JP2008111822A - Gas sensor element and gas concentration measuring device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small-sized gas sensor element with high selectivity for gases and high sensitivity that senses concentration of a gas in an object with gases, such as hydrogen and others, without the use of the presence of a specific gas such as oxygen, and a gas concentration measuring device using the same. <P>SOLUTION: The gas sensor element and the gas concentration measuring device using the same are provided, in which a thin film 10 heat-separated from a substrate 1 is divided to two thin films with thermal resistance sections 45 so that they have thermal resistances respectively, with a heater and a temperature sensor 20a installed in one of the thin films and a gas absorber 5 and a temperature sensor 20b in the other, and changes in temperature resulting from heat absorption and generation in absorption and discharge of a gas to be sensed in the gas absorber 5 are measured by a temperature sensor 20b. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a gas sensor, and a gas sensor element mainly using a temperature change based on a thermal reaction at the time of absorption (including occlusion and adsorption) or release of a specific gas component in gas, particularly hydrogen or water vapor, and the like. The present invention relates to a gas concentration measuring apparatus using the above.

  Conventionally, gas sensors include a catalytic combustion type hydrogen gas detection sensor (see Patent Document 1) in which the temperature of a catalyst such as Pt is raised by a heater and combined with this catalytic action.

Some semiconductor gas sensors use a change in electrical resistance by utilizing a change in carrier density on the semiconductor surface due to reducing gas adsorption.

In addition, there has been a sensor that enhances gas selectivity by utilizing absorption and permeation of specific gases such as hydrogen and oxygen. 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 a temperature sensor capable of measuring an absolute temperature and a temperature sensor capable of measuring only a temperature difference. As a 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, Further, 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. Moreover, as a temperature sensor that can measure only a temperature difference, there are a thermocouple and a thermopile in which the output voltage is increased by connecting the thermocouple in series, and a current detection type thermocouple invented by the present applicant (see Patent Document 6). is there.

The current detection type thermocouple is a pair of thermocouples, and if the internal resistance of the conductor constituting the thermocouple is reduced and the short-circuit current is measured by using the short circuit of the OP amplifier, etc., This is a temperature sensor that can detect a temperature difference that is superior to a thermopile that detects a temperature difference with an open-circuit voltage.

In addition, the applicant first applied for a heat-conducting sensor, and formed two micro heaters and two temperature sensors on a thin film suspended in the air, at least one of which was formed on a cantilever provided at the tip of the thin film. It has been shown that vacuum can be detected with high sensitivity when formed, and that the above-described current detection type thermocouple can be used as this temperature sensor to achieve particularly high accuracy and high sensitivity (Japanese Patent Application No. 20006-224180).
JP 2006-201100 A Japanese Patent Laid-Open No. 10-73530 JP 2005-249405 A Japanese Patent No. 3366590 Japanese Patent No. 3583704 JP 2005-221238 A

The hydrogen gas detection sensor of the contact combustion type shown in Patent Document 1 is heated by a heater and burns at a relatively low temperature using Pt or the like as a catalyst, and uses the reaction heat at that time, and is combustible. If it is a gas, it will react with the gas, and the selectivity of the gas is poor, and even if it is a low temperature, it requires a temperature of 100 ° C. or higher, and uses the action of combustion, so in the atmosphere The presence of oxygen was indispensable.

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 shown in Patent Document 2 and detects the hydrogen concentration from the magnitude of strain when absorbing hydrogen is suitable for detecting a high concentration of hydrogen. In addition to being unsuitable for detecting gas concentrations in a wide range up to the concentration, there is also a problem of fatigue because physical deformation is used.

In addition, the sensor disclosed in Patent Document 3 is a temperature control unit that controls a crystal resonator, which is a detection unit that detects a change in state (weight change) when hydrogen is absorbed, and a detection element at substantially the same temperature. There is a need to incorporate a Peltier element, and there is a problem of high power consumption of the Peltier element and a problem that the sensor itself is inevitably enlarged.

The present invention has been made in view of the above circumstances, and is suitable for absorption (including occlusion and adsorption) and release of specific gas components in gas, particularly hydrogen, oxygen, water vapor, carbon dioxide gas, and other thermal reactions. A small, low temperature operation that does not require the presence of oxygen and the like, is mass-productive, is inexpensive, has high gas selectivity, and has a high sensitivity. It aims at providing the gas concentration measuring apparatus using this.

  In order to achieve the above object, a gas sensor element according to claim 1 of the present invention absorbs one or a plurality of temperature sensors 20 and a gas to be detected (absorbs or absorbs) the thin film 10 thermally separated from the substrate. Gas absorption substance 5 to be included, and the temperature sensor 20 is arranged and formed so as to be able to measure a temperature change accompanying heat absorption or heat generation during absorption or release of the gas to be detected.

Taking hydrogen as an example of the gas to be detected, palladium (Pd), platinum (Pt), and metals called hydrogen storage alloys generally react 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 is heated (endothermic reaction) to raise the temperature, hydrogen is released and the original hydrogen storage alloy is restored. Thus, it is known that a hydrogen storage alloy reversibly absorbs and releases hydrogen, and a large amount of heat enters and exits accordingly. As described above, generally, when the gas absorbing material 5 absorbs the gas to be detected (including adsorption and occlusion), the gas absorbing material 5 rises in temperature because it is an exothermic reaction. Conversely, when the gas to be detected that has been absorbed by the gas absorbing material 5 is released, it is an endothermic reaction, so the temperature of the gas absorbing material 5 will drop. These temperature changes are detected by the temperature sensor 20, and the concentration of the gas to be detected is to be measured. However, when the gas absorption material 5 is heated to release the gas to be detected, the catalytic action of the gas absorption material 5 generates heat in the presence of oxygen at a low temperature as in the catalytic combustion type. A reaction may occur. Actually, the temperature change including this is measured.

The gas sensor element of the present invention has a minute amount due to heat generation and heat absorption during absorption and release of a gas to be detected by a gas absorption material 5 such as a hydrogen storage alloy formed on a thin film (thin film suspended in the air) thermally separated from the substrate. A temperature change is detected by a highly sensitive temperature sensor formed in a thin film floating in the air.

Forming the gas-absorbing material 5 such as a hydrogen-occlusion alloy in a thin film is advantageous because the surface area in contact with the gas to be detected is increased, the adsorption / desorption process is accelerated, the heat capacity is small, and the high-speed response is provided. If the porous film is made more porous, the surface area of contact with the gas to be detected further increases, so that the amount of gas to be detected increases and the sensitivity is increased accordingly.

In addition, when detecting the temperature of the gas absorbing material 5 based on the absorption and desorption of the gas to be detected by a highly sensitive temperature sensor formed in a thin film floating in the air, if there is an air current, the temperature change due to this air current is extremely large. It is necessary to provide an appropriate cap, for example, a porous cap, in the sensing portion where the gas absorbing material 5 is formed to prevent this air flow. However, the gas to be detected such as hydrogen gas needs to reach the gas absorbing material 5 by diffusion through the porous layer. Covering with a porous cap is preferable because it functions as an explosion-proof hydrogen sensor.

As the above-described temperature sensor 20, a pn junction diode or a transistor that can be formed into an IC and can be made into a thin film can be used. Since these can be handled like a thermistor, they are called diode thermistors and transistor thermistors, respectively, which can measure absolute temperature, and can measure the temperature of thin films with extremely high sensitivity, and are small and mass-productive. There is so cheap.

The gas sensor element according to claim 2 of the present invention is a case where at least one of one or a plurality of temperature sensors 20 is formed on the protrusion of the cantilever-like thin film 10 provided with the gas absorbing material 5.

The thin film 10 floating in the air itself may be formed in a cantilever shape from the substrate, and the gas absorbing material 5 may be provided there, or the thin film 10 floating in the air may be divided and connected to the substrate for supporting the thin film 10. The gas absorbing material 5 may be provided in a cantilever shape protruding from the beam portion. The cantilever forming the gas absorbing material 5 is provided with a slit or the like on the substrate or the beam (beam) portion so that the heat generated in the gas absorbing material 5 does not easily escape to the substrate or the beam (beam) portion. It is better to form so as to have thermal resistance. Since the gas absorbing material 5 is formed on the thin film formed in a cantilever shape, the heat generated in the gas absorbing material 5 is difficult to escape to the substrate and the beam (beam) portion. For this reason, the temperature rise of the thin film portion formed in a cantilever shape becomes large even with a small amount of generated heat. Therefore, by providing the temperature sensor on the protrusion of the cantilever-like thin film 10 on which the gas absorbing material 5 is formed, a highly sensitive and highly accurate gas sensor can be obtained.

Two cantilever-like protrusions forming a temperature sensor are provided, and one cantilever-like protrusion is provided with a gas absorbing material 5 so as to be exposed to the gas to be detected as a detection part, and the other cantilever-like protrusion is provided. The protruding portion is used as a reference portion for comparison with the detecting portion, and the gas absorbing material 5 is provided there or not provided so that the mass at the protruding portion of the cantilever-like thin film 10 of the detecting portion is substantially equal. In addition, a film that does not react with the gas to be detected is formed. However, when the gas absorbing material 5 is provided also in the reference portion, it is necessary to shield it from being exposed to the gas to be detected. With this configuration, even if the temperature sensor is an absolute temperature sensor, the film of the gas absorbing material 5 such as Pd exposed to the detection gas such as hydrogen gas absorbs the detection gas and generates heat. Since the reaction occurs, the temperature change is compared with the temperature of the reference portion, and the content of the gas to be detected in the gas can be measured using the calibration curve. Note that the reaction between the gas to be detected and the gas absorbing material 5 has its optimum environmental temperature, so it is preferable to set it to a predetermined appropriate temperature with a heater or the like. Of course, it is also possible to temporarily set the set temperatures to a plurality of different temperatures and use comparison of temperature change data based on the thermal reaction between the gas absorbing material 5 and the gas to be detected at those temperatures.

The gas sensor element according to claim 3 of the present invention is a case where at least one of the temperature sensors 20 is a temperature difference detection sensor. The temperature sensor 20 provided on the thin film 10 provided with the gas absorbing material 5 is preferably configured to detect only a temperature change based on the reaction between the gas to be detected and the gas absorbing material 5 from a predetermined optimum environmental temperature. This is because two temperature sensors that detect a predetermined optimum ambient temperature and a temperature sensor that detects a temperature change based on the reaction between the gas to be detected and the gas absorbing material 5 are required. Both temperature sensors are absolute temperature sensors. If this is the case, problems with poor temperature accuracy at the time of measurement at a temperature that deviates from the temperature at the time of calibration or the temperature change at the time of calibration will surface. In addition, even if it is attempted to detect a very slight temperature change, the problem of being in the error range is encountered.

In such a case, using a temperature difference detection sensor such as a thermocouple or a thermopile that can detect only a temperature change based on the reaction between the gas to be detected and the gas absorbing material 5, for example, this hot junction is connected to the gas absorbing material 5. The temperature change based on the reaction of the gas to be detected to the gas absorbing material 5 can be detected with high sensitivity by installing the cold junction at the location of the temperature reference. That is, when the gas absorption material 5 absorbs or discharges the gas to be detected and no temperature change due to heat generation or heat absorption occurs, the output voltage is essentially zero, so zero is the reference (zero reference method). Temperature detection, that is, detection of the presence of the gas to be detected can be achieved with high sensitivity and high accuracy.

The gas sensor element according to claim 4 of the present invention is a case where the temperature difference detection sensor which is the temperature sensor 20 is a current detection type thermocouple. A current detection type thermocouple is a sensor that detects a short-circuit current based on a detected temperature difference between a pair of thermocouples, instead of measuring an open electromotive force based on a detected temperature difference between a conventional thermocouple and a thermopile, There is a sensor that can detect only the temperature difference with high sensitivity. A hot junction of this thermocouple is formed on the portion of the cantilever-like thin film 10 provided with the gas absorbing material 5, and a cold junction is formed at a specific location such as near the heater of the thin film portion used as the substrate or the reference temperature. Then, accurate temperature detection using the reference temperature as the above-described zero reference method, that is, detection of the presence of the gas to be detected can be achieved with high sensitivity and high accuracy.

The gas sensor element according to claim 5 of the present invention is a case where the thin film 10 can be heated by providing a heater in the thin film 10 or by providing a heater in the vicinity of the thin film 10. The gas absorbing material 5 such as a hydrogen storage alloy absorbs gas at a low temperature and releases the absorbed gas at a high temperature. The temperature can be raised from room temperature or a specific temperature, for example, 30 ° C. to a predetermined high temperature, for example, 100 ° C., and the absorbed gas can be released to return to the initial value. Then, the gas to be detected can be absorbed again in the process of being cooled to room temperature or a specific temperature. In order to do this, a heater is required, and in order to reduce the heat capacity and thermal conductance of the heater to reduce power consumption and improve high-speed response, the heater is formed on the thin film 10 or in proximity. A heater is provided, and this heater is also advantageously a thin film heater that is thermally separated from the substrate. In addition, since flammable hydrogen is detected, it is better to use a so-called explosion-proof structure such as a porous metal film.

In the gas sensor element according to claim 6 of the present invention, the thin film 10 is divided into two or more thin films, one of the divided thin films is provided with a temperature sensor 20a, and the other thin film is provided with the temperature sensor 20a. Includes the temperature sensor 20b and the gas absorbing material 5, the one temperature sensor 20a measures the temperature of the heater and can be used for the temperature control of the heater, and the other temperature sensor 20a. This is a case where the temperature sensor 20b is used to detect a temperature change accompanying heat absorption or heat generation of the gas to be detected.

The temperature sensor that measures the temperature of the heater should be made to know the absolute temperature of the heater. For example, the above-described diode thermistor may be used. On the other hand, the other temperature sensor 20b detects a temperature change accompanying heat absorption or heat generation in the gas absorbing material 5 of the gas to be detected, so that it has a thermal resistance from the thin film region where the heater is formed. A temperature sensor, for example, a current detection type thermocouple, which is formed together with the gas absorbing material 5 on the thin film obtained by dividing the thin film 10 and can detect only the temperature difference is suitable.

A gas sensor element according to a seventh aspect of the present invention is a gas sensor element according to the sixth aspect, or a thermal reaction substance that is a substance that causes a thermal reaction with a gas to be detected instead of the gas absorbing substance 5 of the gas sensor element according to the sixth aspect. The thin film 10 is divided into two equivalent thin films, and one of the two thin films includes a balance film and a temperature sensor 20a as a reference portion. And the other thin film is provided with the gas absorbing material 5 and the temperature sensor 20b as a detection unit, and the heat absorption, heat generation or detection of the detected gas by the differential operation of the temperature sensor 20a and the temperature sensor 20b. This is a case characterized in that it is possible to detect a temperature change accompanying a thermal reaction with a gas.

Since the temperature change due to heat generation and heat absorption as a gas sensor such as hydrogen gas is extremely small when the gas concentration is low, temperature sensors having equivalent temperature characteristics other than the thermal reaction, that is, the temperature sensor of the detection part that reacts thermally ( In this case, by differentially amplifying the output of the temperature sensor 20b) and the reference temperature sensor (in this case, the temperature sensor 20a) that does not thermally react, a highly accurate and highly sensitive gas sensor element is provided. . In particular, when the temperature is raised or lowered by the heater, consideration should be given so that a temperature difference other than a thermal reaction can be ignored between the detection unit and the reference unit. Moreover, since a temperature difference is seen, it is good to use a highly sensitive temperature difference sensor. In addition, when using a temperature difference sensor, it is necessary to mount the absolute temperature sensor used as a reference | standard.

The gas concentration measuring apparatus according to claim 8 of the present invention uses the gas sensor element described above to absorb heat in the gas absorbing material 5 with respect to the gas to be detected, heat generation at the time of release, endothermic reaction, or thermal reaction with the thermal reactant. The temperature change accompanying this is measured, and the concentration of the detected gas in the gas can be measured using the data of the temperature change.

The temperature change associated with heat generation or heat absorption during absorption or release by the gas absorbing material 5 and thermal reaction with the thermal reaction material increases as the concentration of the gas to be detected in the gas increases. Therefore, a calibration curve for the relationship between the temperature change and the concentration of the gas to be detected is prepared, and the concentration of the gas to be detected is measured using the calibration curve.

The gas concentration measuring apparatus according to claim 9 of the present invention heats the thin film 10 with a heater in a predetermined cycle so that the thin film 10 reaches a set temperature at the time of heating. This is a case where temperature change based on reaction is utilized.

The heater may be formed on the thin film 10 on which the gas absorbing material 5 and the temperature sensor are formed as described in claims 5 and 6, or the thin film on which the temperature sensor is formed outside. 10 may be heated. In addition, the heater may be heated by placing the thin film 10 at a predetermined steady temperature, and the thin film 10 may be heated by the heater periodically at a predetermined cycle up to a predetermined temperature based on this temperature. In any case, the gas absorbing material 5 formed on the gas sensor element is heated and cooled from a predetermined temperature with a predetermined power, etc., and the gas absorbing material of the gas to be detected at that time Measures the temperature change based on the adsorption / desorption process in Fig. 5, especially the temperature rise, the temporal change in temperature, and the equivalent change in the thermal time constant, etc., and detects the concentration of the gas to be detected. is there. If the concentration of the gas to be detected is large, the heat of reaction of the gas absorbing material 5 increases even if the same heater is heated, and the temperature rise of the thin film 10 on which the gas absorbing material 5 is formed increases. As soon as the gas to be detected enters the absorbent material 5, it becomes difficult to desorb (release) the gas to be detected even by heating the heater, and the equivalent thermal time constant tends to increase.

As described above, in the present invention, when the gas absorbing material 5 for the gas to be detected is used by heating the thin film 10 with a heater or additional heating, the gas to be detected is desorbed from the gas absorbing material 5. It is expected to act to promote and return to the initial state. In addition, since the ambient temperature varies from measurement to measurement depending on the environment, a low temperature is recommended for storing the gas to be detected. It is better to heat the thin film (10) with a heater at a higher predetermined temperature (for example, 30 ° C.).

In the gas sensor element of the present invention, the thin film 10 thermally separated from the substrate is provided with the temperature sensor 20 and the gas absorbing material 5 such as palladium Pd or hydrogen storage alloy that absorbs the detected gas such as hydrogen. Even with a small amount of heat absorption or heat generation during gas absorption or release, the temperature change becomes large and the temperature change can be measured with a highly sensitive and accurate temperature sensor. There is an advantage that a gas sensor element can be provided.

In the gas sensor element of the present invention, when the gas absorbing material 5 and the temperature sensor are formed on the protrusions of the cantilever-like thin film 10, the temperature sensor is formed at a location where the temperature change is most severe. Therefore, there is an advantage that a highly sensitive gas sensor element 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, the gas absorbing material 5 is not necessarily required to have a protrusion having a reference temperature sensor that does not form the gas absorbing material 5. The only advantage is that only the change in temperature when the gas to be detected can be detected based on the temperature when the gas to be detected does not exist, using only the projections forming the temperature sensor. is there. When a current detection type thermocouple is used as the temperature difference detection sensor, a more sensitive gas sensor element can be provided.

The gas sensor element of the present invention has an advantage that a simple gas sensor element can be provided because the heater is provided in the thin film 10 or close thereto.

In the gas sensor element of the present invention, the thin film 10 is divided into two parts. One thin film is provided with a heater and a temperature sensor 20a, and the other thin film having thermal resistance is provided with a temperature sensor 20b and a gas absorbing material 5. Since the thin film 10 is heated by the heater 25 mounted and the thin film 10 is controlled to a predetermined temperature, the gas absorption material 5 absorbs heat of the gas to be detected, changes in temperature due to heat generation, or the gas to be detected. There is an advantage that the temperature change accompanying the thermal reaction at the time of temperature rise with the thermal reaction substance can be easily detected.

In the gas sensor element of the present invention, the thin film 10 is divided into two equivalent thin films, and one of the two thin films includes a balance film and a temperature sensor 20a as a reference portion. The other thin film is provided with a gas absorbing material 5 and a temperature sensor 20b as a detection unit. In this case, the differential operation of the temperature sensor 20a and the temperature sensor 20b can detect a temperature change associated with heat absorption, heat generation, or thermal reaction with the gas to be detected, so that a highly sensitive gas sensor element can be provided.

In the gas concentration measuring apparatus according to the present invention, when the temperature change caused by the absorption or release of the gas to be detected in the gas absorbing material 5 is used, the temperature change data is used to detect the gas in the gas. The concentration of the detection gas can be measured without requiring the presence of a specific gas such as oxygen as in the contact combustion type gas sensor. Further, since the gas to be detected by the gas absorbing material 5 is selective and can be combined with a highly sensitive temperature sensor, a small gas concentration measuring device can be provided.

In the gas concentration measuring apparatus of the present invention, the thin film 10 is heated with a heater in a predetermined cycle so that the thin film 10 reaches a set temperature during heating, and the temperature change based on the adsorption / desorption process of the detected gas and the thermal reaction at that time Therefore, even if the detected gas is absorbed once and the exothermic reaction is completed, the temperature is raised by heating the heater to promote the desorption of the detected gas, release it, and further cool it. There is an advantage that the initial state can be restored.

  Hereinafter, the gas sensor element of the present invention can be formed on a silicon (Si) substrate that can also form an IC using mature semiconductor integration technology and MEMS technology. A case of manufacturing using this silicon (Si) substrate will be described in detail based on an embodiment with reference to the drawings. The gas concentration measuring apparatus using the gas sensor element of the present invention will be described with reference to the block diagram.

  FIG. 1 is a schematic plan view showing an embodiment of the gas sensor element of the present invention. Here, this is a case where an SOI substrate is used as the substrate 1, and the thin film 10 that is suspended in the air for thermal separation from the substrate 1 has a thin film 10A as one thin film and a thin film 10A as the other thin film. The thin film 10B is divided into two, and the thin film 10B has a structure of a protrusion protruding from the thin film 10A through the thermal resistance portion 45 in a cantilever shape. The thermal resistance part 45 has a structure in which heat conduction is difficult due to the slit 41 provided between the thin film 10A and the thin film 10B, and has thermal resistance. This is a case where a thin film of palladium (Pd) is formed on the thin film 10B serving as the cantilever-like protrusion for hydrogen detection as the gas absorbing material 5.

Here, 20A as the temperature sensor 20a formed on the thin film 10A and the temperature sensor 20B as the temperature sensor 20b formed on the thin film 10B which is the protrusion are both the above-described diode thermistor, and further, the heater 25 This is also the case where the thin film 10 is heated by the forward current of the pn junction diode. Here, the diode thermistor is for observing the temperature dependence of the forward current when a predetermined forward bias (for example, 0.55 V) is applied to the pn junction diode and fixed, and like the thermistor, The reciprocal of the absolute temperature and the logarithm of the forward current have a linear relationship, and the temperature sensor can measure the absolute temperature T with high sensitivity. This diode thermistor is characterized in that its temperature coefficient can be adjusted by the magnitude of the forward bias voltage. Of course, a semiconductor diffusion resistance heater or a metal thin film heater by impurity diffusion may be used as the heater 25.

The case where hydrogen gas, which is a gas to be detected in the atmosphere, is detected as a gas sensor element will be described as follows. The heater 25 that uses Joule heating with a forward current of a pn junction diode formed in the thin film 10A is heated with, for example, 30 ° C. slightly higher than room temperature as a reference temperature. This temperature is measured by a temperature sensor 20A as a diode thermistor which is also formed on the thin film 10A, and a heater control circuit (not shown here) is made constant by 30 ° C. using this output. Control). At this time, if hydrogen gas exists in the atmosphere, it is absorbed by the gas absorbing material 5 and generates heat. This temperature change is detected by a temperature sensor 20B (here, a diode thermistor) formed on the thin film 10B. The greater the temperature rise, the higher the concentration of hydrogen gas. Therefore, the concentration of hydrogen gas in the atmosphere can be calculated based on calibration data.

When the concentration and absorption amount of hydrogen gas in the atmosphere under the heating temperature of the thin film 10A reach equilibrium, the exothermic reaction stops, and after that, it cools and returns to around 30 ° C. of the original temperature. Further, when the thin film 10A is heated to, for example, about 100 ° C. with the heater 25 mounted here, the hydrogen gas absorbed in the gas absorbing material 5 is released, and in this process, the gas absorbing material 5 Endothermic reaction. Thereafter, the heater heating is stopped again, and the temperature is returned to around 30 ° C. by natural cooling. In this process, the gas absorbing material 5 absorbs the hydrogen gas of the gas to be detected and returns to around 30 ° C. while causing an exothermic reaction. However, if the cooling is abrupt due to a thin film having a small heat capacity, the absorption of the hydrogen gas The speed is not in time, the temperature rises slowly after returning to around 30 ° C., and after that, when absorption is saturated, it returns to around 30 ° C. again as described above. In this way, by using the heater 25 to repeat the heating cycle to, for example, about 30 ° C. and about 100 ° C., the data of the temperature change amount and the heating / cooling time ratio of the heating cycle are used while returning to the initial state. From the above, the concentration of hydrogen gas in the atmosphere can be specified.

In the case where the thin film 10 is divided into the thin film 10A and the thin film 10B and the thin film 10B has a projecting portion protruding in a cantilever shape, the thermal resistance portion 45 is formed by the slit 41 formed in the thin film 10 in the above-described embodiment. The thin film 10A and the thin film 10B are divided by giving thermal resistance to the thin film 10 to form a protrusion protruding from the thin film 10 in a cantilever shape. As another method of heating from the outside, for example, a thin film Although 10A and the thin film 10B are the thin films 10 floating in the air, they are not close to each other, but are arranged close to each other through a slit 41 that cuts the thin film 10, and the thin films 10A and 10B While supporting each through the beam 18 from the board | substrate 1, the thin film 10B can also be made into the structure which protrudes from the board | substrate 1 in the shape of a cantilever.

The outline of the manufacturing process for processing the substrate 1 in the gas sensor element of the present invention shown in FIG. 1 will be described as follows. When the SOI layer 11 of the substrate 1 uses n-type, the temperature sensor 20A, the temperature sensor 20B, and the pn junction diode as the heater 25 form the p-type diffusion region 22 by thermal diffusion using a known semiconductor microfabrication technique, Further, the n-type diffusion region 21 is formed in order to obtain a good ohmic contact. Thereafter, when not exposed to a nickel (Ni) or strong alkaline etchant, an aluminum (Al) metal is used to form electrodes 60a, 60b, 61a, 61b, 62a, 62b by sputtering thin film formation and photolithography. The wiring 110 and the electrode pads 70a, 70b, 71a, 71b, 72a, 72b are formed. Further, the portions to be the slits 41 and 42 are removed by etching up to the BOX layer. Next, after forming an insulating film layer of the gas absorbing material 5 by CVD or sputtering, the gas absorbing material 5 is formed into a thin film. The patterning of the gas absorbing material 5 can be formed by conventional techniques such as RIE and photolithography. Next, a cavity 40 is formed from the back surface of the substrate 1 by DRIE, and penetration of the slits 41 and 42 is also achieved. In this way, the thin film 10 floating in the air is divided into two through the thermal resistance portion 45 by the slit 41 to form the thin film 10A and the thin film 10B having a structure protruding from the thin film 10A into a cantilever shape, and the heater 25 And a pn junction diode as the temperature sensor 20A are formed on the thin film 10A, and a temperature sensor 20B of the pn junction diode is formed on the thin film 10B. The thin film 10 including the thin film 10A and the thin film 10B has a structure that is supported from the substrate 1 by a narrow beam 18. The thin film 10B is a region where the gas absorbing material 5 reacts with the gas to be detected to detect heat generation and temperature change due to heat absorption. Therefore, the thin film 10B has a structure like a cantilever to transfer heat to the substrate 1. The structure is difficult to communicate.

Further, here, the temperature sensors 20A and 20B formed on the thin film 10A and the thin film 10B use pn junction diodes like a thermistor, and the heater 25 is also in the order of pn junction diodes. Heating is performed by directional current. These can be conveniently formed in the same process. The electrical wiring between the temperature sensor 20A and the temperature sensor 20B is an electrode pad 71a formed on the substrate 1 by the wiring 110 formed on the silicon oxide film 51 which is an insulating film on the beam 18 supporting the thin film 10A. , 71b, 72a, 72b.

FIG. 2 is a schematic plan view showing another embodiment of the gas sensor element of the present invention. Here, it is a case where it implemented using the SOI substrate as the board | substrate 1 similarly to the above-mentioned Example 1, and the thin film 10 made into the structure suspended in the air for the thermal separation from the board | substrate 1 is used as one thin film. The thin film 10A and the thin film 10B as the other thin film are divided into two, and the thin film 10B protrudes from the thin film 10A through the thermal resistance portion 45 in a cantilever shape, and the thin film 10B absorbs gas. This is a case where the substance 5 is formed. The main difference from the above-described first embodiment is that the thermal resistance portion 45 has a structure confined by a slit 41 provided between the thin film 10A and the thin film 10B, and has a structure in which heat conduction is difficult. The temperature sensor 20B as the temperature sensor 20b formed on the thin film 10B uses a current detection type thermocouple that detects only a temperature difference, and uses the p-type SOI layer 11 as an SOI substrate to diffuse impurities. And the other thermocouple conductor 120a of the current detection type thermocouple formed as an n-type SOI layer having a high impurity density is electrically isolated by a pn junction.

The current detection type thermocouple needs to have an extremely low resistance in order to measure a short circuit current (Seebeck current at the time of a short circuit) that flows due to a thermoelectromotive force (Seebeck voltage) based on a temperature difference. For this purpose, the p-type SOI layer 11 is used to increase the concentration of the n-type impurity in the entire thin film 10B and a portion of the thin film 10A close to the thin film 10B so that the n-type impurity is degenerated. The n-type diffusion region 21 is added to serve as one conductor of the thermocouple, and a pn junction is formed for electrical isolation from the heater 25 and the temperature sensor 20A formed on the thin film 10A. I try to do it. Here, the hot junction of the current detection type thermocouple as the temperature sensor 20B is an ohmic electrode 62a formed at the tip of the thin film 10B on the cantilever, and the cold junction is close to the thin film 10B in the thin film 10A. The n-type diffusion region 21 is continuous from the thin film 10B.

When the gas sensor element of the present invention detects hydrogen gas, which is a gas to be detected in the atmosphere, palladium (Pd) or a hydrogen storage alloy can be used as the gas absorbing material 5, and the measurement method is also described in the first embodiment. The hydrogen gas concentration can be measured in substantially the same way. The thin film 10B has a structure protruding from the thin film 10A in a cantilever shape. As the temperature sensor 20B, a current detection type thermocouple that measures only a temperature difference with respect to the thin film 10A is used, and the temperature of the thin film 10A is used as a reference. Since the so-called zero reference method described above, which detects only the temperature change from there, can be applied, the hydrogen gas concentration of the gas to be detected in the atmosphere can be measured with high accuracy.

Note that the two wirings 110 to the substrate 1 in the current detection type thermocouple as the temperature sensor 20B are branched using the same metal material from a portion of the thin film 10A close to the thermal resistance portion 45 recognized as an equal temperature. It is good to do. In this embodiment, the wiring 110 from the electrode 62b that is in ohmic contact with the high-concentration n-type diffusion region 21, which is one thermoconductor 120a of the current detection type thermocouple, is the same as the other thermoconductor 120b. The material nickel (Ni) is used. Ni was used because hydrazine used at the time of anisotropic etching of silicon is durable and has a Seebeck coefficient opposite to that of an n-type semiconductor. However, since the Seebeck coefficient of the n-type semiconductor is an order of magnitude larger than that of Ni, even if aluminum (Al) having the same sign as the Seebeck coefficient of the n-type semiconductor is used, it does not change so much. In forming the thin film 10 by DRIE without using an anisotropic etchant such as hydrazine, aluminum (Al) may be used. Since Al is widely used as a wiring material in IC technology, the wiring 110 may be made of aluminum (Al) for the purpose of integration with an amplifier or the like.

FIG. 3 is a schematic plan view showing another embodiment of the gas sensor element of the present invention. The major difference from FIG. 2 described in the second embodiment is that the thin film 10B as the other thin film is divided into a thin film 10Ba and a thin film 10Bb, which have the same shape. Is formed with a gas absorbing material 5 and the thin film 10Ba has a mass substantially equal to that of the gas absorbing material 5 of the thin film 10Bb and is a balance film made of a material that does not react with hydrogen gas as a detection gas. The temperature sensor 20Ba as the temperature sensor 20a and the temperature sensor 20Bb as the temperature sensor 20b are current detection type thermocouples as in FIG. The difference is that the temperature difference between 10Ba and the thin film 10Bb is connected in series so that it can be directly measured. In this way, the temperature sensor 20Ba and the temperature sensor 20Bb formed on each of the thin film 10Ba and the thin film 10Bb can apply the above-described zero reference method with respect to heat in the measurement of the hydrogen gas concentration. The method for measuring hydrogen gas, which is a gas to be detected in the atmosphere, can be substantially the same as the method described in the first embodiment, but the gas absorbing material 5 is formed with reference to the temperature of the thin film 10Ba. By measuring the temperature difference of the thin film 10Bb, it is possible to detect a gas concentration such as hydrogen gas with higher sensitivity and accuracy.

Although not shown here, the temperature sensor 20Ba and the temperature sensor 20Bb, which are current detection type thermocouples, are not connected in series as shown in FIG. The short circuit currents of the thin film 10Ba and the thin film 10Bb can be directly measured so that the short circuit currents of the thin film 10Ba and the thin film 10Bb can be directly measured.

FIG. 4 is a schematic side view showing another embodiment of the gas sensor element of the present invention, and FIG. 5 is a schematic plan view thereof. The thin film 10 thermally separated from the substrate 1 is divided into two thin films 10A and 10B in the shape of a cantilever 7, and each further constitutes a temperature sensor 20A as a temperature sensor 20a and a temperature sensor 20B as a temperature sensor 20b. In this case, the heater 25 is an example in the case where the heater 25 is disposed in proximity to the two thin films 10A and 10B. Here, a case where the current detection type thermocouple described in the second and third embodiments is used as the temperature sensors 20A and 20B is shown. In order to differentially operate the temperature sensor 20A and the temperature sensor 20B, which are two current detection type thermocouples, one of the thermocouple conductors (metal thin film wiring) constituting the thermocouple is connected on the way, The short-circuit current flowing between the two electrodes 72a and 72b formed in the high-concentration diffusion n-type diffusion region 21 which is the other thermocouple conductor constituting the pair is detected. In addition, short circuit current detection (short circuit) between the electrode 70 of the thermocouple conductor of one metal thin film wiring and the two electrodes 72a or 72b formed in the other thermocouple consisting of the respective n-type diffusion regions 21 With the Seebeck current detection, each temperature (temperature near the tip of the cantilever) can be measured with reference to an absolute temperature sensor (here, the pn junction diode 23) formed on the substrate 1.

The thin film heater 25 mainly composed of the SOI layer 11 thermally separated from the substrate 100 shown in FIG. 4 has a diaphragm shape and uses the n-type diffusion region 21 as a heater material. When p-type silicon is used for the SOI layer 11 of the substrate 1 and the substrate 100, a pn junction can be formed, which facilitates electrical isolation and is preferable. The diaphragm heater 25 is required to heat the reference unit 8 and the detection unit 9 which are the two cantilevers 7 uniformly and equally, so that the diaphragm heater 25 is disposed close to the two cantilevers 7, A larger area is desirable than the dimensions of these cantilevers 7. Therefore, the degree of proximity is substantially determined by the thickness of the base substrate of the SOI substrate of the substrate 100. Further, the diaphragm heater 25 formed by the MEMS technology has a feature that it can be heated at high speed and with low power consumption because of its small heat capacity.

Of the two thin films 10A and 10B in the shape of cantilever 7, one thin film 10A is used as a reference portion 8, and the other thin film 10B includes a gas absorbing material such as a porous Pd metal thin film as a hydrogen gas sensor. 5 is formed, and is configured to generate a temperature change that is absorbed here to generate heat and dissipate heat to cool. These two cantilever 7-shaped thin films 10A and 10B absorb gas such as a porous Pd metal thin film in the detection section 9 so as to have an equivalent heat capacity and thermal conductance except for reaction heat by the gas to be detected. Instead of the substance 5, the balance film 6 may be formed in the thin film 10 </ b> A on the reference portion 8. As the balance film 6, a thin film material that is stable in the temperature range to be used and has no enthalpy change is desired. For example, a silicon oxide film, a stable metal film, or the like may be used. In some cases, the tip of the cantilever 7 of the SOI layer 11 of the detection unit 9 is slightly removed by etching, and the reference unit 8 performs etching removal accordingly. Instead, an equivalent balance film may be formed so as to leave the SOI layer. In FIG. 5, a groove 130 is for electrically insulating the temperature sensor 20 </ b> A and the temperature sensor 20 </ b> B made of each current detection type thermocouple by forming a groove in the SOI layer 11. Since the operation is almost the same as in the second and third embodiments, the description thereof is omitted here.

FIG. 6 shows a block schematic diagram of the gas concentration measuring apparatus of the present invention. Depending on the application purpose, there are at least a power supply circuit for supplying power to the above-described gas sensor element of the present invention and other circuits, and a gas to be detected in the atmosphere using the output from the gas sensor element. It has a heater control circuit such as an arithmetic circuit for obtaining the hydrogen gas concentration and a heater heating cycle for driving the gas sensor element. Here, a case is also shown in which a display unit for displaying the concentration of the gas to be detected is provided. Yes.

The heater 25 may be heated to about 100 ° C. in a short time by pulse driving, or according to a predetermined temperature program like a thermal analysis device, for example, a differential thermal analysis (DTA) device, Set the temperature rise and fall, further control the temperature rise and fall rate to be constant, detect the deviation from the temperature rise and fall rate by the heat absorption and heat generation by the gas absorbing material 5, The detected gas concentration can be measured. Further, it is possible to measure the concentration of the gas to be detected with higher sensitivity and higher accuracy by performing an operation such as obtaining a derivative with respect to time of temperature change. In addition, the concentration of the gas to be detected can be measured with high sensitivity and high accuracy by expanding the difference by time integration of the temperature change when the temperature rises or falls. Further, after the temperature is raised to a predetermined temperature, the heating of the heater is stopped, and the concentration of the gas to be detected can be obtained by utilizing the temperature drop characteristic in the subsequent cooling process, particularly the change of the thermal time constant.

Although not shown here, two lids for sandwiching and joining the substrates 1 are prepared. At least one lid prevents airflow, and a porous lid for ventilation allows the gas to be detected to pass by diffusion or the like. It is better to have a lid with many fine slits.

In the above description, the temperature difference detection type temperature sensor, in particular, an n-type silicon semiconductor added with an impurity at a high concentration to reduce resistance as one thermocouple conductor of the current detection type thermocouple, and the other thermocouple. The case of using Ni or Al as the conductor has been described, but as these thermocouple materials, the performance index Z of the thermoelectric material (the value obtained by dividing the square of the Seebeck coefficient α by the resistivity ρ and the thermal conductivity κ) is large. A thermocouple may be configured by a combination using materials. Further, as these thermocouple thin films, a thermocouple is formed by using a composite film in which electrically insulating thin films are stacked so as to be sandwiched so that the composite film itself becomes a cantilever temperature sensor 20B. Also good.

In the above-described embodiments, the heater 25 is often heated by Joule heat based on application of a forward bias voltage of a pn junction diode. This is because it is easy to prevent the influence of the voltage application to the heater to the temperature sensor formed in the same thin film, even if a platinum thin film heater or a diffusion resistance heater due to impurity diffusion to the semiconductor is used, Of course it does not matter.

In the above-described embodiment, the hydrogen gas sensor for measuring the concentration of hydrogen gas as the gas to be detected has been described. Therefore, palladium Pd, platinum Pt, nickel Ni, a hydrogen storage alloy, or the like has been used as the gas absorbing material 5. However, in the case of water vapor as the gas to be detected, for example, a thin film in which quick lime is contained in a silica gel thin film can be used as the gas absorbing material 5. Moreover, a porous carbon thin film can also be utilized with respect to the organic solvent as a to-be-detected gas. These exhibit an exothermic reaction when the gas to be detected is adsorbed. In addition, it has been known that a catalyst in which Pd, Pt, and Ni fine particles are supported on an oxide reacts exothermically with a reactive gas when the temperature rises. In this way, it is possible to select the gas absorbing material 5 and the thermal reaction material corresponding to various gases to be detected.

The gas sensor element of the present invention and the gas concentration measuring apparatus using the gas sensor element are not limited to the present embodiment. Naturally, various modifications can be made while the gist, operation and effect of the present invention are the same.

  The gas sensor element of the present invention measures the concentration of a gas to be detected, for example, hydrogen gas in a gas with high sensitivity and high accuracy mainly as a temperature change due to heat generation or heat absorption during the adsorption / desorption process in the gas absorbing material 5. This is a thermal type sensor. For example, when the gas sensor element of the present invention is applied to a sensor for detecting the concentration of hydrogen gas in the atmosphere, it is an ultra-compact device that detects only a temperature difference with high sensitivity and high accuracy in a thin film suspended in a cantilever shape. Since a current detection type thermocouple can be used, it can be used as a hydrogen gas detector having a very wide concentration range. Furthermore, since a very small temperature difference can be detected by the zero reference method, it is suitable for measuring the concentration of a minute amount of a specific gas to be detected. In addition, a gas concentration measurement device equipped with the gas sensor element of the present invention includes a control system, and is suitable for a gas concentration measurement device such as a hydrogen gas concentration measurement.

It is a plane schematic diagram showing one example of the gas sensor element of the present invention. Example 1 It is a schematic plan view showing another embodiment of the gas sensor element of the present invention. (Example 2) It is a schematic plan view showing another embodiment of the gas sensor element of the present invention. (Example 3) It is a side schematic diagram showing another example of the gas sensor element of the present invention. Example 4 FIG. 5 is a schematic plan view of the gas sensor element of the present invention shown in FIG. 4. Example 4 The block schematic diagram of the gas concentration measuring device of the present invention is shown. (Example 5)

Explanation of symbols

1,100 Substrate 5 Gas absorbing material 6 Balance film 7 Cantilever 8 Reference part 9 Detection part 10, 10A, 10B, 10Ba, 10Bb Thin film 11 SOI layer 18 Beam 20, 20A, 20B, 20Ba, 20Bb Temperature sensor 21 n-type diffusion region 22 p-type diffusion region 23 pn junction diode 25 heater 40 cavity 41, 42 slit 45 thermal resistance portion 51 silicon oxide film 60 electrode 60a, 60b electrode 61a, 61b electrode 62a, 62b electrode 70 electrode pad 70a, 70b electrode pad 71a, 71b Electrode pads 72a, 72b Electrode pad 110 Wiring 120 Thermocouple 120a, 120b Thermocouple conductor 130 Groove

Claims (9)

  The thin film (10) thermally separated from the substrate is provided with one or a plurality of temperature sensors (20) and a gas absorbing material (5) that absorbs the gas to be detected, and absorbs or releases the gas to be detected. A gas sensor element, wherein the temperature sensor (20) is arranged and formed so as to be able to measure a temperature change accompanying heat absorption or heat generation. The gas sensor element according to claim 1, wherein at least one of the temperature sensors (20) is formed on a projection of a cantilever-like thin film (10) provided with a gas absorbing material (5). The gas sensor element according to claim 1 or 2, wherein at least one of the temperature sensors (20) is a temperature difference detection sensor. The gas sensor element according to claim 3, wherein the temperature difference detection sensor is a current detection type thermocouple. The gas sensor element according to any one of claims 1 to 4, wherein the thin film (10) can be heated by providing a heater in the thin film (10) or by providing a heater in the vicinity of the thin film (10). . The thin film (10) is divided into two or more thin films, one of the divided thin films is provided with a temperature sensor (20a), and the other thin film is provided with a temperature sensor (20b). A gas-absorbing substance (5), the one temperature sensor (20a) being capable of measuring the temperature of the heater and subjecting it to temperature control of the heater, the other temperature sensor The gas sensor element according to claim 5, wherein a temperature change accompanying heat absorption or heat generation of the gas to be detected can be detected using (20 b). In the gas sensor element according to claim 6, or a gas sensor element provided with a thermal reaction substance that is a substance that causes a thermal reaction with the gas to be detected instead of the gas absorbing substance (5) of the gas sensor element according to claim 6, ) Are divided into equal shapes, and one of the two thin films is provided with a balance film and a temperature sensor (20a) as a reference portion. Is provided with a gas absorbing material (5) and a temperature sensor (20b) as a detection unit, and by the differential operation of the temperature sensor (20a) and the temperature sensor (20b), heat absorption, heat generation or A gas sensor element characterized in that a temperature change accompanying a thermal reaction with a gas to be detected can be detected. Using the gas sensor element according to any one of claims 1 to 7, accompanying the heat generation or endothermic reaction at the time of absorption or release of the gas to be detected by the gas absorbing material (5), or thermal reaction with the thermal reaction material A gas concentration measuring apparatus characterized in that a temperature change is measured, and the concentration of a gas to be detected in the gas can be measured using data of the temperature change. The thin film (10) is heated or cooled with a heater in a predetermined program cycle so that the thin film (10) reaches a set temperature during heating, and the temperature change based on the gas adsorption / desorption process and thermal reaction at that time The gas concentration measuring apparatus according to claim 8, which is used.

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