KR20050058795A - Solid electrolyte carbondioxide sensor and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a solid electrolyte carbon dioxide sensor and a method for manufacturing the same. Among the conventional solid electrolyte carbon dioxide sensors, the bulk sensor structure has to be installed separately from a heater and a thermometer, so that the structure is complicated and bulky, and power consumption is high in heating the sensor. In addition, the thick film or thin-film sensor structure has a heater and a thermometer mounted on the sensor, but because of the structure that heats all the sensor structure, including the solid reference electrode, there is a problem that the power consumption is large, the temperature gradient is high, the life is short, and the reliability is low. In view of the above problems, the present invention locates a carbonate solid electrolyte in a membrane layer formed on a substrate from which a portion of a region in which a sensor structure is to be located is removed, and arranges a solid reference electrode and a sensing electrode in a portion of the upper region. By placing a heater and thermometer insulated with an insulating layer on the top, the sensor structure is thermally insulated from the external environment while providing a solid electrolyte carbon dioxide sensor and a method of manufacturing the same, which effectively and uniformly absorbs the heat provided by the heater. It reduces the power consumption for the initial operation time and heat of the device, and improves the lifetime, characteristics and reliability of the device due to uniform heating.

Description

SOLID ELECTROLYTE CARBONDIOXIDE SENSOR AND MANUFACTURING METHOD THEREOF

The present invention relates to a solid electrolyte carbon dioxide sensor and a method of manufacturing the same. Particularly, a carbon dioxide sensor structure formed using a carbonate having good gas selectivity and manufacturing characteristics as a solid electrolyte is formed on a partially etched substrate and positioned in the etching region. The present invention relates to a solid electrolyte carbon dioxide sensor and a method of manufacturing the same so that the sensor structure is insulated from the external environment.

The rapid development into industrial society has led to the use of more fossil fuels, whereby environmental pollution has reached a level that threatens human survival. Among them, the increase of CO ₂ in the atmosphere is causing serious problems, especially as a major cause of global warming, and countries around the world are making efforts to reduce it in various ways. The measuring .the CO ₂ concentration in a row and, by seek control and troubleshooting of the source and the situation happens urgent to provide a protection and improvement of the air and the working environment, measuring the concentration of CO ₂ as a primary task for this There is a great need for the development of devices.

CO ₂ is a chemically very safe gas in the atmosphere, and its concentration is difficult to measure. Optical sensors are most often used to detect CO _{2. In} this method, light of a specific wavelength of emitted light (laser) is absorbed by CO ₂ in the air, which reduces the intensity of light. The amount of CO ₂ is measured by detecting the amount. This device has the advantage of excellent selectivity, quantification and reproducibility, but requires a closed space for measurement and has a problem of being bulky and very heavy due to the physical size of components and filters. In particular, since the driving unit and the measuring device are very expensive, and the configuration of the processing unit for the control is complicated, the overall measuring equipment is inevitably expensive, and its use is not widely used despite its wide variety of uses.

As another method for measuring CO ₂ concentration, a semiconductor type gas sensor using a semiconductor compound such as SnO ₂ or TiO ₂ is used. It is a principle to measure the concentration. In this case, but the advantage of a sensor manufactured in thin-film devices form are possible, since it is difficult to distinguish between the gas particles of different types to be adsorbed there is disadvantage that the gas selectivity significantly less using the equipment for measuring and selects only CO ₂ Difficult to do

Therefore, in recent years, interest in electrochemical gas sensors using a solid electrolyte has been amplified. Since the sensor using the solid electrolyte material is measured through an electrochemical reaction, the gas selectivity that can selectively detect only a specific gas is excellent, and the gas concentration can be quantitatively measured.

There are two kinds of electrochemical carbon dioxide gas sensors. The first is to use a sodium ion (Na +) conductor material called Nasicon (Na ₃ Zr ₂ Si ₂ PO ₁₂ ) or beta alumina (β-alumina: Na ₂ O · χ Al ₂ O ₃ ) as a solid electrolyte. Due to its characteristics, Nashikon is used the most. However, since Nashicon is a material containing five elements, there are many manufacturing difficulties, such as precisely controlling the content of each element. In addition, since the sintering temperature is about 1000 ° C. Follow.

Second, alkali metal carbonates such as Na ₂ CO ₃ , Li ₂ CO ₃ , and K ₂ CO ₃ are used as solid electrolytes, and ionic conduction is performed by alkali ions generated by decomposition of carbonates in the solid electrolyte. Among these, taking Li ₂ CO ₃ as an example, carbonates are decomposed as follows.

Li ₂ CO ₃ → 2Li ⁺ + CO ₃ ^2-

However, the carbonate solid electrolyte has a low ion conductivity, low sensitivity to carbon dioxide gas, and has a structure that has a lot of pores during sintering, so that the mechanical strength is low, Na ₂ CO ₃ , K ₂ CO ₃ carbonate is water Absorption of the carbon dioxide gas is well absorbed due to the deterioration of the characteristics there is a problem that the use is very limited. However, unlike these, since Li ₂ CO ₃ has a strong resistance to water, many efforts have been made to overcome the disadvantages related to sensitivity and mechanical strength by using it.

As a result, it has been found that the addition of Li ₃ PO ₄ to Li ₂ CO ₃ can increase the ionic conductivity, and it is also known that the addition of Al ₂ O ₃ can improve the mechanical strength (solid Solid State Ionics 100 (1997) see pages 275–281). CO ₂ sensors have emerged using alkali carbonates as solid electrolytes.

1 is a cross-sectional perspective view showing the structure of a carbon dioxide sensor for a solid electrolyte using a conventional reference gas, in which a reference gas (O ₂ , CO ₂ ) is used to detect CO ₂ using an alkali metal carbonate as a solid electrolyte.

This is because the sensing electrode (Pt) (2) is formed on the solid electrolyte (Li2CO3) (1), the electrode (Pt) (3) is formed on the bottom, and the space is enclosed in the lower electrode (3) As a method of providing a reference gas to be provided in the above, its configuration and operation principle are as follows.

CO ₂ [2], O ₂ [2], sensing electrode ∥ solid electrolyte ∥ reference electrode, O ₂ [1], CO ₂ [1]

This simply shows the configuration shown, [1] means the partial pressure of the reference gas side, [2] means the partial pressure of the gas to be measured. In the above configuration, the reaction at the reference electrode is as follows, taking Li ₂ CO ₃ as an example.

^{Li2CO3 → 2Li + + 2e - +} CO 2 [1] + ½ O 2 [1]

The reaction at the sensing electrode is as follows.

_{^{2Li + + 2e - + CO 2}} [2] + ½ O 2 [2] → Li 2 CO 3

The two responses can be expressed as one:

CO ₂ (2) + ½ O ₂ [2] → CO ₂ [1] + ½ O ₂ [1]

The output voltage is generated by the above chemical reaction, and this generated voltage can be expressed as follows by substituting Nernst equation.

EMF = E ^0- (RT / 2F) × ln {P CO ₂ [2] P ½ O ₂ [2] / [P CO ₂ [1] P ½ O ₂ [1]]}

Where EMF is the electromotive force, E ^o is a constant that does not change to the electromotive force when both reactants and products are in standard state, F is the Faraday constant, P CO ₂ , PO ₂ are the CO ₂ and O ₂ Indicates partial pressure. The values of P CO ₂ [1] and PO ₂ [1] are already defined values. If PO ₂ [2] is set to a fixed value, the electromotive force changes according to the partial pressure of CO ₂ . Therefore, the partial pressure of CO ₂ can be known by measuring electromotive force.

However, when the reference gas is used as described above, the structure becomes complicated, and the actual implementation is difficult because the partial pressure of oxygen and carbon dioxide does not change in the reference gas.

As a result, much research has been conducted on the method of not using a reference gas, and a structure using a solid reference electrode having a structure as shown in FIG. 2 has been proposed (Electrochemical and Solid-State Letters, 2). (4) (1999) pages 201-204, Sensors and Actuators B76 (2001) pages 594-599). In addition to LiMn ₂ O ₄ , a Li oxide such as LiCoO ₂ or a pervskite-type oxide such as La _0.9 Sr _0.1 MnO ₃ may be used as the solid reference electrode.

As illustrated, the CO ₂ sensor structure forms a solid reference electrode 11 under the solid electrolyte 10, gold electrodes 12 and 13 on the upper and lower portions of the structure, and the reference electrode 11. The ceramic seal 14 is formed so that) is not exposed. The case where the used solid reference electrode is used as LiMn ₂ O ₄ will be described as an example.

First, the reaction at the reference electrode is as follows.

_{2LiMn 2 O 4 → 2Li 1-} χ Mn 2 O 4 + 2χLi + + 2χe -

The reaction at the sensing electrode is as follows.

_{^{2χLi + + 2χe - + χCO 2}} + ½χO 2 → χLi 2 CO 3

 If the above reaction is expressed as one, it is as follows.

2LiMn ₂ O ₄ + χCO ₂ + ½χO2 → χLi ₂ CO ₃ + 2Li ₁ -χMn ₂ O ₄

The output voltage is generated by the above chemical reaction, and this generated voltage can be expressed as follows by substituting the nonnest equation.

EMF = E ⁰ + (RT / n ₁ F) × ln (P CO ₂ ) + (RT / n ₂ F) × ln (PO ₂ )

Where n ₁ and n ₂ are the number of electrons reacting with CO ₂ and O ₂ , respectively. Therefore, the partial pressure of CO ₂ can be known by measuring electromotive force.

In order to measure the partial pressure of CO ₂ using the solid electrolyte described above, in order to increase the ionic conductivity, Since the driving temperature must be guaranteed, in order to realize this in the form of a sensor, a heater is required to heat the sensor.

The CO ₂ sensor using the solid reference electrode as shown in FIG. 2 is used in the bulk type shown in FIG. 3 or the thin film and thick film type sensor structures shown in FIG. That is, currently used thin film element type CO ₂ sensor has the structure of FIG. 3 or 4.

First, referring to the bulk sensor of FIG. 3, the LiMn ₂ O ₄ solid reference electrode 16 is positioned below the bulk solid electrolyte 15, the electrodes 18 are formed on the upper and lower sides thereof, and then connected to an external electrode. Seal with (17). Here, since the heater 20 for increasing the ion conductivity of the solid electrolyte 15 must be separately installed outside the sensor, the structure becomes complicated. In addition, a lot of power and time are consumed to heat the bulk sensor structure, and the electromotive force measured by the sensor may be different depending on the heated temperature even if the concentration of the gas to be measured is the same. (21) should be installed separately from the outside.

4 as having a structure in which a heater and a thermometer at the same time in FIG. 3, the sol on the substrate 28, such as alumina-using a gel process or a spin coating method such as a thick film or thin film forming method LiMn ₂ O ₄ After forming the solid reference electrode 26 and the solid electrolyte 27, the electrode 27 is disposed on the solid reference electrode 26. In addition, since the heater and the thermometer 29 are implemented by placing a metal wire such as platinum under the substrate 28, the structure may be simplified. The metal wire thermometer measures the resistance change by using the metal wire resistance change characteristic according to the temperature change so that the temperature change can be known.

However, even in the most advanced thick-film and thin-film solid electrolyte CO ₂ sensors, as shown, a heater is formed under the substrate, so that thermal energy from the heater passes through the substrate 28 and the solid reference electrode 26. Since it reaches (25), a lot of power and initial driving time are consumed, and heat is easily spread to the part where the sensor is fixed and mounted, so that the temperature gradient, which means the imbalance of the temperature distribution, is high, and the device deteriorates and reliability is increased. reliability is deteriorated.

As described above, since the bulk sensor structure of the conventional solid electrolyte carbon dioxide sensor has to separately install a heater and a thermometer, the structure is complicated and bulky, and there is a problem in that power consumption is high in heating the sensor. Although the thermometer is mounted on the sensor, the structure of heating all the sensor structures including the solid reference electrode has a problem that the power consumption is large and the temperature gradient is high, so the life is short and the reliability is low.

In view of the above problems, the present invention locates a carbonate solid electrolyte on a membrane layer formed on a substrate from which a portion of a region in which a sensor structure is located is removed, and arranges a solid reference electrode and a sensing electrode on a portion of the upper region. In order to provide a solid electrolyte carbon dioxide sensor and a method of manufacturing the same, by placing a heater and a thermometer insulated with an insulating layer on an upper portion thereof to effectively and uniformly absorb the heat provided by the heater while the carbonate solid electrolyte is insulated from the external environment. The purpose is.

The present invention for achieving the above object, the substrate is removed a predetermined region; A membrane layer formed on the substrate; A carbonate solid electrolyte layer formed on the membrane layer of the region where the substrate is removed; A solid reference electrode and a sensing electrode formed to be spaced apart from the carbonate solid electrolyte layer; And a metal heater and a thermometer insulated from the electrodes and the carbonate solid electrolyte layer through the insulating layer and positioned above the carbonate solid electrolyte layer.

The carbonate solid electrolyte layer includes at least one of alkali metal carbonate materials including Li ₂ CO ₃ , K ₂ CO ₃ , Na ₂ CO ₃ , and the reference electrode contains LiMn ₂ O ₄ and LiCoO ₂ . And at least one of an oxide including a pervskite-type oxide including La _0.9 Sr _0.1 MnO ₃ .

In addition, the present invention comprises the steps of forming a membrane material on the upper and lower substrates and then forming a carbonate solid electrolyte layer thereon; Forming a reference electrode on an upper portion of the carbonate solid electrolyte layer and performing heat treatment; Forming an insulating layer on the entire upper surface of the formed structure and patterning a portion of the upper insulating layer of the carbonate solid electrolyte layer for a sensing electrode to be formed thereafter; Filling the patterned insulating layer with metal to form a sensing electrode, and forming a metal pattern to be used as a heater and a thermometer on the upper insulating layer of the carbonate solid electrolyte layer; Back etching the substrate so that at least a portion of the carbonate solid electrolyte layer is suspended on the membrane layer.

DETAILED DESCRIPTION OF THE EMBODIMENTS Embodiments of the present invention implemented as described above will be described in detail with reference to the accompanying drawings.

5 to 6 are cross-sectional views showing the structure of an embodiment of the present invention, in which the carbonate solid electrolyte layer 32 is sequentially referred to the upper portion of the membrane layer 31 formed on the substrate 30 from which some regions are removed. The heater 33 and the thermometer 36 insulated by the electrode 33, the sensing electrode 35, and the insulating layer 34 are positioned.

In the sensor structure formed as described above, the sensor region to which the heater 36 is to be heated is limited, and since the size of the substrate 30 to which the generated heat is reduced is reduced, heat dissipation through the substrate 30 is small and measured. In order to reduce the time for heating the carbonate solid electrolyte layer 32, the power consumption is also greatly reduced. This makes it easy to apply the CO ₂ sensor proposed in the present invention to a portable equipment, thereby greatly increasing the utility of the present invention. In addition, heat of the heater 36 is easily transmitted to the carbonate solid electrolyte layer 32 through the insulating layer 34, and the temperature gradient is low because the removed substrate region thermally insulates the surrounding environment and the sensor. It is possible to prevent deterioration of the device and to increase reliability.

The carbonate solid electrolyte layer 32 uses an alkali metal carbonate material including Li ₂ CO ₃ , K ₂ CO ₃ , and Na ₂ CO ₃ , which is resistant to moisture and convenient in material composition, which may be a problem in the illustrated structure. Al ₂ O ₃ is preferably added to increase the physical strength. In addition, Li ₃ PO ₄ may be added to improve the low ionic conductivity peculiar to the solid electrolyte.

The reference electrode 33 formed on the carbonate solid electrolyte layer 32 is formed of an oxide, and is formed of a Lithium oxide containing LiMn ₂ O ₄ , LiCoO ₂ , and a Perpsky type including La _0.9 Sr _0.1 MnO ₃ (pervskite-type) oxide and the like can be used.

In addition, the sensing electrode 35 may use a general metal electrode forming material, and may be simultaneously formed by using metal (Pt, Mo, etc.) for forming the heater and the thermometer 36.

FIG. 6 is a structure in which the solid reference electrode 33 of FIG. 5 is exposed, and thus, the process of forming the heater and the thermometer 36 may be easily performed. Whether the solid reference electrode 33 is exposed does not significantly affect driving of the sensor.

7A to 7D are procedure cross-sectional views showing a manufacturing process of an embodiment of the present invention, the left side showing a cross-sectional view and the right side showing a top plan view in the same process.

First, as shown in FIG. 7A, after forming the membrane layer 31 on the substrate 30, the carbonate solid electrolyte layer 32 is formed thereon. The membrane layer 31 may be a low stress nitride film. However, in the present invention, since the sensor structure is a part for supporting the sensor structure in a portion without a substrate, a film having an silicon nitride film or a film having an oxide / nitride / oxide (ONO) structure is used. The formed substrate can also be used. In addition, the carbonate solid electrolyte layer 32 is deposited on the membrane layer 31 by one of a sol-gel process, a spin coating method, and a sputtering deposition method, and heat-treated at a temperature of 500 to 800 ° C. to a sensor size. It can be formed by patterning.

Next, as shown in FIG. 7B, a solid reference electrode 33 is formed on a portion of the carbonate solid electrolyte layer 32. As shown on the right side, the pattern of the solid reference electrode 33 extends from the top of the carbonate solid electrolyte layer 32 to the membrane layer 31 in which the carbonate solid electrolyte 32 is not located. It has a pad portion for. This may form a heat treatment to a temperature of 500 ~ 800 ℃ after depositing the reference electrode material by a sol-gel process, spin coating method, sputtering or the like.

Next, as shown in FIG. 7C, an insulating layer 34 is formed on the entire upper surface of the structure, and a portion of the upper insulating layer of the carbonate solid electrolyte layer 33 is patterned for the sensing electrode to be formed thereafter. Allow layer 33 to be exposed. The pattern may have a lattice shape as shown on the right side, but may have a comb-tooth shape or various shapes in which the surface area of the pattern may be widened. This complex structure is expected to improve detection performance. In addition, a portion of the solid reference electrode 33 positioned on the membrane layer 31 is exposed to serve as an electrode pad for external connection. The pattern structure for the sensing electrode should also include a pad definition portion for connecting the external electrode.

Next, as shown in FIG. 7D, the patterned insulating layer is filled with metal to form a sensing electrode 35, and a metal pattern to be used as a heater and a thermometer 36 is formed on the upper insulating layer of the carbonate solid electrolyte. Afterwards, the substrate is etched back so that at least a portion of the carbonate solid electrolyte layer 32 floats on the membrane layer 31. The sensing electrode 35 and the heater and the thermometer 36 may be formed of different metal materials through separate metal processes, and may be formed using a single metal material (Pt, Mo) that may be used as the heater and the thermometer 36. It can be formed simultaneously with a metal process, reducing process time and costs. In this case, the sensing electrode 35 formed includes a grating-type sensing region and a pad portion for external connection, as shown in the plan view on the right side. The heater and the thermometer 36 may include a heater part 36a structure having a meander structure and a thermometer 36b structure formed at a portion electrically spaced from the heater part.

In addition, the process of etching the rear surface of the substrate 30 may use a variety of process methods and resultant structures, but the vertical etching may be performed through dry etching, but may have a slope for physical support strength and process convenience. It is preferable to use a bulk micromachining process for etching. When the back side is etched using a bulk micromachining process, the membrane layer 31 formed on the back side of the silicon substrate 30 is patterned to define an etching region, and anisotropic wet etching is performed using a KOH bulk etchant to have a slope. Form a cavity.

Through the above process, most of the sensor structure is supported by the membrane layer 31 and floats in the air. As a result, the air layer causes contact with the surrounding structure (for example, the external structure supporting and mounting the entire sensor structure). Heat loss) and heat insulation, thereby enabling efficient and stable heating of the carbonate solid electrolyte layer 32. Thus, the initial heating time of the sensor is reduced, power consumption is reduced, and reliability is increased.

As described above, the solid electrolyte carbon dioxide sensor of the present invention and a method of manufacturing the same include placing a carbonate solid electrolyte on a membrane layer formed on a substrate from which a portion of a sensor structure is removed, and a solid reference electrode on a portion of the upper region. By placing a sensing electrode and placing a heater and a thermometer insulated with an insulating layer on the upper part of the sensor, the sensor structure is thermally insulated from the external environment while effectively and uniformly absorbing the heat provided by the heater, and thus the initial driving time of the sensor. It reduces power consumption for overheating and improves device lifetime, characteristics and reliability due to uniform heating.

1 is a cut perspective view showing the structure of a solid electrolyte carbon dioxide sensor using a conventional reference gas.

Figure 2 is a cross-sectional view showing the structure of a solid electrolyte carbon dioxide sensor using a conventional reference electrode.

Figure 3 is a partially cut perspective view showing the structure of a conventional bulk carbon dioxide sensor.

Figure 4 is a cross-sectional view showing the structure of a conventional thick film and thin film type carbon dioxide sensor.

5 is a cross-sectional view showing a structure of an embodiment of the present invention.

Figure 6 is a cross-sectional view showing the structure of another embodiment of the present invention.

7a to 7d is a cross-sectional view showing a manufacturing process of an embodiment of the present invention.

* Description of the symbols for the main parts of the drawings *

30 substrate 31 membrane layer

32: carbonate solid electrolyte layer 33: reference electrode

34: insulating layer 35: sensing electrode

36: heater and thermometer

Claims (10)

A substrate from which a predetermined region is removed; A membrane layer formed on the substrate; A carbonate solid electrolyte layer formed on the membrane layer of the region where the substrate is removed; A solid reference electrode and a sensing electrode formed to be spaced apart from the carbonate solid electrolyte layer; And a metal heater and a thermometer insulated from the electrodes and the carbonate solid electrolyte layer through an insulating layer and positioned above the carbonate solid electrolyte layer. The solid electrolyte carbon dioxide sensor of claim 1, wherein the carbonate solid electrolyte layer comprises at least one of an alkali metal carbonate material including Li ₂ CO ₃ , K ₂ CO ₃ , and Na ₂ CO ₃ . The method of claim 1, wherein the carbonate solid electrolyte layer is an Li ₃ PO ₄ was added to increase the ionic conductivity of the alkali metal carbonate material, a solid electrolyte carbon dioxide, characterized in that the Al ₂ O ₃ is added in order to improve the mechanical strength sensor. The method of claim 1, wherein the reference electrode is at least one of an oxide containing LiMn ₂ O ₄ , a Li-containing oxide including LiCoO ₂ , and a pervskite-type oxide including La _0.9 Sr _0.1 MnO ₃ . Solid electrolyte carbon dioxide sensor, characterized in that formed. The solid electrolyte carbon dioxide sensor of claim 1, wherein the sensing electrode may be formed of the same material as the heater and the thermometer, and may be a metal material including Pt and Mo. The solid electrolyte carbon dioxide sensor of claim 1, wherein the sensing electrode has a lattice or comb shape to improve sensing performance. Forming a carbonate solid electrolyte layer thereon by depositing a membrane material on upper and lower substrates; Forming a reference electrode on an upper portion of the carbonate solid electrolyte layer and performing heat treatment; Forming an insulating layer on the entire upper surface of the formed structure and patterning a portion of the upper insulating layer of the carbonate solid electrolyte layer for a sensing electrode to be formed thereafter; Filling the patterned insulating layer with metal to form a sensing electrode, and forming a metal pattern to be used as a heater and a thermometer on the upper insulating layer of the carbonate solid electrolyte layer; Back etching the substrate to cause at least a portion of the carbonate solid electrolyte layer to float on the membrane layer. The method of claim 7, wherein the forming of the carbonate solid electrolyte layer comprises depositing carbonate on a membrane layer using one of a method including a sol-gel process, a spin coating method, and a sputter deposition method; After the heat treatment at a temperature of 500 ~ 800 ℃ patterned according to the sensor size to form a carbonate solid electrolyte layer comprising the step of forming a solid electrolyte carbon dioxide sensor. The method of claim 7, wherein the sensing electrode, the heater, and the thermometer are simultaneously formed through a single metal process. The method of claim 7, wherein etching the substrate backside comprises: defining an etching region by patterning a membrane layer formed on a backside of a silicon substrate; Method for producing a solid electrolyte carbon dioxide sensor comprising the step of oblique etching in a silicon bulk micromachining process using a KOH solution.