JP2014190878A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermal conductivity gas sensor with improved accuracy in detecting detection target gas by increasing heat transfer rate from a heating element thereof to the detection target gas.SOLUTION: A thermal conductivity gas sensor 1 includes a thin film section D which is provided on a substrate 13 to form a diaphragm, and a heater 15 which is provided in the thin film section and whose resistance changes with temperature thereof. Through-holes 19 for allowing gas to flow are formed at least in some of the portions of the diaphragm where heater patterns 15a, 15b of the heater face each other. Inner surfaces 19e of the through-holes of the diaphragm are tapered.

Description

  The present invention relates to a gas sensor that detects a gas concentration of a combustible gas or the like present in an atmosphere to be detected.

  In recent years, research on fuel cells has been actively conducted as a high-efficiency, clean energy source due to social demands such as environmental protection and nature conservation. Among them, solid polymer fuel cells (PEFCs) and hydrogen internal combustion engines are expected as energy sources for home use and on-vehicle use due to advantages such as low temperature operation and high output density. In these systems, for example, hydrogen, which is a flammable gas, is used as a fuel, so detection of gas leakage is cited as one of the important issues.

As a gas sensor that detects the gas concentration of the combustible gas present in this type of atmosphere to be detected, a gas detection element is arranged in the atmosphere to be detected, and this gas detection element has a resistance value due to its temperature change (heat generation) There is known a heat conduction type gas sensor including a heating resistor in which the temperature changes (see, for example, Patent Document 1). Specifically, in this gas sensor, heat conduction to the combustible gas occurs when the heating resistor is energized and the heating resistor generates heat. Therefore, when the temperature of the gas detection element is controlled to a constant temperature, the temperature of the heating resistor changes due to heat conduction and the resistance value changes, so that the gas to be detected can be detected based on the change amount. .
The gas detection element is manufactured using a MEMS technique, and has a configuration in which an insulating layer for forming a diaphragm is provided on a substrate and a heating resistor is disposed on the insulating layer. As a result, heat escape from the heating resistor to the substrate is reduced at the diaphragm portion, so that the heat of the heating resistor is easily transferred to the gas to be detected.
Further, a micro heater has been developed in which an etching solution is flowed from the surface side of the gas detection element through the slit to the inside of the substrate to advance the etching, and a cavity is formed below the heating resistor to form a diaphragm (for example, Patent Document 2). In this microheater, a slit (through hole) is formed through the diaphragm toward the cavity, so that the heating area adjacent to the slit increases the contact area with the gas to be detected and heat is easily transferred to the gas to be detected. Become.

JP-A-2005-300452 JP 2008-70153 A

By the way, the technique of patent document 2 forms the slit using RIE (reactive ion etching), and processes the side wall of a slit perpendicularly | vertically with respect to the surface. However, in order to further improve the detection accuracy of the gas to be detected, it is necessary to improve the slit shape and positively increase the heat transfer rate from the heating resistor to the combustible gas.
That is, an object of the present invention is to provide a gas sensor in which the heat transfer rate from the heating resistor to the gas to be detected in the heat conduction type gas sensor is increased and the detection accuracy of the gas to be detected is improved.

In order to solve the above-described problems, a gas sensor of the present invention includes a thin film portion that is provided on a substrate and forms a diaphragm, and a heater that is provided in the thin film portion and has a resistance value that changes due to its own temperature change. In the heat conduction type gas sensor, a through hole through which gas flows is formed in at least a part of a portion of the diaphragm facing the heater pattern of the diaphragm, and an inner side surface of the diaphragm forming the through hole is: Tapered.
According to this gas sensor, since the inner surface of the through-hole is tapered, the contact area between the internal heater and the gas to be detected via the inner surface is increased and adjacent to the through-hole, compared to when the inner surface is vertical. In the heating resistor, heat is more easily transmitted to the detection gas. For this reason, the heat transfer rate from the heating resistor to the gas to be detected is increased, and the detection accuracy of the gas to be detected can be improved.

In the through hole, the opening on the surface side may be wider than the opening on the opposite side.
According to this gas sensor, the through hole can be easily formed by a resist method.

The heater patterns facing each other have inflection points facing each other, and the through holes may be formed at positions sandwiching lines connecting the inflection points.
Since the heater pattern is intricate in the vicinity of the inflection point and the amount of heat generated by the heater is high, if a plurality of through holes are formed across the line, the heat of the heater is more easily transmitted to the gas to be detected.

The substrate may include a recess having a bottom surface, and the diaphragm may be formed on the surface of the recess.
According to this gas sensor, since the concave portion has a bottom surface, the bottom surface portion serves as a reinforcing portion to suppress a reduction in the strength of the substrate. Therefore, this is a case where the size of the gas detection element is small (the thickness of the substrate is thin). Even if strength is secured.

  The gas sensor is preferable because it is practical to detect hydrogen gas.

  According to this invention, the heat transfer rate from the heating resistor to the gas to be detected in the heat conduction type gas sensor can be increased, and the detection accuracy of the gas to be detected can be improved.

It is a whole block diagram of a gas sensor. It is a top view which shows the structure of the gas detection element used as the principal part of a gas sensor. It is sectional drawing of the gas detection element along the AA line and BB line in FIG. It is a figure showing the manufacturing process of a gas detection element. It is a figure following FIG.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is an overall configuration diagram of a gas sensor 1 to which the present invention is applied.

[overall structure]
The gas sensor 1 detects the concentration of combustible gas using a heat-conducting gas detection element 11, and is installed in the cabin of a fuel cell vehicle, for example, for the purpose of detecting leakage of hydrogen. It is done.

As shown in FIG. 1, the gas sensor 1 includes a gas detection element 11 (see FIGS. 2 and 3) and a control circuit 90 that drives and controls the gas detection element 11. The control circuit 90 includes a gas detection circuit 91 and a temperature measurement circuit 93. The control circuit 90 (except for the heating resistor 15 and the resistance temperature detector 80, which will be described later) and the microcomputer 94 are configured on a single circuit board, and the gas detection element 11 is separated from the circuit board. It is configured.
The gas detection circuit 91 calculates a potential difference obtained from the Wheatstone bridge 911 including the heating resistor 15 provided in the gas detection element 11 and the fixed resistors 95, 96, and 97, and the Wheatstone bridge 911. An operational amplifier 912 for amplification is provided.
When a resistor whose resistance value increases as its temperature rises is used as the heating resistor 15, the operational amplifier 912 is configured so that the temperature of the heating resistor 15 is maintained at a predetermined temperature. When the temperature of 15 rises, the output voltage is lowered, and when the temperature of the heating resistor 15 falls, the output voltage is increased.

Since the output of the operational amplifier 912 is connected to the Wheatstone bridge 911, when the temperature of the heating resistor 15 rises above a predetermined temperature, it is output from the operational amplifier 912 to lower the temperature of the heating resistor 15. And the voltage applied to the Wheatstone bridge 911 decreases. At this time, the voltage of the electrode 21 constituting the end of the Wheatstone bridge 911 is detected by the microcomputer 94 as the output of the gas detection circuit 91, and the output value detected by the microcomputer 94 is included in the gas to be detected and is combustible. It is subjected to arithmetic processing for detecting sex gases.
The temperature measurement circuit 93 is obtained from a temperature measuring resistor 80 (described later) provided in the gas detection element 11, a Wheatstone bridge 931 including fixed resistors 101, 102, and 103, and a Wheatstone bridge 931. And an operational amplifier 933 that amplifies the potential difference generated. The output of the operational amplifier 933 is detected by the microcomputer 94, and the output value detected by the microcomputer 94 is used to measure the temperature of the gas to be detected. Further, in order to detect the combustible gas contained in the gas to be detected. It is used for the arithmetic processing.

  The processing for calculating the concentration of the combustible gas executed by the microcomputer 94 based on the output value of the control circuit 90 having the above configuration is as follows. First, a CPU (not shown) provided in the microcomputer 94 is substantially proportional to the combustible gas concentration from the output value of the gas detection circuit 91 based on a program stored in a storage device (not shown) provided in the microcomputer 94. The first output value is output. Since the first output value includes an output change due to an ambient temperature change in the detection space of the gas sensor 1, the second output is obtained by correcting the first output value based on the output from the temperature measurement circuit 93. Output the value. Further, the microcomputer 94 determines the concentration of the combustible gas contained in the detected gas based on the relationship between the second output value stored in the storage device (not shown) of the microcomputer 94 and the concentration of the combustible gas. Is output. Thus, since the 1st output value is amended based on the output of temperature measuring circuit 93, combustible gas can be detected with sufficient accuracy. In addition, the process which calculates the density | concentration of combustible gas is not restricted to said thing, What is necessary is just to use a well-known means suitably.

[Gas detection element]
Next, the configuration of the gas detection element 11 will be described. FIG. 2 is a plan view of the gas detection element 11, and FIG. 3 is an end view of each of the AA line cutting part and the BB line cutting part of FIG. In FIG. 2, the left-right direction of the paper surface is the left-right direction of the plan view. In FIG. 3, the vertical direction of the paper surface is the vertical direction of the sectional view.
As shown in FIG. 2, the gas detection element 11 has a flat plate shape (square shape in plan view), electrodes 21, 23, 88, and 89 are formed at the four corners of the surface, respectively, and the center of the other surface (back surface). A diaphragm D having a rectangular shape in plan view, which will be described later in detail, is formed in the vicinity.
As shown in FIG. 3, the gas detection element 11 includes a substrate 13 made of a silicon semiconductor, and includes insulating layers (upper insulating layer 45 and lower insulating layer 47) on both upper and lower sides of the substrate 13. The upper insulating layer 45 is formed on the surface of the substrate 13, while the lower insulating layer 47 is formed on the back surface of the substrate 13. The upper insulating layer 45 includes a plurality of insulating layers 33 to 39 formed on the surface of the substrate 13.

Then, by removing a part of the substrate 13 so that the upper insulating layer 45 is partially exposed (substantially square when viewed from above), a recess (cavity) is formed below the upper insulating layer 45 as shown in FIG. ) It has a diaphragm structure in which 13C is formed. A region of the upper insulating layer 45 that is the upper portion of the recess 13C and that forms the diaphragm D corresponds to a “thin film portion” in the claims.
In addition, since the recess 13C has a bottom surface, the bottom surface portion serves as a reinforcing portion and can suppress a decrease in strength of the substrate 13. Therefore, this is a case where the size of the gas detection element 11 is small (the thickness of the substrate 13 is thin). Even if strength is secured. Of course, the recess 13 </ b> C may not have a bottom surface and may be removed from the bottom surface side of the substrate 13 toward the diaphragm D in a pyramid shape (quadrangular pyramid shape).

In a region (thin film portion) corresponding to the recess 13 </ b> C in the upper insulating layer 45, a heating resistor (“heater” in the claims) 15 patterned in a spiral shape is embedded.
The heating resistor 15 is a resistor whose resistance value changes due to its own temperature change due to the temperature of the gas to be detected (specifically, heat conduction to the combustible gas). The heating resistor 15 is made of a conductive material having a large temperature resistance coefficient, and is made of platinum (Pt) in this embodiment. When detecting hydrogen gas as a combustible gas, the amount of heat taken away from the heating resistor 15 by heat conduction to the hydrogen gas is in accordance with the hydrogen gas concentration. From this, it is possible to detect the hydrogen gas concentration based on the change in the electric resistance value in the heating resistor 15.
Then, by providing the heating resistor 15 in the diaphragm D, the heating resistor 15 is insulated from the surroundings, so that the temperature is raised or lowered in a short time. For this reason, the heat capacity of the gas detection element 11 can be reduced.

Since the change in resistance value of the heating resistor 15 is affected by the temperature of the gas to be detected, the electrical resistance of the heating resistor 15 is detected using the temperature detected based on the electrical resistance value of the temperature measuring resistor 80 described later. By correcting the concentration of the detected gas detected based on the value change, the detection accuracy of the detected gas concentration can be improved.
Further, the upper insulating layer 45 and the lower insulating layer 47 may be formed of a single material, or may be formed of a plurality of layers using different materials. In the present embodiment, the plurality of layers constituting the upper insulating layer 45 are composed of a protective layer 39 made of a silicon nitride layer (Si ₃ N ₄ layer) and silicon oxide (SiO ₂ ) in order from the surface side to the substrate 13 side. The upper insulating protective layer 37, the lower insulating protective layer 35 made of silicon oxide (SiO ₂ ), and the insulating layer 33 made of a silicon nitride layer (Si ₃ N ₄ layer) are laminated. The upper insulating protective layer 37 and the lower insulating protective layer 35 have the same composition and function as the insulating protective layer 43 as a whole, and the heating resistor 15 is embedded in the insulating protective layer 43.
The outermost protective layer 39 is provided so as to cover the heating resistor 15, the resistance temperature detector 80, and the wiring film 16 in order to prevent contamination and damage.
On the other hand, the lower insulating layer 47 is formed of an insulating layer 34 made of a silicon nitride layer (Si ₃ N ₄ layer).
The gas detection element 11 has a size of several millimeters (for example, 3 mm × 3 mm) in both vertical and horizontal directions, and is manufactured by, for example, a micromachining technique (micromachining process) using a silicon semiconductor substrate.

  Returning to FIG. 2, the left and right ends of the heating resistor 15 are connected to the respective electrodes 21 via the heater lead portions 16 (FIG. 3) forming wirings embedded in the same plane as the heating resistor 15 is formed. , 23. The electrode 23 is a ground electrode. The electrodes 21 and 23 are wiring lead portions connected to the heating resistor 15 and are exposed through the contact holes 41 (FIG. 4), and are formed of, for example, aluminum (Al) or gold (Au). .

The resistance temperature detector 80 is for detecting the temperature of the gas to be detected existing in the detection space of the gas sensor 1, and protects the upper insulating protection layer 37 and the protection along the upper side (one side) of the gas detection element 11. It is buried between the layer 39. The resistance temperature detector 80 is made of a conductive material whose electric resistance value changes in proportion to the temperature (in this embodiment, the resistance value increases as the temperature rises). In this embodiment, platinum (Pt) It is formed with.
The resistance thermometer 80 is connected to the electrode 88 and the ground electrode 89 via a wiring film (not shown) embedded in the same plane as the plane on which the resistance thermometer 80 is formed. The electrode 88 and the ground electrode 89 are exposed through a contact hole (not shown), and are formed of, for example, aluminum (Al) or gold (Au).

Further, the diaphragm D is formed with a plurality of through holes 19 that communicate with the recess 13C and through which gas flows. The through hole 19 only needs to be formed in at least a part of a portion of the heating resistor (heater) 15 where the heater patterns 15a and 15b face each other. The concave portion 13C is etched and formed below the thin film portion. For this reason, it is preferable that the plurality of through-holes 19 are arranged in a distributed manner in the surface direction of the thin film portion to be the diaphragm D so that the etching proceeds uniformly.
Further, as shown in FIG. 3, the inner surface 19e of the through hole 19 (diaphragm D) has a tapered shape, and the opening on the surface side is wider than the opening on the opposite side. Thus, when the inner side surface 19e of the through-hole 19 is tapered, the contact area between the internal heating resistor 15 and the gas to be detected via the inner side surface 19e is increased as compared with the case where the inner side surface 19e is vertical. In the heating resistor 15 adjacent to the through hole 19, heat is further easily transferred to the gas to be detected. For this reason, the heat transfer rate from the heating resistor 15 to the gas to be detected is increased, and the detection accuracy of the gas to be detected can be improved.

The through hole 19 only needs to be tapered, and the opening on the surface side may be narrower than the opening on the opposite side. However, as will be described later, the through hole 19 tapers toward the substrate 13 side. Since the resist hole is used as a guide and formed by RIE (reactive ion etching), the through-hole 19 having a wider opening on the surface side than the opening on the opposite side is easier to produce.
Further, it is preferable that the heater patterns 15a and 15B facing each other have inflection points 15ap and 15bp facing each other, and the through holes 19 are formed at positions sandwiching a line L connecting the inflection points 15ap and 15bp. In the vicinity of the inflection points 15ap and 15bp, the heater patterns 15a and 15B are intricate, and the amount of heat generated by the heating resistor (heater) 15 is high. Therefore, if a plurality of through holes 19 are formed across the line L, heat is generated. The heat of the resistor 15 is more easily transmitted to the gas to be detected. The inflection point is a point at which the sign of curvature changes on a curve (including a straight line) representing the contour of the heater pattern.

  The heater pattern of the heating resistor 15 is preferably present at the position of the center of gravity G of the diaphragm D, and the width of the heater pattern is preferably the widest at the position of the center of gravity G. The more the heater pattern of the heating resistor 15 is on the outer side of the diaphragm D, the easier it is to dissipate heat, so the heater pattern is made narrower and the temperature is increased accordingly. Here, if the heater pattern does not exist at the position of the center of gravity G or the width of the heater pattern is narrow, the temperature in the vicinity of the center of gravity G that is difficult to dissipate heat increases, and the heat uniformity in the planar direction of the heating resistor 15 decreases. . The maximum temperature of the heating resistor 15 is limited by the heat resistance temperature of the thin film portion. However, when the heat uniformity of the heating resistor 15 decreases, the local high temperature portion of the heating resistor 15 increases the maximum temperature of the heating resistor 15. The average temperature of the entire heating resistor 15 is lowered, and heat is hardly transmitted from the heating resistor 15 to the detection gas.

Next, the manufacturing process of the gas detection element 11 will be described with reference to FIGS. 4 and 5, the respective end surface states in the AA line cutting part and the BB line cutting part of the gas detection element 11 in FIG. 1 are shown for each step.
First, a substrate 13 made of a silicon semiconductor is prepared, the substrate 13 is cleaned, and sacrificial layers 31 and 32 made of a Poly-Si film are formed on the upper and lower surfaces of the substrate 13 by LP-CVD, respectively. Then, the unnecessary region is removed (patterned) while leaving the necessary region so that the sacrificial layer 31 on the upper surface side of the silicon substrate 13 coincides with the formation region of the diaphragm D (FIG. 4A). At this time, the sacrificial layer 32 on the lower surface side is also removed.
Next, an insulating layer 33 made of a silicon nitride film (Si ₃ N ₄ film) serving as a tensile stress film is formed on the upper surface (including the sacrificial layer 31) of the substrate 13 by a low pressure CVD method. An insulating layer 34 made of (Si ₃ N ₄ film) is formed on the lower surface of the substrate 13. Further, a lower insulating protective layer 35 made of a silicon oxide layer (SiO ₂ layer) which is a compressive stress film is formed on the surface of the insulating layer 33 by a plasma CVD method (FIG. 4B).

Next, a metal film such as platinum (Pt) is formed on the lower insulating protective layer 35 by sputtering, and then patterned to form the heater wiring portion 15 (FIG. 4C). At this time, the wiring shape (wiring pattern) of the heater wiring portion 15 is determined so that the electrical resistance value of the heater wiring portion 15 becomes a predetermined design value. At this time, the heater lead portion 16 electrically connected to the heater wiring portion 15 is formed by patterning similarly to the heater wiring portion 15.
Subsequently, the upper insulating protective layer 37 made of silicon oxide (SiO ₂ ) is formed by the plasma CVD method so that the film configuration in the vertical direction (stacking direction) is symmetrical about the heater wiring portion 15. After the formation, a protective layer 39 made of a silicon nitride layer (Si ₃ N ₄ layer) is formed on the upper insulating protective layer 37 by LP-CVD. Further, a contact hole 41 communicating with the heater lead portion 16 is formed in the upper insulating protective layer 37 and the protective layer 39 by RIE (reactive ion etching) (FIG. 4D). In this step, the resistance temperature detector 80 is also patterned between the upper insulating protective layer 37 and the protective layer 39 in the same manner as the heating resistor 15.

  Next, a metal film such as gold (Au) is formed on the contact hole 41 by sputtering, followed by patterning to form a contact electrode 21 made of a metal material such as gold (Au) (FIG. 5 ( E)). Although not shown, the electrodes 23, 88, and 89 are formed simultaneously for the other contact holes.

Next, a resist film 50 is formed on the protective layer 39, and a resist hole 51 is formed by etching away a portion to be a through hole 19 described later. Further, by applying heat (for example, at 150 ° C. for 5 minutes) to the remaining resist film 50, the inner surface 50e of the resist hole 50 is tapered such that the opening on the front side is wider than the opening on the opposite side ( FIG. 5 (F)). Here, for example, AZ1500 positive resist (manufactured by AZ Electronic Materials) can be used as the resist.
Next, by a RIE (reactive ion etching) method, the stacked body (thin film portion forming the diaphragm D) of the protective layer 39, the upper insulating protective layer 37, the lower insulating protective layer 35, and the insulating layer 33 is applied. Through holes 19 forming slits leading to the sacrificial layer 31 are formed at a plurality of locations (FIG. 5G).
Here, in the etching by RIE, not only the protective layer 39 but also the resist film 50 around the resist hole 51 in the upper layer of the protective layer 39 is simultaneously etched. Therefore, as the etching progresses, the size of the resist hole 51 is increased, and etching is performed while forming a larger opening on the surface side of the protective layer 39 than on the lower side. In this way, the inner side surface 19e of the through hole 19 is also tapered (in this example, the opening on the surface side is wider than the opening on the opposite side). Thereafter, the resist 50 is removed.
In the through-hole forming step, a plurality of through-holes 19 are formed in at least a part of the portion where the heater patterns face each other so as to be dispersed throughout the sacrificial layer 31 as shown in FIG.

Next, the sacrificial layer 31 is etched using an anisotropic etching solution (for example, TMAH), and the portion of the silicon substrate 13 on which the sacrificial layer 31 is stacked is etched (FIG. 5H).
Thereby, a recess (cavity) 13 </ b> C can be formed in the silicon substrate 13. At this time, since the anisotropic etching solution is injected from the plurality of through holes 19 that are dispersed and arranged in the thin film portion, the etching of the sacrificial layer 31 proceeds substantially uniformly throughout the sacrificial layer 31. . For this reason, when the sacrificial layer 31 is etched and the etching solution reaches the silicon substrate 13, the etching solution comes into contact with the portion of the silicon substrate 13 where the sacrificial layer 31 is laminated almost evenly. As a result, even in the portion of the silicon substrate 13 on which the sacrificial layer 31 is laminated, the etching proceeds substantially uniformly throughout the entire portion.
Strictly speaking, when the etching of the sacrificial layer 31 is finished, the surface of the silicon substrate 13 is slightly etched and is uneven. However, since the depth of the concave portion is shallow and the etching rate of the convex portion is faster than that of the concave portion, the unevenness of the surface of the silicon substrate 13 (the bottom surface of the concave portion 13C) is immediately flattened.
As the etching of the silicon substrate 13 proceeds in this manner, the shape of the recess 13C formed by the etching can be made flat. The recess 13C having a flat bottom surface has a smaller depth dimension than a pyramid-shaped (quadrangular pyramid) cavity.
The material constituting the sacrificial layer 31 is preferably a material that can be isotropically etched. Furthermore, the material constituting the sacrificial layer 31 is preferably a material that can be etched with an etching solution used when the semiconductor substrate 13 is etched. An example of a material that satisfies the above two conditions is polysilicon (Poly-Si) as in this embodiment.

It goes without saying that the present invention is not limited to the above-described embodiment, but extends to various modifications and equivalents included in the spirit and scope of the present invention. For example, although the case where hydrogen gas is detected has been described in the above embodiment, the gas sensor of the present invention can also detect other types of combustible gases.
Further, in the manufacturing process of the gas detection element 11 described above, a thermal oxide film may be formed on the base layer on which the insulating layers 33 and 34 are formed in order to improve the adhesion of the insulating layers 33 and 34. Further, the formation of the contact hole in FIG. 4D and the formation of the through hole in FIG. 5G may be performed simultaneously.

1 Gas sensor 13 Substrate 13C Recessed portion of substrate 15 Heater (heating resistor)
15a, 15b Heater pattern 15ap, 15bp Inflection point of heater pattern 19 Through hole 19e Inner side surface of through hole (diaphragm) D Diaphragm (thin film portion)
L Line connecting inflection points

Claims (5)

A thin film portion provided on a substrate to form a diaphragm;
A heat conduction type gas sensor provided with a heater provided in the thin film portion, the resistance value of which changes according to its own temperature change,
A through-hole through which gas flows is formed in at least a part of a portion of the diaphragm facing the heater pattern of the heater,
A gas sensor in which an inner surface of the diaphragm forming the through hole is tapered.   The gas sensor according to claim 1, wherein the through hole has a wider opening on the surface side than an opening on the opposite side.   3. The gas sensor according to claim 1, wherein the heater patterns facing each other have inflection points facing each other, and the through holes are formed at positions sandwiching lines connecting the inflection points.   The sensor according to any one of claims 1 to 3, wherein the substrate includes a recess having a bottom surface, and the diaphragm is formed on a surface of the recess.   The gas sensor according to claim 1, wherein the gas sensor detects hydrogen gas.

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