Patent Application
20230358704
This application claims the priority of European patent application 20 197 295.7, filed on Sep. 21, 2020, the disclosure of which is incorporated herein by reference in its entirety.
The present invention relates to a method for manufacturing an electrochemical gas sensor.
An electrochemical gas sensor includes at least two electrodes and an electrolyte between the electrodes. A first of the electrodes is referred to as sensing electrode or working electrode. It is in contact not only with the electrolyte but also with a medium to be investigated, e.g. ambient air. The medium may hold a target gas which target gas is desired to be detected by the electrochemical gas sensor as to its presence and/or concentration in the medium. A second of the electrodes is referred to as reference electrode. The reference electrode is in contact with the electrolyte but is out of contact with the medium to be investigated. When approaching the sensing electrode, an oxidation or reduction reaction involving the target gas in the medium takes place at the sensing electrode. Ions resolved from the sensing electrode in response to the electrochemical reaction with the target gas transit the ion-conducting electrolyte towards the reference electrode and chemically interact there. These ions are called mobile ions. An electrical current and/or voltage is generated between the sensing electrode and the reference electrode, in response to the electrochemical reactions.
A majority of electrochemical gas sensors are based on a solid, ion-conducting electrolyte that separates the target gas from the reference electrode. All commercially available electrochemical CO2 sensors are based on solid electrolytes. They are based on a sintered, solid state ion conductor as electrolyte with a thickness of several 100 μm.
Different solid state electrolytes are available, but in general the choice is limited. All of the currently used and investigated solid state electrolytes suffer from at least one, but frequently several of the following drawbacks:
As to the thermal incompatibility of the various components in the sensor system, it is noted that all of the commercially available products use an alumina substrate which typically is dominant in terms of thermal expansion coefficients. As to the incompatibility with established chemical and physical deposition processes, it is noted that due to the complexity of the stoichiometry of the known solid electrolytes it is very difficult to obtain the desired stoichiometry after depositing a thin film by CVD, ALD or PVD. In many applications, the mobile ion is lithium or sodium which have a higher diffusivity than other ions/atoms/molecules used in the solid electrolytes. Therefore, these ions are underrepresented in the thin film after deposition and/or after processing these thin films at temperatures of more than 600° C. because of a disadvantageous surface-to-volume ratio. Any of these drawbacks limits a degree of freedom in design significantly and has a direct impact on the sensor performance.
Accordingly, it is desired to provide an electrochemical gas sensor with a solid electrolyte avoiding one or more of the above drawbacks.
This problem is solved by a method for manufacturing an electrochemical gas sensor according to claim 1. The electrochemical gas sensor is configured to sense a target gas in a medium applied to the gas sensor. The medium may include a carrier gas, such as ambient air. The target gas preferably is one or more of CO2, O2, NOx, and SOx. Accordingly, the electrochemical gas sensor may be used for measuring a presence and/or a concentration of one or more of the listed target gases in a carrier gas.
In conventional solid state electrochemical gas sensors the electrolyte used is deposited as an ion-conducting thick film. I.e. the ion-conducting electrolyte material is deposited onto the reference electrode during manufacturing as a thick film of several 100 μm, and, of course, remains ion-conducting after manufacture of the gas sensor. Instead, now, a non-ion-conducting electrolyte material is deposited as a thin film onto the reference electrode, with a thickness s of the thin film with s≤5 μm, and even more preferably with s≤1 μm.
Hence, the conventional solid state, thick ion-conducting electrolyte material requiring cumbersome manufacturing and deposition techniques is replaced by a thin layer of a non-conducting—i.e. electronically and also ionically non-conducting—ceramic or glass, preferably deposited by one of CVD (Chemical Vapor Deposition), ALD (Atomic Layer Deposition) or PVD (Physical Vapor Deposition). By this means, one of the standard deposition processes can be used. Only after deposition of the ionically non-conducting material i.a. this material is turned into an ionically conducting electrolyte.
The following criteria result in the specified thickness of the thin film: The thin film preferably is thick enough to separate the reference electrode from the ambient atmosphere at operation temperature, which preferably is a temperature between 250° C. and 500° C. And the thin film preferably is thin enough to allow the mobile species, i.e. ions released from a sensing electrode, to traverse the thin film material during operation at elevated temperatures such as at the operation temperature.
The actual thickness strongly depends on the material selected for the thin film, and, even more important, depends on one or more of its conformal deposition ability, its density, its impurity content/defect density and therefore its exact stoichiometry.
Furthermore, it is also preferred to combine more than one layer of an initially non-ion-conducting material in order to tune the properties of the whole gas sensor system, which properties may include one or more of stress, adhesion, etc.
Accordingly, it is envisaged that in the method for manufacturing the gas sensor, a semi-manufactured gas sensor is provided including a non-ion-conducting thin film of the above thickness s. This semi-manufactured gas sensor comprises at least a substrate, a reference electrode, the non-ion-conducting thin film of glass or ceramic, and a sensing electrode, also referred to as working electrode. Hence, an arrangement comprising the thin film, the sensing electrode and the reference electrode is supported by the substrate with the reference electrode facing the substrate. Preferably, the reference electrode is completely encapsulated by the substrate and the thin film without any exposure to the ambient, and preferably, the thin film material is completely confined by the sensing electrode, the reference electrode, and the substrate. Instead, the sensing electrode is exposed to the medium to be investigated, e.g. to ambient air, in order to chemically interact with the target gas if present in the medium. The sensing electrode is porous. The porosity is such that the target gas can reach the thin film electrolyte. The aforementioned chemical reaction takes place at the interface of electrolyte and sensing electrode. Sometimes this interface is referred to as “triple phase boundary”, consisting of electrolyte, target gas, and sensing electrode. Hence, the sensing electrode may also be considered as permeable for at least the target gas.
The sensing electrode preferably comprises a salt the cation of which is the mobile ion in the operation of the gas sensor, and the reference electrode comprises a noble metal that can form chemical compounds with the mobile ions.
In a subsequent manufacturing step, i.e. at least after deposition of the arrangement including the reference electrode, the non-ion-conducting thin film and the sensing electrode, at least this arrangement is heated to or above an annealing temperature at which annealing temperature the thin film becomes irreversibly ion-conducting. The mechanism responsible for turning the thin film into an ion-conductor, in response to heating to the annealing temperature, may comprise diffusion and intercalation of mobile ions resolved from the sensing electrode into the thin film, and/or transformation of the thin film into a ion-conducting compound, and/or formation of a percolating path of ion-conducting compounds in the thin film, and/or formation of an interphase between the thin film and the reference electrode, and/or formation of an interphase between the thin film and the sensing electrode. After annealing—including a subsequent cooling down to ambient temperature—, the thin film irreversibly remains ion-conducting, i.e. it remains long-term ion-conducting during the envisaged lifetime of the gas sensor operation. Hence, a heating step is applied to a combination of the reference electrode, the originally non-ion-conducting thin film material which is a ceramic or a glass material, and which shall serve as electrolyte after annealing, and the sensing electrode. In response to the treatment with sufficient heat when arranged between the sensing electrode and the reference electrode, mobile ions released from the sensing electrode transit into the thin film material and make this material becomes ion-conducting. This process is also referred to as incorporation of the mobile ions into the thin film material. Afterwards, the thin film can serve as electrolyte in a gas sensor application.
Preferably, the annealing temperature is greater than or equal to 300° C., preferably greater than or equal to 400° C., preferably between 400° C. and 500° C. In one embodiment, an oven or any other heating means external to the gas sensor may generate the annealing temperature. In a different embodiment, a heater integrated in the gas sensor, and preferably arranged in or on the substrate of the gas sensor, may arrange for the annealing temperature. This heater preferably is a resistive heater in or on the substrate, and preferably is electrically controllable. The substrate preferably is a semiconductor substrate, such as a silicon substrate.
In both embodiments, at least the thin film, the reference electrode, and the sensing electrode are heated to at least the annealing temperature. However, more likely the entire gas sensor is heated to at least the annealing temperature. The annealing temperature may depend from the materials used for the thin film, the sensing electrode and the reference electrode. Summarizing, in the semi-manufactured gas sensor the thin film is an electronically non-conducting and ionically non-conducting ceramic or glass. However, after heating the semi-manufactured gas sensor to or above the annealing temperature, the thin film is turned into an ion-conducting state from the previous non-ion-conducting state by means of mobile ions being released from the sensing electrode in response to the heating/annealing of the arrangement.
According to another aspect of the present invention, an electrochemical gas sensor is provided as a result of the above manufacturing process. According to a further aspect of the present invention, an electrochemical gas sensor is provided independent from the above manufacturing method, which comprises a substrate and an arrangement supported by the substrate, which arrangement comprises a sensing electrode porous for the target gas and configured to chemically interact with the target gas, a reference electrode facing the substrate, and a thin film arranged between the sensing electrode and the reference electrode. The thin film—given its post-annealing status—is an ionically conducting ceramic or glass, for conducting ions released from and accepted by the sensing electrode in response to an electrochemical reaction with the target gas. A thickness s of the thin film is s≤5 pm, and even more preferably is s≤1 μm. In some embodiments, even a thickness s of the thin film between 80 nm and 500 nm can be achieved, and in particular between 100 nm and 400 nm, in particular by applying one of the depositing methods of CVD, ALD, or PVD.
Advantages of the method and the resulting electrochemical gas sensor include that the manufacturing process of a solid-state electrochemical gas sensor now is significantly simplified by giving more flexibility in the choice of materials used and by applying one of the established thin film deposition methods, including but not limited to CVD, ALD or PVD. This enables a high volume production with established manufacturing methods and compatibility with MEMS technology. The reduced thickness of the thin film glass or ceramic electrolyte compared to state of the art gas sensors allows lower operating temperatures during operation of the gas sensor, because, first, a diffusion length of ions resolved from the sensing electrode—such as of lithium ions or sodium ions or other ions—is much shorter than in thick film solid electrolytes in the state of the art. A diffusion length of less than 1 um may be achieved by the thin film compared to several 100 um in the thick film electrolytes in the state of the art. Second, the operating power can be reduced in view of the lower volume of the thin film to be heated in comparison with the conventional thick film structures. These structures e.g. were deposited by screen printing, drop coating or ink jetting. Additionally, those technologies have their limits in miniaturization, and structures with a lateral dimension<200 um are difficult to realize. In addition, annealing temperatures exceeding 500° C. by far had to be used.
Summarizing, the present electrochemical gas sensor is a robust solid state device with, still, very small, i.e. miniaturized structures, which are easier to manufacture by applying standard processes, which require lower annealing temperatures, and which make the total volume of the gas sensor smaller compared with state of the art gas sensors.
A material for the thin film preferably is selected from the group consisting of SixOy, SixNy, SiOxNy, Al2O3, BaZrO3, and LaAlO3, or any combination thereof. Hence, besides SixOy other intrinsically non-conductive materials, or a combination thereof could be used, such as Si3N4. Any of these materials were found to become ion-conducting in response to the application of heat when arranged between the sensing electrode and the reference electrode, i.e. in response to being exposed to a material specific annealing temperature, while initially these materials are non-ion-conducting.
The reference electrode preferably consists of or comprises a noble metal, in particular platinum or gold. The sensing electrode in turn is preferred to chemically interact with the target gas. For this reason, a material or compound is selected for the sensing electrode which includes a salt the anion of which is adapted to the target gas. This adaptation is to be understood in the following way. The salt of the sensing electrode preferably can be characterized or determined as a product of an electrochemical reaction involving only the target gas and the mobile ion, and optionally one or more of oxygen and water. This enables the gas sensor to convert an electrochemical reaction of the target gas with the material of the sensing electrode into an electrical measure such as the electric current between the sensing electrode and the reference electrode in response to the ion flow from the sensing electrode to the reference electrode through the thin film. Hence, the compound of the sensing electrode preferably is adapted to release the cations, wherein the compound is selected from the group consisting of sodium salts, lithium salts, sodium hydroxides, and lithium hydroxides. Preferably, the sensing electrode also comprises noble metal, in particular as a powder, in particular gold nanoparticles. The cations of these various salts were found to penetrate into the thin film made from ceramic or glass. For the specific target gas of CO2 the salt preferably is selected to comprise a carbonate anion. For the specific target gas of SOx the salt preferably is selected to comprise a sulfate anion. For the specific target gas of NOx the salt preferably is selected to comprise a nitrate/nitrite anion. Preferably, in the sensing electrode the ratio of noble metal to salt is such that after the annealing step the noble metal provides a percolation path for electrons from the thin film to a conductor contacting the sensing electrode. In a preferred embodiment of the sensing electrode, the noble metal comprises a gold powder and the salt comprises a lithium salt, and the ratio of gold powder to lithium salt is at least 10:1 by weight.
By decreasing the thickness of the electrolyte by three orders of magnitude compared to the state of the art, the roughness of its surface will decrease as well and consequently a direct adhesion of the sensing electrode to the electrolyte thin film may suffer. This is especially the case if primary particles used for manufacturing the sensing electrode are in the dimensional range of or even larger than the thickness/roughness of the thin film, and/or if the annealing temperature used is below a melting point of the used sensing electrode materials.
Further, the thinner the elements of a solid-state electrochemical gas sensors are, which elements may include a heater, the reference electrode, the solid electrolyte and the sensing electrode, the more likely buckling of the whole gas sensor may occur during heating and cooling periods in operation of the gas sensor. This results in stress between the different layers representing the above elements. This could cause a detachment of the different layers and a loss of electrical contact which typically may occur between the sensing electrode and the solid electrolyte.
These problems are solved by providing an adhesion promoter between the thin film and the sensing electrode, according to a preferred embodiment of the present invention. The adhesion promoter is configured to promote adhesion between the thin film electrolyte and the sensing electrode. The adhesion promoter preferably is deposited directly on the thin film ceramic or glass layer as a thin film adhesion layer or as multiple thin films, preferably by one of CVD, ALD or PVD deposition methods.
In a preferred embodiment, the adhesion promoter comprises titanium and gold, or chromium and gold. In a preferred embodiment, the adhesion promoter comprises a bilayer. Preferably, in case of the titanium and gold combination, and/or the chromium and gold combination, each combination in particular is provided in form of a bilayer. Preferably, in case of the bilayer, a diffusion barrier is comprised in between the bilayer. The diffusion barrier preferably comprises a transition metal nitride or a transition metal silicon nitride such as TiN, TaN, TaSiN layers, e.g.
Titanium preferably is deposited by means of PVD as a layer of a thickness between 5 nm and 20 nm. Gold preferably is deposited by means of PVD as a layer of a thickness between 140 nm and 160 nm, preferably 150 nm.
The adhesion promoter, and especially the materials introduced above, preferably shows one or more of a good adhesion to the thin film ceramic or glass, a good adhesion to the layer of the sensing electrode, a good permeability for the ion conducting species, such as Li+ or Na+ ions, but not limited, and optionally electrical conductivity. The permeability for the Li+ or Na+ ions can be reached by providing one or more areas between the thin film and the sensing electrode absent the adhesion promoter. These areas then represent areas permeable for the ions released from the sensing electrode. In a different embodiment, the thickness of the adhesion promoter is less than 10 nm effecting the adhesion barrier to be permeable for the ions released from the sensing electrode.
Summarizing, one of the following ways, or a combination thereof, can improve permeability for the ions released from the sensing electrode into the thin film:
The semi-manufactured gas sensor that will be exposed to at least the annealing temperature preferably is manufactured including the following steps: The substrate is provided. The arrangement is built on the substrate by first depositing a reference electrode material on the substrate. This reference electrode material is deposited on the substrate in form of a layer or a patch building the reference electrode. In a next step, a thin film material is deposited onto the reference electrode resulting in the thin film. Preferably, the thin film material is deposited such that the reference electrode is completely encapsulated by the thin film material and the substrate.
In case of an adhesion promoter, such adhesion promoter is deposited onto the thin film in a subsequent step. The adhesion promoter may be structured including etching, for example, in order to generate one or more surface areas on the thin film not covered by the adhesion promoter. In this one or more areas, the thin film is in direct contact with the sensing electrode to be built in the subsequent step, wherein sensing electrode material is deposited onto the adhesion promoter and the one or more areas of the thin film uncovered by the adhesion promoter material if any.
In a specific embodiment, the target gas is CO2 and hence the preferred embodiment refers to a CO2 sensitive electrochemical gas sensor working in an operating temperature range between 400 and 450° C. with a
Nernstian behaviour. First, a platinum reference electrode is deposited on a substrate. A platinum heater may be deposited in the same step on the substrate. The reference electrode preferably is coated with 200 nm SiOx in a silane and nitrous oxide based PECVD process resulting in the non-ion-conducting thin film of SiOx. Other thicknesses of the thin film may be applied according to the above ranges, e.g. between 40 nm and 1000 nm. Then, a mixture of Li2CO3:Au-powder in an weight ratio of 1:10 is deposited onto the thin film. Such electrochemical gas sensor shows a satisfying CO2 sensitivity.
According to another embodiment of the present invention, the gas sensor is further embodied to comprise a filter. The filter preferably is selected from the group consisting of an adsorption filter, in particular an activated carbon filter, and a diffusion filter, in particular a molecular sieve, and a catalytic filter. Hence, the filter may in particular work according to one of the listed principles. In any case, the filter is permeable for the target gas, while other substance, either gaseous or solid ones present in the carrier gas, are filtered. The filtering property may be selected according to the application of the gas sensor. E.g. in case the gas sensor is used in an environment with solid particles in the ambient air, such as particulate matter, the filter property preferably is adapted to filtering out the solid particles for avoiding demining the sensing electrode or other structures of the electrochemical gas sensor. For this purpose, the filter is either arranged on top of the sensing electrode, i.e. in contact with the sensing electrode, and preferably fully covers the surface of the sensing electrode. In a different embodiment, the filter is arranged distant from the sensing electrode, i.e. a gap is provided between the filter and the sensing electrode. In such embodiment the filter and any support thereof can take the shape of a cap, for example.
According to a further aspect of the present invention, a method is provided for operating a gas sensor. It may be preferred that the gas sensor is to be heated to enable an electrochemical reaction in response to a target gas meeting the sensing electrode. In such a scenario, in particular the sensing electrode and the thin film, but more likely the entire arrangement plus the substrate is heated to an operating temperature. The heating then allows to measure a voltage or current between the sensing electrode and the reference electrode dependent on a concentration of the target detected by the sensing electrode. In one embodiment, the operating temperature is between 250° C. and 350° C., and hence may be less than the annealing temperature applied during manufacturing. In a different embodiment, the operating temperature may be in the same range as the annealing temperature.
Other advantageous embodiments are listed in the dependent claims as well as in the description below. It is emphasized that any embodiment disclosed in relation to the method shall be disclosed in relation to the device as well, and vice versa.
The invention will be better understood and objects other than those set forth above will become apparent from the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:
In addition, a heater 4 is provided in the substrate 5. The heater 4 may be a resistive heater generating heat by applying an electric current. In case the substrate 5 is a semiconductor substrate, conductors and/or electronic circuitry may be provided in the substrate 5 for electrically connecting the heater 4, and for controlling the heater 4, including switching the heater on and off. Reference sign 11 refers to a conductor contacting the sensing electrode 1.
In the semi-manufactured state as shown in diagram 1a), the thin film 2 is electrically non-conducting, and in particular does not conduct ions that may be released from the sensing electrode material in response to an envisaged electrochemical reaction with the target gas.
In diagram 1b), the heater 4 is turned on, which is illustrated by the saw-toothed star. By doing so, the sensing electrode 1 and the thin film 2, and very likely all elements of the gas sensor, are heated to an annealing temperature T1. The thin film 2′ irreversibly remains in the ion-conducting state after the heat treatment. Hence, the annealing step prepares the thin film 2 to act as ion conductor for ions released from the sensing electrode 1 to the reference electrode 3 in response of the sensing electrode 1 being exposed to the target gas. The arrangement, including the ion-conducting thin film 2′ may also be heated later during operation for sensing the target gas. And it preferably is heated to an operating temperature T2 by means of the same heater 4 that is provided for the annealing temperature during manufacturing. The operating temperature T2 may be lower than the annealing temperature T1 in one embodiment.
In this embodiment, an adhesion between the sensing electrode 1 and the thin film 2/2′ is improved by a thin adhesion layer 6, also referred to as adhesion promoter 6. Such adhesion promoter 6 promotes a long term adhesion of the sensing electrode 1.
In present
In addition, a further optional feature is illustrated by dashed lines. In case of a finally processed gas sensor, it may contain a package or encapsulation with an opening for allowing access to the sensing electrode 1. In
The adhesion film structure shown in diagram 3c) represents a sufficient thin adhesion layer 6 as is used in the gas sensor of
In diagram 3a) instead, the areas 7 uncovered by the adhesion promoter 6 are rather large and are manufactured in one or more additional steps after applying the adhesion layer 6 onto the thin film 2/2′ thereby fully covering the thin film 2/2′, and being of a thickness typically not permeable for the ions released from the sensing electrode 1. Such adhesion layer may, after being deposited by one of CVD, ALD or PVD, be structured by means of lithography, for example, such that the areas 7 may thereafter be released from the adhesion promoter material and constitute access areas to the underlying thin film material. As a result, in the areas 7 free from the adhesion promoter 6, the sensing electrode 1 is in direct contact with the thin film 2/2′ such that ions released from the sensing electrode 1 enter the thin film 2 via the one or more areas 7.
A cut view of the gas sensor comprising the processed adhesion layer 6 as shown in diagram 3a) is illustrated in
Diagram 3b) illustrates another structured adhesion layer 6 with processed areas 7 absent the adhesion promoter material. These areas 7 are achieved by annealing the adhesion layer 7 resulting in small areas 7 uncovered by the adhesion promoter 6.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20197295.7 | Sep 2020 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/075611 | 9/17/2021 | WO |
