JP2000292395A - Thin film gas sensor - Google Patents

Abstract

(57) 【Summary】 PROBLEM TO BE SOLVED: To provide an overall improvement of a thin film gas sensor having a diaphragm structure. SOLUTION: In a thin film gas sensor having a SnO ₂ film as a gas detection film (sensor portion) and having a gas selective combustion layer made of a sintered body such as alumina on the thin film gas sensor, the size, thickness and diaphragm of the sensor portion are provided. By devising the ratio with the diameter, etc., gas selectivity can be increased and power consumption can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-power-consumption thin-film gas sensor assuming battery operation, and more particularly to a battery-operated gas sensor for detecting methane gas, propane gas and CO gas for city gas.

[0002]

2. Description of the Related Art Generally, a gas sensor is used for CO, C, and C.
A device that selectively responds to H ₄ , C ₃ H _8, etc.
Due to its characteristics, high sensitivity, high selectivity, high response, high reliability, and low power consumption are essential. By the way, gas leak alarms that are widespread for home use include those for detecting flammable gas for city gas or propane gas, those for detecting incomplete combustion gas in combustion equipment, or However, there is a combination of both functions, but the penetration rate is not so high due to cost and installation problems. Under these circumstances, in order to increase the penetration rate, it is desired to improve the installation property, specifically, to achieve cordless operation by battery driving.

[0003] In order to achieve battery driving, it is essential to reduce the size and reduce the power consumption.
It is manufactured by a method of sintering powder such as O _2, and there is a limit to downsizing even if a method such as screen printing is used. On the other hand, a sensor using a semiconductor thin film processing process, which is relatively easy to miniaturize, is also known. In this sensor, an insulating film, a heater, an electrode, and a sensitive film are laminated on a substrate by a CVD method, a vapor deposition method, or a sputtering method. In order to further reduce the heat capacity of the sensor and to achieve thermal insulation, a method is employed in which the substrate is etched to make it thinner or completely removed.

However, it is impossible to completely insulate a sensor manufactured by a semiconductor thin film process, and the size, thickness, and structure of the sensor in consideration of heat conduction from leads such as a support layer and an electrode are considered. There is a need to. Furthermore, intermittent operation is indispensable to operate by battery,
It is also a necessary condition that the device can operate with a duty ratio as small as possible. Also, in that high selectivity is a thin film sensor element due to evaporation or sputtering, doping and the noble metal element into the SnO ₂ film, SnO ₂ film aimed molecular sieve effect due to the difference in size of the gas molecules A method of forming a certain film structure on the upper part is known, but a sufficient effect has not always been obtained.

[0005]

As described above, high gas selectivity cannot be obtained with the conventional thin film sensor element.
Further, no consideration is given to a low power consumption structure and operating conditions sufficient to realize battery driving. As a result, it was difficult to mount the battery-operated gas leak alarm on a battery-operated gas leak alarm for a long period of five years or more. Therefore, an object of the present invention is to provide a gas sensor having a high gas selectivity particularly for CO gas and CH ₄ gas, and having a low power consumption structure and operating conditions for driving a battery.

[0006]

The size of the heater, the gas detection film and the selective combustion layer, and the ratio of the size of these to the diaphragm are taken into consideration. By using an alumina sintered body carrying a Pd catalyst, gas selectivity is improved and low power consumption for battery driving is achieved. In addition, the heating time of the heater is 200 seconds or less, and the duty ratio is 1/100 or less intermittent drive. In particular, when the CO gas is detected, the voltage is switched within the heater heating time and the low voltage (L
o) The gas sensitivity is measured at a predetermined temperature or lower at the time of application.

[0007]

FIG. 1 is a schematic sectional view for explaining an embodiment of the present invention. That is, although the cross-sectional structure itself as shown in the figure is a known one, by setting the size of the sensor portion, the film thickness, the ratio with the diaphragm diameter, and the like to appropriate values, it is possible to obtain high gas selectivity and low gas selectivity. It is intended to reduce power consumption. First, a SiO ₂ thermal oxide film on both sides was 0.3 μm
(0.1-1 μm) on the surface of the Si substrate, a 0.15 μm (500 °-
5000 .mu.m), and a 1 .mu.m (0 to ₂ .mu.m) SiO2 film are sequentially formed by a plasma CVD method. On this, a TaN film as a heater layer 1 is formed to a thickness of 0.5 μm (0.1 to 1 μm).
m) Forming and forming a heater pattern by wet etching. Further, after a SiO ₂ insulating film or the like is formed to a thickness of 0.5 μm (0.2 to 2 μm) by a sputtering method, the joint between the heater and the electrode pad is etched with fluorine (F) to open a window.

Next, a Pt / Ta film (200
0 ° / 500 °: 1000-5000 °) is formed as an electrode pad for a heater and a gas detection film, and is patterned by waist etching. Here, the Ta film has a role as a bonding layer for increasing the adhesion between the Pt layer and the SiO ₂ layer. Further, 5% of Pt is added to SnO ₂ as a gas detection film 2 serving as a sensor portion by a sputtering method.
The supported powder is made into a paste together with a binder, applied to the surface of SnO ₂ by screen printing and baked to form a paste of about 20.
A selective combustion layer 3 of μm (5 to 100 μm) is formed. In this example, the heater and the SnO ₂ layer are formed in a size of 100 μm square, and the selective combustion layer is formed in a 150 μm square so as to completely cover the SnO ₂ layer. Finally, Si is completely removed from the back surface of the substrate by dry etching to a diameter of 300 μm to form a diaphragm structure. At this time, the size of the heater 1, the SnO ₂ layer 2 and the selective combustion layer 3 is 3
It is desirable that the diameter ratio between the diaphragm and the diaphragm (S / D in FIG. 1) should be 以下 or less. In addition,
The diaphragm is formed by supporting the periphery of a thin-film-like support film with a Si substrate and forming the periphery to be thick and the center part to be thin.

Here, the heat loss of the sensor will be considered. In general, the heat loss H _LOSS of a sensor is expressed as: H _LOSS = Qc + Qt + Qr (Qc: heat conduction, Qt: heat transfer, Qr: heat radiation) (1) Considering the diaphragm sensor as a cylindrical model, Qc,
Qt and Qr are as follows. _{Qc = 2π · d 1 · λ} 1 (T H -T 0) / ln (Rd / Rs) + n · W · d 2 · λ 2 (T H -T 0) / (Rd-Rs) ... (2) Qt _{^{= 2πRd 2 · α (T H}} -T 0) ... (3) Qr = 2πRd 2 · σ · ε (T H 4 -T 0 4) ... (4)

The meanings of the symbols in the above equations are as follows. d _1, d _2: thickness of the support layer, the thickness of lambda ₁ of the electrode lead from the heater, lambda _2: support layer, the thermal conductivity of the electrode lead from the heater T _H, T _0: sensor unit, of the atmosphere Each temperature Rd, Rs: Each radius of the diaphragm and the sensor part n: Number of electrode leads W: Electrode lead width α: Heat transfer coefficient σ: Stefan-Boltzmann constant ε: Emissivity Here, Qt and Qr are considerably smaller than Qc. So Q
Considering only c, the larger the ratio of Rd / Rs (the inverse ratio of S / D in FIG. 1) and the thinner the support layer thickness d ₁ ,
It can be seen that the heat loss through the support layer becomes smaller, and the heat loss from the electrode lead becomes smaller as Rd-Rs becomes larger. However, the heat loss from the electrode leads is generally much smaller than the heat loss through the support layer.

Next, assuming that the battery is driven by a battery, an intermittent operation is indispensable for further reducing power consumption. It is self-evident that the shorter the time the heater is energized and the longer the intermittent operation interval, the lower the power consumption. However, especially the heater energization time is the time required for the sensor unit to rise to a certain temperature. And the gas reaction rate. The following equation (5) represents the time t required for the temperature rise of the sensor section in the adiabatic state. _{t = m · Cp (T H} -T 0) / P = π · Rs 2 · ds · ρ · Cp (T H -T 0) / P ... (5) Rs: thickness of the sensor section: radius ds of the sensor unit Ρ: Density of the sensor part Cp: Specific heat of the sensor part P: Power input to the heater In other words, when the sensor part is relatively thin, 1 μm or less, the heating time can be almost ignored compared to the gas reaction time,
If it is thick, it cannot be ignored. As can be seen from equation (5), the time required for raising the temperature of the sensor unit can be reduced by reducing the radius Rs and the thickness ds of the sensor unit.

As described above, from the above equations (2) and (5), the size and thickness of the sensor portion including the selective combustion layer and the ratio between the size of the sensor portion and the size of the diaphragm are limited. . FIG. 2 shows the relationship between the diameter of the diaphragm and the power consumption when the size of the sensor section including the heater and the gas detection element is fixed at 100 μm. It can be seen that the power consumption decreases as the diaphragm increases, that is, as the ratio of the size of the sensor unit to the diaphragm increases. FIG. 3 shows the relationship between the film thickness of the entire sensor section (see T in FIG. 1) and the heating rate of the heater obtained by thermal analysis using the finite element method. The selective combustion layer formed mainly for the purpose of burning H ₂ requires a certain thickness,
As the thickness increases, the rate of temperature rise decreases.

FIG. 4A shows a SnO 2 thin film element (5% Pt).
The graph shows the detection temperature dependency on the CO, CH4, H2 gas sensitivity in the case of only doping). To detect CH ₄ gas, a high temperature of 450 ° C. or higher is required.
High sensitivity to _two gases, resulting in poor gas selectivity. FIG. 4B shows gas sensitivity when a sintered layer in which Pt or Pd is supported on alumina is formed as a selective combustion layer on the SnO ₂ thin film element of FIG. 3A. A sintered body in which Pt or Pd is supported on alumina well burns a gas such as H _2, as used in a contact combustion type gas sensor. Thus, H ₂ and CO gas does not reach the SnO ₂ film is consumed in the selected combustion layer, CH ₄ gas causes the resistance change in SnO ₂ film without being consumed in the selected combustion layer. Further, from FIG. 4A, the CO gas sensitivity is higher in the temperature range relatively lower than that of CH ₄ , and is particularly selective to H ₂ gas at 300 ° C. or lower. From the above,
It can be seen that a sensor having high sensitivity only to a specific gas can be obtained by selecting the selected combustion layer and the operating temperature.

Next, the relationship between the battery life and the intermittent operation interval is expressed as follows. Battery life = Cbat / (Iappl × duty ratio) (6) Cbat: Battery capacity Iappl: Current applied to heater FIG. 5 shows the relationship between battery life and duty ratio according to equation (6). This shows an example of calculation results when the current applied to the heater is 10 mA and the battery capacity is changed from 1000 to 10000 mAh. Considering the operation guarantee of 5 years or more (43,380 hours), when a 5000 mAh battery is used, the duty ratio needs to be 1/100 or less. In addition, when the present sensor is used as a general city gas alarm device, since it is necessary to perform detection at least once every 20 seconds, it is obvious that the heater energizing time is restricted by the relationship with the duty ratio. is there.

FIG. 6 shows gas sensitivities in various gas atmospheres measured under intermittent operation conditions. The resistance value is obtained from the voltage between the shunt resistors incorporated in series with the sensor. The intermittent operation is performed by energizing the heater for 100 msec once every 10 sec. Gas sensitivity in a 2000 ppm atmosphere of CH ₄ gas (ratio to resistance in an air atmosphere: R
air / Rgas) is about 5, while CO gas 1
Both of the sensitivities in the atmosphere of 00 ppm and 1000 ppm of H ₂ gas show 2 or less, indicating that the sensitivity is selective to CH ₄ gas. Further, in this case, since the response speed of the heater up to 450 ° C. is 5 msec or less, most of the time until the resistance value becomes steady is the time for the adsorption of oxygen in Air and the time for various gases. The power consumption of the sensor manufactured by the above process when the temperature was raised to 450 ° C. was 30 mW. Using a 5000 mAh lithium battery, a constant voltage of 3 V was applied to the heater, and the duty ratio = 1 /
When operated at 100, its battery life is estimated to be 5.7 years.

Next, when used as a CO gas sensor, it is necessary to remove the selective combustion layer at all, or to reduce the amount of catalyst in the selective combustion layer, or to use a sensor which is set to zero. This is because the high combustion efficiency of the H ₂ film, CO also because results in significant amounts combustion. FIG. 7 shows a high / low (Hi / L) signal obtained by removing the selective combustion layer at all.
o) The relationship between the gas selectivity of CO (100 ppm) and H ₂ (1000 ppm) (Rgas (H ₂ ) / Rgas (CO)) measured by the method of switching the voltage level. Immediately after the Hi level voltage was applied for 30 msec, the voltage was switched to the Lo level voltage, and the sensitivity was measured after applying 70 msec. In this case, the sensor temperature at the time of applying the Hi voltage is 450 ° C., but it is preferable that the sensor temperature is 300 ° C. or higher at which the oxygen adsorption is sufficiently performed on the SnO ₂ surface and the effect of removing water and organic substances can be expected. 7 ° C from 200 ° C
In the following, it can be seen that the sensitivity of CO to H ₂ is selective.

[0017]

According to the present invention, in a thin-film semiconductor sensor using a sintered body made of alumina or the like as a selective combustion layer, the size, thickness and ratio of the sensor portion to the diaphragm diameter are set to appropriate values, and By driving under appropriate operating conditions, there is obtained an advantage that low power consumption can be realized while having high gas selectivity.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view for explaining an embodiment of the present invention.

FIG. 2 is a diagram illustrating the relationship between power consumption and diaphragm diameter.

FIG. 3 is an explanatory diagram showing a relationship between a film thickness of a sensor portion and a heating rate.

FIG. 4 is an explanatory diagram of gas sensitivity.

FIG. 5 is an explanatory diagram showing a relationship between a battery life and a duty ratio.

FIG. 6 is an explanatory diagram of gas sensitivity in the case of an intermittent operation.

FIG. 7 is an explanatory diagram of H ₂ selectivity of CO sensitivity.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... heater layer, 2 ... detection film (sensor part), 3 ... selective combustion layer.

Continuation of the front page (72) Inventor Fumihiro Inoue 1-1, Tanabe-Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa Prefecture Inside Fuji Electric Co., Ltd. (72) Inventor Koichi Tsuda 1-1-1, Tanabe-Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa Fuji F term (reference) in Denki Co., Ltd. 2G046 AA11 AA19 AA21 BA01 BB02 BB04 BC05 BE03 BE08 DB04 DC09 DD03 EA08 EA09 EA11 EB06 FB02 FE31 FE38 FE41 FE44

Claims (5)

[Claims] 1. A thermally oxidized SiO ₂ film having a thickness of 1000Å on both surfaces.
A central portion of one side of the Si substrate having a thickness of 1 μm is formed on the substrate surface having a hollow like a diaphragm.
1000Å~1μm the thin film heater through the support layer ～5000A) and made of CVD-SiO ₂ film (2 [mu] m or less)
Formed, and SiO ₂ and nitrided S
After the i-film is formed at 2000 to 2 μm, a Pt / Ta or Pt / Ti film is formed at 2000 to 2 μm as an electrode for a heater and a gas detection film, and then a gas selective combustion layer is formed at the outermost surface at 5 to 100 μm. A thin-film gas sensor characterized in that: 2. The gas selective combustion layer according to claim 1, wherein the gas selective combustion layer is made of an alumina sintered body or a sintered body having alumina as a main component and carrying a catalyst containing Pt or Pd. Thin film gas sensor. 3. The method according to claim 1, wherein the size of the thin film heater, the gas detection film and the selective combustion layer is 300 μm or less, and the ratio to the size of the diaphragm is 1/2 or less. Thin film gas sensor. 4. The thin-film heater has a heating time of 200 hours.
4. The driving method according to claim 1, wherein the driving is performed by a pulse signal having an on / off ratio of 1/100 or less in a period of 1 ms or less.
The thin film gas sensor according to any one of the above. 5. When performing CO gas detection, the thin film heater is driven by a pulse voltage signal, and the operating temperature is switched according to the high and low levels of the voltage.
5. The thin-film gas sensor according to claim 1, wherein the temperature is raised to 300 [deg.] C. or more at a high level, and kept at 200 [deg.] C. or less at a low level.