JPH0792123A - Gas sensor - Google Patents

Abstract

PURPOSE:To obtain a small gas sensor of a low power consumption by roughing the surface of a heat-resistant dielectric substrate mounting a thick film gas- sensitive body and enhancing adhesion of the gas-sensitive body to the substrate thereby facilitating microfabrication of an electrode. CONSTITUTION:A thick film of SnO2 is employed as the gas-sensitive body and a thick film of Al2O3-SnO2-Pd is employed as the filter film. An alumina substrate 2 is subjected to ion milling in Ar atmosphere thus providing a roughed part 3. A film is formed by thin plate process from a heater electrode 4 to an interdigital electrode 10. The electrodes 4, 10 employ a substrate formed by sequentially laminating an active metal such as Ti, and Pt and gold. An interlayer insulation film 8 insulates the SnO2 film 12 and the heater 6 and isolates the heater 6 from the atmosphere thus enhancing the stability. The roughed part 3 underlies the heater electrode 4 and a unroughened region surrounded by a groove protects the electrode 10 against opening. The roughed part 3 enhances adhesion of the SnO2 film 12 and the filter film 14 and formed under or around the SnO2 film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor in which a thick film gas sensor and a finely processed electrode are mixed.

[0002]

2. Description of the Related Art The inventors examined a gas sensor in which a thin film and a thick film are mixed. The purpose is to miniaturize the gas sensor, and the electrodes of the gas sensitive body, the electrodes of the heater, the heater, the interlayer insulating film, etc. are formed by a thin film process, and the gas sensitive body and the filter film are formed by a thick film process. The most miniaturized gas sensor is the electrode connected to the gas sensor.
The gas sensitive body and its filter may have a size of, for example, about 100 μm square, and may be a thick film. The properties of the gas sensor and the filter depend on the film thickness, and a thick film is preferable when detecting a gas such as methane or propane. Also, in the case of other gases, the thick film is less likely to be poisoned than the thin film and the relative sensitivity can be easily adjusted. For these reasons, a gas sensor in which a thin film electrode and a thick film gas sensor are mixed is needed.

The smoothness required for the substrate surface differs between the thin film and the thick film, and it is necessary to use a mirror-finished substrate for the thin film. This is because, firstly, a mirror-like substrate is required for the alignment of the mask in forming the thin film, and secondly, it is intended to prevent the thin film electrode from breaking due to the difference in the substrate surface.
However, if a mirror-like substrate is used, the adhesive force of the gas sensor and the filter film will decrease.For example, tape peeling test (when tape is attached to the surface of the completed gas sensor and the tape film is peeled off, the filter film and gas sensor will adhere to the tape. It was found that almost all the gas sensors were damaged. For this reason, it has been impossible to reduce the size of the gas sensor by mixing the thin film and the thick film and to obtain the performance of the thick film gas sensor with the thick film.

Such a problem is not limited to the combination of the thin film electrode and the thick film gas sensor, but also occurs in the combination of the thick film electrode and the thick film gas sensor. For example, when a thick electrode having a film thickness of about several μm is printed and is etched using a resist to form a comb tooth electrode having a line width of 10 μm or less, it is necessary to align a mask for resist exposure. And this can be realized only with a mirror substrate.

[0005]

An object of the present invention is to provide a gas sensor in which a microfabricated electrode and a thick film gas sensor are combined, 1) enhancing the adhesion of the gas sensor to a substrate, and 2) gas sensor. It facilitates microfabrication of the body's electrodes, and 3) makes it a gas sensor that is small, consumes less power, and is easy to handle. 4) Uses a thick film gas sensor to detect flame-retardant gases such as methane and propane. It is to obtain a gas sensor that is easy to perform and has high reliability even when detecting other gases.

[0006]

The structure and function of the present invention is as follows.
In a gas sensor provided with a thick film gas sensor and an electrode connected to the gas sensor, at least the base of the sensor is roughened. Preferably, the base of the electrode is made a smooth surface without roughening to prevent electrode breakage.
Still more preferably, the base of the gas sensitive body and its surroundings are left as a smooth surface to facilitate the alignment of the mask used when forming the electrodes and the like. More preferably, the roughened area is provided with portions having different degrees of roughening. In this case, a difference occurs between the roughened portion and the non-roughened portion. Similarly, if the degree of roughening is changed, a step is created at the boundary between the deeply roughened portion and the shallowly roughened portion, and the gap between them is used to improve the adhesive force of the gas sensitive body. . The electrode of the gas sensitive body is preferably a thin film, but is not limited to this.

[0007]

EXAMPLE As shown in FIGS. 1 to 9, a thick film of SnO2 was used as a gas sensor and Al2O3-SnO2-Pd was used as a filter film.
An example will be described by taking as an example the one using the thick film. The structure of the gas sensor is shown in FIG. 7, 2 is an alumina substrate, and ZrO
2, other heat-resistant insulating substrates such as mullite, spinel, AlN, and quartz glass may be used. In the embodiment, 99% alumina (the surface of which is mirror-polished) is used, and the roughened portion 3 is provided by ion milling in Ar atmosphere. I used the one. 4 is a heater electrode, 6 is a RuO2 heater, 8 is an interlayer insulating film made of SiO2 film, 10 is a comb tooth electrode, 12 is a SnO2 film, 14
Is a filter membrane. The heater electrode 4 to the comb-teeth electrode 10 are formed by a thin film process (here, sputtering), and the film thickness is 0.5 μm for the interlayer insulating film 8 and 0.3 μm for the others.
Is. Further, the electrodes 4 and 10 increase the adhesiveness to the base, so that the first layer of an active metal such as Ti, W, Mo, Cr or the like (Ti in the embodiment), the second layer of Pt, and the gold layer which is easy to bond are used. A laminate of the third layers is used, and the film thickness is the total film thickness thereof. The electrodes 4 and 10 are not limited to gold-based electrodes, and may be thick-film electrodes printed and finely processed by resist development and etching. Interlayer insulation film 8
Has the function of insulating the SnO2 film 12 from the heater 6 and also insulating the heater 6 from the atmosphere to improve the stability. Sn
The O2 film 12 and the filter film 14 are thick films formed by screen printing, and each has a film thickness of 5 to 30 .mu.m. The materials thereof are arbitrary, and the filter film 14 may be omitted. In this specification, a thin film means a film having a film thickness of 1 μm or less, and a thick film means a film having a film thickness of 5 μm or more.

An example of dimensions from the heater electrode 4 to the filter film 14 is shown. The heater electrode 4 has a minimum line width of 30 μm.
The gap G between the electrodes 4 and 4 is 250 μm, and the electrode length L is 3
It is 50 μm. The heater 6 is approximately 350 μm square, and the comb-teeth electrode 10 has a minimum line width and a gap between the electrodes 10 and 10 of 20 μm. SnO2 film 12 and filter film 14
Are both 250 μm × 350 μm, and the relative positions on the substrate 2 are the same. As is clear from the above, it is the comb-teeth electrode 10 that requires the finest processing, and the other films may be thick films. Further, it is not necessary to stack the SnO2 film 12 on the heater 6, and the heater electrode 4 and the heater 6 and the comb tooth electrode 10 and the SnO2 film 12 are separately arranged in parallel.
The SnO2 film 12 may be indirectly heated by the heater 6 arranged next to it. In this case, the thin film process is required only for the comb-teeth electrode 12.

The roughened portion 3 is located below the heater electrode 4,
The pattern is shown in FIG. The roughened portion 3 has a width of 10 μ, for example.
m grooves 15 are regularly arranged, and the gap between the grooves 15 is also 10 μm wide. These line widths and gaps may be values within a range where patterning with a resist is easy. Reference numeral 16 denotes an electrode passage, which is located at the neck position of the heater electrode 4 and the comb-teeth electrode 10, and the electrodes 4, 4, 10, 1.
It was provided in order to prevent 0 from breaking in the groove 15. Further, the non-roughened area 17 surrounded by the groove 15 is not roughened,
The comb-teeth electrode 10 was prevented from breaking. Of course, the groove 15 may be provided in the non-roughened area 17 so as to avoid the pattern of the comb tooth electrodes 10, 10. In addition, the roughened portion is provided with the groove 1.
There is no particular meaning to the formation of No. 5, and for example, a grid-like roughened portion 20 as shown in FIG. 9 may be used.

The roughened portion 3 is for enhancing the adhesion strength of the SnO 2 film 12 and the filter film 14, and is provided on the base of the SnO 2 film or at its widest, for example, from the periphery of the SnO 2 film.
It is provided within the range of 0 μm. The roughening of the entire surface of the substrate 2 makes it difficult to align the mask necessary for the thin film process and reduces the accuracy of the pattern. Next, the roughening unit 3
In order to avoid the comb-teeth electrodes 10 and 10, it is preferable to have a pattern as shown in FIG. Under the SnO 2 film 12, a step is formed between the roughened portion (groove 15) and the non-roughened portion, and this step improves the adhesion strength of the SnO 2 film 12 and the filter film 14. In order to effectively use such steps, a large number of steps are generated, which are the roughened portions 3 and 20 of FIGS. 8 and 9. Here, the groove 15 is roughened and the surface other than the groove 15 is not roughened. However, the thickness of the mask is changed to change the degree of roughening between the groove 15 and other portions, and the groove 15 is deepened. The surface may be roughened and the other portions may be roughened shallowly. The meaning of roughening is to increase the adhesion strength of the SnO2 film 12 and the filter film 14, so that the unevenness generated on the substrate 2 becomes unevenness of the interlayer insulating film 8 and becomes unevenness of the base of the SnO2 film 12. As is clear from this, not the substrate 2 but the interlayer insulating film 8 may be roughened by the pattern of FIG. 8, for example.

An example of manufacturing the gas sensor will be shown. As the substrate 2, alumina (99% alumina), ZrO 2, and AlN, all of which were mirror-polished, were used, and ion milling was performed in an Ar atmosphere (Ar flow rate 22 cc / min, acceleration power supply 260 Watt). For the milling, a resist is printed, exposed, and developed, and then shown in FIG.
Two types were used: one that was milled with the pattern No. 2 (partial milling) and one that was milled over the entire surface of the substrate 2 without a mask (full-surface milling). In the full-face milling, the marker for mask alignment could not be read due to the unevenness formed on the substrate 2, which made mask alignment difficult. After roughening the surface, the heater electrode 4, the heater 6, the interlayer insulating film 8 and the comb-teeth electrode 10 are laminated by sputtering, and SnO is formed by screen printing.
Two films 12 (film thickness 10 μm) are provided and fired at 650 ° C., and filter film 14 (film thickness 10 μm, alumina 70 wt% -S
nO2 20 wt% -Pd 10 wt%) was screen-printed and baked at 650 ° C.

Regarding the gas detection characteristics of the gas sensor, only those using an Al 2 O 3 substrate (milling time 1 hour) were measured. As a comparative example regarding the gas sensitivity, the heater electrodes 4 to 10 manufactured by a thick film process (film thickness of 10 μm each) are used, and since the comb-teeth electrode 10 cannot be formed with a thick film, it is similar to the heater electrode 4. The pattern was an electrode of SnO2 film. However, since electrodes 4 and 10 are thick films, both are 1
It was created by the 00 μm rule. The gas sensitivity does not change in the examples and the comparative examples, and the resistance value of the SnO2 film 12 (electrodes 10, 10
Resistance value between the comb-teeth electrodes 10 is about 1/1 of that of the comparative example.
It dropped to 00. It should be noted that within the scope of the inventors' experiments, SnO
It was not possible to develop methane sensitivity with the sputtered film of 2.

1 and 2, after milling for 1 hour,
The surface of the alumina substrate 2 is shown. FIG. 1 was taken at an angle of 60 degrees, and there is a level difference of about 0.6 μm inside and outside the groove 15, and the portion masked with the resist is not milled and the initial substrate surface remains. In FIG. 2, the roughening unit 3
Shows the whole picture of. 8 pattern (groove 15 and electrode passage 1
6) is generated as it is on the substrate 2, and the first substrate surface remains as it is in the non-roughened region 17 surrounded by the electrode passage 16 and the groove 15. FIG. 3 shows the surface (× 10,000 times) of the specular alumina substrate 2 before roughening.

4 to 6 show the surface roughness of the substrate (milling time: 1 hour, alumina substrate). The surface roughness was measured by a surface roughness meter and magnified 70,000 times in the vertical direction. displayed. In FIG. 4 (partial milling), the inside and outside of the groove 15 is about 0.
6 μm step is generated and maximum surface roughness Rmax is about 0.8 μm
Is. In FIG. 5 (entire milling), the average surface roughness Ra
Is 0.03 μm, and the maximum surface roughness Rmax is about 0.3 μm. In FIG. 6 (mirror surface substrate before milling), Ra is 0.0.
2 μm and Rmax is about 0.07 μm. SnO2 film 12
The increase in the adhesive force of the filter film 14 is as follows: Ra is slightly increased from about 0.02 μm to about 0.03 μm.
It occurs when max increases from about 0.07 μm to about 0.3 μm. Secondly, the increase in the adhesive force occurs at a step difference of about 0.6 μm inside and outside the groove 15.

The adhesion of the SnO2 film 12 and the filter film 14 was tested on these substrates. A tape peeling test was used for the test, and cellophane tape was attached to the gas sensor after firing, and the peeling state of the films 12 and 14 when the tape was peeled off was checked. The results are shown in Table 1. Table 1 shows the heater electrode 4
-Sn is directly formed on the substrate 2 by omitting the film formation of the comb-teeth electrode 10.
The results obtained when the O2 film 12 and the filter film 14 are formed are shown. When the heater electrode 4 to the comb-teeth electrode 10 were provided on the base of the SnO2 film 12, the number of peeled-offs was reduced to about 1/2.

[0016]

[Table 1] Table 1 Adhesion to Milling and Substrate Substrate Material Milling Conditions Adhesion (Peeling Number) Mask Alignment No Al2O3 Mirror Surface 150/560 Good Same as 15 minutes 10/560 Good Same as 30 minutes to 0/560 Good Same part 1 hour to 10/560 Good Good part 2 hours to 10/560 Good Same whole surface 1 hour to 40/560 Bad ZrO2 None Mirror surface Almost all good 30 minutes to 50/560 Good same part 1 hour to 30/560 Good part 2 hours to 30/560 Good part 3 hours to 40/560 Good AlN no mirror surface Almost all good parts 30 minutes to 80/560 Good part 1 hour to 60/560 Good parts 2 hours to 60 / 560 Good * Partial milling is performed by partially milling using the mask in FIG. 8. Full-face milling is performed without the mask. * The number of adhesion is determined from the adhesion status of the powder to the tape after printing and baking the gas sensitive film 12 and the filter film 14 on the substrate, and then peeling off with a cellophane tape. Shows the number of pieces of the filter that have been damaged beyond the root of the filter film 14 peeled off. * After forming the heater electrode 4 to the comb tooth electrode 10 in the thin film process, the gas sensitive film 12 and the filter film 14 are formed. When the film is formed and the same test is performed, the number of damages is reduced to about 1/2.

As is clear from Table 1, the partial milling of the alumina substrate has the highest adhesion of the SnO2 film and the filter film 14, no problem occurs in the mask alignment, and the disconnection of the electrode 10 does not occur. ZrO2 substrate and AlN
Also in the substrate, the adhesion of the SnO2 film 12 and the filter film 14 is increased by milling, but the adhesion is inferior to that of the alumina substrate.
Therefore, the most preferable substrate 2 is an alumina substrate. Further, it is preferable to carry out partial milling so as not to cause disconnection of the comb-teeth electrode 10 and to make mask alignment difficult. Further, the influence of the milling time was small, and for example, in the case of the example, the influence of the milling time was not obvious when milling for 30 minutes or more. The method of roughening is arbitrary, but a method of etching the substrate 2 or the interlayer insulating film 8 is preferable, for example, reverse sputtering may be used, and an insulating film is attached by CVD or the like and the surface roughness of this film is used. However, the adhesive strength of the deposited insulating film becomes a problem, which is not preferable.

[0018]

The present invention provides a gas sensor using a fine electrode and a thick film gas sensor in combination, 1) enhancing the adhesion of the gas sensor to the substrate, and 2) fine processing of the electrode of the gas sensor. 3) This makes it a gas sensor that is small, consumes less power, and is easy to handle. 4) Uses a thick-film gas sensor to facilitate detection of flame-retardant gases such as methane and propane. It is possible to obtain a highly reliable gas sensor when detecting gases other than the above (claims 1 to 3). According to the second aspect of the invention, the surface of the electrode is roughened except for the underlying layer so that the electrode is not broken due to a step generated by the roughening. According to the invention of claim 3, even in the roughened area, the degree of roughening is changed depending on the location, and the step between the portion where the roughening is performed and the portion where the roughening is not performed is used to increase the adhesive force of the gas sensitive body. It can be further increased.

[Brief description of drawings]

FIG. 1 is a 3,500 times electron micrograph showing the surface structure of an alumina substrate after ion milling for 1 hour.

FIG. 2 is a 350 × electron micrograph showing the surface structure of an alumina substrate after ion milling for 1 hour.

FIG. 3 is a 10,000 times electron micrograph showing the surface structure of a mirror-like alumina substrate before HF treatment.

FIG. 4 is a characteristic diagram showing the surface roughness of the alumina substrate after the partial milling of FIG.

FIG. 5 is a characteristic diagram showing the surface roughness of the alumina substrate after the entire surface is milled.

FIG. 6 is a characteristic diagram showing the surface roughness of a mirror-like alumina substrate before milling.

FIG. 7 is a perspective view showing a disassembled state of the gas sensor of the embodiment.

FIG. 8 is a plan view showing a pattern of partial milling.

FIG. 9 is a plan view showing a milling pattern of a modified example.

[Explanation of symbols]

 2 Alumina substrate 3 Roughened part 4 Heater electrode 6 RuO2 heater 8 Interlayer insulation film 10 Comb tooth electrode 12 SnO2 film 14 Filter film 15 Groove 16 Electrode passage 17 Non-roughened region 20 Roughened part

Claims (3)

[Claims] 1. A gas sensor provided with a thick film gas sensor and an electrode connected to the sensor on a heat-resistant insulating substrate, characterized in that at least the underlying region of the sensor is roughened. Yes, a gas sensor. 2. The gas sensor according to claim 1, wherein the roughened region is a region at least a base of the sensitive body excluding a base of the electrode. 3. The gas sensor according to claim 1, wherein the roughened region is provided with portions having different degrees of roughening.