JP2003279523A - Membrane gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly stable membrane gas sensor in which either variations of a resistance value or failure of continuity due to long-duration pulse energization does not occur. <P>SOLUTION: In the membrane sensor, an Si substrate 1 having a through hole, a diaphragm 2 made of at least a first insulating film extended over an opening part of the through hole, and a gas sensing film 6 made of SnO<SB>2</SB>, etc., and having at least a heater 3 and electrodes 5 made of Pt at both ends on the diaphragm, are insulated from one another by a second insulating film 4 made of SiO<SB>2</SB>, etc. A layer made of at least one material among Ta, Cr, and Ti as a main component is interposed on the side of the second insulating film of the electrodes. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diaphragm type thin film gas sensor suitable for driving a battery and having low power consumption.

[0002]

2. Description of the Related Art Generally, a gas sensor is used for a gas leak alarm or the like, and a certain gas such as CO or CH.
It is a device that selectively responds to ₄ , C ₃ H _8, etc., and its sensitivity requires high sensitivity, high selectivity, high response, high reliability, and low power consumption. By the way, gas leak alarms that are popular for household use include those for the purpose of detecting combustible gas for city gas and propane gas and those for the purpose of detecting incomplete combustion gas of combustion equipment, or Some have both functions, but the penetration rate is not so high due to cost and installation problems. In order to improve the penetration rate, it is desired to improve the installability, specifically, to make it battery-operated and cordless.

Low power consumption is the most important for realizing battery drive, but in the catalytic combustion type or semiconductor type gas sensor, it is necessary to detect by heating to a high temperature of 200 ° C to 500 ° C. The conventional gas sensor in the form of sintered powder such as SnO ₂ has a limit in reducing the thickness even if screen printing is applied, and its heat capacity is too large to be used for battery driving.

Therefore, a heater and a gas sensing film, etc.
A thin-film gas sensor has been developed that is formed of a thin film of m or less to reduce the heat capacity. By installing such a thin-film gas sensor on a diaphragm formed by a microfabrication process, a diaphragm-type gas sensor with a further reduced heat capacity has been developed, and its practical application is awaited.

FIG. 1 is a sectional view of a thin film gas sensor. A first insulating film composed of a thermal silicon oxide film 2a, a CVD (chemical vapor deposition) silicon nitride 2b and a CVD silicon oxide film 2c is formed in the opening of a Si substrate 1 having a through hole in the center.
2 is stretched. The first insulating film 2 constitutes a diaphragm. A thin film heater 3 is provided on the first insulating film 2, a second insulating film 4 made of silicon oxide covering the thin film heater 3 is provided, and a sensing film electrode 5 made of Pt is provided on both ends of the second insulating film 4. The gas sensing film 6 made of the SnO ₂ thin film is sequentially formed. Then, a selective combustion layer 7 covering the gas sensing film 6 is formed. In the conventional thin film gas sensor, the sensing film electrode 5 has a bonding layer made of Cr, Ti, Ta or the like in order to enhance the adhesion with the second insulating film 4 (SiO ₂ ).

Even in a gas leak alarm using a low power consumption thin film gas sensor having an ultra-low heat capacity structure of such a diaphragm structure, in order to have a life of 5 years or more without battery replacement, Pulse driving is essential. Normally, the gas leak alarm requires detection once every 20 to 60 seconds, and the detector is heated from room temperature to a high temperature of 200 to 500 ° C in accordance with this cycle. In order to meet the service life requirement of 5 years or more without replacing the battery, the heating time is set to a few 100 ms or less.

[0007]

As a sensing film electrode material for a gas sensing film, a noble metal material such as Pt is generally used because it has little chemical change and is stable. Pt film as Si
In the case of forming on the insulating support layer of O ₂ or the like, excellent adhesion to the oxide film such as SiO _2, yet good adhesion with the Pt film
Pt is formed on top of it through a bonding layer consisting of a thin film of Ta, Ti, Cr, etc.
A film is being formed. After the film formation, SnO ₂ which is a gas sensing film is formed so as to connect the two sensing film electrodes formed by patterning, and the gas sensor part of the thin film gas sensor is completed.

When the thin film heater formed in the diaphragm structure is pulsed and the temperature is raised and lowered repeatedly, the diaphragm having a laminated structure including the support layer, the thin film heater, the sensing membrane electrode and the gas sensing membrane is heated to several μm by thermal expansion and contraction. It vibrates up and down though. This vibration is small, but 10
Approximately 20 when the sensor is driven for 5 years with a detection cycle of once per second.
Reaches, 000,000 times. This minute vibration causes peeling at the joint between the Pt sensing film electrode and the gas sensing film SnO ₂ , causing fluctuations such as an increase in the sensor resistance value, which may cause conduction failure in extreme cases. is there. In a SnO ₂ gas sensor that detects gas based on the sensor resistance value, naturally, fluctuations in resistance value and poor conduction are major problems. That is, due to such peeling, the gas detection accuracy, which is a characteristic of the thin film gas sensor, may be reduced, or in extreme cases, gas detection may be disabled.

It is an object of the present invention to provide a thin film gas sensor having high stability which does not cause fluctuations in resistance value or conduction failure even when pulsed for a long time.

[0010]

In order to achieve the above object, a Si substrate having a through hole, a diaphragm made of at least a first insulating film stretched over an opening of the through hole, and a diaphragm formed on the diaphragm. A thin film gas sensor formed so that at least a heater and a gas sensing film made of SnO ₂ or the like having sensing film electrodes made of Pt on both ends are insulated from each other by a second insulating film made of SiO ₂ or the like. Membrane electrode is at least one of Ta, Cr and Ti
It is assumed that a first intermediate layer made of a material containing two as a main component is provided on the gas sensing film side.

The material is at least one of Ta, Cr and Ti.
It should consist of two. The material is preferably a Hastelloy alloy made of Ni-Mo-Fe-Cr-W. Alternatively, a Si substrate having a through hole, a diaphragm made of at least a first insulating film stretched over the opening of the through hole, and a SnO ₂ having at least a heater and sensing film electrodes at both ends on the diaphragm. In the thin-film gas sensor, the gas sensing film made of SiO ₂ and the second insulating film made of SiO ₂ are insulated from each other.
It is made of a material containing at least one of r and Ti as a main component.

The material is at least one of Ta, Cr and Ti.
It should consist of two. The material is preferably a Hastelloy alloy made of Ni-Mo-Fe-Cr-W. According to the present invention, Ta, between the sensing membrane electrode and the gas sensing membrane sensing membrane electrode as described above,
Since Ti, Cr or an alloy containing these as the main components is interposed, the strong chemical chemistry of SnO ₂ which is a gas sensing film formed on them due to the oxidization of these metals via oxygen. The adhesiveness between the sensing membrane electrode and the gas sensing membrane becomes high, and the thin film gas sensor is highly reliable because there is no separation of the boundary between the sensing membrane electrode and the gas sensing membrane even after long-term pulse energization. Can be expected to be obtained.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Example 1 Although the structure of the thin film gas sensor according to the present invention is the same as that of FIG. 1, the structure of the sensing film electrode is different from that of the conventional thin film gas sensor. The structure will be described below according to the manufacturing process. A Si substrate having a thermal oxide film 2a of 0.3 μm formed on both sides was used. The thickness of the supporting layer of the diaphragm structure on the surface is 0.
A 15 μm SiN film 2b and a 1 μm thick SiO ₂ film 2c were sequentially formed by plasma CVD.

On top of this, Ta was deposited to a thickness of 0.05 μm as a first heater bonding layer, and PtW was formed as a continuous heater.
A (Pt + 4 Wt% W) film was formed to a thickness of 0.5 μm, and Ta was continuously formed to a thickness of 0.05 μm as a second heater bonding layer. Then, patterning was performed by fine processing to form a heater (including leads and pads). As an etchant for wet etching in microfabrication, a mixture of sodium hydroxide and hydrogen peroxide was used for Ta and aqua regia was heated to 90 ° C for Pt.

[0015] thereon, after the SiO ₂ and 1.0 [mu] m deposited thickness by sputtering as the insulating film 4, etching the SiO ₂ in HF sensing membrane electrode pad portion of the heater by microfabrication window (4w) drilling after In order to secure electrical continuity and improve wire bondability, Ta of the second heater bonding layer was removed using a mixed solution of sodium hydroxide and hydrogen peroxide. Next, in order to improve the adhesion between the sensing film electrode 5 for measuring the resistance of the gas sensing film (SnO ₂ ) 6 and the underlying second insulating film (SiO ₂ ) 4,
After forming the sensing membrane electrode intermediate layer of 0.05 μm in thickness, Pt for sensing membrane sensing membrane electrode was continuously deposited in 0.2 μm thickness. Further continuously, Ta was deposited to a thickness of 0.025 μm as a second sensing film electrode intermediate layer for improving the adhesion with the SnO ₂ gas sensing film 6. After film formation, a pair of sensing film electrodes were patterned by wet etching by microfabrication similarly to the heater.

After that, in order to secure continuity and improve the wire bonding property by fine processing again, Ta is the second intermediate layer of the sensing film electrode pad (not shown) for wire bonding of the sensing film electrode. Was removed by etching with a mixed solution of sodium hydroxide and hydrogen peroxide to expose Pt on the surface. Pt in the part where the SnO ₂ gas sensing film 6 is laminated
The 0.025 μm intermediate layer Ta remains on the sensing film electrode, and this part has a structure in which Pt is sandwiched by Ta as shown by Ta / Pt / Ta.

Further, a resist is patterned on this, SnO ₂ is formed as a gas sensing film by sputtering,
A gas sensing film 6 was formed by registry lift-off. Sn
The O ₂ film was formed under the conditions of 300 W, 1 Pa, 100 ° C. in Ar + O ₂ , and the film thickness was about 2 μm. Next, a powder in which Pt and Pd catalysts were supported on alumina particles was made into a paste together with a binder, which was applied to the surface of SnO ₂ by screen printing and baked to about 30 μm.
A thick selective combustion layer (also called a catalyst filter) 7 was formed. The selective combustion layer 7 improves the sensitivity, gas species selectivity, and reliability of the thin film gas sensor.

Finally, dry etching is performed from the back surface of the substrate 1 to face the heater and the gas sensing film and the peripheral portion thereof.
Si with a diameter of 0 μm was completely removed (removed portion 1h) to form a diaphragm structure. When patterning the heater (Ta / PtW / Ta) and the sensing film electrode (Ta / Pt / Ta), it is possible to use a lift-off method that uses two metal layers with the overhanging shape as a mask. Good (Our file number 98P00568). Comparative Example For comparison with the thin film gas sensor produced in Example 1,
Regarding the Pt sensing film sensing film electrode, it has a conventional Pt sensing film electrode (first intermediate layer (Ta)) without the second intermediate layer (Ta) between the gas sensing film 6 in Example 1. The thin film gas sensor of 1) was similarly manufactured as a prototype.

In this case, it is not necessary to remove Ta from the portion of the Pt sensing film electrode on the sensing film electrode pad for wire bonding. In order to confirm the effect of the present invention, pulse conduction was performed in the atmosphere for each of the thin film gas sensor of Example 1 and the conventional thin film sensor of Comparative Example. Test condition is 3V
/ 50mW, energization 100msec ON / 1secOFF (heater temperature at energization is 450 ℃), repeat this for 500, 10 and 20 million times, then 2000ppmCH ₄ at 20 ℃, 60% RH ₄ / sensing in air The change in resistance of the film SnO ₂ (sensor temperature 450 ° C) was investigated. The results are shown in Table 1.

[0020]

[Table 1] The following was found from Table 1. The thin film gas sensor of the present invention is
The resistance value of the sensing film SnO ₂ (sensor temperature 450 ° C) in 2000 ppm CH ₄ / air has changed little after 20 million cycles. On the other hand, in the conventional thin-film gas sensor having the sensing film electrode (Ta / Pt), the resistance value of the sensing film SnO ₂ is largely changed, and further, the resistance value is changed by two digits.

As can be seen from the fact that there is almost no change in resistance of the thin-film gas sensor made in the same manner except for the sensing membrane electrode, this is not caused by the sensing membrane SnO ₂ itself, but by the difference in the sensing membrane sensing membrane electrode. is there. The cross section was processed by FIB (neutral ion beam), and the joint between the gas sensing film SnO ₂ and the sensing film electrode of the present invention and the conventional thin film gas sensor was evaluated by the FIB secondary electron image. _No trace of delamination was observed in the cross section of ₂ / Ta / Pt / Ta / SnO ₂ , but the gas sensing film SnO ₂
Conventional thin-film gas sensor (SiO ₂ / T
In the cross section (No9, 10) of (a / Pt / SnO ₂ ), voids that were thought to have arisen from delamination were observed between the Pt sensing membrane electrode and the gas sensing membrane SnO ₂ .

With respect to oxides such as SiO ₂ , oxidizable metals such as Ti, Ta, Cr, etc., are chemically bonded through oxygen of SiO ₂ , and therefore have high adhesion. The metal such as Pt deposited on the metal has a high adhesiveness because the metals are similar to each other. On the other hand, since noble metals such as Pt are difficult to oxidize, they have low adhesion directly to SiO ₂ . When adhesion is required, therefore, the film is formed through a metal that is easily oxidized, such as Ti, Ta, Cr or the like. In the present invention, if an easily oxidizable metal such as Ti, Ta, Cr, etc., an extremely thin oxide layer is formed immediately after the film is formed, and SnO _{2 is} formed on the oxide layer, the adhesion property is improved. Can also be interpreted as improving.

Although Ta was used as the intermediate layer in Example 1, similar results were obtained by using Cr and Ti, respectively. Example 2 A thin film gas sensor according to the present invention, which is different only in the configuration of the sensing film electrode, will be described in detail below along with the manufacturing process thereof, but the description of the part overlapping with Example 1 will be omitted.

Si substrate having a thermal oxide film of 0.3 μm formed on both sides
1 was used, and thereafter, the same process as in Example 1 was performed until Ta removal in the sensing film electrode pad portion. Next, Cr was sputter deposited to a thickness of 0.2 μm. The sputtering conditions were a temperature of 100 ° C., a pressure of 1 Pa and a power of 500 W. After that, Au was continuously sputter deposited to a thickness of 0.1 μm. The film forming conditions are the same as for Cr. Then, the pair of sensing film electrodes for resistance measurement of the gas sensing film SnO ₂ was patterned by wet etching similarly to the formation of the heater. The etchant used was an iodine + ammonium iodide + water system for Au, and a ceric nitrate aqueous solution for Cr. After that, the gas sensing film is processed again by microfabrication.
Only Au on the sensing film electrode where SnO ₂ was laminated was removed by etching to expose Cr.

Further, SnO ₂ was formed as a gas sensing film on the upper part of the film by sputtering by resist lift-off. SnO ₂
The film forming conditions are 300 W, 1 Pa, Ar + O ₂ , and the film forming temperature is 100 ° C., and the film thickness is about 2 μm. Next, Pt (Pd) / alumina powder in which Pt or Pd catalyst is supported on γ-alumina particles is made into a paste together with a binder, which is applied to the surface of SnO ₂ by screen printing and baked to form a selective combustion layer (catalyst filter) of about 30 μm thickness Was formed. The selective combustion layer improves the sensitivity, gas species 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 form a diaphragm structure. Here, when patterning the heater (Ta / PtW / Ta) and the sensing film electrode (Cr), a kind of lift-off method is used with a mask of two kinds of metal layers formed in a shape in which the upper layer overhangs. Good (Our file number 98P0056
8). Comparative Example 2 For comparison, with respect to the sensing film electrode, a thin film gas sensor using the conventional Ta / Pt film sensing film electrode was manufactured by the manufacturing method of Example 2. The different process is only the formation part of the sensing membrane electrode, and is the following process.

After forming Ta with a thickness of 0.05 μm as an intermediate layer for improving the adhesion to the underlying SiO ₂ , Pt with a thickness of 0.2 μm was continuously formed for the sensing film electrode. After that, a pair of sensing film electrodes for resistance measurement of the gas sensing film SnO ₂ was patterned by wet etching similarly to the formation of the heater. The etching solution is the same as the patterning of the heater. In order to confirm the effect of the present invention, pulse energization was performed in the atmosphere described above. The results are shown in Table 2.

[0028]

[Table 2] From Table 2, all five thin film gas sensors according to the present invention are
It can be seen that the resistance value of the gas sensing film SnO ₂ (sensor temperature 450 ° C.) in 2000 ppm CH ₄ / air hardly changed even after repeating, 000,000 times. On the other hand, in the conventional sensing film sensing film electrode (Ta / Pt) element, it can be seen that there is a large change in the resistance value of the gas sensing film SnO ₂ and there is also a thin film gas sensor in which the resistance value changes by two digits. . As can be seen from the fact that there is almost no change in resistance of the thin film gas sensor according to the same manufacturing method except for the sensing film electrode, it is not due to the change of the sensing film SnO ₂ itself, but due to the difference of the sensing film electrode. .

The cross section was processed by FIB (neutral ion beam) and the junction between the sensing film SnO ₂ and the sensing film sensing film electrode of the present invention and the conventional device was evaluated by FIB secondary electron image.
In the device of the present invention, no trace due to peeling was observed in the cross section of SiO ₂ / Cr / SnO ₂ , but the conventional thin film gas sensor (SiO ₂ / Ta / Pt / SnO _{2) in which} the resistance value of the sensing film SnO ₂ was greatly increased. ₂ )
Sensing Membrane Electrode and Gas Sensing Membrane SnO ₂ in Cross Section (No9, 10)
Voids, which were considered to have arisen from the peeling, were partially observed during this period.

In the second embodiment, an example in which Cr is used as the sensing film electrode has been described. However, instead of Cr, Ta, Ti, or Ni-Mo-Fe-Cr-containing at least one of Cr, Ta, and Ti is used. W
The same results as in Example 2 were obtained even when the twisted Hastelloy alloy was used. With these materials the sensing film electrode, Ti relative oxides such as SiO _2, Ta, oxidizable metals such as Cr, in order to produce a strong chemical bond through oxygen SiO _2, In close contact. Furthermore, when the sensing film SnO ₂ is laminated on the sensing film electrode, if it is a metal such as Ti, Ta, or Cr that is easily oxidized, SnO ₂ that is an oxide also has a strong chemical property through oxygen. It creates a physical bond and adheres closely. Also Ti, Ta, Cr
Also in alloys such as Hastelloy containing Ti, Ti, Ta, and Cr atoms were precipitated on a part of the surface, and adhesion was improved by the same effect as described above.

[0031]

According to the present invention, as described above, Ta, Ti, Cr or an alloy containing them as a main component is interposed between the sensing film electrode and the gas sensing film, or the sensing film electrode itself is provided.
Since Ta, Ti, Cr or an alloy containing these as the main components is used, the strong chemical chemistry of SnO ₂ which is a gas sensing film formed on them due to the oxidization of these metals via oxygen. The adhesiveness between the sensing membrane electrode and the gas sensing membrane increases, and even after 20 million times of pulse energization, there is no separation of the boundary between the sensing membrane electrode and the gas sensing membrane. A gas sensor is obtained.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a thin film gas sensor.

[Explanation of symbols]

1 Si substrate 2 First insulating film 2a Thermal oxide film 2b Silicon nitride film 2c Silicon dioxide film 3 thin film heater 4 Second insulating film 5 Sensing membrane electrode 6 Gas sensing membrane 7 Selective combustion membrane

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Ken Matsubara             1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa             Within Fuji Electric Co., Ltd. (72) Inventor Takuya Suzuki             1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa             Within Fuji Electric Co., Ltd. (72) Inventor Shinji Ogino             1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa             Within Fuji Electric Co., Ltd. F-term (reference) 2G046 AA02 BA01 BA04 BA09 BB02                       BB04 BB06 BB08 BC04 BC05                       BE03 DD03 EA02 EA04 EA07                       EA08 EA09 EA11 FB02 FE38                       FE39

Claims (6)

[Claims] 1. A Si substrate having a through hole, a diaphragm made of at least a first insulating film stretched over an opening of the through hole, a sensing film electrode made of at least a heater and Pt at both ends on the diaphragm. In a thin-film gas sensor formed so that a gas sensing film made of SnO ₂ or the like and isolated from each other by a second insulating film made of SiO ₂ or the like, the sensing film electrode is at least Ta, Cr, or Ti. A thin film gas sensor, comprising a first intermediate layer made of a material containing one as a main component on the gas sensing film side. 2. The material is at least one of Ta, Cr and Ti.
The thin film gas sensor according to claim 1, wherein the thin film gas sensor comprises: 3. The thin film gas sensor according to claim 1, wherein the material is a Hastelloy alloy made of Ni—Mo—Fe—Cr—W. 4. A Si substrate having a through hole, a diaphragm made of at least a first insulating film stretched over the opening of the through hole, and SnO having at least a heater and sensing film electrodes at both ends on the diaphragm. _In the thin film gas sensor, wherein the gas sensing film made of ₂ or the like is insulated from each other by the second insulating film made of SiO ₂ or the like, the sensing film electrode comprises at least one of Ta, Cr and Ti. A thin-film gas sensor, which is made of a material having a main component. 5. The material is at least one of Ta, Cr and Ti.
The thin film gas sensor according to claim 4, wherein 6. The thin film gas sensor according to claim 4, wherein the material is a Hastelloy alloy made of Ni—Mo—Fe—Cr—W.