JP3988999B2

JP3988999B2 - Thin film gas sensor and manufacturing method thereof

Info

Publication number: JP3988999B2
Application number: JP2003184535A
Authority: JP
Inventors: 総一田畑; 勝己檜垣; 博一佐々木; 久男大西; 健二国原; 卓弥鈴木; 健松原; 光男小林
Original assignee: Osaka Gas Co Ltd; Fuji Electric Holdings Ltd
Current assignee: Fuji Electric Co Ltd; Osaka Gas Co Ltd
Priority date: 2003-06-27
Filing date: 2003-06-27
Publication date: 2007-10-10
Anticipated expiration: 2023-06-27
Also published as: JP2005017182A

Description

【０００１】
【発明の属する技術分野】
この発明は、電池駆動を念頭においた低消費電力型薄膜ガスセンサおよびその製造方法に関する。
【０００２】
【従来の技術】
一般的に、ガスセンサはガス漏れ警報器などの用途に用いられ、或る特定のガス、例えばＣＯ，ＣＨ₄，Ｃ₃Ｈ₈，Ｃ₂Ｈ₅ＯＨ等に選択的に感応するデバイスであり、その性格上、高感度，高選択性，高応答性，高信頼性，低消費電力が必要不可欠である。ところで、家庭用として普及しているガス漏れ警報器には、都市ガス用やプロパンガス用の可燃性ガス検知を目的とするものと、燃焼機器の不完全燃焼ガス検知を目的とするもの、または両方の機能を併せ持ったものなどがあるが、いずれもコストや設置性の問題から普及率はそれほど高くはない。
【０００３】
このような事情から、普及率の向上を図るべく、設置性の改善、具体的には電池駆動としコードレス化することが望まれている。電池駆動を実現するためには低消費電力化が最も重要であるが、接触燃焼式や半導体式のガスセンサでは、１００℃〜５００℃の高温に加熱して検知する必要がある。このことから、ＳｎＯ₂などの粉体を燒結する従来の方法では、スクリーン印刷等の方法を用いたとしても厚みを薄くするには限界があり、電池駆動に用いるには熱容量が大きすぎる。そこで、ヒーター，感知膜を１μｍ以下の薄膜で形成し、さらに、深堀エッチング加工プロセスによりダイアフラム構造などの低熱容量，断熱構造とした薄膜ガスセンサの出現が待たれていた。
【０００４】
そこで、ダイアフラム構造などの超低熱容量構造とした薄膜ガスセンサが、例えば特許文献１に提案されたが、このようなガスセンサを用いたガス漏れ警報器においても、電池の交換無しで５年以上の寿命を持たすためには、薄膜ガスセンサのパルス駆動が必須となる。通常、ガス漏れ警報器は３０〜１５０秒の一定周期に１回の検知が必要であり、この周期に合わせた検知部を室温から１００〜５００℃の高温に加熱する。上記電池の交換無しで５年以上の寿命要請にこたえるため、この加熱時間は１００ｍｓ以下が目標となる。
【０００５】
パルス駆動の薄膜ガスセンサにおいても、低消費電力化のためには検出温度の低温化，検出時間の短縮，検出サイクルの長期化（通電をオフ（ｏｆｆ）する時間を長くする）が重要である。薄膜ガスセンサにおける検出温度はガス種に対する検出感度などからＣＯセンサでは〜１００℃、ＣＨ４センサでは〜４５０℃、検出時間はセンサの応答性から〜５００ｍｓ、検出サイクルはＣＨ４センサでは３０秒、ＣＯセンサでは１５０秒とされる。
また、ｏｆｆ時間にセンサ表面に付着する水分その他の吸着物を脱離させＳｎＯ₂表面をクリーニングすることが、電池駆動（パルス駆動）の薄膜ガスセンサの経時安定性を向上する上で重要であり、検出前に一旦センサ温度を４００〜５００℃に加熱し（時間１００ｍｓ）、その直後にそれぞれのガスの検出温度でガス検知を行なっている。
【０００６】
このことから、低消費電力化のため薄膜ガスセンサではセンサのｏｆｆ時間が、センシングのための時間に比べて圧倒的に長い、すなわち、圧倒的に長時間センサは室温状態にあることになる。検知ガスは活性炭を通じてガス拡散で、センサの検知部であるＳｎＯ₂表面に到達するようなセンサ構造であり、センサを劣化させる被毒ガスまたは検知を阻害するＮＯｘ，ＳＯｘもしくは炭化水素系ガスなどは活性炭に吸着することで、検知部であるＳｎＯ₂表面に到達しないような配慮がされており、長期にわたり経時安定性を維持するような工夫がされている。
【０００７】
しかし、このような対策を施した電池駆動（パルス駆動）の薄膜ガスセンサでも、高温高湿雰囲気が長時間継続するような極端な雰囲気にセンサをさらした場合、抵抗値が不安定になり経時安定性が悪くなることがある。その原因は明確ではないが、高湿度に長時間さらされた結果、雰囲気の水分が活性炭層を破過してセンサの検知部であるＳｎＯ₂表面に到達し、ＳｎＯ₂表面が高湿度雰囲気に晒されるためと推定される。また、活性炭層が水分吸着により飽和することで、本来活性炭層で吸着すべき検知阻害物質ＮＯｘ，ＳＯｘまたは炭化水素系ガスなどがＳｎＯ₂表面に到達することによる影響とも推定される。すなわち、センサのｏｆｆ時間に、ガス感度を向上させるため多孔質にしたＳｎＯ₂の細孔への水分の吸着または検知阻害ガスの吸着により、センサ抵抗値が不安定になるものと考えられる。
【０００８】
本来、４００〜５００℃に加熱するクリーニングにより吸着物を脱離し、ＳｎＯ₂表面を常時クリーニングすることで経時安定性が確保されるはずであるため、クリーニング時間を数倍に延長しての試験を行なったが、経時安定性は改善されていない。このような現象は、特に低温でガス検知を行なうＣＯセンサでの経時安定性に顕著に出現する。
そこで、上記のような問題を解決すべく、出願人はＳｎＯ₂薄膜のスパッタ時に、貴金属触媒を同時にスパッタ（ｃｏ−ｓｐｕｔｔｅｒｉｎｇ）する手法を例えば特許文献２で提案している。
【０００９】
【特許文献１】
特開２０００−２９８１０８号公報（図３、第３−４頁）
【特許文献２】
特開２０００−２９２３９８号公報（図１、第３頁）
【００１０】
【発明が解決しようとする課題】
しかし、特許文献２の方法ではＳｎＯ₂最表面だけでなく、ＳｎＯ₂薄膜の内部細孔への貴金属触媒の担持が期待されたが、ほとんどの貴金属触媒成分がＳｎＯ₂結晶格子にとり込まれてしまってアクセプタ的な振るまいを示し、ＳｎＯ₂薄膜の抵抗が顕著に高抵抗化するなどの問題を含むため実用化に到っていない。
この発明はこのような事情に鑑みてなされたもので、その課題は、高温高湿下でも経時安定性に優れた電池駆動（パルス駆動）の薄膜ガスセンサを提供することにある。
【００１１】
【課題を解決するための手段】
このような課題を解決するため、請求項１の発明では、薄膜状の支持膜の外周または両端部をＳｉ基板により支持し、外周部または両端部が厚く中央部が薄く形成されたダイアフラム様の支持基板上に薄膜のヒータを形成し、この薄膜のヒータをＳｉＯ₂を含む電気絶縁膜で覆い、その上にガス感知膜用の電極を形成し、さらに貴金属触媒とＳｎＯ₂を主成分とする薄膜からなるガス感知膜を形成した後、その最表面にガス感知膜を完全に被覆するように形成した触媒担持多孔質アルミナからなる触媒フィルタ層（選択燃焼層）を具備した薄膜ガスセンサにおいて、
前記貴金属触媒とＳｎＯ ₂ を主成分とする薄膜が、ＳｎＯ ₂ 薄膜と貴金属触媒薄膜を交互に成膜した積層構造で、貴金属同士が互いに孤立したアイランド状に成膜されており、貴金属触媒薄膜の面内および面間のいずれの方向にも貴金属同士の電気的な導通がないことを特徴とする。
【００１２】
請求項１の発明においては、前記ＳｎＯ₂ 粒子は粒界を介して相互に接しており、膜面内および膜面間方向で電気的に導通していることができ（請求項２の発明）、請求項１または２の発明においては、前記貴金属触媒はＰｄまたはＰｔ、もしくはＰｄとＰｔの混合物を主成分とするものであることができる（請求項３の発明）。
【００１３】
請求項４の発明では、薄膜状の支持膜の外周または両端部をＳｉ基板により支持し、外周部または両端部が厚く中央部が薄く形成されたダイアフラム様の支持基板上に薄膜のヒータを形成し、この薄膜のヒータをＳｉＯ₂を含む電気絶縁膜で覆い、その上にガス感知膜用の電極を形成し、さらに貴金属触媒とＳｎＯ₂を主成分とする薄膜からなるガス感知膜を形成し、その最表面にガス感知膜を完全に被覆するように形成した触媒担持多孔質アルミナからなる触媒フィルタ層（選択燃焼層）を具備した薄膜ガスセンサを製造するに当たり、
前記ＳｎＯ ₂ からなるガス感知膜を成膜後、貴金属触媒層をアイランド状に成膜形成することを特徴とする。
【００１８】
ＳｎＯ₂薄膜の表面にＰｄまたはＰｔ、もしくはＰｄとＰｔの混合触媒などの貴金属触媒を分散，担持することで、ガス検知前のクリーニングによりセンサのｏｆｆ時間にＳｎＯ₂薄膜表面に吸着した水分または検知阻害ガスを完全に脱離できるため、経時安定性に優れた電池駆動（パルス駆動）の薄膜ガスセンサが得られる。特に、低温動作のＣＯセンサにおいては、貴金属触媒の分散，担持により、ＣＯ検出反応の安定性が向上することの相乗効果で顕著な経時安定性が得られる。
すなわち、図２の模式図に示されるように、貴金属触媒へ解離吸着した原子状酸素がスピルオーバ（あふれるまたはこぼれる）により、ＳｎＯ₂薄膜表面上へ潤沢に供給されることで、クリーニング時のＳｎＯ₂薄膜表面上に吸着している検知阻害ガスの酸化分解が促進、またはＯＨ基の脱離が促進され、ＳｎＯ₂薄膜表面が充分リフレッシュされることで、経時安定性が向上する。さらに、低温動作のＣＯセンサにおいては、貴金属触媒へ解離吸着した原子状酸素がスピルオーバにより、ＳｎＯ₂薄膜表面上へ潤沢に供給することで、ＣＯ酸化またはＣＯ吸着の反応速度が向上し、経時安定性が向上する。
【００１９】
また、積層構造のものでは、各貴金属触媒薄膜層は貴金属粒子が互いに孤立したアイランド状に成膜されており、貴金属触媒薄膜の面内および面間のいずれの方向にも貴金属同士の電気的な導通が無く、また各ＳｎＯ₂薄膜においてはＳｎＯ₂粒子が相互に粒界を介して接しており、膜面内および膜面間のいずれの方向にも電気的な導通を有するように成膜することが重要である。
具体的には、貴金属触媒薄膜の膜厚は０．５ｎｍ以上で１０ｎｍ以下、好ましくは１ｎｍ以上で６ｎｍ以下であり、ＳｎＯ₂薄膜は１ｎｍ以上で１００ｎｍ以下、好ましくは５ｎｍ以上で５０ｎｍ以下であり、少なくとも４層（基板／ＳｎＯ₂／触媒／ＳｎＯ₂／触媒）以上繰り返した積層構造が必要である。積層構造では、貴金属触媒薄膜の膜厚が１０ｎｍ以上では貴金属触媒がアイランド状にならず導通を生じる場合があり、０．５ｎｍ以下では触媒としての機能が弱い。
積層数が多いほど、また貴金属触媒薄膜の膜厚が厚いほど、貴金属触媒が多く担持されることとなり、センサの安定化に効果がある。担持量（貴金属触媒重量のＳｎＯ₂重量に対する割合（ｗｔ％：重量パーセント））は、貴金属触媒薄膜の膜厚とＳｎＯ₂薄膜の膜厚積層数とにより任意に決めることができる。貴金属の担持量は０．１ｗｔ％から７５ｗｔ％、好ましくは０．５ｗｔ％から２０ｗｔ％である。
【００２０】
【発明の実施の形態】
図１はこの発明の実施の形態を示すセンサ断面図である。
両面に熱酸化膜を０．３μｍ形成したＳｉ基板１の表面に、ダイアフラム構造の支持層２となるＳｉＮとＳｉＯ₂膜を、順次プラズマＣＶＤ法にてそれぞれ０．１５μｍ，１μｍ形成する。その上に接合層（１）としてＴａを０．０５μｍ形成後、連続して、ヒータ層３としてＰｔＷ（Ｐｔ＋４ｗｔ％Ｗ）膜を０．５μｍ形成し、さらに連続して接合層（２）としてＴａを０．０５μｍ形成する。成膜はマグネトロンスパッタリング装置を用い、通常のスパッタリング方法によって行なう。条件は、成膜温度１００℃、成膜パワー１００Ｗ、成膜圧力１Ｐａである。
【００２１】
その後、微細加工により、ヒーターパターンを形成する。ウエットエッチングのエッチャントとして、Ｔａには水酸化ナトリウムと過酸化水素混合液を、Ｐｔには王水をそれぞれ９０℃に加熱して用いた。さらに、ＳｉＯ₂などの絶縁膜４をスパッタ法により１．０μｍ形成した後、微細加工により図示されていないヒータの電極パッド部分をＨＦによりエッチングして窓明け後、導通の確保とワイヤボンディング性を向上させるため、接合層（２）のＴａを水酸化ナトリウムと過酸化水素混合液で除去する。
【００２２】
次に、下地のＳｉＯ₂との密着性向上のため中間層Ｔａを０．０５μｍ形成後、連続してＰｔ感知膜電極５を０．２μｍ成膜する。成膜はマグネトロンスパッタリング装置を用い、通常のスパッタリング方法によって行なう。条件は、成膜温度１００℃、成膜パワー１００Ｗ、成膜圧力１Ｐａである。その後、ヒータ層３と同様ウエットエッチにより、一対の感知膜ＳｎＯ₂の抵抗測定用感知膜電極５をパターニングする。
その後、微細加工でＳｎＯ₂感知膜を成膜する部分のみ開口し、それ以外をレジストで被覆してパターンを形成する。次に、上記のパターニングが施されたウエハーをスパッタチャンバーにセットし、ガスの感知膜であるＳｎＯ₂６をスパッタで成膜する。ＳｎＯ₂感知膜６のサイズは１００μｍ²で、成膜条件は３００Ｗ，１Ｐａ，Ａｒ＋Ｏ２中１００℃で、膜厚は約２μｍである。
【００２３】
続いて、この発明に関わるＳｎＯ₂感知膜上へ触媒層を形成する方法について説明する。
ＳｎＯ₂感知膜を成膜後、さらにＰｄをスパッタで成膜する。Ｐｄのスパッタ成膜は、真空を破ることなく連続成膜しても良いし、一旦真空を破って別のスパッタ装置で成膜しても構わない。Ｐｄの成膜条件は３０Ｗ，１Ｐａ，Ａｒ＋Ｏ２中１００℃で、膜厚は▲１▼０．５ｎｍ、▲２▼１ｎｍ、▲３▼５ｎｍ、▲４▼１０ｎｍ、▲５▼１００ｎｍ、▲６▼５００ｎｍで試作した。比較のために、Ｎｏ．１３にＰｄ無しの素子も用意した。なお、丸付き数字は素子の番号を示し、以下同様とする。Ｎｏ．１〜Ｎｏ．９までは丸付き数字「▲１▼〜▲９▼」を使用する場合もあるが、Ｎｏ．１０以上は専ら「Ｎｏ．」のみで表現することとする。
【００２４】
図３に、スパッタで成膜したＰｄ層／ＳｎＯ₂感知膜の模式断面図を示す。同図（ａ）は全体を示し、同図（ｂ）に部分的に拡大して示す。
図３（ｂ）から、スパッタで成膜したＰｄ層は、ＳｎＯ₂感知膜の表層部に殆どが付着し、柱状成長したＳｎＯ₂の細孔内への付着は少ないことが分かる。
Ｐｄ成膜後（Ｐｄ無しの素子は、ＳｎＯ₂成膜後）レジストを剥離液で除去し、Ｐｄ触媒付きＳｎＯ₂感知膜、およびＰｄ触媒無しＳｎＯ₂感知膜を試作した。なお、Ｐｄ触媒無しＳｎＯ₂感知膜素子は、比較のための従来構造の素子として用いるとともに、以下に述べる含浸法により貴金属触媒を担持する素子の実験に用いた。
【００２５】
以下、含浸法について説明する。
Ｐｄ触媒無しＳｎＯ₂感知膜ウエハーを、貴金属の化合物の水溶液に含浸し、熱分解によりＳｎＯ₂感知膜表面に貴金属触媒を分散させるものである。なお、この発明の成膜条件でスパッタ成膜した、ＳｎＯ₂感知膜の細孔がもたらす空孔容積は約２０％である。
１ｗｔ％のＰｄを含むジニトロジアミンＰｄ硝酸水溶液中にウエハーを１０分間浸漬し、ＳｎＯ₂感知膜の細孔へジニトロジアミンＰｄ硝酸水溶液を含浸する。取り出したウエハー表面に余分に付着したジニトロジアミンＰｄ硝酸水溶液を、Ｎ₂ブローなどで吹き飛ばす。このとき、ＳｎＯ₂感知膜の細孔へ含浸されたジニトロジアミンＰｄ硝酸水溶液は毛細管力で保持されており、Ｎ₂ブローで飛散することはない。
【００２６】
その後、ウエハーを電気炉で空気気流中１５０℃で６０分乾燥後、さらに空気気流中で５００℃で１時間分解し、Ｐｄを担持する。１回の操作で約０．３ｗｔ％のＰｄがＳｎＯ₂感知膜に担持される。この操作を繰り返すことで、操作回数×０．３ｗｔ％のＰｄがＳｎＯ₂感知膜に担持されることになる。ジニトロジアミンＰｄ硝酸水溶液中のＰｄ濃度を上昇させることで、１回の操作でのＰｄ担持量を多くすることも可能であり、逆にＰｄ濃度を減少させることで、１回の操作でのＰｄ担持量を少なくすることも可能である。Ｐｄ濃度を上昇させる場合は、ジニトロジアミンＰｄ硝酸水溶液への溶解度から７ｗｔ％以下が好ましい。
【００２７】
上記操作により、▲７▼０．０５ｗｔ％、▲８▼０．１ｗｔ％、▲９▼１ｗｔ％、５ｗｔ％（Ｎｏ．１０）、１０ｗｔ％（Ｎｏ．１１）、２０ｗｔ％（Ｎｏ．１２）のＰｄを担持したＳｎＯ₂感知膜を得ることができた。
図４に、以上のような含浸法で得たＰｄ／ＳｎＯ₂感知膜の模式断面図を示す。同図（ａ）は全体を示し、同図（ｂ）に部分的に拡大して示す。
図４（ｂ）から、含浸法で担持したＰｄ層はＳｎＯ₂感知膜の表層部だけでなく、柱状成長したＳｎＯ₂の細孔内への付着も均一であることが分かる。なお、Ｐｄは空気気流中で分解しているため、大部分がＰｄＯになっている。また、Ｎｏ．１３は、比較のために試作したＳｎＯ₂感知膜にＰｄを担持していない従来構造の素子である。
【００２８】
次に、アルミナ粒子にＰｔおよびＰｄ触媒を担持させた粉末をバインダとともにペーストとし、スクリーン印刷によりＳｎＯ₂の表面に塗布，焼結させて約３０μｍ厚の選択燃焼層（触媒フィルタ）７を形成する。この選択燃焼層７により、ガスセンサの感度，ガス種選択性，信頼性が向上する。最後に、基板の裏面からドライエッチによりＳｉを４００μｍ経の大きさにて完全に除去し、ダイアフラム構造とする。
ここで、ヒータ層（Ｔａ／ＰｔＷ／Ｔａ）と感知膜電極（Ｔａ／Ｐｔ）のパターニングの際には、きのこかさ状に形成された２種のメタル層をマスクとした一種のリフト法（例えば、特開２０００−０６５７７３号公報参照）を用いても良い。
【００２９】
続いて、上記方法により作製されたパルス駆動の薄膜ガスセンサＮｏ．１〜Ｎｏ．１３を４０℃、相対湿度８０％の環境下で動作させた場合の、３００ｐｐｍＣＯ／空気に対する抵抗値の経時変化を表１に示す。なお、スパッタ成膜法でＰｄ層を形成したセンサはＮｏ．１〜Ｎｏ．６（▲１▼〜▲６▼）であり、含浸法でＰｄ層を形成したセンサはＮｏ．７〜Ｎｏ．１２で、比較のための従来構造センサはＮｏ．１３である。また、パルス駆動条件／測定条件は以下の通りである。
検出サイクル：１５０秒
クリーニング温度×時間＝４５０℃×２００ｍｓｅｃ
ＣＯ検出温度×検出タイミング＝１００℃×１００℃に加熱後５００ｍｓｅｃ後
【００３０】
【表１】

【００３１】
上記表１から、この発明の素子Ｎｏ．２〜Ｎｏ．５（▲２▼〜▲５▼）、Ｎｏ．８〜Ｎｏ．１１が高温，高湿の厳しい条件下でも、３００ｐｐｍＣＯ／空気に対する抵抗値の経時変化がなく安定していることが分かる。それに比べＰｄの付着量，担持量が少ないＮｏ．１とＮｏ．７の素子は、比較のために試作した従来構造の素子（Ｎｏ．１３）と同様、試験時間とともに抵抗値が増加し、５０日後には抵抗値が１桁以上変化する。Ｐｄの付着量，担持量が最も多いＮｏ．６，Ｎｏ．１２は初期で抵抗値が１桁低い。これは、本来アイランド状にならないＰｄ粒子が、Ｐｄの付着量，担持量が多すぎてお互いに接するため、Ｐｄに電気的な導通が生じたためと考えられる。Ｎｏ．６，Ｎｏ．１２の素子のＣＯが無い空気中抵抗値を測定したところ、３００ｐｐｍＣＯ／空気に対する抵抗値とほとんど同じであり、ＣＯ感度が無いことも分かっている。２０００ｐｐｍＣＨ４／空気でも３００ｐｐｍＣＯ／空気に対する経時変化ほど顕著ではないが、同様の結果が得られており、ＳｎＯ₂にＰｄ触媒を付着，担持することで、抵抗値の経時変化が無く安定したパルス駆動の薄膜ガスセンサが得られた。
【００３２】
このような高温高湿下での抵抗値の経時変化は、パルス駆動の薄膜ガスセンサに特有の現象であり、Ｎｏ．１３の素子を常時１００℃以上に加熱する駆動モードで試験すると、上記抵抗値の経時変化は起こらない。
高温高湿下でのパルス駆動の薄膜ガスセンサの抵抗値の経時変化は、ｏｆｆ時間における水分などの吸着が原因と推定されるが、明確には分からない。この発明では、ＰｄをＳｎＯ₂表面に付着，担持することで顕著な改善が認められているが、その原因は図２に示すようなＰｄ上でのスピルオーバ酸素のＳｎＯ₂表面への潤沢な供給、それに伴うクリーニング反応の促進、Ｐｄ触媒のＣＯ，ＣＨ４検知反応の促進などいろいろ考えられる。
また、以上ではＰｄ触媒の例で説明したが、その代わりにＰｔ，Ｐｔ−Ｐｄの合金触媒などの貴金属触媒を用いることができる。さらに、含浸法で触媒を担持する化合物としてジニトロジアミン化合物を用いたが、塩化物などの化合物でも良い。
【００３３】
次に、ＳｎＯ₂薄膜と貴金属触媒薄膜を交互に成膜して積層構造の感知膜（Ｐｔ／ＳｎＯ₂）ｎを形成する例について説明する。
感知膜のサイズは、１００μｍ²とする。また、ＳｎＯ₂層の成膜条件は成膜パワー５０Ｗ，成膜圧力１Ｐａ，成膜雰囲気Ａｒ＋Ｏ２中，成膜温度１００℃で、各層の膜厚は成膜時間で制御される。上記条件でＳｎＯ₂とＰｔの成膜レートは、それぞれ５ｎｍ／２．５ｎｍとなる。なお、積層膜（Ｐｔ／ＳｎＯ₂）ｎのトータル膜厚は〜１μｍとする。スパッタ装置はＳｎＯ₂，Ｐｔの両ターゲットを具備しており、ターゲットを切り替えることでＳｎＯ₂，Ｐｔと交互に成膜することができる。
【００３４】
最初にＳｎＯ₂層を、１００ｎｍ成膜する。その後Ｐｔを３ｎｍ成膜し、次にＳｎＯ₂を１８ｎｍ成膜する。さらに、Ｐｔを２ｎｍ、ＳｎＯ₂を１８ｎｍの成膜を４５回繰り返し、最表面はＰｔ成膜で終了する。これにより、トータル膜厚は１μｍの感知膜（Ｐｔ／ＳｎＯ₂）ｎを得る。Ｐｔの担持量は約３０ｗｔ％となる。
図５に積層膜（Ｐｔ／ＳｎＯ₂）₄₅の模式断面図を示す。ＳｎＯ₂はＰｔ上にもＳｎＯ₂上と同様な成膜をするため、Ｐｔの一部はＳｎＯ₂に被覆される。従って、気相と接する（表面にでている）Ｐｔの濃度としては＜１０％と推定される。図５に示すように、ＳｎＯ₂粒子とＰｔ粒子が帯状に成膜され、Ｐｔはアイランド（島）状になる。ＳｎＯ₂粒子は３次元的に連結し、電気的には互いに導通する。スパッタでＰｔを成膜するとその殆どはＳｎＯ₂粒子最表面に堆積し、一部はＳｎＯ₂粒子同士が作る細孔の中にも堆積する。ＳｎＯ₂粒子同士が作る細孔径は１０〜２０ｎｍと狭いため、細孔の深いところまではＰｔ粒子は入り込まない。こうして感知膜を成膜した素子を「Ａ」とする。
【００３５】
比較のため、感知膜を１μｍ厚のＳｎＯ₂にした従来素子（素子Ｃ）と、１μｍ厚のＳｎＯ₂感知膜を成膜後、さらに連続してＰｔを３ｎｍスパッタで成膜した素子（素子Ｂ）とを試作し、この発明の素子Ａとともに後工程を進めた。素子Ｂでは１μｍ厚のＳｎＯ₂の最表面に３ｎｍ厚のアイランド状Ｐｔが堆積した感知膜構造となる。
上記の感知膜構造を成膜後、レジストを剥離液で除去し、積層膜（Ｐｔ／ＳｎＯ₂）₄₅感知膜，ＳｎＯ₂感知膜，３ｎｍＰｔ／ＳｎＯ₂感知膜を有する３種類の素子Ａ，Ｂ，Ｃを試作した。
【００３６】
次に、アルミナ粒子にＰｔおよびＰｄ触媒を担持させた粉末をバインダとともにペーストとし、スクリーン印刷により感知膜の表面に塗布，焼結させ約３０μｍ厚の選択燃焼層（触媒フィルタ）を形成する。選択燃焼層によりガスセンサの感度，ガス種選択性，信頼性が向上する。最後に基板の裏面からドライエッチにより、Ｓｉを４００μｍ径の大きさにて完全に除去し、ダイアフラム構造とする。
ここで、ヒータ層（Ｔａ／ＰｔＷ／Ｔａ）と感知膜電極（Ｔａ／Ｐｔ）のパターニングの際には、先に挙げた特開２０００−０６５７７３号公報のように、きのこかさ状に形成された２種のメタル層をマスクとした一種のリフト法を用いても良い。
続いて、上記方法により作製されたパルス駆動の薄膜ガスセンサＡ，Ｂ，Ｃを４０℃，相対湿度８０％の環境下で動作させた場合の３００ｐｐｍＣＯ／空気に対する抵抗値の経時変化を表２に示す。
【００３７】
【表２】

表２のパルス駆動条件／測定条件は以下の通りである。
検出サイクル：１５０秒
クリーニング温度×時間＝４５０℃×２００ｍｓｅｃ
ＣＯ検出温度×検出タイミング＝１００℃×１００℃に加熱後５００ｍｓｅｃ後
【００３８】
表２から素子Ａが高温高湿の厳しい試験条件下で１５０日経過後も、３００ｐｐｍＣＯ／空気に対する抵抗値の経時変化が無く安定していることが分かる。これに比べ素子Ｃは２桁以上も素子抵抗が上昇する。また、素子Ｂでは５０日程度では安定であるが、それ以上の経過日数でやや抵抗値が上昇する傾向がある。
２０００ｐｐｍＣＨ４／空気でも、３００ｐｐｍＣＯ／空気に対する経時変化ほど顕著ではないが同様の結果が得られており、ＳｎＯ₂薄膜最表面だけでなく内部の細孔内部へ高分散でＰｔを担時することで、抵抗値の経時変化が無く安定したパルス駆動の薄膜ガスセンサを得ることができる。このような高温高湿下での抵抗値の経時変化は、パルス駆動の薄膜ガスセンサに特有の現象であり、上記素子Ｂ，Ｃを常時１００℃以上に加熱する駆動モードで試験しても、抵抗値の経時変化は起こらない。
【００３９】
高温高湿下でのパルス駆動の薄膜ガスセンサの抵抗値の経時変化は、ｏｆｆ時間における水分などの吸着が原因と推定されるが、明確には分からない。この発明ではＳｎＯ₂薄膜最表面だけでなく内部の細孔内部へ高分散でＰｔを担時することで、顕著な改善が認められているが、その原因は先の図２で説明したＰｔ上でのスピルオーバ酸素のＳｎＯ₂表面への均一で潤沢な供給、それに伴うクリーニング反応の促進、Ｐｄ触媒のＣＯ，ＣＨ４検知反応の促進などいろいろ考えられる。
以上では、Ｐｔ触媒の例を説明したが、Ｐｔの代わりにＰｄ，Ｐｔ−Ｐｄの合金触媒などの貴金属触媒を用いても良い。
【００４０】
また、積層構造のものでは、最初にＳｎＯ₂層を１００ｎｍ成膜したが、最初からＰｔを２ｎｍ，ＳｎＯ₂を１８ｎｍの繰り返しでも同様の効果が得られる。また、Ｐｔ／ＳｎＯ₂の膜厚比は一定でなくても良く、さらに成膜の最初をＳｎＯ₂で最後はＰｔとしたがその逆でも良いし、成膜の最初と最後が同一でも良い。
成膜方法もスパッタでなく蒸着やＣＶＤ（化学気相成長）法によっても良い。
【００４１】
【発明の効果】
請求項１〜４の発明によれば、ＰｄまたはＰｔ、もしくはＰｄ＋Ｐｔ混合触媒などの貴金属触媒を、ＳｎＯ₂薄膜最表面だけでなく内部の細孔内部へ高分散で担持することで、ガス検知前のクリーニングによりセンサのｏｆｆ時間にＳｎＯ₂薄膜表面に吸着した水分、または検知阻害ガスを完全に脱離できるため、経時安定性に優れた電池駆動（パルス駆動）の薄膜ガスセンサを得ることができる。特に、低温動作のＣＯセンサにおいては、貴金属触媒の分散，担持により、ＣＯ検出反応の安定性が向上するという相乗効果で顕著な経時安定性が得られる。
【図面の簡単な説明】
【図１】この発明の実施の形態を示す薄膜ガスセンサの断面構成図
【図２】酸素のスピルオーバを説明する説明図
【図３】スパッタ法で製造された感知膜の模式図
【図４】含浸法で製造された感知膜の模式図
【図５】スパッタ法で製造された積層感知膜の模式図
【符号の説明】
１…Ｓｉ基板（ダイアフラム）、２…支持層及び熱絶縁層、３…ヒーター層、４…絶縁層、５…感知層電極、６…感知層、７…選択燃焼層（触媒フィルタ）。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a low power consumption type thin film gas sensor with battery driving in mind and a method for manufacturing the same.
[0002]
[Prior art]
In general, a gas sensor is a device that is used for applications such as a gas leak alarm, and is selectively sensitive to a specific gas such as CO, CH ₄ , C ₃ H ₈ , C ₂ H ₅ OH, and the like. Due to its nature, 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 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 There are some that have both functions, but the penetration rate is not so high due to cost and installation problems.
[0003]
Under such circumstances, in order to improve the penetration rate, it is desired to improve the installation property, specifically, to be battery-driven and cordless. Low power consumption is the most important for realizing battery driving. However, in a catalytic combustion type or semiconductor type gas sensor, it is necessary to detect by heating to a high temperature of 100 ° C. to 500 ° C. For this reason, in the conventional method of sintering powder such as SnO ₂ , 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 appearance 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 and heat insulation structure such as a diaphragm structure by a deep etching process has been awaited.
[0004]
Thus, a thin film gas sensor having an ultra-low heat capacity structure such as a diaphragm structure has been proposed in, for example, Patent Document 1, but a gas leak alarm using such a gas sensor also has a life of 5 years or more without battery replacement. In order to have the above, pulse driving of the thin film gas sensor is essential. Usually, the gas leak alarm needs to be detected once in a fixed period of 30 to 150 seconds, and the detection unit in accordance with this period is heated from room temperature to a high temperature of 100 to 500 ° C. In order to meet the life requirement of 5 years or longer without replacing the battery, the target heating time is 100 ms or less.
[0005]
Even in the pulse-driven thin film gas sensor, it is important to lower the detection temperature, shorten the detection time, and lengthen the detection cycle (increase the time for turning off the energization) in order to reduce power consumption. The detection temperature in the thin film gas sensor is ~ 100 ° C for the CO sensor due to the detection sensitivity to the gas species, ~ 450 ° C for the CH4 sensor, the detection time is ~ 500 ms due to the response of the sensor, the detection cycle is 30 seconds for the CH4 sensor, and the CO sensor 150 seconds.
Moreover, it is 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. Before detection, the sensor temperature is once heated to 400 to 500 ° C. (time 100 ms), and immediately after that, gas detection is performed at the detection temperature of each gas.
[0006]
For this reason, in order to reduce power consumption, in the thin film gas sensor, the sensor off time is overwhelmingly longer than the sensing time, that is, the sensor is overwhelmingly at room temperature for a long time. The detection gas has a sensor structure that reaches the surface of SnO ₂ that is the detection part of the sensor by gas diffusion through activated carbon, and poisonous gas that degrades the sensor or NOx, SOx, or hydrocarbon gas that inhibits detection is activated carbon. Consideration is made so as not to reach the SnO ₂ surface, which is the detection part, by adsorbing to the surface, and contrivances are made to maintain stability over time over a long period of time.
[0007]
However, even with a battery-driven (pulse-driven) thin film gas sensor with such countermeasures, if the sensor is exposed to an extreme atmosphere where the high-temperature and high-humidity atmosphere continues for a long time, the resistance value becomes unstable and stable over time. May be worse. The cause is not clear, but as a result of being exposed to high humidity for a long time, the moisture in the atmosphere breaks through the activated carbon layer and reaches the SnO ₂ surface, which is the sensor detection part, and the SnO ₂ surface becomes a high humidity atmosphere. Presumed to be exposed. It is also estimated that the activated carbon layer is saturated by moisture adsorption, so that the detection inhibitor NOx, SOx or hydrocarbon gas that should be adsorbed by the activated carbon layer reaches the SnO ₂ surface. That is, it is considered that the sensor resistance value becomes unstable due to the adsorption of moisture to the pores of SnO ₂ made porous to improve the gas sensitivity or the detection inhibiting gas during the sensor off time.
[0008]
Originally, the adsorbate should be desorbed by cleaning heated to 400 to 500 ° C., and the stability over time should be ensured by constantly cleaning the SnO ₂ surface. Although performed, stability over time has not been improved. Such a phenomenon appears remarkably in the stability over time in the CO sensor that performs gas detection at a low temperature.
Therefore, in order to solve the above-mentioned problems, the applicant has proposed, for example, Patent Document 2 a method of simultaneously sputtering (co-sputtering) a noble metal catalyst during the sputtering of a SnO ₂ thin film.
[0009]
[Patent Document 1]
JP 2000-298108 A (FIG. 3, page 3-4)
[Patent Document 2]
JP 2000-292398 A (FIG. 1, page 3)
[0010]
[Problems to be solved by the invention]
However, in the method of Patent Document 2, it was expected that the noble metal catalyst was supported not only on the outermost surface of SnO ₂ but also in the internal pores of the SnO ₂ thin film, but most of the noble metal catalyst components were incorporated into the SnO ₂ crystal lattice. It has not been put into practical use because it has a problem such as acceptor behavior and includes a problem that the resistance of the SnO ₂ thin film is remarkably increased.
The present invention has been made in view of such circumstances, and an object thereof is to provide a battery-driven (pulse-driven) thin film gas sensor having excellent temporal stability even under high temperature and high humidity.
[0011]
[Means for Solving the Problems]
In order to solve such a problem, in the invention of claim 1, the outer periphery or both end portions of the thin film-like support film are supported by the Si substrate, the outer periphery portion or both end portions are thick, and the central portion is formed thin. A thin film heater is formed on a support substrate, this thin film heater is covered with an electrical insulating film containing SiO ₂ , an electrode for a gas sensing film is formed thereon, and a noble metal catalyst and SnO ₂ are the main components. In the thin film gas sensor having a catalyst filter layer (selective combustion layer) made of catalyst-supporting porous alumina formed so as to completely cover the gas sensing film on the outermost surface after forming the gas sensing film made of a thin film,
The thin film mainly composed of the noble metal catalyst and SnO ₂ has a laminated structure in which the SnO ₂ thin film and the noble metal catalyst thin film are alternately formed, and the noble metals are formed in islands isolated from each other. There is no electrical continuity between noble metals in both the in-plane and inter-plane directions .
[0012]
In the invention of claim 1, the SnO ₂ particles are in contact with each other through a grain boundary, and can be electrically connected within the film surface and in the direction between the film surfaces (invention of claim 2). In the invention of claim 1 or 2, the noble metal catalyst may be composed mainly of Pd or Pt, or a mixture of Pd and Pt (invention of claim 3 ).
[0013]
In the invention of claim 4 , the outer periphery or both ends of the thin film-like support film are supported by the Si substrate, and the thin film heater is formed on the diaphragm-like support substrate in which the outer periphery or both ends are thick and the central portion is thin. The thin film heater is covered with an electrical insulating film containing SiO ₂ , an electrode for a gas sensing film is formed thereon, and a gas sensing film comprising a thin film mainly composed of a noble metal catalyst and SnO ₂ is formed. In manufacturing a thin film gas sensor having a catalyst filter layer (selective combustion layer) made of catalyst-supporting porous alumina formed so as to completely cover the gas sensing film on the outermost surface ,
After forming the gas sensing film made of SnO ₂ , the noble metal catalyst layer is formed in an island shape.
[0018]
Pd or Pt on the surface of the thin film of SnO _2, or dispersing a noble metal catalyst such as a mixed catalyst of Pd and Pt, by carrying water or sensing adsorbed on SnO ₂ thin film surface to off time of the sensor by the cleaning before gas detection Since the inhibitor gas can be completely desorbed, a battery-driven (pulse-driven) thin film gas sensor with excellent temporal stability can be obtained. In particular, in a CO sensor operating at a low temperature, a remarkable temporal stability can be obtained by a synergistic effect of improving the stability of the CO detection reaction by dispersing and supporting the noble metal catalyst.
That is, as shown in the schematic diagram of FIG. 2, the atomic oxygen dissociated and adsorbed on the noble metal catalyst is abundantly supplied onto the surface of the SnO ₂ thin film by spillover (overflow or spillage), so that SnO ₂ at the time of cleaning is supplied. The oxidative decomposition of the detection inhibiting gas adsorbed on the surface of the thin film is promoted, or the elimination of OH groups is promoted, and the SnO ₂ thin film surface is sufficiently refreshed, thereby improving the temporal stability. Furthermore, in the CO sensor operating at low temperature, atomic oxygen dissociated and adsorbed on the noble metal catalyst is abundantly supplied onto the SnO ₂ thin film surface by spillover, so that the reaction rate of CO oxidation or CO adsorption is improved and stable over time. Improves.
[0019]
In the case of a laminated structure, each noble metal catalyst thin film layer is formed in an island shape in which noble metal particles are isolated from each other, and the noble metal catalyst is electrically connected to each other in both directions within and between the surfaces of the noble metal catalyst thin film. In each SnO ₂ thin film, SnO ₂ particles are in contact with each other through grain boundaries, and are formed so as to have electrical continuity in both directions within the film surface and between the film surfaces. This is very important.
Specifically, the thickness of the noble metal catalyst thin film is 0.5 nm to 10 nm, preferably 1 nm to 6 nm, and the SnO ₂ thin film is 1 nm to 100 nm, preferably 5 nm to 50 nm, A laminated structure in which at least four layers (substrate / SnO ₂ / catalyst / SnO ₂ / catalyst) are repeated is required. In the laminated structure, when the thickness of the noble metal catalyst thin film is 10 nm or more, the noble metal catalyst may not be island-like and may be conductive, and when it is 0.5 nm or less, the function as a catalyst is weak.
The greater the number of layers and the greater the thickness of the noble metal catalyst thin film, the more noble metal catalyst is supported, which is effective in stabilizing the sensor. The supported amount (ratio of precious metal catalyst weight to SnO ₂ weight (wt%: weight percent) ) can be arbitrarily determined by the film thickness of the precious metal catalyst thin film and the number of SnO ₂ thin films stacked. The amount of noble metal supported is 0.1 wt% to 75 wt%, preferably 0.5 wt% to 20 wt%.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a sensor sectional view showing an embodiment of the present invention.
On the surface of the Si substrate 1 having a thermal oxide film of 0.3 μm formed on both surfaces, an SiN and SiO ₂ film to be a support layer 2 having a diaphragm structure are sequentially formed to a thickness of 0.15 μm and 1 μm, respectively, by plasma CVD. On top of this, 0.05 μm of Ta was formed as the bonding layer (1), and then 0.5 μm of PtW (Pt + 4 wt% W) film was continuously formed as the heater layer 3, and Ta as the bonding layer (2). To 0.05 μm. Film formation is performed by a normal sputtering method using a magnetron sputtering apparatus. The conditions are a film formation temperature of 100 ° C., a film formation power of 100 W, and a film formation pressure of 1 Pa.
[0021]
Thereafter, a heater pattern is formed by fine processing. As an etchant for wet etching, a mixed solution of sodium hydroxide and hydrogen peroxide was used for Ta, and aqua regia was heated to 90 ° C. for Pt. Further, after an insulating film 4 such as SiO _{2 is} formed by a sputtering method by 1.0 μm, the electrode pad portion of the heater (not shown) is etched by fine processing by HF to open a window, thereby ensuring conduction and wire bonding properties. In order to improve, Ta in the bonding layer (2) is removed with a mixed solution of sodium hydroxide and hydrogen peroxide.
[0022]
Next, in order to improve adhesion with the underlying SiO ₂ , the intermediate layer Ta is formed with a thickness of 0.05 μm, and then the Pt sensing film electrode 5 is continuously formed with a thickness of 0.2 μm. Film formation is performed by a normal sputtering method using a magnetron sputtering apparatus. The conditions are a film formation temperature of 100 ° C., a film formation power of 100 W, and a film formation pressure of 1 Pa. Thereafter, the resistance measurement sensing film electrodes 5 of the pair of sensing films SnO ₂ are patterned by wet etching in the same manner as the heater layer 3.
Thereafter, only the portion where the SnO ₂ sensing film is formed by microfabrication is opened, and the rest is covered with a resist to form a pattern. Next, the wafer subjected to the above patterning is set in a sputtering chamber, and SnO ₂ 6 which is a gas sensing film is formed by sputtering. The size of the SnO ₂ sensing film 6 is 100 μm ² , the film forming conditions are 300 W, 1 Pa, 100 ° C. in Ar + O 2, and the film thickness is about 2 μm.
[0023]
Next, a method for forming a catalyst layer on the SnO ₂ sensing film according to the present invention will be described.
After the SnO ₂ sensing film is formed, Pd is further formed by sputtering. The sputter deposition of Pd may be performed continuously without breaking the vacuum, or may be performed once with a different sputtering apparatus after breaking the vacuum. The film formation conditions for Pd were 30 W, 1 Pa, 100 ° C. in Ar + O2, and the film thickness was (1) 0.5 nm, (2) 1 nm, (3) 5 nm, (4) 10 nm, (5) 100 nm, (6) 500 nm. Prototype. For comparison, no. An element without Pd was also prepared as 13. The numbers with circles indicate the element numbers, and so on. No. 1-No. Up to 9, there are cases where numbers “1” to “9 ▼” with circles are used. 10 or more is expressed exclusively by “No.”.
[0024]
FIG. 3 is a schematic cross-sectional view of a Pd layer / SnO ₂ sensing film formed by sputtering. The figure (a) shows the whole, and the figure (b) expands partially and shows.
It can be seen from FIG. 3B that most of the Pd layer formed by sputtering adheres to the surface layer portion of the SnO ₂ sensing film, and the columnar grown SnO ₂ adheres little to the pores.
After Pd deposition (element without Pd is, SnO ₂ after its formation) the resist is removed by stripping solution was prototyped Pd catalyzed SnO ₂ sensing film, and Pd catalyst without SnO ₂ sensing film. The Pd catalyst-free SnO ₂ sensing film element was used as an element having a conventional structure for comparison, and was used for an experiment of an element supporting a noble metal catalyst by an impregnation method described below.
[0025]
Hereinafter, the impregnation method will be described.
A Pd catalyst-free SnO ₂ sensing film wafer is impregnated with an aqueous solution of a noble metal compound, and the noble metal catalyst is dispersed on the surface of the SnO ₂ sensing film by thermal decomposition. The pore volume brought about by the pores of the SnO ₂ sensing film formed by sputtering under the film forming conditions of the present invention is about 20%.
The wafer is immersed for 10 minutes in a dinitrodiamine Pd nitric acid aqueous solution containing 1 wt% Pd, and the pores of the SnO ₂ sensing film are impregnated with the dinitrodiamine Pd nitric acid aqueous solution. The dinitrodiamine Pd nitric acid aqueous solution adhering excessively to the removed wafer surface is blown off by N ₂ blow or the like. At this time, the dinitrodiamine Pd nitric acid aqueous solution impregnated in the pores of the SnO ₂ sensing film is held by the capillary force and is not scattered by the N ₂ blow.
[0026]
Thereafter, the wafer is dried in an air stream at 150 ° C. for 60 minutes in an electric furnace, and further decomposed in an air stream at 500 ° C. for 1 hour to carry Pd. About 0.3 wt% of Pd is supported on the SnO ₂ sensing film in one operation. By repeating this operation, the number of operations × 0.3 wt% of Pd is carried on the SnO ₂ sensing film. It is possible to increase the amount of Pd supported in one operation by increasing the Pd concentration in the dinitrodiamine Pd nitric acid aqueous solution, and conversely, by reducing the Pd concentration, the Pd concentration in one operation can be increased. It is also possible to reduce the loading amount. In the case of increasing the Pd concentration, it is preferably 7 wt% or less from the solubility in the dinitrodiamine Pd nitric acid aqueous solution.
[0027]
By the above operation, (7) 0.05 wt%, (8) 0.1 wt%, (9) 1 wt%, 5 wt% (No. 10), 10 wt% (No. 11), 20 wt% (No. 12) An SnO ₂ sensing film carrying Pd could be obtained.
FIG. 4 shows a schematic cross-sectional view of a Pd / SnO ₂ sensing film obtained by the above impregnation method. The figure (a) shows the whole, and the figure (b) expands partially and shows.
From FIG. 4B, it can be seen that the Pd layer supported by the impregnation method is uniform not only in the surface layer portion of the SnO ₂ sensing film but also in the pores of the columnar grown SnO ₂ . Since Pd is decomposed in an air stream, most of it is PdO. No. Reference numeral 13 denotes an element having a conventional structure in which Pd is not supported on a SnO ₂ sensing film, which was prototyped for comparison.
[0028]
Next, a powder in which Pt and Pd catalyst are supported on alumina particles is used as a paste together with a binder, and is applied to the surface of SnO ₂ by screen printing and sintered to form a selective combustion layer (catalyst filter) 7 having a thickness of about 30 μm. . This selective combustion layer 7 improves the sensitivity, gas type selectivity, and reliability of the gas sensor. Finally, Si is completely removed from the back surface of the substrate by dry etching to a size of 400 μm to obtain a diaphragm structure.
Here, when patterning the heater layer (Ta / PtW / Ta) and the sensing film electrode (Ta / Pt), a kind of lift method (for example, using two kinds of metal layers formed in a mushroom shape as a mask) JP, 2000-065773, A) may be used.
[0029]
Subsequently, the pulse-driven thin film gas sensor No. 1 manufactured by the above method was used. 1-No. Table 1 shows the change over time of the resistance value with respect to 300 ppm CO / air when 13 is operated in an environment of 40 ° C. and a relative humidity of 80%. A sensor having a Pd layer formed by sputtering is No. 1-No. No. 6 ((1) to (6)), and the sensor having the Pd layer formed by the impregnation method is No. 6. 7-No. 12, the conventional structure sensor for comparison is No. 12. 13. The pulse driving conditions / measurement conditions are as follows.
Detection cycle: 150 seconds cleaning temperature × time = 450 ° C. × 200 msec
CO detection temperature × detection timing = 100 ° C. × 500 ° C. after heating 500 ms
[Table 1]

[0031]
From Table 1 above, the element No. 2-No. 5 ((2) to (5)), No. 5 8-No. 11 shows that the resistance value against 300 ppm CO / air is stable with no change even under severe conditions of high temperature and high humidity. Compared to that, No. 1 and No. The resistance value of the element No. 7 increases with the test time, and the resistance value changes by an order of magnitude or more after 50 days, as in the case of the element (No. 13) of the conventional structure prototyped for comparison. No. with the most Pd adhesion and loading. 6, no. 12 is an initial resistance value one digit lower. This is presumably because Pd particles that were not originally in the form of islands were in contact with each other due to the amount of Pd adhering and loading, and Pd was electrically connected. No. 6, no. When the resistance value in air without CO of 12 elements was measured, it was almost the same as the resistance value against 300 ppm CO / air, and it was also found that there was no CO sensitivity. Even when 2000 ppm CH4 / air is not as noticeable as the change over time for 300 ppm CO / air, the same result is obtained. By attaching and supporting a Pd catalyst on SnO ₂ , there is no change over time in the resistance value, and stable pulse driving is possible. A thin film gas sensor was obtained.
[0032]
Such a change in resistance value with time under high temperature and high humidity is a phenomenon peculiar to a pulse-driven thin film gas sensor. When the 13 elements are tested in a driving mode in which the elements are constantly heated to 100 ° C. or more, the resistance value does not change with time.
The change over time in the resistance value of the pulse-driven thin film gas sensor under high temperature and high humidity is estimated to be due to adsorption of moisture or the like during the off time, but it is not clearly understood. In this invention, remarkable improvement is recognized by attaching and supporting Pd on the SnO ₂ surface. The cause is abundant supply of spillover oxygen to the SnO ₂ surface on Pd as shown in FIG. There are various conceivable methods such as promotion of the cleaning reaction associated therewith, and promotion of the CO and CH4 detection reaction of the Pd catalyst.
In the above description, the example of the Pd catalyst has been described. Instead, a noble metal catalyst such as a Pt, Pt—Pd alloy catalyst may be used. Furthermore, although the dinitrodiamine compound is used as the compound for supporting the catalyst by the impregnation method, a compound such as chloride may be used.
[0033]
Next, an example in which a SnO ₂ thin film and a noble metal catalyst thin film are alternately formed to form a laminated sensing film (Pt / SnO ₂ ) n will be described.
The size of the sensing film is 100 μm ² . The film formation conditions for the SnO ₂ layer are film formation power 50 W, film formation pressure 1 Pa, film formation atmosphere Ar + O 2, film formation temperature 100 ° C., and the film thickness of each layer is controlled by the film formation time. Under the above conditions, the deposition rates of SnO ₂ and Pt are 5 nm / 2.5 nm, respectively. The total film thickness of the laminated film (Pt / SnO ₂ ) n is set to ˜1 μm. The sputtering apparatus includes both SnO ₂ and Pt targets, and SnO ₂ and Pt can be alternately formed by switching the targets.
[0034]
First, a SnO ₂ layer is formed to a thickness of 100 nm. Thereafter, Pt is deposited to 3 nm, and then SnO ₂ is deposited to 18 nm. Further, the film formation of ₂ nm of Pt and 18 nm of SnO ₂ is repeated 45 times, and the outermost surface is finished with the Pt film formation. As a result, a sensing film (Pt / SnO ₂ ) n having a total film thickness of 1 μm is obtained. The amount of Pt supported is about 30 wt%.
FIG. 5 shows a schematic cross-sectional view of the laminated film (Pt / SnO ₂ ) ₄₅ . Since SnO ₂ is deposited on Pt in the same manner as on SnO ₂ , a part of Pt is covered with SnO ₂ . Therefore, the concentration of Pt in contact with the gas phase (on the surface) is estimated to be <10%. As shown in FIG. 5, SnO ₂ particles and Pt particles are formed in a band shape, and Pt becomes an island shape. SnO ₂ particles are three-dimensionally connected and are electrically connected to each other. When a Pt film is formed by sputtering, most of it is deposited on the outermost surface of the SnO ₂ particles, and a part is also deposited in the pores formed by the SnO ₂ particles. Since the pore diameter formed by the SnO ₂ particles is as narrow as 10 to 20 nm, the Pt particles do not enter the deep pores. The element on which the sensing film is thus formed is designated as “A”.
[0035]
For comparison, the sensing film conventional element in which the SnO ₂ of 1μm thickness (element C), after forming a 1μm thick SnO ₂ sensing film, further continuous element formed by 3nm sputtering Pt with (element B ) And a post process was advanced together with the element A of the present invention. The element B has a sensing film structure in which an island-like Pt having a thickness of 3 nm is deposited on the outermost surface of SnO _{2 having} a thickness of 1 μm.
After forming the above-described sensing film structure, the resist is removed with a stripping solution, and three types of elements A and B having a laminated film (Pt / SnO ₂ ) ₄₅ sensing film, a SnO ₂ sensing film, and a 3 nm Pt / SnO ₂ sensing film. , C was prototyped.
[0036]
Next, a powder in which Pt and Pd catalyst are supported on alumina particles is used as a paste together with a binder, and is applied to the surface of the sensing film by screen printing and sintered to form a selective combustion layer (catalyst filter) having a thickness of about 30 μm. The selective combustion layer improves the sensitivity, gas type selectivity, and reliability of the gas sensor. Finally, Si is completely removed in a size of 400 μm by dry etching from the back surface of the substrate to obtain a diaphragm structure.
Here, when patterning the heater layer (Ta / PtW / Ta) and the sensing film electrode (Ta / Pt), it was formed in the shape of a mushroom as described in Japanese Patent Laid-Open No. 2000-065773. A kind of lift method using two kinds of metal layers as a mask may be used.
Next, Table 2 shows changes over time in the resistance value with respect to 300 ppm CO / air when the pulse-driven thin film gas sensors A, B, and C manufactured by the above method are operated in an environment of 40 ° C. and a relative humidity of 80%. .
[0037]
[Table 2]

The pulse driving conditions / measurement conditions in Table 2 are as follows.
Detection cycle: 150 seconds cleaning temperature × time = 450 ° C. × 200 msec
CO detection temperature × detection timing = 100 ° C. × 100 ° C. After heating 500 msec.
It can be seen from Table 2 that the element A is stable with no change over time in the resistance value against 300 ppmCO / air even after 150 days under severe test conditions of high temperature and high humidity. In comparison with this, the element resistance of the element C increases by more than two digits. Further, although the element B is stable for about 50 days, the resistance value tends to slightly increase after the elapsed days.
Even at 2000 ppm CH4 / air, the same result was obtained although not as noticeable as the change over time with respect to 300 ppm CO / air, and not only the outermost surface of the SnO ₂ thin film but also Pt in high dispersion inside the pores, It is possible to obtain a stable pulse-driven thin film gas sensor with no change in resistance value over time. Such a change in resistance value with time under high temperature and high humidity is a phenomenon peculiar to a pulse-driven thin film gas sensor, and even if the element B or C is tested in a driving mode in which the element B or C is constantly heated to 100 ° C. or higher, The value does not change over time.
[0039]
The change over time in the resistance value of the pulse-driven thin film gas sensor under high temperature and high humidity is estimated to be due to adsorption of moisture or the like during the off time, but it is not clearly understood. In the present invention, not only the outermost surface of the SnO ₂ thin film but also Pt is highly dispersed in the inside pores, and a remarkable improvement is recognized. The cause is on the Pt explained in FIG. There are various possibilities such as uniform and abundant supply of spillover oxygen to the SnO ₂ surface, acceleration of the cleaning reaction associated therewith, and promotion of the CO and CH4 detection reaction of the Pd catalyst.
The example of the Pt catalyst has been described above, but a noble metal catalyst such as an alloy catalyst of Pd, Pt—Pd may be used instead of Pt.
[0040]
In the case of the laminated structure, the SnO ₂ layer is first formed to a thickness of 100 nm, but the same effect can be obtained by repeating Pt of 2 nm and SnO ₂ of 18 nm from the beginning. Further, the film thickness ratio of Pt / SnO ₂ does not have to be constant, and furthermore, the first film formation is SnO ₂ and the last is Pt, but the opposite is also possible, and the first and last film formation may be the same.
The film forming method may be vapor deposition or CVD (chemical vapor deposition) instead of sputtering.
[0041]
【The invention's effect】
According to the first to fourth aspects of the present invention, a noble metal catalyst such as Pd or Pt, or a Pd + Pt mixed catalyst is supported not only on the outermost surface of the SnO ₂ thin film but also inside the pores in a highly dispersed manner, before gas detection. Since the moisture adsorbed on the SnO ₂ thin film surface or the detection-inhibiting gas can be completely desorbed during the sensor off time, a battery-driven (pulse-driven) thin film gas sensor with excellent temporal stability can be obtained. In particular, in a CO sensor operating at a low temperature, remarkable stability over time is obtained by a synergistic effect that the stability of the CO detection reaction is improved by dispersing and supporting the noble metal catalyst.
[Brief description of the drawings]
FIG. 1 is a cross-sectional configuration diagram of a thin film gas sensor showing an embodiment of the present invention. FIG. 2 is an explanatory diagram for explaining oxygen spillover. FIG. 3 is a schematic diagram of a sensing film manufactured by sputtering. Schematic diagram of the sensing film manufactured by the sputtering method. Fig. 5 Schematic diagram of the laminated sensing film manufactured by the sputtering method.
DESCRIPTION OF SYMBOLS 1 ... Si substrate (diaphragm), 2 ... Support layer and thermal insulation layer, 3 ... Heater layer, 4 ... Insulating layer, 5 ... Sensing layer electrode, 6 ... Sensing layer, 7 ... Selective combustion layer (catalyst filter).

Claims

The outer periphery or both ends of the thin film support film are supported by a Si substrate, and a thin film heater is formed on a diaphragm-like support substrate having a thick outer periphery or both ends and a thin central portion. After covering with an electrical insulating film containing SiO ₂ , an electrode for a gas sensing film is formed thereon, and further a gas sensing film comprising a thin film mainly composed of a noble metal catalyst and SnO ₂ is formed, and then a gas is formed on the outermost surface. In a thin film gas sensor having a catalyst filter layer (selective combustion layer) made of catalyst-supporting porous alumina formed so as to completely cover the sensing membrane,
The thin film mainly composed of the noble metal catalyst and SnO ₂ has a laminated structure in which the SnO ₂ thin film and the noble metal catalyst thin film are alternately formed, and the noble metals are formed in islands isolated from each other. A thin film gas sensor characterized in that there is no electrical continuity between noble metals in either the in-plane or between-plane directions .

_2. The thin film gas sensor according to claim 1, wherein the SnO ₂ particles are in contact with each other via a grain boundary and are electrically connected in a film plane and in a direction between the film planes .

3. The thin film gas sensor according to claim 1, wherein the noble metal catalyst is mainly composed of Pd or Pt, or a mixture of Pd and Pt .

The outer periphery or both ends of the thin film support film are supported by a Si substrate, and a thin film heater is formed on a diaphragm-like support substrate having a thick outer periphery or both ends and a thin central portion. Cover it with an electrical insulating film containing SiO ₂ , form an electrode for the gas sensing film on it, and then form a gas sensing film consisting of a noble metal catalyst and SnO ₂ as the main components, and gas sensing on the outermost surface In manufacturing a thin film gas sensor having a catalyst filter layer (selective combustion layer) made of catalyst-supporting porous alumina formed so as to completely cover the membrane,
A method of manufacturing a thin film gas sensor, comprising forming a noble metal catalyst layer in an island shape after forming the gas sensing film made of SnO ₂ .