JP4851610B2 - Thin film gas sensor - Google Patents

Description

  The present invention relates to a low power consumption thin film gas sensor with battery driving in mind.

In general, a gas sensor is a device that is used for applications such as a gas leak alarm and is selectively sensitive to a specific gas, such as CO, CH ₄ , C ₃ H ₈ , CH ₃ OH, etc. High sensitivity, high selectivity, high response, high reliability, and low power consumption are indispensable.
By the way, 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, and those for the purpose of detecting incomplete combustion gases in combustion equipment, or However, the penetration rate is not so high due to cost and installation problems. For this reason, in order to improve the penetration 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 driving. However, in a catalytic combustion type or semiconductor type gas sensor, it is necessary to detect by heating to a high temperature of 100 ° C. to 500 ° C. Thus, in the conventional method in which a powder such as SnO ₂ is sintered, there is a limit to reducing the thickness using a method such as screen printing, and the heat capacity is too large to be used for battery driving. Therefore, the realization of a thin film gas sensor in which a heater and a sensing film are formed with a thin film of 1 μm or less and a low heat capacity structure such as a diaphragm structure by a fine processing process has been awaited. As a result, a thin film gas sensor having a low heat capacity structure is proposed in Patent Document 1, for example.

FIG. 3 shows a cross-sectional structure of a thin film gas sensor which is basically the same as that of Patent Document 1.
This is produced, for example, as follows.
First, on a silicon (Si) wafer 1 having a thermal oxide film on both sides, a Si ₃ N ₄ film and a SiO ₂ film are sequentially formed as a support layer having a diaphragm structure and a thermal insulating layer 2 by a CVD method. Next, the heater layer 3 and the SiO ₂ insulating layer 4 are formed in this order by sputtering. A bonding layer 5, a sensing layer electrode 6, and a sensing layer 7 are formed thereon. Film formation is performed by an ordinary sputtering method using an RF magnetron sputtering apparatus. The film formation conditions are the same for both the bonding layer (Ta or Ti) 5 and the sensing layer electrode (Pt or Au) 6, Ar gas pressure 1 Pa, substrate temperature 300 ° C., RF power 2 W / cm ² , film thickness is bonding layer 5 / The sensing layer electrode 6 = 500/2000 mm.

Next, a sensing layer 7 is formed, and a selective combustion layer 8 in which a catalyst is supported on a porous metal oxide such as Al ₂ O ₃ is applied on the sensing layer 7 by a screen printing method at 500 ° C. Baking for 1 hour or more. It is desirable that the selective combustion layer 8 has a sufficient size to cover the sensing layer 7 and a thickness after firing of about 20 to 30 μm. Finally, silicon is removed from the back surface of the silicon wafer 1 by etching to form a diaphragm structure.

  In order to guarantee the life of the thin film gas sensor having the above structure for 5 years or longer without replacement of the battery, intermittent driving of the heater 3 is essential. When used as a CO sensor for CO detection, detection is required once every 150 seconds, and in order to desorb moisture and other adsorbates adhering to the surface of the sensing layer 7 during the off time, the sensing layer In order to improve the temporal stability, it is important to clean the surface of No. 7 once. Therefore, in the thin film CO sensor, the temperature of the heater 3 is once heated to 400 to 500 ° C. for 50 to 200 msec for the purpose of cleaning before CO detection, and immediately after that, the temperature is about 200 to about 100 ° C. which is the CO detection temperature. CO detection is performed by a temperature pattern of holding for 1000 msec.

When such a thin-film gas sensor is used as a CO sensor for CO detection, it not only has good sensitivity to CO, but also guarantees selectivity for different gases such as CH ₄ and H ₂ and a long life of the sensor. Various requirements such as durability must be satisfied.
Here, the CO sensitivity (CO concentration gradient) is, for example, the sensor resistance value when CO = 300 ppm and CO = 500 ppm, and the CH ₄ selectivity is the ratio of the sensor resistance value when, for example, CH ₄ = 4000 ppm and CO = 100 ppm. , H ₂ selectivity is defined by, for example, a ratio of sensor resistance values when H ₂ = 1000 ppm and CO = 100 ppm.

Then, in order to improve gas selectivity, the applicant has proposed what is shown in Patent Document 2. This is a combination of SnO ₂ layer doped with pentavalent or hexavalent element serving as a donor (first layer), SnO ₂ layer doped with catalyst (second layer).

JP 2000-298108 A (page 4, FIG. 1) JP 2000-292399 A (page 4-5, FIG. 1)

However, there is still room for improvement in the device described in Patent Document 2.
Accordingly, an object of the present invention is to further improve gas selectivity in a thin film gas sensor.

In order to solve such a problem, in the invention of claim 1, the outer periphery or both ends of the thin film-like support film are supported by the Si substrate, and the outer periphery or both ends are thick and the central portion is thin. A thin-film heater is formed on the support substrate, and the thin-film heater layer is covered with an electrical insulating film, and a SnO ₂ layer to which a + 5-valent or + 6-valent element serving as a donor is added as a first layer thereon, and A gas sensing layer having a two-layer structure in which an SnO ₂ layer added with an element Pt as a catalyst is laminated as a second layer is formed thereon, and a sensing electrode made of a pair of noble metals in contact with the gas sensing layer at a predetermined interval. In a thin film gas sensor comprising a layer,
The added amount of the catalyst Pt of the second layer forming the gas sensing layer is 11 wt% or more and 60 wt% or less, and the ratio of the sensor resistance value at CH4 = 4000 ppm and CO = 100 ppm is 1 or more with respect to methane gas. It is characterized by improved selectivity .

  According to the present invention, in a thin film gas sensor having a two-layer structure, it is possible to improve the gas selectivity that has been insufficient in the past by optimizing the concentration of the catalyst added to the second layer.

Sectional drawing which shows embodiment of this invention Explanatory drawing of concentration distribution in the depth direction from the film surface of Pt Sectional view showing a conventional example

FIG. 1 is a block diagram showing an embodiment of the present invention.
As is apparent from FIG. 1, the steps up to the formation of the sensing layer electrode 6 are the same as those of the conventional example, and therefore, different points will be described here. Reference numeral 8 denotes a selective combustion layer.
As the target sensing layer 7, SnO ₂ having 0.5 weight percent (wt%) of Sb is used for the donor-doped first layer 9, and SnO ₂ having 16 wt% of Pt is used for the catalyst-doped second layer 10. ₂ is used.
The film forming conditions are Ar + O ₂ gas pressure 2 Pa, substrate temperature 150 to 300 ° C., RF power 2 W / cm ² , and film thicknesses of both the first and second films are 400 nm. Further, finally, Si from the diaphragm portion was removed by etching from the back surface of the substrate using a dry etcher to obtain a diaphragm structure.

Table 1 shows the conditions for forming the sensing layer in the conventional example and the example.
The first layer of the sensing layer is a SnO ₂ film to which, for example, Sb is added as a donor, and the film thickness is 400 nm. This is the same in the conventional example and the example. For the second layer, the added catalyst is Pt, which is 40 nm in the conventional example, 40 nm in the example, or 400 nm. The value of the second layer catalyst concentration in the formation conditions in the table is determined by measuring the concentration of the catalyst element at a deep position of 30 nm or more in the thickness direction from the outermost surface of the second layer with an X-ray photoelectron spectrometer (ESCA). It is the value converted into weight% from the result. FIG. 2 shows the Pt concentration distribution in the film thickness direction in the conventional example and the example. From the figure, it can be seen that the concentration becomes stable when the depth from the outermost surface of the second layer is about 30 nm or more.

Table 2 shows the characteristics of the gas sensor under each condition by taking up one sample having an average characteristic among the samples produced under each forming condition. The characteristics shown here are the results of detection using the temperature pattern described in the preceding paragraph [0006]. As characteristics, CO concentration dependence, hydrogen selectivity and methane selectivity were compared.

The CO concentration gradient (CO concentration dependency) is defined by the ratio of the sensor resistance value when CO = 300 ppm and CO = 500 ppm, and the methane selectivity is the sensor resistance value when CH ₄ = 4000 ppm and CO = 100 ppm. The hydrogen selectivity is defined by the ratio of the sensor resistance value when H ₂ = 1000 ppm and CO = 100 ppm. A large value of the CO concentration dependency means high sensitivity, and at the same time means that the fluctuation range of the detected concentration becomes small with respect to the variation of the sensor resistance value with time, which is desirable for the gas leak alarm. The gas selectivity is an index for selectively detecting only the target gas, and when it is mounted on the gas leak alarm, the value is at least “1” in order to prevent misreporting by other gases. It is necessary to be above.

In Example 1, the thickness of the second layer is 400 nm, which is 10 times that of the conventional example, and is formed to the same thickness as the first layer. Thereby, as shown in Table 2, the hydrogen selectivity is improved twice.
In Example 2, the film thickness of the second layer is the same as that of the conventional example, but the catalyst concentration was 11 wt% with respect to 4 wt% of the conventional example. According to Table 2, hydrogen selectivity is improved 2.9 times and methane selectivity is improved 5 times.
In Example 3, aiming to significantly increase the catalyst activity, the film thickness of the second layer was 10 times that of the conventional example, and the catalyst concentration was about 2.7 times that of the conventional example. From Table 2, it can be seen that the CO concentration dependency is kept almost the same as the conventional example, and the gas selectivity is improved by 16 times for hydrogen and 50 times or more for methane.

In Examples 4 and 5, the catalyst concentration in the second layer was remarkably increased. In Example 4, both hydrogen selectivity and methane selectivity were significantly improved. Methane selectivity is greatly improved, but hydrogen selectivity is about the same or lower.
That is, the gas selectivity can be improved by increasing the catalyst concentration in the second layer from the comparison between the conventional example and Example 2, and the catalyst concentration in the second layer is the same from the comparison between Examples 2 and 3. If so, Example 3 having a larger film thickness can greatly improve the gas selectivity. Further, from the results of Examples 4 and 5, if the catalyst concentration of the second layer is increased, the gas selectivity can be greatly improved up to a certain point, but this also has an upper limit, which eventually exceeds 10 wt%. It can be seen that the gas selectivity can be improved in the range of 60 wt% or less.

DESCRIPTION OF SYMBOLS 1 ... Si wafer, 2 ... Support layer and thermal insulation layer, 3 ... Heater layer, 4 ... Insulation layer, 5 ... Bonding layer, 6 ... Sensing layer electrode, 7 ... Sensing layer, 8 ... Selective combustion layer, 9 ... Donor addition SnO ₂ layer, 10... Catalyst-added SnO ₂ layer.

Claims (1)

A thin film heater is formed on a diaphragm-like support substrate in which the outer periphery or both ends of the thin support film are supported by a Si substrate, and the outer periphery or both ends are thick and the center is thin. The heater layer is covered with an electric insulating film, and a SnO ₂ layer added with a +5 or +6 valent element serving as a donor as a first layer thereon, and further an element Pt serving as a catalyst as a second layer thereon. _In a thin film gas sensor in which a gas sensing layer having a two-layer structure in which _two layers are stacked is formed and a sensing electrode layer made of a pair of noble metals is provided in contact with the gas sensing layer at a predetermined interval,
The added amount of the catalyst Pt of the second layer forming the gas sensing layer is 11 wt% or more and 60 wt% or less, and the ratio of the sensor resistance value at CH4 = 4000 ppm and CO = 100 ppm is 1 or more with respect to methane gas. A thin film gas sensor characterized by improved selectivity .

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