance.
• Can tips smaller than 2mm be used?

Smaller tips can be easily fabricated. The real question is how stable, mechanically, is the mounting system? The tip-sample interface forms a plane parallel capacitor. The distance between the tip and sample, and the area of the tip, among other values, determine the capacitance. In use, the tip-sample distance is reduced until a useable signal can be measured and that signal intensity is a function of total capacitance. That signal is composed of both the desirable values and stray capacitances from things like fringe fields around the circumference of the tip. As the tip diameter (or circumference) decreases linearly, the tip area decreases exponentially. The result is that the contribution to the total signal from fringe fields increases dramatically. Therefore, the tip must be moved closer to the sample to cancel out the fringe field contributions. As the tip-sample gap decreases, the contribution from external mechanical vibrations can become much more prevalent. Therefore, the mechanical stability of the mounting system is an integral factor and must be considered in selecting tip sizes. Please see "On the Elimination of Spacing Dependent Errors in the Kelvin Probe" for more information.
 
 
• When performing a measurement, is the probe-sample distance constant, e.g. does the software correct the probe position for variations in the height of the sample surface?

Yes. The probe tip is supported on a shaft which is suspended on thin diaphragm springs.  The springs allow considerable axial motion while restricting radial motion very well. The oscillation of the probe is caused by a sine wave supplied to a voice coil-like driver. The software, when this option is selected, automatically varies a superimposed DC Offset to keep the tip-to-sample distance constant. The DC Offset is updated at each data point. Variation increments are as small as ~2 microns. This feature also can be used to compensate for thermal drift due to, for instance, sample heating or cooling.

---