JP3265326B2 - Gas sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power-saving gas sensor such as a thin film and a thick film.

[0002]

2. Description of the Related Art Reducing the power consumption of gas sensors is one of the fundamental issues in this field. In this regard, Japanese Patent Publication No. 4-1301 proposes that a SiO2 film having a thickness of about 1 μm is formed as a bridge on a cavity, and the SiO2 film is used as a substrate of a gas sensor. Then, a Pt heater film is provided on the SiO2 film, which is insulated by the SiO2 film, and a pair of electrodes and a SnO2 thin film are provided on the insulating film. Further, this publication proposes supplying electric power in a pulsed manner to the heater film to pulse-heat the gas sensor.

[0003] Such a gas sensor is disclosed in
Although more than 12 years have passed since the basic proposal of 8750, it has not been put to practical use.
It is presumed that the cause is that the SiO2 film is thermally deformed each time pulse heating is performed, and the heater film, the electrode film, or the metal oxide semiconductor film is peeled off or cracked with this. Therefore, it is necessary to develop a power-saving gas sensor by a method other than using a SiO2 substrate, that is, a method that does not depend on undercut etching of silicon.

[0004]

The objects of the present invention are (1) no deterioration due to thermal deformation of the substrate, (2) suitable for pulse heating drive, (3)
An object of the present invention is to provide a power-saving gas sensor.

[0005]

The gas sensor according to the present invention is a gas sensor having a heater film and a metal oxide semiconductor film provided on an insulating substrate, wherein the substrate has a thickness of 50 to 300 μm and is made of SiO2, ZrO2, MgO.・ SiO2、2MgO ・
A heater film is provided on a substrate using at least one member of the group consisting of SiO2 and TiO2, a metal oxide semiconductor film is stacked on the heater film, and the heater film is pulsed to generate a metal. The oxide semiconductor film is configured to be pulse-heated, an electrode pad is provided on the substrate, a lead wire is connected to the electrode pad, and the substrate is held hollow by the lead wire. Preferably, the thickness of the substrate is 50 to 200 μm, and the material of the substrate is SiO 2 or ZrO 2. For example, an Au wire is used for the lead wire,
For example, a wire having a wire diameter (diameter) of 10 to 30 μm, more preferably 15 to 20 μm is used. The heater film and the electrode film to be used are preferably thin films, and the electrode pads are also preferably thin film pads. The metal oxide semiconductor film may be either a thin film or a thick film.

[0006]

The gas sensor is pulse-heated (pulse drive).
In this case, power consumption is determined by heat radiation to the substrate. The heat radiation to the substrate is determined by the thermal conductivity and the thickness of the substrate. For example, a thin film heater is formed on a 1 mm × 1 mm substrate having a thickness of 100 μm, and an insulating film, a pair of electrodes, and a SnO 2 thin film are formed on the heater film. A heating pulse having a pulse width of 4 mJ and a pulse width of 8 msec was applied to the heater film, and the maximum heating temperature of the metal oxide semiconductor film was measured. The maximum heating temperature is about 470 for a crystalline SiO2 substrate.
° C, about 450 ° C for the ZrO2 substrate. In contrast, the maximum temperature of an Al2O3 substrate having a high thermal conductivity is about 280.
° C. At a maximum heating temperature of 280 ° C., the gas sensor does not operate. On the other hand, at the maximum heating temperature of 450 ° C., CO, hydrogen, alcohol, H 2 S, ammonia, or isobutane can be detected.

The reason that a heating temperature of about 450 ° C. is obtained with SiO 2 or ZrO 2 and a heating temperature of about 280 ° C. is obtained with Al 2 O 3 is due to a difference in thermal conductivity. For example, the thermal conductivity at room temperature is 0.017 J / sec · cm for SiO 2.
・ Deg and ZrO2 are 0.05J / sec ・ cm ・ deg
On the other hand, for Al2O3, 0.3 J / sec.c
m · deg.

[0008] The thickness of the substrate plays a decisive role in the power consumption of the sensor in the case of pulse driving. When the sensor is continuously heated, the temperature of the substrate reaches a steady value, and the problem is not heat absorption by the substrate but heat radiation from the substrate. Therefore, even if the thickness of the substrate changes, only the heat radiation area from the end face of the substrate changes, and the influence on the power consumption is reduced. In the prior art, a thick substrate having a thickness of about 1 mm is used. However, in the case of pulse heating, the substrate temperature drops to about room temperature between pulse heating. For this reason, heat absorption of the substrate during pulse heating becomes a problem. And according to experiments,
The power consumption was proportional to the -1 power or -0.5 power of the substrate thickness. The power consumption is proportional to the negative power of the thickness of the substrate when the temperature distribution in the substrate is linear, which corresponds to the case where the pulse heating time is long. Also, the thickness of the substrate
It is proportional to 0.5 power when the temperature distribution is close to a Gaussian distribution, which corresponds to the case where the pulse width is short. In any case, reducing the thickness of the substrate significantly reduces power consumption.

The inventor has studied the lower limit of the thickness of the substrate which can be practically used. In addition, it was confirmed that a thin substrate could not generally be obtained with a thickness of less than 50 μm. Therefore, the thickness of the substrate is set to 50 μm or more. On the other hand, since power consumption increases with the thickness of the substrate, the upper limit of the thickness of the substrate is set to 300 μm,
Preferably, it is 200 μm.

The types of substrates used are SiO2, ZrO2,
MgO.SiO2, 2MgO.SiO2, or TiO
These are materials that have low thermal conductivity and can be processed into thin plates. Among these, the most preferred substrate material is SiO2 having a low thermal conductivity, and may be crystalline SiO2 or glassy SiO2.
The next preferred material is ZrO2, which has a slightly higher thermal conductivity but is excellent in strength and workability. On the other hand, MgO
SiO2 and 2MgO.SiO2 are inferior in that they have low thermal conductivity, but have low workability, and contain a basic Mg element and may undergo a solid-phase reaction with the metal oxide semiconductor film. TiO2 is excellent in workability and stability,
Among the usable substrates, the thermal conductivity is large.

In the above example, using a 1 mm ² substrate,
It was shown that the gas sensor can be driven with power consumption of 4 mW. That is, if a heating pulse of 4 mJ is applied once per second, the power consumption becomes 4 mJ. When the heater film is further miniaturized,
The power consumption is 1 mW or less. This is approximately the same power consumption as the gas sensor disclosed in Japanese Patent Publication No. 4-1301. That is, according to the present invention, even if a gas sensor is not formed on a SiO2 film having a thickness of about 1 .mu.m formed on a cavity, a sensor having power consumption comparable to this can be obtained.

[0012]

1 and 2 show an embodiment. In the figure,
2 is silica (SiO2), zirconia (ZrO2), titania (TiO2), steatite (MgO.SiO2),
A substrate made of any one of forsterite (2MgO.SiO2) and having a thickness D of 50 μm or more and 300 μm or less, preferably 50 μm or more and 200 μm or less. The width and length L of the substrate 2 are preferably 200 to 800 μm.
m, more preferably 300 to 600 μm. In the present invention, since the substrate 2 is held by the lead wire, the wire bonding pad needs to be about 50 μm. Therefore, the width of the pair of bonding pads is about 100 μm. When a heater film is formed in a portion having a width of 100 μm, the width and length L of the substrate 2 become 200 μm. On the other hand, since the power consumption increases in proportion to the area of the substrate 2, the upper limit of the width or length L is set to 800 μm, more preferably 600 μm.
And When a heater film or the like is provided in an area of 100 μm square,
Since the minimum line width and the minimum gap width when using the comb-shaped electrode are about 10 μm, if the width and the length of the heater film are set to 200 μm or more to avoid this, the width and the length L
Is 300 μm or more. In the embodiment, the thickness D of the substrate 2
Was set to 100 μm, and the width and length L were each set to 500 μm.

Reference numeral 4 denotes a heater film made of RuO 2, and alternatively, a Pt heater film, an IrO 2 heater film, or the like may be used. Numeral 6 denotes an insulating film, for example, using a SiO2 film or a SiO2 film, shielding the RuO2 heater film 4 from the atmosphere to enhance the stability, and forming a metal oxide semiconductor film on the heater film 4 into the heater film 4.
Used to separate from Reference numerals 8 and 8 denote a pair of comb-shaped electrodes, which may be simple parallel electrodes, but are comb-shaped electrodes to obtain a low-resistance gas sensor. Reference numeral 10 denotes a metal oxide semiconductor such as a SnO2 film, which is used as a thin film having a thickness of 1 μm or less or a thin film having a thickness of about 1 to 30 μm. Reference numeral 12 denotes a wire bonding pad, and reference numeral 14 denotes a lead wire. Here, an Au wire having a wire diameter of 10 to 30 μm is used, preferably an Au wire having a wire diameter of 15 to 20 μm. The smaller the diameter of the lead wire 14, the smaller the heat loss, and at the same time, the more difficult the bonding becomes. Therefore, the lower limit of the wire diameter is set to 10 from the limit of wire drawing.
μm, and the lower limit was set to 15 μm from the bonding performance. The bonding pad 12 is actually a part of the electrode 8. The lead wire 12 is welded to a stem (not shown) or the like to hold the substrate 2 hollow. The substrate 2 and the heater film 4
Insulation glass film (film thickness, for example, 10 to 20 μm)
May be provided.

FIG. 2 shows an arrangement of the substrate 2 excluding the metal oxide semiconductor film 10 and the insulating film 6. An insulating film 6 is arranged on the heater film 4, and a pair of comb-shaped electrodes 8, 8
Is provided, and the resistance value of the metal oxide semiconductor film 10 is detected. An electrode of the same material as the electrode 8 is also connected to the heater film 4,
The pad 12 on the left side of the figure is a common pad.

The following test was conducted to examine the relationship between the material of the substrate 2 and power consumption. Width and length L as substrate 2
Is a crystalline SiO 2 having a thickness of 500 μm and a thickness of 100 μm.
2, SiO2 glass, ZrO2, 2MgO.SiO2, Mg
O.SiO2, TiO2 and Al2O3 were used. A heater film 4 made of RuO2 having a thickness of 0.2 .mu.m was formed on the substrate 2 by sputtering, and a SiO2 insulating film 6 having a thickness of 0.3 .mu.m was formed thereon by sputtering. The area of the heater film 4 is 300 μm × 300 μm. Insulating film 6
On the upper side, the tooth-shaped electrodes 8 and 8 of Au were formed by sputtering, and the minimum line width and the minimum line interval were each set to 20 μm. Next, a SnO2 film 10 having a thickness of 0.3 μm was formed by sputtering. The lead portions of the electrodes 8, 8 were used as wire bonding pads 12, and an Au wire having a wire diameter of 18 μm was wire-bonded.

Since the temperature of the heater film 4 of 300 μm square cannot be measured by a radiation thermometer, a sensor having a substrate 2 of 1 mm square, a thickness of 100 μm and a similar area only quadrupled was prepared. A heating pulse having a power of 4 mJ and a pulse width of 8 msec was applied once per second to this sensor, and the maximum temperature of the metal oxide semiconductor film 10 was measured with a radiation thermometer.
Table 1 shows the maximum heating temperature.

[0017]

Table 1 Maximum heating temperature Substrate 2 Maximum heating temperature (° C.) Thermal conductivity (25 ° C.) (J / sec · cm · deg) Crystalline SiO 2 471 0.017 SiO 2 glass 522 0.012 ZrO 2 452 0.05 2 MgO · SiO2 463 0.03 MgO · SiO2 458 0.025 TiO2 418 0.07 Al2O3 * 277 0.3 * The substrate 2 is 1 mm x 1 mm square, the thickness D is 100 µm,
The heater film 4 is 600 μm × 600 μm, and Al 2 O 3 is a comparative example.

As is clear from Table 1, the maximum temperature of the Al2O3 substrate is only about 280.degree. C., and the maximum temperature of about 420.degree. C. can be obtained even with TiO2 having a large thermal conductivity in the embodiment, and ZrO2, SiO2, SiO2, MgO
With SiO2, a heating temperature of 450 ° C. or higher can be obtained. Next, when examining the type of the substrate, ZrO2 has high workability, and no problem occurs even if the substrate 2 is scribed at a side of 200 μm square. In the case of crystalline SiO2 or SiO2 glass, no problem occurred even if the substrate 2 was scribed at a side of 300 μm square. On the other hand, 2MgO.SiO2 or MgO.SiO2 has poor workability,
When scribed to a μm square, cracks and chips occurred at a frequency of about several percent. On the other hand, the maximum heating temperature of the TiO 2 substrate was 420 ° C., which was 30 ° C. or more lower than the others. From these, the most preferable material of the substrate 2 is crystalline SiO 2
2 or SiO2 glass, then ZrO2.

The thickness of the substrate 2 is set to 150 μm, 100 μm,
The maximum heating temperature was measured in exactly the same manner as in the previous test example, except that the three types were 60 μm. Table 2 shows the results.

[0020]

[Table 2] Substrate thickness Substrate 150 μm 100 μm 60 μm crystalline SiO 2 396 471 531 ZrO 2 412 452 503 Al 2 O 3 203 277... * The 60 μm thick Al 2 O 3 substrate could not be cut.

As is apparent from Table 2, the maximum temperature increases as the thickness of the substrate 2 decreases. From the numerical calculation relating to heat conduction, the heat absorption of the substrate 2 is proportional to the thickness of the substrate when the temperature distribution in the substrate 2 is linear, and the heat absorption is thick when the temperature distribution in the substrate 2 is Gaussian. It was proportional to the route. The results in Table 2 do not agree with this
It is considered that there is heat conduction from the substrate and radiation from the front and back of the substrate 2. The thickness of the substrate 2 that can be processed is about 50 μm, so the lower limit of the thickness D of the substrate 2 was set to 50 μm. Further, since the power consumption increases with the thickness D of the substrate 2, the upper limit of the thickness D is set to 300 μm, preferably 200 μm.
μm.

A gas sensor having a thickness D of the substrate 2 of 100 μm and a width and length L of 500 μm was manufactured, and a heating pulse having a pulse width of 8 msec and a power of 1 mJ was applied once per second to measure gas sensitivity. As a measurement gas, 1000 ppm of isobutane was used as a gas requiring detection at a high temperature, and 3 ppm of H2S was used as a gas capable of detection at a low temperature.
In the case of isobutane 1000 ppm, the resistance at 8 msec from the start of pulse heating was detected, and in the case of H2S, the resistance at 6 msec was measured. The results are shown in Table 3 using the ratio of the resistance value in air to the resistance value in gas as sensitivity.

[0023]

Table 3 Gas Sensitivity Substrate Isobutane 1000 ppm H2S 3 ppm Crystalline SiO2 3.2 8.4 SiO2 glass 3.5 9.0 ZrO2 3.1 7.6 Al2O3 * No sensitivity 3.0 * Al2O3 is a comparative example.

As is evident from Table 3, crystalline SiO 2
If SiO2 glass or ZrO2 is used as the material of the substrate 2, gas can be detected even with a power consumption of 1 mW. When a substrate such as SiO2 or ZrO2 is used, the sensor can be driven with substantially the same power consumption as when a SiO2 film having a thickness of about 1 μm provided on the cavity is used. This is considered to be because when the SiO2 film is used as the substrate, heat flows from the base side of the film to the substrate even though the thermal conductivity and heat capacity of the SiO2 itself are small. In the gas sensor of the embodiment, the heater film 4 can be further miniaturized, and the power consumption can be further reduced.
In the embodiment, pulse heating is shown, but the sensor may be used in any manner, and continuous heating may be used.

[0025]

According to the present invention, a power-saving gas sensor can be obtained without using undercut etching of a silicon substrate. Therefore, there is no problem of thermal deformation of the substrate, the gas sensor is suitable for pulse driving, and the power consumption is as small as about 1 mW.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a substrate of a gas sensor according to an embodiment.

FIG. 2 is a plan view showing an electrode arrangement of the gas sensor according to the embodiment.

[Explanation of symbols]

 2 Substrate 4 RuO2 heater film 6 SiO2 insulating film 8 Au electrode 10 Metal oxide semiconductor film 12 Bump 14 Lead wire

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-4-80647 (JP, A) JP-A-2-82150 (JP, A) JP-A-5-126788 (JP, A) JP-A-3- 123845 (JP, A) JP-A-4-127047 (JP, A) JP-A-3-233350 (JP, A) JP-A 4-361148 (JP, A) JP-A-1-316652 (JP, A) JP-A-54-121794 (JP, A) JP-A-59-19622 (JP, U) JP-A-63-18856 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 27/12

Claims (2)

(57) [Claims] 1. A gas sensor in which a heater film and a metal oxide semiconductor film are provided on an insulating substrate, wherein the substrate has a thickness of 50 to 300 μm and a material of Si.
O2, ZrO2, MgO.SiO2, 2MgO.SiO2,
A heater film is provided on a substrate using at least one member of the group consisting of TiO2, a metal oxide semiconductor film is stacked on the heater film, and the heater film is heated in a pulsed manner to generate a metal oxide. A gas sensor, wherein a pulse is applied to a semiconductor film, an electrode pad is provided on a substrate, a lead wire is connected to the electrode pad, and the substrate is held hollow by the lead wire. 2. The gas sensor according to claim 1, wherein the thickness of the substrate is 50 to 200 μm, and the material of the substrate is at least one member of a group consisting of SiO 2 and ZrO 2.