JPS5983046A - Gas sensor and preparation thereof - Google Patents

Abstract

PURPOSE:To obtain a gas sensor especially excellent in gas selectivity, by a method wherein an ultra-fine particle film comprising metal oxide of which the greater part of crystals are oriented to a specific surface direction is formed on a substrate and an electrode is formed on said film. CONSTITUTION:When the division ratio obtained by dividing the peak intensity of the diffraction surface of an X-ray chart by the relative intensity of the same surface of an ASTM card is calculated and the ratio A/B of the max. value A and the min. value B of said ratio is defined as an orientation characteristic, a sensor film 11 being an ultra-fine particle film occupied by metal oxide particles each having an average particle size of 40-200Angstrom in an amount of 95% or more by wt. of the whole film and packed in 27-74% by ratio of the theoretical density of the particle is formed on a substrate 9 made of Al2O3 having a gold comb shaped electrode 10 formed thereon under gas pressure of 8X10<-2>torr or more while adjusting the temp. of said substrate 9 to a low temp. almost equal to a liquid nitrogen temp. After the film is formed, heat treatment is performed at a temp. 100-300 deg.C higher than the operation temp. of the gas sensor. The electrode 10 may be provided on the film 11 formed on the substrate 9.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a gas sensor and a manufacturing method thereof, and more particularly to a gas sensor in which a gas-sensitive part is a metal oxide ultrafine particle film with excellent gas selectivity, and a manufacturing method thereof. [Prior art] There are currently 5 types of gas sensor elements for city gas (CH4).
N02 sintered type semiconductor devices occupy the majority of the market. However, the main problem is that they malfunction in response to alcohol. Although various devices such as catalysts have been developed, there are large variations in the products.
The current situation is that the characteristics deteriorate over time. Therefore, it has high gas selectivity for methane.
7. There is a demand for sensor elements that are highly reliable and do not deteriorate over time.

As a manufacturing method for gas sensors, the particle size of the metal oxide particles constituting the sensor element is made extremely fine, increasing the surface area and increasing the sensitivity. A method of evaporating (evaporation in gas method) is known (Japanese Unexamined Patent Publication No. 55-27925, etc.). In this in-gas evaporation method, evaporated atoms collide with gas molecules and are cooled while reaching the substrate, forming ultrafine particles. High sensitivity has been achieved through the formation of aggregates of ultrafine particles, but gas selectivity is still insufficient.8 Generally, gas selectivity is still controlled by changing the operating temperature or the material composition of the gas-sensitive part. Although this is commonly practiced, there are limits to improving gas selectivity.

[Purpose of the invention]

An object of the present invention is to provide a gas cell with excellent gas selectivity and a method for producing the same.

[Summary of the invention: 1 The present inventors discovered that in an ultrafine particle gas sensor using a semiconductor of an n-metal oxide such as 5nOz or ZnQ, the reactivity of the surface of the fine particles with gas varies depending on the plane orientation of the crystal. Therefore, we focused on the point that gas selectivity could be improved by giving ultrafine particles a specific directionality.

The present invention has been made based on this point of view, and uses an ultrafine particle film, in which most of the crystals of ultrafine metal oxide particles are aggregated in a specific plane direction, as an electrode. In order to obtain such a gas sensor, sputtering is performed at a gas pressure higher than that of normal sputtering to obtain an ultrafine particle film.

Hereinafter, F1 will further explain the present invention in detail.

In the present invention, when the crystals of ultrafine metal oxide particles have random plane orientations, various gases react with each other and the gas selectivity is low. If the nanoparticles have directionality, gases that are likely to react on that surface will preferentially react on the surface of the ultrafine particles, increasing the selectivity of detected gases. It is most desirable that all the ultrafine particles exhibit the same orientation, but in practice, it is sufficient if the orientation defined as described later is above 2υ.

Here, orientation refers to the ratio of the peak intensity of the diffraction surface of the X-ray chart divided by the relative intensity of the same side of the AsTM card, and the ratio of the maximum value A to the minimum value B of this ratio value (A/B). Defined by

That is, in order to examine the crystal orientation of the ultrafine metal oxide particles, X-ray diffraction is used to examine the crystal orientation of the sensitive material. -1 nOz (iio), Dot).

For ZnO, (100) and (1,01) diffraction planes were selected.

These diffraction surfaces are surfaces with relatively strong diffraction intensity depending on the ASI'M card. The XKQ diffraction peak intensity of each diffraction plane when the orientation of the crystal is random is expressed by the relative intensity of the ASTM card. Therefore, in order to see how far the orientation of the crystal deviates from a random crystal on each surface, the peak intensity of the diffraction surface of the measured X-ray chart was divided by the relative intensity of the same surface of the ASTM card. Find the ratio. The ratio (A/B) between the maximum value and the minimum value B of the split ratio obtained in this way is defined as the orientation, and gas selection 4.
I looked at the relationship with sex. The gas selectivity between methane and ethanol was investigated using 450 tl, gas 1% eo, and 5th gear.

When a -C film of ultrafine metal oxide particles is formed by sputtering, the crystal orientation of the fine particles changes depending on the gas pressure. Therefore, when we look at the relationship between orientation and gas selectivity by changing the gas pressure, we find that Fig. 1(a) (SnO2il, :a enemy particle film) and Fig. 1(1)) (Z n Q ultrafine particle film). )
As is clear from the 2-layer film), gas selectivity was improved when the orientation was 2 or more.

In this example, S 1102 is strongly oriented in the (101) direction, resulting in high gas selectivity for methane and CO. However, depending on the manufacturing method, it may be strongly oriented in other plane directions, resulting in I It is also possible to provide high gas selectivity to alcohols and alcohols. In this case as well, the orientation is preferably 2μ or higher.

In the present invention, it is preferable that the average particle diameter of the ultrafine metal oxide particles is 40 to 200 particles.

If the particle size of the fine particles exceeds 40, it is not preferable because the gas sensitivity and gas selectivity, which are characteristics of ultrafine particles, will be significantly impaired. In addition, the particle size of the ultrafine particles is 71.
If it becomes too small, the gaps between the particles will be smaller than the effective size of the detected gas molecules and oxygen molecules, and the gas will not be able to penetrate into the ultrafine particle aggregate. As a result, the particles inside the aggregate are 1! (If no change occurs (the thickness of the detected gas molecule), the gap between the particles), the characteristics will return to normal in nitrogen. ``a;)A, ``7.''-f- +p;v
), the characteristics of the sensor will be significantly impaired. In the backing of -υ mouth η particles, the distance 1iλ between particles is A: * (-about 10 to 15 of the radius (1), but the molecular radius (),
J” :' , /- Lewars radius) is 4.0 people (l\
7 sex), 2.4 people (methane), 3.9 people (j@J, z
Ir1-? The particle size is about 4
08 or higher is desirable.

Next, the packing density of ultrafine particles is 27
~74chi is desirable.

In general, the ultrafine particles V are approximately spherical, and this error can be considered in the same way as when filling a sphere. −)/′ξ1. J1 shift:
l/.

The bond between the particles is A) ``1. The contact part of the weight particle Cl?Cut, 1' The state in which the 1 particle is in contact with as many bars as possible is stable and has high mechanical strength.In addition, Flowing through the ultrafine particle aggregate - [Since the L flow also flows through the contact part, each particle has as many i51 particles and reeds) The characteristics of the gas sensor are stable in an independent state, and ,
Gas sensitivity increases.

Fine particles 15a produced by evaporation in gas or sputtering
Details of the R'7 structure of the body.
Ignoring the & 〇 summer movement, on average, the ultrafine particles are packed in a cubic close-packed structure, a square close-packed structure, a simple cubic structure, etc., or packed in this way. It has been found that the resulting secondary particles can be further classified into those that are filled in the same way, and those that are filled with tertiary particles that are filled with secondary particles. moreover,
In order for the mechanical strength to be as large as possible and the properties of the gas senna to be stable, the aggregation state of the ultrafine particles must have a mixed packing structure of secondary particles. It was found that complete filling or filling with tertiary particles is not preferable for a gas sensor. In other words, on average, the packing density is 27 times the case where ultrafine particles are packed into a simple cubic structure (6) and the secondary particles are also packed into a simple cubic structure (the theory where they are packed without any voids). density to 1
It was found that if the filling degree is lower than this, the filling is insufficient and undesirable.

On the other hand, when ultrafine particles exceed the maximum packing density and fall into a state of fusion, some of the particles become fused, and the resulting gas sensor contains a high gas i, which is a characteristic of ultrafine particle aggregates.・It is not preferable because 6 degrees and gas selectivity cannot be expected. Therefore, it is necessary that the packing density is 74 times less than the theoretical density. In the sputtering method, if the pressure during sputtering is 8X 10-2 Tor+ or higher, an ultrafine particle film with a packing density of 74 or less can be obtained2. When it becomes appropriate, it becomes pregnant. To prevent this, it is desirable to appropriately control the gas pressure, substrate temperature, and substrate distance in the sputtering method. If the gas pressure is increased, the substrate distance is increased, and the substrate temperature is decreased, the density decreases, and vice versa, 76 degrees decreases.

In addition, the following phenomena are observed in the ultrafine particle gas-1 conductor produced by the sputtering method after long-term use: The gas sensitivity to methane remains almost the same, but the gas sensitivity to ethanol gradually increases. However, gas selectivity deteriorates to some extent. As a result of detailed study, it was found that this phenomenon occurs because the particle size gradually increases during use.

We investigated the causes of particle size changes in ultrafine particles.As a result, we found that growth occurs mainly due to the diffusion of the ultrafine particles.The Arrhenius plot of the rate of particle size change shows that the rate of particle size change is approximately 85[
This value will be considered.7. n O+(,:r
,,03, It was found that metal oxides such as SnO2 and Fe2O31TiOz i have almost constant values. It is known that the change in particle size is approximately proportional to the square root of the heat treatment time.

From this point of view, after forming a metal Ffide ultrafine particle film on a substrate by sputtering, it is desirable to perform heat treatment at a temperature 100 to 300 C higher than the operating temperature of the gas sensor. For example, the operating temperature of a gas hair sensor is L 50
'C glue 1 cup, temperature 100C higher than this temperature (zs
The operating temperature of the gas sensor is 4.
(In the case of l OC, the temperature is 300 r higher than this temperature (7
00C), grains grow at a rate of 100 times faster than the operating temperature, but even after continuous use for 10,000 hours at each operating temperature, the grains grow at Changes in characteristics based on this do not pose any problem in practical use.

In addition, during sputtering, by cooling the substrate at a temperature lower than water cooling, near the liquid perforation temperature, the A′q diameter of the ultrafine particles can be maintained within an appropriate range, and gas sensitivity can be improved. 1. [Embodiments of the invention] Hereinafter, the invention will be described in detail based on the drawings.

A schematic diagram of the LP sputtering apparatus (used in FIG. 2) is shown. 1 is a target (S n O2 sintered body here), 2 is an electrode, and 3 is a substrate holder (dOwn method)
. Glow discharges normally even under high gas pressure. (It is the same as a normal device except that there is a slight twist around the sea urchin cuguette. Gas (generally A, r) is passed through the barrier pull leak pulp in 5, Control the Nose King in Belgia. 8 is RF % #, 7 is Matsuboung 7 Iζtux. After applying It F, perform Syria 4 bats, followed by 4 shutter. Sputtering is performed on a substrate set on a substrate holder using M1.

It is believed that ultrafine particles are generated due to collisions between sputtering atoms and gas molecules, but the detailed formation U'('I is not clear at present.Results of actual research 1, L F gravity 100W, substrate distance 40H) 0, 1 'I' if
It has been found that an S n O 2 super-permeable silica film can be produced at a gas pressure of Orr or higher. This was confirmed by X-ray diffraction, microscopic observation, density measurement, electrical resistance measurement, etc. The particle size of the produced ultrafine particles was estimated from the spread of diffraction lines in the X-ray diffraction pattern. According to this, the particle size was approximately 908 mm. FIG. 3 shows the results of determining the density of this ultrafine particle film.

The true density of S II 02 (6,7
g/cm3). 0.1'F
When the gas pressure reaches or+- or higher, the density ratio becomes 60%, F
As a result, there is only a small trace of the maximum light loading (74 cm) of the ball. As described below, ultra-fine particles produced by the spankling method not only have an extremely small particle size, but also
Since the film density is appropriate, the gas W degree is excellent. Figure 3 also shows the knocking density ratio of the S n 02 ultrafine particle film produced by the evaporation method in a vacuum cleaner. , taking the 02 pressure at that time as the negative axis of 7.).It can be seen that the density of 1"C is significantly lower than that of the Snoki Tsukling method.

In addition, the particle size in this case is I To r r (-) about 80 people + 0-1 'J at 20 hours, about 50 hours at `` r rO□O
It's a person. If the gas pressure becomes low, the evaporated Sn and 0
The reaction with 2 will not proceed sufficiently. In fact, in the in-gas evaporation method, even if 02 is excited by applying plasma to increase its reactivity, if the 02 pressure becomes 0.1'in rr or less, more than 50% by weight of Sno will be contained. Further 02
When the pressure is low, metal Sn is included. The ultrafine particle film (b contains at least 95 or more S n 02; good external gas sensitivity cannot be achieved unless it is irritated.)
This means that VJ, super(□“95 times [η,
Sputtering target (e.g. 5no2)
If the composition of C is the same, which is desirable from the viewpoint of gas sensitivity, there is also a method of using 8110 or S n O2 as the evaporation source, ] At 02 pressure below Torr, it is impossible to decompose and increase the proportion of 5 nOz contained. 7 continuous 3nO here by sputtering method? There is almost no deviation from the stoichiometric composition of 95%, and the content can be easily controlled to the above 2%.
, then ↓) the orientation of the ultrafine particle film! The bus selectivity was investigated.

Figure 4 shows the construction of the R particulate sensor for measuring gas sensitivity. (a) Top view, (b) cross section. , A/403χ A comb-shaped electrode 10 is provided on a plate 9 by thick Nu film printing, and a 5nozs fine particle film 11 is sputtered thereon.

For SnO, the X-ray diffraction intensity (Fig. 5) of each diffraction surface of the ultrafine particle gas sensitive material obtained by the conventional method of evaporation in gas is
02 (Ctl i < α) divided by the relative strength of the ASTM card (FIG. 6), there is not much difference, and the orientation, which is the maximum value divided by the minimum value, is almost a value close to 1. At this time, the Nobbs selectivity (R ethanol/methane) is 1
．． It is 4.

On the other hand, each time J)' of the sensitive body (3n02) according to this embodiment
The X-ray diffraction intensity of the i′ plane (Fig. 7) is calculated using S n O2 (C
11Ktx ) divided by the relative strength of the AS'rM card (Figure 6), the maximum and minimum values are (
iol,) surface and (110). The orientation is calculated by dividing this maximum value by the small value of 〒[), and the result is 3.8. The gas selectivity at this time is 11.7.

Next, in the case of ZnO, as with SnO□, the sensitizer made by the conventional gas evaporation method is as shown in FIGS. 8 and 9.

In the case of 5 nOz, when the orientation is similarly determined, it is close to 1, and the gas selectivity is also 2 or less. On the other hand, the sensitive body (
For ZnO), the X-ray diffraction intensity (Fig. 10) is
The maximum value divided by the relative strength of the card (Figure 9) is (1
00) plane takes the minimum value, and the (1, [l],) plane takes the minimum value. At this time, the orientation is 5, and the gas selectivity is 12. Therefore, gas selectivity is also improved in the case of ZnO.

FIG. 11 shows the measurement results of gas sensitivity to methane and ethanol. In FIG. 11, (a) shows the sputtering conditions as Ar gas pressure 0. I Torr, substrate distance 3
The film thickness was approximately 8,000 mm, the density ratio was approximately 55 mm, and the particle size was approximately 90 mm. The horizontal axis of the figure is the gas concentration ( The vertical axis shows the gas sensitivity, which is the value obtained by dividing the electrical resistance of the membrane in the gas by the electrical resistance in the air.The smaller this value is, the higher the gas sensitivity is.The measurement temperature is 450C.Ethanol 1
It has little sensitivity to 2, but high sensitivity to methane 13. In other words, a sensor element with good gas selectivity for methane was obtained.

In addition, FIG. 11(b) shows S n produced by the in-gas evaporation method.
The gas sensitivity of the 02 ultrafine particle film is shown. 0 of ITOrr
This is obtained by resistively heating and evaporating the metal Sn using an At20s coated W boat in No. 2. The substrate, measurement conditions, etc. are the same as in case (a). From this diagram.

The sensitivity to methane is not much different from the case of the present invention, but the sensitivity to ethanol is high and the sensitivity to methane is also high.
1<I can see that the quality is bad.

Also, during sputtering, the /I5 plate was cooled with acid nitrogen.
), the particle size of the deceased particles was 50 (2'C1 is very active. The gas sensitivity to methane was also high, and the gas sensitivity was 71', the same as in the case of water cooling of the substrate! 1.j'1° is irradiated at about 350C, then heat treatment is applied to this element so that it can withstand long-term use. Riko C is heat treated at 500tT for 1 hour.
If gas sensitivity measurement is continued at ζo350C, 30 O
The result was that there was almost no i-conversion at n-hour stability, and good gas selectivity was maintained at 1 t.

Note that Example C has an Al-203 substrate-heat density: i'+f
Al-made comb dog j7.4 installed in L-, L-,''-
31021Pl'i fine particle film ((11) was formed, but the pole is the ultrafine particle film's "IL resistance 'I: j"jlj
Since it is useless to record, j4 fine; particle>+U
There is no problem in providing an electric heater inside or on the top of the ultrafine particle membrane.

The 6-shoulder 4-oxide that forms the ultrafine particles is 8 n 02
IIO, Cr2O3, Fe2O3, T
A good gas sensation can be obtained even at j04. However, noble metals such as platinum and palladium may be contained in these group 2 oxide ultrafine particle aggregates. In particular, in the case of SnO□, at an operating temperature of about 150°C, 1 or 2 is good for ethanol, and it can also be used as an -alcohol sensor.

〔Effect of the invention〕

As described above, according to the present invention, a gas sensor with excellent gas selectivity can be provided.

[Brief explanation of drawings]

Figure 1 (a) shows the relationship between the orientation of the S n Oz super #FL child film and the gas selection notes; Figure 1 (b) shows the relationships between
A diagram showing the relationship between the orientation of 81'17' and gas selectivity, Figure 2 is a block diagram showing an example of a sputtering apparatus for carrying out the present invention, and Figure 3 is a diagram showing the gas pressure during sputtering. Figure 4 (a) shows the relationship between the backing density ratio and the backing density ratio, Figure 4 (a) r, l.
Figure 5 is a sectional view of the main part of a).
2 X-ray diffraction intensity of ultrafine particle film, et al.
Figure 7 shows the relative strength of the card, and Figure 7 shows the X-ray diffraction intensity of the Zn membrane using the 5noz ultra-fine intensity according to the present invention.
The X-ray diffraction intensity of the O ultrafine ladle film (9th house) is Z r+ 0
(C old α) A ST M card usage) degree,
81': ](I r, to the present invention +C-', r ”
'S tI O ultrafine particles 1) (See the X-ray monthly folding intensity, 1 at the 111th close. figure(
b) is the conventional example of Gassure/Rini1,? , the gas sensitivity to methane and ethanol. 1...Target, 2...Electrode, :3...Substrate holder, 1...-/Yatsuta, 5...Variable leak harmonization, 6.M/F valve, 7...Mano Cinque box, 8...f (, source to F 1, 9...A 1
203 board, 1(J...l([+ -d attack(,11・
...+lfl fine particle film 1.12...Eknor, 13
... / Chill concave (drop) distribution 1) Self 1-1 ノ): 1Il- fL vessel (Raw grass 2 eyes Kaya 3 eyes force pressure (Ton-jisei 4 eyes (cL) ZO Kaya 5 eyes Eyes 20 30 40
．． 5't) 60'7
/y zO(+ +3 /, 5 7th ward bud ■ ZO3t) (to 50
60 76 80$ wo Figure (jC) θ ρ, S /
Sword straight cliff (%)

Claims (1)

[Scope of Claims] 1. The ultrafine particle film, which is made up of most of the crystals 75-1 of ultrafine metal oxide particles and is oriented in a specific plane direction, is brought into contact with an electrode. The gas l=nna. 2. The peak intensity of the diffraction surface of
T f~・1 When dividing the ratio by the relative strength of the same side of the card and defining the ratio (A/B) between the maximum value and the minimum value B of this ratio value as the orientation, the metal oxide super 2. The gas sensor according to claim 1, wherein the crystal orientation of the fine particles is 2 or more. 3. Gas-1 according to claim 1 or 2, wherein the metal oxide ultrafine particles have an average particle size of 40 to 200 particles.
Ninsa. 4. Ultrafine metal oxide particles have a theoretical density of 27
The gas sensor according to any one of claims 1 to 3, characterized in that the gas sensor is packed and assembled in 74 units. 5. The gas center according to any one of claims 1 to 4, characterized in that the ultrafine particle film is composed of a totally bent oxide with a total weight of 95% or more of the entire aggregate. 6. A method for producing a gas sensor, comprising sputtering a metal oxide onto a substrate at a gas pressure of 6,8 x 10-"frHr or more, to collect ultrafine particles of a fully bent oxide on the substrate. 7. The method of manufacturing a gas sensor according to claim 6, wherein the substrate is cooled at a lower temperature than water cooling during sputtering. 8. The method of manufacturing a gas sensor according to claim 7, wherein the temperature lower than 8° water cooling is liquid nitrogen temperature. 9. After forming an aggregate of ultrafine metal oxide particles on the substrate by sputtering, the temperature is 10% higher than the operating temperature of the gas sensor.
The method for manufacturing a gas sensor according to any one of claims 6 to 8, characterized in that the heat treatment is performed at a temperature 0 to 300C higher.