JP2006226741A - Manufacturing method of thin film gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a thin film gas sensor that has high controllability in the shape of a selective combustion layer and small variation in a gas sensor characteristic. <P>SOLUTION: The manufacturing method comprises a process of forming the selective combustion layer 7, a process of applying paste of mixed porous Al<SB>2</SB>O<SB>3</SB>impalpable powder carrying noble metal catalyst, an inorganic binder such as silica sol binder, and photoresist to gas sensing film 6 and its rim, a process of patterning the gas sensing film 6 of the paste, and a process of heating it in oxidizing atmosphere, burning off the resist component, and baking the porous Al<SB>2</SB>O<SB>3</SB>impalpable powder and the inorganic binder. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method of manufacturing a low power consumption thin film gas sensor suitable for battery driving.

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 terms of character, high sensitivity, high selectivity, high response, high reliability, and low power consumption are indispensable.
By the way, the gas leak alarms that are widely used for home 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 There are things that have both functions, but the penetration rate is not so high due to cost and installation problems. Therefore, in order to improve the penetration rate, it is desired to improve the installability, specifically, to be battery-driven and cordless.
In contact combustion and semiconductor type gas sensors, it is necessary to detect the gas while the gas detector is heated to a high temperature of 200 ° C to 500 ° C. To achieve battery operation, low power consumption is the most important. is there. Therefore, it is awaited to realize a thin film gas sensor with high heat insulation and low heat capacity as a diaphragm structure by a microfabrication process.

In general, when a semiconductor thin film is used for the gas sensing film, the gas sensing film alone is sensitive to a plurality of reducing gas species and cannot be selectively sensitive to a specific gas. Therefore, it is effective to provide a selective combustion layer made of a noble metal catalyst such as Pd or Pt on the gas sensing film and burn a gas having a stronger oxidation activity than the sensing gas.
FIG. 1 is a cross-sectional view of a thin film gas sensor. A heater 3 is formed on a support film and a thermal insulating film (diaphragm) 2 supported at both ends or peripheral edges by the Si substrate 1, and a gas sensing film 6 having a sensing loss electrode 5 on the insulating layer 4 covering the heater 3. Is formed. The gas sensing membrane 6 is covered by a selective combustion layer 7.
Further, in order to realize low power consumption in the battery-driven gas sensor, an intermittent operation with a duty ratio (ratio of time during which the heater is turned on) of about 1/300 to 1/100 is required.

Further, in the case of a diaphragm structure in which thin films are laminated, distortion of about several μm occurs, so that the selective combustion layer also requires a certain level of mechanical strength. In order to ensure the mechanical strength of the selective combustion layer, the gas sensing membrane is completely covered with a paste in which an inorganic binder such as silica sol or alumina sol is added to porous Al ₂ O ₃ fine powder supporting a noble metal catalyst. As described above, after coating by a technique such as screen printing, baking is performed by baking at a temperature of about 500 ° C. for about 1 hour (see, for example, Patent Document 1 and Patent Document 2).
The film thickness required for the selective combustion layer is 10 to 20 μm, and it is important to completely cover the gas sensing film. On the other hand, if the selective combustion layer is too large, the heat capacity of the selective combustion layer increases the power consumption of the gas sensor, so it is also important to minimize the covering area.
Japanese Unexamined Patent Publication No. 2000-292397 (page 2 to page 3, FIG. 1) JP 2003-262598 A (2nd page-4th page, FIG. 1)

In the case of a thin film gas sensor, the SnO ₂ gas sensing film is suitably about 100 μm □ in consideration of durability and processing accuracy. The minimum size of the selective combustion layer that completely covers this varies depending on the method of manufacturing the selective combustion layer.
In the case of screen printing or the like, considering the alignment accuracy, printing accuracy, etc., it is often the case that the diameter of the selective combustion layer is controlled to about φ170 to 230 μm using a metal mask having a hole diameter of φ200 μm. Further, when viewed over the entire surface of the wafer, the position of the selective combustion layer / SnO ₂ gas sensing film is likely to shift, and the cross-sectional shape is thick at the center and thin at the periphery.
In the case of application using a microsyringe, the positional deviation is small, but it takes time to work, the shape control is difficult, or the porous Al ₂ O ₃ fine powder carrying the noble metal catalyst in the paste clogs the syringe. There were problems such as being easy to occur.

That is, in any method for forming the selective combustion layer, the controllability of the shape of the selective combustion layer is poor and the variation is large, resulting in a problem that the variation in the gas sensor characteristics becomes large as a result.
The objective of this invention is providing the manufacturing method of the thin film gas sensor which is excellent in controllability of the shape of a selective combustion layer, and can obtain the thin film gas sensor with little dispersion | variation in a gas sensor characteristic.

In order to achieve the object of the present invention, the outer periphery or both ends of a thin film diaphragm (support film) are supported by a Si substrate, and after forming at least a gas sensing film as a semiconductor thin film on the diaphragm, In the method of manufacturing a thin film gas sensor, which has a step of forming a selective combustion layer in which a noble metal catalyst such as Pd or Pt is supported on a porous Al ₂ O ₃ fine powder carrier particle on the surface,
Forming the selective combustion layer,
Applying paste mixed with porous Al ₂ O ₃ fine powder supporting noble metal catalyst and inorganic binder such as silica sol binder or alumina sol binder and photoresist to the gas sensing film and its periphery The gas sensing film of the paste is completely applied Patterning the photoresist so that only the portion to be coated remains on,
And a heat treatment in an oxidizing atmosphere to burn off the resist component in the patterned paste, and to burn porous Al ₂ O ₃ fine powder / inorganic binder,
To be made up of.

The photoresist is preferably negative.
The paste is preferably applied by a spin coater.
The heat treatment is preferably performed at 500 ° C. or higher and 700 ° C. or lower.
Volume ratio of the negative photoresist, the porous Al ₂ O ₃ fine powder supporting the noble metal catalyst and the inorganic binder in the paste [negative resist volume / (negative resist volume + porous Al ₂ O ₃ The fine powder and inorganic binder)] are preferably 50% or more and 70% or less.

According to the present invention, the step of forming the selective combustion layer is performed by using a paste in which a porous Al ₂ O ₃ fine powder supporting a noble metal catalyst, an inorganic binder such as a silica sol binder or an alumina sol binder, and a photoresist are mixed. And its peripheral edge, the photoresist can be patterned so that only the part completely covering the gas sensing film of this paste remains, the shape of the selective combustion layer and SnO ₂ gas sensing. The positional accuracy with respect to the membrane is improved. As a result, the size of the selective combustion layer can be minimized, the heat capacity of the thin film gas sensor can be reduced, and the power consumption of the gas sensor can be reduced. In addition, an improvement in manufacturing yield can be expected.
Further, since the heat treatment is performed in an oxidizing atmosphere, the resist component in the patterned paste can be burned out, so that the resist component does not remain in the selective combustion layer, and the function of the selective combustion layer can be sufficiently exhibited. .

  Since the above heat treatment is performed at 500 ° C. or higher, the resist components are burned off, and the heat treatment and the gas sensing electrode are avoided at 700 ° C. or lower, so that the function of the selective combustion layer can be sufficiently exerted as described above. There is no problem with the characteristics.

The manufacturing method of the thin film gas sensor of this invention is demonstrated along the manufacturing process of a thin film gas sensor. The cross-sectional view of the thin film gas sensor to which the manufacturing method according to the present invention is applied is the same as FIG.
Si ₃ N ₄ and SiO ₂ films are sequentially formed by plasma CVD on a Si substrate 1 having a thermal oxide film on both sides as a support film having a diaphragm structure and a thermal insulating film 2.
Next, a heater 3 patterned with a heater layer made of Pt—W and an insulating layer 4 made of SiO ₂ are sequentially formed by sputtering, and a gas sensing film electrode film is formed thereon through a bonding layer and patterned. A gas sensing membrane electrode 5 is formed. Film formation is performed by normal sputtering using an RF magnetron sputtering apparatus. The deposition conditions are the same for the bonding layer (Ta or Ti) and gas sensing film electrode (Pt or Au), Ar gas pressure 1Pa, substrate temperature 300 ° C, RF power density 2 W / cm ² , film thickness is bonding layer / Gas sensing membrane electrode = 50 nm / 200 nm.

Next, an SnO ₂ film that is a gas sensing film is formed. The size of the SnO ₂ film is 100 μm □. Film formation is performed by reactive sputtering using an RF magnetron sputtering apparatus. SnO ₂ containing 0.5 wt% Sb is used as the target. The deposition conditions are Ar + O ₂ for gas, gas pressure 2 Pa, substrate temperature 150 to 300 ° C., RF power density 2 W / cm ² , and film thickness 500 nm.
Next, formation of the selective combustion layer according to the present invention will be described with reference to FIG.
FIG. 2 is a sectional view of the selective combustion layer according to the present invention in the order of manufacturing steps.
Only one portion of the SnO ₂ gas sensing film is taken out from the portion above the SiO ₂ insulating film 4 and shown in the figure, but there are several hundred or more regular portions regularly on the substrate wafer.
Add 5 to 20 wt% of alumina sol to γ-alumina (average particle size 2 to 3 μm) carrying 7.0 wt% of Pd, and mix well to obtain a paste with a viscosity of 800 cP. In a room where ultraviolet light is blocked, a negative photoresist (for example, OMR83 / 800 cP manufactured by Tokyo Ohka Kogyo Co., Ltd.) 1.5 times in volume is added and mixed to be uniform.

Next, the above-mentioned Pd 7.0 wt% supported γ-alumina / alumina sol / photoresist mixture is applied to the wafer formed with a spinner up to the SnO ₂ gas sensing film to form a coating film 7a (FIG. 2A).
The application condition with a spinner is 1000 rpm, and a uniform film with a film thickness of 15 μm can be obtained in one minute. Then, it is dried for about 30 minutes in a clean oven at 85 ° C.
When double thickness is required, it can achieve by performing the same process on the same conditions.
Next, a photomask and a wafer are set in the exposure machine. The photomask is provided with a 120 μm square transparent portion in accordance with the pitch of the SnO ₂ gas sensing film 6. Align with a joystick that exposes the 100μm □ SnO ₂ gas sensing film at the center of the 120μm □ transparent part, contact the wafer and photomask, and irradiate UV light with a high-pressure mercury lamp (Fig. 2 (b) )).
The exposure energy is 200 mW / cm ² (wavelength is 405 nm). Next, develop for 120 seconds with OMR developer, remove the unexposed portion of the photoresist, and 120μm □ (coated part of SnO ₂ gas sensing film) Pd 7.0wt% supported γ-alumina / alumina sol / photoresist mixture Then, the patterned film 7b is left (FIG. 2C). Photosensitive photoresist crosslinks and remains with Pd 7.0 wt% supported γ-alumina / alumina sol / Conversely, the unphotosensitized photoresist dissolved in the developer solution was dispersed because Pd 7.0 wt% supported γ-alumina / alumina sol Removed at the same time.

Thereafter, it is further set in an electric furnace and baked at 550 ° C. for 1 hour while flowing oxygen. The photoresist is an organic substance and is completely removed by oxidative decomposition. The remaining Pd 7.0 wt% supported γ-alumina / alumina sol is sintered and fixed to the substrate (SnO ₂ / SiO ₂ surface) to form the selective combustion layer 7. The The selective combustion layer 7 is a sintered product of Pd 7.0 wt% supported γ-alumina fine powder, and becomes a porous body having good gas permeability (FIG. 2D). The selective combustion layer 7 has a size of 120 μm and completely covers the SnO ₂ gas sensing film 6, and the film thickness is about 10 μm.
According to the manufacturing method according to the present invention, since the film thickness controllability, the position accuracy controllability, and the dimensional accuracy controllability of the selective combustion layer are good, variations in gas sensor characteristics are reduced.
FIG. 3 is a cross-sectional view of the selective combustion layer by screen printing. When the selective selection layer is formed by screen printing, it can be seen that the selective combustion layer size is significantly larger than SnO _{2 as} compared.

In this embodiment, the photoresist is removed by thermal decomposition in an oxidizing atmosphere, but it may be baked at 550 ° C. for 1 hour after decomposition of the photoresist using an asher using oxygen plasma. When the photoresist is decomposed using an asher, a positive photoresist can be used. Finally, Si was removed from the back surface of the substrate by etching to form a diaphragm structure.
The element having the selective combustion layer of the present invention has about 20% lower power consumption than the element having the selective combustion layer formed by screen printing shown in FIG. did it.

  According to the present invention, by using a photoresist / catalyst-supported alumina powder / alumina sol mixed resist to form a selective combustion layer by a microfabrication technique, positional accuracy, dimensional accuracy, and film thickness accuracy are improved. A gas sensor with lower power consumption and less characteristic variation than the selective combustion layer by screen printing can be obtained.Therefore, the number of gas sensors per Si substrate will increase, leading to lower prices in conjunction with improved yield, and the spread of thin film gas sensors. It becomes possible to plan.

It is sectional drawing of a thin film gas sensor. It is sectional drawing of the order of the manufacturing process of the selective combustion layer which concerns on this invention. It is sectional drawing of the selective combustion layer by screen printing.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Si substrate 2 Support film and thermal insulating film 3 Insulating layer 4 Heater 5 Gas sensing film electrode 6 Gas sensing film 7 Selective combustion layer 7a Coating film 7b Film after pattern

Claims (5)

The outer periphery or both ends of the thin film diaphragm (support film) is supported by the Si substrate, and after forming a gas sensing film, which is at least a semiconductor thin film, on the diaphragm, a noble metal catalyst such as Pd or Pt is formed on the outermost surface. In the method of manufacturing a thin film gas sensor, including a step of forming a selective combustion layer in which porous Al ₂ O ₃ fine powder carrier particles are supported,
The step of forming the selective combustion layer includes:
Applying paste mixed with porous Al ₂ O ₃ fine powder supporting noble metal catalyst and inorganic binder such as silica sol binder or alumina sol binder and photoresist to the gas sensing film and its periphery The gas sensing film of the paste is completely applied Patterning the photoresist so that only the portion to be coated remains on,
And a heat treatment in an oxidizing atmosphere to burn off the resist component in the patterned paste, and to burn porous Al ₂ O ₃ fine powder / inorganic binder,
A method for producing a thin film gas sensor, comprising: 2. The method of manufacturing a thin film gas sensor according to claim 1, wherein the photoresist is a negative type. 3. The method of manufacturing a thin film gas sensor according to claim 1, wherein the application of the paste is performed by a spin coater. 4. The method of manufacturing a thin film gas sensor according to claim 2, wherein the heat treatment is performed at a temperature of 500 ° C. or higher and 700 ° C. or lower. Volume ratio of the negative photoresist, the porous Al ₂ O ₃ fine powder supporting the noble metal catalyst and the inorganic binder in the paste [negative resist volume / (negative resist volume + porous Al ₂ O ₃ The method for producing a thin film gas sensor according to any one of claims 1 to 4, wherein the fine powder and the inorganic binder)] are 50% or more and 70% or less.

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