JP6749603B2 - MEMS gas sensor and gas detector - Google Patents

Description

The present invention relates to gas detection by a MEMS gas sensor.

The MEMS gas sensor manufactured by the MEMS technology becomes an ultra-low power consumption sensor when the heater is pulse-driven. Since the MEMS gas sensor can be mounted on various electronic devices such as medical devices and smartphones, new applications such as medical applications and environmental monitoring are expected.

The inventors have investigated the sorption and desorption behavior of oxygen in perovskite type oxides such as Ba1-xLaxFeO3-δ (BLF) by the temperature programmed desorption method. Further, the oxygen permeation characteristics of these perovskite type oxides were measured (for example, Non-Patent Document 1).

"Microstructure Effect on the Oxygen Permeation through Ba0.95La0.05FeO3-δ Membrane Fabricated by Different Methods" J. American Ceramic Soc., 93 (7), 2012-2017 (2010)

The inventor has focused on releasing oxygen when the temperature of an oxygen sorption/desorption material such as BLF is raised to near the detection temperature of the metal oxide semiconductor gas sensor, and has examined increasing the sensitivity of the gas sensor. That is, the gas sensor is heated in a pulsed manner to the detection temperature to release oxygen from the oxygen sorption/desorption material. Then, this oxygen is used to enhance the gas sensitivity of the metal oxide semiconductor. In the MEMS gas sensor, a cycle in which oxygen is effectively released by raising the temperature of the oxygen sorption/desorption material in a short time and cooled in a short time after detecting the gas is possible.

An object of the present invention is to provide a MEMS gas sensor which has high sensitivity due to release of oxygen from an oxygen sorption/desorption material, and a gas detection device using this gas sensor.

This invention provides a MEMS gas sensor in which a heater, an electrode, and a gas sensitive film containing a metal oxide semiconductor are provided in an insulating film on a cavity of a Si substrate,
The gas-sensitive film contains the metal oxide semiconductor and an oxygen sorption/desorption material that releases oxygen at a temperature detected by the metal oxide semiconductor and absorbs oxygen at a temperature lower than the detection temperature. And

Further, the gas detector of the present invention, the MEMS gas sensor,
By controlling the heater power of the MEMS gas sensor, the temperature of the gas sensitive film of the MEMS gas sensor, the detection temperature suitable for detection of gas by the metal oxide semiconductor and release of oxygen from the oxygen sorption/desorption material, and oxygen absorption. Heater control means for changing the desorption material to an absorption temperature at which it absorbs oxygen,
Gas detection means for detecting a gas based on the resistance value of the metal oxide semiconductor at the detection temperature.

The metal oxide semiconductor is SnO2 containing a noble metal such as Pd. The metal oxide semiconductor may be another metal oxide semiconductor such as In2O3, WO3, ZnO or the like, and Pd may be replaced with another noble metal such as Pt, Au or Rh. Further, the metal oxide semiconductor does not need to contain a noble metal. Further, the gas sensitive membrane may contain a third component such as silica, alumina, a catalyst layer on the surface of the sensitive membrane.

The oxygen sorption/desorption material is preferably a perovskite material having a composition formula of ABO3-δ, the A site contains at least one of the rare earth elements of atomic numbers 57-71, Sr, Ba, Ca, Y, and the B site contains Mn. , Co, Ni, Fe, In, Zn, Cu, V, Mo are included. Description of δ, which is a non-stoichiometric factor of oxygen, may be omitted. Exemplifying a preferable oxygen sorption/desorption material,
Ba1-xLaxFeO3 (x is, for example, 0 or more and 0.3 or less, preferably 0 or more and 0.2 or less, particularly preferably 0 or more and 0.06 or less),
BaFe1-xMxO3 (M is, for example, In, Zn, Mn, Co, Ni, Cu, V, Mo, x is, for example, 0 or more and 0.3 or less, preferably 0 or more and 0.2 or less, particularly preferably 0 or more and 0.1 or less),
LnMO3 (Ln is a rare earth element with atomic number 57-71 or Sr, for example, La, Gd, Sr. The rare earth element and Sr may be solid-solved at any ratio, and M is Mn, Co, Ni, Fe. , In, Zn, Cu, V, Mo, etc.), etc. Although these perovskites often have oxygen ion conductivity and electronic conductivity, they generally have lower resistance than SnO2, and are not included in the metal oxide semiconductor in this specification.

The oxygen sorption/desorption material releases oxygen by heating, absorbs oxygen by cooling, and has various temperatures for releasing or absorbing oxygen. However, when a noble metal such as Pd, Pt, or Au is supported on the oxygen sorption/desorption material, the temperature at which oxygen is released or absorbed can be lowered. The mass ratio of the metal oxide semiconductor and the oxygen sorption/desorption material is, for example, 70 to 95 mass% for the metal oxide semiconductor and 30 to 5 mass% for the oxygen sorption/desorption material. When the proportion of the oxygen sorbing/desorbing material is increased, the resistance of the gas sensitive film becomes low and it becomes difficult to measure the output of the gas sensor. Fewer oxygen sorption and desorption materials will reduce the increase in sensitivity due to released oxygen.

For example, the particle diameter of the oxygen sorption/desorption material is made larger than that of the metal oxide semiconductor, and the metal oxide semiconductor particles are attached to the particle surface of the oxygen sorption/desorption material. As a result, a layer of the metal oxide semiconductor is formed on the surface of the oxygen sorption/desorption material particles, and the oxygen released from the oxygen sorption/desorption material is quickly transferred to the metal oxide semiconductor. In addition, by connecting the layers of the metal oxide semiconductor to each other, the electrical conductivity of the metal oxide semiconductor can be easily measured. However, the oxygen sorption/desorption material may be simply mixed with the metal oxide semiconductor, or the small diameter oxygen sorption/desorption material may be supported on the surface of the large diameter metal oxide semiconductor particles.

In the present invention, the gas sensitive membrane is heated to a detection temperature of about 200°C to 500°C to release oxygen from the oxygen sorption/desorption material. The released oxygen is adsorbed on the metal oxide semiconductor as oxygen ions or used for oxidizing the reducing gas. It is considered that when the adsorption amount of oxygen ions increases, the resistance of the metal oxide semiconductor increases, so that the resistance value in air increases and the gas sensitivity increases. It is considered that even if the oxidation of the reducing gas is promoted, the adsorption amount of the reaction intermediate increases and the gas sensitivity increases. When the gas-sensitive membrane cools from the sensing temperature, the oxygen sorption/desorption material reabsorbs oxygen and returns to its original state. Since the MEMS gas sensor can perform heating and cooling in a short time, oxygen can be effectively released.

Sectional view of the MEMS gas sensor of the example Block diagram of the gas detector of the embodiment The figure which shows typically the waveform (a) of the heater voltage and the waveform (b) of the oxygen partial pressure near the metal oxide semiconductor in the example. Characteristic diagram showing oxygen release from Ba0.95La0.05FeO3-δ during temperature programmed desorption Characteristic diagram showing the mass change of Ba0.95La0.05FeO3-δ when the temperature is changed between 50°C and 250°C A diagram showing the sensitivity to 200 ppm C7H8 when the heater voltage is turned on/off every one second between room temperature and heating temperature: the sensitivity of Pd-SnO2 with BLF added in the example, and the oxygen of the comparative example. The sensitivity of Pd-SnO2 without a sorption/desorption material is shown. Diagram showing sensitivity to 200 ppm C7H8 in Example and Comparative Example when heating temperature was constant

The best examples for carrying out the present invention will be shown below.

FIG. 1 shows a MEMS gas sensor 2 of the embodiment. Reference numeral 4 denotes a substrate made of silicon or the like, in which a cavity 6 is provided, and an insulating film 8 is provided on the cavity 6. The cavity 6 may be etched from the insulating film 8 side or may be a through hole.

The insulating film 8 is provided with a heater 10 made of, for example, a Pt film, the surface of the insulating film 8 is provided with electrodes 11, 12 such as a pair of Pt films, and the thick gas-sensitive film 14 is provided with the electrodes 11, 12. Cover. It is also possible to measure the combined resistance of the heater 10 and the gas sensitive film 14 by exposing the heater 10 above the insulating film 8 without providing the electrodes 11 and 12. Further, the gas sensitive film 14 may be a thin film.

The gas sensitive film 14 is made of a mixture of a metal oxide semiconductor and an oxygen sorption/desorption material made of perovskite. The structure of the gas sensitive film 14 is shown in the upper right of FIG. 1, the oxygen sorption/desorption material particles 16 have a larger particle diameter than the particles of the metal oxide semiconductor, and the SnO 2 layer 18 is provided on the surface of the oxygen sorption/desorption material particles 16. The SnO2 layers 18 are connected to each other to develop electrical conductivity.

For example, the particle size of SnO2 and the particle size of the oxygen sorption/desorption material particles 16 may be approximately the same, and these may be mixed to form the gas sensitive film 14. Alternatively, the particle size of the oxygen sorption/desorption material particles 16 may be made smaller than the particle size of SnO2, and the oxygen sorption/desorption material particles may be supported on the SnO2 particles.

FIG. 2 shows a gas detection device 20 of the embodiment, in which a load resistor R1 is connected to the gas sensitive film 14 and a detection voltage Vcc is applied. The heater drive 22 of the microcomputer 21 periodically controls the electric power of the heater 10, and the A/D converter 23 A/D-converts the output voltage to the load resistance R1 when the gas sensitive film 14 is heated, The detector 24 detects the gas.

FIG. 3 schematically shows the voltage waveform of the heater 10 and the oxygen partial pressure in the gas sensitive film 14. In the embodiment, the heater 10 is turned on/off every 1 second, but the time width for turning on the heater 10 is preferably 10 msec or more and 1 minute or less, and the time width for turning off the heater 10 is 0.1 second or more and 1 minute or less. preferable. The heater voltage may be controlled by a sine wave, a ramp wave, or the like instead of turning on/off the heater 10.

The oxygen sorption/desorption material releases oxygen by heating and absorbs oxygen by cooling. Therefore, the oxygen partial pressure in the gas sensitive film rises in synchronization with ON of the heater and decreases in synchronization with OFF of the heater. Oxygen released from the oxygen sorption/desorption material is adsorbed on the metal oxide semiconductor as oxygen ions to increase the resistance value of the gas sensor 2. Alternatively, the reaction between the released oxygen and the reducing gas to be detected produces an intermediate that reduces the resistance value of the metal oxide semiconductor, thereby improving the sensitivity.

FIG. 4 shows the amount of released oxygen when Ba0.95La0.05FeO3 was heated at a rate of 4° C./min. There is an oxygen release peak near 370° C., and this temperature is almost equal to the gas detection temperature of the MEMS gas sensor 2. Note that instead of Ba0.95La0.05FeO3 (BLF), Ln1-xSrxCoO3, La1-xSrxFeO3,
Similar perovskites such as La1-xSrxMnO3, LaMnO3, LaCoO3, LaNiO3 may be used. Further, as in Gd1-xSrxCoO3, etc., the La element of perovskite may be replaced with a Gd element or the like. Furthermore, SrCoO3, SrFeO3, etc. also release oxygen at around 400-500°C.

When the temperature at which the oxygen sorption/desorption material releases oxygen is higher than the temperature at which the gas is detected by the metal oxide semiconductor, the oxygen sorption/desorption material is loaded with a noble metal such as Pd, Pt, or Au, and The absorption temperature can be lowered.

FIG. 5 shows changes in the mass of BLF when the temperature of BLF was cyclically changed between 50° C. and 250° C. The mass of BLF decreases in synchronization with the temperature rise to 250°C, and the mass increases in synchronization with the cooling to 50°C. This indicates that the release of oxygen starts at a temperature lower than the peak in FIG. 4, and the sensitivity of the gas sensor that detects combustible gas at 250° C. can be increased by using BLF.

An example of preparation of the gas sensitive film 14 will be shown. The SnCl4 aqueous solution was neutralized, the precipitate was washed and then centrifuged, and hydrothermal treatment was carried out to obtain SnO2 sol, which was calcined at 700°C to obtain SnO2 powder. The average primary particle diameter of SnO2 was 16 nm, and the SnO2 particles were monodispersed. An aqueous solution of Pd salt was added to SnO2 powder, dried and baked to obtain Pd-SnO2 carrying 1.5 mass% Pd.

A mixed aqueous solution containing Fe (trivalent) and Ba and La nitrates in a molar ratio of 20:19:1 was prepared and neutralized with ammonia to coprecipitate Fe, Ba and La ions. .. This precipitate was dried and calcined so that the maximum temperature was 1050°C to prepare Ba0.95La0.05FeO3. Ba0.95La0.05FeO3 was crushed to obtain BLF powder having an average primary particle size of 50 nm. 85 mass% of Pd-SnO2 powder and 15 mass% of BLF powder were mixed and applied on the insulating film 8 provided with the electrodes 11 and 12, and dried and fired to obtain the gas sensor 2 of FIG. other,
Gas sensor using 1.5 mass% Pd-supported SnO2 as a gas sensitive film (comparative example),
A gas sensor using a mixture of Pd-SnO2 powder 70mass% and BLF powder 30mass% as a gas sensitive film,
Gas sensor with varying x in Ba1-xLaxFeO3,
Gas sensor using BaFe1-xMxO3 (M is In, etc.) and Pd-SnO2,
A gas sensor using BLF and Pd-In2O3 was prepared.

FIG. 6 shows gas sensitivity to C7H8 of 200 ppm when the gas sensor 2 is pulse-driven (heater voltage is turned on/off every one second so as to change the temperature between room temperature and 250° C.). The gas sensitivity was defined by the ratio of the resistance value in air to the resistance value in gas, and the inclusion of BLF significantly increased the gas sensitivity.

FIG. 7 shows the gas sensitivity to 200 ppm of C7H8 when the gas sensor 2 is operated at a constant temperature of 250° C. There is no great difference in gas sensitivity between the gas sensor composed of only Pd-SnO2 and the gas sensor composed of Pd-SnO2 containing BLF. From FIG. 6 and FIG. 7, it can be seen that the BLF repeatedly releases and absorbs oxygen due to the temperature change of the gas-sensitive film, and the gas sensor 2 has high sensitivity.

When a mixture of Pd-SnO2 powder 70mass% and BLF powder 30mass% was used as a gas sensitive film and the temperature was changed between room temperature and 250℃, the resistance value at 250℃ increased. This means that the A/D converter 23 of FIG. 2 needs to be highly accurate. Even when x in Ba1-xLaxFeO3 was set to 0, the same characteristics as in Fig. 6 were obtained. However, when x was set to 0.1, the peak of oxygen release moved to around 450 ℃, and the characteristics suitable for the LPG sensor and CH4 sensor, in which the temperature was changed between room temperature and 350-500 ℃, were obtained.

The gas to be detected is not limited to C7H8, but may be any gas such as H2, LPG, CH4, VOC gas (volatile organic substance gas, C7H8 is a typical compound thereof). The type of oxygen sorption/desorption material and the presence/absence of noble metal loading on the oxygen sorption/desorption material may be changed according to the temperature detected by the gas-sensitive film. For example, the oxygen sorption/desorption material is not limited to BLF, but is a very similar compound BaFe1-xMxO3 (M is In, Zn, Mn, Co, Ni, Cu, V, Mo, x is 0 or more and 0.3 or less, preferably 0.01 or more. 0.2 or less). In this case, when M was In, the peak of oxygen release appeared near 380°C, and gases such as C7H8 could be detected by the temperature change between room temperature and 250°C as in the example. When M was Zn and Ni, the peak of oxygen release shifted to around 450 ℃, and the characteristics suitable for detecting LPG, CH4, etc. at room temperature and the temperature change between 350 and 500 ℃ were obtained.

The A site is a mixture of a rare earth element such as La and at least one of Sr, Ba, Ca and Y, especially a rare earth element such as La and Sr, and the B site is Mn, Co, Ni, Fe, In, Zn, Cu. , V, Mo, La1-xSrxBO3 (x is arbitrary) may be used instead of BLF. Particularly, when the B site was made of Fe, Co, or a mixture of Fe and Co, and x was 0.2 or more and 0.9 or less, a peak of oxygen release occurred at 150 ℃ to 300 ℃.

Even if the mixture of Pd-In2O3 and BLF was used as a gas sensitive film and the temperature was changed between room temperature and 250°C, the sensitivity to C7H8 could be similarly improved. This indicates that the type of metal oxide semiconductor is arbitrary. The type and concentration of the noble metal added to the metal oxide semiconductor are arbitrary.

2 MEMS gas sensor
4 board
6 cavities
8 Insulating film
10 heater
11,12 electrodes
14 Gas sensitive membrane
16 Oxygen sorption/desorption material particles
18 SnO2 layer
20 gas detector
21 Microcomputer
22 heater drive
23 A/D converter
24 Gas detector

R1 load resistance
Vcc detection voltage

Claims (3)

In a MEMS gas sensor in which a heater, an electrode, and a gas sensitive film containing a metal oxide semiconductor are provided in an insulating film on a cavity of a Si substrate,
The gas-sensitive film contains the metal oxide semiconductor, and an oxygen sorption/desorption material that releases oxygen at a detection temperature of gas by the metal oxide semiconductor and absorbs oxygen at a temperature lower than the detection temperature , and A particle size of the oxygen sorption/desorption material is larger than a particle size of the metal oxide semiconductor, and the metal oxide semiconductor particles adhere to the surface of the particles of the oxygen sorption/desorption material . The oxygen sorption/desorption material is a perovskite material having a composition formula of ABO3-δ, Ba1-xLaxFeO3-δ
(X is 0 or more and 0.3 or less) or BaFe1-xMxO3−δ (M is In, Zn, Mn, Co, Ni, Cu, V, or Mo, x is 0 or more and 0.3 or less) Item 1 MEMS gas sensor. The MEMS gas sensor according to claim 1 or 2,
By controlling the heater power of the MEMS gas sensor, the temperature of the gas sensitive film of the MEMS gas sensor, the detection temperature suitable for detection of gas by the metal oxide semiconductor and release of oxygen from the oxygen sorption/desorption material, and oxygen absorption. Heater control means for changing the desorption material to an absorption temperature at which it absorbs oxygen,
A gas detection unit that detects a gas based on a resistance value of the metal oxide semiconductor at the detection temperature.

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