| The SKP100 measures work-function values in atmospheric conditions. It uses a mechanical scanning head (based on stepper motors) to scan the probe over the surface of the specimen. The sample is held fixed and the probe is scanned over the sample surface. This makes sample holding and sample dimensions very flexible. It will perform a work-function distribution measurement over the surface, all under computer control. It also performs topography measurements on the same surface. |
Is the SKP100 a complete system, e.g. can we perform a measurement without the need to buy additional equipment?
The only extra requirement is for a suitable computer. Everything else required to perform Kelvin measurements is supplied, including the environmentally controlled Tri-cell™ to control atmospheric sample conditions. |
What is the maximum (and minimum) sample size?
The system works with an x, y, z, scanning stage, which has maximum movement of 100 mm in the x and 75 mm in y direction. However, it is also possible to scan larger samples by simply positioning the scanning head above different areas of the sample. The size of the smallest sample area is limited to 1 x 0.75 mm. |
What is the accuracy of the height and potential measurements?
In topography height mode, the instrument is accurate to 1 micron resolution. In potential Kelvin mode, the instrument has a resolution of 5 mV. |
What is the accuracy in relation to the Standard Hydrogen Potential (is a potential calibration possible/necessary)?
The relationship between the Kelvin measurement (wf)and corrosion potential (Ecorr) is offset by a single constant such that
Ecorr = Constant + wf
Therefore, if you make a measurement with the Kelvin probe on a system with a known redox potential (such as Cu/CuSO4), it is possible to calibrate the SKP100. |
What is the lateral resolution of the measurements?
The scanning stage has a resolution of 1 micron in the x and y direction (mechanical step resolution). However, the absolute resolution of the work function measurement will ultimately depend on the diameter and geometry of the probe tip. We supply probes having a tip diameter of 500 µm, which would give a resolution in the region of 250 µm (the convention is to halve the probe diameter for final resolution). However, the probe can be customer modified. We supply special connectors for users who want to design and manufacture their own probes. |
How long does it take to perform an area scan?
The maximum scan speed of the SKP100 is 2 mm per second in sweep scan mode. The SKP100 software works on a 4/3 scan area aspect ratio; for an area scan of 10 mm (x direction) x 7.5 mm (y direction) at 48 lines and 64 samples per line, this would take approximately 10 minutes (including time for probe to return to x = 0 before each line scan). Much higher resolution can be achieved by increasing the lines and samples per line (the maximum with the SKP100 is 512 samples per line, 384 lines. Also, greater accuracy will be achieved at slower speeds and in step-scan mode (where the instrument moves to each point, stops, measures, then moves to the next point). |
How is the SKP100 work function measurement actually performed?
The principle of the Kelvin measurement is based on a feedback loop. Where the charge on a capacitor (probe to sample) is measured continuously by vibrating one of the plates (the probe). The resulting AC current is measured via a Lock-In amplifier. This signal is fed into a "Backing Potential Controller" which continuously adjusts the potential on the sample (the other plate of the capacitor) such that the AC current in the capacitor is kept at a minimum (i.e. zero). Thus we have a feedback loop. The function of the Lock-In is to provide a "sensible" signal to the Backing Potential Controller. Adjustment of the Lock-In gain does not and should not be directly reflected in the output, because the output is the potential applied to the sample. Selection of the correct Lock-In settings is essential to "stimulate" the feedback loop correctly. |
| When using the instrument in Topography mode the measured signal "is" the output of the Lock-In amplifier, there is no feedback loop. In this mode of operation the output is directly affected by the gain. |
| The SKP measurement is based on a measure of the charge (Q) between a probe tip and a conducting surface (the sample), which form a capacitor. The main parameters affecting the measurement relate to the physics of this capacitor, the distance between the plates (d), the dielectric constant of the medium between the plates and the surface area of the plates. As the surface area of the probe decreases, so the charge on the capacitor decreases and with it the measurable signal. At some point the measurable signal is obscured by the background noise and the measurement becomes impossible. This background noise is the sum of many influences, the greatest of these is electromagnetic radiation from external electrical apparatus modulating the charge on the capacitor. Minimizing this interference is the key to using the SKP measurement with small diameter probes. In a standard lab environment the use of probes less than 1mm diameter will result in interference problems. The use of a Faraday cage is recommended with any probe diameter less than 1mm. The result of interference on the SKP measurement is not obvious until you consider the operation of the system. The measurement of the capacitors charge is used in a feedback loop to control the backing potential applied to the sample. Thus if the charge measurement is non-resolvable, then the feedback loop will be unstable, resulting in the backing potential (equal to the Work function difference) being unstable and probably resting at its positive or negative full scale value (+/- 5V). In the case of extreme noise the instability may result in oscillation of the backing potential. |
| The spatial resolution of the measurement depends on the geometry of the probe to sample positioning and the probe diameter. The maximum attainable spatial resolution will be half the diameter of the probe, as with any probe technique. This assumes that the distance between the probe and the sample is less than half the probe diameter. At larger probe to sample distances the spatial resolution will be reduced in proportion to the increased distance. |
| It can be seen therefore that the selection of probe diameter for a Kelvin probe measurement is not simply a case of "smallest is best." Consideration of the required environment, amplitude resolution, minimum "probe to sample distance" attainable and what actual surface information may be there to be measured, are required. The Kelvin probe measures variations in surface potential. In a homogeneous metal substrate there will be no variation in surface potential. A coated sample may only have variations due to electrochemical activity occurring at the coating-metal substrate junction. Large variations may be detected on coated samples indicating localized "charge" held within the coating. This charge may easily be misinterpreted as variation in the substrate surface potential and care should be taken in sample preparation to eliminate unwanted charge from any coated sample. One common method is to flow a humidified stream of an inert gas across the sample to help dissipate any unwanted charge while making a measurement. |
| The instrument comes with a specific point by point description on how to setup a complete topography and Kelvin experiment, included the description on how to orient and level the sample and scanning head. |
Does the SKP100 measure the absolute value of the work-function ?
No, it is not possible to measure the absolute work-function value for any sample with an SKP. The SKP can only make relative measurements. The measurement is repeatable given the same operating conditions (both ambient & instrumentation). The value measured is only valid relative to a reference. This is why many users reference a copper sulfate sample in their measurements. To put this another way, if you have a defined sample such as a step change in potential created by connecting a battery between two identical metals, then the measured difference in potential between the metals is equivalent to the battery potential. |
Can the SKP100 be used for other type of scanning measurements?
Yes it can. The SKP100 can be upgraded (factory installment is required) for SVP (scanning vibrating probe) measurements. This way both SKP and SVP data can be mapped on a sample with a single instrument requiring a change in the probe and software program. SVP is a technique to map local electrochemical activity of a surface immersed in an electrolyte. |
