JP3101115B2 - Gas type determining sensor and gas type determining method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas type discriminating sensor and a gas type discriminating method.

[0002]

2. Description of the Related Art Conventionally, when determining the type of gas, a detector utilizing so-called mass spectrometry is used. FIG.
FIG. 2 is a diagram illustrating the principle of a conventional 180 ° deflection type mass spectrometer. The test gas ionized by the ion source 51 is accelerated by the voltage V of the acceleration power supply 52, deflected by the magnetic field 53, focused on the ion collector 54, and detected. At this time,
The orbital radius R of the test gas ion follows the following equation.

R = k · (M · V) ^1/2 / B where M: mass number of test gas ion, V: acceleration voltage, B
(T): magnetic field strength k: constant = 1.445 × 10 ^-4 And since the mass number M differs depending on the type of the gas to be detected, a plurality of ion collectors are installed, and the ion gas to be detected reaches any ion collector Thus, the type of the test gas can be determined.

[0004]

A detector utilizing mass spectrometry has a feature of high measurement accuracy, but has the following problems. One is that the detector itself becomes very large. Equipped with the necessary equipment such as an ion source, a magnet for generating a magnetic field, and a plurality of ion collectors, the scale becomes large.

Another problem is that it is difficult to discriminate between gases having a small mass difference and close masses, and there is a high possibility that the discrimination is erroneous. In view of the above circumstances, the present invention relates to a sensor used for gas type discrimination, and a gas type discrimination sensor having suitability for discrimination between small and close-mass gases, and a gas type discrimination using this sensor. It is an object of the present invention to provide a method for performing the above.

[0006]

In order to solve the above-mentioned problems, a gas type discriminating sensor according to the present invention comprises a substrate having a hollow portion, and a heat insulating member which covers the hollow portion and whose periphery is supported by the substrate. A thin film, and a thermistor provided on a region covering a hollow portion of the heat insulating thin film , wherein the substrate is disposed on a stem, and a test gas is provided.
Mean free path L of gas molecules, heat insulating thin film and Stem
Satisfies the relationship of L> d .

When the type of gas is determined using the sensor of the present invention, the test gas is introduced while the sensor is placed in a reduced pressure atmosphere and the thermistor is energized, and the resistance of the thermistor when the test gas is introduced is measured. The type of the test gas is determined based on the value. Hereinafter, the gas type determination sensor according to the present invention will be specifically described with reference to the drawings.

FIG. 1 shows an example of a configuration of a main part of a gas type determination sensor according to the present invention. In the case of the gas type discriminating sensor 1 shown in FIG. 1, a substrate 2 having a hollow portion 3, a heat insulating thin film 4 covering the hollow portion 3 and having the periphery supported by the substrate 2, and a hollow portion 3 of the heat insulating thin film 4 And a thermistor 5 provided on a region covering the. Reference numeral 7 denotes a stem (base) that constitutes a part of the container. Further, there is a case where the can is combined with the stem 7 in some cases.

As the substrate 2, a semiconductor substrate such as a silicon substrate is used. The hollow portion 3 can be formed by digging the silicon substrate 2 by anisotropic etching. As the heat insulating thin film 4, a heat insulating thin film such as a semiconductor thin film or a dielectric thin film is used. Preferred heat insulating thin films 4 include:
A thin film having a multilayer structure in which a plurality of layers are stacked is exemplified. For example, a silicon oxide (SiO) layer and a silicon nitride layer (Si
In the case of the multilayer structure formed by N), the stress is balanced between the films, and the mechanical strength of the film is high, which is preferable.

In the thermistor 5, a thin-film resistor is used as a temperature-sensitive resistor whose resistance value changes with a temperature change. As the thin-film resistor, a semiconductor thin film, particularly a silicon-based semiconductor thin film, is suitable. is there. Examples of the silicon-based semiconductor thin film include an amorphous silicon (a-Si) thin film and an amorphous silicon carbide (a-SiC) thin film, and a plurality of different semiconductor layers such as an a-Si layer and an a-SiC layer are stacked. In some cases, a semiconductor thin film having a multilayer structure may be used. For example, a-S
An i-layer is laminated, and the conductive thin films serving as upper and lower electrodes are a-
A configuration in which the layers are in contact with each other by an Si layer is exemplified.

The gas type discriminating sensor having the above structure is
It can be manufactured using semiconductor device manufacturing technology.
Materials for the substrate 2, the heat insulating thin film 4, and the thermistor 5
As for the thin film, any of the materials and thin film forming methods used in the manufacture of semiconductor devices can be applied, and the hollow portion 3 and the thermistor 5 can be used.
Can apply the anisotropic etching and fine processing techniques used in the manufacture of semiconductor devices, and as a result, a gas type discrimination sensor having a very small size can be easily realized.

[0012]

The principle of operation of the gas type discrimination sensor according to the present invention will be described with reference to FIG. In the gas type discriminating sensor 1 of the present invention, the back side of the portion where the thermistor 5 is placed on the heat insulating thin film 4 is the hollow portion 3. The heat generated by the thermistor 5 is more predominantly flowing (radiated) to the stem 7 using air or the like as a medium than diffusing to the substrate 2 due to the large resistance.

On the other hand, when there is a certain relationship between the mean free path L of the gas molecules existing in the hollow portion 3 and the dimension d between the heat insulating thin film 4 and the stem 7, the amount of heat transmitted from the thermistor 5 to the stem 7 Q follows the formula below. Q = αΛ ₀ p (273.2 / T) ^1/2 (T ₂ −T ₁ ) · A where α: temperature-dependent coefficient, Λ ₀ : free molecular thermal conductivity (depending on the type of gas) Different), T ₁ : thermistor formation region in the thermally insulating thin film, T ₂ : temperature of the stem A: area of the thermistor formation region in the thermally insulating thin film, T: temperature of the hollow portion 3 The above heat quantity Q is a free molecule as can be seen from the equation. Thermal conductivity す る proportional to ₀ , but this free molecular thermal conductivity Λ
₀ varies depending on the type of gas as shown in Table 1 below.

[0014]

[Table 1]

The change in the amount of heat Q caused by the difference in the thermal conductivity inherent to the gas has a meaning in determining the type of gas because the mean free path L and the dimension d between the heat insulating thin film 4 and the stem 7 are different.
Is the case where the relationship of the mean free path L> the dimension d is satisfied. When there is no relation of mean free path L> dimension d,
Considerable gas molecules already exist in the hollow portion 3, and the appropriate thermal insulation action by the hollow portion 3 has already been broken.
This is because even if the test gas is introduced into the chamber, the amount of heat Q that can be used to determine the type of gas does not change.

The relationship of mean free path L> dimension d is practically difficult to realize unless the atmosphere is a reduced pressure atmosphere.
It seems difficult unless the atmosphere is under a reduced pressure of 1 Torr or less. Since the pressure value (degree of vacuum) of the reduced-pressure atmosphere is also a factor that causes a change in the amount of heat Q, it is usually desired that the pressure value of the reduced-pressure atmosphere in which the sensor is installed be kept constant. The pressure value of the reduced pressure atmosphere may be different depending on the type of the detection gas.

When using the gas type discriminating sensor of the present invention, as described above, the sensor is installed in a reduced-pressure atmosphere, and a current is supplied to the thermistor 5 so that the thermistor 5 is heated to generate heat. On the other hand, the heat quantity Q is determined by the free molecular thermal conductivity Λ ₀ of the gas existing in the hollow portion 3, and the thermistor 5
The temperature is determined by the heat value and the heat value Q. On the other hand, the free molecular thermal conductivity Λ ₀ differs depending on the type of gas. If the type of gas existing in the hollow portion 3 changes, the heat quantity Q changes, and the temperature determined by the thermistor 5 changes. On the other hand, the difference 5 changes the resistance value when the temperature changes. Therefore, the hollow part 3
The resistance value of the thermistor 5 changes depending on the type of gas present in the above, so that the type of gas can be known based on the resistance value of the thermistor 5. As described above, in the present invention, the type of gas is determined by focusing on the thermal conductivity inherent to the gas.

[0018] Besides, for example, as seen in Table 1, may be mass there is a large difference in free molecular heat conductivity lambda ₀ be closer, it can be seen that performed accurately close the mass of gas determination. As described above, the gas type discrimination sensor of the present invention has a configuration in which a semiconductor device manufacturing technology is applied and a very small-sized form can be easily realized, so that a sufficient miniaturization can be achieved.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a gas type discriminating sensor according to the present invention will be described below with reference to the drawings. FIG. 2 is a cross-sectional view illustrating a main configuration of a gas type determination sensor according to the embodiment.

Embodiment The gas type discriminating sensor 1 of the embodiment is provided with a silicon substrate 2 having a hollow portion 3 which is a heat separation space, and the surface of the silicon substrate 1 covers the hollow portion 3 and has a silicon surrounding. This is a diaphragm configuration in which a heat insulating thin film 4 supported on a substrate 2 is provided, and a thermistor 5 is installed on a diaphragm structure area of the heat insulating thin film 4. The heat separation space functions to thermally insulate the thermistor 5 from the silicon substrate 2. Reference numeral 7 denotes a stem that constitutes a part of the container.

The heat insulating thin film 4 has a three-layer structure, and a silicon oxide (SiO) layer 4a having a thickness of 5000.degree.
It is a structure sandwiched between 0 ° silicon nitride (SiO layers) 4b and 4c. Thin films having different characteristics from those of tensile and compression are laminated to balance the stress between the films. And also has electrical insulation. Of course, it may be a heat insulating thin film having only a silicon oxide layer alone.

On the other hand, the thermistor 5 has a configuration in which a lower electrode 5b and an upper electrode 5c are provided on the back and front surfaces of a semiconductor thin film 5a which is a thin film resistor. As the semiconductor thin film 5a,
Formed by a capacitively coupled plasma CVD method,
300-mm thick p-type a-Si layer 51, 10000 mm thick
P-type a-SiC layer 52, p-type a-Si having a thickness of 300 °
It is an amorphous semiconductor thin film formed by stacking layers 53.

The conditions for forming the upper and lower p-type a-Si layers 51 and 53 are as follows: monosilane (B ₂ H ₆ / SiH ₄ = 0.25%) to which 0.25 mol% of diborane is added; Substrate temperature 180 ° C, gas pressure 0.9 Torr, discharge power 20W,
13.56MHz frequency, electrode size 30mm x 30m
m, and the electrode spacing was 25 mm. The conditions for forming the p-type a-SiC layer 52 are as follows: SiH ₄ : 100 sccm, B ₂ H
₆ (0.5% H ₂ base): 50 sccm, CH ₄ : 4
The gas supply amount was 00 sccm, the substrate temperature was 180 ° C., the gas pressure was 0.9 Torr, the discharge power was 20 W, and the frequency was 13.56.
MHz, electrode size 30mm x 30mm, electrode spacing 25m
m.

As the lower electrode 5b, an appropriate conductive thin film having a thickness of about 2000 ° formed by an electron beam evaporation method is used. As the conductive thin film (especially in the case of the lower electrode 5b), a Ni—Cr-based thin film is suitable, but a Cr thin film may be used. As the upper electrode 5c, an appropriate conductive thin film having a thickness of about 2000 ° formed by an electron beam evaporation method is used. Examples of the conductive thin film include a Cr thin film.

Needless to say, the semiconductor thin film and the conductive thin film are formed into a predetermined pattern shape by patterning using a fine processing technique. The a-Si layer 51 and a-S
Between the iC layer 52 and between the a-Si layer 53 and the a-SiC layer 52, it is possible to insert a layer (buffer layer) continuously or stepwise shifted from the a-Si composition to the a-SiC composition. It is desirable for obtaining good ohmic properties.

Further, a-Si layers 51 and 53 and a-SiC
The conditions for forming the layer 52 are not limited to the above conditions. For example, the gas pressure is 0.1 to 10 Torr, the discharge power is 10 to 150 W, the substrate temperature is 100 to 300 ° C., and B ₂ H ₆ / SiH ₄ = 0.0.
Appropriate conditions are selected from the range of 1 to 1%. a-SiC
The thickness of the layer 52 can also be selected from the range of several hundreds of μm to several μm. When such a thin film is used, a thermistor 5 having a B constant of about 5000 can be obtained.

Usually, after the heat insulating thin film 4 and the thermistor 5 are completed, the back surface of the silicon substrate 1 is HF-H
Drilling is performed by anisotropic etching using an etchant such as NO ₃ or KOH so as to leave the heat insulating thin film 4 to form the hollow portion 3 to complete the diaphragm structure. Thereafter, the back surface of the silicon substrate 1 was joined to the surface of the stem 7 using a silicone resin or the like, thereby completing a gas type discrimination sensor. In the case of the gas type discrimination sensor of the present invention, the state without the stem 7 may be used. In the case of the embodiment, the surface of the stem 7 is closest, and the dimension d of the distance between the surface of the stem 7 and the heat insulating thin film 4 needs to be smaller than the mean free path L. A configuration may be adopted in which a container surface facing closely is arranged, and the mean free path L is larger than the dimension of the distance between the container surface and the thermistor surface.

In the case of the embodiment, since the thickness of the silicon substrate 1 is about 300 μm, the surface of the stem 7 and the heat insulating thin film 4
Is about 300 μm, which is the distance on the back side of. FIG.
The relationship between the gas type and the temperature rise of the thermistor in the gas type determination sensor of the embodiment is shown below. The abscissa in FIG. 3 indicates the type of gas, and the ordinate indicates the degree of temperature rise when the gas is introduced into the thermistor in a depressurized atmosphere. FIG.
In FIG. 7, it is understood that the temperature of the thermistor 5 differs depending on the type of gas, and as a result, the type of gas can be determined.

[0029]

As described above, the gas type discriminating sensor according to the present invention has a configuration in which an extremely small form can be easily realized. If there is a large difference in conductivity, appropriate gas type discrimination can be performed, so it is suitable for gas discrimination between gases having similar masses and is highly useful, and gas type discrimination is performed using this sensor. In the case of the method of the present invention, a reduced-pressure atmosphere is necessary, and a large-scale apparatus configuration is not required, so that the practicality is remarkable.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing an example of a basic configuration of a gas type determination sensor according to the present invention.

FIG. 2 is a cross-sectional view illustrating a configuration of a main part of a gas type determination sensor according to the embodiment.

FIG. 3 is a graph showing the relationship between the type of gas and the temperature rise of the thermistor in the sensor of the embodiment.

FIG. 4 is an explanatory view showing the principle of a conventional mass spectrometer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Gas type discrimination sensor 2 Substrate 3 Hollow part 4 Thermal insulating thin film 5 Thermistor 7 Stem

Continuation of the front page (56) References JP-A-1-96549 (JP, A) JP-A-4-355699 (JP, A) Jiko 2-39249 (JP, Y2) (58) Fields surveyed (Int .Cl. ⁷ , DB name) G01N 27/18 G01N 25/18

Claims (6)

(57) [Claims] 1. A thermistor provided on a region having a hollow portion and a heat insulating thin film covering the hollow portion and having a periphery supported by the substrate, and provided on a region covering the hollow portion of the heat insulating thin film. The substrate is arranged on a stem
Mean free path L of gas molecules of the test gas
The dimension d between the edge thin film and the stem is defined by the relation of L> d.
Satisfactory gas type sensor. 2. The gas type discrimination sensor according to claim 1, wherein the heat insulating thin film is a thin film having a multilayer structure in which a plurality of layers are laminated. 3. The gas type discrimination sensor according to claim 2, wherein the multilayer structure is formed by laminating a silicon oxide layer and a silicon nitride layer. 4. The gas type discriminating sensor according to claim 1, wherein a resistor made of a semiconductor thin film is used as the temperature-sensitive resistor whose resistance value changes with a temperature change in the thermistor. . 5. The gas type discrimination sensor according to claim 4, wherein the semiconductor thin film has at least an amorphous silicon carbide thin film. 6. A gas type discriminating sensor according to claim 1, wherein the gas type discriminating sensor is disposed in a reduced-pressure atmosphere, and a test gas is introduced while the thermistor is energized. A gas type determining method for determining the type of the test gas based on the resistance value of the gas.