JP2004037224A - Gas sensor, gas leakage alarm using the same, gas leakage alarm system, and manufacturing method of gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas sensor with a gas detecting film having satisfactory homogeneity and excellent gas sensitivity characteristic. <P>SOLUTION: This gas sensor is equipped with a substrate, the gas detecting film formed on the substrate, an electrode formed in contact with the detecting film, and a heater for heating. A platinum-added zinc oxide thin film formed by a sol-gel process is used as the detecting film. <P>COPYRIGHT: (C)2004,JPO

Description

【００２８】
次いで、このガス検知膜材料の溶液を用いて、洗浄、熱処理をしたシリコン基板の表面に塗布した後、３００℃で１分間乾燥する。これを１０回繰り返し、最後に５００℃で１時間熱処理する。得られる、約１００ｎｍ厚の、白金を添加した酸化亜鉛薄膜をガス検知膜３とする。シリコン基板の表面への塗布法としては、たとえば、スピンコーティング法、ドクターブレード法、浸漬塗布法等が用いられる。
【００２９】
ガス検知膜３の作成時に、亜鉛／白金（モル比）が１００／０．１〜１００／１０の割合で用いるのが好ましく、この亜鉛１００に対する白金の添加割合（モル）が０．１未満の場合には、ガス感度の発現に問題があり、１０を越える場合には、ゾルゲル溶液の安定性が低くなる。
【００３０】
こうして得られる、ガス検知膜３上に、たとえば、白金をスパッタリング法で堆積させ、エッチングによりパターニングして、電極４を形成する。この電極４には、電極用リード線１１・１２を取り付ける。
【００３１】
一方、前記シリコン基板２の下面に、たとえば、白金をスパッタリング法で堆積させ、エッチングによりパターニングして、ヒータ５を形成する。このヒータ５には、ヒータ用リード線１３・１４を取り付ける。
【００３２】
こうして得られるこの発明のガスセンサ１のセンサ特性を、次ぎのようにして測定した。
【００３３】
５００ｍｍ×５００ｍｍ×５００ｍｍのアルミニウム製箱の中に、上記例で得られたガスセンサ１を入れ、密封する。電極４にはガスセンサ１の抵抗を測定するために、約１Ｖの定電圧をかけ、ガス検知膜３に流れる電流を読み、抵抗に換算する。箱内を窒素８０％および酸素２０％からなる合成空気にしたときのガス検知膜３の抵抗と、この合成空気にそれぞれイソブタンガス濃度０〜１８００ｐｐｍになるようにイソブタンガスを加えた評価用気体中のガス検知膜３の抵抗との比をガス感度（Ｒａｉｒ／Ｒｇａｓ）とした。
【００３４】
ヒータ５には、ガス検知膜３を任意の温度に加熱できるように電圧をかけ、非接触の温度計でガス検知膜３の表面温度を測定した。
【００３５】
図３に、ガス検知膜として、その添加量を変えて白金を添加した酸化亜鉛薄膜を用いるこの発明のガスセンサと、白金を添加しない酸化亜鉛薄膜を用いるガスセンサ（比較）とについて、ガス感度を比較した結果を示す。いずれもヒータは約４００℃になるように加熱した。
【００３６】
図３から、白金を添加した酸化亜鉛薄膜を用いる、この発明のガスセンサの方が、白金を添加しない酸化亜鉛薄膜を用いるガスセンサ（比較）より、ガス感度が高いこと、特に亜鉛／白金（モル比）が１００／０．２５の割合で亜鉛に白金を添加した酸化亜鉛薄膜を用いる場合に、ガス感度が著しく高いことが明らかである。ガス感度の点から、亜鉛／白金（モル比）が１００／０．１〜１００／０．７５の割合で亜鉛に白金を添加した酸化亜鉛薄膜が好ましいことが明らかである。
【００３７】
次ぎに、５００ｍｍ×５００ｍｍ×５００ｍｍのアルミニウム製箱内を、窒素８０％および酸素２０％からなる合成空気にし、ガス検知膜表面が室温から４００℃になるようにヒータ５を加熱して、上記と同様に作成したガスセンサについて、電気特性を測定し、その結果を図４に示す。
【００３８】
図４から、室温から２２０℃位迄で、酸化亜鉛薄膜の抵抗が低下していることが明らかである。これは、高温での半導体の特性であり、キャリアの運動エネルギーの影響で、イオン化不純物の散乱が弱くなるためと考えられる。そして、２２０℃を越えると、ガス検知膜表面に酸素が活性化吸着し、抵抗が増大すると考えられる。
【００３９】
さらに、前記箱内を、窒素８０％および酸素２０％からなる合成空気にイソブタンガス濃度１０００ｐｐｍになるようにイソブタンガスを加えた評価用気体について、前記と同様に測定して、ガスセンサの電気特性を得た。この測定結果を、図４に示す窒素８０％および酸素２０％からなる合成空気の電気特性と比較して図５に示す。図６は、図５内の四角部分の拡大図である。
【００４０】
表面温度が約３１０℃を越えると、窒素８０％および酸素２０％からなる合成空気中の抵抗値と比較して、評価用気体中の抵抗値が減少していることが図５および図６より明らかである。これは、吸着酸素が可燃性ガスと反応して除去されたためと考えられる。
【００４１】
表２に、膜表面温度が３００℃、３５０℃および４００℃のときの、湿度を変化させた場合のガス感度の値を示す。この際、窒素８０％および酸素２０％からなる合成空気中のガス検知膜の抵抗と、この合成空気にイソブタンガス濃度１０００ｐｐｍになるようにイソブタンガスを加えた評価用気体中のガス検知膜の抵抗との比をガス感度とした。
【００４２】
【表２】

【００４３】
表２より、ガス検知膜の膜表面温度が３５０℃を越えると、湿度の影響を殆ど受けることなく、安定したガス感度が得られることが明らかである。
【００４４】
図７および図８は、それぞれ、この発明でガス検知膜として用いる白金を添加した酸化亜鉛薄膜、および白金を添加しない酸化亜鉛薄膜（比較）の表面の結晶構造を示す電子顕微鏡写真である。
【００４５】
図７および図８より、白金を添加した酸化亜鉛薄膜は、白金を添加しない酸化亜鉛薄膜（比較）より粒径が小さく、白金粒子が酸化亜鉛に均一に固溶しており、均質性が良好であることが明らかである。
【００４６】
上記例では、基板２として、絶縁膜処理を施したものを使用し、絶縁層を特に設けていないが、図９〜図１１に示すように、絶縁処理を施さない基板２を使用し、基板２の下面または上面の少なくとも一部に絶縁層６を設けることもできる。
【００４７】
たとえば、図９に示すガスセンサ１では、基板２の下面に絶縁層６を設け、その絶縁層６にヒータ５を形成してなり、そして、前記例と同様に、基板２の上面にはガス検知膜３を形成し、そのガス検知膜３上には電極４を形成してなる。
【００４８】
このガスセンサ１のヒータ５に電力を加えて発熱させ、ガス検知膜３を３５０℃以上に加熱させると、高感度のガスセンサ１となる。
【００４９】
また、図１０に示すガスセンサ１では、ガス検知膜３を、ヒータ５を含む絶縁層６が挟む構成となっている。すなわち、このガスセンサ１は、基板２の上面にヒータ５を形成し、少なくともヒータ５を含む基板２の上面の一部に絶縁層６を設け、基板２の上面の残部にガス検知膜３を形成してなり、そして、前記例と同様に、ガス検知膜３上には電極４を形成してなる。
【００５０】
この図１０のガスセンサ１を作成するには、たとえば、シリコン基板２上に白金をスパッタリング法で堆積させ、ヒータ５をパターニングする。次いで、少なくともヒータ５を含む基板２の上面の一部にＴａ_２Ｏ_５、ＳｉＯ_２等の絶縁層６を堆積し不要な部分をエッチングにより削って形成する。
【００５１】
このように形成した、隣接する絶縁層６の間の基板２の上面に、前記例と同様に、ガス検知膜３を形成する。
【００５２】
このガス検知膜３上に、前記例と同様に、電極４を形成して図１０に示す構成のガスセンサ１を得る。
【００５３】
さらに、図１１に示すガスセンサ１では、基板２とガス検知膜３との間に絶縁層６を設けた構成となっている。すなわち、このガスセンサ１は、基板２の上面にヒータ５を形成し、そのヒータ５を含む基板２の上面の全部に絶縁層６を設け、その絶縁層６上にガス検知膜３を形成してなり、そして、前記例と同様に、ガス検知膜３上には電極４を形成してなる。
【００５４】
この図１１のガスセンサ１を作成するには、たとえば、シリコン基板２上に白金をスパッタリング法で堆積させ、ヒータ５をパターニングする。次いで、ヒータ５を含む基板２の上面の全部にＴａ_２Ｏ_５、ＳｉＯ_２等の絶縁層６を堆積し不要な部分をエッチングにより削って形成する。
【００５５】
このように形成した絶縁層６上に、前記例と同様に、ガス検知膜３を形成する。
【００５６】
このガス検知膜３上に、前記例と同様に、電極４を形成して図１１に示す構成のガスセンサ１を得る。
【００５７】
この発明のガスセンサでは、低消費電力ガスセンサを目的としたマイクロガスセンサへの適用が可能なように、ガス検知膜のガス検知部分（電極およびヒータが存在する部分）を空中に浮かした構造とするように構成することもできる。たとえば、図１２に示すガス検知部分を橋状に空中に浮かした構造や、図１３に示すガス検知部分を梁状に空中に浮かした構造とすることができる。
【００５８】
上記のいずれの構造とする場合にも、マイクロマシニングプロセス上の工程は同様であるので、以下に、図１２のガス検知部分を橋状に空中に浮かした構造とする場合について説明する。図１４はその製造工程の一例を示す。なお、図１４中ではヒータを省略している。また、図１４中のＣＤＥは、ケミカルドライエッチングを意味する。
【００５９】
まず、アンダーカットエッチングが容易な、たとえば結晶方位（１１０）の絶縁膜処理を施したシリコン基板２２を利用し、そのシリコン基板２２の上面の酸化膜をフッ酸を用いて除去する（図１４の工程イ）。このシリコン基板２２の表面に、Ｔａ_２Ｏ_５をスパッタリング法で、約１．２μｍの厚さで製膜して絶縁層２６ａを形成する（図１４の工程ロ）。
【００６０】
次いで、この絶縁層２６ａ上に電極２４やヒ−タ２５を形成するため、たとえば、白金等の金属薄膜をスパッタリング法で製膜し（図１４の工程ロ）、さらにマスクとしてたとえばＴａ_２Ｏ_５をスパッタリング法で製膜し（図１４の工程ロ）、電極やヒ−タの形状にフォトリソグラフィでパターニングした（図１４の工程ハ）後、ケミカルドライエッチングによりＴａ_２Ｏ_５を除去して、電極やヒータ形状のマスクとする。このマスクを利用して、逆スパッタリング等のドライエッチング法により、電極２４およびヒータ２５を形成する（図１４の工程ニ）。
【００６１】
こうして形成した電極２４およびヒータ２５を保護するため、たとえばＴａ_２Ｏ_５をスパッタリング法で製膜して保護用の絶縁層２６ｂを形成する（図１４の工程ホ）。
【００６２】
次いで、後に形成するガス検知膜２３が直接電極２４に接触できるように、フォトリソグラフィでパターニングした（図１４の工程ヘ）後、ケミカルドライエッチング等のドライエッチングにより、ガス検知膜用窓２７を形成する（図１４の工程ト）。
【００６３】
さらに、堀形状にフォトリソグラフィでパターニングした後、ケミカルドライエッチング等のエッチングにより、堀のマスクを形成する（図１４の工程チ）。次いで、エッチャントとして、たとえばＫＯＨ溶液を用いて、エッチャント温度７５〜８０℃で２１０分間シリコン基板２２をアンダーカットエッチング（異方性エッチング）して、堀部分２８を形成する（図１４の工程リ）。
【００６４】
この際、エッチャントとしては、上記ＫＯＨ溶液以外に、たとえば、ＮａＯＨ溶液、ヒドラジン溶液、エチレンジアミンーピロカテコールー水溶液、水酸化テトラメチルアンモニウム溶液等を用いてもよい。
【００６５】
上記で得られる構造体に、前記と同様にして調製したガス検知膜材料の溶液を用いて塗布した後、焼結する（図１４の工程ヌ）。塗布法としては、たとえば、スピンコーティング法、ドクターブレード法、浸漬塗布法等が用いられる。
【００６６】
最後に、電極２４のパッド部分に電極用リード線２９・２９を取り付け、ヒータ２５のパッド部分にヒータ用リード線３０・３０を取り付けると、ガス検知部分を橋状に空中に浮かした橋状部３１上に設ける構造とする、この発明のガスセンサが得られる。
【００６７】
上記例では、絶縁層としてＴａ_２Ｏ_５を用いるが、Ｓｉ_３Ｎ_４、ＳｉＯ_２等を用いてもよい。また、上記例では、電極やヒータとして白金を用いるが、Ｎｉ−Ｃｒ、Ｃｒ等の金属薄膜を用いてもよい。
【００６８】
ここで、センサ前面にガス検知膜材料の溶液を塗布することによりリード線とパッド間の導電性が懸念されるが、ガス検知膜は十分薄いため、通常のワイヤーボンディングで貫通し導電性には何ら問題を生じない。また、電極やヒータへの電圧印加時に、ガス検知膜を介したリークが懸念されるが、電極やヒータ材料は、ガス検知膜材料より抵抗が小さいため、ガス検知膜材料への電流のリークは無視できる程度であり、何ら問題はない。しかし、これらの懸念を無視できない場合には、橋状構造を感光性材料で覆い、フォトリソグラフィおよびエッチングにより、ガス検知膜用窓２７以外の部分のガス検知膜材料を取り除けばよい。
【００６９】
図１３に示すガス検知部分を梁状に空中に浮かした構造とするガスセンサも、上記で説明した図１２に示すガス検知部分を橋状に空中に浮かした構造とするガスセンサの場合と同様な工程により製造することができる。図１３中、３２はシリコン基板、３３はガス検知膜、３４は電極、３５はヒータ、３６ａおよび３６ｂは絶縁層、３７はガス検知膜用窓、３８は堀部分、３９は電極用リード線、４０はヒータ用リード線、４１は梁状部を示す。
【００７０】
【発明の効果】
この請求項１〜７に記載の発明によれば、ガス検知膜として、ゾルゲル法により形成した、白金を添加した酸化亜鉛薄膜を用いるから、白金粒子をガス検知材料中に均質に分散することができ、均質性が良好で、しかもガス感度特性に優れたガス検知膜を有するガスセンサを提供することができる。
【００７１】
また、請求項４〜６に記載の発明によれば、ガス検知膜のガス検知部分を空中に浮かした構造とするから、消費電力を低く抑えることができるマイクロガスセンサへの適用が可能なガスセンサを提供することができる。
【００７２】
さらに、請求項８〜９に記載の発明によれば、請求項１〜７のいずれかに記載のガスセンサを用いることにより、ガス漏れ時に単体で警報を発したりガスの供給を停止させたりするガス漏れ警報器や、ガス漏れ時に、たとえばガスメータ等と連動して警報を発したりガスの供給を停止させたりするガス漏れ警報システムを、簡易に提供することができる。
【００７３】
さらに、請求項１０に記載の発明によれば、ガス検知膜の検知部分を空中に浮かした構造とするとき、ゾルゲル法により形成した、白金を添加した酸化亜鉛薄膜材料の溶液を塗布してガス検知膜を形成することにより、均質性が良好で、しかもガス感度特性にも優れたガス検知膜を有するガスセンサを簡易に製造することができる。
【図面の簡単な説明】
【図１】この発明の一実施例であるガスセンサの概略構成説明図である。
【図２】そのガスセンサの斜視図で、（ａ）は表側から、（ｂ）は裏側から見た説明図である。
【図３】この発明のガスセンサの、白金の添加量を変えた酸化亜鉛薄膜を用いるときのガス感度を示す図である。
【図４】この発明のガスセンサの、窒素８０％および酸素２０％からなる合成空気中における電気特性を示す図である。
【図５】この発明のガスセンサの、窒素８０％および酸素２０％からなる合成空気にイソブタン濃度１０００ｐｐｍになるようにイソブタンガスを加えた評価用気体中における電気特性を示す図である。
【図６】その四角部分の拡大図である。
【図７】この発明のガスセンサのガス検知膜である白金を添加した酸化亜鉛薄膜の表面の結晶構造を示す電子顕微鏡写真である。
【図８】白金を添加しない酸化亜鉛薄膜の表面の結晶構造を示す電子顕微鏡写真である。
【図９】この発明の別のガスセンサの概略構成説明図である。
【図１０】この発明のまた別のガスセンサの概略構成説明図である。
【図１１】この発明のさらにまた別のガスセンサの概略構成説明図である。
【図１２】ガス検知部分を橋状に空中に浮かした構造とするこの発明のガスセンサの、（ａ）は上面概略構成説明図、（ｂ）はそのＡ、Ａ断面概略構成説明図である。
【図１３】ガス検知部分を梁状に空中に浮かした構造とするこの発明のガスセンサの、（ａ）は上面概略構成説明図、（ｂ）はそのＢ、Ｂ断面概略構成説明図である。
【図１４】ガス検知部分を橋状に空中に浮かした構造とするこの発明のガスセンサの製造工程の一例を示す工程説明図である。
【符号の説明】
１　　ガスセンサ
２、２２、３２　　基板
３、２３、３３　　ガス検知膜
４、２４、３４　　電極
５、２５、３５　　ヒータ
６、２６ａ、２６ｂ、３６ａ、３６ｂ　　絶縁層
２７、３７　　ガス検知膜用窓
２８、３８　　堀部分
１１、１２、２９、３９　　電極用リード線
１３、１４、３０、４０　　ヒータ用リード線[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a gas sensor that can be used for various gas detections and quantifications.
[Prior art]
[0002]
Conventionally, there has been known a semiconductor-type gas sensor that detects the type and concentration of a gas by measuring the resistance value by utilizing the fact that when a reducing gas comes into contact with the surface of an oxide semiconductor, the resistance of the oxide semiconductor surface changes. ing.
[0003]
In particular, in recent years, various studies have been made on a gas sensor using zinc oxide as a gas sensor material for practical use. For example, a gas sensor in which a catalytic metal such as platinum or palladium is added to zinc oxide has been studied. In the case of zinc oxide containing platinum, the sensitivity of a hydrocarbon-based gas is increased. In the case of zinc oxide containing palladium, CO and H ₂ are added. Is known to increase the sensitivity of the ceramics ("Ceramics" Vol. 18, No. 11 (November 1983, published by The Ceramic Society of Japan), pages 941-945, entitled "Zinc Oxide Gas Sensor") .
[0004]
[Problems to be solved by the invention]
Generally, in the liquid phase method (impregnation method) using a metal salt, which is known as a method of adding these catalyst metal particles, since the catalyst particles are fine and easily aggregated, the catalyst particles are added to the gas detection material. Since it cannot be dispersed uniformly, there has been a problem that a gas detection film having good homogeneity cannot be obtained.
[0005]
Also, in a gas phase method such as a sputtering method, which is known as a method of forming a thin film element of a semiconductor gas sensor element, the film quality of a thin film formed by an atmosphere, sputtering conditions, or the like changes or a unique fine structure appears. There is a possibility that the gas sensitivity characteristics may change significantly.
[0006]
Therefore, an object of the present invention is to provide a gas sensor having a gas detection film having good homogeneity and excellent gas sensitivity characteristics.
[0007]
[Means for Solving the Problems]
Therefore, the invention according to claim 1 is a gas sensor comprising a substrate, a gas detection film formed on the substrate, an electrode formed in contact with the gas detection film, and a heater for heating. The detection film is characterized by using a zinc oxide thin film to which platinum is added, which is formed by a sol-gel method.
[0008]
According to a second aspect of the present invention, in the gas sensor according to the first aspect, as the gas detection film, zinc / platinum (molar ratio) is formed by adding platinum to zinc at a ratio of 100 / 0.1 to 100 / 0.75. It is characterized by using a zinc oxide thin film to which platinum is added, which is obtained by addition.
[0009]
According to a third aspect, in the gas sensor according to the first aspect, an insulating layer is provided on at least a part of a lower surface or an upper surface of the substrate.
[0010]
According to a fourth aspect of the present invention, there is provided the gas sensor according to any one of the first to third aspects, wherein a gas detection portion of the gas detection film is floated in the air.
[0011]
According to a fifth aspect of the present invention, in the gas sensor according to the fourth aspect, a gas detection portion of the gas detection film is configured to be suspended in the air like a bridge.
[0012]
According to a sixth aspect of the present invention, there is provided the gas sensor according to the fourth aspect, wherein the gas detection portion of the gas detection film has a structure in which it is suspended in the air like a beam.
[0013]
According to a seventh aspect, in the gas sensor according to any one of the first to fourth aspects, the gas detection film operates at 350 ° C. or higher.
[0014]
According to the present invention, since the zinc oxide thin film to which platinum is added, which is formed by the sol-gel method, is used as the gas detection film, the homogeneity is good, and the gas sensitivity characteristics are also high. The gas sensor has an excellent gas detection film.
[0015]
An eighth aspect of the present invention is a gas leak alarm using the gas sensor according to any one of the first to seventh aspects. Here, the gas leak alarm device refers to a device that issues, for example, an alarm or stops the gas supply by itself when a gas leaks.
[0016]
A ninth aspect of the present invention is a gas leak alarm system using the gas sensor according to any one of the first to seventh aspects. Here, the gas leak alarm system refers to a system that issues an alarm or stops the supply of gas in conjunction with, for example, a gas meter or the like instead of a single unit when a gas leaks.
[0017]
The invention according to claim 10 forms an insulating layer on a substrate, forms an electrode and a heater on the insulating layer,
A protective insulating layer is formed thereon, and a gas detection film window is formed on the insulating layer.
Next, a moat portion is formed on the substrate,
To the resulting structure, separately prepared by a sol-gel method, apply a solution of zinc oxide thin film material with added platinum, sinter after forming a gas detection film,
The method of manufacturing a gas sensor according to claim 4, comprising attaching an electrode lead wire to the electrode and a heater lead wire to the heater.
[0018]
According to the tenth aspect of the present invention, when the detection portion of the gas detection film has a structure floating in the air, a solution of a zinc oxide thin film material added with platinum formed by a sol-gel method is applied. By forming the gas detection film, a gas sensor having a gas detection film having good homogeneity and excellent gas sensitivity characteristics can be easily manufactured.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0020]
FIG. 1 is a schematic configuration diagram of a gas sensor 1 according to an embodiment of the present invention.
FIGS. 2A and 2B are perspective views of the gas sensor 1, wherein FIG. 2A is an explanatory diagram viewed from the front side, and FIG.
[0021]
The gas sensor 1 includes a substrate 2, a gas detection film 3 formed on the substrate 2, an electrode 4 formed on the upper surface of the substrate 2 in contact with the gas detection film 3, and a heating electrode formed on the lower surface of the substrate 2. And a heater 5. In this example, the electrode 4 is formed on the gas detection film 3, but the electrode 4 may be formed in any manner as long as the electrode 4 is in contact with the gas detection film 3.
[0022]
As the gas detection film 3, a zinc oxide thin film to which platinum is added, which is formed by a sol-gel method, is used. The gas detection film 3 is formed on the substrate 2 by a sol-gel method.
[0023]
As the substrate 2, for example, a silicon substrate is used. In this example, a silicon substrate subjected to an insulating film treatment is used.
[0024]
As the electrode 4, a metal electrode such as platinum is used, and electrode leads 11 and 12 for measuring resistance are attached to the electrode 4.
[0025]
As the heater 5, platinum or the like is used, and heater leads 13 and 14 for applying electric power are attached to the heater 5.
[0026]
In manufacturing the gas sensor of the present invention, first, a gas detection film 3 is formed on a substrate 2 by a sol-gel method. In order to form the gas detection film 3 by the sol-gel method, first, zinc / platinum (molar ratio) and Zn = 0.5 mol / l shown in Table 1, for example, 11 g of zinc acetate dihydrate, 100 ml of 2-methoxyethanol, 6 ml of monoethanolamine, and bisacetylacetonatoplatinum are each placed in a flask as shown in Table 1, and heated at 70 ° C. for 1 hour to prepare a solution of a gas detection membrane material.
[0027]
[Table 1]

[0028]
Next, using the solution of the gas detection film material, the solution is applied to the surface of a silicon substrate that has been cleaned and heat-treated, and then dried at 300 ° C. for 1 minute. This is repeated 10 times, and finally heat treatment is performed at 500 ° C. for 1 hour. The resulting zinc oxide thin film having a thickness of about 100 nm and containing platinum is used as the gas detection film 3. As a coating method on the surface of the silicon substrate, for example, a spin coating method, a doctor blade method, a dip coating method, or the like is used.
[0029]
When forming the gas detection film 3, it is preferable to use zinc / platinum (molar ratio) in a ratio of 100 / 0.1 to 100/10, and the addition ratio (mol) of platinum to zinc 100 is less than 0.1. In this case, there is a problem in the expression of gas sensitivity, and when it exceeds 10, the stability of the sol-gel solution becomes low.
[0030]
On the gas detection film 3 thus obtained, for example, platinum is deposited by a sputtering method and patterned by etching to form an electrode 4. The electrode leads 11 and 12 are attached to the electrode 4.
[0031]
On the other hand, for example, platinum is deposited on the lower surface of the silicon substrate 2 by a sputtering method and patterned by etching to form the heater 5. The heater lead wires 13 and 14 are attached to the heater 5.
[0032]
The sensor characteristics of the gas sensor 1 of the present invention thus obtained were measured as follows.
[0033]
The gas sensor 1 obtained in the above example is put in a 500 mm × 500 mm × 500 mm aluminum box and sealed. To measure the resistance of the gas sensor 1, a constant voltage of about 1 V is applied to the electrode 4, the current flowing through the gas detection film 3 is read, and the resistance is converted into resistance. The resistance of the gas detection film 3 when the inside of the box is made of synthetic air composed of 80% of nitrogen and 20% of oxygen, and the gas for evaluation obtained by adding isobutane gas to the synthetic air so that the isobutane gas concentration becomes 0 to 1800 ppm, respectively. The ratio with the resistance of the gas detection film 3 was defined as gas sensitivity (Rair / Rgas).
[0034]
A voltage was applied to the heater 5 so that the gas detection film 3 could be heated to an arbitrary temperature, and the surface temperature of the gas detection film 3 was measured with a non-contact thermometer.
[0035]
FIG. 3 shows a comparison of gas sensitivity between a gas sensor of the present invention using a zinc oxide thin film to which platinum is added in a different amount and a gas sensor using a zinc oxide thin film to which no platinum is added (comparative). The results obtained are shown. In each case, the heater was heated to about 400 ° C.
[0036]
FIG. 3 shows that the gas sensor of the present invention using the zinc oxide thin film to which platinum is added has higher gas sensitivity than the gas sensor using the zinc oxide thin film to which platinum is not added (comparison), and in particular, zinc / platinum (molar ratio). It is clear that gas sensitivity is remarkably high when a zinc oxide thin film obtained by adding platinum to zinc in a ratio of 100 / 0.25 is used. From the viewpoint of gas sensitivity, it is clear that a zinc oxide thin film in which zinc is added to platinum at a zinc / platinum (molar ratio) of 100 / 0.1 to 100 / 0.75 is preferable.
[0037]
Next, the inside of a 500 mm × 500 mm × 500 mm aluminum box was made into synthetic air composed of 80% of nitrogen and 20% of oxygen, and the heater 5 was heated so that the surface of the gas detection film became 400 ° C. from room temperature. The electrical characteristics of the gas sensor produced in the same manner were measured, and the results are shown in FIG.
[0038]
It is apparent from FIG. 4 that the resistance of the zinc oxide thin film decreases from room temperature to about 220 ° C. This is a characteristic of the semiconductor at a high temperature, and it is considered that scattering of ionized impurities becomes weak due to the influence of kinetic energy of carriers. If the temperature exceeds 220 ° C., it is considered that oxygen is activated and adsorbed on the surface of the gas detection film, and the resistance increases.
[0039]
Further, in the box, an evaluation gas obtained by adding isobutane gas to a synthetic air composed of 80% of nitrogen and 20% of oxygen and isobutane gas at a concentration of 1000 ppm was measured in the same manner as described above, and the electric characteristics of the gas sensor were measured. Obtained. FIG. 5 shows the measurement results in comparison with the electrical characteristics of the synthetic air composed of 80% nitrogen and 20% oxygen shown in FIG. FIG. 6 is an enlarged view of a square portion in FIG.
[0040]
5 and 6 that when the surface temperature exceeds about 310 ° C., the resistance value in the evaluation gas decreases as compared with the resistance value in synthetic air consisting of 80% nitrogen and 20% oxygen. it is obvious. This is probably because the adsorbed oxygen reacted with the combustible gas and was removed.
[0041]
Table 2 shows gas sensitivity values when the humidity is changed when the film surface temperature is 300 ° C., 350 ° C., and 400 ° C. At this time, the resistance of the gas detection film in the synthetic air consisting of 80% of nitrogen and 20% of oxygen and the resistance of the gas detection film in the evaluation gas obtained by adding isobutane gas to the synthetic air so that the isobutane gas concentration becomes 1000 ppm. And the ratio was defined as gas sensitivity.
[0042]
[Table 2]

[0043]
From Table 2, it is apparent that when the film surface temperature of the gas detection film exceeds 350 ° C., stable gas sensitivity can be obtained without being substantially affected by humidity.
[0044]
FIGS. 7 and 8 are electron microscope photographs showing the crystal structures of the surfaces of a zinc oxide thin film to which platinum is added and a zinc oxide thin film to which platinum is not used (comparative) used as a gas detection film in the present invention.
[0045]
7 and 8, the zinc oxide thin film to which platinum was added had a smaller particle size than the zinc oxide thin film to which platinum was not added (comparative), and the platinum particles were uniformly dissolved in zinc oxide, and the homogeneity was good. It is clear that
[0046]
In the above example, as the substrate 2, a substrate subjected to an insulating film treatment is used, and no insulating layer is particularly provided. However, as shown in FIGS. The insulating layer 6 can be provided on at least a part of the lower surface or the upper surface of the second 2.
[0047]
For example, in the gas sensor 1 shown in FIG. 9, the insulating layer 6 is provided on the lower surface of the substrate 2 and the heater 5 is formed on the insulating layer 6. A film 3 is formed, and an electrode 4 is formed on the gas detection film 3.
[0048]
When electric power is applied to the heater 5 of the gas sensor 1 to generate heat and the gas detection film 3 is heated to 350 ° C. or higher, the gas sensor 1 has high sensitivity.
[0049]
Further, the gas sensor 1 shown in FIG. 10 has a configuration in which the gas detection film 3 is interposed between the insulating layers 6 including the heater 5. That is, in the gas sensor 1, the heater 5 is formed on the upper surface of the substrate 2, the insulating layer 6 is provided on at least a part of the upper surface of the substrate 2 including the heater 5, and the gas detection film 3 is formed on the rest of the upper surface of the substrate 2. The electrode 4 is formed on the gas detection film 3 in the same manner as in the above example.
[0050]
To produce the gas sensor 1 of FIG. 10, for example, platinum is deposited on the silicon substrate 2 by a sputtering method, and the heater 5 is patterned. Next, an insulating layer 6 such as Ta ₂ O ₅ or SiO ₂ is deposited on at least a part of the upper surface of the substrate 2 including the heater 5, and unnecessary portions are formed by etching.
[0051]
The gas detection film 3 is formed on the upper surface of the substrate 2 between the adjacent insulating layers 6 thus formed in the same manner as in the above example.
[0052]
An electrode 4 is formed on the gas detection film 3 in the same manner as in the above example to obtain the gas sensor 1 having the configuration shown in FIG.
[0053]
Further, the gas sensor 1 shown in FIG. 11 has a configuration in which an insulating layer 6 is provided between the substrate 2 and the gas detection film 3. That is, in the gas sensor 1, the heater 5 is formed on the upper surface of the substrate 2, the insulating layer 6 is provided on the entire upper surface of the substrate 2 including the heater 5, and the gas detection film 3 is formed on the insulating layer 6. Then, the electrode 4 is formed on the gas detection film 3 in the same manner as in the above-described example.
[0054]
In order to produce the gas sensor 1 shown in FIG. 11, for example, platinum is deposited on the silicon substrate 2 by a sputtering method, and the heater 5 is patterned. Next, an insulating layer 6 such as Ta ₂ O ₅ or SiO ₂ is deposited on the entire upper surface of the substrate 2 including the heater 5, and unnecessary portions are formed by etching.
[0055]
On the insulating layer 6 thus formed, the gas detection film 3 is formed as in the above-described example.
[0056]
An electrode 4 is formed on the gas detection film 3 in the same manner as in the above example, to obtain the gas sensor 1 having the configuration shown in FIG.
[0057]
In the gas sensor according to the present invention, the gas detection portion (the portion where the electrodes and the heater are present) of the gas detection film has a structure floating in the air so that the gas sensor can be applied to a micro gas sensor intended for a low power consumption gas sensor. Can also be configured. For example, a structure in which the gas detection portion shown in FIG. 12 is suspended in the air like a bridge or a structure in which the gas detection portion shown in FIG. 13 is suspended in the air like a beam can be used.
[0058]
Since the steps in the micromachining process are the same in any of the above structures, a description will be given below of a case in which the gas detection portion in FIG. 12 has a structure floating in the air like a bridge. FIG. 14 shows an example of the manufacturing process. Note that the heater is omitted in FIG. Further, CDE in FIG. 14 means chemical dry etching.
[0059]
First, a silicon substrate 22 which is easily subjected to undercut etching, for example, subjected to an insulating film treatment with a crystal orientation (110) is used, and an oxide film on the upper surface of the silicon substrate 22 is removed using hydrofluoric acid (FIG. 14). Step a). An insulating layer 26a is formed on the surface of the silicon substrate 22 by sputtering using Ta ₂ O ₅ to a thickness of about 1.2 μm (step b in FIG. 14).
[0060]
Next, in order to form the electrode 24 and the heater 25 on the insulating layer 26a, for example, a metal thin film of platinum or the like is formed by a sputtering method (step b in FIG. 14), and a mask such as Ta ₂ O _{5 is formed.} Is formed by a sputtering method (step b in FIG. 14), patterned into a shape of an electrode or a heater by photolithography (step c in FIG. 14), and then Ta ₂ O ₅ is removed by chemical dry etching. An electrode or a heater-shaped mask is used. Using this mask, the electrode 24 and the heater 25 are formed by a dry etching method such as reverse sputtering (step d in FIG. 14).
[0061]
In order to protect the electrode 24 and the heater 25 thus formed, for example, a film of Ta ₂ O ₅ is formed by a sputtering method to form a protective insulating layer 26b (Step E in FIG. 14).
[0062]
Next, after patterning by photolithography so that a gas detection film 23 to be formed later can directly contact the electrode 24 (following the process in FIG. 14), a gas detection film window 27 is formed by dry etching such as chemical dry etching. (Step g in FIG. 14).
[0063]
Further, after patterning into a moat shape by photolithography, a moat mask is formed by etching such as chemical dry etching (Step H in FIG. 14). Next, the silicon substrate 22 is undercut-etched (anisotropically etched) at an etchant temperature of 75 to 80 ° C. for 210 minutes using, for example, a KOH solution as an etchant to form a moat portion 28 (step in FIG. 14). .
[0064]
In this case, as the etchant, for example, a NaOH solution, a hydrazine solution, an ethylenediamine-pyrocatechol-water solution, a tetramethylammonium hydroxide solution, or the like may be used in addition to the KOH solution.
[0065]
After applying to the structure obtained above using the solution of the gas detection film material prepared in the same manner as above, sintering is carried out (Step n in FIG. 14). As the coating method, for example, a spin coating method, a doctor blade method, a dip coating method, or the like is used.
[0066]
Finally, when the electrode lead wires 29 and 29 are attached to the pad portions of the electrodes 24 and the heater lead wires 30 and 30 are attached to the heater 25 pad portions, the gas detection portion is bridged like a bridge. The gas sensor of the present invention having a structure provided on the base 31 is obtained.
[0067]
In the above example, Ta ₂ O ₅ is used as the insulating layer, but Si ₃ N ₄ , SiO _2, or the like may be used. Further, in the above example, platinum is used for the electrodes and the heater, but a metal thin film such as Ni-Cr or Cr may be used.
[0068]
Here, by applying a solution of the gas detection film material to the front surface of the sensor, there is a concern about conductivity between the lead wire and the pad. However, since the gas detection film is sufficiently thin, it penetrates by ordinary wire bonding and has a low conductivity. Does not cause any problems. In addition, when voltage is applied to the electrodes and the heater, there is a concern about leakage through the gas detection film. However, since the resistance of the electrodes and the heater material is smaller than that of the gas detection film material, the leakage of the current to the gas detection film material is small. It is negligible and there is no problem. However, if these concerns cannot be ignored, the bridge-like structure may be covered with a photosensitive material, and the gas detection film material other than the gas detection film window 27 may be removed by photolithography and etching.
[0069]
The gas sensor having a structure in which the gas detection portion shown in FIG. 13 is suspended in the air like a beam is also similar to the gas sensor having a structure in which the gas detection portion shown in FIG. 12 is suspended in the air like a bridge. Can be manufactured. In FIG. 13, 32 is a silicon substrate, 33 is a gas detection film, 34 is an electrode, 35 is a heater, 36a and 36b are insulating layers, 37 is a gas detection film window, 38 is a moat portion, 39 is an electrode lead wire, Numeral 40 denotes a heater lead wire, and numeral 41 denotes a beam-shaped portion.
[0070]
【The invention's effect】
According to the present invention, since the zinc oxide thin film to which platinum is added, which is formed by the sol-gel method, is used as the gas detection film, it is possible to uniformly disperse the platinum particles in the gas detection material. It is possible to provide a gas sensor having a gas detection film having good gas homogeneity and excellent gas sensitivity characteristics.
[0071]
Further, according to the invention described in claims 4 to 6, since the gas detection portion of the gas detection film has a structure floating in the air, a gas sensor which can be applied to a micro gas sensor which can reduce power consumption is provided. Can be provided.
[0072]
Furthermore, according to the invention described in claims 8 to 9, by using the gas sensor according to any one of claims 1 to 7, a gas that issues an alarm alone or stops supply of gas when a gas leaks is provided. It is possible to easily provide a leak alarm device and a gas leak alarm system that issues an alarm or stops gas supply in conjunction with, for example, a gas meter when a gas leaks.
[0073]
Furthermore, according to the invention as set forth in claim 10, when the detection portion of the gas detection film has a structure floating in the air, a solution of a zinc oxide thin film material to which platinum is added, formed by a sol-gel method, is applied. By forming the detection film, a gas sensor having a gas detection film having good homogeneity and excellent gas sensitivity characteristics can be easily manufactured.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a gas sensor according to an embodiment of the present invention.
FIGS. 2A and 2B are perspective views of the gas sensor, wherein FIG. 2A is an explanatory diagram viewed from a front side and FIG.
FIG. 3 is a graph showing gas sensitivity of the gas sensor of the present invention when using a zinc oxide thin film in which the amount of added platinum is changed.
FIG. 4 is a diagram showing electric characteristics of the gas sensor of the present invention in synthetic air composed of 80% of nitrogen and 20% of oxygen.
FIG. 5 is a diagram showing the electrical characteristics of the gas sensor of the present invention in an evaluation gas obtained by adding isobutane gas to synthetic air consisting of 80% of nitrogen and 20% of oxygen to give an isobutane concentration of 1000 ppm.
FIG. 6 is an enlarged view of the square portion.
FIG. 7 is an electron micrograph showing a crystal structure of a surface of a zinc oxide thin film to which platinum is added, which is a gas detection film of the gas sensor of the present invention.
FIG. 8 is an electron micrograph showing the crystal structure of the surface of a zinc oxide thin film to which no platinum is added.
FIG. 9 is a schematic diagram illustrating the configuration of another gas sensor according to the present invention.
FIG. 10 is an explanatory diagram showing a schematic configuration of still another gas sensor according to the present invention.
FIG. 11 is an explanatory view showing a schematic configuration of still another gas sensor according to the present invention.
FIGS. 12A and 12B are schematic explanatory diagrams of the top surface of the gas sensor of the present invention having a structure in which the gas detecting portion is suspended in the air like a bridge, and FIGS.
FIGS. 13A and 13B are schematic explanatory views of the upper surface of the gas sensor of the present invention having a structure in which the gas detecting portion is suspended in the air in the form of a beam, and FIG.
FIG. 14 is a process explanatory view showing an example of a manufacturing process of the gas sensor of the present invention having a structure in which a gas detection portion is suspended in the air like a bridge.
[Explanation of symbols]
1 Gas sensors 2, 22, 32 Substrates 3, 23, 33 Gas detecting films 4, 24, 34 Electrodes 5, 25, 35 Heaters 6, 26a, 26b, 36a, 36b Insulating layers 27, 37 Gas detecting film windows 28, 38 Moat parts 11, 12, 29, 39 Lead wires for electrodes 13, 14, 30, 40 Lead wires for heater

Claims (10)

In a gas sensor including a substrate, a gas detection film formed on the substrate, an electrode formed in contact with the gas detection film, and a heater for heating, the gas detection film is formed by a sol-gel method. A gas sensor using a zinc oxide thin film to which is added. As the gas detection film, a platinum / zinc oxide thin film obtained by adding platinum to zinc at a ratio of zinc / platinum (molar ratio) of 100 / 0.1 to 100 / 0.75 is used. The gas sensor according to claim 1. The gas sensor according to claim 1, wherein an insulating layer is provided on at least a part of a lower surface or an upper surface of the substrate. The gas sensor according to any one of claims 1 to 3, wherein the gas detection portion of the gas detection film has a structure floating in the air. The gas sensor according to claim 4, wherein the gas detection portion of the gas detection film has a structure in which it is suspended in the air like a bridge. The gas sensor according to claim 4, wherein the gas detection portion of the gas detection film has a structure in which it is suspended in the air like a beam. The gas sensor according to claim 1, wherein the gas detection film operates at 350 ° C. or higher. A gas leak alarm using the gas sensor according to claim 1. A gas leak alarm system using the gas sensor according to claim 1. Forming an insulating layer on the substrate, forming an electrode and a heater on the insulating layer,
A protective insulating layer is formed thereon, and a gas detection film window is formed on the insulating layer.
Next, a moat portion is formed on the substrate,
To the resulting structure, separately prepared by a sol-gel method, apply a solution of zinc oxide thin film material with added platinum, sinter after forming a gas detection film,
The method of manufacturing a gas sensor according to claim 4, further comprising attaching an electrode lead wire to the electrode and a heater lead wire to the heater.

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