JPH1183777A - Thin film gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform battery drive by reducing the heat capacity of a heater and miniaturizing the heater itself. SOLUTION: In this thin film gas sensor for which a sensitive film 7 for gas detection composed of a metal oxide thin film is formed on the upper surface of a film heater 4 for keeping an operation temperature through an electric insulation layer 5, a heat insulation layer 2 is laminated on the through hole part upper surface of an Si substrate 1 for supporting where a through hole 2 is formed and the film heater 4 is formed on the upper surface by a sputtering method. The material of the film heater 4 is one of an Ni-Cr group alloy, an Fe-Cr group alloy, SiC, RuO2 , MoSi2 , WSi2 and TaSi2 . Also, to the film heater 4, a power unit for applying a pulse-like voltage with a fixed interval is connected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film gas sensor for detecting a reducing gas using a thin film of a metal oxide semiconductor such as tin oxide (SnO ₂ ), which is used for a household gas leak alarm or the like. In particular, the present invention relates to a structure of a thin film gas sensor capable of driving a battery with low power consumption.

[0002]

Generally, a gas sensor is used for a gas leak alarm or the like, and a specific gas such as carbon monoxide (CO), methane gas (CH ₄ ), and propane gas (C ₃ H) is used. ₈ ) It is a device that selectively responds to methanol vapor (CH ₃ OH), etc. Due to its characteristics, high sensitivity, high selectivity, high responsiveness, high reliability, and low power consumption are indispensable. By the way, gas leak alarms installed for household use include those for detecting flammable gas for city gas and propane gas and those for detecting incomplete combustion gas in combustion equipment, or Some have both functions. However, the spread of these household gas leak alarms is not so advanced. One of the reasons is that the installability is not good. For example, it was necessary to improve the installability in order to spread household gas leak alarms.

[0003] In order to improve the installation property, it is conceivable that the conventional commercial power supply system requiring a power cord is replaced with a battery drive system using pulse driving to achieve cordless operation. The biggest condition for achieving battery operation is to greatly reduce the power consumption of the gas sensor itself.
Must be detected heated to a high temperature of to 400 ° C., from these facts, in the conventional structure by sintering the powder, such as SnO ₂ to form a gas detector, even using the method of screen printing There was a limit to reducing the thickness. The inability to reduce the size of the gas detector itself has a limit in reducing the heat capacity of the gas detector, and the heat capacity must be heated using a heater, and a certain amount of drive power is absolutely necessary. And that power was far beyond what a battery could provide. That is, it was impossible to switch the power supply to the battery with the structure of the conventional sensor. By the way, in the thin-film gas sensor, the length L of the resistor used as a heater for heating is restricted by the following equation.

[0004]

[Equation 1] L ≧ Vappl / (Jmax × ρ)

Where Vappl is the applied voltage, Jmax is the maximum current density, and ρ is the specific resistance of the heater. Here, two batteries are used as a power source, and the applied voltage Vapp
The l and 1.8V, the maximum current density Jmax and ^{5 × 10 9 A / m 2} , generally assumed platinum (Pt) which may be used as the thin-film heater that specific low anti ρ as 1 × 10 ^-7 Ωm By substitution, L ≧ 3.6
mm. Here, in consideration of the configuration with low heat capacity,
Assuming a sensor chip of 100 μm × 100 μm, the line width of the heater is about 1 μm, and a sophisticated fine processing technology is required to form the heater.

As described above, when Pt is used as a heater material as in the prior art, it is necessary to make the length relatively long. Therefore, it is difficult to form a heater by patterning on a microchip by fine processing. It was difficult. Therefore, in practice, since a heater having a required length is formed on a chip having a relatively large area to form a heater, the heat capacity increases, and the time required to increase the temperature from room temperature to a temperature zone required for detection becomes longer. There was a problem. That is,
Heating must be continuously performed by a method such as constantly applying a constant voltage, and it has been difficult to employ a pulse driving method that consumes less power and is driven by a battery.

[0007]

In order to solve the above-mentioned problems, the present invention realizes battery driving by reducing the heat capacity of the heater to reduce power consumption until pulse driving is possible. In other words, the invention of claim 1 is a thin film gas sensor in which a gas detecting portion made of a metal oxide thin film is formed on an upper surface of a heater for maintaining an operating temperature via an electrical insulating layer, and a through hole of a support substrate having a through hole is provided. A heat insulating layer is laminated on the upper surface of the part and a heater is formed by a thin film on the upper surface, and the material of the thin film heater is Ni-Cr based alloy, Fe-Cr based alloy, silicon carbide (SiC), ruthenium oxide (RuO ₂ ) , Molybdenum silicide (MoSi ₂ ), tungsten silicide (WSi ₂ ), or tantalum silicide (TaSi ₂ ).

According to a second aspect of the present invention, in the first aspect, the heater is formed by a sputtering method.

According to a third aspect of the present invention, in the first or second aspect of the present invention, a heater power supply device for applying a pulsed voltage to the heater at regular intervals is provided.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing an embodiment of the thin-film gas sensor according to the present invention. In the figure, reference numeral 1 denotes a silicon (Si) substrate, and a through hole 2 is formed in the center.
A heat insulating layer 3 is formed on the upper surface of the Si substrate 1 around the through hole 2 by a silicon dioxide (SiO ₂ ) film by thermal oxidation or a SiO ₂ film by CVD. In addition, it is also possible to form the heat insulating layer 3 from silicon nitride (Si ₃ N ₄ ). On its upper surface, a film heater 4 made of a material described later.
Is formed by sputtering, an electrical insulating layer 5 such as a SiO ₂ film is formed on the upper surface, a sensor electrode 6 made of Pt or the like is formed, and a sensing film 7 such as SnO ₂ is formed. It is.

Further, lead wires 8 and 9 are connected to both ends of the film heater 4 and the sensor electrode 6, respectively, to form a sensor body. Here, for the film heater 4, a high melting point metal silicide such as Ni—Cr alloy, Fe—Cr alloy, RuO ₂ , SiC and MoSi ₂ , WSi ₂ , TaSi ₂ is used. Further, as described above, since the heat insulating layer 3 is formed in a diaphragm shape on the through hole 2 of the Si substrate 1 and the sensor main body is supported on the upper surface thereof, the heat capacity of the film heater 4 and the like can be reduced along with the miniaturization. Becomes smaller.

In the thin-film gas sensor thus configured, a heater power supply device (not shown) is connected to the lead wire 8 of the film heater 4 and pulse driving for repeating voltage application only for a very short time at regular intervals is performed. ,
The temperature of the sensing film 7 is raised to a predetermined temperature at which operation is possible. In the embodiment of FIG. 1, the sensing film 7 is formed of SnO ₂ , but the present invention can be similarly applied to a sensor using another metal oxide.

Further, the present invention realizes battery driving by reducing the heat capacity of the heater portion of the thin-film gas sensor and thereby reducing the power consumption until pulse driving is possible. A specific example of the reduction will be described.

FIG. 2 shows a graph illustrating the relationship of equation 1 already described. When the heater is formed by fine processing, if the processing is to be performed relatively easily, the length is 1 mm.
It is desirable that the specific resistance is 40 × 10
It is desirable to be at least ^-8 Ωm. Therefore, the battery life is obtained by simulation using the specific resistance as a parameter. First, the heating rate Q generated by the heater per unit volume
Is represented by the following equation.

[0015]

[Equation 2] Q = Jmax ² × ρ

Next, a specific configuration of the sensor is as shown in FIG. That is, the plane dimension of the entire sensor chip is 100 μm
Assuming that the thickness is × 100 μm, a SiO ₂ film having a thickness of 2 μm is formed on the lowermost layer, a thin film heater having a thickness of 0.3 to ₂ μm is further formed on the SiO ₂ film, an SiO ₂ film, and SnO ₂ are sequentially stacked. The heat conduction equation in this model is represented by the following equation.

[0017]

[Equation 3] ρc · ∂T / ∂t = λ (∂ ² T / θx ² + ∂ ² T / ∂y ² ) + Q

Here, numerical analysis is performed by the finite element method using Equation 3. Only heat transfer to air is considered as a boundary condition. FIG. 4 shows the result of calculating the time required to raise the temperature to 400 ° C. using the specific resistance and the thickness of the heater as variables. From the figure, it can be seen that the time required for the heater to increase in temperature decreases as the specific resistance of the heater increases and as the thickness of the heater increases up to 2 μm.

Next, using the time required for the temperature increase obtained in FIG. 4, the life of the battery is obtained assuming that a pulsed voltage is applied for a short period once every 10 seconds. 5000 mA battery capacity
FIG. 5 shows the result obtained by calculation when h is set. From these results, a specific resistance of 20 × 10 ⁻⁸ Ωm or more is required to achieve a life of 45,000 hours, which is about 5 years. In any case, it is necessary to use a material having a higher specific resistance than Pt as the heater film, from both the heater length and the battery life.

Here, the following conditions are mentioned as properties desired for the constituent material of the heater thin film. (1) The specific resistance is large and is at least twice as large as Pt. (2) Heat stability is high, that the use of around 400 ° C. which is believed to have the most sensitive to CH ₄ are possible. (3) The internal stress generated in the film is sufficiently small. (4) The adhesion strength of the film is sufficiently large. (5) Easy etching in fine processing.

Table 1 shows the material properties which meet the above conditions (1) and (2).

[0022]

[Table 1]

Although the single metals W and Mo described in this table have higher melting points than Pt, they are unsuitable because they are oxidized in the air and have low specific resistance. Alloy Ni-Cr or Fe-Cr
The system can be used because of its relatively low melting point but high specific resistance. Although SiC has a negative temperature coefficient of resistance, it has a high melting point and high specific resistance and can be used satisfactorily. RuO ₂ and silicides such as MoSi ₂ , WSi ₂ and TaSi ₂ are optimal because of their high melting points and high specific resistances. As a result, Ni-Cr alloy, Fe-Cr alloy, SiC, R
_{uO 2, MoSi 2, WSi 2} , it is possible to adopt the TaSi _2.

[0024]

As described above, according to the present invention, a through hole is provided in a thin film gas sensor in which a gas detecting portion made of a metal oxide thin film is formed on an upper surface of a heater for maintaining an operating temperature via an electric insulating layer. A heat insulating layer is laminated on the upper surface of the through hole portion of the formed support substrate, and a thin film heater is formed on the upper surface by sputtering, and the material of the heater is made of Ni-Cr alloy, Fe-Cr alloy, SiC, RuO ₂ , MoSi ₂ , WSi ₂ , or TaSi ₂ . As a result, the heat capacity of the heater itself is reduced, and power consumption is reduced. Furthermore, by providing a heater power supply device that applies a pulsed voltage to the heater at regular intervals, power consumption is significantly reduced, and long-term operation with a battery is enabled.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an embodiment of a thin-film gas sensor according to the present invention.

FIG. 2 is a graph showing a relationship between a specific resistance and a heater length according to the embodiment of the present invention.

FIG. 3 is a schematic diagram of a sensor unit according to the embodiment of the present invention.

FIG. 4 is a graph showing the relationship between the specific resistance of the heater and the time required for temperature rise in the embodiment of the present invention.

FIG. 5 is a graph showing the relationship between the specific resistance of the heater and the battery life in the embodiment of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 silicon (Si) substrate 2 through hole 3 heat insulating layer 4 film heater 5 electric insulating layer 6 sensor electrode 7 sensing film 8, 9 lead wire

Continuation of front page (72) Inventor Fumihiro Inoue 1-1, Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa Prefecture Inside Fuji Electric Co., Ltd.

Claims (3)

[Claims] 1. A thin-film gas sensor in which a gas detecting portion made of a metal oxide thin film is formed on an upper surface of a heater for maintaining an operating temperature via an electrical insulating layer, wherein a heat is formed on an upper surface of the through-hole portion of the supporting substrate having the through-hole. Insulating layers are laminated and a heater is formed by a thin film on the upper surface,
The material of the thin film heater is Ni-Cr alloy, Fe-Cr alloy, SiC,
A thin-film gas sensor comprising any one of RuO ₂ , MoSi ₂ , WSi ₂ , and TaSi ₂ . 2. The thin film gas sensor according to claim 1, wherein the heater is formed by a sputtering method. 3. The thin-film gas sensor according to claim 1, further comprising a heater power supply for applying a pulsed voltage to the heater at regular intervals.