JP4585756B2 - Semiconductor gas sensor and gas monitoring method using semiconductor gas sensor - Google Patents

Description

  The present invention relates to a semiconductor gas sensor, and more particularly to a low power consumption semiconductor gas sensor with battery driving in mind, and a gas monitoring method using the semiconductor gas sensor.

Generally the gas sensor is used in applications such as gas leak alarms, gas to be detected, for example, carbon monoxide (CO), methane (CH _4), propane (C ₃ H _8), methanol (CH ₃ OH) and other devices that are selectively sensitive. A gas sensor is required to have high sensitivity, high selectivity, high response, high reliability, and low power consumption due to its character. Here, gas leak alarms that are widely used for household use are intended for detection of flammable gases such as city gas and propane gas, and for the detection of incomplete combustion gases from combustion equipment. Or a combination of both of these functions. However, the penetration rate of these gas sensors is not so high due to cost and installation problems. Under such circumstances, in order to improve the diffusion rate of the gas sensor, it is desired to improve the installation property, specifically, to make the gas sensor battery-driven and cordless.

  In order to realize battery driving, it is important to reduce the power consumption of the gas sensor. For this purpose, it has been proposed to intermittently operate a thin film semiconductor gas sensor having a high heat insulation and low heat capacity structure such as a diaphragm structure formed using a microfabrication process in accordance with a detection cycle.

  For example, in the case of a CO sensor, the temperature of the sensor is once raised to a high temperature state (about 450 ° C.), the sensor is cleaned, and then the temperature is lowered to a low temperature state (about 100 ° C.) for detection. By doing so, it is known that the sensitivity and selectivity to CO increase. There is also a sensor capable of detecting not only cleaning but also methane at a high temperature, and detecting both methane and CO with a single gas sensor, together with CO detection at a low temperature.

  As such an example, Non-Patent Document 1 describes a driving method for periodically detecting high temperature and low temperature and detecting methane at high temperature and CO at low temperature for a small bead type semiconductor CO / methane composite gas sensor. There is. Patent Document 1 describes a method of detecting a methane at 450 ° C. and CO at 300 ° C. by switching the detection temperature between 450 ° C. and 300 ° C. for a hot-wire semiconductor CO / methane composite gas sensor.

  Here, in particular, in the case of a battery-driven gas sensor, it is necessary to perform measurement in a much shorter time than a normal sensor due to power consumption restrictions. For this reason, the gas reaction must be measured in a transitional state that does not reach a steady state, and as a result, sufficient selectivity for the target gas may not be obtained.

  The battery-driven thin-film semiconductor gas sensor is adjusted so that the sensor temperature becomes a standard condition as shown in FIG. 4 in the air, in the presence of 100 ppm of CO, in the presence of 1000 ppm of hydrogen, and in the presence of 4000 ppm of methane. Set the heater to High-Low so that the sensor temperature is high (about 450 ° C.) for 0 to 0.2 seconds after the start of measurement and low temperature (about 100 ° C.) for 0.2 to 0.7 seconds. FIG. 5 shows changes with time in the sensor resistance when driven. Here, the presence of 100 ppm of CO means that the air contains CO at a concentration of 100 ppm.

First, it can be seen that in a high temperature state, even in any gas, a steady state is reached in about 100 ms and a fast response can be obtained. Only in the presence of 4000 ppm of methane, the resistance is lower by an order of magnitude than in the air, and the resistance is high in the presence of CO 100 ppm or 1000 ppm of hydrogen. Therefore, it can be said that the methane sensor has sufficient sensitivity and selectivity. On the other hand, in a low temperature state, the response is slow, and the sensor resistance in the presence of 100 ppm of CO decreases monotonously with time. Even when the detection point is 0.7 seconds (0.5 seconds after being lowered to a low temperature), the sensor resistance in CO 100 ppm is almost the same as the resistance value in the presence of 1000 ppm of hydrogen and 4000 ppm of methane. That is, the CO sensor has a problem in selectivity.
Chemical Sensors Vol.16 Supl.A (2000) JP-A-7-174725

  An object of the present invention is to improve gas selectivity when monitoring a gas using a semiconductor gas sensor, preferably a semiconductor gas sensor with low power consumption.

  According to one aspect of the present invention, there is provided a method of monitoring a test gas using a semiconductor gas sensor including a resistor, the step of maintaining the temperature of the resistor at a first temperature for a predetermined time; A first measurement step of measuring a resistance value of the resistor in the test gas at one temperature, and changing the temperature of the resistor from the first temperature to a second temperature; Maintaining a second temperature for a predetermined time; a second measuring step of measuring a resistance value of the resistor in the test gas at the second temperature; and a temperature of the resistor. A step of changing the temperature from the second temperature to the third temperature and keeping the temperature at the third temperature for a predetermined time, and measuring the value of the electric resistance of the resistor in the test gas at the third temperature. A third measuring step, the first, Based on the value of the electrical resistance obtained by second and third measurement step, said method comprising determining if it contains a target gas in the test gas is provided.

  According to another aspect of the present invention, there is provided a resistor made of a semiconductor, the resistor having a value of electrical resistance that varies depending on the gas contained in the test gas and the temperature of the resistor, and the resistor. Temperature control means capable of controlling the temperature of the at least three types of temperature, electrical resistance measurement means capable of measuring the value of the electrical resistance at the at least three types of temperature, and the at least three types of temperature. A semiconductor-type gas sensor is provided that includes determination means for determining whether or not the gas to be detected contains a gas to be detected based on the value of the electrical resistance measured in step (1).

  As described in detail below, according to the present invention, when a gas is monitored using a semiconductor gas sensor, preferably a semiconductor gas sensor with low power consumption, the sensitivity and selectivity to the gas can be improved. Further, by selecting the temperature at which the sensor resistance is minimum for each gas, it is possible to obtain more stable sensor characteristics against temperature changes.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. However, the embodiments described below do not limit the present invention.

  As described above, according to one aspect of the present invention, a method for monitoring a test gas using a semiconductor gas sensor including a resistor, the temperature of the resistor being maintained at a first temperature for a predetermined time. A first measurement step of measuring a resistance value of the resistor in the test gas at the first temperature, and a temperature of the resistor from the first temperature to a second temperature. And maintaining the second temperature for a predetermined time; a second measuring step for measuring a value of electric resistance of the resistor in the test gas at the second temperature; and the resistance Changing the temperature of the body from the second temperature to the third temperature and maintaining the temperature at the third temperature for a predetermined time; and at the third temperature, the electrical resistance of the resistor in the test gas A third measurement step that measures the value of And a step of determining whether the detection target gas is contained in the detection gas based on the electric resistance values obtained by the first, second and third measurement steps. The

  Although not particularly limited, the semiconductor gas sensor utilizes the fact that the movement of electrons in the semiconductor of the sensor is limited by the gas to be detected. Any type of semiconductor gas sensor can be used in the method according to the present invention. More specifically, the semiconductor gas sensor is a resistor made of a semiconductor, and a resistor whose value of electric resistance changes according to the gas contained in the test gas and the temperature of the resistor, and the resistor It is preferable to include temperature control means that can control the temperature of the resistor and electrical resistance measurement means that can measure the value of the electrical resistance of the resistor.

  As described in detail below, the test gas is specified by using a semiconductor type gas sensor having a resistor made of a semiconductor and having a resistance whose electric resistance varies depending on the gas and temperature contained in the test gas. It can be measured as a change in the value of the electrical resistance of the resistor. Further, the value of this electric resistance changes depending on the temperature of the resistor, and generally, the pattern of this change also changes depending on the type of gas to be detected. Therefore, based on the pattern of the value of electric resistance at each temperature, the test gas It is possible to selectively monitor whether or not the gas to be detected is contained in.

  Although not particularly limited, as a semiconductor gas sensor applicable to the present invention, a thin film semiconductor gas sensor (see Japanese Patent Publication No. 6-43978), a hot-wire semiconductor gas sensor (see Non-Patent Document 1 and Patent Document 1). ) And the like. The semiconductor gas sensor is preferably a battery-driven type. This is because according to the present invention, it is possible to improve the selectivity with respect to the detection target gas, which has been a problem in the battery-driven semiconductor gas sensor as described above.

  Here, the semiconductor gas sensor is preferably a thin film semiconductor gas sensor. This is because the thin-film semiconductor gas sensor can realize a high heat insulation and low heat capacity structure by a diaphragm structure formed by using a microfabrication process, and is suitable as a battery-driven gas sensor.

  Although not particularly limited, as will be described in detail below, the thin film type semiconductor gas sensor is formed of a resistor, a heater, a selective combustion layer, a thermal insulator, an electrical insulator, etc. in a thin film shape. A diaphragm structure whose periphery is supported by a substrate is preferable. By using a thin film semiconductor gas sensor having a diaphragm structure, the heat capacity of the sensor, particularly the resistor, can be reduced, and the power consumption of the sensor can be reduced.

  The test gas is not particularly limited, and the method according to the present invention can be applied to any gas. That is, the method according to the present invention can be applied to any gas that is desired to be monitored as to whether or not the detection target gas is contained therein. For example, it is preferable that the test gas is air and the gas to be detected may be contained therein.

  The detection target gas is not particularly limited, and the method according to the present invention can apply any gas as the detection target gas. For example, the detection target gas may be a combustible gas used as a fuel, an incomplete combustion gas generated when the fuel is incompletely combusted, or the like. If the flammable gas is not burned and remains in the air at a high concentration, it may cause a fire or explosion, and if the flammable gas is inhaled for a long time at a high concentration, it may be due to insufficient oxygen. This is because there is a risk of causing harm such as suffocation to the human body. In addition, inhaling carbon monoxide generated by incomplete combustion may cause symptoms such as headache, nausea, and feeling bad, and may lead to death if the symptoms are severe.

  The combustible gas is preferably hydrogen, hydrocarbon gas, alcohol or the like, and particularly preferably hydrocarbon gas having 1 to 4 carbon atoms. Specifically, although not particularly limited, hydrogen, methane, ethane, propane, butane (n-butane, isobutane), methanol, ethanol, and the like are preferable as combustible gases. This is because these gases are usually used for city gas, LP gas, and the like. Further, it is preferable to use carbon monoxide generated by incomplete combustion of hydrocarbon gas as the incomplete combustion gas. This is because carbon monoxide may cause harm to the human body as described above.

  Note that the number of types of detection target gas is not particularly limited, and the method according to the present invention can apply any number of gases as detection targets. However, the number of types of detection target gas is preferably 1 or more, more preferably 2 or 3. Although not particularly limited, the detection target gas is preferably methane, hydrogen, and carbon monoxide, and more preferably methane and carbon monoxide when hydrogen is considered as an interfering gas. This is because methane is a combustible gas generally used as a city gas and the like, and carbon monoxide is a gas generated when a combustible gas such as a city gas is incompletely combusted.

  As described above, the method according to the present invention includes the step of maintaining the temperature of the resistor at the first temperature for a predetermined time, and changing the temperature of the resistor from the first temperature to the second temperature. A step of maintaining the temperature for a predetermined time, and a step of changing the temperature of the resistor from the second temperature to the third temperature and maintaining the temperature at the third temperature for a predetermined time, as described in detail below. The electric resistance value of the resistor in the test gas is measured at each temperature. According to the present invention, the characteristics of the test gas can be measured in more detail than before by measuring the resistance value of the resistor in the test gas at at least three different temperatures. Accordingly, it is possible to determine whether or not the detection target gas is included in the test gas based on the characteristics of the test gas measured in more detail, so that the accuracy of the determination is improved. Furthermore, according to the present invention, the influence of background and noise generated from air and other gases is relatively reduced, so that it is possible to selectively determine only whether or not the detection target gas is included. The determination method will be described later.

  The first, second and third temperatures at which the electrical resistance of the resistor in the test gas is measured should be appropriately determined according to the detection target gas, the measurement method, the environment in which the test gas is monitored, etc. is there. However, in order to further improve the accuracy of measurement and further improve the selectivity with respect to the detection target gas, the value of the electrical resistance of the resistor in the presence of one predetermined concentration of the detection target gas is At least one of the first, second and third temperatures has a temperature that makes a significant difference with respect to the value of the electrical resistance of the resistor in the presence of a predetermined concentration of a particular gas or in air. Preferably it is set. In this way, by determining the first, second and third temperatures, the value of the electric resistance caused by the detection target gas can be increased with respect to the background and noise, and the detection sensitivity and The accuracy and selectivity of determination can be further improved.

  That is, when there is one kind of detection target gas, the value of electrical resistance in the presence of a predetermined concentration of this detection target gas is the value of electrical resistance in the presence of a predetermined concentration of other specific gas or in air. On the other hand, it is preferable to set the temperature so as to produce a significant difference. In addition, when there are a plurality of detection target gases, the electric resistance value in the presence of one type of detection target gas is in the presence of a specific gas including other detection target gases or in the air. Therefore, it is preferable to set the temperature so as to cause a significant difference. Here, the other specific gas is not particularly limited, but the other specific gas or air generates background and noise when measuring one of the detection target gases. It is preferable that there is a strong possibility of hindering detection of the detection target gas. Further, the predetermined concentration can be arbitrarily determined as the concentration to be detected for each gas.

  For example, when the detection target gas is carbon monoxide and the concentration to be detected is 100 ppm, the electric resistance value in the presence of 100 ppm of carbon monoxide at 240 to 450 ° C. is the electric resistance in the atmosphere. However, at room temperature or higher and lower than 150 ° C., the value of electrical resistance in the presence of 100 ppm of carbon monoxide is significantly different from the value of electrical resistance in the atmosphere. For this reason, if at least one of the first, second and third temperatures is set to room temperature or higher and lower than 150 ° C., the presence of carbon monoxide can be suitably detected. Similarly, when the detection target gas is methane and the concentration to be detected is 4000 ppm, the electrical resistance value in the presence of 4000 ppm of methane is the electrical resistance in the presence of 1000 ppm of hydrogen at room temperature or higher and less than 300 ° C. However, at 450 ° C. or higher, the value of electrical resistance in the presence of 4000 ppm of methane is significantly different from the value of electrical resistance in the presence of 1000 ppm of hydrogen. For this reason, if at least one of the first, second, and third temperatures is 450 ° C. or higher, the presence of methane can be suitably detected.

  Here, the significant difference means that the electric resistance value of the resistor in the presence of one predetermined concentration of the detection target gas and the presence of a predetermined concentration of another specific gas or in the air In the presence of one predetermined concentration of the detection target gas and a predetermined value of the other specific gas. It is preferable to say that the difference between the electrical resistance value in the presence of the concentration or in the air is larger than the error (standard deviation) in the electrical resistance value when measured multiple times under the respective conditions. The error (standard deviation) in the value of electrical resistance may be a logarithmic error (standard deviation) in the value of electrical resistance. This is because the value of electrical resistance generally changes exponentially according to the type and temperature of the gas. Further, a significant difference is that the resistance value of the resistor in the presence of one predetermined concentration of the detection target gas is different from that of the resistor in the presence of a predetermined concentration of another specific gas or in the air. It is preferable that the electric resistance is 1/10 times or less. This is because the error in the electric resistance value is generally smaller than 1/10 of the electric resistance value.

  Although not particularly limited, it is preferable that the first temperature is higher than the second temperature and the second temperature is higher than the third temperature. By making the first temperature the highest temperature, i.e. before making the first measurement, or at the time of making the first measurement, the highest temperature can be used to clean the sensor and increase the sensitivity and selectivity. It is because it can improve more. That is, as described above, the semiconductor gas sensor utilizes the fact that the movement of electrons in the resistor is restricted by the test gas, and the resistor gas is maintained at a high temperature for a predetermined time. The gas adsorbed on can be removed. In addition, when making 1st temperature the highest, control of temperature can be simplified more because 2nd temperature is made higher than 3rd temperature, and it is preferable.

  Further, although not particularly limited, the first, second and third temperatures are set to the temperatures at which the electric resistance value of the resistor in the presence of the predetermined concentration of the detection target gas is lowest. It is preferable that at least one is set. By determining the temperature in this way, it is possible to minimize the error of the electric resistance value due to the temperature error at the time of measurement, and it is possible to make the sensor characteristics more stable against temperature changes. .

  More specifically, each of the first, second and third temperatures is preferably selected from room temperature to 150 ° C., 150 ° C. to 350 ° C., 350 ° C. to 500 ° C. More preferably, it is selected from any one of 150 ° C., 200 ° C. and 300 ° C. More preferably, the first temperature is 350 ° C. or more and 500 ° C. or less, the second temperature is 150 ° C. or more and less than 350 ° C., and the third temperature is room temperature or more and less than 150 ° C. Particularly preferably, the first temperature is 450 to 500 ° C., the second temperature is 200 to 300 ° C., and the third temperature is 100 to 150 ° C. These will be described in detail in Examples. In addition, a preferable temperature can be similarly determined for gases other than methane, hydrogen, and carbon monoxide. Moreover, although it does not specifically limit, Room temperature says 20 degreeC preferably.

  Here, in the present invention, the time for maintaining the temperature of the resistor at the first, second or third temperature is appropriately determined according to the detection target gas, the measurement method, the environment in which the test gas is monitored, and the like. It should be. However, when the semiconductor gas sensor is used, considering the accuracy and selectivity of measurement and the reduction in power consumption of measurement, the time for maintaining the first temperature is preferably 0.05 to 10 seconds. Similarly, the time for maintaining the second temperature is preferably 0.05 to 10 seconds, and the time for maintaining the third temperature is preferably 0.05 to 10 seconds. It usually takes 0.05 seconds or more for the temperature to change, and is preferably 10 seconds or less due to power consumption constraints.

  The method for controlling the temperature of the resistor in the semiconductor gas sensor is not particularly limited, and the temperature of the resistor can be controlled by any method. In particular, when the semiconductor gas sensor includes a resistor and a temperature control unit that can control the temperature of the resistor, the temperature of the resistor can be suitably controlled by the temperature control unit. More specifically, as described in detail below, the semiconductor gas sensor includes, as temperature control means, a heater that is in thermal contact with the resistor, and a heater temperature control means that controls the temperature of the heater. In the case where the semiconductor gas sensor includes a heater or the like as described above, the temperature of the resistor can be increased by heating the heater. Note that the semiconductor gas sensor may not have a specific configuration for lowering the temperature of the resistor. That is, by stopping or weakening the heating of the heater, the temperature of the resistor can be lowered by natural heat dissipation.

  As described above, the method according to the present invention includes the first measurement step of measuring the value of the electrical resistance of the resistor in the test gas at the first temperature, and the test gas in the test gas at the second temperature. The second measuring step of measuring the value of the electric resistance of the resistor in the step of the above, and the third measuring step of measuring the value of the electric resistance of the resistor in the test gas at the third temperature. As described above, according to the present invention, by measuring the electric resistance value of the resistor in the test gas at at least three different temperatures, the characteristics of the test gas can be measured in more detail than before. Based on the obtained three types of electrical resistance values, it can be determined more accurately and more selectively whether or not the detection target gas is contained in the detection target gas.

  The method for measuring the value of electric resistance is not particularly limited, and can be measured by any method. In particular, a semiconductor gas sensor is a resistor made of a semiconductor, and a resistor whose electric resistance varies depending on the gas contained in the test gas and the temperature of the resistor, and the temperature of the resistor is controlled. In the case of including a temperature control means capable of measuring and an electric resistance measurement means capable of measuring the electric resistance value of the resistor, the electric resistance is measured by measuring the electric resistance value of the resistor by the electric resistance measurement means. Can be measured. As described above, the gas to be detected includes the gas to be detected by using the semiconductor gas sensor having a resistor made of a semiconductor and having a resistance whose electric resistance varies depending on the gas and temperature contained in the gas to be detected. It can be measured as a change in the value of the electrical resistance of the resistor. Further, the value of the electric resistance changes depending on the temperature of the resistor, and generally, the pattern of this change also changes depending on the type of gas to be detected. Therefore, by detecting the pattern of the electric resistance value at each temperature, Whether or not the detection target gas is included in the detected gas can be selectively monitored.

  As described above, the method according to the present invention includes the step of determining whether the detection target gas is included in the test gas based on the values of the electrical resistance obtained by the first, second, and third measurement steps. Including. Here, the determination of whether or not the detection target gas is contained in the test gas is a qualitative determination that determines whether or not the detection target gas is contained in the detection gas at a predetermined concentration or more. And quantitative determination for determining the concentration of the detection target gas in the test gas.

  The determination as to whether the detection target gas is included in the gas to be detected based on the obtained electric resistance value is not particularly limited, and can be performed by any method. For example, the determination can be made by comparing the data to be compared with the value of the electrical resistance obtained by measuring the test gas at each temperature. In particular, the value of electric resistance in the presence of the gas to be detected is measured in advance at each temperature at a predetermined concentration of the gas, and a threshold value determined based on this electric resistance value is compared. By using this data, it is possible to qualitatively determine whether or not the detection target gas is contained in the detected gas at a specific concentration or higher. In addition, the value of the electric resistance in the presence of the detection target gas is measured in advance at each temperature at a plurality of predetermined concentrations of the gas, and based on the value of the electric resistance, the concentration of the gas and the electric resistance are measured. By obtaining a relationship between values and using this relationship as data to be compared, it is possible to quantitatively determine the concentration of the detection target gas in the detection gas. Note that the value of the electrical resistance in the presence of the detection target gas at a predetermined concentration can be calculated using known data or by using known data without measurement in advance.

  The means for performing the above determination is not particularly limited, and can be performed by any means. For example, the determination can be made by performing an artificial calculation based on the obtained value of electrical resistance. However, as will be described in detail below, the semiconductor gas sensor preferably includes a determination unit that determines whether or not the detection target gas is included in the gas to be detected based on the value of the electrical resistance measured at each temperature. It is preferable to determine whether or not the detection target gas is contained in the detection gas by comparing the obtained electric resistance value with the data to be compared by this determination means.

  In the above, a series of steps for determining whether or not a gas to be detected contains a detection target gas by measuring at at least three kinds of temperatures has been described. In the method according to the present invention, these series of steps are described. It is preferable to repeat this step. In other words, by performing these series of steps, it is possible to monitor whether or not the gas to be detected contains the detection target gas at that time, but by performing this series of steps repeatedly, the detection gas is detected. Whether or not the target gas is included can be continuously monitored. Although not particularly limited, when considering low power consumption, this series of steps is preferably performed intermittently, for example, once every 30 to 150 seconds.

  In addition, although the above demonstrated the aspect which mainly measures the test gas in three types of temperature, the method concerning this invention is not limited to three types of temperature, In four or more types of temperature, Measurement of the test gas can also be performed. That is, the method according to the present invention includes a step of changing the temperature of the resistor to a fourth or higher temperature and maintaining the temperature at the fourth or higher temperature for a predetermined time, and the test gas at the fourth or higher temperature. And a fourth or more measurement step for measuring the electric resistance value of the resistor in the determination step, wherein the determination step includes the first, second and third measurement steps and the fourth or more measurement step. Based on the obtained value of the electrical resistance, it can also be determined whether the detection target gas is contained in the detection gas.

  In addition, as described above, according to another aspect of the present invention, a resistor made of a semiconductor, the resistance of which the value of electric resistance changes according to the gas contained in the test gas and the temperature of the resistor Temperature control means capable of controlling the temperature of the resistor to at least three kinds of temperatures; electrical resistance measurement means capable of measuring the value of the electrical resistance at the at least three kinds of temperatures; There is provided a semiconductor type gas sensor including determination means for determining whether or not a gas to be detected is contained in the gas to be detected based on the values of the electrical resistance measured at at least three kinds of temperatures. According to the semiconductor gas sensor according to the present invention, the detection target gas can be selectively monitored using the method according to the present invention.

The semiconductor gas sensor according to the present invention preferably includes a resistor made of a semiconductor, and includes a resistor whose value of electric resistance varies depending on the gas contained in the test gas and the temperature of the resistor. As described above, the semiconductor type gas sensor utilizes the fact that the movement of electrons in the semiconductor of the sensor is limited by the test gas. By using such a resistor, it is detected in the test gas. This is because whether or not the target gas is contained can be measured as a change in the resistance value of the resistor. Further, the value of the electric resistance varies depending on the temperature of the resistor, and generally, the pattern of this change also varies depending on the type of gas to be detected. Therefore, by measuring the pattern of the electric resistance value at each temperature, Whether or not the detection target gas is included in the detected gas can be selectively monitored. In particular but not limited to, a semiconductor having such characteristics, it is possible to use SnO ₂ or the SnO ₂ of +5 or +6 valent element serving as a donor is added.

  The semiconductor gas sensor preferably includes a temperature control means capable of controlling the temperature of the resistor to at least three types of temperatures. By using such temperature control means, the temperature of the resistor is controlled, and the value of the electrical resistance of the resistor in the test gas at each temperature is suitably measured, and the method according to the present invention is carried out. it can. The temperature control means is not particularly limited, and the temperature can be controlled by any means.

  For example, the semiconductor gas sensor preferably includes, as temperature control means, a heater that is in thermal contact with the resistor and a heater temperature control means that controls the temperature of the heater. As described above, when the semiconductor gas sensor includes a heater or the like as described above, the temperature of the resistor can be controlled via the heater. Note that that the resistor and the heater are in thermal contact means that the resistor can be heated by the heat from the heater. Although not particularly limited, the temperature of the heater can be controlled electrically. That is, the temperature of the heater can be raised by energizing the heater, and the temperature of the resistor can be controlled by controlling the amount of current to be energized.

  Moreover, it is preferable that the semiconductor gas sensor includes an electrical resistance measuring unit capable of measuring the value of the electrical resistance at the at least three types of temperatures. The electrical resistance measuring means is not particularly limited, and the electrical resistance can be measured by any means.

  The semiconductor gas sensor preferably includes a determination unit that determines whether the detection target gas is included in the detection gas based on the electrical resistance values measured at the at least three types of temperatures. The determination means is not particularly limited, and it can be determined whether the detection target gas is included in the detection gas by any means. For example, the determination can be made by electrically comparing the data to be compared with the value of the electrical resistance obtained by measuring the test gas at each temperature. The data to be compared can be determined as described above.

The resistor may be disposed so as to be in direct contact with the test gas, but is preferably disposed so as to be indirectly contacted with the test gas via the selective combustion layer. Specifically, the semiconductor gas sensor according to the present invention is a selective combustion capable of selectively burning a specific gas so as to be disposed on the surface of the resistor, that is, between the resistor and the test gas. Preferably it includes a layer. In this case, the selective combustion layer is provided on at least a part, preferably all, of the surface of the resistor. In general, when the resistor is used, it may be difficult for the resistor to respond to a plurality of gases and to selectively detect only the detection target gas. However, by including the selective combustion layer, a gas having a stronger oxidation activity than the detection target gas can be selectively burned, and only the detection target gas can be detected more selectively. Although not particularly limited, a noble metal catalyst such as Pd or Pt can be used as the selective combustion layer. Specifically, when the detection target gas is methane and a selective combustion layer made of a noble metal catalyst such as Pd or Pt is used, a gas such as H ₂ , CO, or ethanol is selectively burned by the selective combustion layer. Therefore, it is possible to suitably measure only the target detection target gas.

In order to minimize the influence of the external environment on the temperature of the resistor and to make the heating of the resistor by the heater more effective, the semiconductor gas sensor is provided between the heater and the external environment. It is preferable that a thermal insulator is included at a position that does not hinder the thermal contact between the heater and the resistor. For example, the thermal insulator is preferably located between the heater and the external environment, on the opposite side of the heater from the resistor. Although not particularly limited, Si ₃ N ₄ or SiO ₂ can be used as the thermal insulator.

The semiconductor gas sensor preferably includes an electric insulator between the resistor and the electric resistance measuring means and the heater. That is, it is preferable to electrically insulate them so that energization for controlling the temperature of the heater does not affect the measurement of the electrical resistance of the resistor. Although not particularly limited, SiO ₂ can be used as an electrical insulator.

  The semiconductor gas sensor preferably includes control means for performing measurement at three types of temperatures and repeatedly performing a series of steps for determining whether or not the detection target gas includes the detection target gas. In addition, the semiconductor gas sensor can include a buzzer, a lamp, and the like for outputting the detection target gas when the detection target gas is detected. In addition, the semiconductor gas sensor has a shutoff valve for shutting off the gas supply source when the detection target gas is detected in the detection gas, such as when the detection target gas is a combustible gas used as fuel. Can be included.

  Here, the semiconductor gas sensor preferably has a diaphragm structure in order to reduce power consumption. That is, it is preferable that the semiconductor gas sensor has a diaphragm structure in which a resistor and a heater, and if necessary, a selective combustion layer, a thermal insulator, and an electrical insulator are formed in a thin film shape and the periphery thereof is supported by a substrate. By adopting the diaphragm structure, it is possible to reduce the heat capacity of the sensor, particularly the resistor, and the power consumption of the sensor can be reduced. Note that although not particularly limited, Si can be used as the substrate. These resistors, heaters, electrical resistance measuring means, selective combustion layers, thermal insulators, electrical insulators, and the like can be the same as conventional ones.

Moreover, the manufacturing method of a semiconductor type gas sensor is not specifically limited, Arbitrary methods can be used. For example, as shown below, the heater 3 for raising temperature, the resistor film 7 and the selective combustion layer 8 electrically insulated from the heater 3 illustrated in FIG. 3 are provided to reduce power consumption. A thin film semiconductor gas sensor having a diaphragm structure with a low heat capacity and high heat resistance as a whole can be manufactured. That is, it is preferable to form a Si ₃ N ₄ and SiO ₂ film sequentially as a support film having a diaphragm structure and a thermal insulating film 2 on the Si substrate 1 having a thermal oxide film on both sides by a plasma CVD method. Next, it is preferable that the Pt—W heater layer 3 and the SiO ₂ electrical insulating layer 4 are formed in this order by sputtering, and the bonding layer 5 and the resistor film electrode 6 are formed thereon. Film formation is preferably performed by an ordinary sputtering method using an RF magnetron sputtering apparatus. The film formation conditions can be the same for the bonding layer (Pt or Au) and the resistor film electrode (Ta or Ti). For example, the Ar gas pressure is 1 Pa, the substrate temperature is 300 ° C., and the RF power is 2 W / cm ² . be able to. The film thickness is preferably 500 mm and 2000 mm for the bonding layer and the resistor film electrode, respectively. Next, it is preferable to form SnO ₂ as the resistor film 7. Film formation is preferably performed by a reactive sputtering method using an RF magnetron sputtering apparatus. It is preferable to use SnO ₂ having 0.5 wt% Sb as the target. The film forming conditions can be, for example, Ar + O ₂ gas pressure 2 Pa, substrate temperature 150 to 300 ° C., RF power 2 W / cm ² . The film thickness is preferably 5000 mm. Subsequently, it is preferable to form the selective combustion layer 8. Pd was added 7.0 wt% .gamma.-alumina was added 5 to 20 wt% of alumina sol (average particle diameter 2 to 3 [mu] m) and a paste, screen printed directly on the resistor film in which SnO _2, thereafter 500 ° C. It can be formed by baking for one hour. The film thickness of the selective combustion layer after firing is preferably 30 to 35 μm. Finally, Si can be removed by etching from the back surface of the substrate to form a diaphragm structure.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. However, the embodiments described below do not limit the present invention. In this example, the thin film type semiconductor gas sensor illustrated in FIG. 3 was used. The detection target gas was methane, hydrogen, and carbon monoxide.

  In FIG. 1, the control pattern of the temperature of the resistor in a present Example is shown. In contrast to the conventional drive mode, a medium temperature state is included between the high temperature and the low temperature. That is, the sensor temperature is high (about 450 ° C.) for 0 to 0.2 seconds after the start of measurement, medium temperature (about 240 ° C.) for 0.7 to 1.2 seconds, and 0.7 to 1.2 for 0.2 to 0.7 seconds. The heater was driven so that the temperature was low (about 120 ° C.) for 2 seconds.

  FIG. 2 shows the relationship between the sensor temperature (resistor temperature) and the resistance value of the resistor in the presence of 4000 ppm of methane, 1000 ppm of hydrogen, or 100 ppm of carbon monoxide. As shown in FIG. 2, when the measurement temperature is preferably room temperature or higher and lower than 150 ° C., more preferably 100 ° C. or higher and lower than 150 ° C., the value of electric resistance in air and methane 4000 ppm, hydrogen 1000 ppm or carbon monoxide 100 ppm There is a significant difference from the value of electrical resistance in the presence of. Similarly, when the measurement temperature is preferably 150 ° C. or higher and lower than 350 ° C., more preferably 200 ° C. or higher and lower than 300 ° C., the value of electric resistance in the presence of 100 ppm of carbon monoxide or in air, and methane 4000 ppm or hydrogen 1000 ppm There is a significant difference from the value of electrical resistance in the presence of. When the measurement temperature is preferably 350 ° C. or more and 500 ° C. or less, more preferably 450 ° C. or more and 500 ° C. or less, in the presence of 100 ppm of carbon monoxide or 1000 ppm of hydrogen or in the presence of 4000 ppm of methane There is a significant difference from the value of electrical resistance at.

  Based on these, the first, second and third temperatures were determined to be 450 ° C., 240 ° C. and 120 ° C., respectively. Note that 240 ° C. is the temperature at which the value of electrical resistance is lowest in the presence of methane and hydrogen. Similarly, 120 ° C. is the temperature at which the value of electrical resistance is lowest in the presence of carbon monoxide. At this time, it can be determined whether or not the detection target gas is contained in the detection gas as follows.

That is, as described above, at a measurement temperature of 120 ° C. (low temperature), there is a significant difference between the value of electrical resistance in the air and the value of electrical resistance in the presence of 4000 ppm of methane, 1000 ppm of hydrogen, or 100 ppm of carbon monoxide. . Here, the threshold value of the electric resistance value at 120 ° C. is between the electric resistance value in the air and the electric resistance value in the presence of 4000 ppm of methane, 1000 ppm of hydrogen or 100 ppm of carbon monoxide, It can be determined to be 7.6 × 10 ⁴ Ω, which is a value near the center of these values. Here, as a result of measuring the test gas, if the value of electric resistance at 120 ° C. is larger than this threshold value, it is determined that methane, hydrogen and carbon monoxide are not present at concentrations of 4000 ppm, 1000 ppm or 100 ppm, respectively. can do. Conversely, if the value of electrical resistance at 120 ° C. is smaller than this threshold, it can be determined that methane, hydrogen, and carbon monoxide are present at concentrations of 4000 ppm, 1000 ppm, or 100 ppm, respectively.

Similarly, based on these data, the threshold values of the electrical resistance values at 240 ° C. and 450 ° C. can be determined as 7.7 × 10 ³ Ω and 9.8 × 10 ³ Ω, respectively. Here, as a result of measuring the test gas, if the value of electrical resistance at 240 ° C. is larger than this threshold value, it can be determined that methane and hydrogen are not present at concentrations of 4000 ppm or 1000 ppm, respectively. Conversely, if the value of electrical resistance at 240 ° C. is smaller than this threshold value, it can be determined that methane and hydrogen are present at a concentration of 4000 ppm or 1000 ppm or more, respectively. As a result of measuring the test gas, if the value of electrical resistance at 450 ° C. is larger than this threshold value, it can be determined that methane does not exist at a concentration of 4000 ppm or more. Conversely, if the value of electrical resistance at 450 ° C. is smaller than this threshold, it can be determined that the methane is present at a concentration of 4000 ppm or more.

  These are summarized in Table 1. For example, as shown in case 1, when the electric resistance value is larger than the threshold value at any of high temperature, medium temperature, and low temperature, the test gas is a reducing gas, that is, methane, hydrogen, and carbon monoxide. It can be determined that none of these are included. Similarly, as shown in case 2, it can be determined that the test gas contains carbon monoxide when the value of the electrical resistance is smaller than the threshold value only at a low temperature. Similarly, in case 3 and case 4, it can be determined that the test gas contains hydrogen or methane, respectively.

−：しきい値より大きい電気抵抗の値
＋：しきい値より小さい電気抵抗の値

−: Value of electrical resistance greater than threshold value +: Value of electrical resistance less than threshold value

  As described above, according to the present invention, when a gas is monitored using a semiconductor gas sensor, preferably a semiconductor gas sensor with low power consumption, the sensitivity and selectivity for the gas can be improved. Further, by selecting the temperature at which the sensor resistance is minimum for each gas, it is possible to obtain more stable sensor characteristics against temperature changes.

The control pattern of the temperature of the resistor in the Example of this invention is shown. The relationship between the sensor temperature in the presence of 4000 ppm of methane, 1000 ppm of hydrogen, or 100 ppm of carbon monoxide and the value of electrical resistance of the resistor is shown. It is sectional drawing of a battery drive type semiconductor gas sensor. It is a schematic diagram which shows the relationship between the time after the measurement start by a semiconductor type gas sensor, and sensor temperature in the conventional High-Low type drive system. It is a schematic diagram which shows the relationship between the time after the measurement start by a semiconductor type gas sensor, and the resistance value which a sensor shows, when various gas is measured with the conventional High-Low type drive system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Support film and thermal insulation film 3 Heater 4 Electrical insulation film 5 Bonding layer 6 Resistor film electrode 7 Resistor film 8 Selective combustion layer

Claims (6)

A method for monitoring a test gas using a semiconductor gas sensor including a resistor,
Maintaining the temperature of the resistor at a first temperature for a predetermined time;
A first measuring step for measuring a value of an electric resistance of the resistor in the test gas at the first temperature;
Changing the temperature of the resistor from the first temperature to a second temperature, and maintaining the temperature at the second temperature for a predetermined time;
A second measuring step for measuring a value of electric resistance of the resistor in the test gas at the second temperature;
Changing the temperature of the resistor from the second temperature to a third temperature and maintaining the temperature at the third temperature for a predetermined time;
A third measuring step for measuring a value of electric resistance of the resistor in the test gas at the third temperature;
Determining whether or not a gas to be detected is contained in the gas to be detected based on the values of the electrical resistance obtained in the first, second and third measuring steps ,
The method wherein the first temperature is higher than the second temperature and the second temperature is higher than the third temperature.   The method according to claim 1, wherein the detection target gas is at least one selected from the group consisting of methane, hydrogen, and carbon monoxide.   The at least one of said 1st, 2nd, and 3rd temperature is set to the temperature from which the value of the electrical resistance of the said resistor in the presence of the predetermined density | concentration of the said detection target gas becomes the lowest. 2. The method according to 2.   Each of said 1st, 2nd and 3rd temperature is any one of the Claims 1-3 selected from room temperature or more and less than 150 degreeC, 150 degreeC or more and less than 350 degreeC, 350 degreeC or more and 500 degrees C or less. the method of.   The value of the electrical resistance of the resistor in the presence of one predetermined concentration of the detection target gas is relative to the value of the electrical resistance of the resistor in the presence of a predetermined concentration of another specific gas or in air. The method according to claim 1, wherein at least one of the first, second, and third temperatures is set to a temperature that causes a significant difference. Changing the temperature of the resistor to a fourth or higher temperature and maintaining the fourth or higher temperature for a predetermined time; and
A fourth or more measuring step of measuring a value of electric resistance of the resistor in the test gas at the fourth or higher temperature, and in the determining step, the first, second and third The determination according to any one of claims 1 to 5 , wherein whether or not a detection target gas is contained in the gas to be detected is determined based on the value of the electrical resistance obtained by the measurement step and the fourth or more measurement steps. Method.

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