JP2004294127A - Humidity sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a humidity sensor capable of keeping measurement accuracy even in long-term use, and having excellent durability. <P>SOLUTION: This humidity sensor is equipped with a substrate (silicon substrate 111), a gas-sensitive layer (117) provided on the substrate, and a detection electrode (115) in contact with the gas-sensitive layer. The gas-sensitive layer includes an acidic polymer electrolyte, and at least the detection electrode surface part in contact with the gas-sensitive layer of the detection electrode is mainly composed of Ta. The whole detection electrode (115) may be mainly composed of Ta, and a fluorine-based resin may be used as the acidic polymer electrolyte. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a humidity sensor. More specifically, it relates to a humidity sensor having excellent durability.
INDUSTRIAL APPLICABILITY The humidity sensor of the present invention detects moisture contained in the atmospheric atmosphere (measurement of environmental humidity, etc.), exhaust gas from internal combustion engines such as vehicles, ships, and airplanes (particularly, it does not substantially contain oxygen and contains a reducing gas. Detection of moisture contained in atmosphere), detection of moisture for control of home air conditioner, detection of moisture for control of humidity of air purifier, detection of moisture for control of humidifier and dehumidifier Detection, moisture detection in horticultural or agricultural humidity management systems, detection of moisture in industrial gas piping, detection of humidity in environmental monitor humidity monitors (constant temperature and humidity), drying of semiconductor industrial gas Detection for humidity control, humidity detection in a humidity monitor in a constant environment room (clean room, etc.), detection of humidity for humidity control in precision electronic component manufacturing process, in anode gas and cathode gas of fuel cells etc. Included in Moisture detection, and it can be used for moisture detection, such as for food quality control.
[0002]
[Prior art]
Conventionally, noble metals having excellent corrosion resistance and the like and excellent heat resistance have been widely used as detection electrodes. On the other hand, the gas-sensitive layer for humidity sensor, Nafion (registered trademark) polymer system feeling Shimezai such, as well as _Al 2 _{O 3} system, MgCr ₂ O ₄ -TiO ₂ systems and ceramic humidity-sensitive material such as Nasicon It has been known. Above all, as the polymer-based moisture sensitive material, a material composed of a SiO ₂ / Nafion (registered trademark) mixed film disclosed in Non-Patent Document 1 below is known. Non-Patent Document 1 below discloses a humidity sensor having a structure in which an alumina substrate is used as a substrate, a detection electrode made of platinum is provided on the alumina substrate, and the mixed film is brought into contact with the detection electrode.
[0003]
[Non-patent document 1]
C. D. Feng, 4 others, Humidity sensing properties of Nafion and sol-gel derived SiO ₂ / Nafion composite thin films, “Sensors and Actibusiness Chemicals B.C. A. , May 1997, Vol. 40, No. 1-2, p. 217-222
[0004]
[Problems to be solved by the invention]
The present inventors have studied the humidity sensor having the structure disclosed in Non-Patent Document 1 described above. As a result, it was found that this humidity sensor showed excellent performance in measuring humidity, but the resistance (impedance) gradually increased when measurement was repeated. Then, when the humidity sensor whose resistance (impedance) was increased by repeated use was examined in detail, it was found that agglomeration of platinum was recognized in a part of the detection electrode and there was a portion where the detection electrode disappeared. As described above, it has been known that when the gas-sensitive membrane is an acidic polymer electrolyte such as Nafion (registered trademark), such a phenomenon occurs even though platinum is used for the detection electrode. It was a surprising finding that was not done.
An object of the present invention is to solve the above-mentioned problems, and an object of the present invention is to provide a gas sensor that can maintain measurement accuracy even during long-term use and has excellent durability.
[0005]
[Means for Solving the Problems]
(1) A substrate, a gas-sensitive layer provided on the substrate and containing an acidic polymer electrolyte, and a detection electrode provided in contact with the gas-sensitive layer, wherein at least the gas-sensitive layer of the detection electrode is provided. A humidity sensor characterized in that a surface of a detection electrode in contact with a layer contains Ta as a main component.
(2) The humidity sensor according to (1), wherein the entirety of the detection electrode is mainly composed of Ta.
(3) The humidity sensor according to (1) or (2), wherein the substrate is a silicon substrate.
(4) The humidity sensor according to any one of (1) to (3), wherein the acidic polymer electrolyte is a fluororesin.
[0006]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to the humidity sensor of this invention, the durability of a detection electrode is improved effectively and humidity can be measured accurately over a long period of time.
When the entire sensing electrode contains Ta as a main component, particularly high durability can be exhibited.
When the substrate is a silicon substrate, it is excellent in workability and can be easily reduced in size.
When the acidic polymer electrolyte is a fluororesin, it can be a humidity sensor capable of measuring particularly high humidity.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention will be described in detail below.
(1) Substrate The "substrate" supports the detection electrode and the gas-sensitive layer. The material constituting the substrate is not particularly limited, and one or two or more of silicon, silicon carbide, silicon nitride, alumina, and a resin having an active group (epoxy group, hydroxyl group, isocyanate group, carboxyl group, or the like) are used. be able to. Among these, silicon which is excellent in workability, particularly easy to reduce the size of the humidity sensor, and which is excellent in bonding property with an inorganic skeleton or the like described later is preferable. Further, among the above materials, the surface of the substrate formed of silicon, hydrocarbon, and silicon nitride may be subjected to an oxidation treatment to form a silica layer, and may be subjected to a silanolation treatment to form a silanol group. Is also good. Thereby, the adhesiveness with the gas sensitive layer can be improved. Further, the substrate may be composed of only one layer, or may be composed of two or more layers.
[0008]
(2) Gas Sensitive Layer The above “gas sensitive layer” contains an acidic polymer electrolyte described later, and causes an electrical change by the action of water molecules. Other arrangements of the gas-sensitive layer are not particularly limited as long as the gas-sensitive layer is formed on the substrate and is in contact with the detection electrode (usually having a pair).
[0009]
The "acidic polymer electrolyte" causes a change in resistance (impedance) due to adsorption and desorption of water molecules. Examples of the acidic polymer electrolyte include fluorine resins such as Nafion (registered trademark), polystyrenesulfonic acid, vinylsulfonic acid, polytrifluorostyrenesulfonic acid, and polyacrylsulfonic acid. Among them, Nafion is suitable as a polymer electrolyte for a humidity sensor because its proton conductivity changes according to humidity and can measure humidity up to high humidity.
[0010]
The amount of the acidic polymer electrolyte contained in the gas-sensitive layer is not particularly limited, but is preferably 15% by mass or more when the entire gas-sensitive layer is 100% by mass. If it is 15% by mass or more, sufficient sensitivity can be exhibited.
In addition, examples of the gas-sensitive layer other than the acidic polymer electrolyte include an inorganic skeleton having a three-dimensional network structure for fixing the acidic polymer electrolyte. A network skeleton is preferred. Therefore, as the inorganic skeleton, for example, polysiloxane is preferable. When a substrate having a silica layer formed on its surface (for example, a silicon substrate having a silica layer formed thereon) is used as the substrate, it is preferable to include a polysiloxane in the gas-sensitive film because the adhesion to the silica layer is improved.
[0011]
(3) Detection electrode The “detection electrode” is a portion of the conductor layer formed on the substrate, which is in contact with the gas-sensitive layer. This sensing electrode usually includes a pair, but may include two or more pairs. Furthermore, both the one sensing electrode and the other sensing electrode of each pair may be in contact with one surface of the gas-sensitive layer, or may be in contact with one surface and the other surface of the gas-sensitive layer. Further, the detection electrode may be composed of only one layer, or may be composed of two or more layers. Further, the sensing electrode may be made of only one kind of constituent material, or may be made of two or more kinds of constituent materials. The positional relationship between the detection electrode and the substrate is not particularly limited.
[0012]
In addition, at least the sensing electrode surface portion of the sensing electrode which is in contact with the gas sensitive layer contains Ta as a main component. When the surface of the sensing electrode is mainly composed of Ta, even if the inside of the sensing electrode is mainly composed of another metal such as a noble metal, a problem caused by migration or corrosion such as aggregation and dispersion of the metal. Can be effectively prevented. For this reason, the detection electrode exhibits high durability.
The thickness of the sensing electrode surface is not particularly limited, but is preferably 0.01 μm or more. Further, the term “having Ta as a main component” is not particularly limited as long as the effect of containing Ta is exhibited, but is preferably 50% by mass or more. However, Ta contained in the surface portion may be contained in a metal state and function as a conductor, or may be contained as an oxide or the like and not function as a conductor.
[0013]
The sensing electrode may be entirely composed of Ta as a main component. In this case, particularly high durability is exhibited. When platinum is formed on the silica layer as a part of the detection electrode, it is necessary to form an adhesion layer of Ta or the like in order to improve the adhesion between the platinum and the silica layer. On the other hand, when the entirety of the detection electrode is mainly composed of Ta, it is not necessary to form the above-mentioned adhesion layer, and the manufacturing process of the humidity sensor can be simplified.
[0014]
The constituent material of the detection electrode is not particularly limited except for the above-mentioned Ta, and for example, Pt, Pd, Au, Rh, Cu, Ag, Ru, Re, Ir, and the like can be used. Among these, Pt or the like is preferable because of its high conductivity, excellent heat resistance and the like. These constituent materials may be used alone or in combination of two or more. For example, when Pt and Rh are used in combination, the volatilization of Pt at a high temperature can be suppressed.
The shape of the detection electrode is not particularly limited, and may be a comb-like shape, a meandering shape, or the like.
[0015]
Further, a lead for a detection electrode is usually extended from the detection electrode. The lead for the detection electrode is a portion for establishing conduction between the detection electrode and the connection means of the external circuit. Further, a bonding pad may be provided at an end of the detection electrode lead that is not connected to the detection electrode. The bonding pad is a terminal portion of a lead for a sensing electrode which is formed wide so as to easily connect a bonding wire. The detection electrode, the lead for the detection electrode, and the bonding pad may be made of the same material or different materials.
[0016]
The humidity sensor of the present invention can include other elements in addition to the substrate, the gas-sensitive layer, the detection electrode, the lead for the detection electrode, the bonding pad, and the like. As another element, there may be mentioned an adhesion improving layer for improving the adhesion between the gas sensitive layer and the substrate. Although the substrate and the gas-sensitive layer may be directly bonded to each other, it is preferable to have an adhesion improving layer in order to further improve the adhesion. Examples of the adhesion improving layer include a porous layer capable of improving the adhesion between the substrate and the gas-sensitive layer by an anchor effect.
In addition, when the substrate has conductivity and the detection electrode is provided on the substrate, an insulating layer that insulates the substrate from the detection electrode can be given. When the substrate is a silicon substrate, a silica layer formed by oxidizing the surface of the silicon substrate functions as the insulating layer.
[0017]
【Example】
Hereinafter, the present invention will be specifically described with reference to examples.
[1] Production of Humidity Sensor (1) Example 1 (The whole sensing electrode is made of tantalum)
Using a silicon substrate (111) as a substrate, a first silica film (112) having a thickness of 200 nm was formed on the surface of the silicon substrate (111) by a plasma CVD method. Next, a meandering electrode (113) having a thickness of 200 nm and a line width of 5 μm was formed on the surface of the first silica film (112) by a sputtering method. Thereafter, a second silica film (114) having a thickness of 200 nm was formed again by the plasma CVD method. Next, by a sputtering method, a comb-shaped detection electrode (115) having a thickness of 60 nm, a line width of 20 μm, and made of tantalum on the surface of the second silica film (114), and a detection having a bonding pad at an end. The electrode lead (116) was formed integrally. Thereafter, each humidity sensor element (11) was cut out by dicing, and a predetermined portion was masked. Then, a fluorine-containing resin liquid (manufactured by DuPont, trade name "Nafion") containing 20% by mass of a fluorine-based resin and tetraethoxysilane (manufactured by Wako Pure Chemical Industries, Ltd.) are mixed and obtained through a predetermined process. The sol solution thus obtained was spin-coated on the surface of the mask provided with the humidity sensor element. Next, the mask was removed and heat treatment was performed at 150 ° C. for 2 hours to form a gas-sensitive layer (117). Thereafter, each humidity sensor element is bonded to the package base (12), and bonding is performed from a bonding pad of the lead (116) for the detection electrode to a predetermined pin (121) using a bonding wire (13) made of gold. Sensor (1) was obtained.
[0018]
(2) Comparative Example 1 (the lower layer of the detection electrode is made of tantalum and the upper layer is made of platinum)
On the second silica film obtained in the same manner as in the above (1), a lower layer is a 20 nm-thick tantalum layer and an upper layer is a 40 nm-thick platinum layer by sputtering. The same detection electrode (115) and detection electrode lead (116) as in (1) above were formed. Otherwise, the humidity sensor of Comparative Example 1 was obtained in the same manner as in the above (1). This humidity sensor differs from the humidity sensor of the present invention in that the surface of the detection electrode is a platinum layer.
[0019]
[2] Evaluation of Humidity Sensor (1) Measurement of Impedance First, the impedance between the initial detection electrodes of the respective humidity sensors of Example 1 and Comparative Example 1 obtained in [1] (1) and (2) above ( (Applied voltage AC 1 V, measurement frequency 1 kHz) was measured at 80 ° C. in the air atmosphere. Further, the relative humidity at the time of measurement was changed to each of 40 RH%, 55 RH%, 70 RH%, 85 RH%, and 95 RH%, and the impedance between the detection electrodes was measured. Then, the relative humidity was changed in the order of 40 RH% → 55 RH% → 70 RH% → 85 RH% → 95 RH%, and a cycle of holding for 1 hour at each relative humidity was defined as one cycle, and a 30 cycle durability test was performed. Then, the impedance under the same conditions as the initial measurement was measured. The result is shown in FIG.
[0020]
(2) Observation by Optical Microscope and EPMA In each humidity sensor obtained in the above [1] (1) and (2), an enlarged image of the detection electrode by an optical microscope before the endurance test and an image after the endurance test of 30 cycles. An enlarged image of the detection electrode by an optical microscope was obtained. These enlarged images are shown in FIGS. The humidity sensor of Comparative Example 1 was obtained by measuring the same part of each detection electrode before and after the durability test with an electron probe microanalyzer (hereinafter simply referred to as “EPMA”). The resulting color map (black and white converted) is shown in FIGS.
[0021]
[3] Effect of Example From the above [2] (2), FIG. 5 shows the state of the detection electrode of the humidity sensor of Comparative Example 1 before the durability test. FIG. 6 shows the humidity sensor of Comparative Example 1 after a 30-cycle durability test. The thin white line (meandering shape) in each figure is a temperature measurement electrode, and the thick comb-shaped white line (115 in the figure) is a detection electrode. Comparing the detection electrodes of the two optical microscope images, it can be seen in FIG. 6 that most of the comb-shaped detection electrodes have disappeared. It can also be seen that a new black line (2 in FIG. 6) is observed.
[0022]
FIGS. 7 and 8 show portions with particularly high Pt concentrations in lighter colors. According to this, a tip portion of the comb-shaped detection electrode is recognized in a portion surrounded by a white line ellipse in FIG. 7, while a comb-shaped detection electrode recognized in FIG. It can be seen that the tip is not recognized and a line (2 in FIG. 8) which is not recognized in FIG. This is the black line observed in FIG. 6, and it can be seen that platinum was formed by aggregation. That is, it can be confirmed that the detection electrode formed in a comb shape before the endurance test aggregates into a unique shape after the endurance test, and most of the tip of the detection electrode has disappeared.
[0023]
According to the graph of FIG. 2, the impedance of the comparative example 1 after the durability test was 2.3 times at 45 RH%, 6.5 times at 55 RH%, and 12 times at 70 RH% compared to the impedance before the durability test. It can be seen that they are increased 12 times at 85 RH% and 8.8 times at 90 RH%.
From these results, in the humidity sensor (Comparative Example 1) in which the surface portion of the detection electrode has Pt as the main component and has the detection electrode formed in direct contact with the acidic polymer electrolyte, the platinum gradually becomes higher as the measurement is repeated. It is considered that aggregation occurs, the impedance increases, and the measurement accuracy decreases.
[0024]
On the other hand, in the humidity sensor having the detection electrode made of only tantalum (Example 1), FIGS. 3 and 4 which are explanatory diagrams using an optical microscope image show the detection electrode both before and after the durability test. No difference is recognized. FIG. 2 also shows that the impedance decreases as the relative humidity increases, and that the lower layer conventionally used is made of tantalum and the upper layer shows the same impedance characteristics as the detection electrode made of platinum.
From these results, it can be seen that the humidity sensor of Example 1 having the detection electrode made of only tantalum can exhibit high durability and exhibit excellent measurement performance.
[Brief description of the drawings]
FIG. 1 is a schematic sectional view of an example of a gas sensor (humidity sensor) of the present invention.
FIG. 2 is a graph showing a correlation between relative humidity and impedance measured in an example.
FIG. 3 is an explanatory diagram based on an optical microscope image before a durability test of the humidity sensor of Example 1.
FIG. 4 is an explanatory diagram based on an optical microscope image after a durability test of the humidity sensor of Example 1.
FIG. 5 is an explanatory diagram based on an optical microscope image before a durability test of a humidity sensor of Comparative Example 1.
FIG. 6 is an explanatory diagram based on an optical microscope image after a durability test of the humidity sensor of Comparative Example 1.
FIG. 7 is an explanatory diagram using an EPMA element map before a durability test of the humidity sensor of Comparative Example 1.
FIG. 8 is an explanatory diagram based on an EPMA element map after a durability test of the humidity sensor of Comparative Example 1.
[Explanation of symbols]
1; gas sensor (humidity sensor), 11; gas sensor element (humidity sensor element), 111; substrate (silicon substrate), 112; first insulating layer (first silica film), 113; electrode for temperature measurement, 114; Insulating layer (second silica film), 115; detecting electrode, 116; lead for detecting electrode, 117; gas-sensitive layer, 12; package base, 121; pin, 13; bonding wire, 2: aggregate of platinum.

Claims (4)

Board and
A gas-sensitive layer provided on the substrate and containing an acidic polymer electrolyte,
A detection electrode provided in contact with the gas-sensitive layer,
A humidity sensor characterized in that at least a surface of the sensing electrode in contact with the gas-sensitive layer has Ta as a main component. 2. The humidity sensor according to claim 1, wherein the entirety of the detection electrode is mainly composed of Ta. The humidity sensor according to claim 1, wherein the substrate is a silicon substrate. The humidity sensor according to any one of claims 1 to 3, wherein the acidic polymer electrolyte is a fluororesin.

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