Patent Description:
Electrochemical sensors are known which typically comprise three electrodes in contact with an electrolyte. These electrodes are normally identified as a working electrode, a counter electrode and a reference electrode. Generally speaking, in such sensors, the reference electrode is held at a constant potential with respect to the working electrode. The presence of substances which interact with the working electrode/electrolyte interface can invoke current flow between the working electrode and the counter electrode as a result of reduction/oxidation (REDOX) reactions at the working electrode. Other electrochemical sensors may only have a working electrode and a counter electrode and in such sensors the potential difference, current flow or resistance between those electrodes may be measured.

Generally speaking, such electrochemical sensors are made on a one by one basis or by using techniques that are quite variable. As a result, the sensors tend to vary from one to another. In some fields of use, such as carbon monoxide sensors, this is not too much of an issue as the trigger threshold for the associated electronics to issue an alarm is set so high that there can be no doubt that an unsafe level of carbon monoxide has been reached. However, for situations where greater precision and/or resolution are required then the sensors have to be calibrated prior to use. This is generally expensive and/or time consuming. Furthermore, it is generally desirable to reduce the size of sensors, in order to increase the range devices and applications with which they can be used.

<CIT> discloses a sensor for gaseous and vaporous species. The sensor comprises a substrate having a surface having an opening therein. A gas and vapor permeable sensing electrode having front and back sides is located across the opening with the front side facing generally the same direction as does the surface. A gas flow path leads to the back side of the sensing electrode. An electrolytic medium is in contact with the front side of the electrode. An additional electrode is in contact with the electrolytic medium and is electronically isolated from the sensing electrode other than via the electrolytic medium. A gas sensor as described has a very fast response time and an extended lifetime and can be made selective for any of a number of different gaseous species.

An electrochemical sensor is provided which may be formed using micromachining techniques commonly used in the manufacture of integrated circuits. This is achieved by forming microcapillaries in a silicon substrate and forming an opening in an insulating layer to allow environmental gases to reach through to the top side of the substrate. A porous electrode is printed on the top side of the insulating layer such that the electrode is formed in the opening in the insulating layer. The sensor also comprises at least one additional electrode. The electrolyte is then formed on top of the electrodes. A cap is formed over the electrodes and electrolyte. This arrangement may easily be produced using micromachining techniques.

According to a first aspect of this disclosure there is provided an electrochemical sensor in accordance with independent claim <NUM> comprising a substrate having a first upper surface and a second lower surface, and also having a plurality of microcapillaries formed therein, the microcapillaries extending from the first surface to the second surface; an insulating layer, formed on or adjacent the first surface of the substrate, the insulting layer having an opening aligned with the plurality of microcapillaries and extending through the insulating layer; at least two electrodes, at least one the electrodes being a porous electrode formed in the opening in the insulating layer and a portion of the at least one porous electrode is formed on or adjacent the first surface of the substrate in an area defined by the opening in the insulating layer; one or more conductive tracks formed over the insulating layer, extending onto side-wall of the opening in the insulating layer and/or a side-wall of the at least one hole formed in the substrate, a passivation layer, formed over the insulating layer and the conductive tracks; an electrolyte comprising a liquid solution, conductive polymer or hydrogel formed as a layer over the electrodes and the insulating layer; a cap formed over the insulating layer, wherein the electrolyte is provided within the cap, and at least partially fills the cap; and at least one recess, formed in the substrate, in communication with the space defined by the cap, wherein the electrolyte is also provided within the recess, wherein the sensor is arranged such that the at least one porous electrode is exposed to the environment through the plurality of microcapillaries.

According to a second aspect of this disclosure there is provided a method of manufacturing an electrochemical sensor in accordance with independent claim <NUM>, comprising: forming an insulating layer on or adjacent a substrate, the substrate having a first upper surface and a second lower surface and the insulating layer being formed on or adjacent the first surface; providing a plurality of microcapillaries in the substrate, the microcapillaries extending from the first surface to the second surface; providing an opening in the insulating layer, the opening aligned with the plurality of microcapillaries and extending through the insulating layer; forming at least one porous electrode in the opening in the insulating layer, a portion of the at least one porous electrode being formed on or adjacent the first surface of the substrate in an area defined by the opening in the insulating layer, and such that the at least one electrode is exposed to the environment through the substrate; forming at least one additional electrode, over the insulating layer; forming one or more conductive tracks over the insulating layer that extend onto side-wall of the opening in the insulating layer and/or a side-wall of the at least one hole formed in the substrate, forming a passivation layer over the insulating layer and the conductive tracks; forming a layer of electrolyte comprising a liquid solution, conductive polymer or hydrogel over the electrodes and the insulating layer; forming a cap over the insulating layer, wherein the electrolyte is provided within the cap, and at least partially fills the cap; and forming at least one recess in the substrate, in communication with the space defined by the cap, wherein the electrolyte is also provided within the recess.

Further features of the disclosure are defined in the appended claims.

The teachings of this disclosure will be discussed, by way of non-limiting examples, with reference to the accompanying drawings, in which:.

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> shows a cross-section through an electrochemical sensor <NUM> formed on silicon using micromachining techniques. The electrochemical sensor is formed on a silicon substrate <NUM>. In this example, a single sensor is formed on the silicon substrate <NUM>. 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 micro capillaries <NUM> are formed in the substrate <NUM>. In <FIG>, six microcapillaries are shown in cross-section. However, the microcapillaries <NUM> are 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 substrate <NUM>, and extends from an upper surface to a lower surface of the substrate. Each microcapillary is approximately <NUM> in diameter, although each microcapillary may be in the range of <NUM> to <NUM> in diameter. The group of microcapillaries <NUM> is approximately <NUM> across, but may be in the range of <NUM> to <NUM> across.

An insulating layer <NUM> is formed on the upper surface of the substrate <NUM>. The insulating layer <NUM> may be formed from silicon oxide (SiO<NUM>) and is approximately <NUM> thick. An electrode opening <NUM> is formed in the insulating layer <NUM> in a position that is aligned with the microcapillaries <NUM>. The opening is described as being aligned in the sense that the micro capillaries are formed in an area defined by the opening in the insulating layer. The walls of the opening <NUM> are not necessarily precisely aligned with the walls of the microcapillaries. In this example, the opening <NUM> is approximately circular, but may be square or rectangular. The opening <NUM> may be <NUM> to <NUM> across. The side walls of the opening <NUM> are 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 tracks 105A, 105B are formed on a top surface of the insulating layer <NUM>. The conductive tracks 105A, 105B are adhered to the insulating layer <NUM> by an adhesion layer 106A, 106B. The conductive tracks 105A, 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 <NUM> from the edge of the opening <NUM>. The tracks may stop anywhere between a few microns to a few millimeters from the edge of the opening. The conductive tracks 105A, 105B are for connecting the electrodes to external circuit elements. The conductive tracks may extend into the opening formed in the insulating layer <NUM>. Additionally the conductive tracks may extend into the capillaries in order to improve contact resistance.

A passivation layer <NUM> is formed over the insulating layer <NUM> and the conductive tracks 105A, 105B. An opening <NUM> is formed in the passivation layer <NUM>. The opening <NUM> is the same size as the electrode opening <NUM>, and is aligned with the opening <NUM>. Additional holes 109A, 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.

As <FIG> shows a cross-section through the sensor <NUM>, only a working electrode 110A and a counter electrode 110B are shown. The working electrode 110A is formed in the openings <NUM> and <NUM>. The electrode completely fills the openings <NUM> and <NUM> and abuts the top surface of the substrate <NUM>. The working electrode 110A extends approximately <NUM> above the top of the passivation layer <NUM>. The working electrode 110A also extends into hole 109B. This provides an electrical connection to conductive track 105B, allowing connections to external circuit elements via hole 109A. A counter electrode 110B is formed in hole 109C. Counter electrode 110B also extends <NUM> above the passivation layer <NUM>. The counter electrode 110B also extends into hole 109C. This provides an electrical connection to conductive track 105A, allowing connections to external circuit elements via hole 109D. The electrode 110A is printed directly on the microcapillaries <NUM>. As such, the electrolyte <NUM> may be liquid. The electrode 110A 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 electrode 110A thus provides the <NUM>-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 cap <NUM> is formed over the electrodes 110A, 110B. In embodiments where additional electrodes are used, the cap <NUM> would also be formed over those electrodes. The cap may be formed from glass, ceramic, silicon or plastic. The cap <NUM> is sealed to the passivation layer <NUM> by epoxy/adhesive or frit glass 112A, 112B. Other bonding techniques may be used. A hole <NUM> is formed in the top of the cap <NUM>. An electrolyte <NUM> is provided within the cap <NUM>. Alternatively, two or more holes may be formed in the cap <NUM>. This would enable the electrolyte to be vacuum filled. The electrolyte <NUM> may 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 electrolyte <NUM> completely covers the electrodes, but when using liquid electrolytes, does not completely fill the cap <NUM>. Instead, a void space <NUM> is left towards the top of the cap <NUM>. The void space <NUM> may not be required when using conductive polymer electrolytes and hydrogels. Epoxy glue <NUM> is formed over the hole <NUM> to prevent any pollutants entering the cap, and also to prevent the electrolyte <NUM> from leaving the cap. Other options may be utilized for sealing. If two holes are provided in the cap <NUM>, 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 cap <NUM> is made from plastic, the plastic material must be compatible with the electrolyte <NUM>. 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.

In <FIG>, the conductive tracks 105A, 105B are provided over the insulating layer <NUM>. The openings 109A, 109D are provided outside of the cap <NUM> in order to allow the sensor to be connected to external devices. It may be preferable to omit the portion of the substrate <NUM> and insulting layer <NUM> that extend outside of the cap <NUM>, in order to reduce the size of the sensor <NUM>. 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 substrate <NUM>. Additionally, the size of the substrate <NUM> may be reduced to the size of the cap <NUM>.

The microcapillaries <NUM> may be lined with an insulating material. The purpose of this would be to electrically insulate the silicon substrate <NUM> from the electrodes.

<FIG> is a top view of the sensor <NUM> with certain components removed for clarity. In particular, the cap <NUM>, the electrolyte <NUM>, and the electrodes <NUM>, are not shown. Holes 109A, 109B, 109C, 109D are shown in passivation layer <NUM>. Additionally, holes 109E, 109F are formed in passivation layer <NUM>. These additional holes are for accommodating an additional electrode not shown in <FIG>. Holes 109E and 109F would not be needed in a two-electrode sensor. Opening <NUM> is also shown in <FIG>, as well as a top surface of a portion of the substrate <NUM>. The microcapillaries <NUM> are shown in the substrate <NUM>. The sensor <NUM> also includes conductive tracks 105A, 105B, 105C. Broken line A-A represents the cross-section shown in <FIG>.

<FIG> shows the same top view as <FIG>, with the addition of the electrodes <NUM>. In particular, <FIG> shows working electrode 110A, counter electrode 110B and reference electrode 110C. Hole 109A provides access to a conductive track which is coupled to electrode 110A. Hole 109D provides access to a conductive track which is coupled to counter electrode 110B. Hole 109F provides access to a conductive track which is coupled to reference electrode 110C.

<FIG> show a sensor with particular dimensions. These dimensions may be altered. For example, the opening <NUM> may be much larger than shown in <FIG>, and in particular may cover much of the area occupied by the electrode 110A. The length and width of each sensor may be in the range of <NUM> to <NUM>. The overall thickness, including the substrate <NUM> and the cap <NUM> may be <NUM>. As such, on a typical <NUM> wafer, in excess of <NUM> 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 electrode 110A may come into contact with environmental gases via the microcapillaries <NUM>. As the electrode 110A is porous, the environmental gases are able to pass through the electrode to a point where they come into contact with the electrolyte <NUM>. A three-phase junction is therefore formed within the electrode. An advantage of using a printed, solid electrode 110A, is that it prevents the electrolyte <NUM> from escaping through the micro capillaries <NUM> in the substrate <NUM>.

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 sensor <NUM> will now be described with reference to <FIG>.

<FIG> shows the first step in the fabrication process. A silicon wafer is used as the silicon substrate <NUM>. 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 substrate <NUM> is used for mechanical support, and could be substituted for another type of material, such as glass.

An oxide insulating layer <NUM> is deposited on the wafer, as shown in <FIG>. 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 microcapillaries <NUM> are 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 in <FIG>.

<FIG> shows formation of inert metal layers which form the conducting tracks <NUM>. They are deposited on the insulation layer, on the front side of the wafer. An adhesive layer <NUM> is first deposited on the insulating layer <NUM>, and is used to attach the metal layer to the insulating layer <NUM>. 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> shows the sensor after deposition and definition of the passivation layer <NUM>. The insulating oxide <NUM> on the front side of the wafer <NUM> is removed in the region of the microcapillaries <NUM> using a wet etch, as shown in <FIG>.

A porous electrode material is deposited on the wafer using screen printing, stencil printing, electroplating, or other lithographic deposition techniques to form electrodes 110A and 110B, as shown in <FIG>. Electrode 110A covers the microcapillaries <NUM>, and connection is made to the conductive tracks.

The cap <NUM> is then placed over the sensor <NUM>, as shown in <FIG>. As described above, the cap <NUM> may 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 cap <NUM> is 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 cap <NUM> may be attached by other means such as ultrasonics. The electrolyte <NUM> is dispensed through the cap hole <NUM> and the hole is sealed, as shown in <FIG>. As noted above, the cap <NUM> may have more than one hole.

<FIG> shows an alternative embodiment in accordance with the present disclosure. Most of the structure is the same as that shown in <FIG>. An electrochemical sensor <NUM> includes a substrate <NUM> having a plurality of microcapillaries <NUM> formed therein. An insulating layer <NUM> is formed on a top surface of the substrate <NUM>, and has an electrode opening <NUM> formed in a position aligned with the microcapillaries <NUM>. A conductive pad 205A is formed on the insulating layer <NUM> and is attached thereto by adhesion layer 206A.

A passivation layer <NUM> is formed over the insulating layer <NUM>, and has an opening <NUM> formed therein. The opening <NUM> is aligned with the opening <NUM>. In addition, the passivation layer includes a hole 209A aligned with the conductive pad 205A. A working electrode 210A is formed in the openings <NUM> and <NUM>, as well as in the opening 209A. A cap <NUM> is formed over the electrode 210A, and is attached to the passivation layer using adhesive 212A and 212B, or through wafer bonding. A hole <NUM> is formed in the top of the cap <NUM>, and is for providing electrolyte <NUM> within the cap <NUM>. A void space <NUM> may be formed above the electrolyte <NUM> if a liquid electrolyte is used, and an epoxy glue cap is provided over the opening <NUM>. As noted above, other sealing techniques may be used to cover the opening <NUM>.

<FIG> shows a top view of the electrochemical sensor <NUM> with various components removed. In particular, the electrode 210A, the electrolyte <NUM> and the cap <NUM> are not shown. The broken line B-B represents the cross-section of <FIG>. As can be seen in <FIG>, opening <NUM> and hole 209A are formed in passivation layer <NUM>. Additionally, holes 209B, 209C, 209D, 209E, 209F are also formed in passivation layer <NUM>.

<FIG> also shows the same top view of electrochemical sensor <NUM>. In this example, electrode 210A is shown formed over opening <NUM> and hole 209A. Additionally, counter electrode 210B and reference electrode 210C are also shown.

<FIG> shows a further alternative embodiment in accordance with the present disclosure. Most of the structure is the same as that shown in <FIG> and <FIG>. An electrochemical sensor <NUM> includes a substrate <NUM> having a plurality of microcapillaries <NUM> formed therein. An insulating layer <NUM> is formed on a top surface of the substrate <NUM>, and has an electrode opening <NUM> formed in a position aligned with the micro capillaries <NUM>.

Conductive tracks 305A, 305B are formed on the insulating layer <NUM> and are attached thereto by adhesion layers 306A, 306B. The purpose of these tracks is to connect electrodes (not shown in <FIG>) to external connections (also not show in <FIG>). These tracks will be made clearer below in connection with <FIG>. A conductive pad 305C is also formed on the insulating layer <NUM>, and is attached to the insulting layer using adhesion layer 306C. Conductive pad 305C is formed around the opening <NUM> in the insulating layer <NUM>. This will be shown more clearly below in connection with <FIG>.

A passivation layer <NUM> is formed over the insulating layer <NUM> and the conductive tracks 305A, 305B and conductive pad 305C. The passivation layer <NUM> is not formed over the entire conductive pad 305C, and as such has an opening <NUM> formed therein. The passivation layer <NUM> overlaps the edges of the pad 305C by around <NUM> to <NUM>. The opening <NUM> is aligned with, but wider than, the opening <NUM>. A working electrode 310A is formed in the openings <NUM> and <NUM>, as well as over the conductive pad 305C.

A cap <NUM> is formed over the electrode 310A (and the other electrodes that are not shown), and is attached to the passivation layer <NUM> using adhesive 312A and 312B, or using wafer bonding. A hole <NUM> is formed in the top of the cap <NUM>, and is for providing electrolyte <NUM> within the cap <NUM>. A void space <NUM> may be formed above the electrolyte <NUM> if a liquid electrolyte is used, and an epoxy glue cap <NUM> is provided over the opening <NUM>.

<FIG> shows a top view of the electrochemical sensor <NUM> with various components removed. In particular, the electrodes 310A, 310B, 310C, the electrolyte <NUM> and the cap <NUM> are not shown. The broken line C-C represents the cross-section of <FIG>. The conductive pad 305C is formed towards the middle of the sensor <NUM>. The conductive pad 305C is formed around the opening <NUM> in the insulating layer <NUM>. As such, the microcapillaries <NUM> may be seen in the substrate <NUM>. The sensor <NUM> also includes two further conductive pads 305D and 305E. These pads are for having electrodes formed thereon. The sensor <NUM> also includes three external connection pads 305F, <NUM>, <NUM>. These pads are connected to respective electrode pads by respective tracks 305A, 305I and 305B. These tracks are shown as hashed lines as they are not visible through the passivation layer <NUM>. The passivation layer <NUM> is formed over the insulating layer <NUM>. The passivation layer covers the tracks 305A, 305I, and 305B, but does not cover the conductive pads 305C, 305D, 305E, 305F, <NUM>, <NUM>.

<FIG> also shows the same top view of electrochemical sensor <NUM>. In this figure, electrode 310A is shown formed over openings <NUM>, <NUM>, and conductive pad 305C. Additionally, counter electrode 310B and reference electrode 310C are also shown.

<FIG> shows an example of an electrochemical sensor <NUM> in accordance with a further embodiment in the present disclosure. Most of the structure is the same as that shown in <FIG>. The electrochemical sensor <NUM> includes a substrate <NUM> having a plurality of microcapillaries <NUM> formed therein. An insulating layer <NUM> is formed on a top surface of the substrate <NUM>, and has an electrode opening <NUM> formed in a position aligned with the micro capillaries <NUM>. Conductive pads <NUM> are formed on the insulating layer <NUM> and are attached thereto by adhesion layer <NUM>.

A passivation layer <NUM> is formed over the insulating layer <NUM>, and has an opening <NUM> formed therein. The opening <NUM> is aligned with the electrode opening <NUM>. In addition, the passivation layer includes a hole 409A aligned with the conductive pad 405A. A working electrode 410A is formed in the openings <NUM> and <NUM>. A counter electrode 410B is formed in the opening 409A. A cap <NUM> is formed over the electrodes <NUM>, and is attached to the passivation layer <NUM> using adhesive 412A and 412B, or using wafer bonding processes). A hole <NUM> is formed in the top of the cap <NUM>, and is for providing electrolyte <NUM> within the cap <NUM>. A void space <NUM> may be formed above the electrolyte <NUM>, when liquid electrolytes are used, and an epoxy glue cap <NUM> is provided over the opening <NUM>. As with sensor <NUM>, sensor <NUM> may include an additional reference electrode formed over an additional hole in the passivation layer.

In contrast with the previous embodiments, recesses 417A and 417B are formed in the substrate <NUM>, the insulating layer <NUM>, and the passivation layer <NUM>. The electrolyte <NUM> fills recesses 417A and 417B and part of the cap volume. In <FIG>, the substrate <NUM> forms the walls of the recesses 417A, 417B. As an alternative, the walls and base of the recesses 417A, 417B may be covered in a layer of insulating material. This layer may be the insulating layer <NUM>. 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> shows a top view of the electrochemical sensor <NUM> shown in <FIG>. Here, the recesses 417A and 417B may be seen. The recesses together form a microwell <NUM>. The microwell <NUM> forms a 'C' shape around the top, left and bottom sides of the electrodes <NUM>. Other shapes may be used depending on the design of the sensor.

<FIG> show the function of the microwell <NUM>. In <FIG>, electrochemical sensor <NUM> is upright, and the electrolyte <NUM> fills the microwells such that the electrolyte also covers the electrode 410A (as well as electrodes 410B and 410C which are not shown).

In <FIG>, the electrochemical sensor <NUM> is upside down. As can be seen, the electrolyte <NUM> now fills the cap <NUM>, and a void space <NUM> is formed in the microwells <NUM>. As such, the electrodes 410A, 410B and 410C are completely covered by the electrolyte <NUM>.

In <FIG>, the electrochemical sensor <NUM> is shown on its side. Here, the electrolyte <NUM> fills the microwell <NUM> and the cap <NUM> such that a void space is provided in a portion of the cap <NUM> and the void space <NUM>. As such, the electrodes <NUM> are completely covered by electrolyte <NUM>. 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.

<FIG> show a substrate <NUM> in accordance with a further embodiment of the present disclosure. <FIG> shows a cross-section through a substrate <NUM>. <FIG> shows an end-view of the substrate <NUM>. The substrate <NUM> is the same as the substrate <NUM> in most respects. In particular, the substrate <NUM> includes microcapillaries <NUM>. However, in contrast to substrate <NUM>, the substrate <NUM> includes a trench <NUM>. The trench is formed in an underside of the substrate <NUM>, and extends from the microcapillaries <NUM> to an edge of the substrate <NUM>. The purpose of the trench is to enable environmental gases to reach the micro capillaries in the event the substrate <NUM> is placed on a solid surface. This would enable several sensors to be stacked, as shown in <FIG>. Alternatively, this would enable the sensor to be placed on another die, such as a microcontroller.

<FIG> shows multiple caps formed in a wafer. In the process described above in connection with <FIG>, 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> shows a wafer <NUM> which includes a plurality of caps <NUM>. Several caps are formed in each of a number of parallel lines.

If forming the caps <NUM> from 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 caps <NUM> from 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.

<FIG> show cross sections through a wafer. In <FIG>, the wafer <NUM> includes a plurality of caps <NUM>. Each cap includes two holes 602A, 602B. In this example, the caps are adjacent each other, with no spacing between the caps. In <FIG>, the same wafer is shown but with the cross-section orthogonal to the one shown in <FIG>. The wafer <NUM> includes a plurality of caps <NUM>. Each cap includes a two holes, but only one of the holes 602A can be seen in this cross-section. In this cross-section, the caps include a spacing section <NUM>, between each cap. In each case, the positioning <NUM> of 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 aforementioned sensors may be provided. Alternatively an integrated circuit comprising the above-described sensor array may be provided.

In the above-described embodiments, a <NUM>-electrode system has been described. The disclosure is also applicable to <NUM>-electrode systems and systems with more than <NUM> 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> is a top view of certain components of a sensor <NUM> in 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 sensor <NUM> is the same as in the previous embodiments, and is not shown here in detail. The sensor <NUM> includes a plurality of capillaries <NUM> which 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 electrode 704A, a counter electrode 704B and a reference electrode 704C. The working electrode 704A is circular, and is arranged above the capillaries <NUM> such that the circle formed by the working electrode 704A is co-axial with the circle formed by the capillaries <NUM>. The working electrode 704A has a diameter that is greater than the diameter of the circle formed by the capillaries <NUM>, in order that sufficient space is provided to form connections with the conductive tracks.

The counter electrode 704B and the reference electrode 704C are both semi-annular in shape, and formed on either side of the working electrode 704A. 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 electrode 704B and the reference electrode 704C 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 electrode 704A is in contact with the upper surface of the silicon substrate, in the area of the capillaries <NUM>. 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 sensor <NUM> includes conductive tracks 706A, 706B and 706C. The conductive tracks are shown in broken lines, as they are all positioned below the passivation layer. Conductive track 706A is for connecting the working electrode 704A. The conductive track includes a ring-shaped portion, which is located around the capillaries <NUM>, but within the outer edge of the working electrode 704A. The ring-shaped portion is co-axial with the working electrode 704A. A ring-shaped opening is formed in the passivation layer, and is aligned with the ring-shaped portion of the conductive track 706A, in order to allow the working electrode 704A to connect to the conductive track 706A. A rectangular connecting portion of track 706A is formed at the bottom edge of the ring-shaped portion, to provide a connection to external circuitry.

Conductive tracks 706B and 706C are formed partially underneath counter electrode 704B and reference electrode 704C 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 tracks 706B and 706C to connect to the working electrode 704B and reference electrode, respectively. These openings are similar in size and shape to the semi-annular portions of the conductive tracks 706B and 706C. In a similar manner to the conductive track 706A, the conductive tracks 706B and 706C 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> shows the sensor <NUM>. In this embodiment, the sensor <NUM> includes all of the components shown in <FIG>, and additionally includes shielding tracks 708A, 708B and 708C. 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 sensor <NUM> from electromagnetic interference. The shielding tracks may be used with any of the above-described embodiments.

<FIG> shows a sensor package <NUM> in accordance with a further embodiment of the disclosure. In this embodiment, the sensor package includes two sensors 800A and 800B. The working electrodes 804A-<NUM> and 804A-<NUM> are semicircular, rather than circular, and are positioned adjacent each other. Two groups of capillaries 802A and 802B are formed under each respective working electrode. Each working electrode is connected to a respective conductive track 806A-<NUM> and 806A-<NUM>. The conductive tracks include semi-annular portions, corresponding to the semicircular electrodes, rather than ring-shaped portions.

The sensor package <NUM> also includes two counter electrodes 804B-<NUM> and 804B-<NUM>. 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 track 806B-<NUM> and 806B-<NUM>. 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 electrode 804C, which is the same size and shape as the reference electrode 704C. The reference electrode is shared by both sensors 800A and 800B and is connected to conductive track 806C.

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> is a sensor package <NUM> in accordance with a further embodiment of the disclosure. Sensor package <NUM> is the same as sensor package <NUM> except for the counter electrodes. In sensor package <NUM>, both sensors 900A and 900B share the same counter electrode 904B. Sensor package <NUM> also includes working electrodes 904A-<NUM>, 904A-<NUM> and reference electrode 904C. The conductive tracks include two tracks 906A-<NUM>, 906A-<NUM> for the working electrode, one track 906B for the counter electrode 904B, and one track 906C for the reference electrode 904C. 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.

Claim 1:
An electrochemical sensor comprising:
a substrate having a first upper surface and a second lower surface, and also having a plurality of microcapillaries formed therein, the microcapillaries extending from the first surface to the second surface;
an insulating layer, formed on or adjacent the first surface of the substrate, the insulting layer having an opening aligned with the plurality of microcapillaries and extending through the insulating layer;
at least two electrodes, at least one the electrodes being a porous electrode formed in the opening in the insulating layer and a portion of the at least one porous electrode is formed on or adjacent the first surface of the substrate in an area defined by the opening in the insulating layer;
one or more conductive tracks formed over the insulating layer, extending onto side-wall of the opening in the insulating layer and/or a side-wall of the at least one hole formed in the substrate,
a passivation layer, formed over the insulating layer and the conductive tracks;
an electrolyte comprising a liquid solution, conductive polymer or hydrogel formed as a layer over the electrodes and the insulating layer;
a cap formed over the insulating layer, wherein the electrolyte is provided within the cap, and at least partially fills the cap; and
at least one recess, formed in the substrate, in communication with the space defined by the cap, wherein the electrolyte is also provided within the recess, wherein the sensor is arranged such that the at least one porous electrode is exposed to the environment through the plurality of microcapillaries.