JP2849588B2 - Thin film gas sensor and method of manufacturing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a gas sensor for detecting gas in air, and more particularly to a gas sensor for detecting a reducing gas by a change in electric resistance of a semiconductor element.

[Problems to be solved by conventional technology and invention]

Semiconductor gas sensors used to detect various reducing gases, such as gas leak alarms such as LP gas,
When a reducing gas is adsorbed on a semiconductor element heated to a predetermined temperature, oxygen in the adsorbed gas is desorbed, and the phenomenon that the electric resistance of the element is reduced is used.

As the sensor element, there are a sensor element in which an electrode is attached to a porous sintered body made of an n-type semiconductor such as SnO ₂ or ZnO, and a sensor in which these substances are fixed on an insulating substrate to form a thin film.

The former is called the bulk type, and as shown in FIG.
A metal oxide semiconductor 5 such as SnO ₂ is sintered around a platinum wire coil heater 3 and a catalyst is supported on the surface thereof. However, in order to manufacture this element, the time required for the coil winding step and the sintering step becomes long, so that the productivity was poor. In addition, there has been a problem that the element becomes large and power consumption increases.

On the other hand, there are two types for forming the latter thin film, one called a counter electrode type and one called a hot wire type. As shown in FIG. 1, the opposing electrode type element has opposing comb-shaped electrodes 4, 4 'formed on an insulating substrate 2, and a gas-sensitive oxide semiconductor thin film 6 is fixed thereon. Things.

On the other hand, as shown in FIG. 2, the hot-wire element has a meandering heater 8 also serving as an electrode formed on an insulating substrate 2 and an oxide semiconductor thin film 6 fixed thereon. It is used by being incorporated in a circuit together with a temperature compensation element manufactured from a gas-insensitive oxide.

In the case of these counter-electrode type and hot-wire type elements, since the electrodes and the thin film are formed by using a printing technique or a vapor deposition technique, there are advantages that the productivity is higher and the power consumption is lower than that of the bulk type. However, the gas sensitivity is generally poor and the electronic resistance is high, so that the reproducibility and stability of the gas sensitivity characteristics remain uneasy.

On the other hand, Japanese Patent Publication No. 60-29064 proposes a method of reducing the electric resistance of a sensor element as a two-layer thin film having a gas-sensitive high electric resistance layer and a low electric resistance layer. However, in this method, the thickness of the two thin films must be controlled quite strictly, and there is a concern that the characteristics may change due to the diffusion of the components depending on the operating temperature. However, there is a problem that it is difficult to perform a high-temperature heat treatment for the heat treatment.

Accordingly, an object of the present invention is to provide a thin-film gas sensor having good gas sensitivity, stable performance and high productivity, and a method for manufacturing the same.

[Means for solving the problem]

In view of the above problems, as a result of intensive research, the present inventors have found that a proper amount of Nb and / or Sb and Pt and / or Pd are added to tin oxide to form a thin film material, and the thin film material is formed on a substrate by an evaporation method such as sputtering. By using a semiconductor element on which a thin film having a fine homogeneous mixed phase is formed, it has been found that a gas sensor having low electric resistance and good gas sensitivity can be obtained, and the present invention has been completed.

That is, the hot-wire thin-film gas sensor of the present invention has a heater which also serves as an electrode formed on an insulating substrate, and a gas-reactive semiconductor thin film is fixed thereon, and the thin film contains Nb in tin oxide. and / or Sb is in a phase in which dissolved 0.1 to 5% by weight Nb ₂ O ₃ or Sb ₂ O ₃ in terms of values, 0.01 to 15 wt% of P
It is characterized in that t and / or Pd are uniformly dispersed.

Further, the first method for producing the thin film gas sensor of the present invention comprises forming an electrode on an insulating substrate, and forming a gas-sensitive semiconductor thin film on the electrode as a thin film of Nb and / or Sb in tin oxide. A thin film gas sensor is manufactured by forming a thin film in which 0.01 to 15 wt% of Pt and / or Pd is uniformly dispersed in a 5 wt% (Nb ₂ O ₃ or Sb ₂ O ₃ converted) solid solution phase. The target is obtained by mixing and solidifying all the components constituting the thin film, and forming the thin film on the substrate by a sputtering method.
It is characterized by heat treatment at ~ 1200 ° C.

Further, a second method for producing the thin film gas sensor of the present invention comprises forming an electrode on an insulating substrate, and forming a gas-sensitive semiconductor thin film on the electrode as Nb and / or Sb in the tin oxide. A thin film gas sensor is manufactured by forming a thin film in which 0.01 to 15 wt% of Pt and / or Pd is uniformly dispersed in a 5 wt% (Nb ₂ O ₃ or Sb ₂ O ₃ converted) solid solution phase. The target is obtained by mixing and solidifying all the components constituting the thin film, and when forming the thin film on the substrate by a sputtering method, the substrate is heated to 200 to 700 ° C.
It is characterized by heating to.

 Hereinafter, the present invention will be described with reference to the drawings.

The thin film gas sensor of the present invention is configured as a counter electrode type as shown in FIG. 1 or a hot wire type as shown in FIG. That is, opposing comb-shaped electrodes 4 and 4 'made of platinum wire or the like, or a meandering heater 8 also serving as an electrode, which is also a platinum wire, is formed on a substrate 2 made of alumina or the like having heat resistance and insulation. ing. Further, a semiconductor thin film 6 mainly composed of tin oxide is provided thereon.

Tin oxide is a typical n-type semiconductor exhibiting good gas sensitivity, and its electric resistance decreases in response to a reducing gas.
However, when tin oxide is thin-filmed in a single phase, the electric resistance is high and is not practical. Therefore, Nb in the form of Nb ₂ O ₃ or S
The electric resistance is reduced by adding Sb in the form of b ₂ O ₃ and dissolving it in tin oxide. The chemical structure of tin oxide after solid solution is
SnO _2-X (where 0 <X ≦ 1). Here, X changes according to the added amount of Nb and / or Sb. Further, Pt and / or Pd is supported as a catalyst in order to increase the gas sensitivity.

The thin film is preferably porous, but it is technically difficult to disperse the catalyst inside the pores after film formation. Therefore, components such as tin oxide, Sb, and Pt are simultaneously deposited on the substrate by sputtering or ion plating.

For example, in the case of using a sputtering method, when all components constituting the thin film are mixed and solidified to form a cathode target, while a substrate is used as an anode and a voltage is applied between the electrodes in a low-pressure gas atmosphere, the gas is ionized. The gas ions collide with the target to cause the thin film components to fly out and be fixed on the substrate to form a thin film having a thickness of 0.05 to 10 μm. Thereby, the structure of the thin film becomes a mixed structure in which Pt and / or Pd are uniformly dispersed in a matrix in which Nb and / or Sb is dissolved in tin oxide.

One or both of Nb ₂ O ₃ and Sb ₂ O ₃ are added, and the addition amount is preferably 0.1 to 5% by weight based on the weight of tin oxide. That is, the relationship between the amount of addition and the electrical resistance generally has a tendency to be represented by a reverse parabola as shown in FIG. 3, and the resistance increases when the amount of addition is too small or too large. It is desirable to show an electric resistance (specific resistance) of about 500Ωcm or less.
₂ O ₂ and / or Sb ₂ O ₃ should be in the range of 0.1 to 5% by weight. More preferably, the content is set to 1 to 3%.

One or both of Pt and Pd are added, and the addition amount is preferably 0.01 to 15% by weight based on the weight of tin oxide. As shown in FIG. 4, the addition amount and the electric resistance show a proportional relationship in a range where a small amount is added in consideration of economy. To make the electric resistance 500 Ω · cm or less, it is necessary to make it 15% by weight or less. On the other hand, if it is less than 0.01% by weight, a sufficient catalytic effect cannot be obtained. More preferably 0.
It is good to be 1-5%.

The semiconductor thin film applied to the gas sensor of the present invention has good gas responsiveness by setting the amount of the additive to the above-described appropriate amount range and forming a mixed structure in which the catalyst is uniformly dispersed by the above-described highly-productive deposition method. Thus, the electric resistance is low enough to be practical.

In addition, a device in which a thin film is formed on a substrate is
Nb or Sb by heat treatment at a temperature of 200 ° C
Is promoted, the structure of the thin film becomes dense, and the electric resistance can be further reduced. In particular, heat treatment is suitable for a hot-wire sensor requiring a low-resistance gas-sensitive film. If the temperature is lower than 300 ° C., a remarkable effect cannot be obtained. If the temperature is higher than 1200 ° C., the film is softened and a uniform film thickness cannot be obtained.

In order to reduce the electric resistance of the film, an effect can be obtained by heating the substrate to 200 to 700 ° C. when depositing the thin film on the substrate. Below 200 ° C, no remarkable effect is obtained, while 700
If the temperature is higher than ° C., the restriction of the heating device is increased, which is not practical.

The semiconductor device obtained by the above manufacturing method is incorporated into an electric circuit to operate. First, oxygen gas is adsorbed on catalytic metals such as Pt and Pd dispersed in tin oxide. When the reducing gas in the gas to be measured is adsorbed there, the already adsorbed oxygen gas is consumed in the oxidation reaction of the reducing gas, or the oxygen is desorbed, and the conduction trapped strongly in the tin oxide. The band electrons are released. SnO ₂
The semiconductor receives the electron and its resistance value decreases.

〔Example〕

The present invention will be described in more detail with reference to the following specific examples.

Example 1 An alumina substrate having a thickness of about 50 μm was cut into 5 mm squares, and a comb-shaped opposing electrode was patterned by screen printing using Pt paste on one side, and a heater pattern was similarly formed on the opposite side. After printing, firing was performed to form an electrode. Next, a thin film having a thickness of about 1 μm is formed so as to cover the electrodes by a high-frequency sputtering method using a target mixed and solidified at a ratio (by weight) of SnO ₂ 96%, Sb ₂ O ₃ 2%, and Pt 2%. A gas sensor element was manufactured.

A predetermined voltage was applied to a heater portion of the device to heat the device to about 350 ° C. On the other hand, a voltage of 5 V was applied between the opposing electrodes, and a reference resistor was connected in series with the voltage, so that the resistance change of the element was read by the voltage change across the reference resistor. FIG. 5 shows the result of detecting 1000 ppm of isobutane gas with this sensor element. As shown in the figure, a voltage of about 50 mV was measured for about 1.5 hours immediately after the measurement, indicating that the element had sufficient sensitivity.

Example 2 After forming a heater pattern with a Pt paste on the same alumina substrate as in Example 1, a thin film having the same composition as in Example 1 was formed to a thickness of about 1 μm on the substrate by high frequency sputtering. Thus, a hot wire gas sensor element was obtained. After heat-treating the device in an air atmosphere at a temperature of 800 ° C. for about 1 hour, a bridge circuit was formed so as to constitute the temperature compensating device and the adjacent side, and the change in electrical resistance was measured with the device.

FIG. 6 shows the results obtained by examining the gas sensitivity to 1000 ppm of isobutane. As shown in the figure, a voltage of about 120 to 150 mV was measured for about 2 hours immediately after the measurement. From this result, it can be seen that the device using the thin film of the present invention has a low electric resistance enough to operate as a hot-wire device and has a sufficient gas sensitivity.

〔The invention's effect〕

As described above, the thin-film gas sensor of the present invention uses a sensor element in which one layer of thin film is formed on a substrate, and the thin film has a mixed phase in which the catalyst in the semiconductor matrix is uniformly dispersed.

Therefore, it can be easily manufactured by a sputtering method or the like, and has low electric resistance and good gas reactivity. Further, the electric resistance can be further reduced by heat treatment or the like.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a counter electrode type sensor element used in the gas sensor of the present invention, FIG. 2 is a schematic view showing a hot wire type sensor element used in the gas sensor of the present invention, FIG. And FIG. 4 are graphs showing the relationship between the additive components in the thin film and the electrical resistance of the thin film, and FIGS. 5 and 6 are graphs showing the results of measuring the output voltage using the gas sensor of the present invention. FIG. 7 is a schematic view showing a conventional gas sensor element. 2 ... substrate 3 ... coil heater 4 ... electrode 5 ... semiconductor 6 ... thin film 8 ... heater

Continuation of the front page (72) Inventor Yukio Nakanouchi 810 Kumagaya, Kumagaya-shi, Saitama Riken Co., Ltd. Kumagaya Works (56) References JP-A-62-75245 (JP, A) JP-A-62-67437 (JP, A) JP-A-54-139598 (JP, A) JP-A-55-126851 (JP, A) JP-A-53-19093 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01N 27/12 G01N 27/16

Claims (3)

(57) [Claims] 1. A hot-wire thin-film gas sensor comprising a heater serving also as an electrode formed on an insulating substrate, and a thin film of a gas-sensitive semiconductor fixed on the heater, wherein the thin film comprises Nb and / or Or Sb is 0.1 in Nb ₂ O ₃ or Sb ₂ O ₃ conversion value
A hot-wire type thin-film gas sensor characterized in that 0.01 to 15% by weight of Pt and / or Pd is uniformly dispersed in a phase in which a solid solution of about 5% by weight is formed. 2. An electrode is formed on an insulative substrate, and Nb is contained in tin oxide as a gas-sensitive semiconductor thin film on the electrode.
And / or Sb to form a 0.1 to 5 wt% film (Nb ₂ O ₃ or Sb ₂ O ₃ conversion value) in the solid solution phase of 0.01 to 15 wt% Pt and / or Pd is uniformly dispersed In the method of manufacturing a thin film gas sensor, by mixing and solidifying all components constituting the thin film as a target, forming the thin film on the substrate by a sputtering method, and then heating the thin film in air at 300 to 1200 ° C. A method for producing a thin-film gas sensor, comprising: 3. An electrode is formed on an insulating substrate, and a thin film of a semiconductor having gas sensitivity is formed on the electrode as a thin film of Nb in tin oxide.
And / or Sb to form a 0.1 to 5 wt% film (Nb ₂ O ₃ or Sb ₂ O ₃ conversion value) in the solid solution phase of 0.01 to 15 wt% Pt and / or Pd is uniformly dispersed In the method of manufacturing a thin film gas sensor, all the components constituting the thin film are mixed and solidified to form a target, and when the thin film is formed on the substrate by a sputtering method, the substrate is heated to 200 to 700 ° C. A method for manufacturing a thin-film gas sensor.