JP3846616B2 - Thin film gas sensor - Google Patents

Description

【００１４】
ＴｉＮ，ＮｂＮヒーターを、４５０℃で２０００万回間欠動作させたときの抵抗の変化（２０００万回間欠動作後／間欠動作前の比）を表２に示す。
【表２】

表２から、ＴｉＮ，ＮｂＮのいずれも、２０００万回以上の間欠動作でも、ヒーターの破損は認められなかったことが分かる。
【００１５】
次に、第２のグループの例について説明する。
まず、Ｗ−Ｐｔ膜を図１と同様に形成した例では、Ｗの含有率を２，４，６，８，１２および２０ｗｔ％（重量百分率）の６種類としたときの比抵抗，熱膨張係数を表３に示す。
【表３】

表３に示すように、Ｗ−Ｐｔ系合金の熱膨張係数は、個々の熱膨張係数の重量分率の和で表わされるのに対し、比抵抗はＷの含有率約６ｗｔ％に極大値が見られる。すなわち、２０℃での比抵抗が３０μΩｃｍ以上の組成範囲は、Ｗが３〜１６ｗｔ％であった。
【００１６】
表３で比抵抗が極大値を示す６ｗｔ％Ｗ−Ｐｔヒーターについて、４５０℃加熱前後の最大変位量と、２０００万回間欠動作前後の抵抗の変化を表４に示す。
【表４】

表４から、Ｗ−Ｐｔヒーターは第１のグループの窒化物系ヒーターと同等またはそれ以上の間欠動作安定性を示すことが分かる。
【００１７】
ヒーター層としてＲｕ−Ｐｔ系合金を用いる場合について説明する。
この場合も、Ｒｕの含有率を５，１０，２０，３０ｗｔ％の４種類としたときの比抵抗，熱膨張係数を表５に示す。
【表５】

表５から、Ｗ−Ｐｔ系の場合と同様、熱膨張係数は個々の熱膨張係数の重量分率の和で表わされるのに対し、比抵抗はＲｕの含有率約１０ｗｔ％に極大値が見られる。すなわち、２０℃での比抵抗が３０μΩｃｍ以上の組成範囲は、Ｒｕが４〜２０ｗｔ％であった。
【００１８】
次に、ヒーター層としてＩｒ−Ｐｔ系合金を用いる場合について説明する。
この場合も、Ｉｒの含有率を１０，２５，４０ｐ５５ｗｔ％の４種類としたときの比抵抗，熱膨張係数を表６に示す。
【表６】

表６から、Ｗ−Ｐｔ系の場合と同様、熱膨張係数は個々の熱膨張係数の重量分率の和で表わされるのに対し、比抵抗はＩｒの含有率約２５ｗｔ％に極大値が見られる。すなわち、２０℃での比抵抗が３０μΩｃｍ以上の組成範囲は、Ｉｒが１５〜４０ｗｔ％であった。
【００１９】
ここで、１０ｗｔ％Ｒｕ−Ｐｔと２５ｗｔ％Ｉｒ−Ｐｔの２種類のヒーターについて、４５０℃加熱前後の最大変位量と、２０００万回間欠動作前後の抵抗の変化を表７に示す。
【表７】

表７に示すように、Ｒｕ−Ｐｔ，Ｉｒ−Ｐｔ系ヒーターのいずれも、窒化物系ヒーターと同等またはそれ以上の間欠動作安定性を示すことが分かる。
【００２０】
【発明の効果】
この発明によれば、薄膜ヒーター材としてＰｔの５倍、つまり５０μΩｃｍ以上の比抵抗を有し、かつ熱膨張係数が１０×１０^-6／℃以下、望ましくは７×１０^-6／℃以下の材料を用いることで、従来よりも低消費電力で間欠動作寿命の長い薄膜ガスセンサを得ることが可能となる利点がもたらされる。
【図面の簡単な説明】
【図１】この発明の実施の形態を示す構造断面図である。
【図２】図１で用いられるヒーター層の形状例説明図である。
【図３】ヒーター部の間欠動作回数に対する抵抗変化説明図である。
【図４】ヒーター部の電圧印加前後での変位の最大値と温度との関係説明図である。
【図５】ヒーター部の間欠動作回数に対する抵抗変化の別の例の説明図である。
【図６】ヒーター部の電圧印加前後での変位の最大値と温度との関係の別の例の説明図である。
【符号の説明】
１…ヒーター層、２…電極、３…ＳｎＯ₂ 膜（感応膜）。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a low power consumption thin film gas sensor suitable for battery driving.
[0002]
[Prior art]
Among general gas sensors, those used for applications such as household gas leak alarms are devices that are selectively sensitive to CO, CH ₄ , C ₃ H _8, etc. Performance, high responsiveness, 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, 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. Under such circumstances, it is desired to improve the installation property, specifically, to be cordless as battery driving in order to increase the diffusion rate.
[0003]
In order to realize battery driving, it is indispensable to reduce the size and reduce the power consumption. However, conventionally used catalytic combustion type and semiconductor type gas sensors have a sensitive film sensitive to gas such as SnO ₂ . The powder is manufactured by sintering, and there is a limit to downsizing even if a method such as screen printing is used.
On the other hand, a sensor using a semiconductor microfabrication process that is relatively easy to downsize is also known. This is because an insulating film, a heater, an electrode, a sensitive film, etc. are formed on a substrate by a chemical vapor deposition (CVD) method or the like. In order to reduce the heat capacity of the sensor and to achieve thermal insulation, the substrate is thinned by etching or completely removed in order to reduce the heat capacity of the sensor and to achieve thermal insulation.
[0004]
[Problems to be solved by the invention]
In a sensor using the microfabrication process as described above, Pt is often used as a heater layer.
By the way, the length L of the heater resistor is restricted by the following equation.
L ≧ Vappl / (Jmax × ρ) (1)
Here, Vappl is the applied voltage, Jmax is the maximum current density, and ρ is the specific resistance (resistivity) of the heater.
[0005]
Now, assuming that the applied voltage is 2 V and the maximum current density is 5 × 10 ⁹ A / m ² , L ≧ 4 mm at Pt (ρ≈10 μΩcm) from the above equation. Considering responsiveness to temperature, the sensor preferably has a chip area of about 100 μm × 100 μm, and the heater line width in that case is about 1 μm. This is difficult to realize by a relatively simple wet etching photo process, and it is necessary to use a material having a specific resistance of at least about 5 times Pt for the heater.
[0006]
In addition, assuming driving with a battery, intermittent driving by applying a pulse voltage is indispensable. However, when a general metal heater layer is formed on an insulating film such as SiO ₂ or SiN, the coefficient of thermal expansion is There is also a problem that it is easily broken by deformation or stress concentration due to the difference.
Accordingly, an object of the present invention is to reduce power consumption and extend the intermittent operation life.
[0007]
[Means for Solving the Problems]
To solve such problems, in the invention of claim 1, on the substrate surface which one side central portion is hollowed out in the diaphragm-like Si substrate, a thermal oxide SiO ₂ film, CVD-SiO ₂ film and the Si nitride film A thin film heater is formed by a sputtering method or a vapor deposition method through a support film made of, and then a sensitive film electrode is formed by Pt / Ta or Pt / Ti through an electric insulating film of a SiO ₂ film or a Si nitride film, Further, a sensitive film containing SnO ₂ is formed , and as the thin film heater, a Fe—Ni-based invar alloy having a thermal expansion coefficient of 10 × 10 ⁻⁶ / ° C. or less and a specific resistance of 50 μΩcm or more, or TiB ₂ , ZrB ₂ , metal boride containing HFB ₂ , refractory metal silicide containing MoSi ₂ , WSi ₂ , TaSi ₂ , TiSi ₂ , or SiC, TiN, NbN Pt containing 2 to 16 wt% of W, Pt containing 4 to 20 wt% of Ru, or Pt containing 15 to 40 wt% of Ir.
In this invention of Claim 1, the said thin film heater can be driven by the pulse which repeats a voltage application only for a very short time for every fixed time (Invention of Claim 2).
[0008]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a cross-sectional structural view showing an embodiment of the present invention.
That is, a SiN and SiO ₂ film as a support film having a diaphragm structure and a thermal insulating film are sequentially formed on a Si substrate having a thermal oxide film on both sides by a plasma CVD method. A TiSi ₂ film of 0.5 μm was formed thereon as the heater layer 1 according to the present invention. Furthermore, after the electrodes 2 for the heater and the sensitive film were formed of Pt / Ta through the SiO ₂ insulating film, the SnO ₂ film 3 serving as the sensitive film was formed by sputtering. Finally, Si was removed from the back surface of the substrate by etching to form a diaphragm structure. In addition, the diaphragm means that the periphery of the thin film-like support film is supported by the Si substrate and the periphery is thick and the central portion is thin.
The TiSi ₂ film as the heater layer 1 was formed by sputtering using a mixed crystal target at a film forming temperature of 200 ° C. After sputtering, heat treatment was performed at 850 ° C. for 3 hours in an N ₂ atmosphere in order to crystallize the TiSi ₂ film.
[0009]
FIG. 2 shows an example of the shape of the heater layer.
This is an example in which a line width of 10 μm and a length of about 500 μm are formed, and the resistance value of the heater layer is about 200Ω.
FIG. 3 shows an example of the relationship between the maximum displacement value and temperature before and after voltage application in the heater. The solid line is an example where the material is NiCr, and the dotted line is an example of TiSi ₂ according to the present invention. When NiCr was used, destruction occurred due to intermittent operation of several million times or less at 400 ° C. or more, whereas TiSi ₂ did not show destruction even at 450 ° C. after 20 million times of intermittent operation. Here, the thermal expansion coefficients of NiCr and TiSi ₂ are 18 × 10 ⁻⁶ / ° C. and 7 × 10 ⁻⁶ / ° C., respectively. This is considered to be because the larger the difference in thermal expansion coefficient between the insulating film and the heater material, the larger the deformation amount before and after voltage application, and the fatigue failure of the insulating film due to repeated operations.
[0010]
FIG. 4 is an explanatory diagram of changes in resistance value with respect to the number of intermittent operations of the heater material, and shows a comparison between the case of using the heater as described above and the case of a conventional NiCr heater. The operating temperature is 400 ° C.
As is clear from the figure, the NiCr heater broke down about 2 million times, whereas the TiSi ₂ heater according to the present invention only showed a resistance value change of about 3% after 20 million times. You can see that it wo n’t break.
[0011]
A material having a specific resistance of 3 times the specific resistance of Pt, that is, about 30 μΩcm or more, preferably 5 times the specific resistance of Pt, that is, about 50 μΩcm or more, and having a thermal expansion coefficient of 10 × 10 ⁻⁶ / ° C. or less as the heater layer The following may be mentioned.
As a material having a specific resistance of 50 μΩcm or more, a one-component nitride (first group) selected from tantalum (Ta), titanium (Ti) or niobium (Nb), or a material having a specific resistance of 30 μΩcm or more, It has been found that there is an alloy (second group) containing platinum (Pt) as a main component and containing one component of tungsten (W), ruthenium (Ru) or iridium (Ir).
[0012]
Hereinafter, these will be described. As the first group of nitrides, those having a metal / nitrogen atomic ratio of 1: 1 can be used, but the metal / nitrogen atomic ratio is not necessarily limited to 1: 1. As described in Japanese Utility Model Laid-Open No. 9-201965, nitrides having a metal to nitrogen ratio other than 1: 1, for example, 1: 0.8 (TaN0.8) can be used.
FIG. 5 shows the relationship between the maximum displacement corresponding to FIG. 3 and the heater temperature when TaN (TaN heater) is used as the heater material, and FIG. 5 shows the relationship between the resistance change and the number of intermittent operations corresponding to FIG. 6 respectively. The thermal expansion coefficient of TaN is 8 × 10 ⁻⁶ / ° C. 5 and 6, the solid line is Ta, and the dotted line is NiCr. As is apparent from FIGS. 5 and 6, the characteristics are almost the same as those of TiSi ₂ .
[0013]
These characteristics are exactly the same when TiN or NbN is used. The thermal expansion coefficients of TiN and NbN are 9.4 × 10 ⁻⁶ / ° C. and 10 × 10 ⁻⁶ / ° C.
Table 1 shows the maximum value of displacement at 450 ° C. in the case of TiN and NbN heaters in comparison with NiCr heaters. For both TiN and NbN, the maximum displacement before and after voltage application is smaller than that of NiCr, and is about 75% of the displacement of NiCr.
[Table 1]

[0014]
Table 2 shows the change in resistance when the TiN and NbN heaters are intermittently operated 20 million times at 450 ° C. (ratio after 20 million times intermittent operation / before intermittent operation).
[Table 2]

From Table 2, it can be seen that neither TiN nor NbN was found to be damaged by the heater even in intermittent operation of 20 million times or more.
[0015]
Next, an example of the second group will be described.
First, in the example in which the W—Pt film is formed in the same manner as in FIG. 1, the specific resistance and thermal expansion when the W content is six types of 2, 4, 6, 8, 12 and 20 wt% (weight percentage). The coefficients are shown in Table 3.
[Table 3]

As shown in Table 3, the thermal expansion coefficient of the W-Pt alloy is expressed by the sum of the weight fractions of the individual thermal expansion coefficients, whereas the specific resistance has a maximum value at a W content of about 6 wt%. It can be seen. That is, in the composition range where the specific resistance at 20 ° C. was 30 μΩcm or more, W was 3 to 16 wt%.
[0016]
Table 4 shows the maximum displacement amount before and after heating at 450 ° C. and the change in resistance before and after 20 million intermittent operations for the 6 wt% W-Pt heater having the maximum specific resistance in Table 3.
[Table 4]

It can be seen from Table 4 that the W-Pt heater exhibits an intermittent operation stability equivalent to or higher than that of the first group of nitride heaters.
[0017]
The case where a Ru—Pt alloy is used as the heater layer will be described.
Also in this case, Table 5 shows the specific resistance and the thermal expansion coefficient when the Ru content is 4, 10, 20, and 30 wt%.
[Table 5]

From Table 5, as in the case of the W-Pt system, the thermal expansion coefficient is represented by the sum of the weight fractions of the individual thermal expansion coefficients, whereas the specific resistance has a maximum value at a Ru content of about 10 wt%. It is done. That is, in the composition range where the specific resistance at 20 ° C. is 30 μΩcm or more, Ru was 4 to 20 wt%.
[0018]
Next, a case where an Ir—Pt alloy is used as the heater layer will be described.
Also in this case, Table 6 shows specific resistance and thermal expansion coefficient when the Ir content is 10, 25, and 40 p55 wt%.
[Table 6]

From Table 6, as in the case of the W-Pt system, the thermal expansion coefficient is expressed as the sum of the weight fractions of the individual thermal expansion coefficients, whereas the specific resistance has a maximum value at an Ir content of about 25 wt%. It is done. That is, in the composition range where the specific resistance at 20 ° C. is 30 μΩcm or more, Ir was 15 to 40 wt%.
[0019]
Table 7 shows the maximum displacement before and after heating at 450 ° C. and the change in resistance before and after 20 million intermittent operations for two types of heaters of 10 wt% Ru—Pt and 25 wt% Ir—Pt.
[Table 7]

As shown in Table 7, it can be seen that any of the Ru-Pt and Ir-Pt heaters exhibits an intermittent operation stability equivalent to or higher than that of the nitride heater.
[0020]
【The invention's effect】
According to the present invention, the thin film heater material has a specific resistance of 5 times Pt, that is, 50 μΩcm or more, and a thermal expansion coefficient of 10 × 10 ⁻⁶ / ° C. or lower, preferably 7 × 10 ⁻⁶ / ° C. or lower. By using the material, there is an advantage that it is possible to obtain a thin film gas sensor with lower power consumption and longer intermittent operation life than in the past.
[Brief description of the drawings]
FIG. 1 is a structural sectional view showing an embodiment of the present invention.
FIG. 2 is an explanatory diagram of a shape example of a heater layer used in FIG. 1;
FIG. 3 is an explanatory diagram of resistance change with respect to the number of intermittent operations of a heater unit.
FIG. 4 is a diagram for explaining the relationship between the maximum value of displacement before and after voltage application to the heater and the temperature.
FIG. 5 is an explanatory diagram of another example of a resistance change with respect to the number of intermittent operations of the heater unit.
FIG. 6 is an explanatory diagram of another example of the relationship between the maximum value of displacement before and after voltage application to the heater and the temperature.
[Explanation of symbols]
1 ... heater layer, 2 ... electrode, 3 ... SnO ₂ film (sensitive film).

Claims (2)

A thin film heater is formed by sputtering or vapor deposition on a substrate surface in which one side central portion of a Si substrate is hollowed out like a diaphragm through a support film made of a thermally oxidized SiO ₂ film, a CVD-SiO ₂ film, and a Si nitride film. Then, a sensitive film electrode is formed of Pt / Ta or Pt / Ti through an electric insulating film of SiO ₂ film or Si nitride film, and further, a sensitive film containing SnO ₂ is formed , and the thin film heater As follows: Fe—Ni-based Invar alloy having a thermal expansion coefficient of 10 × 10 ⁻⁶ / ° C. or less and a specific resistance of 50 μΩcm or more, or a metal boride containing TiB ₂ , ZrB ₂ , HFB ₂ , or MoSi ₂ , Refractory metal silicide containing WSi ₂ , TaSi ₂ , TiSi ₂ , or Pt and Ru containing 2 to 16 wt% of SiC, TiN, NbN, and W 4 to 20 wt% A thin film gas sensor using any one of Pt containing 15% and Pt containing 15 to 40 wt% Ir . 2. The thin film gas sensor according to claim 1, wherein the thin film heater is driven by a pulse that repeats voltage application for a very short time every predetermined time .

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