Gas detection equipment

Abstract

IGZO thin film ozone detector eliminates the need for a resistivity recovery mechanism. [Solution] The detection unit 10 consists of a conductive substrate 1, an insulating film 2, and an IGZO thin film 3. The IGZO thin film 3 is provided with a source electrode 4 and a drain electrode 5, and the conductive substrate 1 is connected to a gate voltage source 7. NaOH is added to the precursor solution of the IGZO thin film. In the embodiment and the comparative example that does not contain Na, the drain current decreases without ozone exposure, as shown by the solid line and dashed-dotted line 11 starting from time 0. When ozone exposure is performed for 1 minute and ended at 2 minutes, the drain current does not recover even after 2 minutes in the comparative example (curve 12), while the drain current recovers rapidly after 2 minutes in the embodiment (curve 13). A heating mechanism and recovery mechanism for sensor operation are not required, making it possible to realize a small, low-power ozone sensor.

Description

[Technical Field]

[0001] The present invention relates to a gas detector that measures the concentration of an oxidizing gas such as ozone gas, converts the measured value into an electrical signal, and outputs the signal. More specifically, the gas detector includes an oxide semiconductor thin film as an oxidizing gas detection means, and measures the concentration of the oxidizing gas by changing the electrical conductivity of the oxide semiconductor thin film upon exposure to the oxidizing gas. The gas detector can restore the electrical conductivity that has changed after exposure to the oxidizing gas to its pre-exposure value without the need for heat treatment or light irradiation, which have conventionally been required, thereby enabling the sensor to be reused. This provides a small, low-cost, and low-power oxidizing gas sensor. [Background technology]

[0002] The main conventional methods for detecting ozone gas are: (1) a method that utilizes the absorption of 250 nm light in the ultraviolet region by ozone gas and measures the ozone concentration by measuring the absorbance of that light with high precision (detection method (1)); and (2) a method that brings ozone into contact with a thin film of a conductor or semiconductor that is reactive to ozone, and measures the ozone concentration from the resulting change in conductivity (detection method (2)).

[0003] Patent Document 1 discloses an invention relating to detection method (1). This invention comprises a means for introducing a gas into a measurement sample cell, a means for irradiating the introduced gas with ultraviolet light, and a means for measuring light transmittance from the intensity of light transmitted through the gas. However, the cost of constructing these means is generally higher than that of detection method (2) described below.

[0004] Non-patent documents 1, 2 and 3 disclose inventions relating to the detection method (2). The invention disclosed in Non-Patent Document 1 relates to an ozone sensor that uses a heated indium tin oxide (ITO) conductive film as an ozone reactant. In this sensor, the ITO conductive film is heated to activate it so that it can detect ozone, and ozone is detected by measuring the change in resistivity of the ITO conductive film that occurs only when it comes into contact with ozone. Here, to activate the ITO conductive film, it is necessary to heat it to a temperature of 300°C or higher.

[0005] Non-Patent Document 2 is part of a catalog of commercially available ozone sensors that use ITO conductive films, and was disclosed on the Internet. This product also has a heater, and although the temperature is unknown, it is presumed that heating the ITO conductive film activates it so that it can detect ozone.

[0006] Non-Patent Document 3 discloses an invention of an ozone sensor that uses an indium (In) gallium (Ga) zinc (Zn) oxide (IGZO) thin film deposited by sputtering as an ozone reactant. The IGZO thin film does not require heating, which was necessary in the inventions disclosed in Non-Patent Documents 1 and 2, and is capable of detecting ozone even at room temperature. Meanwhile, the sensors of the inventions disclosed in Non-Patent Documents 1 and 2 undergo a change in resistivity only when in contact with ozone, whereas the invention of Non-Patent Document 3, once exposed to ozone and undergoing a change in resistivity, does not return to its original state for a long time, even after the sensor is no longer in contact with ozone. Therefore, before gas detection can be resumed, the resistivity change must be restored by means of ultraviolet light irradiation, heating, or other means. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent Publication No. 2012-13573 [Non-patent literature]

[0008] [Non-Patent Document 1] Ozone sensor made by dip coating method, Akiyoshi Hattori, Hirokazu Tachibana, Nobuyuki Yoshiike, Akihiko Yoshida: Sensors and Actuators 77 1999 120 125 [Non-patent document 2] https: / / www.fisinc.co.jp / common / pdf / J _A1320301-SP361-xx.pdf [Non-patent document 3] Kuen-Lin Chen, Guo-Jhen Jiang, Kai-Wei Chang, Jan-Han Chen, Chiu-Hsien Wu: Gas sensing properties of indium gallium zinc oxide gas sensors in different light intensity, Analytical Chemistry Research 4 (2015) 8 12. Summary of the Invention [Problem to be solved by the invention]

[0009] In the inventions disclosed in Non-Patent Documents 1 and 2, when ozone detection is desired, the ITO conductive film must be heated to activate it for ozone detection. In other words, the ITO conductive film must be kept constantly heated during ozone detection. This heating requires a large amount of power, and when the ozone sensor is configured as a portable device, battery capacity is limited due to size, weight, and other constraints, resulting in a short battery operating time. Furthermore, the ITO conductive film must be covered with a metal can (casing) to protect it from accidental contact with the human body or clothing when heated to a high temperature, making it difficult to miniaturize the entire device.

[0010] In recent years, the invention disclosed in Non-Patent Document 3 has attracted attention as an ozone detector that solves the above-mentioned problems of the inventions disclosed in Non-Patent Documents 1 and 2, and extensive research is being conducted in various fields. The invention disclosed in Non-Patent Document 3 uses an IGZO thin film as an ozone detector instead of an ITO conductive film. However, this IGZO thin film is active against ozone without heating, so no special heating mechanism is required for ozone detection. This eliminates the need for power for heating, and a significantly longer battery life can be expected compared to the inventions disclosed in Non-Patent Documents 1 and 2. Furthermore, the inventions disclosed in Non-Patent Documents 1 and 2 required a can to house the ITO conductive film to prevent inadvertent contact with the hot ITO conductive film, but this elimination of this requirement is advantageous for miniaturization.

[0011] However, IGZO thin films have another problem: once their resistivity changes due to contact with ozone, it does not return to its original state even after the film is no longer in contact with ozone. For this reason, irradiation with ultraviolet light is required to restore the changed resistivity (hereinafter referred to as the recovery operation). Although not as powerful as the heating used in the inventions of Non-Patent Documents 1 and 2, power consumption due to light irradiation is unavoidable, and the addition of a light irradiation mechanism also increases the size and costs involved.

[0012] The present invention has been made in light of the above-described conventional technologies and problems, and aims to realize and provide a small, low-power gas detection device (e.g., an ozone sensor) by using an IGZO thin film that does not require heating for ozone detection, thereby eliminating the need for light irradiation for recovery operation and avoiding the increase in size, cost, and power consumption that would otherwise result from light irradiation. [Means for solving the problem]

[0013] The gas detection device according to claim 1 is a gas detection device that detects In, Ga, Z n The sensor is characterized by using an oxide semiconductor thin film containing constituent elements and one or more additive elements selected from alkali metals and alkaline earth metals, and whose resistivity changes upon reaction with the gas to be detected.

[0014] The gas detection device according to claim 2 is the gas detection device according to claim 1, The alkali metal element is K or Na, and the alkaline earth metal is Ca or Mg.

[0015] The gas detection device according to claim 3 is the gas detection device according to claim 2, The gas to be detected is an oxidizing gas containing a gas selected from ozone gas, nitrogen monoxide, and nitrogen dioxide.

[0016] The gas detection device according to claim 4 comprises: a conductive substrate; an insulating film formed on the surface of the conductive substrate; In, Ga, Z n an oxide semiconductor thin film formed on the insulating film, the oxide semiconductor thin film containing constituent elements and one or more additive elements selected from alkali metals and alkaline earth metals, and the resistivity of which changes upon reaction with a gas to be detected; a current detector for detecting a current flowing through the oxide semiconductor thin film; It is characterized by having: [Effects of the Invention]

[0017] The gas detector of the present invention uses an oxide semiconductor thin film (e.g., an IGZO thin film) containing one or more constituent elements selected from In, Ga, and Zn and one or more additive elements selected from alkali metals (e.g., Na and K) and alkaline earth metals (e.g., Ca and Mg) as a detector for detecting oxidizing gases (e.g., ozone gas, nitric oxide, and nitrogen dioxide). According to the present invention, this configuration allows the resistivity of the oxide semiconductor thin film, which increases upon contact with an oxidizing gas, to immediately return to its pre-contact value after the contact ends. This eliminates the need for a heating mechanism, which was necessary for sensor operation in the inventions of Non-Patent Documents 1 and 2, or a special recovery mechanism such as an ultraviolet light irradiation mechanism, which was necessary in the invention of Non-Patent Document 3. As a result, it is possible to realize a small, low-power ozone sensor. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic perspective view of a gas detector according to an embodiment; [Figure 2] 1 is a schematic cross-sectional view of a gas detector according to an embodiment. [Figure 3] 10 is a graph showing the detection characteristics of ozone gas in the gas detectors of the embodiment and the comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0019] An embodiment of the present invention will be described with reference to FIGS. The present invention relates to a gas detection device for detecting an oxidizing gas containing ozone gas and a gas selected from nitrogen monoxide and nitrogen dioxide, but the embodiment described below relates to an ozone detector that particularly detects ozone gas.

[0020] As shown in FIGS. 1 and 2, the ozone detector of this embodiment includes an ozone detection unit 10. The ozone detection unit 10 is composed of a conductive substrate 1 made of p-type single crystal silicon, an insulating film 2 made of SiO deposited on the conductive substrate 1, and an IGZO thin film 3 as an oxide semiconductor thin film deposited on the insulating film 2. The conductive substrate 1 must be sufficiently smooth to accurately and reliably perform the process of forming the SiO insulating film 2 on its surface and then forming the IGZO thin film 3 thereon. It must also be conductive so that the substrate can function as a gate electrode, as described below. Therefore, in this embodiment, a p-type single crystal silicon substrate, which is a conductive semiconductor, is used. However, a metal plate with a mirror-finished surface on which the insulating film 2 is formed, may also be used. The thickness of the conductive substrate 1 is not particularly limited. The insulating film 2 made of SiO may have a thickness sufficient to insulate the conductive substrate 1 from the IGZO thin film 3 and to ensure the strength required to stably form the IGZO thin film 3 thereon. In this embodiment, the thickness is, for example, approximately 300 nm. The thickness of the IGZO thin film 3 is, for example, approximately 10 to 20 nm. Na, K, Ca, Mg, or ions thereof, either singly or in any combination, or various compounds of Na, K, Ca, and Mg, which will be described later, are added to the IGZO thin film 3. The method for producing the IGZO thin film 3 will be described in detail later.

[0021] As shown in Figures 1 and 2, a source electrode 4 and a drain electrode 5 are provided on the surface of the IGZO thin film 3 to apply a voltage in a direction parallel to the IGZO thin film 3 and cause a current to flow within the IGZO thin film 3. In Figure 2, the current flowing within the IGZO thin film 3 is indicated by a dashed line. The source electrode 4 is connected to ground, i.e., a point at a potential of 0 V, and the drain electrode 5 is connected to a drain voltage source 6 via a current detector 8 for measuring the drain current. The conductive substrate 1 is connected to a gate voltage source 7 and is given the function of a gate electrode.

[0022] Next, a method for manufacturing the IGZO thin film 3 will be described in detail. The oxide semiconductor thin film of the gas detection device of the present invention has one or more metals selected from In, Ga, and Zn as its constituent elements. In this embodiment, the oxide semiconductor thin film is an IGZO thin film 3, and its constituent elements are the three elements In, Ga, and Zn.

[0023] First, indium nitrate, gallium nitrate, and zinc nitrate are dissolved in 2-methoxyethanol so that the metal ion concentration is 0.1M (mol / L) and the solution is thoroughly stirred to obtain three solutions containing the constituent elements In, Ga, and Zn ions. These three solutions are then mixed so that the molar ratio of In, Ga, and Zn ions is 6:1:3 to obtain an IGZO precursor solution.

[0024] Although the molar ratio of In, Ga, and Zn ions in the IGZO precursor solution was set to 6:1:3, experiments have shown that even if this molar ratio is changed to 6:0:3 or 6:0:0 and an oxide semiconductor thin film is fabricated using the process described below and an ozone detector is constructed, the resulting effect is equivalent to or better than that of this embodiment. That is, the oxide semiconductor thin film of this embodiment may contain any ratio of the constituent elements In, Ga, and Zn, as long as it contains one or more constituent elements selected from the group consisting of these three constituent elements. Furthermore, in this embodiment, the IGZO thin film 3 is formed by applying the IGZO precursor solution to the insulating layer 3 of the conductive substrate 1 by spin coating, as described below. However, if a sputtering method is used instead of a liquid-phase method, the molar ratio of In, Ga, and Zn ions should be 1:1:1.

[0025] Next, the additive element was added to the IGZO precursor solution. First, NaOH was dissolved in 2-methoxyethanol to a concentration of 0.1M (mol / L) and added to the IGZO precursor solution. The amount added was such that the molar ratio of Na ions in the IGZO precursor solution after addition was 5%. Here, Na ions can be added by adding substances other than NaOH. For example, NaCl (sodium chloride), NaHCO3 (sodium bicarbonate), Na2CO3 (sodium carbonate), or Na2CH3O6 (sodium percarbonate) are also acceptable.

[0026] K ions may also be added instead of Na ions. K ions can be added using KOH, but they may also be added by adding compounds of the same type as the other substances exemplified for Na. For example, KCl (potassium chloride), KHCO3 (potassium bicarbonate), K2CO3 (potassium carbonate), or K2CH3O6 (potassium percarbonate) may be used. When K ions are added, the same results as when Na ions are added are obtained.

[0027] Furthermore, the same effect as when Na ions or K ions are added can be obtained by adding alkaline earth metal ions such as Ca or Mg instead of the alkali metal elements Na or K. When Ca or Mg ions are added, they can be added in the form of hydroxide, chloride, or carbonate compound, similar to when Na or K ions are added.

[0028] Furthermore, any combination of Na, K, Ca, and Mg ions may be added, or any combination of Na, K, Ca, and Mg compounds may be added. When multiple types of ions or compounds are added to the IGZO precursor solution, it is preferable that the total molar ratio of the added ions be 5%.

[0029] As shown in FIG. 1, the IGZO precursor coating solution is applied to the surface of the insulating film 2 on the conductive substrate 1. Spin coating can be used as the coating method, but other liquid-phase methods are also acceptable. The conductive substrate 1 is then heated to 250-300°C on a hot plate and held there for 60 minutes. Prior to this heating, the substrate may be heated at 90-130°C for 5 minutes or more to dry the solvent. This allows an IGZO thin film 3 to be formed on the surface of the insulating film 2 on the conductive substrate 1. Furthermore, two Al electrodes are formed at a distance on the thus-formed IGZO thin film 3 by vapor deposition or other methods, forming the source electrode 4 and the drain electrode 5, respectively.

[0030] Next, the operation of the ozone detector of this embodiment when detecting ozone will be described. With the output voltage of the drain voltage source 6 shown in FIG. 1 set to 40 V and the output voltage of the gate voltage source 7 set to 40 V, the magnitude of the current flowing from the drain electrode 5 to the source electrode 4 is measured by a current detector 8 for measuring the drain current.

[0031] FIG. 3 is a graph showing the time dependence of the drain current flowing from the drain electrode 5 to the source electrode 4 of an ozone detector according to the embodiment, which includes an IGZO thin film 3 doped with Na ions by the manufacturing method described above, and an ozone detector according to a comparative example, which includes an IGZO thin film not doped with Na ions or other additive elements, when exposed to ozone gas and when not exposed to ozone gas. In other words, this graph shows the characteristics of both the ozone detector according to the embodiment and the comparative example. The horizontal axis of FIG. 3 represents the elapsed time (in minutes) after the start of voltage output from the drain voltage source 6 and the gate voltage source 7. The vertical axis represents the current ratio, on a logarithmic scale, where the magnitude of the drain current at 0 minutes elapsed is set to 1.

[0032] 3, the solid curve starting from the point where the elapsed time is 0 minutes and the drain current ratio is 1, and the dashed-dotted curve 11 that smoothly continues thereto, show the change in the drain current ratio common to the embodiment and the comparative example without exposure to ozone. According to the solid and dashed-dotted curves 11, the drain current gradually decreases with the passage of time after the drain voltage source 6 and the gate voltage source 7 start to output voltage, even without exposure to ozone. This is a characteristic common to the ozone detector of the embodiment and the ozone detector of the comparative example.

[0033] Next, an operation will be described in which the ozone detector is exposed to ozone gas when one minute has elapsed in FIG. 3, and the exposure to ozone gas is terminated when two minutes has elapsed. When an ozone detector of the comparative example, which uses an IGZO thin film that does not contain added elements such as Na, is exposed to ozone, the drain current decreases rapidly as shown by the solid line one minute after the start of ozone exposure. Even after the ozone exposure is stopped at two minutes, the drain current remains reduced and does not recover, as shown by the dashed curve 12.

[0034] On the other hand, when the ozone detector of the embodiment including the Na-doped IGZO thin film 3 is exposed to ozone, the drain current rapidly decreases as shown by the solid line from 1 minute after the start of exposure, just as in the comparative example. However, when the ozone exposure is interrupted at 2 minutes, unlike the comparative example, the drain current rapidly recovers as shown by the solid curve 13, and returns to a value equivalent to that of the curve 11 shown by the dashed-dotted line in the case of no ozone exposure. In the example of Figure 3, the time required to return to a value equivalent to that of no ozone exposure is just under 1 minute, but even when various experimental conditions are changed so that it takes longer, it is generally within 5 minutes.

[0035] From the above experimental results, the ozone detector of this embodiment has properties in which the resistivity of the IGZO thin film 3 increases and the drain current decreases only when exposed to ozone, and therefore has advantageous properties as an ozone sensor in that the resistivity decreases and the drain current recovers immediately when the ozone exposure is discontinued, without the need for recovery operations such as UV light irradiation. Therefore, it is possible to measure the concentration of ozone gas in real time using the drain current value as an index. Furthermore, since no heating mechanism is required for sensor operation, and no UV light irradiation mechanism or other recovery mechanisms are required for the sensor, it is possible to realize a small, low-power ozone sensor.

[0036] Thus, the property that the drain current of an IGZO thin film, which has decreased due to ozone exposure, immediately recovers when the exposure is discontinued, is thought to be obtained by adding a specific additive substance (or its compound) to the IGZO thin film. The detailed mechanism by which Na, K, Ca, Mg or their ions cause a rapid recovery in resistivity is unknown. However, the inventors of the present application presume that in the case of an IGZO thin film manufactured by adding NaOH to an IGZO precursor solution to add Na as an additive element, the above property appears through the following mechanism. That is, in the IGZO thin film, the OH contained therein - It is thought that current flows when ozone and OH are the source of electrons, but when an IGZO thin film is exposed to ozone, -Because OH bonds - When the ozone exposure is stopped, the O2 and H2O (water) in the air and the Na in the IGZO thin film are released. + The resistivity returns to the value before exposure to ozone, and the drain current recovers. This is thought to be the case for compounds other than NaOH.

[0037] The ozone detection using the ozone detector of this embodiment is carried out as follows. An ozone sensor different from the ozone detector of this embodiment is used as the reference sensor. First, the ozone concentration is measured using the reference sensor. The ozone detector of this embodiment is exposed to ozone of known concentration, assuming the concentration measured by the reference sensor, and the rate of decrease in drain current is measured. This measurement is performed similarly for ozone of various concentrations. This results in reference data (or a graph showing this correspondence) indicating the correspondence between the ozone concentration measured by the reference sensor and the rate of decrease in drain current (the rate of decrease from the reference value) detected by the ozone detector of this embodiment. The ozone detector of this embodiment is exposed to ozone of unknown concentration to obtain measurement results such as those shown by the solid line in Figure 3. The rate of decrease in drain current is calculated from the ratio of the drain current at the start of exposure to that at the end of exposure, and the reference data is referenced to determine the ozone concentration. The process of calculating the ozone concentration from the measurement results and the reference data can be performed automatically by software.

[0038] In the embodiment described above, ozone was used as the gas to be detected, but other oxidizing gases, such as nitrogen monoxide and nitrogen dioxide, can also be detected in the same way, and gases containing a mixture of multiple types of oxidizing gases can also be detected.

[0039] In the embodiment described above, a gate voltage is applied to the conductive substrate 1 to increase the sensitivity of ozone detection, but it is possible to detect ozone by measuring the drain current even without applying this voltage. [Explanation of symbols]

[0040] 1. Conductive substrate 2. Insulating film 3 IGZO thin film 4. Source electrode 5. Drain electrode 6 Drain voltage source 7 Gate voltage source 8 Current detector for drain current measurement 10 Ozone detector 11 Curve when not exposed to ozone 12 Curve when an ozone detector using the comparative IGZO thin film is exposed to ozone 13 Curve when an ozone detector using the IGZO thin film of the embodiment is exposed to ozone

Claims

1. A gas detection device characterized by using an oxide semiconductor thin film containing the constituent elements In, Ga, and Zn and one or more additive elements selected from alkali metals and alkaline earth metals, and whose resistivity changes upon reaction with the gas to be detected.

2. 2. The gas detection device according to claim 1, wherein the alkali metal element is K or Na, and the alkaline earth metal element is Ca or Mg.

3. 3. The gas detection device according to claim 2, wherein the gas to be detected is an oxidizing gas containing a gas selected from the group consisting of ozone gas, nitrogen monoxide, and nitrogen dioxide.

4. a conductive substrate; an insulating film formed on the surface of the conductive substrate; an oxide semiconductor thin film formed on the insulating film, the oxide semiconductor thin film containing constituent elements of In, Ga, and Zn and one or more additive elements selected from alkali metals and alkaline earth metals, the resistivity of which changes upon reaction with a gas to be detected; a current detector for detecting a current flowing through the oxide semiconductor thin film; A gas detection device comprising:

Images