The Kelvin Probe Product Range

Introduction to Kelvin Probes

The Kelvin Probe is a non-contact, non-destructive vibrating capacitor device used to measure the work function (wf) of conducting materials or surface potential (sp) of semiconductor or insulating surfaces. The wf of a surface is typically defined by the topmost 1-3 layers of atoms or molecules, so the Kelvin Probe is one of the most sensitive surface analysis techniques available. KP Technology Systems offer very high wf resolution of 1-3 meV, currently the highest achieved by any commercial device.

The Kelvin Probe does not actually touch the surface; rather an electrical contact is made to another part of the sample or sample holder. The probe tip is typically 0.2 - 2.0 mm away from the sample and it measures the 'traditional work function', i.e. that found in literature tables. Other techniques, using very sharp tips some 10's of nanometers away from the sample, measure very reduced and distorted work functions due to the close separation of tip and sample.

The physical form of a Kelvin Probe is a head unit containing a voice coil driving system and integral amplifier suspended above a sample. The vibrating tip and the sample forms a capacitor, having ideal, or parallel-plate, geometry. As the tip vibrates electric charge is pushed around the external detection circuit. By careful control of the tip potential and automatic capture and analysis of the resulting waveform both the potential across the capacitor and the capacitor spacing can be calculated to very high resolution. In scanning form the tip is steered across the sample surface using a high resolution 3-axis translator. The spatial resolution of the tip is approximately the tip diameter, we typically use tips of 2 and 0.05 mm as standard tips in air and anything from a sharp tip to 10 mm diameter in Ultrahigh Vacuum. The resulting 3D surface work function images contain information about surface structures, surface composition, thin films, defects, etc. In time-varying mode artifacts such as oxidation (corrosion), defect relaxation, etc can be observed.


Silicon Wafer with Si3N4 layer Sample analysed using
Surface Photovoltage and scanning system

For semiconductor surfaces, both organic (Polymer) and inorganic (Si, Ge, CdS, etc) the Kelvin Probe is the only way to directly measure the Fermi-level. Changes in Fermi-level, caused by illumination with white or monochromatic light, results in energy band shifts which can be used to characterize interface and bulk defect states. These techniques are termed Surface Photovoltage (SPV) and Surface Photovoltage Spectroscopy (SPS) for which KP Technology can supply both software and accessories for our systems.

The traditional kelvin probe actually produces the work function difference between the tip and sample. The kelvin method was first postulated by the renowned Scottish scientist Lord Kelvin, in 1861. Typically the tip is calibrated against a reference surface, such as gold first. However KP Technology is the only company to offer absolute Kelvin Probes, which combine the Kelvin method and Einstein Photoelectric effect to produce absolute work function values (in eV). KP Technology has developed dedicated head units for ambient, controlled atmosphere, relative humidity and ultrahigh vacuum environments. All of our systems share the Off-Null, Height Regulation (ONHR) method invented by Prof. Baikie and consequently produce stable high signal levels, repeatable and high-resolution measurements. The rapid growth of the company since 2000 means the KP Technology Systems are found in leading materials research laboratories worldwide. We pride ourselves on our rapid post-sales support and our ability to assist technical and data interpretation queries.

The renowned Scottish physicist Sir William Thomson, later Lord Kelvin showed over a century ago that a potential is generated between the surfaces of two conductors when they are brought into electrical contact. This contact potential depends on the work functions of the materials being used. The work function is the amount of energy needed to release electrons from a material's surface, and is related to the material's optical, electrical and mechanical properties. When two materials with different work functions are brought together, electrons in the material with the lower work function flow to the one with the higher 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 wf difference between two surfaces can be found by measuring the flow of charge when the two conducting materials are connected (Figure B). However, this produces a once only measurement, as the surfaces become charged, and the charge must dissipate before another measurement can be made. By using a vibrating probe, a varying capacitance is produced. This is given by: C=Q/V=(epsilon.A)/t (Equation 1) Where C is the capacitance, Q is the Charge, V is the Potential, Epsilon 0 is the permittivity of the dielectric (in an air probe the dielectric is air). A is the surface area of the capacitor and d is the separation between the plates. Therefore, as the separation d increases the capacitance C decreases. As the charge remains constant, the voltage V must increase.


As the probe oscillates above the sample the voltage change is recorded. The peak-to-peak output voltage Vptp, is given by: Vptp = (Delta V - Vb)R.C0.w.e.sin(w.t + f) (Equation 2) Where Delta V represents the voltage difference between probe tip and sample, Vb is the externally applied emf used to balance (or null) the circuit, R is the I/V converter feedback resistance, C0 is the mean Kelvin Probe capacitance, Omega the angular frequency of vibration and Theta the phase angle. Epsilon is the modulation index (d1/d0) where d0 is the average distance between the sample and the probe tip and d1 is the amplitude of oscillation of the probe.


The backing potential Vb is set to a range of potentials and a plot of Vptp versus Vb is made. As can be seen from Equation 2 this is a straight line. When Vptp = 0 the Work Function or Contact Potential of the surface is equal and opposite to Vb. Traditional phase sensitive detection methods utilise a LIA (lock in amplifier) to detect the null output condition. However, these systems have the disadvantage that at the balanced point, the signal to noise (S/N) ratio reaches a minimum, and noise creates an offset voltage. A better method is to set Vb to a range of voltages and then extrapolate to the null signal condition, thus working to a high S/N ratio. Figures A to C (below) show various electron energy diagrams for two conducting specimens, 1 and 2.
 

Electron Energy Level Diagram

Figure A Electron energy level diagram for two conducting specimens, where Theta 1 and Theta 2 are the work functions of the materials, and Epsilon 1 and Epsilon 2 represent their fermi levels.

Equalised Fermi Level Diagram

Figure B If an external electrical contact is made between the two electrodes their fermi levels equalise and the resulting flow of charge (in direction indicated) produces a potential gradient, termed the contact potential Vc, between the plates. The two surfaces become equally and oppositely charged.

Null Signal Diagram

Figure C Inclusion of a variable "backing potential" Vb in the external circuit, permits biasing of one electrode with respect to the other. At the unique point where the (average) electric field between the plates vanishes, resulting in a null output signal.

For more information and definition of terms, please visit our knowledge database.

Kelvin Probe Variants

Product Families

Our Kelvin Probes are split into 4 different product families. They are as follows:


The KP (Kelvin Probe) Range

Our Kelvin Probe range is the best equipment available for quick and easy relative work function or surface potential measurements; capable of determining an accurate result (resolution 2mV) for a single point on the surface of your sample material. Our Kelvin Probe is a non contact vibrating capacitor which offers the flexibility to measure over seconds to hours with real time results. This is a desktop system and includes our easy to use interface software and Data Acquisition System.




The SKP (Scanning Kelvin Probe) Range

Our large range of Scanning Kelvin Probes gives the user full access to 2D and 3D Work Function plots of samples ranging in size from 5mm to 300mm. With Work Function resolution at less than 3meV, and the resolution of the 3-axis scanning stages as low as 317.5nm, the Scanning Kelvin Probe gives reliable, repeatable measurements.




The RHC (Relative Humidity/Corrosion) Range

The Relative Humidity Kelvin Probe Systems are the ideal solution for researchers investigating environmentally sensitive or corrosive materials in a controlled atmosphere. As a natural upgrade from our Kelvin Probe/ Scanning Kelvin Probe packages, our Relative Humidity Chambers produces reliable, accurate and repeatable data for over a range of measurement points. The Relative Humidity is automatically controlled with a range of 10-90% (accuracy 1%), allowing for quick and easy RH control to the conditions you require.






The UHV (Ultra High Vaccum) Range,

Offers scanning or single point measurements for an exsisting vacuum system or as a stand alone system






 

Kelvin Probe Acronyms


Single Point Kelvin Probes

Single Point Kelvin Probe (KP010)

Our most basic system for Kelvin Probe measurements, no enclosure.

Single Point Kelvin Probe (KP020)

Our most basic system for Kelvin Probe measurements, enclosed in a faraday cage with tracking to account for distance from tip to sample


Relative Humidity/Corrosion

Relative Humidity Chamber (RHC010)

Relative Humidity control with a Single Point Kelvin Probe (KP020)

Relative Humidity Chamber (RHC020)

Relative Humidity Control with a Scanning Kelvin Probe (SKP5050)

Relative Humidity Chamber (RHC030)

A Single Point Kelvin Probe (KP020) inside an environmental system controlling both Relative Humidity and Controlled Gas environments.

Relative Humidity Chamber (RHC040)

A Scanning Kelvin Probe (SKP5050) inside an environmental system controlling both Relative Humidity and Nitrogen environments.


Scanning Kelvin Probe

Scanning Kelvin Probe (SKP5050)

Scanning Kelvin Probe with 50mm*50mm range

Advanced Scanning Kelvin Probe (ASKP5050)

Scanning Kelvin Probe with 100mm*100mm range with advanced and more capable software and hardware including limit, speed and 3D scan advantages

Advanced Scanning Kelvin Probe (ASKP150100)

Scanning Kelvin Probe with 150mm*100mm range with advanced and more capable software and hardware including limit, speed and 3D scan advantages

Advanced Scanning Kelvin Probe (ASKP200150)

Scanning Kelvin Probe with 200mm*150mm range with advanced and more capable software and hardware including limit, speed and 3D & Circular scan advantages

Advanced Scanning Kelvin Probe (ASKP350350)

Scanning Kelvin Probe with 350mm*350mm range with advanced and more capable software and hardware including limit, speed and 3D & Circular scan advantages


Ultra High Vacuum Kelvin Probe

Ultra High Vacuum Kelvin Probe (UHV020)

A Kelvin Probe to add to an exsisting UHV set up.

Ultra High Vacuum Kelvin Probe (UHV030)

UHV Cell System that can be used for Ambient, UHV or Gaseous scanning measurements without the requirement for a preexsisting vacuum chamber.

 

Bespoke Kelvin Probes

KP Technology has a sound background in scientific measurement and instrumentation, including first hand experience in the requirements of the scientific community. Our wealth of experience, from the needs of the user through the design process and on to manufacture, is invaluable to our working methods and practice.

We are able to supply bespoke systems to meet the technical needs of the customer; from varying sizes of Kelvin Probe tips to completely bespoke systems. Our in-house team of experts are here to help get the right system for you.


UHV Multi system


Nanyang Technologcal University, Singapore

We provided the client with a customised UHV Multi system with fast-access door, Sample Heater Stage and motorised Z-Axis for Automatic Tip approach. The client then upgraded to an SPS system which we had to design a fibre holder for the front of the Multi-System to allow light injection into the vacuum chamber





Loyola

Ambient Kelvin Probe with Multi-Wavelngth LEDs


Loyola University Chicago, USA



The customer wanted a bespoke SPS system using a series of multi-wavelngth LEDs. The system was designed with 6 LEDs total, 2 pairs of visible spectrum LEDs and a pair of LEDs in the UV spectrum.






CERN

Bespoke Tip Solution for UHV Kelvin Probe


CERN, Switzerland

The client needed a UHV Kelvin Probe to perform their measurements at CERN, the problem they had was the positioning of the chamber relative to the UHV Probe. We designed a bespoke tip for the Kelvin Probe that allowed the user to measure the Work Function at a specific angle (i.e. not perpindicular ot the probe).





MBraun

SPS and Advanced Scanning Kelvin Probe


M.Braun, Germany

The client wanted to shine multiple wavelengths of light upon their large Si samples. We redesigned our SPS and ASKP systems to do this. This gave the client the ability to perform 150mm x 200mm scans with varying wavelength and intensity. The system was to be placed insie their glovebox systems so we designed a Tri-Fold door for ease of use on the front of the enclosure.






IMEC

Multi-System for Gas flow through and LED


IMEC, Holst Centre, Netherlands

The customer had samples that had to be measured under varying gases. We based our design on a UHV Cell and added swagelok adapters for gas flow-through. The sample was mounted on a bespoke 25mm manual translator. The system also contained SPV LEDs mounted inside the Cell.






Portland

Rotational Axis to measure Spent Brass Bullet Casings


Portland Police Department, USA

The customer was a Criminologist with the portland Police Department who needed to measure latent fingerprints from spent 9mm rounds. We designed and built a Kelvin Probe system that had the 9mm casing mounted on a rotational axis and scanned in 3 Axes. The Kelvin Probe tips were also designed at a dimension to pick up fingerprint ridge detail




Oxford instruments

UHV Kelvin Probe Mounted on Bespoke Flange


Oxford Instruments, USA

The client is a producer of large multi-systems that require a bespoke mounting solution. We had designed a bespoke flange with various adapters for the client. This allows them to mount our UHV Kelvin Probe onto their own system and perform Work Function Measurements.






Designed Sample Holders



We can tailor our Sample Holders to the client's specification.Shining light from top and bottom, rotational, angled, held with spring clips, 4",6" or 12" wafers or tapped sample holders for multi-sample mounting we can do a large range. Three examples of these are below.




SampleHolder Sampleholder2 SampleHolder3

Customised Tips


We design Kelvin Probe tips in-house. From sizes of 0.05mm to 20mm. Sharp tip, flat tip, angled, holed, mesh. Gold, Gold Coated, Platinum, Stainless Steel, we offer an extensive selection.


Tips1

Kelvin Probe Specifications

System KP020 SKP5050 ASKP100100 ASKP200250 ASKP350350 UHVKP020 UHVKP030 RHC010 RHC020 RHC030 RHC040
Measurement Principle Off-null Kelvin Probe Off-null Kelvin Probe Off-null Kelvin Probe Off-null Kelvin Probe Off-null Kelvin Probe Off-null Kelvin Probe Off-null Kelvin Probe Off-null Kelvin Probe Off-null Kelvin Probe Off-null Kelvin Probe Off-null Kelvin Probe
CPD Resolution 1-3meV 1-3meV 1-3meV 1-3meV 1-3meV 1-3meV 1-3meV 1-3meV 1-3meV 1-3meV 1-3meV
Scan Area Single Point 50*50mm 100*100mm 200*250mm 350*350mm Single Point Single Point Single Point 50*50mm Single Point 50*50mm
Translation Z Axis Motorised X, Y, Z axis Motorised X, Y, Z axis Motorised X, Y, Z axis Motorised X, Y, Z axis Manual or Motorised z axis Manual or Motorised z axis Z Axis Motorised X, Y, Z axis Motorised Z axis Motorised X, Y, Z axis
Operating Conditions Air at ambient pressure Air at ambient pressure Air at ambient pressure Air at ambient pressure Air at ambient pressure Vacuum (UHV Cell not included) Vacuum (UHV Cell included) Air at ambient pressure with RH control Air at ambient pressure with RH control Air at ambient pressure with RH control and with Oxygen to <1% Air at ambient pressure with RH control and with Oxygen to <1%
Power Requirement 100 – 240 V 100 – 240 V 100 – 240 V 100 – 240 V 100 – 240 V 100 – 240 V 100 – 240 V 100 – 240 V 100 – 240 V 100 – 240 V 100 – 240 V
Control Supplied PC control with dedicated software PC control with dedicated software PC control with dedicated software PC control with dedicated software PC control with dedicated software PC control with dedicated software PC control with dedicated software PC control with dedicated software PC control with dedicated software PC control with dedicated software PC control with dedicated software
Surface Photovoltage Module Not Included. Upgrade Possible Not Included. Upgrade Possible Not Included. Upgrade Possible Not Included. Upgrade Possible Not Included. Upgrade Possible Not Included. Upgrade Possible Not Included. Upgrade Possible Not Included. Upgrade Possible Not Included. Upgrade Possible Not Included. Upgrade Possible Not Included. Upgrade Possible
Surface Photovoltage Spectroscopy Module Not Included. Upgrade Possible. Not Included. Upgrade Possible. Not Included. Upgrade Possible. Not Included. Upgrade Possible. Not Included. Upgrade Possible. ASKP350350 Not Included. Upgrade Possible. Not Included. Upgrade Possible. Not Included. Upgrade Possible. Not Included. Upgrade Possible. Not Included. Upgrade Possible. Not Included. Upgrade Possible.

Product Overview

Provided here is a brief summary of KP Technology Ltd's product range. Bespoke systems are also available to meet the technical needs of our customers. Click here to learn more.

Knowledge Database

This webpage provides information on the science behind the Kelvin probe and the air photoemission systems, ideas for applications, examples of measurements and some comments on current apparatus available. All have been compiled by Professor Iain D. Baikie, who has extensive experience in these techniques. Click here to learn more.

Brochure & Downloads

Find Application Notes on OLEDs, Solar Cells, Diagnosis of Disease, Photovoltaics and a lot more here. Click here to learn more.

Accessories Overview

KP Technology offers a wide array of modular accessories to complement our systems. These can be applied to any system. Our current accessories include the Surface Photovoltage Spectroscopy (SPS), Surface Photovoltage (SPV), LED Rings and Sample Heaters. Click here to learn more.

Support

We pride ourselves on support, offering a dedicated support page for our clients. The first port of call when there is a problem is to contact our support team here.

Contact Us

Whatever your query, please feel free to get in contact with us by phone, email or post. Click here to be taken to our contact page.