JPH1151893A - Contact combustion type gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the sensitivity by forming a heater and a temperature measuring resistor independently at a detecting part and preventing deterioration even after long term use. SOLUTION: A heater 4 comprising a silicon oxide layer 2, an insulation layer 3 of gas impermeable insulator, and polysilicon is arranged at a detecting part, i.e., a diaphragm part 1a. An insulation layer 5 is formed between the heater 4 and a temperature measuring resistor 6 of platinum while covering the heater 4 to protect against contact with the outer air. An oxidizing catalyst layer 7 of porous alumina is arranged around the temperature measuring resistor 6. A uniform heater 8 composed of boron diffused silicon is arranged on the silicon oxide layer 2 and contributes to make uniform the temperature at a diaphragm part 1a. Since the heater 4 is prevented from touching the outer air, it is protected against deterioration. Furthermore, since the electric resistance can be set independently for the temperature measuring resistor 6 and the heater 4, a most sensitive resistance can be set for a measured gas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalytic combustion type gas sensor for detecting flammable gas.

[0002]

2. Description of the Related Art FIG. 6 shows an example of a conventional catalytic combustion type gas sensor. This figure is a partially transparent perspective view in which the platinum wire inside can be seen. A porous alumina support having a catalyst is arranged around a thin platinum wire in a coil shape. In use, the alumina carrier having an oxidation catalyst such as a noble metal is kept at a temperature suitable for its catalytic ability by supplying electricity to a platinum wire at the time of use. When the combustible gas reaches the gas sensor, it is burned by a catalyst disposed on the alumina carrier and generates heat. By measuring the change in heat at this time by the change in resistance of the platinum wire, the detection of flammable gas becomes possible. However, such a catalytic combustion type gas sensor has a drawback that power consumption is large because of its large heat capacity. For this reason, there has been a problem in application to a small flammable gas measuring device using a battery.

[0003] As a sensor for improving such disadvantages,
In Japanese Patent Application Laid-Open No. Hei 8-94561, a heat capacity is reduced by a sensor as shown in FIGS. 7A and 7B to which a silicon semiconductor technology is applied. That is, an insulating layer made of silicon oxide is provided on a silicon substrate, a platinum wire is provided thereon, and a catalyst layer made of a carrier having an oxidation catalyst covering a part of the surface of the platinum wire is provided. The platinum wire side has the same function as the contact combustion type gas sensor of FIG. 6, and the platinum wire side works as a heat correction element. Note that a cavity is provided in the silicon substrate to measure a decrease in heat capacity near the measurement unit. As described above, this sensor has the advantages of low power consumption and easy downsizing, but has the disadvantage that it tends to deteriorate as the usage time increases. This also had the disadvantage that the output for combustible gas was small.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a contact combustion type gas sensor which does not deteriorate even when used for a long time and has high sensitivity.

Means for Solving the Problems The present inventors have focused on the sensor according to the prior art described above, in particular, the gas sensor described in JP-A-8-94561, and examined the cause of its deterioration. As a result, the platinum wire of this sensor is a thin-film heater with extra-fine wiring, and since the film thickness is smaller than the wiring width, current concentration tends to occur in a specific portion, causing local heat generation. As a result, it was found that the resistance changed due to high-temperature oxidation of the heater, crack generation, and the like. The contact combustion type gas sensor according to the present invention solves the above problem.
As described in (1), the detector has a heater and a resistance temperature detector as separate components.

[0005]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, it is necessary to separately provide a heater and a resistance temperature detector in a detection unit.
With this configuration, it is possible to perform measurement with a resistance temperature detector without being affected by a heater that is easily deteriorated and has a large change in resistance with time. Further, the resistance of the heater is limited in the setting of the resistance value in order to reduce the power consumption. However, in the present invention, since the heater has a temperature measuring resistor separately from the heater,
The electric resistance of the resistance temperature detector can be set to an optimum resistance value for measurement, and therefore, high gas sensitivity can be obtained.

In the catalytic combustion type gas sensor of the present invention,
If an insulator is placed between the heater and the resistance temperature detector,
This is desirable because heat conduction is performed by the insulator. Such insulators include tantalum pentoxide, silicon nitride, silicon carbide, aluminum nitride, and hafnium oxide. Note that among these, silicon nitride, silicon carbide, and aluminum nitride are preferable because of high thermal conductivity. In addition, the conventional contact combustion type gas sensor,
Since the heater was also used as a resistance temperature detector, it was covered with a porous carrier having an oxidation catalyst by its nature, and was usually heated in an atmosphere containing oxygen such as air. However, since the contact combustion type gas sensor of the present invention has the heater and the temperature measuring resistor separately as described above, the heater does not need to be in contact with the outside air. Therefore, it is possible to prevent the outside air from contacting the heater with the gas impermeable insulator. With such a configuration, the life of the heater can be significantly extended, the occurrence of temperature unevenness can be prevented, and the aging can be made extremely small. As described above, tantalum pentoxide, silicon nitride, silicon carbide, aluminum nitride, or phosphoborosilicate glass can be used as the gas impermeable insulator that prevents contact between the heater and the outside air.

In the catalytic combustion type gas sensor of the present invention,
It is desirable to have a heat equalizer for reducing the temperature distribution of the detection unit. The heat equalizer needs to be made of a material having excellent heat conductivity, and by this function, the temperature distribution of the detecting portion of the sensor is made uniform, and the sensor sensitivity is stabilized. It is preferable that the heat equalizer is a silicon layer formed by diffusing boron, because it is easy to form and has good thermal conductivity. Next, an example α of the catalytic combustion type gas sensor according to the present invention will be described with reference to the drawings.

FIG. 1A is a top view of a catalytic combustion type gas sensor α according to the present invention, and FIG. 1B is a sectional view thereof. In the figure, reference numeral 1 denotes silicon serving as a substrate, which has a diaphragm 1a serving as a detection unit. The diaphragm 1a includes a silicon oxide layer 2, an insulating layer 3 made of a silicon nitride film which is a gas impermeable insulator, and a heater 4 made of polysilicon (polycrystalline silicon) on the insulating layer 3. Are arranged.
In this sensor α, polysilicon is used as the material of the heater. However, P ⁺⁺ silicon (silicon in a region in which p-type impurities are diffused at a high concentration), ruthenium oxide, platinum, or the like can be used. However, by using polysilicon, the thermal expansion behavior of the material due to heat generation can be made very close to the thermal expansion behavior of the silicon oxide film and the thermal expansion behavior of the silicon nitride film. Can be enhanced.

An insulating layer 5 made of a silicon nitride film, which is a gas impermeable insulator, is disposed between the heater 4 and a resistance temperature detector 6 made of platinum. The insulating film 5 covers the heater 4 to prevent outside air from contacting the heater 4. A catalyst layer 7 made of porous alumina having palladium as an oxidation catalyst is arranged around the temperature measuring resistor 6.

The oxide film 2 has a heat equalizer 8 made of silicon in which boron is diffused, which contributes to uniformizing the temperature of the diaphragm portion 1a serving as a detection portion. It should be noted that the diaphragm portion 1a has a thin silicon substrate 1, and the temperature control is facilitated by a decrease in the heat capacity of the detection portion. With such a configuration, the heater 4 of the contact combustion type gas sensor α is hardly deteriorated because the heater 4 is prevented from contacting with outside air. Furthermore, since the electric resistance of the resistance temperature detector 6 can be set separately from the electric resistance of the heater 4, it is possible to obtain a resistance value that provides the best sensitivity to the gas to be measured. Further, even when the heater is deteriorated to some extent, it is possible to perform highly accurate measurement with almost no influence.

Such a catalytic combustion type gas sensor α is a sensor S having a resistance value Rs, similar to the contact combustion type gas sensor α, except that a temperature compensating sensor D (electric resistance value) having no oxidation catalyst in the catalyst layer is used. Rd) is used by being incorporated in a bridge circuit as shown in FIG.

The contact combustion type gas sensor α is manufactured, for example, as follows. One side of the silicon substrate as indicated by reference numeral 1 in FIG. 3A is oxidized by thermal oxidation to form an oxide film 1 ′ (see FIG. 3B). Next, the oxide film 1 'is patterned by a photoresist processing method to open a window as shown in FIG. Boron is diffused into the window by a vapor phase diffusion method to form a soaking body 8 (see FIG. 3D). The oxide film 1 ′ is removed by peeling off by wet etching (see FIG. 4A), and the entire surface of the silicon substrate 1 on which the heat equalizer 8 is provided is again oxidized by thermal oxidation to form the silicon oxide layer 2. Forming (FIG. 4B)
reference). Next, an insulating layer 3 made of silicon nitride is formed on the oxide layer 2 by a CVD (chemical vapor deposition) method (see FIG. 4C). Further, a heater 4 made of polysilicon is formed at a position corresponding to the heat equalizer 8 by a CVD method (see FIG. 4D).

An insulating layer 5 made of silicon nitride, which is a gas impermeable insulator, is formed by CVD so as to cover the heater 4.
5 (a), and a platinum resistance temperature detector 6 is disposed on the insulating layer 5 by sputtering (see FIG. 5 (b)). Next, a catalyst layer 7 made of porous alumina containing palladium as an oxidation catalyst is provided by vapor-depositing a target made of a mixture of palladium and alumina so as to cover the resistance bulb 6 (see FIG. 5C). Here, the catalyst layer 7 can be obtained by sputtering instead of vapor deposition. Finally, the silicon substrate is etched with potassium hydroxide to form the diaphragm 1a. Here, tetramethylammonium hydroxide may be used instead of potassium hydroxide. Thus, the contact combustion type gas sensor α can be obtained (FIG. 5).
(D)).

[0014]

According to the present invention, since the detector has a heater and a resistance temperature detector separately from each other, it is easy to deteriorate and the resistance is greatly changed with time. Becomes possible. Further, the heater has a limitation in the setting of its resistance value in order to reduce power consumption. However, in the present invention, since the heater has a temperature measuring resistor separately from the heater, an optimal resistance value for measuring the electric resistance of the temperature measuring resistor is used. , So that high gas sensitivity can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example α of a contact combustion type gas sensor according to the present invention. (A) Top view (b) Cross-sectional view

FIG. 2 is a diagram showing an example of a detection circuit in which an example α of a catalytic combustion type gas sensor according to the present invention is incorporated as a sensor S.

FIG. 3 is a view showing a manufacturing process of an example α of the catalytic combustion type gas sensor according to the present invention.

FIG. 4 is a view showing a manufacturing process of an example α of the catalytic combustion type gas sensor according to the present invention.

FIG. 5 is a diagram showing a manufacturing process of an example α of the catalytic combustion type gas sensor according to the present invention.

FIG. 6 is a diagram showing an example of a conventional catalytic combustion type gas sensor.

FIG. 7 is a diagram showing another example of a conventional catalytic combustion type gas sensor. (A) Cross-sectional view (b) Top view

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Silicon substrate 1 'Oxide film 1a Diaphragm part 2 Silicon oxide layer 3 Insulating layer 4 Heater 5 Insulating layer 6 Resistance thermometer 7 Catalyst layer 8 Soaking body

Claims (7)

[Claims] 1. A catalytic combustion type gas sensor, wherein a detection unit has a heater and a resistance temperature detector separately. 2. The catalytic combustion type gas sensor according to claim 1, wherein an insulator is arranged between the heater and the resistance temperature detector. 3. The contact combustion type gas sensor according to claim 1, wherein the heater is prevented from contacting with outside air by a gas impermeable insulator. 4. The method according to claim 1, wherein the gas impermeable insulator is tantalum pentoxide, silicon nitride, silicon carbide, aluminum nitride,
And one selected from hafnium oxide and phosphoborosilicate glass
The catalytic combustion type gas sensor according to any one of claims 1 to 3, wherein the gas sensor comprises at least one. 5. The catalytic combustion type gas sensor according to claim 2, wherein the insulator is a thin film made of silicon nitride. 6. A heat equalizing body for reducing a temperature distribution of the detecting section.
4. The contact combustion gas sensor according to claim 1. 7. The catalytic combustion type gas sensor according to claim 6, wherein the heat equalizing body is made of silicon in which boron is diffused.