JPH063310A - Gas sensor - Google Patents

Abstract

PURPOSE:To obtain a thin film sensor which is small in change with lapse of time by adding the oxide of one element selected from among the group of W, Mo, and V to an SnO2 thin film at a specific rate. CONSTITUTION:In the title gas sensor using the resistance of an SnO2 thin film, the oxide of one element selected from among the group of W, Mo, and V is added to the thin film at a rate of 0.1-40atm.% against Sn atoms. All of the W, Mo, and V stabilize the resistance of the SnO2 and, regarding the gas sensitivity of the sensor, the W improves the sensitivity of the sensor against HS and its derivatives and the Mo improves the sensitivity against oxygen- containing organic compounds, such as alcohol, etc., and ammonia and its derivatives. The V balances the sensitivity of the sensor against compounds, such as alcohol, etc., and ammonia-based compounds with the sensitivity of the sensor against H2S and its derivatives. Of the W, Mo, and V, the W and Mo highly improve the sensitivity, but the V reduces the sensitivity. When the adding amount of the element is controlled within the range of 0.1-40%, a thin gas sensor which is small in change with lapse of time and has high sensitivity against derivatives of hydrogen sulfide, oxygen-containing organic compounds, and derivatives of ammonia can be obtained.

Description

Detailed Description of the Invention

[0001]

This invention relates to thin film gas sensors,
In particular, it relates to improvement of its aging characteristics.

[0002]

2. Description of the Related Art The inventors examined a gas sensor using the resistance value of a SnO2 thin film. However, the obtained gas sensor has low stability over time, and the resistance value changes about 10 times when used for about one month (see FIG. 5). Therefore, the inventor tried to improve the temporal stability by using an additive to the SnO2 thin film, and screened 23 kinds of additive elements. As a result, they have found that W, Mo and V are effective for improving the stability of SnO2 thin films over time, and arrived at the present invention.

As a related prior art, Japanese Patent Application Laid-Open No. 63-313,048 shows that addition of Zn or Pb to a SnO2 thin film is effective for improving H2S sensitivity. In our experiment, we screened 23
Among the elements of the species, Zn is SnO2 after W, Mo, V
It was effective in stabilizing the thin film. FIG. 4 shows the time-dependent characteristics of the SnO2 / Zn-based sensor. Although the time-dependent change is suppressed as compared with the SnO2 simple thin film, there is still a large time-dependent change.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a thin film gas sensor which has a small change over time.

[0005]

According to the present invention, in a gas sensor using the resistance value of a SnO2 thin film, an oxide of at least one member of the group consisting of W, Mo and V is added to the SnO2 thin film in an atomic ratio of 0.1 to Sn atoms. ˜40% added. In this specification, the amount of addition is expressed by the atomic ratio between the metal element in the additive and the SnO2 Sn element.

All of W, Mo and V stabilize the resistance value of the SnO2 thin film. Regarding the gas sensitivity, W enhances the sensitivity to H2S and its derivatives, and Mo is an oxygen-containing organic compound such as alcohol and ammonia and its Increase sensitivity to derivatives.
Further, V balances the sensitivity to compounds such as alcohol and ammonia compounds and the sensitivity to H2S and its derivatives. Comparing W, Mo, and V, large gas sensitivity is obtained with W and Mo, whereas gas sensitivity is reduced with V. Regarding the amount of addition, we conducted experiments centering on 4%,
Although the range is 40%, if the amount added exceeds 10%, the gas sensitivity decreases and the resistance increases. On the other hand, add 0.5
If it is less than%, the change over time will be insufficiently suppressed. As a result, the most preferable additive elements are W and Mo, and the added amount is generally 0.1 to 40%, preferably 0.5 to 10%.

[0007]

【Example】

[0008]

[Structure and Preparation of Gas Sensor] FIG. 7 shows the structure of a thin film gas sensor. In the figure, 2 is an alumina substrate, 4 is Sn
O2 type thin films 6 and 8 are thin film electrodes, and 10 is a heater. The structure of the gas sensor itself is arbitrary.

The gas sensor of the example was manufactured as follows. After the substrate 2 was reverse-sputtered and cleaned using a reactive sputtering apparatus with two chambers, a SnO2 film was formed in a thickness of about 600 nm by reactive sputtering using a SnO2 target in the first sputtering chamber. In the second sputtering chamber, a metal target of W, Mo, V, etc. is used, and W is formed by reactive sputtering.
A thin film of O3, MoO3, V2O5, etc. was laminated on the SnO2 film. In the sputtering process, the substrate was water-cooled and the sputtering gas used was a mixed gas of O2 30% -Ar 70%.
The sputtering was performed by RF sputtering, and the input power was 400W. After film formation, use a gas sensor in air
It was heated to 00 ° C. for 1 hour to remove strain during film formation and to grow crystals.

The composition of the sputtering gas is, for example, O 2
You may change in the range of 20% -Ar80% -O2 80% -Ar20%. Instead of sputtering, another film forming method such as CVD or vacuum evaporation may be used, and cooling or heating of the substrate is optional. Instead of sputtering in two chambers, a SnO2 / W mixed target or the like may be used to form a film having a substantially uniform composition from the beginning.
After the two films have been formed, the SnO2 film may be immersed in a solution of a salt of W, Mo, V or an organic metal compound solution of these elements and thermally decomposed to carry an oxide of W, Mo, V. .

The SnO 2 -based thin film 4 preferably has a total film thickness of W, Mo and V oxides of 100 to 2000 nm, and the thin film 4 is made of a noble metal catalyst such as Pt or Pd. You may add as a component.

[0012]

[Measurement conditions] The obtained gas sensor is heated to 340 ° C,
The measurement is started after aging for 2 days. The data after 2 days of aging are the initial characteristics and correspond to day 0 of the characteristics over time. The characteristics are expressed by an average value of 4 to 5 sensors, and the gas concentration used for the measurement is 3 ppm for H2S and CH3SH, and 300 ppm for gases other than H2S such as ethanol, ammonia, and H2. The sensor was constantly kept at 340 ° C. by the heater 10.

[0013]

[Results] FIG. 1 to FIG. 5 show data on the characteristics over time. The resistance value is shown based on the initial resistance value in 3 ppm of H2S. In addition to the plain SnO2 film, the sensors include W, Mo, V in the examples, Zn, Cu, and
Al, In, Sb, Bi, Pb, Ce, Nb, Ta, Mn, Re,
A total of 23 kinds of Ni, Y, S, Mg, Fe, Cr, Ru and Co were examined. These addition elements were added so that the addition amount was changed centering on the addition amount of 4% and all of them were supported as oxides.

Of these, only W, Mo and V were substantially effective in suppressing the change with time. S in Figure 1
nO2 / W (W / Sn atomic ratio 4%) based sensor, 9
The characteristics over time of 0 days are shown. Similarly, in Fig. 2, SnO2 / M
9 shows the time-dependent characteristics of an o (Mo / Sn atomic ratio of 4%) based sensor for 90 days. Further, in FIG. 3, SnO2 / V (V / Sn
2 shows the characteristics over time of 90 days of a sensor having an atomic ratio of 2%).

The most effective additive element other than W, Mo and V is Zn. In Fig. 4, SnO2 / Zn (Zn
/ Sn atomic ratio is 4%). Among the comparative examples, with the additives other than Zn, a result in which the change with time was more remarkable than that in FIG. 4 was obtained. Moreover, in FIG.
The time characteristic of a simple sensor is shown.

As is apparent from FIG. 5, plain SnO2
The stability of the sensor over time is extremely low, and even if Zn is added to this, the stability over time is still insufficient. For example, SnO in FIG.
In the 2 / Zn sensor, the ethanol sensitivity and the H2S sensitivity reverse and cross. 90 days later 300 pp ethanol
The resistance value in m is equal to the initial resistance value in air.

On the other hand, SnO2 / W shown in FIGS.
The SnO2 / Mo and SnO2 / V sensors are much more stable, and the SnO2 / W sensor has H2S sensitivity and CH3SH.
A characteristic relative sensitivity of high sensitivity was obtained. The SnO2 / Mo sensor shown in FIG. 2 has high sensitivity to ethanol, ammonia, methylamine, aniline, acetone, and ethyl acetate, and relative sensitivity to oxygen and its derivatives, and oxygen-containing organic compounds such as alcohol. SnO in Figure 3
With the 2 / V sensor, the H2S sensitivity and the ethanol sensitivity were balanced, and low gas selectivity was obtained.

The effects of the amounts of W and Mo added are shown in Tables 1 and 2. As is clear from the table, additives such as W, Mo and V are actually effective in the range of the added amount of 0.2 to 36%,
The characteristics over time are remarkably improved with the addition of 0.6% or more (Table 2), and the sensitivity is drastically lowered with the addition of more than 10% (Tables 1 and 2). Therefore, the addition amount is preferably 0.5 to 10%.

[0019]

[Table 1] Table 1 W additive amount dependency W additive amount Initial sensitivity Time characteristics (70 days) Initial resistance (% atom / Sn atom) H2S (3 ppm) (after 70 days / initial value) H2S (3 ppm) ... 40 135 7 0.5 0.2 30 25 8 0.8 12 3.5 10 2 11 2.6 8 4 9 9 1.7 10 10 8 8 1.5 50 35 35 5 1.3 800 * Sensitivity in air and in H2S3ppm. Resistance value ratio, measurement temperature is 340 ℃. * The aging characteristics show the change of resistance value in H2S3ppm. * Resistance value is in KΩ.

[0020]

[Table 2] Table 2 Mo Addition Dependency Mo Addition Initial Sensitivity Aging characteristics (70 days) Initial resistance (% atom / Sn atom) Ethanol 300ppm (70 days later / initial value) in H2S3ppm ... 8.5 145 75 0. 3 10 18 10 0.6 0.6 16 2.6 30 4 9.5 1.7 1.7 25 8 8.5 8.5 1.4 40 15 15 6 1.5 120 36 36 4 1.3 1500 * Sensitivity in air and 300 ppm ethanol The ratio of the resistance value of, the measurement temperature is 340 ℃. * Time-dependent characteristics show changes in resistance value in 300 ppm of ethanol. * Resistance value is in KΩ.

FIG. 6 shows an SnO2 / W sensor (W / Sn =
4%) is shown. Measurement temperature is 34
At 0 ° C, when the temperature was lowered further, the relative sensitivity to H2S and CH3SH increased, and the relative sensitivity to ethanol and ammonia decreased. In all of the results shown in FIGS. 1 to 6, the H2 sensitivity was low, and in addition to this, the CO sensitivity was also low as was the H2 sensitivity.
Further, the sensitivity of iso-C4H10 is extremely low, and according to the results to date, H2S and its derivative, oxygen-containing organic compounds such as alcohol, ammonia and its derivative are the main detection targets.

[0022]

According to the present invention, the change with time is small, and
It is possible to obtain a thin film gas sensor having high sensitivity to hydrogen sulfide derivatives such as S and CH3SH, alcohols such as ethanol, oxygen-containing organic compounds such as acetone and acetaldehyde, and ammonia derivatives such as ammonia, methylamine and aniline.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing a 90-day aging characteristic of an SnO 2 / W-based thin film sensor of an example at an operating temperature of 340 ° C.

FIG. 2 shows a SnO 2 / Mo based thin film sensor of the embodiment,
Characteristic diagram showing aging characteristics for 90 days at operating temperature of 340 ° C

FIG. 3 is a characteristic diagram showing the aging characteristics of the SnO 2 / V type thin film sensor of the example for 90 days at an operating temperature of 340 ° C.

FIG. 4 shows a conventional SnO 2 / Zn-based thin film sensor,
Characteristic diagram showing aging characteristics for 90 days at operating temperature of 340 ° C

FIG. 5 is a characteristic diagram showing a time-dependent characteristic of a SnO 2 pure thin film sensor of a conventional example for 90 days at an operating temperature of 340 ° C.

FIG. 6 is a characteristic diagram showing gas concentration characteristics of an SnO 2 / W thin film sensor of an example at an operating temperature of 340 ° C.

FIG. 7 is a cross-sectional view of a thin film gas sensor of an example.

[Explanation of symbols]

 2 substrate 4 metal oxide semiconductor thin film 6,8 thin film electrode 10 heater

Claims (3)

[Claims] 1. A gas sensor using the resistance value of a SnO2 thin film, wherein the SnO2 thin film contains an oxide of at least one element of the group consisting of W, Mo and V in an atomic ratio of 0.1 to Sn atom. A gas sensor characterized by being added up to 40%. 2. The gas sensor according to claim 1, wherein the additive element is W. 3. The gas sensor according to claim 1, wherein the additive element is Mo.