GB2439439A

GB2439439A - Measurement device and method

Info

Publication number: GB2439439A
Application number: GB0711754A
Authority: GB
Inventors: Iain Douglas Baikie
Original assignee: KP TECHNOLOGY Ltd
Current assignee: KP TECHNOLOGY Ltd
Priority date: 2006-06-17
Filing date: 2007-06-18
Publication date: 2007-12-27
Anticipated expiration: 2027-06-18
Also published as: GB2439439A9; GB0711754D0; GB0612072D0; GB2439439B

Abstract

A measurement device capable of measuring a contact potential difference between a first surface and a second surface separated by an insulator, through the application of an external potential. The measurement device comprises a calibration system for determining an absolute value of work function of the first surface. The calibration system has a calibration sample and the measurement device further comprising a translocation means actuable to cause relative movement of the first surface and the calibration sample. The first surface and the calibration sample are typically provided within a housing. The calibration sample can have an absolute work function value lower then that of the first surface. The measurement device comprises a Kelvin probe 10 with tip 12 and light source 18.

Description

MEASUREMENT DEVICE AND METHOD

The present invention relates to a measurement device capable of measuring a contact potential difference between a first surface and a second surface and a method for calibrating the first surface of a measurement device. In particular, the method provides an absolute value for the work function of the first surface of the measurement device.

The invention also provides a method of calibrating a first surface of a measurement device in a gaseous environment.

The Kelvin probe is a non-contact, non-destructive measurement device used to investigate properties of materials. The Kelvin probe is generally used for measuring the work function difference between a specimen and a reference material (typically a vibrating tip). The work function is a sensitive indicator of surface condition and is affected by adsorbed or evaporated layers, surface construction, surface charging, oxide layer imperfections and surface and bulk contamination, as well as many other factors. The work function is a material property that can be defined as the minimum amount of energy that must be applied to a surface of a material in order to remove an electron from the material so that it can just exist outside the boundary of the material in vacuum conditions.

A technique for measuring work function of a surface involves bringing two * p. conducting materials into electrical contact and quantifying the flow of charge from one material to the other. One of the conducting materials is typically a reference material having a documented value for work function and the other conducting material has a value of work function which is required to be measured relative to the reference.

When two conducting materials with different values of work function are electrically connected to one another, electrons in the material with the lower work function flow to the material with the higher work function. If the conducting materials are assembled to form the plates of a parallel plate capacitor, equal and opposite surface charges form on the plates.

The potential difference developed between the plates of the capacitor is called the contact potential and it is measured by applying an external backing potential to the capacitor until the surface charges on the plates disappear. At this point, commonly referred to as the null output, the backing potential is equal to the contact potential difference (CPD). CPD can be defined as the measured change in the contact potential between the reference material and the specimen surface.

The GPO can be equated to the difference in Fermi-levels of the capacitor plate materials. In the case of a metal, the Fermi level is the energy of the most energetic electron within the outer electron band with respect to the vacuum level, Evac. Changes in CPD can be wholly ascribed to changes at the specimen surface, assuming that the Fermi level of the reference material is unchanged during the measurement. This assumption does not take into account the fact that certain specimens could cause the reference material to be altered, nor does it account for the possibility that the reference can be damaged or otherwise modified during *: . experimentation.

Further, the data obtained using Kelvin probe measurements are all relative to the reference material. The Kelvin probe is an inherently relative technique since it measures the contact potential difference between a reference material and the specimen surface under investigation. Thus, the absolute work function of the reference material must be known in order to obtain an absolute value for work function of the specimen surface.

According to a first aspect of the present invention, there is provided a measurement device capable of measuring a contact potential difference between a first surface and a second surface separated by an insulator, through the application of an external potential, wherein the measurement device comprises a calibration system for determining an absolute value of work function of the first surface, the calibration system having a calibration sample, the measurement device further comprising a translocation means actuable to cause relative movement of the first surface and the calibration sample.

The first surface and the calibration sample are typically provided within a housing.

The translocation means can be actuable to cause three-dimensional relative movement of the calibration sample and the first surface. The translocation means can be actuable to selectively cause relative rotational and/or axial movement of the first surface and the calibration sample. The translocation means can be actuable to move the calibration sample relative to the first surface. * *I ** S * 8*

The measurement device can comprise a specimen retainer for retaining a specimen having a second surface relative to the first surface in a first measurement position, in which the contact potential difference between the first and second surfaces is measurable, and wherein the specimen retainer is provided within the housing.

The measurement device can comprise a second translocation means actuable to cause relative movement of the specimen retainer and the first surface. The second translocation means can be actuable to cause relative three-dimensional movement of the specimen retainer and the first surface. The second translocation means can be actuable to move the specimen retainer relative to the first surface.

The housing can comprise a port for accommodating a specimen therethrough. The second translocation means can be actuable to move the specimen retainer proximate the port for receiving the specimen therethrough.

The translocation means for relative movement of the calibration sample and the first surface and the second translocation means for relative movement of the specimen retainer and the first surface can be one and the same translocation means.

The first surface can be referred to as a reference' or tip'. The first surface typically comprises a chemically and thermodynamically stable material, having a known structure such as gold or stainless steel. The second surface or specimen can be any material for which the work function is required to be measured. The insulator between the first and second surfaces can be any environment that offers relatively high resistance to the passage of electric current, such as vacuum, air or any * *5 other gaseous mixture at a controlled humidity. The external potential difference is typically applied by a circuit electrically connecting the first * surface to the second surface. The measurement device can be a probe such as a Kelvin probe.

"Absolute value" as used herein is intended to refer to a self existent value, rather than a value determined solely with reference to another value.

The measurement device can be capable of measuring the contact potential difference of the first surface relative to the second surface and calibrating the first surface in a gaseous environment. A portion of the calibration sample and the first surface can be positioned within a predetermined maximum distance for measurements in a gaseous environment. The predetermined maximum distance is preferably less than the mean free path of the gas molecules of the gaseous environment.

Accordingly, the housing can be provided with one or more inlets for purging the enclosure with a gas or mixture thereof.

The measurement device can also be capable of measuring contact potential difference of the first surface relative to the second surface and calibrating the first surface in a vacuum. Thus the housing can be arranged to form a sealed chamber for use as a vacuum chamber.

The measurement device can further comprise a shielding means actuable to substantially shield at least a portion of the calibration system from the first surface within the housing, when the first surface and the specimen retainer are arranged relative to one another in the first measurement position. The shielding means can comprise a shutter * operable to provide a physical barrier substantially separating the first surface and calibration sample.

I..... * .

The calibration sample can comprise a material having an absolute work function value lower than that of the first surface. The calibration sample can comprise a material coating having a work function value lower than that of the first surface of the measurement device. The material or material coating can comprise gadolinium (Gd), lanthanum sulphide (LaS) or calcium (Ca) or any other low work function material.

The calibration system may further comprise an electromagnetic radiation source arranged to emit electromagnetic radiation for interaction with the calibration sample such that electrons are emitted from the calibration sample.

The calibration system can comprise a light source arranged to illuminate the calibration sample. The light source can illuminate the sample with light in the ultraviolet (UV) range.

The light source can be arranged such that the calibration sample is illuminated through the first surface. In this case, the first surface can be substantially transparent. The first surface can comprise glass coated with a conducting material. The glass can be coated with gold. At least 90% of the surface of the glass can be coated with gold. Alternatively, the glass can be coated with indium tin oxide (ITO) or any other transparent *.20 conductor. S...

The light source can advantageously emit quasimonochromatic or * monochromatic light. The light source can comprise one or more light emitting diodes (LEDs). The light source can comprise one or more : 25 ultraviolet LED(S). I..

The LED(s) can be powered by a direct current (DC). The LED(s) can be modulated at a predetermined frequency. The measurement device can comprise a vibration means actuable to cause vibration of the first surface within a range of predetermined frequencies. The predetermined frequency at which the LED(s) are modulated can be outwith the range of the frequency of vibration of the first surface. The LED(s) can be switched at a frequency in the range 1.5-2.5 kHz. Alternatively, the LED(s) can be switched at any frequency in the range of MHz.

The measurement device can also comprise a first detection circuit to detect the frequency of vibration of the first surface and a second detection circuit for detecting the frequency of modulation of the LED(s).

Alternatively, the light source can comprise light filtered from a mercury lamp.

The first surface can be shaped to allow light from the light source to illuminate a portion of the calibration sample proximate the first surface.

Thus the first surface can be provided with a cut-out portion to allow light to travel therethrough. The first surface can have a semicircular cut-out portion. The first surface can be shaped to be semicircular. Other geometries of the first surface would also be suitable.

According to the first aspect of the invention there is provided a method of obtaining an absolute value for work function of a first surface of a measurement device, comprising the steps of: *:*::* providing a measurement device capable of measuring a contact potential difference between a first surface and a second surface * 25 separated by an insulator, through the application of an external potential, :.: the measurement device comprising a translocation means and a calibration system, having a calibration sample; locating the first surface and the calibration sample within a housing; actuating the translocation means to thereby cause relative movement of the first surface and the calibration sample within the housing; and actuating the calibration system to obtain an absolute value for the work function of the first surface.

According to a second aspect of the invention, there is provided a method of obtaining an absolute value for work function of a first surface of a measurement device in a gaseous environment, the method comprising the steps of: providing a measurement device capable of measuring a contact potential difference between a first surface and a second surface separated by an insulator, through the application of an external potential, the measurement device also comprising a calibration system including a calibration sample; positioning a portion of the calibration sample within a predetermined maximum distance of the first surface; and actuating the calibration system to obtain an absolute value for the "Gaseous environment", as discussed with reference to the second aspect of the invention can be taken to include any environment that is not a vacuum. For example, gaseous environment can comprise any ambient vapour, gas, or any mixture thereof. Thus, the second aspect of the invention enables the first surface of the measurement device to be calibrated in air or purged gas or in a humidity chamber allowing control of *.* the humidity.

The predetermined maximum distance is preferably less than the mean free path of the gas molecules in the gaseous environment.

The mean free path can be defined as the average distance travelled by a molecule in a gas between collisions with other molecules.

Preferably, the measurement device is a Kelvin probe.

The calibration system and the first surface can be combined in the same housing in a manner previously described for the first aspect of the invention. All other characteristics of the measurement device, calibration system or first surface previously mentioned in relation to the first aspect of the invention can also be applicable to the second aspect of the invention.

Embodiments of the present invention will now be described with reference to and as shown in the following drawings in which: Fig. 1 is a sectional schematic view of part of a measurement device in accordance with one embodiment of the present invention; Figs. 2(a) to 2(c) are energy level diagrams of two different materials, : (a) not in contact with one another, S." (b) in external electrical contact, and *:*::* (c) with an external potential applied; Figs. 3(a) to 3(c) are energy level diagrams of a reference material and a calibration sample of a measurement device in photocurrent measurement mode with, (a) no electrons collected at the reference, (b) the onset of the photocurrent, and (c) the saturation current; Fig. 4 is a sectional schematic view of part of a measurement device according to a second embodiment of the invention; Fig. 5a is a schematic sectional view of a Kelvin probe tip and a calibration sample; Fig. 5b is a plan view of the base of the tip of Fig. 5a; Fig. 6 is a sectional schematic view of a transparent tip; and Fig. 7 is a schematic view of a measurement device and a processing means in ambient mode in accordance with another embodiment of the invention.

In the embodiments descnbecf herein, the measurement device is exemplified by a Kelvin probe. A Kelvin probe head is indicated generally at 10 in Fig. 1. The head 10 is provided with a first surface, reference or tip 12. The tip 12 comprises a gold substantially flat circular reference electrode.

A light source 18 comprising ultraviolet (UV) LEDs (not shown) is positioned adjacent the head 10 and provides light with a known wavelength (in the range 250 to 350nm). The light emitted by the LEDs is transmitted along a coated quartz tube 20 which directs the light towards a calibration sample in the form of a gadolinium foil 28, although the calibration sample can be any suitable material or material coating with a work function lower than that of the tip 12. An ultraviolet light source such as the Fibrepen' source from Sentronic GmbH can be used instead of LEDs. * ** * * * I.. *

Within the Kelvin probe head 10 there is provided a digital oscillator to cause oscillation of the tip 12, a tip actuator, a signal amplifier and a scan controller. The Kelvin probe head 10 is coupled to a computer (not shown) with a data acquisition system used to control the probe and the tip 12 and capture and process measured data.

The tip 12 and the gadolinium fofl 28 are sealed within a housing 14, part of which is shown in Fig. 1. The housing 14 also accommodates a specimen retainer (not shown) for retaining a specimen.

The gadolinium foil 28 is held by a calibration sample retainer having a shaft 26 passing through the housing 14, and moveable by actuation of a translocation means in the form of a knob 30, located at the end of the shaft 26 outside the housing 14. According to the present embodiment, the knob 30 can be turned to rotate the calibration sample retainer by 180 around the long axis of the shaft 26 and/or pulled or pushed to move the calibration sample retainer axially. A shutter (not shown) is retractably positioned between the gadolinium foil 28 and the tip 12 once the knob 30 has been actuated to move the gadolinium foil 28 remote from the tip 12.

The shutter is arranged to substantially isolate the gadolinium foil 28 and prevent interference with the subsequent contact potential difference measurements between the tip 12 and a specimen (not shown).

:.:::: The specimen can be inserted into the housing 14 through a port (not shown). The port is accessible through a door (not shown) sealed over the port. The specimen is retained on a movable tray (not shown). The * ..

specimen is placed with the surface of interest (the second surface) facing * 25 outwardly. Actuation of the movable tray can position the specimen :.: . surface relative to the tip 12 in a position suitable for obtaining CPD measurements.

When a measurement for work function of the specimen surface is required to be taken, the tip 12 is suspended a predetermined distance above and substantially parallel to the specimen surface, thereby creating a capacitor. In this position CPD measurements can be performed to obtain a relative value for work function of the specimen surface and the tip 12.

The Kelvin probe is operational in two modes: the CPD measurement mode and the photocurrent measurement mode, both of which are described below. The housing 14 is sealed to create a vacuum chamber and the CPD and photocurrent measurements described below are performed in ultra high vacuum (UHV).

The CPD measurement method is explained with reference to the energy level diagrams in Figs. 2(a) to 2(c), in which the tip 12 is represented on the left hand side and the specimen is represented on the right hand side.

E indicates the electrical field between the tip 12 and the specimen, which act as the plates of a capacitor. Et,p and Es refer to the Fermi levels of the tip and the specimen respectively.

If an external electrical contact is made between the tip 12 and the specimen, for example, when an external circuit C connecting the tip 12 and the specimen is closed, their Fermi levels equalise and the resulting flow of electrons from the metal with the lower work function (the tip 12) to that with a higher work function (the specimen) produces the contact potential difference between the plates as shown in Fig 2(b): * 25 eVCPD=S - * where e is the electronic charge, VCPD is the contact potential difference S....

* and 4 and %,, are the work functions of the specimen and the tip 12 respectively.

Actuation of the digital oscillator within the Kelvin probe head 10 causes vibration of the probe in the direction indicated by an arrow 34 in Fig. 1, and causes a resultant vibration of the tip 12, which is parallel to the specimen, thereby producing a varying capacitance. The varying capacitance causes a current to flow back and forth between the plates of the capacitor. Inclusion of a variable backing potential Vb in the external circuit C permits biasing of one plate of the capacitor with respect to the other.

At the unique point where: Vb = -VCPD the electrical field between the plates is zero (see Fig. 2(c)) resulting in a null output. The work function difference between the plates is thus equal and opposite to the potential necessary to produce a zero output signal.

For an improved signal to noise ratio, an off-null method is employed in the present embodiment to measure CPD. Data from the off-null method are extrapolated to the null output in order to reduce the effect of noise on the results. The Kelvin probe incorporates advanced off -null detection * 20 with a resolution of better than 1 mV, where the null output is determined by linear extrapolation. The tip actuator is capable of automatically controlling the tip-to-sample spacing which is calculated using data acquired by the computer. Measurements are performed with a frequency of up to one data point per second. The scan controller can cause relative movement of the tip 12 and the specimen surface to obtain CPD data :.:. across the surface of interest. This method is a conventional way of obtaining CPD data between the tip 12 and the specimen and gives a relative value of work function of the specimen surface.

At any point during experimentation, the calibration system of the Kelvin probe can be actuated to perform photocurrent measurements in order to obtain an absolute value for work function of the tip 12 of the Kelvin probe and hence obtain an abso'ute va'ue of work function of the specimen surface.

In the present embodiment, the polycrystalline gadolinium foil 28 acts as a photoelectron source. Before use, the gadolinium foil 28 is cleaned by repeated resistive heating up to 1 200 K for several minutes. The shutter ensures that the tip 12 is unaffected by the gadolinium foil 28 thermal processing. Gadolinium is useful for the photocurrent measurements since it has a relatively low work function (-.3.1 eV) and is available in the form of thin foils.

Actuation of the movable tray moves the specimen remote from the tip 12 and the knob 30 is actuated to move the gadolinium foil 28 into the photocurrent measurement position in which it is shown in Fig. 1.

In the photocurrent measurement mode, the distance between the tip 12 and the gadolinium foil 28 is maintained constant. The light source 18 is used to illuminate the gadolinium foil 28. Arrow 22 indicates the light leaving the coated quartz tube 20 and illuminating the gadolinium foil 28.

The monochromatic light causes photoelectrons, indicated by arrow 36, to be emitted from the low work function surface of the gadolinium foil 28.

The monochromatic light has energy: Eph = hv (with 4> hv > where h is Planck's constant and v is the frequency of the monochromatic light.

The photoelectrons 36 are collected by the Kelvin probe tip 12 and measured as a function of the applied backing potential Vb. Figures 3(a) to 3(c) are energy level diagrams of different stages of the photocurrent measurements, with the tip 12 represented on the left hand side and the gadolinium foil 28 represented on the right hand side of each diagram.

At the onset (Fig. 3b) of the photocurrent collected at the Kelvin probe tip 12, the absolute work function of the tip 12 equals the photon energy E plus a required bias potential supplied by Vb: = hv -eVb (onset) With increasing bias potential (analogous to the Fowler theory of photoemission from metals near threshold) the photocurrent increases with the square of Vb: 1ph is proportional to Vb 2* When Vb= -VCPD (Fig. 3c) electrons emitted from energy states below the Fermi level, , of the gadolinium foil 28 with virtually zero velocity are just able to reach the tip 12 and at this point the photocurrent will saturate. * *. *. S * *5

The photocurrent measurement technique is independent of the value for work function of the gadolinium foil 28. The only requirement for the gadolinium foil 28 is that it must provide a source of photoelectrons when * 1 illuminated with quasimonochromatjc or monochromatic light.

For both CPD and photocurrent measurements the signal collected from the gold tip 12 is converted into a voltage signal by the signal amplifier, which is typically a high gain (-1 Or), low noise operational amplifier. The signal is filtered and digitisec! by an analogue to digital (AD) converter for further signal processing by the computer. A computer program and the digital oscillator controls oscillation of the tip 12 and the application of the backing potential.

The photocurrent measurement method provides an absolute value for work function of the tip 12, which can be used to obtain an absolute value of work function for the specimen surface.

The gadolinium foil 28 and the specimen can be interchanged in the region of the tip 12 as frequently as required to obtain respective data from the gadolinium foil 28 in the photocurrent measurement mode and the specimen in the CPD mode.

The tip 12 is provided with an isolation system to electrically isolate the tip 12 from a suspension mechanism holding the tip 12 in position. An insulator 13 forms part of the isolation system. However, charges can accumulate on the surface of the insulator 13 and this charged surface can interact with the tip 12 and adversely affect the measurements of the Kelvin probe. S... * I S...

*:*:* According to a separate aspect of the invention, there is provided a measurement device capable of measuring a contact potential difference between a first surface and a second surface separated by an insulator, through the application of an external potential, the measurement device comprising an isolation system for substantially electrically isolating portions of the measurement device wherein at least a portion of an outer surface the isolation system is covered with conducting material.

The isolation system can electrically isolate portions of the measurement device from the first surface. The isolation system can include insulators and the conducting material can be a thin layer of gold. The conducting material covering can be broken in places to maintain the normal function of the isolating system. Any outer surface of an insulator within the housing 14 which the tip 12 can see' (i.e. which can therefore interact with the tip 12) may be coated with conducting material. The conducting material substantially prevents charges from accumulating on the insulator 13.

Fig. 4 shows part of a Kelvin probe head 110. A light source 118 used for the photocurrent measurements is a 100W Hg-Cd-Zn spectral lamp directed through a quartz or sapphire viowport 119 to illuminate a calibration sample in the form of lanthanum sulphide 128 which has a low work function surface. The light from the light source 118 illuminates the lanthanum sulphide 128 and photocurrent measurements can be obtained in the same manner as previously described for the first embodiment.

In the present embodiment, a translocation means (not Shown) is provided in the form of a movable support which accommodates the lanthanum sulphide 128. A port (not shown) selectively sealed by a door (not shown) is provided in a housing 114. The door allows the specimen of interest to

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be placed on a specimen retainer also provided on a movable support, * .* which can be actuated to selectively position either the lanthanum sulphide 128 or the specimen surface proximate the tip 112 for the * ** photocurrent or CPD measurements respectively. A shutter (not shown) can shield the tip 112 and the specimen from the lanthanum sulphide 128. * *

The method of calculating an absolute value for work function of the tip 112 is achieved by measuring the current voltage (I-V) characteristic of photoelectric emission from the lanthanum sulphide 128, illuminated by monochromatic light of a fixed wavelength from the light source 118. Thespecimen can then be positioned within the housing 114 so that the work function of the specimen surface can be obtained by subsequent Kelvin probe CPD measurements under the same conditions.

The housing 14, 114 allows UHV to be used during the measurements if required. The port in each embodiment can function as an entry or exit port for the specimens. A single entry/exit port is advantageous as it minimises contamination. The housing 14, 114 provides a single instrument solution for a Kelvin probe with an inbuilt calibration system for ease of use. Further, the tip 12, 112, calibration sample and specimen contained within a single housing 14, 114 ensures the same measurement conditions for both CPD measurements and photocurrent measurements.

The shutter is actuable to substantially shield the specimen and prevent contamination between the specimen and the lanthanum sulphide 28, 128.

In general, use of an LED light source 18 has advantages over a mercury (Hg) source 118 since an Hg source can be noisy, hazardous and melt the filters typically used in conjunction with the Hg lamps. LEDs are advantageous since they are low power and generate less electromagnetic noise and cause less interference. I., S * .5

The Hg source typicay runs from a standard aflernating current (AC) mains voltage which causes the source to switch polarity at a frequency of around 50-60 Hertz. Since this is similar to the frequency at which the tip 112 is typically vibrated there is therefore likely to be some S.....

* 5 electromagnetic interference with the tip 112. However, the LEDs can be powered by a direct current (DC) through an intermediary transformer rectifier to convert the mains power source from AC to DC. Thus, this will effectively reduce electromagnetic noise so that there are fewer spurious readings at the tip 112. Further, the LEDs can be switched at a frequency of 2 kHz or an alternative frequency distinct from that at which the tip 112 is vibrating. A first detection circuit is incorporated into the tip 112 to detect the frequency of vibration of the tip 112 and a second detection circuit can be incorporated into the tip 112 for detection of the frequency at which the LEDs are modulated so that the Kelvin probe tip 112 can detect whether it is operational in the photocurrent measurement mode or the CPD measurement mode.

Figs. 5(a) and 5(b) show a tip 212 adapted for use with incident light from a range of angles. Light from a UV light source is shown by arrow 222 in Fig. 5a. This incident UV light can be from a filtered mercury lamp or UV LEDs. The cat ibration sample in this embodiment is calcium 228. The tip 212 on the end of a tip shaft 244 is shaped to be a semicircular plate as shown in Fig. 5b. The shape of the tip 212, having a semicircular portion removed therefrom, facilitates the illumination of the calcium calibration sample 228 and enables a greater proportion thereof to be illuminated from a wider range of angles. A semicircular tip 212 is therefore particularly suited for use with the photocurrent measurement method. I. * *

:..::: Fig. 6 shows an alternative tip 312 and method of illumination. A tip mount 352 holds a glass substrate 354, through which incident light, indicated by arrow 322 from a source similar to those previously described, can travel.

The conducting layer of the tip 312 is provided by an indium tin oxide (ITO) : **. layer 360 on a glass plate 352. The photocurrent measurement is ** * conducted in the same manner as previously described, except that in the 0S**** * photocurrent measurement mode, the gadolinium foil calibration sample is illuminated through the tip.

Fig. 7 shows a schematic of an ambient Kelvin probe system. A computer 480 is coupled to a digital control unit (DCU) 482 for processing the data acquired from the Kelvin probe shown generally at 400. A calibration sample 428 is electrically connected to the DCU 482 by an electrical connection 484. An electrical connection 486 electrically connects the DCU 482 and the Kelvin probe head 410.

The Kelvin probe head 410 is axially moveable along a post 490.

Movement of the Kelvin probe along the post 490 provides a long range travel mechanism for positioning the tip 412 near the calibration sample 427. A manual translator 491 allows the Kelvin probe user a medium range mechanism (approximately 1 inch (2.54 cm)) for travel of the tip 412 relative to the calibration sample 428. The fine positioning control of the tip 412 is governed by the computer 480 through the DCU 482 and an electrical connection 488. The electrical connection 488 also allows a driver coil (not shown) to cause the tip 412 to vibrate as required during GPO measurements.

The distance indicated by arrow 492 corresponds to the distance between the tip 412 and the calibration sample 428. For photocurrent measurements in a gaseous environment, the maximum distance 492 is less than the mean free path of the gas molecules in the environment in which the measurements are being conducted so that the photoelectrons * ** ** are less likely to collide with gas molecules.

The calibration sample 28 can be provided with a means of refreshing the * surface. Thus, the outer coating of the calibration sample 28 can be S S. * removed where it may have been tarnished or otherwise damaged so that the surface is refreshed.

Every feature that is unique from the prior art, alone or in combination with other features, should also be considered a separate description of further inventions by the applicant including the structural and/or functional concepts embodied by such feature(s).

Modifications and improvements can be made without departing from the scope of the invention. For example, the coated quartz tube 20 (Fig. 1) can be substituted for another suitable optical fibre along which light can be transmitted towards the calibration sample 28. The quartz tube 20 can be substituted for a quartz window. The LED(s) can emit different light, such as blue light. Alternatively, combined LED fibre optic cables can be used. The tip 12 can be made from stainless steel rather then gold.

The ambient measurement mode in which CPD and photocurrent measurements are taken in an ambient environment can be used with the first and second embodiments of the invention whereby the interior of the housing 14 can be purged with gas and the gadolinium foil 28 or lanthanum sulphide 128 is positioned within a predetermined maximum distance of the tip 12, 112. * * * *** * * * .s *. * * ** * ** * S S *** *

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S..... * S

Claims

CLAIMS

1. A measurement device capable of measuring a contact potential difference between a first surface and a second surface separated by an insulator, through the application of an external potential, wherein the measurement device comprises a calibration system for determining an absolute value of work function of the first surface, the calibration system having a calibration sample, the measurement device further comprising a translocation means actuable to cause relative movement of the first surface and the calibration sample.

2. A measurement device according to claim 1, wherein the first surface and the calibration sample are provided within a housing.

3. A measurement device according to claim 1 or claim 2, wherein the translocation means is actuable to cause three-dimensional relative movement of the calibration sample and the first surface.

4. A measurement device according to any preceding claim, wherein the translocation means is actuable to selectively cause at least one of relative rotational and axial movement of the first surface and the calibration sample. * .e

5. A measurement device according to any preceding claim, wherein the translocation means is actuable to move the calibration sample relative * *. to the first surface. * S 5 *** * *

* SS SI * 6. A measurement device according to any preceding claim, when dependent on claim 2, further comprising a specimen retainer for retaining a specimen having a second surface relative to the first surface in a first measurement position, in which the contact potential difference between the first and second surfaces is measurable, and wherein the specimen retainer is provided within the housing.

7. A measurement device according to any preceding claim, further comprising a second translocation means actuable to cause relative movement of the specimen retainer and the first surface.

8. A measurement device according to claim 7, wherein the second translocation means is actuable to cause relative three- dimensional movement of the specimen retainer and the first surface.

9. A measurement device according to claim 7 or claim 8, wherein the second translocatlon means is actuable to move the specimen retainer relative to the first surface.

10. A measurement device according to any preceding claim dependent on claim 2, wherein the housing comprises a port for accommodating a specimen therethrough.

11. A measurement device according to claim 10, when dependent on claim 7, wherein the second translocation means is actuable to move the specimen retainer proximate the port for receiving the specimen * ** therethrough.

: ** 12. A measurement device according to any of claims 7 to 9 and 11, * wherein the translocation means for relative movement of the calibration * sample and the first surface and the second translocation means for relative movement of the specimen retainer and the first surface are one and the same translocation means.

13. A measurement device according to any preceding claim, wherein the first surface comprises a chemically and thermodynamically stable material.

14. A measurement device according to claim 13, wherein the first surface comprises a material selected from the group consisting of: gold and stainless steel.

15. A measurement device according to any preceding claim, wherein the insulator between the first and second surfaces is selected from the group consisting of: vacuum, air and a gaseous mixture.

16. A measurement device according to any preceding claim, wherein the external potential difference is applied by a circuit electrically connecting the first surface to the second surface.

17. A measurement device according to any preceding claim, wherein the measurement device is a Kelvin probe.

18. A measurement device according to any preceding claim, when dependent on claim 2, wherein the housing is provided with one or more inlets for purging the enclosure defined by the housing. * ** * . * * *.

19. A measurement device according to any preceding claim, when dependent on claim 2, wherein the housing is arranged to form a sealed chamber for use as a vacuum chamber. * S

20. A measurement device according to any preceding claim, compnsing a shielding means actuable to substantially shield at least a portion of the calibration system from the first surface, when the first surface and the specimen retainer are arranged relative to one another in the first measurement position.

21. A measurement device according to claim 20, wherein the shielding means comprise a shutter operable to provide a physical barrier substantially separating the first surface and calibration sample.

22. A measurement device according to any preceding claim, wherein the calibration sample comprises a material having an absolute work function value lower than that of the first surface.

23. A measurement device according to any preceding claim, wherein the calibration sample comprises a material coating having a work function value lower than that of the first surface of the measurement device.

24. A measurement device according to claim 20, wherein the material coating can comprise a material selected from the group consisting of: gadolinium (Gd), lanthanum suiphide (LaS) and calcium (Ca).

25. A measurement device according to any preceding claim, wherein the calibration system further comprises an electromagnetic radiation source arranged to emit electromagnetic radiation for interaction with the calibration sample such that electrons are emitted from the calibration sample. * ** p

26. A measurement device according to any preceding claim, wherein the calibration system further comprises a light source arranged to illuminate the calibration sample.

27. A measurement device according to claim 26, wherein the fight source illuminates the sample with light in the ultraviolet (UV) range.

28. A measurement device according to claim 26 or claim 27, wherein the light source is arranged such that the calibration sample is illuminated through the first surface.

29. A measurement device according to claim 28, wherein the first surface is substantially transparent.

30. A measurement device according to claim 28 or claim 29, wherein the first surface comprises glass coated with a conducting material.

31. A measurement device according to claim 30, wherein the glass is coated with a conducting material selected from the group consisting of: gold and indium tin oxide (ITO).

32. A measurement device according to any one of claims 26 to 31, wherein the light source emits a light selected from the group consisting of: quasimonochromatjc and monochromatic light. * * * .*.

33. A measurement device according to any one of claims 26 to 32, I...

wherein the light source comprises one or more light emitting diodes (LEDs).

34. A measurement device according to claim 33, wherein the LED(s) can be powered by a direct current (DC).

35. A measurement device according to claim 33, wherein the LED(s) can be modulated at a predetermined frequency.

36. A measurement device according to any preceding claim, comprising a vibration means actuable to cause vibration of the first surface within a range of predetermined frequencies.

37. A measurement device according to claim 36, when dependent on claim 35, wherein the predetermined frequency at which the LED(s) are modulated is outwith the range of the frequency of vibration of the first surface.

38. A measurement device according to claim 37, wherein the LED(s) are switched at a frequency in the range of 1.5-2.5 kHz.

39. A measurement device according to any one of claims 36 to 38, comprising a first detection circuit to detect the frequency of vibration of the first surface and a second detection circuit for detecting the frequency of modulation of the LED(s).

40. A measurement device according to any one of claims 26 to 32, wherein the light source comprises light filtered from a mercury lamp. I. * * * S..

41. A measurement device according to any one of claims 26 to 40, S...

wherein the first surface is shaped to allow light from the light source to * S. illuminate a portion of the calibration sample proximate the first surface.

: **** 42. A measurement device according to claim 41, wherein the first surface is provided with a cut-out portion to allow fight to travel therethrough.

43. A method of obtaining an absolute value for work function of a first surface of a measurement device, comprising the steps of: providing a measurement device capable of measuring a contact potential difference between a first surface and a second surface separated by an insulator, through the application of an external potential, the measurement device comprising a translocation means and a calibration system, having a calibration sample; actuating the translocation means to thereby cause relative movement of the first surface and the calibration sample; and actuating the calibration system to obtain an absolute value for the 44. A method according to claim 43, wherein the measurement device is arranged to measure the contact potential difference of the first surface relative to the second surface and calibrate the first surface in a gaseous environment.

45. A method according to claim 43 or claim 44, wherein a portion of the calibration sample and the first surface are positioned within a predetermined maximum distance of one another for measurements in a gaseous environment. * * *.

* 46. A method according to claim 45, including selecting the predetermined maximum distance such that it is less than the mean free path of the gas molecules of the gaseous environment. * S. * S S SSI *

47. A method of obtaining an absolute value for work function of a first surface of a measurement device in a gaseous environment, the method comprising the steps of: providing a measurement device capable of measuring a contact potential difference between a first surface and a second surface separated by an insulator, through the application of an external potential, the measurement device also comprising a calibration system including a calibration sample; positioning a portion of the calibration sample within a predetermined maximum distance of the first surface; and actuating the calibration system to obtain an absolute value for the 48. A measurement device as substantially hereinbefore described with reference to and as shown in Figure 1.

49. A measurement device as substantially hereinbefore described with reference to and as shown in Figure 4. * S * S.. S... * * * .* S. S * ** * S. * * S *5S *

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