JP2007057254A - Thin-film gas sensor and its manufacturing method - Google Patents
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この発明は、電池駆動に適した低消費電力型薄膜ガスセンサとその製造方法に関する。 The present invention relates to a low power consumption thin film gas sensor suitable for battery driving and a method for manufacturing the same.
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、ある特定ガス、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したものと燃焼機器の不完全燃焼ガス検知を目的としたもの、または、両方の機能を合わ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ない。そういった事から普及率の向上をはかるべく、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事が望まれている。
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℃〜450℃の高温に加熱し検知する必要がある。これから、SnO2などの粉体を焼結した従来の方法では、スクリーン印刷等の方法を用い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た。その対策として、微細加工プロセスによりダイヤフラム構造などの低熱容量構造とした薄膜ガスセンサの実現が待たれている。
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. For this reason, in order to improve the diffusion 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 drive. However, in a catalytic combustion type or semiconductor type gas sensor, it is necessary to detect it by heating it to a high temperature of 100 ° C. to 450 ° C. From this, the conventional method of sintering powder such as SnO 2 has a limit in 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. As a countermeasure, it is expected to realize a thin film gas sensor having a low heat capacity structure such as a diaphragm structure by a microfabrication process.
ダイヤフラム構造などの超低熱容量構造とした低消費電力薄膜ガスセンサを適用し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せる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秒の一定周期に一回の検知が必要であり、この周期に合わせ検知部を室温から100℃〜450℃の高温に加熱する。前記の電池の交換無しで5年以上の寿命要請に応えるため、この加熱時間は数100ms以下が目標となる。
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の違いから、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〜500msec、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。
Even in a gas leak alarm using a low power consumption thin film gas sensor with an ultra-low heat capacity structure such as a diaphragm structure, pulse driving of the thin film gas sensor is indispensable in order to have a life of 5 years or more without replacing the battery. .
Normally, the gas leak alarm needs to be detected once in a fixed cycle of 30 to 150 seconds, and the detector is heated from room temperature to 100 ° C. to 450 ° C. in accordance with this cycle. In order to meet the life requirement of 5 years or longer without replacing the battery, the heating time is set to several hundreds ms or less.
Even in a pulse-driven thin film gas sensor, in order to reduce power consumption, 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).
Detection temperature in the thin film gas sensor from differences in detection sensitivity to gas species, is to 100 ° C. in a CO sensor, CH 4 to 450 ° C. in the sensor, the detection time ~500msec from the response of the sensor, the detection cycle 30 is CH 4 sensor Second, 150 seconds for CO sensor.
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約450℃に加熱(加熱時間〜100msec)し、その直後に、それぞれのガスの検出温度でガス検知を行っている。
図４は従来の薄膜ガスセンサの断面図である。外周または両端部がSi基板Bにより支持された支持膜L上に、薄膜ヒーターHが形成され、薄膜ヒーターHは絶縁膜L4により被覆され、その上にガス感知膜S用の感知膜電極Eが形成され、そのガス感知膜Sを完全に被覆するように触媒を担持する多孔質アルミナから成る選択燃焼層Cが形成されている。通常最終工程でSi基板の裏面からのエッチングによりガス感知膜等の領域より大きい径のSiを完全に除去しダイヤフラム構造としている。
In addition, it is important to remove the moisture and other adsorbate adhering to the sensor surface during the off time and clean the SnO 2 surface to improve the stability over time of the battery-driven (pulse-driven) thin film gas sensor. Before, the sensor temperature is once heated to about 450 ° C. (heating time to 100 msec), and immediately after that, gas detection is performed at the detection temperature of each gas.
FIG. 4 is a cross-sectional view of a conventional thin film gas sensor. A thin film heater H is formed on the support film L whose outer periphery or both ends are supported by the Si substrate B. The thin film heater H is covered with an insulating film L4, and a sensing film electrode E for the gas sensing film S is formed thereon. A selective combustion layer C made of porous alumina that supports the catalyst so as to completely cover the gas sensing film S is formed. Usually, Si having a diameter larger than the area of the gas sensing film or the like is completely removed by etching from the back surface of the Si substrate in the final process to form a diaphragm structure.
上記の感知膜電極の材料としては一般的にはPtなど貴金属材料を、SiO2等の絶縁性の支持層の上に、SiO2等の酸化膜との密着性に優れしかもPtとも密着性の良いTa、Ti、Cr等の接合層を介してPt層を成膜、パターニングしてPt感知膜電極として用いる。従来の薄膜ガスセンサでは、その後、その上にガス感知層であるSnO2を形成し、ガス感知層の裏面のSiを除去して、ダイヤフラム構造として薄膜ガスセンサを完成する。
薄膜ヒータをパルス駆動させ昇降温を繰り返すと、支持層、薄膜ヒータ、感知膜電極、感知膜を含む積層構造からなるダイヤフラムは熱膨張/収縮により数μmであるが上下に振動する。この振動は微小ではあるが10秒に１回の検知周期でセンサを6年間駆動させると約2000万回に達する。
この微少な振動の継続によって、Pt感知膜電極と感知膜SnO2の接合部に剥離を生じセンサ抵抗値が上昇するなどの変動を生じ、極端な場合には導通不良を発生することがある。センサ抵抗値によりガス検知を行うSnO2ガスセンサにおいては当然、抵抗値の変動/導通不良は大きな問題になる。
General noble metal material such as Pt in as the material of the sensing film electrode, on the insulating support layer such as SiO 2, excellent adhesion to the oxide film such as SiO 2 addition of adhesion with Pt A Pt layer is formed and patterned through a good bonding layer of Ta, Ti, Cr, etc., and used as a Pt sensing film electrode. In the conventional thin film gas sensor, SnO 2 which is a gas sensing layer is then formed thereon, and Si on the back surface of the gas sensing layer is removed to complete the thin film gas sensor as a diaphragm structure.
When the thin film heater is pulse-driven and the temperature rise and fall is repeated, the diaphragm composed of the laminated structure including the support layer, the thin film heater, the sensing film electrode, and the sensing film vibrates up and down although it is several μm due to thermal expansion / contraction. This vibration is small, but reaches about 20 million times when the sensor is driven for 6 years with a detection cycle of once every 10 seconds.
Due to the continuation of this minute vibration, fluctuations such as peeling at the junction between the Pt sensing film electrode and the sensing film SnO 2 and an increase in the sensor resistance value occur, and in an extreme case, conduction failure may occur. Naturally, in the SnO 2 gas sensor that performs gas detection based on the sensor resistance value, variation in resistance value / conductivity failure is a serious problem.
このような剥離によって、薄膜ガスセンサの特性であるガス検知精度が低下したり、極端な場合には、ガス検知が不能になるような重大な影響を与える場合がある。
上記の問題であるPt感知膜電極とSnO2ガス感知膜の接合部の剥離は、PtとSnO2の密着性の弱さに起因している。
Pt感知膜電極とその上のSnO2ガス感知膜の間に、金属であるPtと密着性が良く、酸化物であるSnO2ガス感知膜との密着性も良い中間薄膜層を設けるたり、SnO2ガス感知膜形成前に感知膜電極表面を前処理することで上記技術課題に対しての対策が公開されている。
その１としては、Pt感知膜電極作製後、SnO2ガス感知膜のスパッタ成膜前に真空処理 or 逆スパッタ処理などによりPt感知膜電極表面のクリーニング処理により密着性の向上を狙った製造方法が公開されている（特許文献１参照。）。
Such peeling may deteriorate the gas detection accuracy, which is a characteristic of the thin film gas sensor, or, in extreme cases, may have a serious effect that disables gas detection.
The peeling of the joint between the Pt sensing film electrode and the SnO 2 gas sensing film, which is the above problem, is caused by the poor adhesion between Pt and SnO 2 .
Between the Pt sensing film electrode and the SnO 2 gas sensing film on the Pt sensing film electrode, an intermediate thin film layer having good adhesion to the metal Pt and good adhesion to the oxide SnO 2 gas sensing film is provided, or SnO 2 A countermeasure for the above technical problem is disclosed by pretreating the surface of the sensing film electrode before forming the gas sensing film.
The first is a manufacturing method aimed at improving adhesion by cleaning the surface of the Pt sensing film electrode by vacuum treatment or reverse sputtering treatment after the Pt sensing film electrode is prepared and before the sputtering of the SnO 2 gas sensing film. (See Patent Document 1).
同様に、Pt感知膜電極表面のクリーニング処理としてSnO2ガス感知膜のスパッタ成膜前に紫外線照射を行う製造方法も公開されている（特許文献２参照。）。
しかしながら上記のようなPt感知膜電極表面のクリーニング処理のみでは、再現性良く密着性の良いPt感知膜電極/SnO2ガス感知膜構造を得ることは困難である。その対策として、Pt感知膜電極とSnO2ガス感知膜の間に中間薄膜層として、Ptとは金属同士で接合性が良く、表面に酸化膜を作り易い性質を有しSnO2ガス感知膜との良好な接合が行えるTa、Cr、Ti等の金属を挟み込む特許が公開されている（特許文献３参照。）。
しかしながら上記方法では良好な接合が行えるがTa、Cr、Ti等がセンサ駆動を行っていると徐々に酸化が進行しPt感知膜電極とSnO2ガス感知膜の間の中間薄膜層が絶縁物化するため、センサ抵抗が徐々に上昇すると言う問題が発生する。上記問題を解決するための方法としてPt感知膜電極とSnO2ガス感知膜の間の中間薄膜層にPt/SnO2混合物を取り入れることで、アンカー効果により両方の膜と接合性に優れ、しかもセンサ駆動を行っても酸化が進行することなくセンサ抵抗の変動がないセンサ構造が公開されている（特許文献４参照。）。
Similarly, a manufacturing method in which ultraviolet irradiation is performed before sputtering of the SnO 2 gas sensing film is disclosed as a cleaning process for the surface of the Pt sensing film electrode (see Patent Document 2).
However, it is difficult to obtain a Pt sensing film electrode / SnO 2 gas sensing film structure with good reproducibility and good adhesion only by cleaning the surface of the Pt sensing film electrode as described above. As a countermeasure, an intermediate thin layer between the Pt sensitive membrane electrode and the SnO 2 gas sensitive film, good bondability in the metal between the Pt, and SnO 2 gas sensing film has a property of easily creating an oxide film on the surface Patents for sandwiching metals such as Ta, Cr, Ti, etc. that can be satisfactorily bonded are disclosed (see Patent Document 3).
However, with the above method, good bonding can be achieved, but when Ta, Cr, Ti, etc. are driving the sensor, oxidation gradually proceeds and the intermediate thin film layer between the Pt sensing film electrode and the SnO 2 gas sensing film becomes an insulator. Therefore, a problem that the sensor resistance gradually increases occurs. As a method to solve the above problem, by incorporating a Pt / SnO 2 mixture into the intermediate thin film layer between the Pt sensing membrane electrode and the SnO 2 gas sensing membrane, both the membrane and the bonding property are excellent due to the anchor effect. A sensor structure has been disclosed in which oxidation does not proceed even when driven and the sensor resistance does not vary (see Patent Document 4).
また、Pt感知膜電極をSnO2ガス感知膜中にはさみ込むことで同じ効果を狙った構造特許も公開されている（特許文献５参照。）。さらに中間薄膜層にAu-Sn、Ni-Snの金属間化合物を用いた特許も公開されている（特許文献６参照。）。上記の特許では、一定の効果が認められるが、第３金属のSnO2ガス感知膜への拡散に伴う信頼性低下、製造工程が複雑化するためコストアップするなどの欠点を有している。
本発明の目的は、長期のパルス駆動においてもPt感知膜電極とSnO2ガス感知膜の間に問題の発生しない薄膜ガスセンサおよびその製造方法を提供することにある。
An object of the present invention is to provide a thin film gas sensor which does not cause a problem between a Pt sensing film electrode and a SnO 2 gas sensing film even in a long-term pulse drive, and a method for manufacturing the same.
本発明の目的を達成するため、Si基板の貫通孔の一端を覆って張られ、その周縁がSi基板に固定された、酸化ケイ素または／および窒化ケイ素などからなる支持膜（ダイヤフラム構造という）上に、少なくとも、薄膜ヒータが形成され、これを被覆する絶縁膜上に一対のPt感知膜電極を有するSnO2ガス感知膜が形成されてなる薄膜ガスセンサにおいて、前記Pt感知膜電極と前記SnO2ガス感知膜の間に、Sn-Ptからなる合金中間層を有することとする。
前記Sn-Ptからなる中間薄膜層を有する薄膜ガスセンサの製造方法において、前記Pt感知膜電極の形成の後、SnOからなる中間薄膜層を成膜し、さらに、SnO2ガス感知膜を形成し、さらに熱処理を行うこととする。
あるいは、前記Sn-Ptからなる中間薄膜層を有する薄膜ガスセンサの製造方法において、前記Pt感知膜電極の形成の後、SnOからなる中間薄膜層を成膜し、熱処理を行った後、SnO2ガス感知膜を形成しても良い。
In order to achieve the object of the present invention, on a support film (referred to as a diaphragm structure) made of silicon oxide or / and silicon nitride, which is stretched so as to cover one end of a through hole of a Si substrate and whose periphery is fixed to the Si substrate. In addition, in the thin film gas sensor in which at least a thin film heater is formed and an SnO 2 gas sensing film having a pair of Pt sensing film electrodes is formed on an insulating film covering the thin film heater, the Pt sensing film electrode and the SnO 2 gas An alloy intermediate layer made of Sn—Pt is provided between the sensing films.
In the method of manufacturing a thin film gas sensor having an intermediate thin film layer made of Sn-Pt, after the formation of the Pt sensing film electrode, an intermediate thin film layer made of SnO is formed, and further, a SnO 2 gas sensing film is formed, Further heat treatment is performed.
Alternatively, in the method of manufacturing a thin film gas sensor having an intermediate thin film layer made of Sn—Pt, after the formation of the Pt sensing film electrode, an intermediate thin film layer made of SnO is formed, heat-treated, and then SnO 2 gas A sensing film may be formed.
前記中間薄膜層の厚みは1 nm以上、20 nm以下であると良い。
前記熱処理は、酸素を含む窒素ガス雰囲気中で400℃以上、800℃以下で行われると良い。
前記熱処理の酸素分圧(Po2)は0.01気圧以上、0.2気圧以下あると良い。
前記Sn-Ptからなる中間薄膜層を有する薄膜ガスセンサの製造方法において、前記Pt感知膜電極の形成の後、Snからなる中間薄膜層を成膜し、さらに、SnO2ガス感知膜を形成し、さらに熱処理を行うこととする。
あるいは、前記Sn-Ptからなる中間薄膜層を有する薄膜ガスセンサの製造方法において、前記Pt感知膜電極の形成の後、Snからなる中間薄膜層を成膜し、熱処理を行った後、SnO2ガス感知膜を形成しても良い。
The thickness of the intermediate thin film layer is preferably 1 nm or more and 20 nm or less.
The heat treatment is preferably performed at 400 ° C. or higher and 800 ° C. or lower in a nitrogen gas atmosphere containing oxygen.
The oxygen partial pressure (Po 2 ) of the heat treatment is preferably 0.01 atm or more and 0.2 atm or less.
In the method of manufacturing a thin film gas sensor having an intermediate thin film layer made of Sn-Pt, after the formation of the Pt sensing film electrode, an intermediate thin film layer made of Sn is formed, and further, a SnO 2 gas sensing film is formed, Further heat treatment is performed.
Alternatively, in the method of manufacturing a thin film gas sensor having an intermediate thin film layer made of Sn-Pt, after the formation of the Pt sensing film electrode, the intermediate thin film layer made of Sn is formed, and after heat treatment, the SnO 2 gas A sensing film may be formed.
前記中間薄膜層の厚みは1 nm以上、10nm以下であると良い。
前記熱処理は酸素を含む窒素雰囲気中で、400℃以上、800℃以下で行われると良い。。
前記熱処理の酸素分圧(Po2)は0.01気圧以上、0.2気圧以下であると良い。
The thickness of the intermediate thin film layer is preferably 1 nm or more and 10 nm or less.
The heat treatment is preferably performed at 400 ° C. or higher and 800 ° C. or lower in a nitrogen atmosphere containing oxygen. .
The oxygen partial pressure (Po 2 ) of the heat treatment is preferably 0.01 atm or more and 0.2 atm or less.
本発明によれば、Pt感知膜電極とSnO2ガス感知膜の界面にPt-Snからなる合金中間層を有するようにしたため、Pt感知膜電極とSnO2ガス感知膜の密着性が向上し、長期間パルス駆動に対してもセンサ抵抗特性は安定する、すなわち、薄膜ガスセンサの信頼性は向上する。
その製造方法として、Pt感知膜電極の形成の後、SnOからなる中間薄膜層を成膜し、さらに、SnO2ガス感知膜を形成し、さらに熱処理を行う、あるいは、前記Pt感知膜電極の形成の後、SnOからなる中間薄膜層を成膜し、熱処理を行った後、SnO2ガス感知膜を形成するようにしたため、
または、Pt感知膜電極の形成の後、Snからなる中間薄膜層を成膜し、さらに、SnO2ガス感知膜を形成し、さらに熱処理を行う、あるいは、Pt感知膜電極の形成の後、Snからなる中間薄膜層を成膜し、熱処理を行った後、SnO2ガス感知膜を形成するようにしたため、このような簡便なプロセスによって、Pt感知膜電極とSnO2感知膜とのあいだにPt-Sn合金中間層を形成することができるので、Pt感知膜電極とSnO2感知膜の接合部剥離の発生を防止する事で、信頼性の高い薄膜ガスセンサを得ることが出来る。
According to the present invention, because you have an alloy intermediate layer made of Pt-Sn at the interface Pt sensitive membrane electrode and the SnO 2 gas sensitive film improves the adhesion of the Pt sensing film electrode and SnO 2 gas sensitive film, The sensor resistance characteristics are stable even for long-term pulse driving, that is, the reliability of the thin film gas sensor is improved.
As a manufacturing method thereof, after the formation of the Pt sensing film electrode, an intermediate thin film layer made of SnO is formed, and further, a SnO 2 gas sensing film is formed, and further heat treatment is performed, or the Pt sensing film electrode is formed. After that, after forming an intermediate thin film layer made of SnO and performing a heat treatment, an SnO 2 gas sensing film was formed,
Alternatively, after the formation of the Pt sensing film electrode, an intermediate thin film layer made of Sn is formed, and further, an SnO 2 gas sensing film is formed and further heat treatment is performed, or after the formation of the Pt sensing film electrode, Sn is formed. After forming an intermediate thin film layer made of and heat-treating, an SnO 2 gas sensing film was formed, so by such a simple process, Pt is formed between the Pt sensing film electrode and the SnO 2 sensing film. Since a -Sn alloy intermediate layer can be formed, a highly reliable thin film gas sensor can be obtained by preventing the occurrence of peeling of the joint between the Pt sensing film electrode and the SnO 2 sensing film.
図１は本発明に係る薄膜ガスセンサの断面図である。ガス感知膜電極Eの形成までは従来の薄膜ガスセンサ（図４）と同じであるので説明は省略する。
図２は本発明に係る薄膜ガスセンサの要部の断面図であり、（ａ）は熱処理前であり、（ｂ）は熱処理後である。
本発明に係る薄膜ガスセンサを製造工程に従って説明する。
感知膜電極Eの形成の後、SnO2ガス感知膜Sと感知膜電極Eとの間の密着性向上のための中間薄膜層Iの形成の準備のため、1〜20nm厚みのSnOからなる極薄の薄膜スパッタ膜層Ilを設ける。さらに連続してSnO2ガス感知膜Sをスパッタ成膜する（図２（ａ））。
その後、酸素分圧（Po2）0.01〜0.2気圧の窒素ガス雰囲気中で、400℃〜800℃を熱処理を行う。熱処理後のPt感知膜電極とSnO2ガス感知膜の界面の状態を図２（ｂ）に示す。
FIG. 1 is a cross-sectional view of a thin film gas sensor according to the present invention. Since the process up to the formation of the gas sensing film electrode E is the same as that of the conventional thin film gas sensor (FIG. 4), the description thereof is omitted.
FIG. 2 is a cross-sectional view of a main part of the thin film gas sensor according to the present invention, where (a) is before heat treatment and (b) is after heat treatment.
The thin film gas sensor according to the present invention will be described according to the manufacturing process.
After the formation of the sensing film electrode E, an electrode made of SnO having a thickness of 1 to 20 nm is prepared in preparation for the formation of the intermediate thin film layer I for improving the adhesion between the SnO 2 gas sensing film S and the sensing film electrode E. A thin thin sputtered film layer Il is provided. Further, an SnO 2 gas sensing film S is continuously formed by sputtering (FIG. 2A).
Thereafter, heat treatment is performed at 400 ° C. to 800 ° C. in a nitrogen gas atmosphere having an oxygen partial pressure (Po 2 ) of 0.01 to 0.2 atm. The state of the interface between the Pt sensing film electrode and the SnO 2 gas sensing film after the heat treatment is shown in FIG.
この熱処理は中間薄膜層を成膜後行い、その後SnO2ガス感知膜をスパッタ成膜しても良い。
Pt感知膜電極とSnO2ガス感知膜の界面の状態が図２（ｂ）のように変化する理由は以下のように考えられる。Ptの触媒作用と関連していると推定されるが、熱処理時にSnO薄膜の最下層近傍のPt感知膜電極と直接接しているSnOは変質し、Pt-Snの化合物（金属間化合物）を形成し強固に接合する。一方上記変質部以外のSnOは酸化により安定相のSnO2（SnO2ガス感知膜）になる。Pt-Snの化合物とSnO2相の界面はステップ状に組成が変化しているわけではなく、Pt-Sn/Pt-Sn-O/SnO2-X（Snの不定比化合物）が入り組んだ混合層として存在するためPt感知膜電極とSnO2ガス感知膜の密着性は飛躍的に上昇する。Pt-Snの生成は熱処理時にSnO薄膜の最下層に酸素の供給が少なくSnO→Sn+1/2O2の分解反応がPt触媒により促進されSnが析出しPtとの間でSn-Ptの合金が生成されたとものと推定される。一方同じ酸素分圧で処理されたにも関わらずSnO薄膜の表面側はPt（触媒）が無いことと、酸素供給がSnO薄膜の最下層より十分な量供給されたためSnO+1/2O2→SnO2の反応が優勢のためSnO2が生成されたものと考えられる。またPt感知膜電極間ではSnO薄膜の下地はSiO2であるが、SiO2上に成膜されたSnO薄膜は熱処理後SiO2/SnO界面層を含め全てSnO+1/2O2→SnO2の反応によりSnO2になる。
This heat treatment may be performed after forming the intermediate thin film layer, and then the SnO 2 gas sensing film may be formed by sputtering.
The reason why the state of the interface between the Pt sensing film electrode and the SnO 2 gas sensing film changes as shown in FIG. 2B is considered as follows. It is presumed to be related to the catalytic action of Pt, but SnO in direct contact with the Pt sensing film electrode in the vicinity of the bottom layer of the SnO thin film is altered during heat treatment to form a Pt-Sn compound (intermetallic compound). And firmly joined. On the other hand, SnO other than the above-mentioned altered portion becomes stable phase SnO 2 (SnO 2 gas sensing film) by oxidation. The interface between the Pt-Sn compound and the SnO 2 phase does not change in a step-like manner, and Pt-Sn / Pt-Sn-O / SnO 2-X (Sn non - stoichiometric compound) is mixed. Since it exists as a layer, the adhesion between the Pt sensing film electrode and the SnO 2 gas sensing film is dramatically increased. Pt-Sn is produced by Sn-Pt alloy between Sn and Pt-catalyzed decomposition reaction of SnO → Sn + 1 / 2O 2 with little oxygen supplied to the bottom layer of SnO thin film during heat treatment. Is presumed to be generated. On the other hand, despite the treatment with the same oxygen partial pressure, SnO + 1 / 2O 2 → SnO thin film has no Pt (catalyst) on the surface side and a sufficient amount of oxygen was supplied from the bottom layer of SnO thin film. It is considered that SnO 2 was produced because the reaction of SnO 2 was dominant. In the inter-Pt sensing film electrode foundation of SnO thin film is a SiO 2 but, SnO thin film formed on the SiO 2 is all including SiO 2 / SnO interface layer after the heat treatment SnO + 1 / 2O 2 → SnO 2 reaction become SnO 2 by.
Pt感知膜電極/Pt-Sn合金層/SnO2ガス感知膜の構造により、Pt感知膜電極とSnO2ガス感知膜との間の高い密着性が複雑なプロセスを行うことなく簡単に得られる。
また、中間薄膜層としてSnOに替えてSnの極薄膜を用いていも良い。図３は本発明に係る他の薄膜ガスセンサの要部の断面図であり、（ａ）は熱処理前であり、（ｂ）は熱処理後である。
Pt感知膜電極EとSnO2ガス感知膜Sの間の密着性向上のための中間薄膜層Iの形成の準備として、Pt感知膜電極Eの形成後、1〜10nm厚さのSn薄膜Ilを成膜する。さらに連続してSnO2ガス感知膜をスパッタ成膜（図３（ａ））する。そして、酸素分圧（Po2）0.01〜0.2気圧の窒素ガス雰囲気中で、400℃〜800℃の熱処理を行う（図３（ｂ））。
この熱処理は中間薄膜層（Sn）を成膜後に行い、その後SnO2ガス感知膜をスパッタ成膜しても良い。
Due to the structure of the Pt sensing film electrode / Pt-Sn alloy layer / SnO 2 gas sensing film, high adhesion between the Pt sensing film electrode and the SnO 2 gas sensing film can be easily obtained without performing a complicated process.
Further, an Sn thin film may be used as the intermediate thin film layer instead of SnO. FIG. 3 is a cross-sectional view of the main part of another thin film gas sensor according to the present invention, where (a) is before heat treatment and (b) is after heat treatment.
In preparation for the formation of the intermediate thin film layer I for improving the adhesion between the Pt sensing film electrode E and the SnO 2 gas sensing film S, the Sn thin film Il having a thickness of 1 to 10 nm is formed after the formation of the Pt sensing film electrode E. Form a film. Further, an SnO 2 gas sensing film is continuously formed by sputtering (FIG. 3A). Then, heat treatment is performed at 400 ° C. to 800 ° C. in a nitrogen gas atmosphere having an oxygen partial pressure (Po 2 ) of 0.01 to 0.2 atm (FIG. 3B).
This heat treatment may be performed after forming the intermediate thin film layer (Sn), and then the SnO 2 gas sensing film may be formed by sputtering.
Pt感知膜電極EとSnO2ガス感知膜Sの界面の状態が図３（ｂ）のように変化する理由は以下のように考えられる。熱処理時にSn薄膜の最下層近傍のPt感知膜電極と直接接しているSnはPt感知膜電極と反応し、Pt-Snの化合物（金属間化合物）を形成し強固に接合する。一方上記変質部以外のSnは酸化によりSnO2（SnO2ガス感知膜）になる。Pt-Snの化合物とSnO2相の界面はステップ状に組成が変化しているわけではなく、Pt-Sn/Pt-Sn-O/SnO2-X（Snの不定比化合物）が入り組んだ混合層として存在するためPt感知膜電極とSnO2ガス感知膜の密着性は飛躍的に上昇する。電極間のSn薄膜の下地はSiO2であるが、SiO2上に成膜されたSnO薄膜は熱処理後SnO+1/2O2→SnO2の反応によりSnO2になるため電極間はショートしない。
上記のように、Pt感知膜電極E/Pt-Sn合金層I/SnO2ガス感知膜Sの構造により、Pt感知膜電極とSnO2ガス感知膜との間の高い密着性が複雑なプロセスを行うことなく簡単に得られる。
The reason why the state of the interface between the Pt sensing film electrode E and the SnO 2 gas sensing film S changes as shown in FIG. 3B is considered as follows. Sn that is in direct contact with the Pt sensing film electrode in the vicinity of the lowermost layer of the Sn thin film during the heat treatment reacts with the Pt sensing film electrode to form a Pt—Sn compound (intermetallic compound) to be strongly bonded. On the other hand, Sn other than the altered portion becomes SnO 2 (SnO 2 gas sensing film) by oxidation. The interface between the Pt-Sn compound and the SnO 2 phase does not change in a step-like manner, and Pt-Sn / Pt-Sn-O / SnO 2-X (Sn non - stoichiometric compound) is mixed. Since it exists as a layer, the adhesion between the Pt sensing film electrode and the SnO 2 gas sensing film is dramatically increased. The base of the Sn thin film between the electrodes is SiO 2 , but the SnO thin film formed on the SiO 2 becomes SnO 2 by the reaction of SnO + 1 / 2O 2 → SnO 2 after the heat treatment, so the electrodes are not short-circuited.
As described above, due to the structure of the Pt sensing membrane electrode E / Pt-Sn alloy layer I / SnO 2 gas sensing membrane S, the high adhesion between the Pt sensing membrane electrode and the SnO 2 gas sensing membrane is a complicated process. Easy to get without doing.
図1は本発明に係る薄膜ガスセンサの断面図である。以下その製造方法に沿って詳しく説明する。
両面に熱酸化膜L1を0.3μm厚に形成されたSi基板Bの表面に、ダイヤフラム構造の支持層となるSiN膜L2とSiO2膜L3を順次プラズマＣＶＤにより、それぞれ厚さ0.15μmと1μm形成した。
この上にPt感知膜電極接合Eの第１の接合層として、Taを0.05μm形成後、連続して、ヒータH用の金属層としてPtW（Pt＋4Wt％W）膜を0.5μm形成し、さらに連続して第２の接合層としてTaを0.05μm形成した。
そして微細加工によりパターニングしヒータHを形成した。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、Ptには王水をそれぞれ90℃に加熱して用いた。
FIG. 1 is a cross-sectional view of a thin film gas sensor according to the present invention. Hereinafter, it explains in detail along the manufacturing method.
The SiN film L2 and SiO 2 film L3, which are the support layers of the diaphragm structure, are sequentially formed by plasma CVD on the surface of the Si substrate B with the thermal oxide film L1 formed on both sides with a thickness of 0.3 μm, and thicknesses of 0.15 μm and 1 μm, respectively. did.
On top of this, 0.05 μm of Ta is formed as the first bonding layer of Pt sensing film electrode bonding E, and then 0.5 μm of PtW (Pt + 4 Wt% W) film is formed continuously as the metal layer for heater H, and further continuous. Then, 0.05 μm of Ta was formed as the second bonding layer.
Then, the heater H was formed by patterning by fine processing. As the 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.
その上にSiO2絶縁膜をスパッタにより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Hの電極パッド部分をHFにてエッチングし窓明け後、導通の確保とワイヤボンディング性を向上のため、第２の接合層のTaを水酸化ナトリウムと過酸化水素混合液を用いて除去した。
その後、スパッタでPt感知膜電極Eを以下のようにして形成する。第１の接合層としてTaを0.05μm形成後、連続して、Pt感知膜電極E用のPt薄膜を0.2μmスパッタ成膜した。なお、Pt/Taの成膜条件は１Pa、Ar中で、投入電力100W、成膜温度100℃とした。さらに微細加工によりヒータパターンを形成した。ウエットエッチングのエッチャントとしてPtには王水をTaには水酸化ナトリウムと過酸化水素混合液、それぞれ90℃に加熱して用いた。
つづいて本発明に係る密着性向上のための中間薄膜層SnO層IおよびSnO2ガス感知膜Sをレジストリフトオフ法に従って形成する。具体的には以下のような工程で形成する。
On top of that, a SiO 2 insulating film is formed by sputtering to 1.0 μm, then the electrode pad part of the heater H (not shown) is etched with HF by fine processing, and after opening the window, to ensure conduction and improve wire bonding The Ta of the second bonding layer was removed using a mixed solution of sodium hydroxide and hydrogen peroxide.
Thereafter, the Pt sensing film electrode E is formed by sputtering as follows. After forming 0.05 μm of Ta as the first bonding layer, a Pt thin film for the Pt sensing film electrode E was continuously formed by sputtering with 0.2 μm. The film formation conditions for Pt / Ta were 1 Pa, Ar, input power 100 W, and film formation temperature 100 ° C. Further, a heater pattern was formed by fine processing. As wet etchants, aqua regia was used for Pt, and sodium hydroxide and hydrogen peroxide mixed solution was used for Ta, each heated to 90 ° C.
Subsequently, the intermediate thin film layer SnO layer I and the SnO 2 gas sensing film S for improving adhesion according to the present invention are formed according to a registry ftoff method. Specifically, it is formed by the following steps.
先ず、レジストを塗布し、微細加工により、1対のPt感知膜電極上およびそのPt感知膜電極間のSnO2ガス感知膜を形成する部分のレジストを除去し、開口したパターンにレジストを加工する。
そして、スパッタ成膜で中間薄膜層SnOおよびSnO2ガス感知膜をスパッタ成膜により形成した。ここで中間薄膜層SnOおよびSnO2ガス感知膜のスパッタ成膜は真空を破ることなく連続して成膜しても良いし、一旦真空を破り、レジストのリフトオフを行い、後述する熱処理を行った後、再度レジストパターンニング後SnO2ガス感知膜を成膜してもかまわない。中間薄膜層SnOの成膜条件は１PaのAr中で、投入電力100W、成膜温度100℃であり、SnO2ガス感知膜の成膜条件は、１PaのAr+O2中で、投入電力100W、成膜温度100℃とした。なおスパッタターゲットとしてSnO2を用い、スパッタガス組成のみ変えれば中間薄膜層SnOおよびSnO2ガス感知膜の成膜が可能である。SnO、SnO2は結晶形態が異なるためＸ線回折により両相の確認は容易に行うことができる。
First, a resist is applied, and by fine processing, the resist on the pair of Pt sensing film electrodes and the portion forming the SnO 2 gas sensing film between the Pt sensing film electrodes is removed, and the resist is processed into an opened pattern. .
Then, intermediate thin film layers SnO and SnO 2 gas sensing films were formed by sputtering deposition. Here, the sputter deposition of the intermediate thin film layer SnO and SnO 2 gas sensing film may be performed continuously without breaking the vacuum, or once the vacuum is broken, the resist is lifted off, and the heat treatment described later is performed. Thereafter, an SnO 2 gas sensing film may be formed after resist patterning again. The deposition condition of the intermediate thin film layer SnO is 1W Ar, input power 100W, deposition temperature 100 ° C, and the SnO 2 gas sensing film deposition condition is 1Pa Ar + O 2 with input power 100W. The film forming temperature was 100 ° C. If SnO 2 is used as the sputtering target and only the sputtering gas composition is changed, the intermediate thin film layer SnO and SnO 2 gas sensing film can be formed. Since SnO and SnO 2 have different crystal forms, both phases can be easily confirmed by X-ray diffraction.
なお中間薄膜層SnOの膜厚が1nm未満ではSnOが島状に形成されたり、後述する熱処理後も十分なPt-Sn合金層が得られない。また、20 nmを越えると熱処理後もSnO内部に未反応部(酸化でSnO2に変態しない部分)が残り、SnO2の電気抵抗に影響する場合があるのでSnO中間薄膜層厚みは1nm以上、20nm以下であることが必要である。
その後、レジストのリフトオフを行い、電気炉中で熱処理を行った。熱処理は酸素分圧（Po2）0.01〜0.2気圧でバランスガスはN2、Arなどの不活性ガスとした。
Po2が0.01気圧未満ではSnO+1/2O2→SnO2の反応において、SnOが未反応で残る場合がある。またPo2が0.2気圧を超えると、Pt感知膜電極と接しているSnO界面のSnOがSnO→Sn+1/2O2の分解反応が充分進む前に酸化が進行し、十分なPt-Sn合金層が得られない。
また、熱処理温度は400℃以上800℃以下で熱処理することにより目的が達成できる。熱処理温度が400℃未満の場合SnO→Sn+1/2O2なる反応が十分進まない。また800℃を超えた場合、多孔質であるSnO2ガス感知部の比表面積の低下によるガス感知感度の低下が発生し不具合が起こる。熱処理時間は400℃の場合60分、800℃の場合10分で十分な効果が得られる。
When the thickness of the intermediate thin film layer SnO is less than 1 nm, SnO is formed in an island shape, or a sufficient Pt—Sn alloy layer cannot be obtained even after the heat treatment described later. In addition, if it exceeds 20 nm, an unreacted part (part that does not transform into SnO 2 due to oxidation) remains in the SnO even after heat treatment, and the SnO 2 electrical resistance may be affected, so the SnO intermediate thin film layer thickness is 1 nm or more, It must be 20 nm or less.
Thereafter, the resist was lifted off, and heat treatment was performed in an electric furnace. The heat treatment was an oxygen partial pressure (Po 2 ) of 0.01 to 0.2 atm, and the balance gas was an inert gas such as N 2 or Ar.
When Po 2 is less than 0.01 atm, SnO may remain unreacted in the reaction of SnO + 1 / 2O 2 → SnO 2 . Also, when Po2 exceeds 0.2 atm, oxidation proceeds before SnO at SnO interface in contact with the Pt sensing membrane electrode sufficiently proceeds to decompose SnO → Sn + 1 / 2O 2 and a sufficient Pt-Sn alloy layer Cannot be obtained.
The purpose can be achieved by heat treatment at a temperature of 400 ° C. or higher and 800 ° C. or lower. When the heat treatment temperature is less than 400 ° C., the reaction SnO → Sn + 1 / 2O 2 does not proceed sufficiently. When the temperature exceeds 800 ° C., the gas sensing sensitivity decreases due to the decrease in the specific surface area of the porous SnO 2 gas sensing unit, which causes a problem. A sufficient effect can be obtained when the heat treatment time is 60 minutes at 400 ° C and 10 minutes at 800 ° C.
ここまでは中間薄膜層としてSnOを用いた実施例で説明してきたが、金属Sn薄膜を中間薄膜層として用いても同様の効果が得られる。但し、金属Sn薄膜を中間薄膜層として用いた場合、膜厚としては1nm以上10nm以下と薄くすることが適切であることが判った。Po2、熱処理温度、時間などの条件は若干Sn中間薄膜層とは異なる。
実験に用いた本発明の素子Aは厚み10nmSnOと厚み400nmSnO2を連続的に成膜して得た。熱処理はPo2=0.1気圧/N2バランス、温度700℃、時間30分の条件で行った。比較のため、中間薄膜層の無い従来素子Bも比較のため試作した。
いずれの素子においても、その後、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混合したペーストを用い、スクリーン印刷によりSnO2の表面に塗布、焼成させ約30μm厚の選択燃焼層（触媒フィルター）Cを形成した。選択燃焼層C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ングによりSiを400μm径の大きさだけ完全に除去し、ダイヤフラム構造とした。
So far, the embodiment using SnO as the intermediate thin film layer has been described, but the same effect can be obtained even when the metal Sn thin film is used as the intermediate thin film layer. However, when a metal Sn thin film was used as an intermediate thin film layer, it was found that it was appropriate to make the film thickness as thin as 1 nm or more and 10 nm or less. Conditions such as Po 2 , heat treatment temperature, and time are slightly different from those of the Sn intermediate thin film layer.
The element A of the present invention used in the experiment was obtained by successively forming a film having a thickness of 10 nm SnO and a thickness of 400 nm SnO 2 . The heat treatment was performed under the conditions of Po 2 = 0.1 atm / N 2 balance, temperature 700 ° C., and time 30 minutes. For comparison, a conventional device B without an intermediate thin film layer was also prototyped for comparison.
In any element, a selective combustion layer (catalyst) having a thickness of about 30 μm is then applied to the surface of SnO 2 by screen printing using a paste in which a powder in which Pt and Pd catalysts are supported on alumina particles is mixed with a binder, and fired by screen printing. Filter) C was formed. The selective combustion layer C improves the sensitivity, gas type selectivity, and reliability of the gas sensor. Finally, Si was completely removed from the back surface of the substrate by dry etching by a diameter of 400 μm to obtain a diaphragm structure.
表１は本発明のセンサ（素子A）と従来のセンサ（素子B）を各５個ずつ大気中でパルス通電(試験条件3V/50mW 、通電100msec ON/1secOFF（通電時ヒータ温度450℃）)を500、1000、2000万回繰り返した後の20℃、60％RHでの2000ppmCH4/空気中におけるSnO2ガス感知膜（センサ温度450℃）の抵抗値の変化を示したものである。
本発明の素子と従来素子で抵抗変化が大きく変化した素子について、ＦＩＢ（中性集束イオンビーム）による断面加工を行い、SnO2ガス感知膜とPt感知膜電極の接合部をＦＩＢ２次電子像により評価した。
本発明の素子では、Pt感知膜電極／SnO2ガス感知膜の断面に何ら剥離による痕跡が認められなかったが、抵抗値が大きく上昇した従来素子ではPt感知膜電極／SnO2ガス感知膜の断面に剥離から生じたと考えられる空隙が部分的に認められた。
Cross section processing by FIB (neutral focused ion beam) is performed on the element of the present invention and the element in which the resistance change has greatly changed, and the junction between the SnO 2 gas sensing film and the Pt sensing film electrode is obtained by FIB secondary electron image. evaluated.
In the element of the present invention, no trace due to peeling was observed on the cross section of the Pt sensing film electrode / SnO 2 gas sensing film, but in the conventional element in which the resistance value increased greatly, the Pt sensing film electrode / SnO 2 gas sensing film In the cross section, voids thought to be caused by peeling were partially observed.
本発明によれば、Pt感知膜電極とその上に形成するSnO2ガス感知膜の間に、Pt-Sn合金を主成分とした中間薄膜層を設けることで、Pt感知膜電極とSnO2ガス感知膜との密着性が向上し感知膜SnO2と感知膜電極との間に剥離を生じないため、長期間安定した抵抗値（センサ特性）が得られたものであり、このようなガス薄膜ガスセンサを用いて、ガス警報器の普及が促進できる。 According to the present invention, between the SnO 2 gas sensing film formed thereon with Pt sensitive membrane electrode, by providing the intermediate thin layer composed mainly of Pt-Sn alloy, Pt sensitive membrane electrode and the SnO 2 gas Since the adhesion with the sensing film is improved and no separation occurs between the sensing film SnO 2 and the sensing film electrode, a stable resistance value (sensor characteristics) is obtained for a long time. The spread of gas alarms can be promoted using a gas sensor.
B Si基板
L1 酸化ケイ素膜
L2 窒化ケイ素膜
L3 酸化ケイ素膜
L 支持膜
L4 酸化ケイ素膜
H ヒータ
E SnO2ガス感知膜電極
I Pt-Sn合金層
Il 中間薄膜層
S ガス感知膜
C 選択燃焼層
B Si substrate
L1 silicon oxide film
L2 silicon nitride film
L3 silicon oxide film
L Support membrane
L4 silicon oxide film
H heater
E SnO 2 gas sensing membrane electrode
I Pt-Sn alloy layer
Il Intermediate thin film layer
S gas sensing membrane
C selective combustion layer
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| CN102441375A (en) * | 2011-11-03 | 2012-05-09 | 华东理工大学 | Homogeneous mesoporous rhodium oxide / alumina composite catalysis material, preparation method and application thereof |
| US8389996B2 (en) | 2009-03-06 | 2013-03-05 | Canon Kabushiki Kaisha | Method for forming semiconductor film, method for forming semiconductor device and semiconductor device |
| JP2018031685A (en) * | 2016-08-25 | 2018-03-01 | フィガロ技研株式会社 | Mems gas sensor and gas detector |
| JP2022030596A (en) * | 2020-08-07 | 2022-02-18 | 新コスモス電機株式会社 | MEMS type semiconductor gas detection element |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN105606661B (en) * | 2016-03-09 | 2019-02-12 | 中国科学院微电子研究所 | Film-type MOS gas sensor of integrated nanometer structure and preparation method thereof |
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