Field of Invention

Certain embodiments of the present invention relate to diamond based electrical conductivity sensors and methods of measuring the electrolytic conductivity of a solution using a diamond based electrolytic conductivity sensor.

Background of invention

The measurement of electrolytic conductivity is a fundamental and ubiquitous measurement whenever the quality, nature, composition, and/or properties of a solution are interrogated across a broad range of commercial and environmental applications. Solutions of interest may be based on an aqueous solvent, an organic solvent, or a mixture thereof. Examples of applications requiring solution conductivity measurements include chemical synthesis processes such as pharmaceuticals fabrication, drinking water monitoring, various forms of industrial effluent, river and sea water monitoring, and oil and gas applications both in terms of surface measurements of oil/water mixtures and down hole measurements. It will be noted that in many applications other solution measurements are required such as measurement of the presence and concentration of specific target chemical species within the solution, e.g. specific heavy metal pollutants such as lead and mercury. However, electrolytic conductivity is usually the first basic measurement which is taken to give an overall indicator of a solutions quality, nature, composition, and/or properties. As such, the measurement of a solution's electrical conductivity is important across a range of industries and environmental bodies and in some cases is needed to meet legal requirements.

A large number of conductivity sensors are commercially available and these are often tailored for a particular application with a required accuracy, conductivity range, and/or chemical robustness. Currently available conductivity sensors can be divided according to their mechanism of conductivity measurement. There are three basic types: 2-point probes; 4-point probes; and inductive probes. These three basic types of sensors are summarized in the table below including the pros and cons of each basic sensor type:

Of the three types of commercially available conductivity sensors indicated in the table, 2-point probes are the most accurate and conceptually simple. However, the narrow active range of 2-point probes limits their utility in many applications. 4-point probes address this by avoiding polarisation/electrolysis at the sensing electrode, which enables a wider range of measurements to be performed. However, 4-point probes are more complex and less accurate, which again limits their utility in many applications. Further still, both 2-point and 4-point conductivity sensors involve direct electrode-solution contact and, since commercially available conductivity sensors have electrodes fabricated using materials such as metals or graphite, they are not chemical resistant to solutions which react with such materials. In this regard, inductive sensors measure conductivity inductively through an inert dielectric material which is in contact with the solution. These sensors are used when the solution of interest would otherwise chemically react with a metal or graphite electrode of a 2-point or 4-point probe. As such, inductive conductivity sensors enable measurement of conductivity in solutions which would otherwise attack electrode materials as they involve putting an inert dielectric between the electrode and the solution being measured. However, inductive conductivity sensors are only suitable for highly conductive solutions as they are unable to accurately measure differences between solutions of low conductivity. Furthermore, although the use of inert plastics improves chemical resistance, they are only as resistant as the plastic used, which is PEEK (Polyetheretherketone) or ETFE (Ethylene/Tetrafluoroethylene Copolymer) in most cases. The use of ETFE at high temperatures can result in the release of hydrofluoric acid, for instance. As such, the working temperature of ETFE or PEEK is only up to 150°C. Again, this can limit the utility of inductive conductivity sensors in many applications. Inductive devices can be made for high temperature applications, e.g. using ceramics, but these don't work well for high resistivity solutions.

Further still, all three types of commercially available sensor as described above require operational protocols including: regular calibration; refresh in water to hydrate the sensor prior to measurement if stored dry; ex situ cleaning using a damp cloth and surfactant; 5-10% HCl for inductive sensors or <1% HCl for 2/4-point sensors. Such protocols limit the utility of conductivity sensors, particularly in applications which require in situ, remote, and/or long duration conductivity monitoring.

The present inventors have identified that it would be desirable to provide an electrical conductivity sensor which combines a broad operating range, high accuracy across such a range, and high chemical robustness. In addition, it would be useful to provide such a conductivity sensor in a form which does not require regular calibration, a re-hydration treatment, ex situ cleaning, or particular concentrations of HCl. That is, it would be desirable to provide a conductivity sensor which effectively combines all the benefits of the 2-point, 4-point, and inductive probes already available while having none of the disadvantages as described above.

The present inventors consider that a diamond based electrical conductivity sensor will address the problems outlined above with respect to currently available conductivity sensors. In this regard, it has already been proposed in the prior art to provide a diamond based sensor for measuring the electrochemical properties of a solution. Diamond can be doped with boron to form semi-conductive or fully metallic conductive material for use as an electrode. Diamond is also hard, inert, and has a very wide potential window making it a very desirable material for use as a sensing electrode for an electrochemical cell, particularly in harsh chemical, physical, and/or thermal environments which would degrade standard metal based electrochemical sensors. In addition, it is known that the surface of a boron doped diamond electrode may be functionalized to sense certain species in a solution adjacent the electrode.

One problem with using diamond in such applications is that diamond material is inherently difficult to manufacture and form into suitable geometries for sophisticated electrochemical analysis. To date, diamond electrodes utilized as sensing electrodes in an electrochemical cell have tended to be reasonably simple in construction and mostly comprise the use of a single piece of boron doped diamond configured to sense one physical parameter or chemical species at any one time. More complex arrangements have involved introducing one or more channels into a piece of boron doped diamond through which a solution can flow for performing electrochemical analysis. However, to date the present applicant is unaware of sophisticated diamond based electrochemical sensors which can perform multiple sensing functions at the same time, particularly configured for use in harsh environments. Due to the inherent difficulties involved in manufacturing and forming diamond into multi- structural components, even apparently relatively simple target structures can represent a significant technical challenge.

In terms of prior art arrangements, the present applicant already has a number of patent publications relating to diamond based electrochemical sensor structures and methodologies including: WO2005/012894; WO2007/107844; WO2012/156203; and WO2012/126802.

WO2005/012894 describes a microelectrode comprising a diamond layer formed from electrically non-conducting diamond and containing one or more pin-like projections of electrically conducting diamond extending at least partially through the layer of non-conducting diamond and presenting areas of electrically conducting diamond at a front sensing surface.

WO2007/107844 describes a microelectrode array comprising a body of diamond material including alternating layers of electrically conducting and electrically nonconducting diamond material and passages extending through the body of diamond material. In use, fluid flows through the passages and the electrically conducting layers present ring-shaped electrode surfaces within the passages in the body of diamond material.

Both WO2005/012894 and WO2007/107844 indicate that the microelectrode structures described therein are suitable for performing electrochemical sensing measurements on solutions although measurement of electrical conductivity is not disclosed.

WO2012/156203 relates to an electrochemical sensor comprising a diamond reference electrode in addition to a diamond sensing electrode, i.e. a robust all-diamond sensor structure. Since the diamond reference electrode is not a standard reference electrode which is independent of solution type, an in situ calibration system is provided for assigning peaks in voltammetry data to chemical species thereby allowing the type and concentration of chemical species in the solution to be determined. This document mentions that such an all-diamond sensor structure may be used to measure or control properties of the fluid environment such as conductivity although no details are given regarding the specific electrode configuration or methodology required to measure solution conductivity across a large range at high accuracy.

WO2012/126802 describes a diamond based electrochemical band sensor comprising a plurality of boron doped diamond band electrodes disposed within a diamond body. Each boron doped diamond electrode has a length / width ratio of at least 10 at a front sensing surface of the sensor. It is described that it is advantageous from a functional perspective for certain sensing applications to provide band electrodes which have a high aspect ratio at the sensing surface such that a length of a band electrode across the sensing surface is very much larger than a width of the band electrode.

WO2012/126802 also suggested that a larger number of electrodes can be provided within the diamond body to support a range of sensing capabilities. For example, a plurality of boron doped diamond band electrodes may be configured to sense one or more of the following properties of a solution adjacent the sensing surface: pH; conductivity; temperature; individual or total heavy metal concentration; and H ₂ S. It is described that the plurality of boron doped diamond band electrodes may be grouped into sets, each set comprising one or more boron doped diamond band electrodes, wherein each set is configured to sense a different parameter.

Having regard to conductivity measurements, WO2012/126802 suggests an electrode set comprising at least two spaced apart boron doped diamond electrodes configured to sense electrical conductivity of a solution disposed adjacent the sensing surface between the electrodes. It is described that more preferably, the electrode set may comprise at least three spaced apart boron doped diamond electrodes wherein a spacing between the at least three spaced apart electrodes is different. Providing multiple electrodes with different spacing can aid in providing a more accurate reading for solution electrical conductivity. The specific spacing required can be selected and optimized according to an approximate magnitude of electrical conductivity for solutions to be analysed and/or an address voltage to be applied to the electrodes. By providing multiple electrodes with different spacing it is possible for one sensor to be used in a range of conductive environments. A small spacing might be sufficient for one conductive range whilst a larger spacing would be more sensitive for a different conductive range. That said, WO2012/126802 does not provide a specific electrode configuration or methodology required to measure solution conductivity across very large ranges at high accuracy.

In light of the above, it will be evident that the present applicant's previous work has disclosed the use of a diamond electrode structure for measuring solution conductivity. It is an aim of certain embodiments of the present invention to provide diamond electrode configurations and associated methodology which is optimized towards measuring solution conductivity across very large ranges at high accuracy.

Summary of Invention

While the present applicant has previously disclosed the use of diamond electrodes for measuring solution conductivity, recent work on optimisation of diamond electrode configurations and associated methodology has resulted in surprisingly large operating ranges and accuracies for solution conductivity measurements. According to one aspect of the present invention there is provided a conductivity sensor comprising:

a plurality of boron doped diamond electrodes including a first set of boron doped diamond electrodes having a first surface area and a first electrode spacing and a second set of boron doped diamond electrodes having a second surface area and a second electrode spacing,

wherein the first surface area is larger than the second surface area and the second electrode spacing is larger than the first electrode spacing, and

wherein the plurality of boron doped diamond electrodes is configured such that in use the conductivity sensor can measure conductivity of a solution at 25°C over at least a range of 1 x 10 ^"4 S m ^"1 to 1 x 10 ^"1 S m ^"1 .

In relation to the above, it may be noted that the surface area of the first set of electrodes may be larger than that of the second set of electrodes by providing individual boron doped diamond electrodes in the first set which each have a larger surface area than individual boron doped diamond electrodes in the second set of electrodes. For example, the first and second sets of electrodes may comprise equal numbers of electrodes but with individual boron doped diamond electrodes in the first set each having a larger surface area than individual boron doped diamond electrodes in the second set of electrodes. Alternatively, the individual electrodes of the first and second sets of electrodes may have an identical surface area. In this case the first set of electrodes can be formed of a larger number of individual boron doped diamond electrodes connected such that the overall surface area of the first set of electrodes is larger than that of the second set of electrodes.

In addition to the above, it may be noted that the first and second sets of boron doped diamond electrodes may be supported on a common substrate or on separate substrates. Alternatively, or additionally, individual boron doped diamond electrodes within a particular set may be supported on a common substrate or on separate substrates.

The present inventors have found that by providing two sets of boron doped diamond electrodes, one set having a larger inter-electrode spacing and a lower surface area and another set having a smaller inter-electrode spacing and a larger surface area, it is possible to fabricate a conductivity sensor structure which is capable of measuring solution conductivities over a very wide conductivity range of 1 x 10 ^"4 S m ^"1 to 1 x 10 ^{" 1} S m ^"1 . In fact, it has been found that such diamond sensor structures are capable of measuring conductivity across a range of 1 x 10 ^"5 S m ^"1 to 1 S m ^"1 and even 5 x 10 ^"6 S m ^"1 to 5 S m ^"1 or more. In such arrangements, each set of boron doped diamond electrodes is configured to take a 2-point conductivity measurement. Each 2-point conductivity measurement is sensitive to a different conductivity range and the two ranges overlap to give a wide overall operating range.

For certain applications such as in the oil and gas industry, for example down a drill hole, such a wide operating range is required. A high degree of accuracy may not be so critical in such applications. For example, an accuracy of better than 50%, 30%, 20%), 15%), or 10%) can be provided across these broad ranges by embodiments of this invention and is sufficient for some applications. Furthermore, it has also been found that embodiments of the present invention are capable of a higher degree of accuracy, e.g. an accuracy better than 5%, 3%, 2%, or 1%.

Such diamond based sensors have thus been found to provide a much broader operating range than commercially available conductivity sensors with a higher degree of accuracy. In addition, such diamond based sensors are more chemically robust than commercially available sensors. Further still, diamond based conductivity sensors can be cleaned in-situ, e.g. due to the wide potential solvent window diamond electrodes are able to generate OH and/or H ⁺ ions and/or other strongly oxidizing species to clean the electrodes. As such, the diamond based conductivity sensors are capable of being operated in situ for long time periods without being required to be removed, cleaned, and recalibrated. As such, these sensors can be left in remote and/or inaccessible locations for extended periods of time. Furthermore, as the surface of diamond is stable then its capacitance does not change significantly over time. Such diamond based conductivity sensor structures can also be made very small and compact in size. For example, the first set of boron doped diamond electrodes may be disposed between the boron doped diamond electrodes of the second set of electrodes to provide a compact arrangement.

Brief Description of the Drawings For a better understanding of the present invention and to show how the same may be carried into effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 shows a plan view of a conductivity sensor according to one configuration;

Figure 2 illustrates a graph showing the conductivity range for the conductivity sensor configuration shown in Figure 1 extending from distilled water through to sea water;

Figure 3 shows a cross-sectional view of the conductivity sensor configuration shown in Figure 1 ;

Figures 4(A) to 4(F) illustrate the method steps for fabricating a conductivity sensor configuration as shown in Figures 1 and 3;

Figure 5 shows a cross-sectional view of a portion of a conductivity sensor comprising an electrode which has an inverted top-hat profile with a deeper central portion which can be filled with good quality boron doped diamond material while also being more easy to rear contact without drilling through the entire electrode thickness;

Figure 6 shows an alternative to the arrangement of Figure 5 comprising an electrode which has an inverted triangular profile with a deeper central portion for ease of filling and rear contacting;

Figure 7 shows a plan view of an alternative conductivity sensor configuration to that shown in Figure 1 comprising a plurality of central electrodes which may be contacted in such a way as to provide an interdigitated electrode structure in place of the central pair of electrodes shown in Figure 1;

Figure 8 shows a picture of a prototype structure comprising a plurality of boron doped diamond electrodes formed in an intrinsic diamond support matrix; Figure 9 shows an SEM image of an overgrown boron doped diamond electrode illustrating good quality growth within a trench which has a profile according to that shown in Figure 5;

Figure 10 shows a Raman spectrum of the overgrown boron doped diamond material indicating that the material has substantially no sp2 carbon; and

Figure 11 shows a schematic diagram illustrating an experimental set-up for performing conductivity measurements.

Detailed description of Certain Embodiments

As described in the summary of invention section, embodiments of the present invention provide a conductivity sensor comprising:

a plurality of boron doped diamond electrodes including a first set of boron doped diamond electrodes having a first surface area and a first electrode spacing and a second set of boron doped diamond electrodes having a second surface area and a second electrode spacing,

wherein the first surface area is larger than the second surface area and the second electrode spacing is larger than the first electrode spacing, and

wherein the plurality of boron doped diamond electrodes is configured such that in use the conductivity sensor can measure conductivity of a solution at 25°C over at least a range of 1 x 10 ^"4 S m ^"1 to 1 x 10 ^"1 S m ^"1 , 1 x 10 ^"5 S m ^"1 to 1 S m ^"1 , and even 5 x 10 ^"6 S m ^"1 to 5 S m ^"1 or more with an accuracy of better than 50%, 30%, 20%, 15%, 10%), 5%), 3%), 2%), or 1% across said conductivity range (the precise operating range and accuracy required being determined by the requirements of the end application).

The operating range and the accuracy of the conductivity sensor can be measured against readily available standard solutions having known conductivities determined by adding known quantities of a salt to highly distilled water. For example, a suitable standard of known conductivity at a specified temperature can be prepared by adding a known weight of KCl to distilled water of known volume, in accordance with Kolrausch's Law. Dilutions of this standard can then be used to generate calibration curves from which the performance of the conductivity sensor can be determined. In one configuration the first set of boron doped diamond electrodes consists of only a pair of boron doped diamond electrodes and the second set of boron doped diamond electrodes consists of only a pair of boron doped diamond electrodes. With such a configuration a closely spaced pair of large area electrodes can be located in the inter- electrode space between a widely spaced pair of small area electrodes.

In an alternative configuration the first set of boron doped diamond electrodes consists of more than two boron doped diamond electrodes and the second set of boron doped diamond electrodes consists of a pair of boron doped diamond electrodes. For example, the first set of boron doped diamond electrodes comprises an interdigitated array of boron doped diamond electrodes. In such a configuration the interdigitation may be provided by electrical contacts rather than fixed into the structure of the first set of boron doped diamond electrodes. Such a structure provides more flexibility for controlling the and changing the electrode spacing and active area of the first set of electrodes by controlling the way the plurality of electrodes in the first set area addressed. Furthermore, by providing an interdigitated set of electrodes it has been found that the conductivity range and accuracy of measurement can be increased. The interdigitated array of electrodes can be disposed between the second set of electrodes. For example, the structure may comprise an interdigitated array disposed between a widely spaced pair of small area electrodes. In such a configuration the interdigitated electrodes replace the pair of closely spaced, wide area electrodes of the previously described arrangement. In either configuration, the sensor is configured such that each set of boron doped diamond electrodes takes a 2- point conductivity measurement. Each 2-point conductivity measurement is sensitive to a different conductivity range and the ranges of the different sets of boron doped diamond electrodes overlap to give a wide overall operating range.

Advantageously, the conductivity sensor structure is an all-diamond structure where the boron doped diamond electrodes are embedded with an intrinsic diamond support matrix. In this regard, it has been found that such structures can be fabricated by forming trenches in a block of intrinsic diamond material, overgrowing boron doped diamond material into the trenches using a CVD diamond growth technique, and then surface processing the overgrown boron doped diamond material such that the boron doped diamond material is removed from a top surface of the intrinsic diamond material and remains in the trenches to form the desired electrode structure. In this case, the electrodes may then be contacted from a rear side by drilling through the intrinsic diamond support matrix to a rear surface of the boron doped diamond electrodes. An ohmic contact is formed on the rear surface of the boron doped diamond electrodes within each blind hole drilled in the intrinsic diamond support matrix and an electrical connection made to each of the ohmic contacts via the blind holes.

As such, the conductivity sensor may be configured such that the plurality of boron doped diamond electrodes are in the form of band electrodes disposed in an intrinsic diamond support matrix and the plurality of boron doped diamond electrodes are individually addressable. Each boron doped diamond electrode has an associated through-hole passing through the intrinsic diamond support matrix to an ohmic contact disposed within the blind hole on a rear surface of the boron doped diamond electrode. Furthermore, each blind hole contains an electrically conductive pathway such that the boron doped diamond electrodes are individually addressable via associated electrically conductive pathways and ohmic contacts.

One problem with the aforementioned configuration is that it is difficult to form a metal ohmic contact on the rear surface of the boron doped diamond electrodes through narrow laser-drilled blind holes in the intrinsic diamond support matrix. In this regard, it has been surprisingly found that the rear surface of the boron doped diamond electrodes can be graphitized using a laser and such a graphitized surface within the through-hole provides an ohmic contact which is suitable for this conductivity sensor application.

Yet another problem with the aforementioned method of fabricating the conductivity sensor by patterning, over-growth, and rear contacting, is that the boron doped diamond electrodes formed by overgrowth into laser cut trenches tend to be very thin. Indeed, if laser cut trenches are formed too deeply, then it has been found that the trenches are not filled with good quality boron doped diamond material, e.g. voids and poor quality diamond material form in narrow, deep trenches. As such, shallow trenches are preferred for fabrication of good boron doped diamond electrode fabrication via overgrowth. However, this causes problems in terms of electrical connection to a rear surface of the electrodes. As previously described, this requires a blind hole to be formed by laser drilling to a rear surface of the electrode. If the electrode is very thin, this requires very precise control of the laser drilling procedure or a hole will be drilled through the electrode to the outer surface. It has been found that this problem can be alleviated by providing an electrode which has a region which is thicker and thus easier to contact without drilling through the entire electrode. Furthermore, an entire trench does not need to be made deeper during electrode fabrication therefore allowing good overgrowth and filling of the trench to form such an electrode. Accordingly, each boron doped diamond electrode may have a first region and a second region, the first region having a greater thickness than the second region, and wherein the ohmic contacts are disposed on a rear surface of the first region.

In addition to the above, it is known in the art that conductivity measurements are sensitive to temperature. As such, in order to make an accurate conductivity measurement when the temperature of the solution significantly varies relative to room temperature, the temperature at which the measurement is taken should be measured and used to calculate the true conductivity of the solution at room temperature from the measured conductivity. In this regard, the fact that diamond has the highest thermal conductivity of any known material can be used advantageously. A temperature sensor, such as a thermocouple, can be readily placed in thermal contact with the plurality of boron doped diamond electrodes and the diamond material will be in thermal equilibrium with the fluid at the surface of the conductivity sensor due to diamonds extremely high thermal conductivity. For example, if the plurality of boron doped diamond electrodes are disposed in an intrinsic diamond support matrix, the temperature sensor can be disposed on a side of the intrinsic diamond support matrix which is not exposed to the solution in use while at the same time being in intimate thermal contact with the sensing surface of the conductivity sensor via the diamond material.

It has also been found that it is advantageous to provide boron doped diamond electrodes which are formed of a boron doped diamond material which has an sp2 carbon content sufficiently low as to not exhibit non-diamond carbon peaks in a Raman spectrum of the boron doped diamond material while also having a boron concentration of at least 1 x 10 ²⁰ atoms/cm ³ . Such a material has been found to be advantageous for electrochemical sensing applications combining the desirable properties of a wide potential window, a low capacitance, and good reversibility. For many applications it is advantageous for the boron doped diamond electrodes to be provided without any coating, i.e. a bare diamond surface terminated with, for example, an oxygen termination. However, it is also envisaged that it is possible for the surface of the boron doped diamond electrodes to be coated with a dielectric material.

Figure 1 shows a plan view of a conductivity sensor comprising a plurality of boron doped diamond band electrodes 2, 4 embedded within a non-conductive diamond support matrix 6. A first set of electrodes 2 are closely spaced and have a relatively large surface area. A second set of electrodes 4 are widely spaced and have a relatively small surface area. For example, the first set of electrodes 4 may have electrode dimensions wi = 1 mm, Si = 0.1 mm, h = 10 mm and the second set of electrodes may have electrode dimensions w ₂ = 0.1 mm, s ₂ = 8.0 mm, 1 ₂ = 10 mm. The first set of electrodes 2 forms a first conductivity sensor for measuring low conductivity solutions whereas the second set of electrodes 4 forms a second conductivity sensor for measuring high conductivity solutions. Together, the sensor structure is capable of measuring solution conductivities across a range from distilled water to sea water.

Figure 2 illustrates a graph showing the theoretical conductivity range for the conductivity sensor configuration shown in Figure 1 extending from distilled water through to sea water. Actual measurements closely match this range.

Figure 3 shows a cross-sectional view of the conductivity sensor configuration shown in Figure 1. The first and second sets of electrodes 2, 4 are contacted via blind holes drilled through the rear of the intrinsic diamond support matrix 6. Ohmic contacts 8 are formed on a rear surface of the electrodes and electrical connections 10 are provided in the blind holes to connect the electrodes 2, 4 via the ohmic contacts 8. Figures 4(A) to 4(F) illustrate the method steps for fabricating a conductivity sensor configuration as shown in Figures 1 and 3.

In step (A) a piece of intrinsic diamond material 6 is provided as a substrate. The intrinsic diamond material may be a wafer of polycrystalline CVD diamond material which has low electrical conductivity. This may be provided by a thermal or optical grade of polycrystalline CVD diamond material which has little or no boron dopant. Alternatively, a significant quantity of boron dopant is present in the material this may be compensated with a nitrogen dopant such that low electrical conductivity is maintained. As such, it will be understood that in the context of the present specification the term "intrinsic diamond" is not to be construed narrowly. The diamond may contain impurities and dopants so long as the material does not have a significant electrical conductivity during operation of the electrodes during conductivity measurements to interfere or short circuit the measurements.

In step (B), the intrinsic diamond substrate 6 is patterned to form trenches 12 with a geometry corresponding to the desired electrode geometry. This may be achieved using a laser although it is envisaged that other patterning techniques may be utilized such as focussed ion beam. As will be described in more detail later in this specification, the cross-sectional profile of the trenches is controlled such that the trenches can be filled with high quality boron doped diamond material with substantially no voids and also so that the electrodes can be contacted from the rear without forming holes in the surface of the electrodes.

In step (C), boron doped diamond material 14 is grown over the patterned intrinsic diamond substrate 6 using a chemical vapour deposition (CVD) technique. A microwave plasma active CVD diamond synthesis technique is preferred with the CVD growth chemistry along with power, pressure and substrate temperature conditions optimized to growth high quality boron doped diamond material (high boron content; low sp2 carbon content) while at the same time achieving substantially complete filling of the trenches with substantially no voids. Details of suitable growth conditions to achieve this are given later in this specification. In step (D), the boron doped diamond material 14 is processed back to electrically isolate individual electrodes and form a planar polished surface comprising the first and second sets of electrodes 2, 4 as illustrated in Figures 1 and 3.

In step (E), through holes 16 are drilled through a rear of the intrinsic diamond support matrix 6 to a rear surface of each electrode 2, 4. This may be achieved using a laser and it has been found that pulsed laser systems can give a relatively precise depth of cut for drilling to a rear surface of the electrode without drilling through the electrode to a front surface thereof. That said, since the electrodes tend to be very thin as it has been found that deep trenches cannot be easily filled with high quality boron doped diamond material without forming voids. As such, even using a carefully controlled pulse laser drilling technique it is advantage to tailor the cross-sectional profile of the trenches such that each electrode has a portion of thicker boron doped diamond material to which the through hole can be formed, thus reducing the possibility of forming holes in the electrodes.

Finally, electrical contacts are formed to the rear surface of each electrode through the through-holes. Each electrical contact comprises an ohmic contact 16 and an electrical connector 18. The ohmic contact 16 may comprise a carbide forming metal such as a titanium-gold contact. However, it has been found to be difficult to provide such a contact within the narrow through-holes and as an alternative it has been found that a graphitic layer formed on the rear of the boron doped diamond electrodes by a laser can be used as a suitable ohmic contact for this conductivity sensor application. The electrical connectors 18 may be provided by standard wiring, e.g. with silver paint.

As mentioned above, one problem with providing conductivity sensors as described herein is that of providing a rear contact to the electrodes without forming holes in the electrodes. Figure 5 shows a cross-sectional view of a portion of a conductivity sensor comprising an electrode which has an inverted top-hat profile with a deeper central portion. It has been found that such a structure which can be filled with good quality boron doped diamond material without substantial void formation while also being easier to rear contact without drilling through the entire electrode thickness. As can be seen in Figure 5, the structure includes an intrinsic diamond support matrix 50 and a boron doped diamond electrode 52 as previously described. The electrode 52 has a thicker central portion and an electrical contact 54 is formed through the intrinsic diamond support matrix 50 to this thicker central portion which is more easy to do in a reproducibly manner without forming pin holes in the electrode 52.

Figure 6 shows an alternative to the arrangement of Figure 5 comprising an electrode 60 within an intrinsic diamond support 62, the electrode 60 having an inverted triangular profile with a deeper central portion for ease of filling and rear contacting 64;

Figure 7 shows a plan view of an alternative conductivity sensor configuration to that shown in Figure 1 comprising a plurality of central electrodes 70 disposed within a pair of electrodes 72. The plurality of central electrodes 70 which may be contacted in such a way as to provide an interdigitated electrode structure in place of the central pair of electrodes shown in Figure 1. Such an electrode structure can be made smaller than the previously described electrode structure and has minimal overlap of sensing envelopes. Furthermore, such a structure can have a wider operating range and more flexibility in how to address the central electrodes.

Figure 8 shows a picture of a prototype structure comprising a plurality of boron doped diamond electrodes formed in an intrinsic diamond support matrix. This prototype has a larger number of pairs of electrodes in order to test operating ranges and target optimum electrode spacings and surface areas for the final commercial device.

Figure 9 shows an SEM image of an overgrown boron doped diamond electrode illustrating good quality growth within a trench which has a profile according to that shown in Figure 5. Figure 10 shows a Raman spectrum of the overgrown boron doped diamond material indicating that the material has a high boron content and substantially no sp2 carbon.

Finally, Figure 11 shows a schematic diagram illustrating an experimental set-up for performing conductivity measurements. The set-up comprises a conductivity solution-containing cell and an all-diamond conductivity sensor as described previously. A sensor switch links the electrodes to an AC conductance meter. The AC conductance meter is connected via a suitable processor to a computer which can be used to control the sensor and to receive and process data from the sensor to measure conductance of the sample solution.

Certain embodiments utilize boron doped diamond electrodes fabricated from a boron doped synthetic diamond material which has the following characteristics:

a solvent window meeting one or both of the following criteria as measured by sweeping a potential of the boron doped synthetic diamond material with respect to a saturated calomel reference electrode in a solution containing only deionised water and 0.1 M KN0 ₃ as a supporting electrolyte at pH 6:

the solvent window extends over a potential range of at least 3.8 V wherein end points of the potential range for the solvent window are defined when anodic and cathodic current density measured at the boron doped synthetic diamond material reaches 38 raA cm ^"2 ; and the solvent window extends over a potential range of at least 3.0 V wherein end points of the potential range for the solvent window are defined when anodic and cathodic current density measured at the boron doped synthetic diamond material reaches 0.4 raA cm ^"2 ;

a peak-to-peak separation ΔΕ _Ρ (for a macroelectrode) or a quartile potential ΔΕ ₃ 4.ι 4 (for a microelectrode) of no more than 90 mV as measured by sweeping a potential of the boron doped synthetic diamond material at a rate of 100 mV s ^"1 with respect to a saturated calomel reference electrode in a solution containing only deionised water, 0.1 M KNO ₃ supporting electrolyte, and 1 mM of FcTMA ⁺ or Ru( H ₃ ) ₆ ³⁺ at pH 6; and

a capacitance of no more than 10 μ¥ cm ^"2 as measured by sweeping a potential of the boron doped synthetic diamond material with respect to a saturated calomel reference electrode between 80 mV and -80 mV in a solution containing only deionised water and 0.1 M KN0 ₃ supporting electrolyte at pH 6, measuring resultant current, subtracting a current value at 0 V when sweeping towards negative potentials from a current value at 0 V when sweeping towards positive potentials, dividing the subtracted current value by 2, and then dividing the result by an area (cm ² ) of the boron doped synthetic diamond material and by a rate at which the potential is swept (Vs ^"1 ) to give a value for capacitance in F cm ^"2 .

Boron doped diamond materials meeting the aforementioned definition are described in PCT/EP2013/055170. Such synthetic diamond materials include both polycrystalline (pBDD) and single crystal (scBDD) boron doped synthetic diamond materials which have been optimized for their electrochemical sensing performance. Such materials are ideal for implementing examples of the present invention as the boron doped diamond materials can be driven to extreme potentials without substantial gas bubble formation and without causing adverse electrochemical reactions which interfere with an electrochemical sensing measurement.

For boron doped diamond materials, a low boron dopant content can aid in providing a large solvent window, flat electrochemical response, and low capacitance as desired. However, such material will not show metallic like properties resulting in nonreversible electrochemical characteristics for simple fast electron transfer outer sphere redox couples in the both positive and negative potential windows and is thus not desirable for electrochemical sensing applications. Increasing the boron dopant content significantly will cause the solvent window to shrink and the capacitance to increase, which is undesirable for the aforementioned reasons. As such, it has been found that an optimum range of boron concentration exists which balances the requirement of reversible electrochemistry for simple fast electron transfer outer sphere redox couples versus the desirable characteristics of a large solvent window, a flat electrochemical response, and a low capacitance.

In addition to the above, it has been found that sp2 carbon content within the boron doped diamond material is undesirable as this also tends to shrink the solvent window, increase capacitance, and increase non-uniformities in the electrochemical response of the electrode material. If the boron dopant content becomes too high then it is more difficult to control the presence of non-diamond carbon, e.g. sp2 carbon, providing an additive detrimental effect on the performance of the electrode material in terms of providing a wide, flat baseline for species detection. The present inventors have developed polycrystalline CVD synthetic diamond materials and single crystal CVD diamond materials which have optimized boron concentrations and substantially no sp2 carbon (as detectable via Raman spectroscopy). That said, for certain applications some sp2 carbon can be tolerated.

The boron doped synthetic diamond materials have been defined above in terms of their functional electrochemical properties as this is the most convenient, clear, and concise way to characterize the materials. In practice, the present inventors have fabricated a number of different types of boron doped synthetic diamond materials which fulfill these functional electrochemical properties. The materials may be categorized into three main types:

bulk boron doped single crystal synthetic diamond materials which comprise a suitable boron dopant content and crystallographic quality to achieve the previously described functional electrochemical properties throughout a majority volume of the single crystal synthetic diamond materials;

capped boron doped single crystal synthetic diamond materials which comprise a capping layer having a suitable boron dopant content and crystallographic quality to achieve the previously described functional electrochemical properties and a support layer having a lower boron content; and

boron doped polycrystalline synthetic diamond materials which comprise a plurality of boron doped synthetic diamond grains with a sufficient portion of the grains at an exposed surface of the material having a suitable boron dopant content, while maintaining phase purity (i.e. substantially no sp2 carbon content), to achieve the previously described functional electrochemical properties.

Whichever of the aforementioned types of material is provided, it has been found to be advantageous to fabricate a boron doped synthetic diamond material in which at least a portion of an exposed surface layer comprises boron doped synthetic diamond material having a boron content in a range 1 x 10 ²⁰ boron atoms cm ^"3 to 7 x 10 ²¹ boron atoms cm ^"3 . Preferably at least 50%, 70%, 90%, or 95% of the exposed surface layer comprises boron doped synthetic diamond material having a boron content in a range

1 x 10 20 boron atoms cm ^" 3 to 7 x 1021 boron atoms cm ^" 3. It has been found that boron doped synthetic diamond material can be fabricated with a boron content over 1 x 10 ²² boron atoms cm ^"3 . However, while such material can provide a low ΔΕ _Ρ it possesses a relatively high capacitance (e.g. greater than 10 μΡ cm ^"2 ). Furthermore, as the boron content is increased the sp2 carbon content and crystallographic defects tend to increase which also detrimentally increases the capacitance of the material in addition to lowering the solvent window. Accordingly, it has been found that a suitable upper limit for boron concentration is 7 x 10 ²¹ boron atoms cm ^"3 when taking all these factors into account.

Conversely, if the boron content is lowered below 1 x 10 ²⁰ boron atoms cm ^"3 the material is found to have a large solvent window (up to 8 V if the boron content is sufficiently low that the material exhibits p-type semi-conductive behavior) and a low capacitance with a flat electrochemical response. However, such material possesses a large ΔΕ _Ρ (over 70 mV and even up to several hundred mV). As such, it has been found to be advantageous to select a boron content falling within the range 1 x 10 ²⁰ boron atoms cm ^"3 to 7 x 10 ²¹ boron atoms cm ^"3 . That said, for certain applications a lower boron content can be tolerated.

In addition to controlling boron dopant content, in order to achieve material having optimized electrochemical characteristics it has been found to be important to minimize the formation of sp2 carbon during growth of the boron doped synthetic diamond material. Raman spectroscopy has been found to be a particularly useful technique for measuring sp2 carbon content. Non-diamond carbon peaks include: 1580 cm ^"1 - graphite; 1350-1580 cm ^"1 - nanocrysallite graphite; and 1550 - 1500 cm ^"1 - amorphous carbon and graphitic phases. It has been found that if sp2 carbon is evident in a Raman spectrum of a material then the material will have a smaller solvent window, a higher capacitance, and surface oxidation/reduction features. Accordingly, preferably the sp2 carbon content is sufficiently low as to not exhibit non-diamond carbon peaks in a Raman spectrum of the material. An sp2 carbon signature in Raman spectra has been correlated with higher capacitance, non-Faradaic surface processes, and a reduced solvent window. Micro-Raman spectroscopy can be performed at room temperature with a Renishaw inVia Raman microscope using an excitation wavelength of 514.5 nm, an Ar laser with a power of 10 mW, and a CCD detector. Magnification for Raman spectroscopy can be x5, xlO, x20, x50 and xlOO objectives at visible and near infrared (NIR) frequencies and x5 and x20 at ultraviolet (UV) frequencies. With x50 magnification the xy-spot size (and hence resolution) is approximately 5x5 microns and with xlOO the xy-spot size is approximately 2x2 microns. Typical values for magnification objectives in air thus range from x5 to xlOO.

In addition to the boron and sp2 carbon compositional requirements discussed above, it has also been found to be desirable to fabricate material which comprises little or no crystallographic defects observable by DIC (Normaski) visible microscopy at a magnification of up to xlOO. For example, samples of boron doped single crystal synthetic diamond material have been found to exhibit small rod-like features, extended polycrystalline inclusions, which are visible using microscope imaging. Such defects can result in variable electrochemical behavior and are thus considered undesirable. As such, it is considered desirable to minimize such defects either by controlled growth or by processing material by selecting areas of material which are substantially free of such defects to form electrochemical electrodes. As such, advantageously an exposed surface of the single crystal boron doped synthetic diamond material may comprise no more than 5%, 3%, 1%, 0.5%, or 0.3% by area of crystallographic defects observable by visible microscopy at a magnification of up to xlOO. That said, for certain applications a higher crystallographic defect concentration can be tolerated.

Materials as described above have been fabricated using a microwave plasma activated chemical vapour deposition (CVD) synthesis process. A microwave plasma activated CVD diamond synthesis system typically comprises a plasma reactor vessel coupled both to a supply of source gases and to a microwave power source. The plasma reactor vessel is configured to form a resonance cavity supporting a standing microwave, typical frequencies used for this heating application include 2.45 GHz and approximately 900 MHz depending on the RF spectrum allocation of each country. In this work the example conditions are given for a system equipped with a 2.45 GHz microwave source. Source gases including a carbon source and molecular hydrogen are fed into the plasma reactor vessel and can be activated by the standing microwave to form a plasma in high field regions. If a suitable substrate is provided in close proximity to the plasma, reactive carbon containing radicals can diffuse from the plasma to the substrate and be deposited thereon. Atomic hydrogen can also diffuse from the plasma to the substrate and selectively etch off non-diamond carbon from the substrate such that diamond growth can occur. If a source of boron such as diborane gas is introduced into the synthesis atmosphere then boron doped synthetic diamond material can be grown. Single crystal synthetic diamond materials are typically fabricated via homoepitaxial growth on single crystal diamond substrates. In contrast, polycrystalline synthetic diamond wafers can be grown on silicon or refractory metal substrates.

Important growth parameters include the microwave power density introduced into the plasma chamber (typically ranging from less than or equal to 1 kW to 5 kW or more for a substrate area < 20 cm ² ), the pressure within the plasma chamber (typically ranging from less than or equal to 50 Torr (i.e. 6.67 kPa) to 350 Torr (i.e. 46.66 kPa) or more), the gas flow velocity flowing through the plasma chamber (typically ranging from a few 10s of seem (standard cm ³ per minute) up to hundreds or even thousands of seem), the temperature of the substrate (typically ranging from 700 to 1200°C) , and the composition of the synthesis atmosphere (typically comprising 1 to 20% by volume of carbon containing gas (usually methane) with the remainder of the synthesis atmosphere been made up of hydrogen). For boron doping the synthesis atmosphere will typically comprise a boron containing gas such as diborane at a concentration from equal to or less than 0.01% up to several % by volume.

The problem to be solved is what growth parameters to select in order to fabricate synthetic boron doped diamond materials with optimized electrochemical sensing properties. Suitable growth parameters for both single crystal and polycrystalline diamond materials are discussed below.

Single Crystal Boron Doped Diamond Materials

As previously described, a boron dopant concentration in a range 1 x 10 ²⁰ boron atoms cm ^"3 to 7 x 10 ²¹ boron atoms cm ^"3 has been found to be desirable to achieve high performance synthetic diamond material for electrochemical sensing applications. However, it has also been found that electrochemical performance of single crystal boron doped diamond materials can be affected by the presence of crystallographic defect features which are observable by visible microscopy at a magnification of up to xlOO. It has been found that growing single crystal CVD synthetic diamond material at higher power and pressure (e.g. 250 Torr (i.e. 33.33 kPa); 5.0 kW at an operating frequency of 2.45 GHz with a 5 cm diameter carrier substrate area) produces better crystal quality material but at high powers and pressures the uptake of boron dopant is reduced such that the required levels cannot be achieved. Conversely, if the power and pressure are reduced (e.g. 100 Torr (i.e. 13.33 kPa); 2kW) then boron uptake is increased to the desired level but the crystal quality of the material is reduced such that the desired electrochemical parameters are not achieved. It has been found that there is a narrow operating window within which the power and pressure are sufficiently high to achieve the required crystal quality and sufficiently low to achieve the desired level of boron dopant uptake. Preferably the pressure is controlled to lie in a range 120 Torr to 160 Torr (i.e. 16.00 kPa to 21.33 kPa), more preferably in a range 130 Torr to 150 Torr (i.e. 17.33 kPa to 20.00 kPa), and most preferably around 140 Torr (i.e. 18.67 kPa). In addition, preferably the power is controlled to lie in a range 3.1 kW to 3.9kW, more preferably in a range 3.3 kW to 3.8kW, and most preferably around 3.6 kW. The temperature of the substrate may be controlled to lie in a range 750°C to 850°C.

In addition to the above, boron incorporation has been found to be increased by manipulating the gas flow firstly by using a reactor configured with a co-axial gas injection system, for example comprising a nozzle positioned between 50 mm and 180 mm above the substrate to direct gas towards the substrate, and secondly by increasing the gas velocity via a combination of the total gas flow rate and the chamber gas injection nozzle diameter. Using an axial gas injection nozzle with a diameter of 2 mm, positioned 75 mm above the substrate, the total gas flow rate may be at least 500 seem, more preferably at least 600 seem, and most preferably over 650 seem. For example, a hydrogen gas flow of between 500 and 700 seem may be utilized with a methane gas flow of between 25 and 40 seem and a diborane gas flow between 15 and 30 seem. Argon gas may also be introduced into the synthesis atmosphere, for example at a flow rate in a range 20 to 30 seem. Only by providing these growth parameters in combination has it been found to be possible to achieve single crystal diamond growth which meets both the boron content and crystallographic quality requirements which result in a material having the electrochemical performance characteristics as described herein. It has also been found that providing a shallow miss-cut angle on the single crystal diamond substrates relative to a crystallographic plane can aid in promoting step-flow growth leading to higher crystallographic quality material for a given power and pressure.

Poly crystalline Boron Doped Diamond Materials

Similar comments as those set out above for single crystal boron doped diamond also apply for polycrystalline boron doped diamond material. Having regard to polycrystalline material, the problem is how to achieve the high levels of boron doping while avoiding incorporation of sp2 carbon during growth. This has been achieved by controlling substrate temperature in a range 700 to 1150°C (for example, 1050 to 1120°C), using a synthesis atmosphere which has a relatively low concentration of carbon containing gas (e.g. in a range 1% to 5% of total gas flow, for example, 1% to 3%), a high power density (e.g. 5 to 6 kW over a 50 mm diameter substrate) in combination with a relatively high reactor pressure (e.g. in the range 150 to 330 Torr, i.e. 20.00 kPa to 44.00 kPa, for example, 200 to 330 Torr, i.e. 26.66 kPa to 40.00 kPa) using a high gas flow configuration and total flow rate as previously described for single crystal growth.

While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appendant claims.