METHOD FOR MANUFACTURING AN ELECTROCHEMICAL GAS SENSOR

Abstract

In a method for manufacturing an electrochemical gas sensor for sensing a target gas, a semi-manufactured gas sensor is provided. The semi-manufactured gas sensor comprises a substrate supporting an arrangement comprising a thin film of a thickness s≤5 pm arranged between a sensing electrode configured to chemically interact with the target gas and a reference electrode facing the substrate. The thin film is an electronically non-conducting and ionically non-conducting ceramic or glass. The arrangement then is heated to an annealing temperature for irreversibly turning the thin film into an ionic conductor by incorporating mobile ions released from the sensing electrode in response to the heating.

Claims

1. A method for manufacturing an electrochemical gas sensor for sensing a target gas, comprising the steps of providing a substrate supporting an arrangement comprising a thin film of a thickness s≤5 pm arranged between a sensing electrode configured to chemically interact with the target gas and a reference electrode facing the substrate, wherein the sensing electrode is porous at least for the target gas, wherein the thin film is an electronically nonconducting and ionically non-conducting ceramic or glass, and heating the arrangement to an annealing temperature for irreversibly turning the thin film into an ionic conductor by incorporating mobile ions released from the sensing electrode in response to the heating.

2. The method according to claim 1, wherein an adhesion promoter is provided between the thin film and the sensing electrode.

3. The method according to claim 2, wherein one or more areas between the thin film and the sensing electrode are provided absent the adhesion promoter representing areas permeable for the ions released in the sensing electrode.

4. The method according to claim 1, wherein the thin film is selected from the group consisting of Si.sub.xO.sub.y, Si.sub.xN.sub.y, SiO.sub.xN.sub.y, Al.sub.2O.sub.3, BaZrO.sub.3, and LaAlO.sub.3, or any combination thereof.

5. The method according to claim 1, wherein the annealing temperature is greater than or equal to 300° C.

6. The method according to claim 1, wherein the sensing electrode comprises a compound selected from the group consisting of sodium salts, lithium salts, sodium hydroxides, and lithium hydroxides, preferably in a mixture with a noble metal powder, in particular a gold powder, in particular nanoparticles, in particular gold nanoparticles.

7. The method according to claim 1, wherein the thickness s of the thin film is between 80 nm and 500 nm.

8. The method according to claim 1, wherein a heater is provided in or on the substrate, and wherein the step of heating the arrangement to the annealing temperature is effected by the heater.

9. The method according to claim 1, wherein the sensing electrode is configured to chemically interact with the target gas by comprising a salt that is characterized as a reaction product of at least a mobile ion released form the sensing electrode and the target gas.

10. The method according to claim 2, prior to providing the substrate supporting the arrangement, building the arrangement on the substrate by: depositing reference electrode material on the substrate for building the reference electrode, depositing thin film material onto the reference electrode for building the thin film, depositing an adhesion promoter onto the thin film, and depositing sensing electrode material onto the adhesion promoter and areas of the reference electrode free from the adhesion promoter material if any, for building the sensing electrode.

11. The method according to claim 2, wherein the adhesion promoter comprises titanium and gold, or chromium and gold, in a form of a bilayer.

12. The method according to claim 11, comprising a diffusion barrier in between the bilayer comprising a transition metal nitride or a transition metal silicon nitride.

13. The method according to claim 2 wherein the adhesion promoter comprises one or more PVD-deposited thin films.

14. The method according to claim 2, wherein a thickness of the adhesion promoter is less than 10 nm effecting the adhesion promoter to be permeable for the mobile ions releases from the sensing electrode.

15. The method according to claim 2, wherein one or more areas between the thin film and the sensing electrode are provided absent the adhesion promoter representing areas permeable for the ions released in the sensing electrode, and wherein a thickness of the adhesion promoter is less than 10 nm effecting the adhesion promoter to be permeable for the mobile ions released from the sensing electrode.

16. The method according to claim 4, wherein the reference electrode consists of or comprises a noble metal, in particular platinum.

17. The method according to claim 7, wherein the thin film is a CVD-, ALD-, or PVD-deposited thin film.

18. The method according to claim 9, wherein the salt is selected to comprise a carbonate anion and the target gas is CO.sub.2, and/or the salt is selected to comprise a sulfate anion and the target gas is SO.sub.x, and/or the salt is selected to comprise a nitrate/nitrate anion and the target gas is NO.sub.x.

19. The method according to claim 10, further comprising structuring the deposited adhesion promoter thereby effecting one or more areas on the reference electrode absent adhesion promoter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] 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:

[0048] FIG. 1 illustrates a method for manufacturing an electrochemical gas sensor according to an embodiment of the present invention, in cut views of the electrochemical sensor;

[0049] FIG. 2 illustrates an electrochemical gas sensor according to an embodiment of the present invention, in a cut view;

[0050] FIG. 3 illustrates three different top views on an adhesion promoter structure as used in an electrochemical gas sensor and a method for manufacturing an electrochemical gas sensor according to embodiments of the present invention;

[0051] FIG. 4 illustrates an electrochemical gas sensor including an adhesion promoter according to FIG. 3a), and

[0052] FIG. 5 shows data of an indoor field measurement of CO2 measured with a calibrated electrochemical gas sensor in accordance with an embodiment of the present invention.

MODES FOR CARRYING OUT THE INVENTION

[0053] FIG. 1 illustrates a method for manufacturing an electrochemical gas sensor according to an embodiment of the present invention, in two cut views a) and b) of the electrochemical sensor. In cut view 1a), a semi-manufactured electrochemical gas sensor is shown, that may be provided for final processing. The semi-manufactured electrochemical gas sensor comprises a substrate 5. The substrate 5 preferably is a semiconductor substrate. The substrate 5 supports an arrangement comprising a reference electrode 3 deposited onto the substrate 5, and hence facing the substrate 5. For this purpose, reference electrode material such as platinum is deposited on the substrate 5 and the material then is structured to form the reference electrode 3. On top of the reference electrode 3, a thin film 2 is deposited which in the present example not only fully covers the reference electrode 3 but also a small portion of the substrate 5. Hence, the reference electrode 3 is fully encapsulated by the thin film 2 and the substrate 5. This avoids the reference electrode 3 being in contact with the sensing electrode 1 which is deposited onto the thin film 2, and also parts of the substrate 5. Hence, the reference electrode 3 is out of contact with the ambient and out of contact with the sensing electrode 1, given that the substrate 5 preferably is gas tight.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] FIG. 2 illustrates an electrochemical gas sensor according to an embodiment of the present invention, in a cut view. The illustration may either show the finally processed gas sensor, hence comprising the ion-conducting thin film 2′, or may show the semi-manufactured gas sensor prior to the annealing step, and hence comprising the non-ion-conducting thin film 2.

[0058] 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.

[0059] In present FIG. 2, the adhesion promoter 6 is not illustrated in real scale with respect to the other elements. In this example, the adhesion promoter 6 is manufactured as a thin adhesion layer that it is permeable for the ions envisaged to be released from the sensing electrode 1. Given that there are no gaps provided in the adhesion layer 1 for allowing direct contact between the sensing electrode 1 and the thin film 2, the adhesion layer 6 itself must be permeable since on the other hand the entire surface of the thin film 2 not in touch with the substrate 5 is covered by the adhesion layer 6.

[0060] 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 FIG. 2 such package in form of a housing is illustrated by 51. The opening in the package 51 preferably is spanned by a filter 52 which at least allows the target gas, and more preferably also gaseous medium to be investigated including the carrier gas to pass. Hence, the filter 52 may prevent particles and/or liquids to reach and impact the arrangement.

[0061] FIG. 3 illustrates three different top views on an adhesion promoter 6 as used in an electrochemical gas sensor and a method for manufacturing according to embodiments of the present invention. In any of the diagrams a) to c), the area including an adhesion promoter 6 is illustrated in grey, while gaps in the adhesion promoter 6, i.e. areas where no adhesion promoter 6 is provided and hence the surface of the underlying thin film 2/2′ is accessible from the top, are illustrated in white and are denoted by the reference sign 7. In those areas 7, the sensing electrode material applied on top of the adhesion promoter 6 gets into direct contact with the thin film 2 exposed in these areas 7.

[0062] The adhesion film structure shown in diagram 3c) represents a sufficient thin adhesion layer 6 as is used in the gas sensor of FIG. 2. The ion permeability shall is illustrated in this diagram by multiple small holes in the adhesion layer 6, which is a simple graphical representation of the ion permeable property of the adhesion layer 6.

[0063] 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.

[0064] A cut view of the gas sensor comprising the processed adhesion layer 6 as shown in diagram 3a) is illustrated in FIG. 4. The three areas 7 absent of adhesion promoter material are visible in this cut view.

[0065] 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.

[0066] FIG. 5 shows data of an indoor field measurement of CO2 measured with a calibrated electrochemical gas sensor in accordance with an embodiment of the present invention. A calibration function (transforming a raw output voltage into a CO2 concentration) is determined beforehand in a gas mixing system. During a time period of four days the CO2 level in a meeting room was measured by said gas sensor (dashed line) and an infrared-optical (non-dispersive infrared, NDIR) reference device (thin solid line). Agreement to within 10% is found.