Patent ID: 12188893

DETAILED DESCRIPTION

The present disclosure provides an electrochemical sensor formed on a silicon substrate. In particular, the disclosure is concerned with a back-side sensor, in which exposure to environmental gases is through the back of the sensor. As such, in order for the gases to reach the electrode, or electrodes, and electrolyte formed on the top of the substrate, microcapillaries are formed in the substrate. Additionally, being silicon, an insulating layer must be formed on the top side of the substrate, in order to isolate the conductors from the substrate. In order to allow the gases to reach the electrodes, an opening is formed in the insulating layer, and the opening is aligned with the microcapillaries. The electrodes are screen or stencil printed onto the insulating layer, such that one of the electrodes is formed in the opening in the insulating layer, and against the top surface of the substrate. As an alternative, the electrodes may be deposited using lithographic deposition techniques. In order for the gases and the electrolyte to interact, the electrode is porous. A benefit of such an arrangements is that it is easily manufactured using micromachining techniques. As such, the sensors may be reduced in size, and produced in such a manner that multiple sensors have the same characteristics. Process variations are not as great as for prior art sensors that are made individually.

FIG.1shows a cross-section through an electrochemical sensor100formed on silicon using micromachining techniques. The electrochemical sensor is formed on a silicon substrate101. In this example, a single sensor is formed on the silicon substrate101. However, in practice, several sensors may be formed on a single substrate, in a similar manner to the way in which multiple integrated circuits may be formed on a single silicon substrate. As an alternative to silicon, the substrate may be made from glass, ceramic or plastic. A plurality of microcapillaries102are formed in the substrate101. InFIG.1, six microcapillaries are shown in cross-section. However, the microcapillaries102are also formed across the width of the substrate, and as such there are typically ten or more microcapillaries. Each microcapillary is formed in a direction orthogonal to the surface of the substrate101, and extends from an upper surface to a lower surface of the substrate. Each microcapillary is approximately 20 μm in diameter, although each microcapillary may be in the range of 1 μm to 2 mm in diameter. The group of microcapillaries102is approximately 1 mm across, but may be in the range of 0.001 mm to 3 mm across.

An insulating layer103is formed on the upper surface of the substrate101. The insulating layer103may be formed from silicon oxide (SiO2) and is approximately 4 μm thick. An electrode opening104is formed in the insulating layer103in a position that is aligned with the microcapillaries102. The opening is described as being aligned in the sense that the microcapillaries arc formed in an area defined by the opening in the insulating layer. The walls of the opening104are not necessarily precisely aligned with the walls of the microcapillaries. In this example, the opening104is approximately circular, but may be square or rectangular. The opening104may be 1 to 2 mm across. The side walls of the opening104are semi-circular in shape. However, it will be appreciated that the side walls may be straight or may be formed from any other shape that increases the surface area of the side walls.

Conductive tracks105A,105B are formed on a top surface of the insulating layer103. The conductive tracks105A,105B are adhered to the insulating layer103by an adhesion layer106A,106B. The conductive tracks105A,105B may be made of gold or any other suitable conductive material. For example, the conductive tracks may be made from metal or conductive plastic. The conductive tracks are arranged such that they stop approximately 25 μm from the edge of the opening104. The tracks may stop anywhere between a few microns to a few millimeters from the edge of the opening. The conductive tracks105A,105B are for connecting the electrodes to external circuit elements. The conductive tracks may extend into the opening formed in the insulating layer103. Additionally the conductive tracks may extend into the capillaries in order to improve contact resistance.

A passivation layer107is formed over the insulating layer103and the conductive tracks105A,105B. An opening108is formed in the passivation layer107. The opening108is the same size as the electrode opening104, and is aligned with the opening104. Additional holes109A,109B,109C,109D are formed in the passivation layer to allow connections to be made between the electrodes (discussed below) and external circuit elements. Additional holes may be added for sensors with more than two electrodes.

AsFIG.1shows a cross-section through the sensor100, only a working electrode110A and a counter electrode110B are shown. The working electrode110A is formed in the openings104and108. The electrode completely fills the openings104and108and abuts the top surface of the substrate101. The working electrode110A extends approximately 25 μm above the top of the passivation layer107. The working electrode110A also extends into hole109B. This provides an electrical connection to conductive track105B, allowing connections to external circuit elements via hole109A. A counter electrode110B is formed in hole109C. Counter electrode110B also extends 25 μm above the passivation layer107. The counter electrode110B also extends into hole109C. This provides an electrical connection to conductive track105A, allowing connections to external circuit elements via hole109D. The electrode110A is printed directly on the microcapillaries102. As such, the electrolyte114may be liquid. The electrode110A prevents the electrolyte passing through the microcapillaries. The electrodes are porous and are made of a catalyst, such as platinum, and a fluoropolymer, such as polytetrafluoroethylene (PTFE). The electrode110A thus provides the 3-phase porous surface required for the chemical reactions to take place. The catalyst is a medium to high surface area porous catalyst, such as platinum black. Sufficient catalyst is provided to ensure sufficient catalytic activity throughout the sensors lifetime. The catalyst may be one of platinum, gold, ruthenium, carbon black or iridium. Other appropriate materials may be used.

A cap111is formed over the electrodes110A,110B. In embodiments where additional electrodes are used, the cap111would also be formed over those electrodes. The cap may be formed from glass, ceramic, silicon or plastic. The cap111is sealed to the passivation layer107by epoxy/adhesive or frit glass112A,112B. Other bonding techniques may be used. A hole113is formed in the top of the cap111. An electrolyte114is provided within the cap111. Alternatively, two or more holes may be formed in the cap111. This would enable the electrolyte to be vacuum filled. The electrolyte114may be made from a liquid solution, such as a conductive aqueous electrolyte or organic electrolyte, a conductive polymer, such as Nafion or PEDOT:PSS. The electrolyte may also be a hydrogel or a room temperature ionic liquid. In one example, the electrolyte may be sulfuric acid solution and may include a wicking material or wicking substructure. The electrolyte may be a two-layer electrolyte. The electrolyte114completely covers the electrodes, but when using liquid electrolytes, does not completely fill the cap112. Instead, a void space115is left towards the top of the cap111. The void space115may not be required when using conductive polymer electrolytes and hydrogels. Epoxy glue116is formed over the hole113to prevent any pollutants entering the cap, and also to prevent the electrolyte114from leaving the cap. Other options may be utilized for sealing. If two holes are provided in the cap111, a seal may be formed over both holes. Alternatively, a larger hole could be covered with an adhered lid, once the cavity is filled.

If the cap111is made from plastic, the plastic material must be compatible with the electrolyte114. Various plastic materials may be used. For example, the cap may be made from acrylonitrile butadiene styrene (ABS), PTFE, polycarbonate (PC), polyethylene (PE), amongst other plastics. Important properties of the plastic are its chemical resistance and its compatibility with the electrolytes.

InFIG.1, the conductive tracks105A,105B are provided over the insulating layer103. The openings109A,109D are provided outside of the cap111in order to allow the sensor to be connected to external devices. It may be preferable to omit the portion of the substrate101and insulting layer103that extend outside of the cap111, in order to reduce the size of the sensor100. In order to facilitate this, the conductive tracks may be omitted, and conductive vias may be formed through the substrate instead. This would enable connections to be made on the underside of the substrate101. Additionally, the size of the substrate101may be reduced to the size of the cap111.

The microcapillaries102may be lined with an insulating material. The purpose of this would be to electrically insulate the silicon substrate101from the electrodes.

FIG.2is a top view of the sensor100with certain components removed for clarity. In particular, the cap111, the electrolyte114, and the electrodes110, are not shown. Holes109A,109B,109C,109D are shown in passivation layer107. Additionally, holes109E,109F are formed in passivation layer107. These additional holes are for accommodating an additional electrode not shown inFIG.1. Holes109E and109F would not be needed in a two-electrode sensor. Opening108is also shown inFIG.2, as well as a top surface of a portion of the substrate101. The microcapillaries102are shown in the substrate101. The sensor100also includes conductive tracks105A,105B,105C. Broken line A-A represents the cross-section shown inFIG.1.

FIG.3shows the same top view asFIG.2, with the addition of the electrodes110. In particular,FIG.3shows working electrode110A, counter electrode110B and reference electrode110C. Hole109A provides access to a conductive track which is coupled to electrode110A. Hole109D provides access to a conductive track which is coupled to counter electrode110B. Hole109F provides access to a conductive track which is coupled to reference electrode110C.

FIGS.2and3show a sensor with particular dimensions. These dimensions may be altered. For example, the opening108may be much larger than shown inFIG.2, and in particular may cover much of the area occupied by the electrode110A. The length and width of each sensor may be in the range of 1 mm to 10 mm. The overall thickness, including the substrate101and the cap111may be 1 mm. As such, on a typical 200 mm wafer, in excess of 1000 sensors may be produced.

In use, the sensor would be connected to a micro-controlled measurement system in a manner familiar to those skilled in the art. The sensor output may be continuously monitored and used to determine the concentration of analyte in the environment. The electrode110A may come into contact with environmental gases via the microcapillaries102. As the electrode110A is porous, the environmental gases are able to pass through the electrode to a point where they come into contact with the electrolyte114. A three-phase junction is therefore formed within the electrode. An advantage of using a printed, solid electrode110A, is that it prevents the electrolyte114from escaping through the microcapillaries102in the substrate101.

An advantage of the above-described structure is that silicon micromachining techniques can be used in its construction. As such, manufacturing of the sensor is compatible with fabrication techniques used to manufacture integrated circuits. By manufacturing multiple sensors in parallel, variations in the parameters of the sensors are reduced.

A further advantage of using silicon fabrication techniques is that the cost of each device is reduced. This is because each process step is applied to multiple sensors in parallel, so the processing cost per device is small. Additionally, micromachining techniques enable very small devices to be produced. As such, the sensors may be more easily incorporated into handheld devices. Furthermore, the sensors all see the same processing steps at the same time. As such, matching between devices is very good when compared with serially produced devices.

A method of fabricating the electrochemical sensor100will now be described with reference toFIGS.4A to4I.

FIG.4Ashows the first step in the fabrication process. A silicon wafer is used as the silicon substrate101. In the following, the process for forming one device will be described, however several hundred devices may be formed in parallel on the same wafer. The silicon substrate101is used for mechanical support, and could be substituted for another type of material, such as glass.

An oxide insulating layer103is deposited on the wafer, as shown inFIG.4B. The oxide layer serves as a “landing” oxide to stop the through wafer etch, and also serves as a layer to insulate the conductive tracks from the substrate to prevent shorting.

The microcapillaries102are defined in the wafer by photolithography. The microcapillaries are etched through the wafer using an isotropic dry etch. They are etched from the backside of the wafer and stop at the oxide layer once the silicon wafer has been etched through, as shown inFIG.4C.

FIG.4Dshows formation of inert metal layers which form the conducting tracks105. They are deposited on the insulation layer, on the front side of the wafer. An adhesive layer106is first deposited on the insulating layer103, and is used to attach the metal layer to the insulating layer103. The conductive tracks may be defined by photolithography and then etched. The thickness of the inert metal can be increased by electroplating in specific areas, as defined by photolithography.

FIG.4Eshows the sensor after deposition and definition of the passivation layer107. The insulating oxide103on the front side of the wafer101is removed in the region of the microcapillaries102using a wet etch, as shown inFIG.4F.

A porous electrode material is deposited on the wafer using screen printing, stencil printing, electroplating, or other lithographic deposition techniques to form electrodes110A and110B, as shown inFIG.4G. Electrode110A covers the microcapillaries102, and connection is made to the conductive tracks.

The cap111is then placed over the sensor100, as shown inFIG.4H. As described above, the cap111may be made of plastic, ceramic, silicon or glass, amongst other materials. If the cap is made of plastic, it is prefabricated by injection molding. The recess and holes may be formed during the injection molding process. If the cap is made from glass, silicon or ceramic, the cap would typically be fabricated using wafer level processing techniques. For glass or ceramic caps, cavities can be made in the cap by firstly using photolithography to pattern the cap cavity. Then one of, or a combination of, wet etching, dry etching, sand blasting and laser drilling may be used to create the cavities in the cap. For silicon caps, cavities can be made in the cap by firstly using photolithography to pattern the cap cavity. Then one of, or a combination of, wet etching, dry etching, sand blasting, and laser drilling may be used to create the cavities in the cap.

The cap111is attached to the wafer through wafer bonding (wafer processing) or through placement with epoxy/adhesive on the sensor wafer (single cap placement process). Alternatively, the cap111may be attached by other means such as ultrasonics. The electrolyte114is dispensed through the cap hole113and the hole is sealed, as shown inFIG.4I. As noted above, the cap111may have more than one hole.

FIG.5shows an alternative embodiment in accordance with the present disclosure. Most of the structure is the same as that shown inFIG.1. An electrochemical sensor200includes a substrate201having a plurality of microcapillaries202formed therein. An insulating layer203is formed on a top surface of the substrate201, and has an electrode opening204formed in a position aligned with the microcapillaries202. A conductive pad205A is formed on the insulating layer203and is attached thereto by adhesion layer206A.

A passivation layer207is formed over the insulating layer203, and has an opening208formed therein. The opening208is aligned with the opening204. In addition, the passivation layer includes a hole209A aligned with the conductive pad205A. A working electrode210A is formed in the openings204and208, as well as in the opening209A. A cap211is formed over the electrode210A, and is attached to the passivation layer using adhesive212A and212B, or through wafer bonding. A hole213is formed in the top of the cap211, and is for providing electrolyte214within the cap211. A void space215may be formed above the electrolyte214if a liquid electrolyte is used, and an epoxy glue cap is provided over the opening213. As noted above, other sealing techniques may be used to cover the opening213.

FIG.6shows a top view of the electrochemical sensor200with various components removed. In particular, the electrode210A, the electrolyte214and the cap211are not shown. The broken line B-B represents the cross-section ofFIG.5. As can be seen inFIG.5, opening204and hole209A are formed in passivation layer207. Additionally, holes209B,209C,209D,209E,209F are also formed in passivation layer207.

FIG.7also shows the same top view of electrochemical sensor200. In this example, electrode210A is shown formed over opening204and hole209A. Additionally, counter electrode210B and reference electrode210C are also shown.

FIG.8shows a further alternative embodiment in accordance with the present disclosure. Most of the structure is the same as that shown inFIGS.1and5. An electrochemical sensor300includes a substrate301having a plurality of microcapillaries302formed therein. An insulating layer303is formed on a top surface of the substrate301, and has an electrode opening304formed in a position aligned with the microcapillaries302.

Conductive tracks305A,305B are formed on the insulating layer303and are attached thereto by adhesion layers306A,306B. The purpose of these tracks is to connect electrodes (not shown inFIG.8) to external connections (also not show inFIG.8). These tracks will be made clearer below in connection withFIGS.9and10. A conductive pad305C is also formed on the insulating layer303, and is attached to the insulting layer using adhesion layer306C. Conductive pad305C is formed around the opening304in the insulating layer303. This will be shown more clearly below in connection withFIGS.9and10.

A passivation layer307is formed over the insulating layer303and the conductive tracks305A,305B and conductive pad305C. The passivation layer307is not formed over the entire conductive pad305C, and as such has an opening308formed therein. The passivation layer307overlaps the edges of the pad305C by around 10 to 100 μm. The opening308is aligned with, but wider than, the opening304. A working electrode310A is formed in the openings304and308, as well as over the conductive pad305C.

A cap311is formed over the electrode310A (and the other electrodes that are not shown), and is attached to the passivation layer307using adhesive312A and312B, or using wafer bonding. A hole313is formed in the top of the cap311, and is for providing electrolyte314within the cap311. A void space315may be formed above the electrolyte314if a liquid electrolyte is used, and an epoxy glue cap316is provided over the opening313.

FIG.9shows a top view of the electrochemical sensor300with various components removed. In particular, the electrodes310A,310B,310C, the electrolyte314and the cap311are not shown. The broken line C-C represents the cross-section ofFIG.8. The conductive pad305C is formed towards the middle of the sensor300. The conductive pad305C is formed around the opening304in the insulating layer303. As such, the microcapillaries302may be seen in the substrate301. The sensor300also includes two further conductive pads305D and305E. These pads are for having electrodes formed thereon. The sensor300also includes three external connection pads305F,305G,305H. These pads are connected to respective electrode pads by respective tracks305A,305I and305B. These tracks are shown as hashed lines as they are not visible through the passivation layer307. The passivation layer307is formed over the insulating layer303. The passivation layer covers the tracks305A,305I, and305B, but does not cover the conductive pads305C,305D,305E,305F,305G,305H.

FIG.10also shows the same top view of electrochemical sensor300. In this figure, electrode310A is shown formed over openings304,308, and conductive pad305C. Additionally, counter electrode310B and reference electrode310C are also shown.

FIG.11shows an example of an electrochemical sensor400in accordance with a further embodiment in the present disclosure. Most of the structure is the same as that shown inFIG.1. The electrochemical sensor400includes a substrate401having a plurality of microcapillaries402formed therein. An insulating layer403is formed on a top surface of the substrate401, and has an electrode opening404formed in a position aligned with the microcapillaries402. Conductive pads405are formed on the insulating layer403and are attached thereto by adhesion layer406.

A passivation layer407is formed over the insulating layer403, and has an opening408formed therein. The opening408is aligned with the electrode opening404. In addition, the passivation layer includes a hole409A aligned with the conductive pad405A. A working electrode410A is formed in the openings404and408. A counter electrode410B is formed in the opening409A. A cap411is formed over the electrodes410, and is attached to the passivation layer407using adhesive412A and412B, or using wafer bonding processes). A hole413is formed in the top of the cap411, and is for providing electrolyte414within the cap411. A void space415may be formed above the electrolyte414, when liquid electrolytes are used, and an epoxy glue cap416is provided over the opening413. As with sensor100, sensor300may include an additional reference electrode formed over an additional hole in the passivation layer.

In contrast with the previous embodiments, recesses417A and417B are formed in the substrate401, the insulating layer403, and the passivation layer407. The electrolyte414fills recesses417A and417B and part of the cap volume. InFIG.11, the substrate401forms the walls of the recesses417A,417B. As an alternative, the walls and base of the recesses417A,417B may be covered in a layer of insulating material. This layer may be the insulating layer403. The purpose of this is to insulate the electrolyte from any galvanic path that may be formed through the silicon substrate to the electrodes. The insulating layer may be provided using thermal oxidation or vapor deposition.

FIG.12shows a top view of the electrochemical sensor400shown inFIG.11. Here, the recesses417A and417B may be seen. The recesses together form a microwell418. The microwell418forms a ‘C’ shape around the top, left and bottom sides of the electrodes410. Other shapes may be used depending on the design of the sensor.

FIGS.13A to13Cshow the function of the microwell418. InFIG.13A, electrochemical sensor400is upright, and the electrolyte414fills the microwells such that the electrolyte also covers the electrode410A (as well as electrodes410B and410C which are not shown).

InFIG.13B, the electrochemical sensor400is upside down. As can be seen, the electrolyte414now fills the cap411, and a void space419is formed in the microwells418. As such, the electrodes410A,410B and410C are completely covered by the electrolyte414.

InFIG.13C, the electrochemical sensor400is shown on its side. Here, the electrolyte414fills the microwell418and the cap411such that a void space is provided in a portion of the cap411and the void space418. As such, the electrodes410are completely covered by electrolyte414. As can be seen, the advantage of providing microwells in this manner is that the electrodes are always covered by electrolyte no matter their orientation. A further benefit of using microwells is that an additional wicking material is not required in the sensor. Additionally, sensor life is improved because the sufficient electrolyte and void space are provided, even in extreme temperatures and humidity.

FIGS.14A to14Cshow a substrate501in accordance with a further embodiment of the present disclosure.FIG.14Ashows a cross-section through a substrate501.FIG.14Bshows an end-view of the substrate501. The substrate501is the same as the substrate101in most respects. In particular, the substrate501includes microcapillaries502. However, in contrast to substrate101, the substrate501includes a trench503. The trench is formed in an underside of the substrate501, and extends from the microcapillaries502to an edge of the substrate501. The purpose of the trench is to enable environmental gases to reach the microcapillaries in the event the substrate501is placed on a solid surface. This would enable several sensors to be stacked, as shown inFIG.14C. Alternatively, this would enable the sensor to be placed on another die, such as a microcontroller.

FIG.15shows multiple caps formed in a wafer. In the process described above in connection withFIG.4, the sensor dies and the caps are described as being formed on an individual level. Advantageously, the above-described sensors may be manufactured as multiple identical units using one or more wafers. Specifically, the substrate may be formed from a single wafer, and the sensors may be built up using parallel processing. Furthermore, a plurality of caps may be formed in a single wafer.FIG.15shows a wafer600which includes a plurality of caps601. Several caps are formed in each of a number of parallel lines.

If forming the caps601from plastic, the entire wafer may be injection moulded to include the plurality of caps. The plastic caps in the wafer can then be bonded to the sensor wafer by epoxy glue, heat treatment or other means. Each cap may then be diced by laser cutting or other wafer dicing techniques. The caps could also be “partially” bonded to the sensor die by e.g. glue or heat treatment, then diced, and then doing the “complete” bonding with more glue, or other means. If forming the caps601from silicon, traditional wafer processing techniques may be used to form the caps.

Access for bond wires through the cap wafer to contact the sensor bondpads would be required. This could be done by forming holes which go through the cap wafer and align with the bondpads on the sensor wafer prior to wafer bond. Alternatively, vias may be formed in the substrate, as noted above, to avoid the need for openings in the cap. In the case of silicon, removal of unwanted silicon on the cap wafer to give access the bondpads on the sensor wafer could be done through a dicing and singulation process.

FIGS.16A to16Bshow cross sections through a wafer. InFIG.16A, the wafer600includes a plurality of caps601. Each cap includes two holes602A,602B. In this example, the caps are adjacent each other, with no spacing between the caps. InFIG.16B, the same wafer is shown but with the cross-section orthogonal to the one shown inFIG.16A. The wafer600includes a plurality of caps601. Each cap includes a two holes, but only one of the holes602A can be seen in this cross-section. In this cross-section, the caps include a spacing section603, between each cap. In each case, the positioning603of the dicing procedure is shown.

The above-described sensors have a broad application space. For example, they are suitable for mobile sensing, smart phones, watches, wearables, etc. This is because of their small size, low manufacturing cost, and accuracy.

In a further embodiment of the disclosure, a sensor array may be provided. The sensor array may include two or more of the above-described sensors. All of the sensors in the array may be the same, and for detecting the same gases. The additional sensors may be included to provide redundancy. Alternatively the sensors may be for detecting different gases.

In a further embodiment, an integrated circuit comprising one of the afore-mentioned sensors may be provided. Alternatively an integrated circuit comprising the above-described sensor array may be provided.

In the above-described embodiments, a 3-electrode system has been described. The disclosure is also applicable to 2-electrode systems and systems with more than 3 electrodes. Different numbers and combinations of electrodes can be used to detect different gases. Furthermore, in above-described embodiments, only one of the electrodes is exposed to environmental gases, through one set of microcapillaries. As an alternative, two or more of the electrodes may be exposed to environmental gases via two or more sets of microcapillaries.

In the above-described embodiments, generally the working electrode is porous. As an alternative, there may be two porous electrodes, and two openings may be provided in the insulating layer. A respective one of each porous electrode may be formed in a respective opening. The substrate may include two sets of microcapillaries, each one aligned with a respective opening in the insulating layer.

The above-described sensor has been described primarily in the context of gas sensing. However, the sensor may be used for liquid sensing.

FIG.17is a top view of certain components of a sensor700in accordance with a further embodiment of the disclosure. In this embodiment, the electrodes and the conductive tracks are annular or semi-annular. The construction of the various layers of the sensor700is the same as in the previous embodiments, and is not shown here in detail. The sensor700includes a plurality of capillaries702which are formed in a silicon substrate layer. The capillaries are formed in rows such that the outer capillaries roughly form a circle. The sensor also includes a working electrode704A, a counter electrode704B and a reference electrode704C. The working electrode704A is circular, and is arranged above the capillaries702such that the circle formed by the working electrode704A is co-axial with the circle formed by the capillaries702. The working electrode704A has a diameter that is greater than the diameter of the circle formed by the capillaries702, in order that sufficient space is provided to form connections with the conductive tracks.

The counter electrode704B and the reference electrode704C are both semi-annular in shape, and formed on either side of the working electrode704A. The diameter of the inner edge of each of the semi-annular electrodes is slightly greater than the diameter of the outer edge of the working electrode, such that they may be insulated from the working electrode. A gap is also formed between the adjacent edges of the ends of each semi-annular portion, in order to ensure that the counter electrode704B and the reference electrode704C are insulated from each other.

An insulating layer and a passivation layer are formed between the electrodes and the silicon substrate. However, openings (not shown) are provided in these layers, in the same manner as the pervious embodiments, such that the working electrode704A is in contact with the upper surface of the silicon substrate, in the area of the capillaries702. Further openings are provided in the passivation layer (but not the insulating layer) to allow the electrodes to make contact with the conductive tracks, as described below.

The sensor700includes conductive tracks706A,706B and706C. The conductive tracks are shown in broken lines, as they are all positioned below the passivation layer. Conductive track706A is for connecting the working electrode704A. The conductive track includes a ring-shaped portion, which is located around the capillaries702, but within the outer edge of the working electrode704A. The ring-shaped portion is co-axial with the working electrode704A. A ring-shaped opening is formed in the passivation layer, and is aligned with the ring-shaped portion of the conductive track706A, in order to allow the working electrode704A to connect to the conductive track706A. A rectangular connecting portion of track706A is formed at the bottom edge of the ring-shaped portion, to provide a connection to external circuitry.

Conductive tracks706B and706C are formed partially underneath counter electrode704B and reference electrode704C respectively. Each track includes a semi-annular portion which is the same shape as the corresponding electrode, but slight smaller in size. As such, the semi-annular portions fit within the perimeters of their respective electrodes. Openings are provided in the passivation layer to enable the conductive tracks706B and706C to connect to the working electrode704B and reference electrode, respectively. These openings are similar in size and shape to the semi-annular portions of the conductive tracks706B and706C. In a similar manner to the conductive track706A, the conductive tracks706B and706C include rectangular portions which extend from an outer edge of the semi-annular portions to provide connections to external circuitry.

The purpose of using a circular and semi-annular arrangement is to reduce and optimise the distance and spacing between the electrodes. This reduces the resistance path between the electrodes, which can affect the sensor performance, including speed of response. For example, in a carbon monoxide sensor, there's ion movement, or transport, between the electrodes in the sensor. Ideally, therefore, the electrodes (including the entire electrode area) should be as close together as possible. Using circular and semi-annular electrodes makes this easier to achieve.

FIG.18shows the sensor700. In this embodiment, the sensor700includes all of the components shown inFIG.17, and additionally includes shielding tracks708A,708B and708C. The shielding tracks are formed around, but insulated from, the conductive tracks. The shielding tracks are also formed from conductive material. The shielding tracks are for shielding the sensor700from electromagnetic interference. The shielding tracks may be used with any of the above-described embodiments.

FIG.19shows a sensor package800in accordance with a further embodiment of the disclosure. In this embodiment, the sensor package includes two sensors800A and800B. The working electrodes804A-1and804A-2are semicircular, rather than circular, and are positioned adjacent each other. Two groups of capillaries802A and802B are formed under each respective working electrode. Each working electrode is connected to a respective conductive track806A-1and806A-2. The conductive tracks include semi-annular portions, corresponding to the semicircular electrodes, rather than ring-shaped portions.

The sensor package800also includes two counter electrodes804B-1and804B-2. Each counter electrode corresponds to a respective one of the working electrodes and groups of capillaries. The counter electrodes are quarter-annular, rather than semi-annular. Each counter electrode is connected to a respective conductive track806B-1and806B-2. The conductive tracks includes quarter-annular portions, rather than semi-annular portions, in a similar manner to the electrodes.

The sensor package includes a single reference electrode804C, which is the same size and shape as the reference electrode704C. The reference electrode is shared by both sensors800A and800B and is connected to conductive track806C.

Alternatively, two separate reference electrodes may be used, in the same manner as the counter electrode. Additionally, both sensors share the same electrolyte, meaning no changes need to be made to the cap design.

FIG.20is a sensor package900in accordance with a further embodiment of the disclosure. Sensor package900is the same as sensor package800except for the counter electrodes. In sensor package900, both sensors900A and900B share the same counter electrode904B. Sensor package900also includes working electrodes904A-1,904A-2and reference electrode904C. The conductive tracks include two tracks906A-1,906A-1for the working electrode, one track906B for the counter electrode904B, and one track906C for the reference electrode904C. While the above-described embodiments have been shown with two sensors in one sensor package, it will be appreciated that three or more sensors could be incorporated into a single sensor package, with appropriate changes to the design of the electrodes. By providing two sensors in a single package, applications which require multiple sensors can be provisioned using a single package, rather than using multiple packages. A multi-sensor package is smaller and more cost effective than using separate packages.

In the above-described embodiments, various different shapes have been used for the electrodes, conductive elements, and openings. Tt will be appreciated that the present disclosure is not limited to any of these shapes, although certain shapes do have particular advantages, as described above. Other shapes may fall within the scope of the claims.