JP2014132232A - Hydrogen gas sensor using hydrogen absorption membrane containing oxygen - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrogen gas sensor which is small-sized, operates in a low temperature, can be mass produced, is inexpensive, has high selectivity of a gas, high sensitivity and high accuracy, and can measure a hydrogen gas of an extensive range of concentration.SOLUTION: A thin film separated from a substrate by thermal separation is made highly sensitive especially under an extreme low concentration of hydrogen, by introducing oxygen into a heater, a temperature sensor and a hydrogen absorption membrane as well as near the surface of the membrane and obtaining a larger added temperature rise due to exothermic reaction of reductive reaction near the surface of the hydrogen absorption membrane in addition to a temperature rise caused by heat evolution at the time of absorption of hydrogen in an atmosphere gas. Heating by the heater makes the hydrogen absorption membrane discharge hydrogen and, after heating by the heater is stopped, a hydrogen gas concentration in an atmosphere gas is obtained with the use of an output of the temperature sensor after a lapse of time which is equal to or larger than a thermal time constant of the thin film of the time when there is no hydrogen in the heater. A heat conduction type sensor without the hydrogen absorption membrane is provided as well if needed, so that the measuring range of a hydrogen gas concentration can be widened.

Description

The present invention relates to a hydrogen gas sensor, which is a hydrogen gas sensor that measures a temperature rise due to an exothermic reaction when a hydrogen absorption film absorbs hydrogen gas (H ₂ ) with a temperature sensor and converts it to a hydrogen gas concentration. The present invention relates to a hydrogen gas sensor that utilizes heat generation based on a reduction reaction accompanied by heat generation with oxygen introduced into a hydrogen absorption film by hydrogen for high sensitivity.

  Hydrogen gas has been found to be an explosion hazard in a very wide range of 4.0 to 75.0% (volume%) in air. Therefore, it is important to measure the hydrogen gas concentration at a low concentration below the explosion limit of 4.0%. Conventionally, gas sensors include a catalytic combustion type hydrogen gas detection sensor (see Patent Document 1) that increases the temperature of a catalyst such as Pt by a heater and measures the temperature during heating of the heater in combination with the catalytic action.

Some semiconductor gas sensors use a change in electrical resistance during heating of the heater by utilizing a change in carrier density on the surface of the semiconductor due to adsorption of a reducing gas. However, if it is a reducing gas other than hydrogen gas, it reacts with anything, so the lack of selectivity for hydrogen has been a problem.

In addition, there has been a sensor that enhances gas selectivity by utilizing absorption and permeation of a specific gas such as hydrogen. For example, as a device for detecting hydrogen using a hydrogen storage alloy, a hydrogen storage alloy is fixed to one surface of a substrate and a strain gauge is attached to the other surface. A hydrogen detector (see Patent Document 2) that expands, detects strain of a substrate generated at that time with a strain gauge, and detects a hydrogen absorption amount based on the detected magnitude of the strain is known.

Using a hydrogen storage alloy with high hydrogen selectivity, it detects the state change (weight change) when absorbing hydrogen while maintaining the hydrogen storage alloy at a constant temperature, and determines the concentration of hydrogen gas contained in the gas. A hydrogen detector for detection (see Patent Document 3) has also been proposed.

Conventionally, as a temperature sensor, there are an absolute temperature sensor capable of measuring an absolute temperature and a temperature difference sensor capable of measuring only a temperature difference. As an absolute temperature sensor capable of measuring absolute temperature, a thermistor, a transistor thermistor using a transistor invented by the present applicant as a thermistor (Patent Document 4, Japanese Patent No. 3366590), and a diode thermistor using a diode as a thermistor (Patent Document 5) Patent No. 3583704), and there is an IC temperature sensor in which the temperature is linearly related to the forward voltage of the diode and the voltage between the emitter and base of the transistor. As a temperature difference sensor that can measure only the temperature difference, there are a thermocouple and a thermopile in which the output voltage is increased by connecting them in series.

  Conventionally, a microcapsule means for coating powder particles of a hydrogen storage alloy with a metal film, a temperature detection end means by a thermocouple, a powder of the hydrogen storage alloy coated by the microcapsule means, and a thermocouple of the temperature detection end means There has been proposed a hydrogen sensor that is mainly composed of an integration unit housed in an electronic control unit including an electronic control unit including a power source (Patent Document 6).

The inventor previously invented a “gas sensor element and a gas concentration measuring device using the same” (see Patent Document 7), and formed one or a plurality of temperature sensors on a thin film thermally separated from the substrate. And a gas-absorbing substance that absorbs the gas to be detected, and is intended to measure the concentration of hydrogen gas that is arranged and formed so that the temperature sensor can measure the temperature change associated with the absorption and heat generation during absorption and release of the gas to be detected A gas sensor element and gas concentration measuring device were proposed. Thereafter, the present inventor further invented a “specific gas concentration sensor” (PCT / JP2011 / 070427), and used an ultra-small cantilever-like thin film provided with a hydrogen absorption film, and a thermal time constant after the heater heating was stopped. We propose a high-speed response hydrogen gas sensor that measures the hydrogen gas concentration by measuring the temperature several times after the elapse of time. Furthermore, in the hydrogen gas concentration measurement in the high concentration region of 3% or more, the heat conduction type is also used. A hydrogen gas sensor was proposed. Thereafter, various experiments and improvements were repeated, and in particular, the present invention was the result of obtaining the best mode for achieving high sensitivity in an extremely low concentration range of about 1 ppm of hydrogen (H2) gas.

JP 2006-201100 A Japanese Patent Laid-Open No. 10-73530 JP 2005-249405 A Japanese Patent No. 3366590 Japanese Patent No. 3583704 JP 2004-233097 A JP 2008-111182 A

In the contact combustion type hydrogen gas detection sensor disclosed in Patent Document 1, it is heated with a heater so that fine particles such as Pt are supported on an oxide so that it can be burned at a relatively low temperature as a catalyst. If it is a flammable gas, it will react with the gas, the gas selectivity is poor, and even if it is said to be a low temperature by the catalyst, a temperature of 100 ° C. or higher is required. Because of the action of combustion, the presence of oxygen in the atmosphere was indispensable. In particular, since a very small amount of hydrogen gas concentration is measured during heating of the heater, it is necessary to control the heater heating temperature so as to be stable, and a minute temperature rise at a high temperature will be measured. Therefore, the problem of accuracy of the control circuit and the detection circuit has been exposed. In addition, a catalytic reaction is used to burn at as low a temperature as possible. In the catalytic reaction, the surface state of the catalyst is important, and it is made porous to increase the surface area, or platinum (Pt In order to form a catalyst by dispersing fine particles of), repeated heating and cooling may cause changes in the catalyst characteristics, such as changes in the surface state of the catalyst over time or changes in the particle size of platinum (Pt). There was also a problem of doing so. Accordingly, there has been a demand for a stable hydrogen gas sensor that can ignore the change with time and operates at a low temperature without using a catalyst.

Conventionally, there is a semiconductor gas sensor that utilizes gas adsorption on the semiconductor surface, but there is a problem that anything reacts if it is a reducing gas. A sensor that uses the hydrogen storage alloy disclosed in Patent Document 2 and detects the hydrogen gas concentration from the magnitude of strain when absorbing hydrogen is suitable for detecting high concentration hydrogen, In addition to being unsuitable for detecting a wide range of gas concentrations from concentration to high concentration, there is also a problem of fatigue because physical deformation is used. In the sensor disclosed in Patent Document 3, the high power of the Peltier element The problem of consumption and the problem that the sensor itself is inevitably enlarged, the sensor disclosed in Patent Document 6 requires a microcapsule means for coating the hydrogen storage alloy powder particles with a metal film, and mass production The sensor has a problem that the heat capacity is large and the time required for detecting the hydrogen gas concentration takes several minutes or more. It was.

Further, in the hydrogen gas sensor proposed by the present inventor disclosed in Patent Document 7, the hydrogen gas concentration cannot be determined only from the temperature rise due to heat generation, and it is necessary to measure the temperature rise using a different mechanism. In order to solve this problem, the present inventor has invented a “specific gas concentration sensor” (PCT / JP2011 / 070427) and is equipped with a hydrogen absorption film that measures in a low concentration hydrogen gas region of 3% or less. Using a small cantilever-like thin film, we propose a hydrogen gas sensor with a high-speed response within 1 second that measures temperature after several times the thermal time constant after the heater stops heating, and measures the hydrogen gas concentration. In the hydrogen gas concentration measurement in the above high concentration region, we proposed a hydrogen gas sensor that can be used in combination with a heat conduction type. However, there has been a demand for a highly sensitive hydrogen gas sensor that has low hydrogen gas sensitivity in a low concentration range of about 1 ppm or less of hydrogen (H 2) gas and can detect and measure low concentration hydrogen gas.

The present invention has been made in view of the above-mentioned problems. In particular, the hydrogen gas sensor of the “specific gas concentration sensor” (PCT / JP2011 / 070427), which is the present invention of the present inventor, has a low concentration of about 1%. It has been improved by improving the sensitivity so that hydrogen gas can be detected even in the region, and it is small in size that uses temperature changes based on the thermal reaction in the hydrogen absorption film after the heater stops heating, operates at low temperatures, and is mass-produced. An object of the present invention is to provide a hydrogen gas sensor that is productive, inexpensive, has high gas selectivity, is highly sensitive and highly accurate, and can expand the hydrogen gas measurement concentration range.

  In order to achieve the above object, a hydrogen gas sensor according to claim 1 of the present invention includes a thin film 10 thermally separated from a substrate 1 of a hydrogen gas sensor element, a heater 25, a temperature sensor 20, and a hydrogen absorption film 5. Heating and cooling of the thin film 10 are repeated with the heater 25 so that the hydrogen absorbed in the hydrogen absorbing film 5 is released during the cooling process, and the temperature due to the heat generation in the hydrogen absorbing film 5 during the cooling process. In the hydrogen gas sensor in which the hydrogen gas concentration in the atmospheric gas is obtained by using the output of the temperature sensor 20 for the increase, oxygen is contained at least near the surface of the hydrogen absorbing film 5. To do.

There are exothermic reactions and endothermic reactions when metals are combined with hydrogen, such as palladium (Pd) and niobium (Nb), which are somewhat unstable intermetallic hydrates due to exothermic reactions with hydrogen. And a metal in which hydrogen mainly forms a solid solution by an endothermic reaction, such as nickel (Ni), silver (Ag), copper (Cu), and gold (Au), and an exothermic reaction as the hydrogen absorbing film 5 When these are used, it is necessary to make an exothermic reaction as a whole with a single element or alloy of these metals. In general, metals called hydrogen storage alloys, organic materials, ceramics, and the like are designed to generate an exothermic reaction when absorbing hydrogen (including storage and adsorption). For example, the reaction heat of a hydrogen storage alloy of LaNi ₅ is about 7 kcal per mole of hydrogen and a large value of about 0.048 kcal per gram of hydrogen. Conversely, when the metal hydride compound is heated to raise the temperature (at this time, an endothermic reaction occurs), hydrogen is released and the original hydrogen storage alloy is restored. As described above, it is known that the hydrogen absorbing film 5 reversibly absorbs and releases hydrogen, and a large amount of heat enters and leaves accordingly.

On the palladium (Pd) film as the hydrogen absorption film 5, the hydrogen gas molecule (H ₂ ) has both a molecular adsorption state and a dissociative adsorption state. Thus, it is known that dissociated hydrogen atoms are absorbed into the hydrogen absorbing film, and further, when the temperature rises, the dissociated hydrogen atoms can be released again from the hydrogen absorbing film as hydrogen gas molecules (H ₂ ). On the other hand, platinum (Pt) is known to exist only in a dissociative adsorption state. Therefore, it is also known that when a platinum (Pt) film is used as the hydrogen absorbing film 5, it is difficult to smoothly absorb and desorb hydrogen as in the case of a palladium (Pd) film.

Oxygen (O) introduced into the hydrogen absorbing film 5 is generally in a negative ionized state and works to promote dissociative adsorption of hydrogen gas in the atmospheric gas. When oxygen (O) is present in the hydrogen absorbing film 5 such as a palladium (Pd) film in a state of, for example, PdO, the dissociated and adsorbed hydrogen is highly reactive, and the oxygen (O) of the hydrogen absorbing film 5 and Combine and reduce this. At this time, since an exothermic reaction further occurs, the exothermic reaction heat and the exothermic reaction heat when absorbed by the hydrogen absorbing film 5 after the dissociative adsorption of the hydrogen gas to the hydrogen absorbing film 5 are added together to generate a calorific value. To increase. Therefore, by detecting this increased calorific value with a temperature sensor, it becomes possible to detect highly sensitive hydrogen gas, particularly ultra-low concentration hydrogen gas. In particular, a hydrogen (Pd) film having a dissociative adsorption state necessary for absorbing hydrogen gas molecules and a molecular adsorption state capable of releasing dissociated hydrogen atoms again as hydrogen gas molecules (H ₂ ) is used as the hydrogen absorbing film 5. When used, if oxygen (O) is introduced here, reduction reaction by hydrogen occurs near room temperature at a very low temperature, which is not seen with other reducing gases, and heat is generated, maintaining the selectivity of hydrogen gas. Is done.

The hydrogen absorption of the hydrogen absorbing film 5 is more easily absorbed at a low temperature than at a high temperature, and the absorbed hydrogen is released by increasing the temperature. On the other hand, the reduction reaction in the presence of oxygen is more likely to react at higher temperatures. Therefore, both of these exothermic reactions have an optimum temperature, and the maximum exotherm is obtained at that temperature. In order to measure the combined exothermic reaction in the hydrogen absorption process, the temperature sensor 20 provided in the vicinity of the hydrogen absorption film 5 is measured. In the cooling process after the heater 25 is stopped, It is preferable to measure in a time zone that should return to room temperature from the viewpoint of optimum temperature and hydrogen gas selectivity, and from the viewpoint of electrical noise during heater heating.

When the hydrogen absorbing film 5 is heated in the air, a thin oxide film such as oxygen (O) dissociative adsorption or PdO is also formed on the palladium (Pd) surface which is difficult to oxidize. When hydrogen gas (H2) is present in the air, these dissociated and adsorbed oxygen atoms and dissociated and adsorbed states (hydrogen atoms released from the hydrogen absorbing film 5 and dissociated and adsorbed hydrogen atoms introduced from the atmosphere gas) ) Are combined with each other to cause an exothermic reaction, and the hydrogen absorbing film 5 is reduced. Depending on the balance between the heater heating temperature, the amount of oxygen gas in the air, and the amount of hydrogen gas in the air, how the hydrogen absorbing film 5 is reduced varies. That is, when the amount of hydrogen gas in the air is relatively large under the same heater heating temperature, the vicinity of the surface of the hydrogen absorbing film 5 is reduced, and the amount of oxygen is reduced. The amount is reduced and the hydrogen detection sensitivity is reduced. In order to prevent this, introduction of oxygen from the beginning near the surface of the hydrogen absorbing film 5 at least in the amount of oxygen in the air prevents the deterioration of sensitivity. Thus, the main point of the present invention is to introduce oxygen (O) at least near the surface of the hydrogen absorbing film 5 during the deposition process of the hydrogen absorbing film 5. Of course, even if other oxides or metal films are present on the hydrogen absorbing film 5, the presence is allowed if the hydrogen absorbing film 5 does not impair the absorption of hydrogen into the hydrogen absorbing film 5 due to porosity or the like. Is.

When the hydrogen absorbing film 5 is formed into a thin film, it can be easily formed by sputtering, ion plating, electron beam evaporation, etc., oxygen can be easily introduced, the surface area in contact with hydrogen gas is increased, the heat capacity is small and the response speed is high. It is possible to adjust the time until the absorption of hydrogen gas is completed by controlling the thickness, and therefore, it is possible to adjust the temperature rise time by exothermic reaction after heating is stopped, and it is not always necessary to make it porous or fine particles This is advantageous because a flat thin film is sufficient.

After stopping the heating of the thin film 10 on which the hydrogen absorbing film 5 is mounted, after a time of about four times the thermal time constant τ when the hydrogen gas of the thin film 10 is not present, heat is generated by hydrogen on the basis of room temperature. Measuring the temperature increase ΔT based on the reaction is preferable because the zero-position method can be used as it is, and the hydrogen gas concentration can be measured with high sensitivity and high accuracy. However, if the temperature rise is increased by increasing the thermal resistance by providing a slit or the like in the substrate or beam portion as described above, the thermal time constant τ of the thin film 10 increases accordingly, As a result, the response speed as a gas sensor becomes slow. In general, it is known that the thermal time constant τ of the thin film 10 floating in the air is proportional to the square of the length of the same material and the same thickness. For this reason, it is important to shorten the length of the thin film 10 so as to achieve a high-speed response. This is because hydrogen in the hydrogen gas concentration range below the hydrogen gas concentration (peak hydrogen gas concentration) at which the temperature of the temperature sensor 20 reaches the peak is slowly absorbed by the hydrogen absorbing film 5 during the cooling process. In this temperature difference detection, if the thermal time constant τ of the thin film 10 is reduced, the time after about 4 times the thermal time constant τ after the heating is stopped can be shortened. At the same hydrogen gas concentration, the absorption rate does not change, so that a large temperature difference is obtained, leading to the advantage of high sensitivity, high accuracy, and high-speed response. For example, the thermal time constant τ of a cantilever with a length of about 200 micrometers (μm) made of an SOI layer is about 2 milliseconds in the air, although it depends on its thickness. Is measured in about 10 milliseconds, which can be said to be a high-speed response.

After the heating of the heater 25 is stopped, the output of the temperature sensor is used in the atmospheric gas by using the output of the temperature sensor at the time when the time of the thermal time constant τ or more of the thin film 10 when the hydrogen of the heater 25 is not present. When the time of the thermal time constant τ after the heating of the heater 25 is stopped, the temperature at the time when the heating of the heater 25 is stopped is approximately 2.718 times (about 3 minutes). 1) There is a temperature when the heater is heated, and the influence remains. In addition, when about 4 to 5 times the thermal time constant τ has elapsed, the residual temperature (temperature deviating from room temperature) can be regarded as almost zero. It is better to measure the hydrogen gas concentration by using the zero method as it is at the time when about 5 times has passed from the time of double.

The hydrogen gas sensor according to claim 2 of the present invention is a case where the hydrogen absorbing film 5 is a palladium film.

As described above, the palladium (Pd) film as the hydrogen absorption film 5 is different from the platinum (Pt) film in that the hydrogen absorption process is an exothermic reaction, and the hydrogen gas molecules (H ₂ ) are absorbed by molecules. Both dissociative and dissociative adsorption states exist, and the dissociated hydrogen atoms are absorbed into the hydrogen absorbing film through the dissociative adsorption state of the hydrogen gas molecules to the hydrogen absorbing film. It can be released from the absorption film as hydrogen gas molecules (H ₂ ). Therefore, a smooth hydrogen absorption / desorption reaction (absorption and release) is obtained as the hydrogen absorption film 5. Palladium (Pd) is suitable for the hydrogen absorbing film 5 because it is difficult to oxidize and has the property of being easily reduced even when oxidized. Further, as described above, the activated oxygen introduced into the palladium (Pd) membrane and the dissociated and adsorbed hydrogen cause an exothermic reaction even at room temperature and exhibit an excellent effect in hydrogen selectivity. Further, oxygen can be easily introduced during deposition such as sputtering of the palladium (Pd) film as the hydrogen absorbing film 5.

The hydrogen gas sensor according to claim 3 of the present invention is a case where oxygen is introduced in the deposition process of the hydrogen absorbing film 5 so that oxygen is contained at least near the surface of the hydrogen absorbing film 5.

By containing oxygen in at least the vicinity of the surface of the hydrogen absorbing film 5, an exothermic reaction is performed using oxygen in the hydrogen absorbing film 5 in an oxidized or oxygen adsorbed state and dissociated and adsorbed hydrogen. This reaction heat is also used to increase the detection sensitivity of hydrogen gas particularly in a low concentration region. As described above, in the case where the palladium (Pd) film is used as the hydrogen absorbing film 5, as a matter of course, when the hydrogen absorbing film 5 such as another hydrogen storage alloy is used, the hydrogen absorbing film 5 is deposited. At the same time, oxygen can be easily introduced, for example, in the form of co-sputtering.

The hydrogen gas sensor according to claim 4 of the present invention is a case where oxygen is introduced by depositing the hydrogen absorbing film 5 by sputtering deposition in an atmosphere gas containing oxygen.

The deposition of the hydrogen absorption film 5 includes sputtering deposition and vacuum deposition (including electron beam deposition and ion plating). Here, oxygen is introduced at the time of sputtering deposition, and argon gas (sputtering atmosphere gas) ( When Ar gas is mixed with a small amount of oxygen gas (O 2 gas) and deposited, the hydrogen absorption film 5 such as palladium (Pd) reacts with oxygen to deposit oxide such as palladium oxide (PdO). . It is also possible to control the oxidation state of the palladium (Pd) film depending on the amount of mixed oxygen gas. Needless to say, oxygen can be introduced during the deposition of the hydrogen absorbing film 5 in both the vacuum deposition and the reactive deposition in an atmosphere of a small amount of oxygen gas. In order to improve the adhesion between the thin film 10 and the hydrogen absorbing film 5, it is better to insert an active metal such as titanium (Ti) very thin and insert between them. The hydrogen absorbing film 5 is deposited by depositing a hydrogen absorbing film 5 of Pd or the like with a thickness of about 1 micrometer and then introducing a palladium oxide (PdO) layer in the vicinity of the surface by introducing oxygen to 0.01. It is good to introduce it with a thickness of about micrometer. Of course, a pure palladium (Pd) film may be further deposited thereon with a thickness of about 0.01 micrometer, for example.

The hydrogen gas sensor according to claim 5 of the present invention is a case where oxygen is introduced by simultaneously depositing a metal oxide in the deposition process of the hydrogen absorbing film 5.

The hydrogen absorption film 5 has a problem of volume expansion or pulverization due to hydrogen embrittlement due to absorption of hydrogen, and in order to prevent this, silver (Ag), copper (Cu), or the like is used. It is known that about 10% is mixed (including alloying) with palladium (Pd) or the like. Here, when silver (Ag), copper (Cu) or the like for preventing hydrogen embrittlement is deposited by sputtering or the like, oxides such as silver (Ag) and copper (Cu) are also partly included. This is the case where sputtering deposition is performed simultaneously. The oxide is introduced in the case where the oxide is deposited in the vicinity (near the surface) of the deposited hydrogen absorption film 5.

The hydrogen gas sensor according to claim 6 of the present invention includes a thin film 11 that is thermally separated from the same substrate 1 and independent of the thin film 10, and the thin film 11 includes a heater 26 and a temperature sensor 21. The hydrogen absorption film 5 is not provided, and the heater 26 is heated under a predetermined power, voltage, or current, and the temperature is based on the difference in thermal conductivity depending on the hydrogen gas concentration in the atmospheric gas due to the heating of the heater 26. This is a case in which a hydrogen gas sensor element as a heat conduction sensor that can measure the concentration of hydrogen gas at least 1% to 100% by using the output of the sensor 21 is also provided.

The hydrogen gas sensor described above includes the heater 25, the temperature sensor 20, and the hydrogen absorption film 5 having an oxygen-containing layer on the thin film 10. The concentration of hydrogen gas of about 5% or less is reduced during the cooling process after heating. In this case, the hydrogen absorption film 5 absorbs hydrogen gas and reduces the hydrogen gas concentration by measuring the temperature rise based on the heat generation, but the hydrogen gas concentration is different from this mechanism. A hydrogen gas sensor that can be used in combination with a gas sensor, that is, a hydrogen gas sensor as a so-called heat-conducting sensor that utilizes the difference in heat dissipation due to the hydrogen gas concentration of the heated thin film 11, and has a hydrogen gas concentration of at least A wide range from 1% to 100% can be detected.

After the elapse of time of the original thermal time constant τ due to the temperature rise based on the combustion heat of the hydrogen gas due to heating and the slow cooling process based on the absorption of the hydrogen gas in the hydrogen absorption film 5 during cooling after the above-mentioned heating stop. Only the hydrogen gas concentration by temperature measurement has a peak in the characteristics of the hydrogen gas concentration and the measured temperature in the vicinity of the hydrogen absorption film 5 due to the magnitude of the thermal conductivity of the hydrogen gas (the presence of the peak hydrogen gas concentration, Hydrogen gas concentration exists in the vicinity of 5%). Therefore, the same temperature rise exists at the hydrogen gas concentrations on both sides of this peak, and it is impossible to identify a wide range of hydrogen gas concentrations by only measuring the temperature rise once after heating is stopped. It was. For this reason, it is necessary to use a hydrogen gas concentration measurement method having a different mechanism such as a heat conduction type sensor.

Since hydrogen gas has the highest thermal conductivity among gases, mixing other gases with hydrogen gas has a lower thermal conductivity than pure hydrogen gas (hydrogen gas concentration is 100%). It has been found that the thermal time constant when heating and cooling the thin film 11 floating on the surface increases accordingly. Accordingly, conversely, when hydrogen gas is mixed into an atmospheric gas such as pure air or nitrogen gas, and if an exothermic reaction such as combustion is negligible, the thin film 11 formed by the heater 26 under the same power supply is ignored. It has been found that in heating, the heat radiation from the thin film 11 increases in proportion to the hydrogen gas concentration, and the temperature reached by the thin film 11 decreases. Also, in the cooling process after heating, the heat radiation from the thin film 11 also increases in proportion to the hydrogen gas concentration, so that it cools quickly and the thermal time constant decreases. Thus, since the thin film 11 does not have an exothermic reaction based on the absorption of hydrogen gas in the hydrogen absorption film 5, the value of the saturation temperature during heating of the thin film 11 by the heater 26 according to the hydrogen gas concentration, immediately after heating, or heating A hydrogen gas concentration of at least 1% or more can be accurately measured by measuring the passage of a predetermined time in the subsequent cooling process or the passage of time to reach a predetermined temperature. However, in the hydrogen gas concentration range below the peak hydrogen gas concentration (about 5%), the hydrogen gas concentration can be accurately measured from the temperature measurement at the time of cooling when a predetermined time elapses immediately after the heating is stopped, so at least 1% or more It is sufficient if it is possible to measure the hydrogen gas concentration of the hydrogen, and the hydrogen absorption concentration of these hydrogen absorption films 5 is measured by a low hydrogen gas concentration of about 5% or less due to the exothermic reaction by hydrogen and the high hydrogen gas concentration of at least 1% to 100% In this measurement, a wide range of hydrogen gas concentrations can be measured using one hydrogen gas sensor element. Furthermore, by introducing oxygen (O) in the vicinity of the surface of the hydrogen absorbing film 5 according to the present invention, the sensitivity can be increased and measurement of ultra-low concentration hydrogen gas of 1 ppm or less is possible using one hydrogen gas sensor element. become. The hydrogen concentration in the natural air is said to be 0.5 ppm.

The hydrogen gas sensor according to claim 7 of the present invention is a case where the thin film 10 is formed into a cantilever shape.

When the heater 25, the hydrogen absorbing film 5 and the temperature sensor 20 are formed in the cantilever-like thin film 10, the temperature sensor can be formed at the tip of the thin film 10 where the temperature changes most drastically. Therefore, there is an advantage that a highly sensitive hydrogen gas sensor can be provided.

The hydrogen gas sensor according to claim 8 of the present invention is a case where the temperature sensors 20 and 21 are temperature difference sensors.

If a temperature difference sensor that can detect only a temperature difference such as a thermopile or a thermocouple is used as the temperature sensor 20, a reference sensor that does not form the hydrogen absorption film 5 is not necessarily required, and the hydrogen absorption film 5, the temperature sensor 20, and the like. With only one cantilever-like thin film 10 formed with a hydrogen gas concentration below the “peak hydrogen gas concentration” can be measured based on the temperature when no hydrogen gas is present. Further, when the substrate 1 is a cold junction and a thermocouple or a thermopile temperature difference sensor is used as a hot junction in a region of the thin film 10 where the hydrogen absorbing film 5 is provided or a region in the vicinity thereof, In particular, since the temperature difference between the room temperature and the hydrogen absorption film 5 can be taken out as it is, it is very convenient because it can be differentially amplified as it is and the zero method can be applied. These temperature sensors are small and inexpensive, because they are mass-productive. The temperature sensor 21 formed on the thin film 11 has the same effect.

The hydrogen gas sensor according to a ninth aspect of the present invention is a case where a current is supplied to the temperature sensors 20 and 21 to be used as the heaters 25 and 26.

The heater 25 is not provided independently of these temperature sensors, and the temperature sensors 20 and 21 and the heaters 25 and 26 are used as micro heaters by heating them by energizing them.

The hydrogen gas sensor according to claim 10 of the present invention is a case where an absolute temperature sensor is provided on the substrate 1 for measuring the temperature of the atmospheric gas.

The amount of hydrogen gas absorbed into the hydrogen absorption film 5 and the amount of heat generated therewith depend on the ambient gas temperature. In general, the lower the ambient gas temperature, the more hydrogen is absorbed and the more heat is generated. Further, when the temperature reaches about 100 ° C., heat generation based on contact combustion of hydrogen gas occurs, and it is necessary to know the ambient gas temperature. Also, when used as a heat conduction sensor as a hydrogen gas sensor, it is greatly affected because the thermal conductivity of the atmospheric gas changes depending on the magnitude of the atmospheric gas temperature (ambient temperature). A temperature sensor for measuring the temperature of the atmospheric gas is required. The substrate 1 is exposed to the atmospheric gas temperature for a long time. If the temperature sensors 20 and 21 are temperature difference sensors, the cold junction is formed on the substrate. For example, it is preferable to provide the substrate 1 with an absolute temperature sensor for measuring the temperature of the atmospheric gas. As described above, the absolute temperature sensor includes platinum, a semiconductor resistor, a diode, a transistor, and the like.

In the hydrogen gas sensor according to claim 11 of the present invention, the substrate 1 is a semiconductor substrate, and the thin film 10 and the thin film 11 are formed through a sacrificial layer formed over the substrate 1. This is a case where a cavity is formed by etching and an electronic circuit related to a hydrogen gas sensor can be formed on the substrate 1 as necessary.

When a semiconductor substrate is used as the substrate 1, various electronic circuits such as an OP amplifier, a memory circuit, an arithmetic circuit, a heater driving circuit, and a display circuit can be formed here by a mature semiconductor IC technology. If the substrate itself is three-dimensionally processed by MEMS technology using anisotropic etching technology etc., the space for forming these electronic IC circuits will be insufficient, and the substrate will tend to be large, Further, since the anisotropic etching or the like is performed after the formation of the IC electronic circuit in the process, the wiring of the IC electronic circuit or the like may not be able to withstand these anisotropic etching chemicals. In such a case, using the sacrificial layer etching technique, as in the present invention, the thin film 10 or the thin film 11 that is thermally separated from the substrate in the form of being stacked on the substrate and floating in the stacked shape. Forming a thin film of the temperature sensors 20, 21 and the heater 25 and the hydrogen absorption film 5 and forming an IC electronic circuit also on the substrate (for example, a single crystal silicon substrate) corresponding to the lower part, Therefore, a compact hydrogen gas sensor can be provided. In addition, when the thin film 10 and the thin film 11 are formed of polysilicon, an insulating film such as an oxide film can be easily provided, it can be formed like a thermocouple as a temperature difference sensor, the temperature sensor can be used as a heater, hydrogen As the absorption film 5, palladium (Pd) can be easily formed by a dry process using a known MEMS technique, such as being easily formed by sputtering.

The hydrogen gas sensor according to claim 12 of the present invention is a case where at least an electronic circuit is provided to measure the hydrogen gas concentration in the atmospheric gas so that the heaters 25 and 26 can be heated in a predetermined cycle.

The hydrogen gas sensor of the present invention includes an electronic circuit such as an amplifier circuit, an arithmetic circuit, and a memory circuit so that the heaters 25 and 26 can be heated in a predetermined cycle so as to follow a predetermined program by clock pulse generation or transistors. In addition, a modular hydrogen gas sensor is included, and a power supply, an arithmetic circuit, and a display circuit can also be mounted so that the hydrogen gas concentration can be displayed by mounting the module hydrogen gas sensor main body. It refers to a hydrogen gas sensor. The electronic circuit may be provided on the substrate by adopting a semiconductor substrate as the substrate, or may be provided in the vicinity of the hydrogen gas sensor element and modularized. The predetermined cycle is not necessarily a constant cycle, and may be repeated.

In the hydrogen gas sensor of the present invention, since the hydrogen absorption film 5 is formed on the thin film 10 floating in the air from the substrate, the thermal time constant τ as the response speed is several milliseconds although it depends on the dimensions of the thin film 10. High speed operation. Accordingly, since the hydrogen absorbing film 5 is also thin, the process of releasing the hydrogen gas by heating the heater may be 10 milliseconds in this case. Also, in the case of such a cooling process, it takes only 10 milliseconds, and even including the heating / cooling process, about 30 milliseconds is sufficient, and an unprecedented high-speed hydrogen gas sensor can be used. It can be provided.

In the heater heating, the thin film 10 may be heated to a predetermined steady temperature, and the thin film 10 or the thin film 11 may be heated by the heater periodically at a predetermined cycle up to a predetermined temperature based on this temperature. . In any case, the hydrogen absorbing film 5 formed on the hydrogen gas sensor is heated at room temperature, which is the temperature of the atmospheric gas, or at a predetermined power from a predetermined temperature, and the heating is stopped and cooled. The temperature change based on the temperature change based on the adsorption / desorption process of hydrogen gas in the hydrogen absorption film 5 at that time and the temperature change due to the exothermic reaction with oxygen in the vicinity of the surface of the hydrogen absorption film 5, The hydrogen gas concentration is detected by measuring the temperature rise, the temporal change in temperature, and the equivalent change in the thermal time constant. Even if the heater heating is the same when the hydrogen gas concentration is high, in general, the reaction heat in the hydrogen absorption film 5 increases accordingly, and the temperature rise of the thin film 10 on which the hydrogen absorption film 5 is formed increases. . However, if the hydrogen gas concentration is about 5% or more, the cooling effect due to heat radiation based on the high thermal conductivity of the hydrogen gas must be taken into consideration. Although depending on the thickness of the hydrogen absorbing film 5, it is difficult to desorb (release) the hydrogen gas even when the heater is heated, and the equivalent thermal time constant tends to appear large. It is in.

As described above, the present invention is expected to promote the desorption of the hydrogen gas from the hydrogen absorbing film 5 by heating the thin film 10 with the heater 25 and return it to the initial state. . Moreover, since the room temperature which is the temperature of atmospheric gas changes for every measurement by the environment, it is necessary to measure this temperature. In particular, when the cap is covered with an explosion-proof type, the temperature without the cap may be several tens of degrees C. unlike the outside air temperature. Accordingly, the temperature of the substrate 1 is also this temperature, and this is the room temperature that is the reference for the cold junction of the temperature sensor 20 that is a temperature difference sensor. Absorption including absorption and adsorption of hydrogen gas is greater at lower temperatures, but in order to make the initial state or initial conditions constant, the predetermined temperature (for example, a little higher than the ambient temperature of the place where measurement is normally performed) , 30 ° C.), the thin film 10 may be heated by the heater 25.

A hydrogen gas sensor according to a thirteenth aspect of the present invention includes a hydrogen gas sensor that uses reaction heat using a thin film 10 provided with a hydrogen absorption film 5 and heat conduction using the thin film 11 according to the concentration of hydrogen gas in the atmospheric gas. This is a case where it is possible to switch the type of hydrogen gas sensor.

The hydrogen gas sensor of the present invention uses one hydrogen gas sensor element from an ultra-low hydrogen concentration region of 1 ppm or less to a low hydrogen concentration region of about 5% to a high hydrogen concentration region of about 1% or more and 100%. Although it is detected or measured, it is based on a different principle mechanism for measuring the high hydrogen concentration region and the ultra-low hydrogen concentration region. Therefore, in order to measure these, for example, first, a heat conduction type hydrogen gas sensor using the thin film 11 is driven, and based on the output result, it is determined that the hydrogen gas concentration is about 1% or less. In this case, it is possible to switch to a hydrogen gas sensor using reaction heat using the thin film 10 provided with the hydrogen absorbing film 5 having an oxygen-containing layer. In addition, it is preferable to switch the driving voltage of the heater (eventually heating power) according to the hydrogen gas concentration. In addition, the hydrogen gas sensor using reaction heat using the thin film 10 provided with the hydrogen absorption film 5 and the heat conduction type hydrogen gas sensor using the thin film 11 are always operated, and the hydrogen gas is taken into consideration in consideration of sensitivity accuracy. It is also possible to determine which hydrogen gas sensor displays the concentration display. However, when the hydrogen absorbing film 5 is heated at a high temperature and exposed to a high concentration of hydrogen gas, water embrittlement is promoted or the reduction reaction proceeds to cause sensitivity degradation. In order to overcome such problems, the heater heating temperature in the hydrogen gas sensor using reaction heat using the thin film 10 provided with the hydrogen absorbing film 5 is heated at least with a predetermined small electric power, and the output level is taken into consideration. However, when high sensitivity is required, it is preferable to switch and adjust the voltage applied to the heater 25 so that a predetermined optimum higher heating temperature that can achieve high sensitivity is obtained.

A hydrogen gas sensor according to a fourteenth aspect of the present invention is a case where the hydrogen gas sensor element is covered with a cap having a mesh structure, thereby blocking the air flow and making it an explosion-proof type.

As described above, it has been found that the hydrogen gas concentration in the air is explosive in a wide range of 4.0-75.0%. In the present invention, since the hydrogen gas is discharged from the hydrogen absorbing film 5 by the heater 25 or the heater is heated as a heat conduction type sensor, measurement of these hydrogen gas concentration ranges is indispensable. Therefore, the hydrogen gas sensor of the present invention needs to be an explosion-proof type. Conventional technology can be applied to the explosion-proof structure. That is, a mesh structure of metal or the like is suitable, and the hydrogen gas sensor of the present invention measures the temperature of a thin film floating in the air, and therefore extremely dislikes the influence of airflow. Therefore, although the airflow is blocked, it is necessary for the hydrogen gas to reach the detection unit smoothly. For this purpose, a mesh-structured cap that is porous such as metal that blocks airflow is suitable.

In the hydrogen gas sensor of the present invention, the thin film 10 thermally separated from the substrate is provided with a temperature sensor 20 and a hydrogen absorption film 5 such as palladium Pd that absorbs hydrogen gas, and oxygen is present in the vicinity of the surface of the hydrogen absorption film 5. Since it is introduced in advance, an exothermic reaction due to the reduction reaction of the hydrogen absorbing film 5 is added to the exothermic reaction at the time of absorption of hydrogen gas even at a very small amount of hydrogen, so that the temperature change becomes large and ultra-low hydrogen It is possible to provide a hydrogen gas sensor that is highly sensitive and accurate with respect to the concentration, and that oxygen near the surface of the hydrogen absorbing film 5 is not easily lost even under a high concentration of hydrogen gas, so that a stable hydrogen gas sensor can be provided. There are advantages.

In the hydrogen gas sensor of the present invention, oxygen can be introduced in the process of forming the hydrogen absorbing film 5 such as palladium Pd by sputtering, etc., so that there is an advantage that oxygen can be easily introduced near the surface of the hydrogen absorbing film 5.

In the hydrogen gas sensor of the present invention, even if oxygen is simply introduced near the surface of the hydrogen absorbing film 5, the hydrogen absorbing film 5 can be formed into a thin film, so that the surface area in contact with the hydrogen gas is increased and the heat capacity is increased. There are advantages that it is small and has high-speed response, and that the hydrogen gas in the atmospheric gas has a high speed of absorption and desorption from the hydrogen absorption film 5, so that it also has a high-speed response. In addition, the hydrogen absorbing film 5 does not necessarily need to be porous, and may be flat, so that there is an advantage that a hydrogen gas sensor with little change with time can be provided.

In the hydrogen gas sensor of the present invention, the exothermic reaction heat based on the absorption of hydrogen in the cooling process after the heater is heated to release hydrogen from the hydrogen absorption film 5, and the hydrogen absorption film 5 containing oxygen in the cooling process. Since the temperature rise of the thin film 10 based on the sum of the exothermic reaction heat due to the reduction reaction of can be measured after a predetermined time after the original thermal time constant τ when no hydrogen is present, highly sensitive hydrogen Since a zero position method that can be a sensor and has a high S / N ratio and can be extremely accurate can be applied, it is possible to detect and measure an extremely low concentration of hydrogen gas.

In the hydrogen gas sensor of the present invention, a heat conduction type hydrogen gas sensor for measuring a hydrogen gas concentration in a wide range of at least 1% to 100% can be used together. There is an advantage that measurement can be performed with high accuracy in a very wide range of hydrogen gas concentrations up to 100%.

In the hydrogen gas sensor of the present invention, when the heater 25, the hydrogen absorption film 5 and the temperature sensor 20 are formed on the cantilever-like thin film 10, the temperature sensor can be formed at the tip of the thin film 10 where the temperature changes most drastically. Therefore, there is an advantage that a highly sensitive hydrogen gas sensor can be provided. In particular, when a sensor capable of detecting only a temperature difference, such as a thermopile or a thermocouple, is used as the temperature sensor 20, the hydrogen absorption film 5 and the temperature sensor are not necessarily required without a reference sensor not forming the hydrogen absorption film 5. There is an advantage that only one cantilever-like thin film 10 formed can measure a low concentration of hydrogen gas with reference to the temperature when no hydrogen gas is present. At this time, as the heater 25, there is an advantage that the temperature sensor 20 can be used as a heater / temperature sensor by Joule heating. In particular, when the temperature sensor 20 is a thermocouple, the temperature sensor is used as a temperature difference sensor in the cooling process after being heated using the heater 25, so that the zero method can be applied as it is. It is.

In the hydrogen gas sensor of the present invention, the absolute temperature sensor is always exposed to the atmospheric gas, and is provided on a substrate that is substantially the same temperature as the room temperature that is the atmospheric gas temperature. It can be used as a certain room temperature. In particular, when a temperature difference sensor such as a thermocouple is used as the temperature sensor, the substrate is used as the reference temperature of the cold junction, and only the temperature rise from this temperature can be accurately measured. Further, since the amount and speed of the hydrogen gas absorbed by the hydrogen absorbing film 5 are affected by the temperature of the atmospheric gas, the measurement value of the reference temperature is necessary for correction in the measurement of the hydrogen gas concentration. By using the measurement value of the reference temperature, a highly accurate hydrogen gas sensor can be provided. In particular, when the substrate is formed of a semiconductor, there is an advantage that a diode or other semiconductor temperature sensor can be formed by a mature IC technology.

In the hydrogen gas sensor of the present invention, the cantilever-like thin film 10 floating in the air is heated by the thin film heater 25 mounted on the cantilever-like thin film 10, so that there is an advantage that it can be heated and cooled at high speed with low power consumption. Further, since the thin hydrogen absorbing film 5 is used, there is an advantage that complete release of hydrogen by heating is easy and can be performed at high speed.

In the hydrogen gas sensor of the present invention, the substrate 1 is a semiconductor substrate, and the thin film 10 and the thin film 11 formed through a sacrificial layer formed above the substrate 1 can be used. This sacrificial layer is removed by etching. Since a cavity is formed and an electronic circuit can be formed on the substrate 1 as required, there is an advantage that an electronic circuit can be effectively formed even on a small semiconductor substrate and a compact hydrogen gas sensor can be provided.

In the hydrogen gas sensor of the present invention, the hydrogen gas sensor element can be formed in a small size of, for example, about 1 mm square, and can be mass-produced by MEMS technology. Therefore, an extremely small cap having a porous mesh structure is attached to provide an explosion-proof type. There is an advantage that an inexpensive hydrogen gas sensor can be provided.

1 is a schematic plan view showing one embodiment of a hydrogen gas sensor element 100 portion which is a feature of the hydrogen gas sensor of the present invention. FIG. Example 1 It is the cross-sectional schematic in the XX line of FIG. Example 1 It is the plane schematic which shows another Example of 100 parts of hydrogen gas sensor elements used as the characteristics of the hydrogen gas sensor of this invention. (Example 2) It is the cross-sectional schematic which shows another Example of the hydrogen gas sensor element 100 part used as the characteristic of the hydrogen gas sensor of this invention. (Example 3) It is the cross-sectional schematic which shows one Example of the hydrogen gas sensor package which made the hydrogen gas sensor of this invention explosion-proof type. Example 4 It is the block diagram which showed one Example of the hydrogen gas sensor of this invention. (Example 5) It is the block diagram which showed other one Example of the hydrogen gas sensor of this invention. (Example 6)

  The hydrogen gas sensor element, which is the basis of the hydrogen gas sensor of the present invention, can be formed on a silicon (Si) substrate on which an IC can also be formed using mature semiconductor integration technology and MEMS technology. A case where the hydrogen gas sensor element is manufactured using a silicon (Si) substrate will be described in detail below based on an embodiment with reference to the drawings. In addition, a case where the hydrogen gas sensor of the present invention is modularized and a hydrogen gas sensor that is used as a hydrogen gas concentration meter using this module will be described with reference to a block diagram thereof.

  FIG. 1 is a schematic plan view showing an embodiment of a chip-like hydrogen gas sensor element 100 manufactured using a silicon single crystal, which is a feature of the hydrogen gas sensor of the present invention, and FIG. FIG. Here, this is a case where an SOI substrate is used as the substrate 1, and one thin film 10 having a structure floating in the air for thermal separation from the substrate 1 and the thin film 11 as the other thin film are the substrate 1. To cantilever 7 are formed in the same manner. One thin film 10 includes a heater 25, a temperature sensor 20, and a hydrogen absorption film 5. As shown in FIG. 2, the surface of the hydrogen absorption film 5 contains oxygen in the hydrogen absorption film 5 into which oxygen is introduced. Layer 6 is formed. Here, a case where a thermocouple that is a temperature difference sensor that serves as both the heater 25 and the temperature sensor 20 is formed, and a current is passed through the thermocouple to generate Joule heat so that it can also be used as a heater. Show. In this thermocouple, the low-resistance n-type semiconductor SOI layer 12 constituting the cantilever 7 is one thermocouple conductor 120a, and the electrically insulating silicon oxide formed by thermally oxidizing the surface of the SOI layer 12 is used. A metal layer such as nickel formed by sputtering through the film 50 is the other thermocouple conductor 120b. As the hydrogen absorption film 5, palladium (Pd) is deposited by sputtering to a thickness of approximately 1 μm. Thereafter, as the oxygen-containing layer 6 of the hydrogen absorption film 5, palladium (Pd) is vacuum-argon (at the time of sputtering deposition). Ar) A small amount of oxygen gas (O2 gas) is introduced into the atmosphere and discharged, and is deposited to a thickness of about 0.1 micrometers by reactive sputtering deposition. Therefore, pure hydrogen (Pd) is formed in the lower layer as the hydrogen absorption film 5, and the oxygen-containing layer 6, which is palladium (Pd) containing ultrathin palladium oxide (PdO), is formed on the hydrogen absorption film 5. 5 is formed in the vicinity of the surface.

On the other thin film 11, the heater 26 and the temperature sensor 21 are formed in the same manner as the thin film 10 described above. However, since the thin film 11 is used as a heat conduction type hydrogen gas sensor described later, the hydrogen absorbing film 5 is not formed. However, when the thin film 10 is used as a hydrogen gas sensor, if necessary, the hydrogen absorption film 5 is made to have the same shape so that the thin film 11 can be used as a reference sensor, and in an atmosphere gas without hydrogen gas. The thermal time constant τ is almost the same. This is a case where a thin film of palladium (Pd) is formed as the hydrogen absorbing film 5. In addition, current is passed through these temperature sensors to cause Joule heating so that the temperature can be raised to about 200 ° C. Thereafter, in the cooling process after the heater heating is stopped, the original operation as a temperature sensor is used. The temperature sensors 20 and 21 may be absolute temperature sensors such as platinum resistors or pn junction diodes, but here, a temperature difference sensor as a thermocouple that can be used as it is is used as a thermocouple. Gas concentration can be measured. The substrate 1 that is considered to be equivalent to the room temperature that is the temperature of the ambient gas as the reference temperature of the temperature difference sensor, and the thermocouple electrode pads 70 and 71 and the thermocouple so as to be the cold junction of the thermocouple that is the temperature difference sensor. Common electrode pad 75 is provided. Further, in this example, an absolute temperature sensor 23 is provided on the substrate in order to measure the temperature of the substrate 1 which is the reference temperature. Here, the absolute temperature sensor 23 is a pn junction diode.

The operation of the hydrogen gas sensor will be described as follows. When the length of the thin film 10 and the thin film 11 is about 300 micrometers (μm) and the thickness of the SOI layer 12 is about 10 μm, the thermal time constant τ of the thin film 10 and the thin film 11 of the cantilever 7 is 5 milliseconds (mSec). ) When the SOI layer is n-type and a resistivity of about 0.01 Ωcm is used, the thermocouple 120 has a resistance value of about 100 Ω and is heated to about 200 ° C. with a heating power of about 100 milliwatts. First, the thin film 10 and the thin film 11 are simultaneously heated in an atmospheric gas containing hydrogen gas (H ₂ gas), for example, in air. The terminal at this time applies a voltage between the common electrode pad 75 and the electrode pad 70 from the SOI layer 12 of the thin film 10 and causes the temperature sensor 20 to act as a heater so that the temperature from about 150 ° C. By raising the temperature, hydrogen absorbed in the hydrogen absorbing film 5 provided with the oxygen-containing layer 6 near the surface is released.

Next, the applied voltage for heating is set to zero, the heater heating is stopped, and the Seebeck electromotive force between the electrode pad 70 as the temperature sensor 20 and the common electrode pad 75 is measured. After the heating is stopped, the output voltage of the Seebeck electromotive force of the temperature sensor 20 that is a thermocouple becomes zero when hydrogen gas is not present at a time point of about 4 to 5 times the thermal time constant τ, but the thin film 10 Because of the hydrogen absorption film 5, for the exothermic reaction based on the absorption of hydrogen gas during cooling and the exothermic reaction based on the reduction reaction in the oxygen-containing layer 6 existing near the surface of the hydrogen absorption film 5. The temperature rise is observed, and the output voltage (Seebeck electromotive force of the temperature sensor 20) between the electrode pad 70 and the common electrode pad 75 is observed. This value is observed as a monotonous function in the low hydrogen gas concentration range up to the peak hydrogen gas concentration described above, and the hydrogen gas in the atmospheric gas prepared in advance at a specific time elapsed after stopping the heating is observed. The hydrogen gas concentration can be obtained using the relationship data (calibration data) between the concentration and the output voltage. In this case, if the hydrogen gas concentration is 0%, the output voltage of the Seebeck electromotive force of the temperature sensor 20 becomes essentially zero at a time point about 4 to 5 times the thermal time constant τ after the heating is stopped. Since the zero position method can be applied, it is particularly suitable for hydrogen gas concentration measurement in a low hydrogen gas concentration region.

When the hydrogen gas concentration is measured only with the thin film 10 provided with the hydrogen absorbing film 5, the maximum sensitivity is obtained when the hydrogen gas concentration is about 5%. However, if the hydrogen gas concentration is higher than that, the thermal conductivity of the hydrogen gas is reduced. It has been found that, due to the effect, if the hydrogen gas concentration is rather high, the temperature of the thin film 10 starts to drop even when the heater 25 is driven with the same power. Thus, since the hydrogen gas concentration having the peak sensitivity exists in the hydrogen gas concentration, the thin film 10 and the thin film 11 are heated at the same time, and the temperature reached by the thin film 11 immediately after the heating is stopped is determined by the temperature sensor 20 and the temperature sensor 21. Then, the output voltage is measured, and the relational data between the hydrogen gas concentration at this time and the temperature reached when heated at a predetermined power is obtained in advance, and the heat conduction type hydrogen gas sensor of the thin film 11 is used as a reference. Thus, it is preferable to obtain a rough value of a wide range of hydrogen gas concentrations in the atmospheric gas.

During the heater heating time, there is a rise in temperature due to the release of hydrogen gas from the hydrogen absorbing film 5 and contact combustion. This heating period is also set as a predetermined time, and under that, the hydrogen gas concentration and the predetermined power are used. It is necessary to obtain in advance the relationship data (calibration data) with the temperature reached when heated. If the heating time is several times the thermal time constant τ of the thin film 10, for example, 4 to 5 times, stable and reproducible data can be obtained. Further, the output information of the temperature rise based on the combustion of the hydrogen gas when heated can also be used as confirmation information in the concentration region higher or lower than the peak hydrogen gas concentration of the hydrogen gas.

The outline of the manufacturing process for processing the substrate 1 in the hydrogen gas sensor of the present invention shown in FIGS. 1 and 2 will be described as follows. When the SOI layer 12 of the substrate 1 is n-type, since the thermocouple 120 is used as the temperature sensors 20 and 21 and the heater 25, the ohmic contact is obtained to obtain a good ohmic contact by a known semiconductor microfabrication technique. An n-type thermal diffusion region is preferably formed at the location of the conductive electrode 60. Further, although a pn junction diode is formed as the absolute temperature sensor 23 provided on the substrate 1, it can be easily formed by a known diffusion technique. As the thermocouple conductor 120b of the thermocouple metal, differential amplification is performed. Therefore, in consideration of the Seebeck effect, all the wirings and electrode pads must be made of the same metal. Nichrome and nickel (Ni) based metals are suitable because they are resistant to strong alkaline etchants. When not exposed to a strong alkali-based etchant by dry etching or the like, an ohmic electrode or wiring 110 and an electrode pad are preferably formed by using an aluminum (Al) -based metal and forming the sputtering thin film and photolithography. There is a dedicated etchant for patterning the Pd hydrogen absorbing film 5, and dry etching is performed as necessary. The cavity 40 and the slit 41 formed in the substrate 1 are formed from the back surface by an etchant or DRIE and penetrated. In this case, the terminals on the n-type SOI layer 12 side serving as cold junctions on the substrate side of the thermocouples of the temperature sensor 20 and the temperature sensor 21 formed on the thin film 10 and the thin film 11 are one common electrode pad 75. It is said.

The hydrogen gas sensor shown in FIG. 1 and FIG. 2 includes a thin film 11 that is thermally separated from the same substrate 1 and is independent of the thin film 10. The thin film 11 includes a heater 26 and a temperature sensor 21. The hydrogen absorbing film 5 is not provided, and the heater 26 is heated under a predetermined power, voltage, or current, and is based on the difference in thermal conductivity depending on the hydrogen gas concentration in the atmospheric gas due to the heating of the heater 26. Although the concentration of hydrogen gas from 0 to 100% can be measured using the output of the temperature sensor 21, the heat conduction is such that the concentration of hydrogen gas from at least substantially 1% to 100% can be measured. It can be operated as a mold sensor. This is because the heater 26 is driven with a predetermined power, for example, 200 milliwatts, and the temperature of the thin film 11 during heating or the temperature after a predetermined elapsed time after heating is used by using the output of the temperature sensor 21. This is based on the principle that the hydrogen gas concentration in the atmospheric gas is measured, and that the higher the hydrogen gas concentration, the smaller the temperature rise of the thin film 11 under the same heater power. Since hydrogen gas has the highest thermal conductivity in the gas, it can be measured even with a minute hydrogen concentration of about 1%, and shows a monotonic temperature change from 0% to 100% concentration under a predetermined heater power. It can operate as a stable hydrogen gas concentration sensor.

FIG. 3 is a schematic plan view showing another embodiment of the hydrogen gas sensor element 100 characteristic of the hydrogen gas sensor of the present invention. The hydrogen gas sensor element 100 shown in FIGS. 1 and 2 of Example 1 is greatly different from the hydrogen gas sensor element 100 shown in FIGS. 1 and 2 in which the thin films 10 and 11 have a cantilever structure, whereas in FIG. This is a point constituted by beams 18 supported at both ends. By doing so, there is a problem that heat conduction to the substrate 1 is likely to increase, and even if heat generation due to hydrogen absorption is the same, there is a problem that the temperature rise is reduced, but the response speed is increased accordingly, and the thin films 10 and 11 are formed. The temperature sensors 20 and 21 are also provided with electrode pads 70 and 71 and a common electrode pad 75 on one side of the substrate 1 as in the case of FIGS. 1 and 2 of the first embodiment. It is an example. However, when complicated wiring is required to separate the heaters 25 and 26 and the temperature sensors 20 and 21, these electrodes are provided on both sides of the both-end support portion in the bridge structure 3 of the substrate 1 as necessary. It is suitable for forming a pad. Here, the heaters 25 and 26 and the temperature sensors 20 and 21 are formed on the thin films 10 and 11, respectively, but these are also cases where they serve as heater / temperature difference sensors. Note that the method for measuring the hydrogen gas concentration is the same as that in the first embodiment, and a description thereof will be omitted here.

Although the thin film 10 and the thin film 11 have the crosslinked structure 3, in order to obtain a large temperature rise even with a slight heat generation, it is preferable that the length of the beam 18 of the crosslinked structure 3 is long. In addition, when an SOI substrate made of a silicon single crystal is used for the substrate 1, the orientation of the crystal is important for performing three-dimensional processing such as forming the cavity 40 or the slit 41 with an etchant on the substrate 1 by MEMS technology. . This is because the crystal orientation is used to form a high-precision cavity 40, such as applying an etch stop, utilizing the fact that the etching rate of the (111) plane of the crystal is extremely slower than other orientations. It is. In order to form the beam 18 having the long bridge structure 3 in the narrow cavity 40, the crystalline silicon is etched in as short a time as possible in consideration of the angle and the width with respect to the crystal orientation of the beam 18 of the bridge structure 3. Thus, it is necessary to form the beam 18 having the long bridge structure 3. In this embodiment, the beam 18 of the long bridge structure 3 has an angle of 45 degrees with respect to the length direction of the cavity 110 in the (110) direction with respect to the surface of the (100) crystal plane of the substrate 1. It can also be considered that the beam 18 of the bridging structure 3 with respect to the both end support portions is like a cantilever as a whole. In the thin film 10, the hydrogen absorbing film 5 having the oxygen-containing layer 6 near the surface is provided in a region that is also the tip of the cantilever and the central part of the beam 18 of the bridge structure 3. The absorption film 5 is not provided. In this case, the temperature sensors 20 and 21 also use thermocouples which are temperature difference sensors, and the heaters 25 and 26 can also be used. These thermocouples are cases where the n-type SOI layer 12 as the thermocouple conductor 120a and the metal film such as nickel or nichrome as the thermocouple conductor 120b are used. The terminals of the n-type SOI layer 12 as the thermocouple conductor 120a are common to the temperature sensors 20 and 21, and are the common electrode pad 75 through the ohmic electrode 60 at the same location.

The manufacturing process of the hydrogen gas sensor element 100 of FIG. 3 is the same as in the case of the above-described first embodiment by a known MEMS technique, and is easy, so detailed description thereof is omitted here. Further, the hydrogen gas concentration measurement is substantially the same as in the first embodiment. In the drawing of FIG. 3 here, the absolute temperature sensor 23 for knowing the absolute temperature of the substrate 1 is also omitted.

FIG. 4 is a schematic cross-sectional view showing another embodiment of the hydrogen gas sensor element 100 characteristic of the hydrogen gas sensor of the present invention. In the above-described embodiment, the thin film 10 and the thin film 11 provided in the hydrogen gas sensor element 100 are thermally separated from the substrate 1 by three-dimensionally processing the substrate 1 itself such as silicon to have the cavity 40 in the lower part. Was. In this embodiment, a sacrificial layer etching technique known in the MEMS technology is used, and a sacrificial layer film (not shown here; embedded in the cavity 40 but etched away when the cavity is formed). A thin film to be a cantilever or a bridge 18 of the bridge structure 3 is formed on top of it, and then the sacrificial layer is removed by etching to create 40 cavities on the substrate 1 to form the thin film 10 or the thin film 11 ( 4 is formed in the shape of the cantilever 7 or the bridge structure 3 to achieve thermal separation from the substrate 1. When the thin film 10 and the thin film 11 are formed on such a substrate 1, various IC electronic circuits 600 for operating as a hydrogen gas sensor on the substrate 1 corresponding to the lower portion thereof, for example, an OP amplifier, Various amplifier circuits, driving circuits for heaters 25 and 26, arithmetic circuits, memory circuits, control circuits, display circuits, and the like are created in advance, and then the thin film 10 and the thin film 11 are formed. In this way, a hydrogen gas sensor can be provided in which the shape of the hydrogen gas sensor element is compact and the electronic circuit is integrated. If the main body of the thin film 10 or the thin film 11 is made of polysilicon, the heaters 25 and 26, the thermocouple, and the like can be easily formed. In the embodiment of FIG. 4, the thin film 10 and the thin film 11 are formed of an n-type low-resistance polysilicon thin film (here, it is used as the thermocouple conductor 120 a and is also a part of the heater 25). ), As a result of the sacrificial layer etching, the cavity 40 is formed, and the anchor portion 360 is fixed to the silicon single crystal substrate 1 to form a structure that is thermally separated from the self-supporting substrate 1. The hydrogen absorbing film 5 provided with the oxygen-containing layer 6 in the vicinity of the surface also uses palladium (Pd), and can be formed by sputtering in the same manner as in the above-described embodiment. The operation as a hydrogen gas sensor is also the same as that of the above-described embodiment using the thin film 10 and the thin film 11 formed by processing the substrate 1 itself, and the description thereof is omitted here.

FIG. 5 shows the hydrogen gas sensor package of the explosion-proof hydrogen gas sensor package, in which the hydrogen gas sensor element 100 described in the above-described embodiment of the hydrogen gas sensor of the present invention is covered with a cap 200 having a mesh structure, thereby blocking airflow. It is the cross-sectional schematic which shows one Example. The hydrogen gas sensor element 100 is bonded to an element holder 500 made of an alumina substrate or a flame retardant plastic substrate, and an electrically insulating lead holder 350 having leads 300 is bonded to electrically connect the lead bonding portion 310. The lead 300 and the electrode pads of the hydrogen gas sensor element 100 are joined to each other. Further, power supply and input / output of electric signals are performed between the outside and the hydrogen gas sensor element 100 through the lead 300. Further, the cap 200 having a mesh structure is joined so that the porous mesh structure 210 is located near the cavity 40 where the thin film 10 and the thin film 11 of the hydrogen gas sensor element 100 are provided. Further, it is more effective if the element holder 500 is also provided with the mesh structure portion 210 as necessary. In the present embodiment, the element holder 500 is also provided with a mesh structure 210.

FIG. 6 is a block diagram showing another embodiment of the hydrogen gas sensor of the present invention, in which a hydrogen gas sensor package incorporating a characteristic hydrogen gas sensor element 100 and an electronic circuit are integrated into a module. It is. In this embodiment, a heater drive circuit, an amplifier circuit, and an arithmetic circuit are used as electronic circuits. Here, it is a case where a power source is obtained from the outside, and an output signal terminal is provided which can take out a signal related to the hydrogen gas concentration to the outside.

FIG. 7 is a block diagram showing another embodiment of the hydrogen gas sensor of the present invention. A hydrogen gas sensor obtained by integrating the hydrogen gas sensor package and the electronic circuit shown in the embodiment 7 into a module is further shown. This is a hydrogen gas sensor that can be provided as a hydrogen gas concentration meter, in which a power supply circuit, a control circuit, and a display circuit that can display the hydrogen gas concentration are also incorporated into a device. Furthermore, this is a case where an output signal terminal that can output a hydrogen gas concentration signal is provided outside.

In the above description, the hydrogen gas sensor element is obtained when the n-type SOI layer 12 is used. Of course, a similar sensor can be achieved even when the p-type SOI layer 12 is used.

The hydrogen gas sensor of the present invention is not limited to the present embodiment, and various modifications can naturally be made while the gist, operation and effect of the present invention are the same.

  In the hydrogen gas sensor of the present invention, an oxygen-containing layer formed in the vicinity of the surface of the hydrogen absorption film 5 and the exothermic reaction due to absorption in the hydrogen absorption film 5 formed in the thin film 10 floating in the air, and the concentration of hydrogen gas in the atmospheric gas. The temperature increase due to the sum of the exothermic reaction based on the hydrogen reduction of 6 is measured by the temperature sensor 20 which is a temperature difference sensor formed there, so that a very small amount of hydrogen concentration of about 1 ppm can be detected. It is a thermal sensor for measuring with high sensitivity and high accuracy. Further, a high-concentration hydrogen gas region of at least 1% to 100% is also formed on the same substrate 1 by using a thin film 11 which is the same as the thin film 10 but does not have the hydrogen absorption film 5. It can be used together as a conduction type hydrogen gas sensor. For example, when the hydrogen gas sensor of the present invention is applied to measure the concentration of hydrogen gas in the atmosphere, an ultra-small thermocouple that detects only a temperature difference with high sensitivity and high accuracy on a thin film suspended in the air. It is possible to apply the zero method by measuring the Seebeck electromotive force of the temperature difference sensor after a predetermined time after stopping the heater heating, especially at a low hydrogen concentration of 4% or less of the explosion limit in air or about 1 ppm. The ultra-low hydrogen concentration can be measured with extremely high accuracy. In this way, a wide range of hydrogen gas concentrations from 0.5 ppm to 100%, which is a hydrogen gas concentration that exists in nature, can be detected and measured by one hydrogen gas sensor element, so that its industrial application is wide.

DESCRIPTION OF SYMBOLS 1 Substrate 3 Crosslinking structure 5 Hydrogen absorption film 6 Oxygen-containing layer 7 Cantilever 10, 11 Thin film 12 SOI layer 13 BOX layer 18 Beam “
20, 21 Temperature sensor 23 Absolute temperature sensor 25, 26 Heater 40 Cavity 41 Slit 50 Silicon oxide film 51 Electrical insulating film 60 Ohmic electrode 70, 71 Electrode pad 75 Common electrode pad 100 Hydrogen gas sensor element 110 Wiring 120a, 120b Thermocouple conductor 200 Cap 210 Mesh structure part 300 Lead 310 Lead joint part 350 Lead holder 360 Anchor part 400 Air gap 500 Element holder 600 IC-ized electronic circuit

Claims (14)

The thin film (10) thermally separated from the substrate (1) of the hydrogen gas sensor element is provided with a heater (25), a temperature sensor (20), and a hydrogen absorption film (5), and the thin film (10) is heated by the heater (25). And repeating the cooling so that the hydrogen absorbed in the hydrogen absorbing film (5) during the cooling process is released during the heating process, and the temperature rise due to the exothermic action in the hydrogen absorbing film (5) during the cooling process. In the hydrogen gas sensor that uses the output of the sensor (20) to determine the hydrogen gas concentration in the atmospheric gas, oxygen is contained at least near the surface of the hydrogen absorbing film (5). A hydrogen gas sensor. The hydrogen gas sensor according to claim 1, wherein the hydrogen absorbing film is a palladium film. 3. The hydrogen gas sensor according to claim 1, wherein oxygen is introduced during the deposition of the hydrogen absorption film (5) so that oxygen is contained at least near the surface of the hydrogen absorption film (5). The hydrogen gas sensor according to claim 3, wherein the hydrogen absorption film (5) is deposited by sputtering deposition in an atmosphere gas containing oxygen, and oxygen is introduced. The hydrogen gas sensor according to claim 3, wherein oxygen is introduced by simultaneously depositing a metal oxide in the deposition process of the hydrogen absorbing film (5).   While being thermally separated from the same substrate (1) and provided with a thin film (11) independent of the thin film (10), the thin film (11) includes a heater (26) and a temperature sensor (21). The hydrogen absorbing film (5) is not provided, the heater (26) is heated under a predetermined power, voltage or current, and the heat due to the hydrogen gas concentration in the atmospheric gas due to the heating of the heater (26). Also equipped with a hydrogen gas sensor element as a thermal conductivity sensor that can measure the concentration of hydrogen gas at least 1% to 100% using the output of the temperature sensor (21) based on the difference in conductivity. The hydrogen gas sensor according to claim 1. The hydrogen gas sensor according to any one of claims 1 to 5, wherein the thin film (10) has a cantilever shape. The hydrogen gas sensor according to any one of claims 1 to 6, wherein the temperature sensor (20, 21) is a temperature difference sensor. The hydrogen gas sensor according to any one of claims 1 to 8, wherein an electric current is passed through the temperature sensor (20, 21) to be used as the heater (25, 26). The hydrogen gas sensor according to any one of claims 1 to 9, wherein an absolute temperature sensor is provided on the substrate (1) for measuring the temperature of the atmospheric gas. The substrate (1) is a semiconductor substrate, and a thin film (10) or a thin film (11) is formed through a sacrificial layer formed above the substrate (1), and the sacrificial layer is removed by etching. 11. The hydrogen gas sensor according to claim 1, wherein a cavity is formed, and an electronic circuit related to the hydrogen gas sensor can be formed on the substrate (1) as necessary. The hydrogen gas sensor according to any one of claims 1 to 11, wherein at least an electronic circuit is provided to measure the hydrogen gas concentration in the atmospheric gas so that the heaters (25, 26) can be heated in a predetermined cycle. A hydrogen gas sensor using reaction heat using a thin film (10) provided with a hydrogen absorption film (5) and a heat conduction type hydrogen gas sensor using a thin film (11) according to the hydrogen gas concentration in the atmospheric gas. The hydrogen gas sensor according to claim 1, wherein the hydrogen gas sensor can be switched. The hydrogen gas sensor according to any one of claims 1 to 13, wherein the hydrogen gas sensor element is covered with a cap having a mesh structure to block an air flow and to be an explosion-proof type.

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