JP2014190847A - Gas sensor - Google Patents

Abstract

The present invention provides a gas sensor in which the thermal conductivity from a heating resistor to a gas to be detected in a heat conduction gas sensor is increased, and the detection accuracy of the gas to be detected is improved.
SOLUTION: A thin film portion provided on a substrate so as to cover an opening 62 formed in the substrate 61, and a heater 71 provided in the thin film portion, the resistance value of which changes with its own temperature change, are provided. In the heat-conducting gas sensor 1 provided, a metal layer 75 having irregularities is formed on at least a part of the surface of the heater forming region 71R in the thin film portion.
[Selection] Figure 3

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 own temperature change (heat generation). There is known a heat conduction type gas sensor provided with a heating resistor in which the temperature changes (see, for example, Patent Documents 1 and 2). Specifically, in this gas sensor, when the heating resistor is energized and the heating resistor generates heat, heat conduction to the combustible gas (gas heat conduction) occurs. 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.

JP-A-2005-300452 JP 2010-96727 A

However, the diaphragm described above is merely a reduction in heat escape from the contact portion between the insulating layer and the semiconductor substrate, and in order to further improve the detection accuracy of the gas to be detected, the heating resistor is changed to a combustible gas. It is necessary to positively increase the thermal conductivity (degree of gas thermal conduction).
That is, an object of the present invention is to provide a gas sensor in which the thermal conductivity 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 problems, the gas sensor of the present invention is provided with a thin film portion provided on the substrate so as to cover an opening formed on the substrate, and the resistance value due to its own temperature change. A heat conduction type gas sensor including a heater in which a metal layer having irregularities is formed on at least a part of the surface of the heater formation region in the thin film portion.
According to this gas sensor, heat is easily transferred from the heating resistor to the gas to be detected through the metal layer having the surface area increased from that of the planar structure due to the provision of the unevenness, and from the heating resistor to the gas to be detected. The thermal conductivity of the gas can be increased, and the detection accuracy of the gas to be detected can be improved. Further, since a heat transfer path from the heating resistor to the metal layer side is formed, heat escape from the contact portion between the thin film portion and the substrate is reduced.

The metal layer is preferably formed in a region inside the thin film portion with respect to the periphery of the opening.
According to this gas sensor, it becomes difficult for the heat of the metal layer to escape to the substrate side, so that it is possible to suppress a decrease in heat conduction from the heating resistor to the gas to be detected.

The metal layer is preferably formed by sputtering Al, Cu, Au, or an alloy thereof.
According to this gas sensor, irregularities can be easily formed in the metal layer. In addition, by forming the metal layer from Al, Cu, Au or an alloy thereof, the metal layer itself has a high thermal conductivity, and in combination with the structure provided with unevenness, the heating resistor to the gas to be detected The degree of heat conduction can be increased.

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

  According to this invention, the thermal conductivity 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 in FIG. It is a figure which shows the method of calculating | requiring the average height of 10 points | pieces from the highest peak of a metal layer. 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. FIG. 2 is a plan view showing the configuration of the gas detection element 60 which is the main part of the gas sensor 1 (however, the internal configuration is also partially shown), and FIG. 4 is a gas detection element along the line AA in FIG. FIG.

[overall structure]
The gas sensor 1 detects the concentration of combustible gas using a heat-conducting gas detection element 60. For example, the gas sensor 1 is installed in a passenger room of a fuel cell vehicle and used 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 60 (see FIGS. 2 and 3) and a control circuit 90 that drives and controls the gas detection element 60. The control circuit 90 includes a gas detection circuit 91 and a temperature measurement circuit 93. The control circuit 90 (except for a heating resistor 71 and a 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 60 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 71 provided in the gas detection element 60 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 71, the operational amplifier 912 is configured so that the temperature of the heating resistor 71 is maintained at a predetermined temperature. When the temperature of 71 rises, the output voltage is lowered, and when the temperature of the heating resistor 71 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 71 rises above a predetermined temperature, it is output from the operational amplifier 912 to lower the temperature of the heating resistor 71. And the voltage applied to the Wheatstone bridge 911 decreases. At this time, the voltage of the electrode 85 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 detected gas and 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 60, 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 calculated value is calculated. Since this first calculated value changes depending on the ambient temperature change in the detection space of the gas sensor 1, the first calculated value is corrected based on the output from the temperature measurement circuit 93 that is separately input to the CPU. A second calculated value is calculated. Furthermore, the microcomputer 94 determines the concentration of the combustible gas contained in the detected gas based on the relationship between the second calculated value stored in the storage device (not shown) of the microcomputer 94 and the concentration of the combustible gas. Is calculated and output to an external circuit. Thus, since the 1st calculation value (output value of the gas detection circuit 91) is correct | amended based on the output of the temperature measurement circuit 93, combustible gas can be detected accurately. 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 60 will be described. 2 shows a plan view of the gas detection element 60, and FIG. 3 shows a cross-sectional view taken along the line AA 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 60 has a flat plate shape (square shape in plan view), electrodes 85, 86, 88, 89 are formed at the four corners of the surface, respectively, and the center of the other surface (back surface). A concave portion 62 having a rectangular shape in plan view that is recessed toward the surface is formed in the vicinity.
As shown in FIG. 3, the gas detection element 60 includes a substrate 61 made of a silicon semiconductor, and includes insulating layers (an upper insulating layer 67 and a lower insulating layer 66) on both upper and lower sides of the substrate 61. The upper insulating layer 67 is formed on the surface of the substrate 61, while the lower insulating layer 66 is formed on the back surface of the substrate 61. The upper insulating layer 67 includes a first insulating layer 68 formed on the surface of the substrate 61, an insulating protective layer 69 formed on the surface of the insulating layer 68, and a second insulating layer formed on the surface of the insulating protective layer 69. 64.
Then, a part of the substrate 61 is removed so that the upper insulating layer 67 is partially exposed (substantially square when viewed from above), thereby forming a recess 62 (an opening of the substrate 61) as shown in FIG. The diaphragm structure is made. A region of the upper insulating layer 67 where the recess 62 is formed (in other words, a partition wall portion that blocks the opening formed in the substrate 61) corresponds to a “thin film portion” in the claims.

In a region (thin film portion) corresponding to the recess 62 in the upper insulating layer 67, a heating resistor 71 (“heater” in the claims) 71 is embedded in a spiral pattern.
The heating resistor 71 is a resistor whose resistance value changes due to its own temperature change accompanying the temperature of the gas to be detected (specifically, gas thermal conduction to the combustible gas). The heating resistor 71 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 71 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 resistance value in the heating resistor 71.
Then, by providing the heating resistor 71 inside the region (thin film portion) corresponding to the recess 62 in the upper insulating layer 67, the heating resistor 71 is thermally insulated from the surroundings, so that the temperature rises in a short time or Lower the temperature. For this reason, the heat capacity of the gas detection element 60 can be reduced.

Note that the upper insulating layer 67 may be formed of a single material or may be formed of a plurality of layers using different materials. In the present embodiment, among the plurality of layers constituting the upper insulating layer 67, the first insulating layer 68 is made of silicon nitride (Si ₃ N ₄ ), the insulating protective layer 69 is made of silicon oxide (SiO ₂ ), and the second insulating layer 68 is formed. The layer 64 is made of silicon nitride (Si ₃ N ₄ ), and the heating resistor 71 is embedded in the insulating protective layer 69.
The gas detection element 60 has a size of about several mm (for example, 3 mm × 3 mm) both vertically and horizontally, and is manufactured by, for example, a micromachining technique (micromachining process) using a silicon semiconductor substrate.

  Returning to FIG. 2, the left end of the heating resistor 71 is connected to the electrode 85 via a wiring 713 and a wiring film 711 (FIG. 3) embedded in the same plane as the plane on which the heating resistor 71 is formed. The right end of the heating resistor 71 is connected to the ground electrode 86 via a wiring 714 and a wiring film 712 (FIG. 3) embedded in the same plane as the plane on which the heating resistor 71 is formed. The electrode 85 and the ground electrode 86 are lead portions of wiring connected to the heating resistor 71, and are exposed through the contact hole 84 (FIG. 3), and are formed of, for example, aluminum (Al) or gold (Au). ing.

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 along the upper side (one side) of the gas detection element 60, the upper insulating layer 67 and the protective layer. 64 is embedded. The resistance temperature detector 80 is made of a conductive material whose resistance value changes in proportion to the temperature (in this embodiment, the resistance value increases as the temperature increases). In this embodiment, the resistance temperature detector 80 is made of platinum (Pt). Is formed.
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, as shown in FIG. 3, a metal layer 75 having irregularities (rough surface) is formed on the surface of the second insulating layer 64 in the region where the heating resistor 71 is formed. This makes it easier for heat to be transferred from the heating resistor 71 to the gas to be detected through the metal layer 75 having an increased surface area due to the formation of irregularities, and the thermal conductivity (gas) from the heating resistor 71 to the gas to be detected. The degree of heat conduction) is increased, and the detection accuracy of the gas to be detected is improved. Further, since a heat transfer path from the heating resistor 71 to the metal layer 75 side is formed, heat escape from the contact portion between the thin film portion (upper insulating layer 67) and the substrate 61 is reduced.
When viewed in the plane direction of the gas detection element 60, the metal layer 75 may be provided inside the formation region 71R of the heating resistor 71, or the metal layer 75 may protrude outside the formation region 71R. In order to increase the conductivity, it is preferable to provide the metal layer 75 so as to overlap with the formation region 71R as shown in FIG. Further, in the upper insulating layer 67, it is preferable that a metal layer 75 is formed in a region inside the concave portion (opening portion of the substrate 61) 62 when viewed in the planar direction of the gas detection element 60. The reason is that a part of the metal layer 75 is formed up to a position corresponding to the periphery of the opening of the substrate 61 (the formation region end of the thin film portion) as viewed in the plane direction of the gas detection element 60, or If the metal layer 75 is formed so as to protrude beyond the peripheral edge (end of the thin film portion formation region), the heat of the metal layer 75 may escape to the substrate 61 side, and heat conduction from the heating resistor 71 to the detection gas may be reduced. Because.
The metal layer 75 may be a single metal or an alloy. The metal layer 75 is preferably made of Al, Cu, Au, or an alloy thereof having high thermal conductivity. In particular, the metal layer 75 is preferably formed by sputtering Al, Cu, Au, or an alloy thereof.

As shown in FIG. 4, when the cross section of the metal layer 75 is observed with a laser microscope (magnification 100 times), an average of 10 points from the highest peak h1 to h10 in one visual field S of 0.128 mm × 0.096 mm. When the height is equal to or greater than ½ of the thickness D of the region (thin film portion) corresponding to the recess 62 (opening portion of the substrate 61) in the upper insulating layer 67, the heating resistor 71 to the detection gas This is preferable because the thermal conductivity is increased. The reason why the average height T of 10 points from the peak h1 to h10 is defined is that the heat is preferentially transferred from the high-height convex portion of the metal layer 75 to the gas to be detected. Is to reflect. In FIG. 4, for the sake of easy understanding, the highest peak h1 to the fourth peak h4 are specified and schematically shown.
The heights of the peaks h1 to h10 are the heights from the outermost surface of the second insulating layer 64 (thin film portion).

  As a manufacturing method of the metal layer 75, a thin film method such as a method of printing or plating a paste containing a metal constituting the metal layer 75 (thick film method) or a sputtering (sputtering method) can be given. As a method of making the surface of the metal layer 75 uneven, there is a method in which an impurity different from the material constituting the metal layer 75 is present in the atmosphere gas at the time of sputtering, and the metal layer 75 is abnormally grown like a dendrite. For example, an atmosphere in which nitrogen is mixed in Ar gas is exemplified.

Next, the manufacturing process of the gas detection element 60 will be described with reference to FIGS. In addition, the composition and thickness of the following coating films are merely examples.
Step of forming a part of the first insulating layer 68, the lower insulating layer 66, and the insulating protective layer 69 (FIG. 5A)
A substrate 61 made of a silicon semiconductor is prepared, and after cleaning the substrate 61, a silicon nitride film (Si ₃ N ₄ film) is formed to a thickness of 200 nm on each of the upper and lower surfaces of the substrate 61 by low pressure CVD. Thereby, the silicon nitride film on the upper side of the substrate 61 is formed as the first insulating layer 68, while the silicon nitride film on the lower side of the substrate 61 is formed as the lower insulating layer 66.
Next, on the surface of the insulating layer 68, a lower insulating protective layer 69a forming a part of the insulating protective layer 69 made of a silicon oxide layer (SiO ₂ layer) is formed with a thickness of 100 nm by plasma CVD. .

Step of forming the heating resistor 71 and the wiring films 711 and 712 (FIG. 5B)
Next, in an atmosphere at a temperature of 300 ° C., a tantalum film (Ta film) is formed by sputtering on the surface of the lower insulating protective layer 69a with a thickness of 20 nm, and then a platinum film (Pt film) is formed with a thickness of 150 nm. A layer is formed on the tantalum film by thickness by sputtering. Thereafter, portions of the tantalum film and the platinum film other than the corresponding portions corresponding to the heating resistor 71 and the wiring films 711 and 712 are removed by photolithography. As a result, the heating resistor 71 and the wiring films 711 and 712 are formed on the surface of the lower insulating protective layer 69a. The resistance temperature coefficient of the wiring films 711 and 712 and the heating resistor 71 is about 2500 [ppm / ° C.]. In this step, the resistance temperature detector 80 is also formed on the surface of the lower insulating protective layer 69a by the same method as the heating resistor 71.

Step of forming the entire insulating protective layer 69 and the second insulating layer 64 (FIG. 5C)
Next, an upper insulating protective layer 69b having the same composition is formed on the surface of the lower insulating protective layer 69a so as to cover each heating resistor 71 and the wiring films 711 and 712 by a plasma CVD method to a thickness of 100 nm. Form. Each heating resistor 71 and wiring films 711 and 712 are embedded between the lower insulating protective layer 69a and the upper insulating protective layer 69b, and the lower insulating protective layer 69a and the upper insulating protective layer 69b are an integral layer. A protective layer 69 is formed.
Further, a second insulating layer 64 made of a silicon nitride layer (Si ₃ N ₄ layer) is formed on the insulating protective layer 69 in a laminated shape with a thickness of 200 nm by a low pressure CVD method.

Contact hole formation process (FIG. 6D)
Next, each part corresponding to the wiring films 711 and 712 in the stacked layer of the insulating protective layer 69 and the second insulating layer 64 is removed by etching under a photolithography process. As a result, a contact hole 84 is formed on the wiring films 711 and 712. Further, a contact hole (not shown) for the resistance temperature detector 80 is formed by the same etching.

Step of forming electrodes 85 and 86 (FIG. 6E)
After forming the second insulating layer 64 as described above, a chromium film (Cr film) is formed in a layer shape on the second insulating layer 64 with a thickness of 20 nm by sputtering, and then on this chromium film, A gold film (Au film) is formed in a laminated form with a thickness of 400 nm by sputtering.
Thereafter, the portions other than the corresponding portions corresponding to the contact holes 84 in the electrode layer composed of the gold film and the chromium film laminated in this manner are removed by etching under a photolithography process. As a result, electrodes 85 and 86 are formed corresponding to each contact hole 84. In this process, the electrode 88 and the ground electrode 89 are also formed corresponding to contact holes (not shown).

Formation process of the diaphragm 62 (FIG. 6F)
After the electrodes 85 and 86 are formed as described above, the portions of the lower insulating layer 66 corresponding to the heating resistor 71 are removed by etching, and then the portions of the substrate 61 corresponding to the removed portions are removed. Etching is performed using tetramethylammonium hydroxide to expose the portion of the insulating layer 68 corresponding to the heating resistor 71 to the outside. As a result, an opening is formed in the substrate 61, and the recess 62 is formed in a portion of the substrate 61 and the lower insulating layer 66 corresponding to the heating resistor 71.

Step of forming metal layer 75 (FIG. 6G)
An electrode protective layer made of a silicon oxide layer (SiO ₂ layer) is formed by plasma CVD so as to cover the second insulating layer 64, the electrodes 85 and 86, the electrode 88, and the ground electrode 89. This electrode protective layer is for protecting the electrodes 85, 86, 88, and 89 when forming the metal layer 75.
Next, Al is sputter-formed on the surface of the electrode protective layer in an atmosphere in which nitrogen is mixed in Ar gas, and then removed by etching leaving a portion corresponding to the heating resistor 71. The nitrogen mixed in the Ar gas causes the metal layer 75 to grow abnormally in a dendrite shape, and the surface thereof becomes uneven (rough surface).
Finally, the electrode protection layer is removed by etching, and the gas detection element 60 is completed.

  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. In the above embodiment, the metal layer 75 is formed directly on the surface of the second insulating layer 64. However, the metal layer 75 is formed on the second insulating layer 64 so as to protect the second insulating layer 64. ) An insulating protective layer (for example, an insulating protective layer formed of alumina) may be formed, and the metal layer 75 may be formed on the insulating protective layer.

1 Gas sensor 61 Substrate 62 Recessed portion (substrate opening)
71 Heater (heating resistor)
71R Heater formation area 75 Metal layer

Claims (4)

A thin film portion provided on the substrate so as to cover the opening formed in the substrate;
A heat conduction type gas sensor comprising a heater provided inside the thin film portion and having a resistance value that changes due to a temperature change of the thin film portion;
The gas sensor in which the metal layer which has an unevenness | corrugation is formed in at least one part of the surface of the formation area of the said heater among the said thin film parts.   2. The gas sensor according to claim 1, wherein the metal layer is formed on a surface of a region of the thin film portion located inside a periphery of the opening.   3. The gas sensor according to claim 1, wherein the metal layer is formed by sputtering Al, Cu, Au, or an alloy thereof.   The gas sensor according to any one of claims 1 to 3, wherein the gas sensor detects hydrogen gas.

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