JP4022822B2 - Thin film gas sensor - Google Patents

Description

  The present invention relates to a low power consumption thin film gas sensor with battery driving in mind.

In general, a gas sensor is a device that is selectively used for a specific gas such as CO, CH ₄ , C ₃ H ₈ , C ₂ H ₅ OH, etc. In addition, high sensitivity, high selectivity, high response, high reliability, and low power consumption are indispensable.
By the way, gas leak alarms that are widely used for household use include those for the purpose of detecting flammable gases for city gas and propane gas, and those for the purpose of detecting incomplete combustion gases in combustion equipment, or However, the penetration rate is not so high due to cost and installation problems. Under such circumstances, it is desired to improve the installation property, specifically, to be battery-driven and cordless in order to improve the diffusion rate.

Low power consumption is the most important for realizing battery driving. However, in catalytic combustion and semiconductor type gas sensors, it is necessary to detect by heating to a high temperature of 100 ° C. to 500 ° C. Thus, in the conventional method in which powder such as SnO ₂ is sintered, there is a limit to reducing the thickness even if a method such as screen printing is used, and the heat capacity is too large to be used for battery driving. Therefore, the realization of a thin film gas sensor in which a heater and a sensing film are formed with a thin film of 1 μm or less and a low heat capacity structure such as a diaphragm structure by a fine processing process has been awaited. As a result, a thin film gas sensor having a low heat capacity structure has been proposed in Patent Document 1, for example.

  By the way, even in a gas leak alarm device to which a low power consumption thin film gas sensor having a low heat capacity structure such as a diaphragm structure is applied, pulse driving of the thin film gas sensor is indispensable in order to have a life of 5 years or more without replacing the battery. . Usually, the gas leak alarm needs to be detected once in a fixed cycle of 30 to 150 seconds, and the detector is heated from room temperature to a high temperature of 100 ° C. to 500 ° C. according to this cycle. In order to meet the life requirement of 5 years or longer without replacing the battery, the heating time is targeted to be several hundred ms.

Even in a pulse-driven thin film gas sensor, it is important to lower the detection temperature, shorten the detection time, and lengthen the detection cycle (lengthen the time for turning off the energization) in order to reduce power consumption.
It is also important to improve the time-dependent stability of the battery-driven (pulse-driven) thin film gas sensor by desorbing moisture and other adsorbates adhering to the sensor surface during the off time and cleaning the SnO ₂ surface. The sensor temperature is once heated to 400 ° C. to 500 ° C. (time to 100 msec) before detection, and immediately after that, gas detection is performed at the detection temperature of each gas.

FIG. 2 shows a cross-sectional structure of a general thin film gas sensor.
Si ₃ N ₄ and SiO ₂ films are successively formed by CVD on the Si wafer 1 with the thermal oxide films on both sides as a support layer having a diaphragm structure and a thermal insulating layer 2. Next, the heater layer 3 and the SiO ₂ insulating layer 4 are formed in this order by sputtering. A bonding layer 5, a sensing layer electrode 6, and a sensing layer 7 are formed thereon. Film formation is performed by an ordinary sputtering method using an RF magnetron sputtering apparatus. The film formation conditions are the same for both the bonding layer (Ta or Ti) 5 and the sensing layer electrode (Pt or Au) 6, Ar gas pressure 1 Pa, substrate temperature 300 ° C., RF power 2 W / cm ² , film thickness is bonding layer 5 / The sensing layer electrode 6 = 500/2000 mm.

Next, a sensing layer 7 is formed, and a selective combustion layer in which a catalyst is supported on a porous metal oxide such as Al ₂ O ₃ is applied on the sensing layer 7 by a screen printing method. Baked for more than an hour. The size of the selective combustion layer is preferably sufficient to cover the sensing layer 7 and the thickness after firing is preferably about 20 to 30 μm. Finally, silicon is removed from the back surface of the Si wafer 1 by etching to form a diaphragm structure.

  In order to guarantee the life of the thin film gas sensor having such a structure for 5 years or longer without replacement of the battery, intermittent driving of the heater layer 3 is essential. When used as a CO sensor for CO detection, detection is required once every 150 seconds, and in order to desorb moisture and other adsorbates adhering to the surface of the sensing layer 7 during the off time, the sensing layer In order to improve the temporal stability, it is important to clean the surface of No. 7 once. Therefore, in the thin film CO sensor, the temperature of the heater layer 3 is once heated to 400 to 500 ° C. for 50 to 200 msec for the purpose of cleaning before CO detection, and immediately after that, the temperature is about 200 ° C., which is the CO detection temperature. CO detection is performed by a temperature pattern of holding for ˜1000 msec.

When such a thin film gas sensor is used as a CO sensor for CO detection, not only the sensitivity to CO is good, but also selectivity for different gases such as CH ₄ and H ₂ and the long life of the sensor are guaranteed. Various requirements such as durability must be satisfied.
Here, the CH ₄ selectivity is defined by the ratio of the sensor resistance value at CH ₄ = 4000 ppm and CO = 100 ppm, and the H ₂ selectivity is defined by the ratio of the sensor resistance value at H ₂ = 1000 ppm and CO = 100 ppm. To do.

The gas detection principle of the SnO ₂ sensor is that the adsorbed oxygen (O ²⁻ ) chemically adsorbed on the SnO ₂ surface reacts (oxidizes) with combustible gas such as CH ₄ , H ₂ , and CO, and the adsorbed oxygen (O ²⁻ ). This utilizes the fact that the electrons trapped in are injected as free electrons into the SnO ₂ crystal and the resistance value changes (decreases). Below, the reaction formula with each detection gas is shown.
CH ₄ , + 4O ²⁻ (ad) ⇒CO ₂ + 2H ₂ O + 8e (1)
H ₂ + O ²⁻ (ad) ⇒H ₂ O + 2e (2)
CO + O ^2- (ad) ⇒ CO ₂ + 2e (3)

The SnO ₂ of the gas detection film of the thin film gas sensor has a lower heat capacity as the geometric size is smaller. Therefore, it is advantageous for low power consumption, but it has limitations such as long-term stability, fine processing accuracy, and thin film deposition method / time. Thus, the practical size is ˜several hundred μm □, and the thickness is about several μm.
As the SnO ₂ film of the thin film gas sensor, as disclosed in Japanese Patent Publication No. 06-043978, SnO ₂ formed by sputtering is most excellent in terms of sensor characteristics and mass productivity.

In the CO sensor, the higher the CH ₄ selectivity and H ₂ selectivity are, the better. However, as can be seen from the above reaction formula, since CH ₄ , H ₂ and CO are all flammable gases, the sensor exhibits the same resistance change. To do. Therefore, in order to improve the gas type selectivity that the CO sensor does not react with CH ₄ and H ₂ but reacts only with CO, various ideas have been made.

Regarding CH ₄ selectivity, SnO ₂ and CO react at low temperature (~ 100 ° C) (sensing / resistance decrease), but CH ₄ operates at low temperature by utilizing the high reaction temperature (~ 400 ° C). (˜100 ° C.) ensures the selectivity.
Regarding the H ₂ selectivity, there is no significant difference in the reaction temperature between SnO ₂ and H ₂ , and SnO ₂ and CO. Therefore, the method using the difference depending on the detection temperature as described above is not effective. The H ₂ selectivity utilizes the difference in catalytic oxidation power with respect to the gas species of a catalyst filter (selective combustion layer) in which a noble metal catalyst formed so as to cover SnO ₂ is supported on porous Al ₂ O ₃ . That is, H ₂ gas is selectively burned in the selective combustion layer, and the arrival of H ₂ gas to SnO ₂ is suppressed. On the other hand, since CO gas is suppressed in the selective combustion layer, the H ₂ selectivity is improved.

Although the CH ₄ selectivity and the H ₂ selectivity are secured by the above-described measures, it cannot be said that there is a sufficient margin when considering the environmental temperature and humidity, long-term stability, and the like.
In a sensor that performs intermittent energization such as a battery-driven CO gas sensor, after heating (cleaning) to a high temperature of 400 ° C. or higher, the sensor temperature is rapidly cooled to 100 ° C., and CO gas detection is performed within one second. During high temperature cleaning at 400 ° C. or higher, CO and H ₂ are completely burned in the selective combustion layer, but most of CH ₄ passes through the selective layer and reaches SnO ₂ . On the other hand, at 100 ° C., CH ₄ does not burn in the selective combustion layer and almost passes through the selective layer and reaches SnO ₂ , but does not react with SnO ₂ because of low temperature.

In such a battery-powered gas sensor, since CH ₄ is sensed during 400 ° C. cleaning and the resistance decreases, even if the sensor temperature is rapidly cooled to 100 ° C., the resistance decreases at a high temperature, as if at 100 ° C. The resistance value is such that CH ₄ is detected. The above is presumed to be the reason for the lack of CH ₄ selectivity margin in the battery-driven gas sensor. On the other hand, as for the H ₂ selectivity properties, CO selected combustion layer upon heating (cleaning) to temperatures higher than 400 ° C., H ₂ is complete combustion, there is no resistance decreases the sensor (not sensing) Therefore, the CH ₄ The problem does not occur. At the CO gas detection temperature (100 ° C.), the combustion ratio of CO in the selective combustion layer is low and the combustion ratio of H ₂ is large, so that H ₂ selectivity is obtained mainly by CO reaching SnO ₂ . Since CO and H ₂ cannot be completely selectively burned in the selective combustion layer, the margin of H ₂ selectivity is insufficient.

The applicant has proposed a method as described in Patent Document 2 as a method for increasing CH ₄ selectivity and H ₂ selectivity by increasing the amount of the noble metal catalyst added to the SnO ₂ layer doped with the catalyst. This is to combine the SnO ₂ layer with the addition of +5 or +6 valent element serving as a donor, and a SnO ₂ layer catalyst was doped. As a result, the practical range has been reached, but further improvement in performance is required to ensure long-term reliability over five years. In particular, with respect to H ₂ selectivity, in semiconductor gas sensors using SnO ₂ , hydrogen sensitization, which gradually improves sensitivity to hydrogen, is likely to occur, so that improvement is important.

JP 2000-298108 A (page 4, FIG. 1) JP 2000-292399 A (page 4-5, FIG. 1)

Accordingly, an object of the present invention is to improve H ₂ selectivity particularly in a thin film gas sensor.

In order to solve such a problem, in the invention of claim 1, the outer periphery or both ends of the thin film-like support film are supported by the Si substrate, and the outer periphery or both ends are thick and the central portion is thin. A thin film heater is formed on a support substrate, the thin heater layer is covered with an electrical insulating film, and a gas composed of a pair of noble metal electrodes for a gas sensing film and a thin film mainly composed of SnO _2. After forming the sensing film, the outermost surface has a catalyst filter layer (selective combustion layer) made of catalyst-supporting porous alumina formed so as to completely cover the gas sensing film, and the gas sensing film comprises: SnO ₂ layer with the addition of +5 or +6 elements of Pt electrode side gas sensing film serving as a donor as the first layer, and further laminating a SnO ₂ layer was added an element as a catalyst as the second layer thereon Two-layer structure The thin film gas sensor by forming a scan sensing film,
A third SnO ₂ layer having a catalyst addition amount different from that of the _second layer is provided on the SnO ₂ layer to which the element serving as a catalyst of the second layer of the gas sensing film is added. It is characterized by.

In the first aspect of the invention, Sb, As, Ta or Nb can be added as the donor of the first layer of the gas sensing film (the second aspect of the invention). In the first or second aspect of the invention, , Pt or Pd or a mixture thereof is added as a catalyst for the second layer of the gas sensing film, and the amount of catalyst added may be 1 at% or more and less than 20 at%, preferably 10 at% or more and 15 at% or less (claims). Item 3).
In the first to third aspects of the invention, Pt or Pd or a mixture thereof may be added as a catalyst for the third layer of the gas sensing film, and the amount of catalyst added may be 20 at% or more (the invention of claim 4). In the invention of claim 1 or 2, the catalyst addition amount of the second layer is 20 at% or more with respect to the catalyst addition amount of the second layer and the third layer of the gas sensing film, and the catalyst addition of the third layer The amount can be 1 at% or more and less than 20 at%, preferably 10 at% or more and 15 at% or less (Invention of Claim 5).

That is, the first layer that forms the sensing layer of the thin film gas sensor for CO detection is a SnO ₂ layer to which a +5 or 6 valent element is added as a donor, and the _second layer that is formed thereon is a precious metal such as Pt. A SnO ₂ layer doped at 1 at% or more and less than 20 at%, and a third layer formed thereon are composed of a SnO ₂ layer doped with a noble metal such as Pt at 20 at% or more, so that a sufficiently high CH ₄ (methane ) Selectivity and H ₂ (hydrogen) selectivity.

According to the present invention, in a thin film gas sensor having a three-layer structure, a low Pt doped layer (second layer) of 1 at% or more and less than 20 at% on an Sb-doped SnO ₂ layer (first layer) for detecting gas. Layer) and a high Pt doped layer (third layer) of 20 at% or more, or a high Pt doped layer (second layer) of 20 at% or more and a low Pt doped layer (third layer) of 1 at% or more and less than 20 at% By using this three-layer structure, high gas selectivity, particularly H ₂ selectivity can be improved.

FIG. 1 is a cross-sectional structural view showing an embodiment of the present invention.
As is apparent from FIG. 1, until the sensing layer electrode 6 is formed and after the sensing layer 7 is formed, the selective combustion layer 8 having a catalyst supported on a porous metal oxide such as Al ₂ O ₃ is screened. Since it is the same as that of the conventional example that it is applied by a printing method and baked at 500 ° C. for 1 hour or more, different points will be described below.
As the target sensing layer 7, SnO ₂ having 0.5 weight percent (wt%) of Sb was used for the donor-doped first layer 9. The catalyst-doped second layer 10 has a target in which 20 Pt wires of φ0.3 mm × 20 mm are arranged on a SnO ₂ target. In particular, the highly catalyst-doped third layer 11 according to the present invention has a target on the SnO ₂ target. SnO ₂ containing Pt 13.0 at% and Pt 27.0 at% was obtained using a target in which 60 Pt wires of φ0.3 mm × 20 mm were arranged.

The film formation conditions are Ar + O ₂ gas pressure 2 Pa, substrate temperature 150 to 300 ° C., RF power 2 W / cm ² , and film thicknesses of both the first, second and third layers are 400 nm. Further, finally, Si1 in the diaphragm portion was removed by etching using a dry etcher from the back surface of the substrate to obtain a diaphragm structure.
The first layer 9 of the sensing layer 7 is a SnO ₂ film to which Sb is added as a donor, and the film thickness is 400 nm. For the second layer 10, the added catalyst is Pt, the Pt doping amount in the film is Pt 13.0 at%, and the film thickness is 400 nm. For the third layer 11, the added catalyst is Pt, the Pt doping amount in the film is Pt 27.0 at%, and the film thickness is 400 nm. The Pt doping amount is a result of measurement with an X-ray photoelectron spectrometer (ESCA).

The element thus obtained is designated as A. As a comparative example for the element A, the element B is formed by the film formation up to the first layer and the second layer, the film formation is performed up to the first layer and the second layer, and the film thickness of the second layer is 800 nm. Element C was formed as an element, and the element D was formed as an element with film formation up to the first layer and the second layer, with a Pt doping amount of Pt 27.0 at%, and a film thickness of 400 nm. Further, as a comparative example, elements E and F are obtained by forming the elements with 400 nm and 1200 nm with only the first layer. As a further comparative example, the first layer is a SnO ₂ film to which Sb is added, the film thickness is 400 nm, the second layer is Pt doped in the film with Pt 27.0 at%, the film thickness is 400 nm, the third layer An element G in which the eye has a Pt doping amount of Pt of 13.0 at% and a film thickness of 400 nm was fabricated. In addition, the first layer is a SnO ₂ film to which Sb is added and the film thickness is 400 nm. The third layer is Pt 27.0 at% in the film, the film thickness is the same as 400 nm, and the film thickness is 400 nm. The elements H, I, J, and K were respectively prototyped by changing the Pt concentration of the second layer to Pt 5 at%, Pt 10 at%, Pt 15 at%, and Pt 18 at%. Table 1 shows the film thickness / dope concentration of each layer of the prototype device. The film thickness is expressed in nm, the Sb concentration in wt%, and the Pt concentration in at%.

Various characteristics of the gas sensor under each condition are shown in Table 2 for one sample having an average characteristic among samples prepared under each film forming condition.

The characteristics shown in Table 2 are the results of detection with a temperature pattern as in paragraph [0008]. As characteristics, H ₂ (hydrogen) selectivity and CH ₄ (methane) selectivity were compared. CH ₄ selectivity is defined as the ratio of sensor resistance values when CH ₄ = 4000 ppm and CO = 100 ppm, and H ₂ selectivity is defined as the ratio of sensor resistance values when H ₂ = 1000 ppm and CO = 100 ppm. The balance gas is air. Gas selectivity is an index that selectively detects only the target gas, and when it is mounted on a gas leak alarm, in order to prevent misreporting by other gases as much as possible, at least "1" or more is required. It is necessary, and when a change with time, environmental conditions, and the like are taken into consideration, it is desired that the value be 5 or more.

  When the device A according to the present invention was compared with the conventional device B, the methane selectivity was not changed, but the hydrogen selectivity was improved about twice. The element D is obtained by increasing the amount of Pt doping to the second layer to 27 at%, but it can be seen that the methane selectivity is lower than that of the conventional element B. The element C is the one in which the Pt doping amount to the second layer is 13 at% and the film thickness is doubled, but the methane selectivity is slightly improved as compared with the conventional element B, but the same as the conventional element. It turns out that it is. In the devices E and F having no Pt doped layer, both hydrogen selectivity and methane selectivity are small. The element G is obtained by reversing the Pt concentrations of the second layer and the third layer with respect to the element A, but it can be seen that the element G has the same high hydrogen selectivity as the element A of the present invention. Further, when comparing the elements A, H, I, J, and K, it can be seen that the elements A, I, and J have the best characteristics with methane selectivity of 5 or more with respect to methane selectivity.

From the above results, the following can be understood.
(1) From the comparison between the elements B and D and the comparison between the elements A, H, I, J, and K, it can be seen that there is an optimal range of 10 to 15 at% for the Pt doping amount to the second layer.
(2) When the film thickness of the second layer is increased from the comparison between the elements B and C, the hydrogen selectivity and the methane selectivity are slightly improved. If the film thickness is increased, the sensor gas response will be delayed. Therefore, it is not preferable to increase the film thickness in terms of response.
(3) From the comparison of the elements A and G, even if the Pt concentrations in the second layer and the third layer are reversed, the same characteristics as those of the element A according to the present invention can be obtained.

The Pt doped layer is expected to have a function of suppressing oxidative combustion of CO and selectively oxidizing and burning hydrogen and methane. From comparison between the elements B and D in Table 2, it can be seen that the oxidative combustion power of hydrogen and methane is remarkably different between the layers having the Pt doping amount of 13 at% and 27 at%. Therefore, structural analysis by X-ray diffraction and evaluation of catalytic activity (oxidation activity) with respect to hydrogen and methane gas were performed on the layers with Pt doping amounts of 13 at% and 27 at%. In the structural analysis by X-ray diffraction, in 13at% Pt-doped SnO ₂ , the diffraction peak due to SnO ₂ is clearly recognized, but since the diffraction peak due to Pt is not recognized, Pt is dissolved in the SnO ₂ crystal. I found out. On the other hand, in 27 at% Pt-doped SnO ₂ , a diffraction peak due to SnO ₂ and Pt is not observed, and it is estimated that the state is like a mixture of Pt—Sn—O.

Further, in the evaluation of catalytic activity (oxidation activity) for hydrogen and methane gas, it was found that 13 at% Pt-doped SnO ₂ showed high methane oxidation activity, but 27 at% Pt-doped SnO ₂ showed low methane oxidation activity. Both showed activity against hydrogen, but 27 at% Pt-doped SnO ₂ showed higher activity. Indicate 13 atomic% Pt-doped high oxidation activity SnO ₂ is relative to methane is estimated to catalyst metal dissolved in a highly dispersed in SnO ₂ (Pt atoms) and synergy SnO _2. On the other hand, 27at% Pt-doped SnO ₂ becomes a mixture of Pt-Sn-O, and the chemical properties (catalytic activity) as SnO ₂ crystals are suppressed, and as a result, the synergy effect of SnO ₂ and the catalytic metal is suppressed. Presumed. It is presumed that the Pt—Sn—O mixture has a strong Pt property and therefore has a strong oxidation activity against hydrogen. For these reasons, it is considered that the results shown in Table 2 were obtained.

In the above, a low Pt doped layer (second layer) of 1 at% or more and less than 20 at% and a high Pt doped layer (20 at% or more) (on the Sb doped SnO ₂ layer (first layer) for detecting gas) Although the third layer is described as being formed, the second layer and the third layer may be repeated. As the first layer donor, As, Ta or Nb can be used in place of Sb, and Pd or a mixture thereof can be used in place of Pt in the second and third layers. .

Structural sectional view showing an embodiment of the present invention Structural cross-sectional view showing a conventional example

Explanation of symbols

1 ... Si wafer (silicon substrate), 2 ... support layer and heat insulating layer, 3 ... heater layer, 4 ... SiO ₂ insulating layer, 5 ... bonding layer, 6 ... sensing layer electrode, 7 ... sensitive layer, 8 ... selection combustion Layers 9, donor-added SnO ₂ layer (first layer) 10, catalyst-added SnO ₂ layer (second layer) 11, high-catalyst-added SnO ₂ layer (first layer)

Claims (5)

A thin film heater is formed on a diaphragm-like support substrate in which the outer peripheral portion or both end portions of the thin support film are supported by a Si substrate, and the outer peripheral portion or both end portions are thick and the central portion is thin. The layer is covered with an electrical insulating film, and a gas sensing film consisting of a pair of noble metal electrodes for gas sensing film and a thin film mainly composed of SnO ₂ is formed thereon, and then the gas sensing film is completely formed on the outermost surface. And a catalyst filter layer (selective combustion layer) made of catalyst-supporting porous alumina formed so as to cover the gas, and the gas sensing film serves as a donor as a first layer from the Pt electrode side for the gas sensing film. In a thin film gas sensor formed by forming a gas sensing film having a two-layer structure in which a SnO ₂ layer to which a valent or +6 valent element is added and a SnO ₂ layer to which an element serving as a catalyst is added as a _second layer are laminated thereon,
A third SnO ₂ layer having a catalyst addition amount different from that of the _second layer is provided on the SnO ₂ layer to which the element serving as a catalyst of the second layer of the gas sensing film is added. A thin film gas sensor.   The thin film gas sensor according to claim 1, wherein Sb, As, Ta, or Nb is added as a donor of the first layer of the gas sensing film.   Pt or Pd or a mixture thereof is added as a catalyst for the second layer of the gas sensing film, and the amount of catalyst added is 1 at% or more and less than 20 at%, preferably 10 at% or more and 15 at% or less. Item 3. The thin film gas sensor according to Item 1 or 2.   The thin film gas sensor according to any one of claims 1 to 3, wherein Pt, Pd or a mixture thereof is added as a catalyst of the third layer of the gas sensing film, and a catalyst addition amount is 20 at% or more. Regarding the catalyst addition amount of the second layer and the third layer of the gas sensing film, the catalyst addition amount of the second layer is 20 at% or more, and the catalyst addition amount of the third layer is 1 at% or more and less than 20 at%, preferably The thin film gas sensor according to claim 1, wherein the gas gas sensor is 10 at% or more and 15 at% or less.

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