Sensor device, method for manufacturing sensor device, and gas determination system

Abstract

[Problem] To provide a sensor device, a production method for the sensor device, and a gas determination system that make it possible to increase the sensitivity with which gases are detected. [Solution] The sensor device according to one embodiment of the present invention comprises a gate electrode, an insulating film, a graphene layer, a source electrode and a drain electrode, and a conductive porous layer. The insulating film is formed on the gate electrode. The graphene layer is formed on the insulating film. The source electrode and the drain electrode are arranged on the insulating film so as to be opposite each other with the graphene layer therebetween. The porous layer is formed on the graphene layer.

Description

[Technical Field]

[0001] The present invention relates to a sensor device, a method for manufacturing a sensor device, and a gas determination system. [Background technology]

[0002] The sensor described in Patent Document 1 has a FET structure including a gate electrode, an insulating film provided on the gate electrode, a graphene film provided on the insulating film, a first electrode, and a second electrode. In the sensor described in Patent Document 1, before measuring the detection object, a constant voltage is applied between the first and second electrodes, the gate voltage of the gate electrode is increased or decreased, and a current value Id is measured. The same operation is then performed during measurement of the test object. The change ΔVg in the gate voltage Vg before and after the measurement, at which the current value Id is minimized, is used to evaluate the test object. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-163146 Summary of the Invention [Problem to be solved by the invention]

[0004] It is desirable for a gas sensor to be able to accurately determine the type of gas. In view of the above circumstances, an object of the present invention is to provide a sensor device, a method for manufacturing a sensor device, and a gas determination system that can improve gas detection sensitivity. [Means for solving the problem]

[0005] A sensor device according to one aspect of the present invention includes a gate electrode, an insulating film, a graphene layer, a source electrode, a drain electrode, and a conductive porous layer. The insulating film is formed on the gate electrode. The graphene layer is formed on the insulating film. The source electrode and the drain electrode are disposed on the insulating film so as to face each other with the graphene layer interposed therebetween. The porous layer is formed on the graphene layer.

[0006] A gas determination system according to one aspect of the present invention includes a sensor device and an information processing device. The sensor device includes a gate electrode, an insulating film formed on the gate electrode, a graphene layer formed on the insulating film, a source electrode and a drain electrode arranged on the insulating film with the graphene layer interposed between them, and a conductive porous layer formed on the graphene layer. The information processing device has a control unit that controls the voltage applied to the gate electrode, and a determination unit that determines the gas adsorbed in the porous layer based on the measurement results of the current flowing between the source electrode and the drain electrode. [Effects of the Invention]

[0007] According to the present invention, the gas detection sensitivity can be improved. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic diagram showing a configuration of a gas determination system according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing the configuration of a sensor device that constitutes a part of the gas determination system. [Figure 3] 10 is a partially enlarged schematic diagram of the vicinity of the channel layer, illustrating the state of the graphene layer and CO 2 molecules when a first voltage and a second voltage are applied to the gate electrode. FIG. [Figure 4] FIG. 2 is a schematic diagram showing the configuration of a channel layer. [Figure 5] 10 is a graph showing a change in the current flowing between the source electrode and the drain electrode when a sweep voltage is applied to the gate electrode after application of a first voltage and after application of a second voltage in the gas determination system. [Figure 6] The figure shows the amount of charge transfer between the channel layer and gas molecules in the CNPD range when CO2, C6H6, CO, NH3, and O2 are used as gases. [Figure 7] 10 is a graph showing the results of measuring the range of CNPD of acetone and ammonia as gases by varying the gas concentration using the gas determination system. [Figure 8] FIG. 4 is a flowchart illustrating an outline of a procedure for gas determination in the gas determination system. [Figure 9] FIG. 10 is a flow chart illustrating a gas determination method. [Figure 10] FIG. 3 is a diagram showing signal waveforms of a first voltage, a second voltage, and a sweep voltage in the gas sensor of the gas determination system. [Figure 11] FIG. 11 shows the experimental results of the graphene sensor. [Figure 12] (A) shows the experimental results of a gas sensor having a channel layer not decorated with activated carbon, and (B) shows the experimental results of a gas sensor having a channel layer decorated with activated carbon. [Figure 13] (A) is the experimental result showing the detection ability of ammonia gas (5 ppb) in the presence of air in a gas sensor having a channel layer decorated with activated carbon, and (B) is the experimental result showing the detection ability of ammonia gas (500 ppt) in the presence of air in a gas sensor having a channel layer decorated with activated carbon. [Figure 14] 5A to 5C are schematic cross-sectional views illustrating steps in a method for manufacturing a sensor device according to an embodiment of the present invention. [Figure 15] 5A to 5C are schematic cross-sectional views illustrating steps in a method for manufacturing a sensor device according to an embodiment of the present invention. [Figure 16] 5A to 5C are schematic cross-sectional views illustrating steps in a method for manufacturing a sensor device according to an embodiment of the present invention. [Figure 17] 5A to 5C are schematic cross-sectional views illustrating steps in a method for manufacturing a sensor device according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [Gas detection system overview] Fig. 1 is a schematic diagram showing the configuration of a gas determination system, and Fig. 2 is a schematic diagram showing the configuration of a sensor 10 that constitutes a part of the gas determination system.

[0010] As shown in FIG. 1, gas determination system 1 includes sensor device 2, information processing device 4, display device 5, and storage unit 6. The sensor device 2 includes a housing chamber 20, a sensor 10 (sensor device), a UV (ultraviolet) light source 23, and a heating unit .

[0011] The storage chamber 20 accommodates the sensor 10, a UV light source 23, and a heating unit 26. The storage chamber 20 has an intake port 21 for drawing in gas from the outside, and an exhaust port 22 for exhausting gas introduced into the storage chamber 20 from the storage chamber 20 to the outside. The intake port 21 is provided with a valve 24 for regulating the inflow of gas into the storage chamber 20, and the exhaust port 22 is provided with a valve 25 for regulating the outflow of gas from the storage chamber 20 to the outside.

[0012] The UV light source 23 emits ultraviolet (UV) light that is irradiated onto the sensor 10. By irradiating the channel layer 15 of the sensor 10 with UV light, which will be described later, the channel layer 15 is cleaned. The heating unit 26 is, for example, a heater, and heats the sensor 10 .

[0013] As shown in FIG. 2, the sensor 10 includes a gate electrode 13, an insulating film 14, a source electrode 11, a drain electrode 12, and a channel layer 15. The gate electrode 13 is made of highly doped conductive silicon. The insulating film 14 is formed on the gate electrode 13. The insulating film 14 is made of, for example, SiO2. The source electrode 11 and the drain electrode 12 are formed on an insulating film 14 and are arranged opposite to each other via a channel layer 15. The source electrode 11 and the drain electrode 12 are configured, for example, with a laminated structure of a Cr film and an Au film.

[0014] The channel layer 15 is formed on the insulating film 14 and is disposed opposite the gate electrode 13 via the insulating film 14. As shown in FIG. 4, the channel layer 15 has a graphene layer 16 and a porous layer 17 formed on the graphene layer 16.

[0015] In this embodiment, the graphene layer 16 is a single layer, but may be a multilayer, and some layers of the multilayer may be made of a conductive material other than graphene.

[0016] The porous layer 17 is formed by decorating the surface of the graphene layer 16 with activated carbon. The material constituting the porous layer 17 is not limited to activated carbon, and may be any material as long as it is more porous than the graphene layer 16 and has conductive properties similar to those of the graphene layer 16. The conductive material constituting the porous layer 17 may be, for example, a carbon-based material, a conductive polymer, a conductive ceramic, or porous silicon.

[0017] The porous layer 17 functions to promote gas adsorption, and because it is porous, it increases the adsorption surface area and has many dangling bonds, allowing it to adsorb gas efficiently. Since the porous layer 17 has conductive properties, the adsorption surface area is larger than when a non-conductive film is provided on the graphene layer 16 as an adsorption film. In addition, the porous layer 17 has many dangling bonds on its surface, which promotes gas molecule adsorption and increases the shift of the charge neutral point.

[0018] Preferably, the porous layer 17 is made of the same carbon-based material as the graphene layer 16. A carbon-based material is a substance whose main component is carbon. In this embodiment, the porous layer 17 is porous and made of a carbon-based material, i.e., activated carbon. This reduces the formation of a potential barrier between different materials with different electron affinities, and increases the difference in charge neutrality points to facilitate charge transfer, compared to when a non-carbon-based material is used for the porous layer 17.

[0019] The thickness of the porous layer 17 is not particularly limited, and may be the same as the thickness of the graphene layer 16, or may be thinner or thicker than the thickness of the graphene layer 16. Typically, the thickness of the graphene layer 16 is 0.35 nm, and the thickness of the porous layer 17 is 30 nm. The porous layer 17 is formed so as to cover the entire surface of the graphene layer 16, but is not limited to this, and may be formed so as to cover at least a portion of the surface of the graphene layer 16.

[0020] As shown in FIG. 1, the information processing device 4 includes an acquisition unit 41, a determination unit 42, an output unit 43, and a control unit 44. The acquiring unit 41 acquires information about changes in the current flowing between the source electrode 11 and the drain electrode 12. Hereinafter, the current flowing between the source electrode 11 and the drain electrode 12 may be referred to as a drain current. The determination unit 42 determines the type of gas using the current change information acquired by the acquisition unit 41. Specifically, the information processing device 4 acquires current change information for each of a plurality of different types of gases in advance and stores it in the storage unit 6. The determination unit 42 identifies and determines the type of gas detected by the sensor 10 by referring to the current change information stored in the storage unit 6. The determination unit 42 can also determine the gas concentration. Details will be described later. The output unit 43 outputs the current change information acquired by the acquisition unit 41 and the determination results such as the type and concentration of gas determined by the determination unit 42 to the display device 5. As shown in FIG. 2, the control unit 44 controls the voltage applied to the gate electrode 13 of the sensor 10.

[0021] The display device 5 has a display unit, and displays on the display unit the type and concentration of gas output from the information processing device 4. The user can understand the gas determination result by checking the display unit. Storage unit 6 acquires in advance current change information for each of a plurality of different known gases detected by gas determination system 1 and stores the information as reference data. Storage unit 6 may be located on a cloud server with which information processing device 4 can communicate, or may be included in information processing device 4.

[0022] (Sensor details) The sensor 10 is a field effect transistor type sensor device. 3A and 3B are enlarged schematic diagrams of a portion near the channel layer 15, which explains the channel layer 15, whose state changes depending on the voltage applied to the gate electrode 13, and the charge state of CO2, an example of a gas adsorbed to the channel layer 15.

[0023] FIG. 3A shows a state in which a first tuning voltage V T1 In this embodiment, the first tuning voltage V T1 is a constant voltage at a given time, which is −40 V. The first tuning voltage V T1 The value of is not limited to -40V, and the first tuning voltage V T1 The voltage may be set to a value such that negative charges are supplied to the channel layer 15 and the channel layer 15 has a valence band by applying the voltage. FIG. 3B shows a second tuning voltage V T2 In this embodiment, the second tuning voltage V T2 is a constant voltage at a given time, 40 V. The second tuning voltage V T2 The value of is not limited to 40V, and the second tuning voltage V T2 The voltage may be set to a value such that a positive charge is supplied to the channel layer 15 and the channel layer 15 has a conduction band by applying the voltage. In this embodiment, the first and second tuning voltages are constant voltages, and an example has been given in which the voltages change in a rectangular wave shape as shown in Fig. 11, but the present invention is not limited to this. For example, the voltage value may fluctuate slightly within a predetermined time, such as by reducing the rise and fall of the voltage or by changing the voltage value with a slight gradient, as long as the voltage value is such that the channel layer 15 has a valence band or a conduction band when applied.

[0024] The channel layer 15 attracts gas when the first tuning voltage is applied and when the second tuning voltage is applied. As shown in FIG. 3, when the first tuning voltage is applied and when the second tuning voltage is applied, the gas molecules (CO2 molecules in this case) adsorbed to the channel layer 15 have different bonding states, such as the distance and bond angle, with the channel layer 15. As a result, when the first tuning voltage V T1 When the second tuning voltage V T2 When voltage is applied, CO2 acts as an acceptor.

[0025] When a gas is supplied to the channel layer, gas molecules naturally adsorbed to the channel layer exist in the state where no voltage is applied to the gate electrode, but the number of such molecules is thought to be small. In contrast, in this embodiment, by applying a first tuning voltage and a second tuning voltage to the gate electrode, gas molecules that come close to the channel layer are guided to the surface of the channel layer by the electric field indicated by the arrows in Figure 3, accelerating gas adsorption. Furthermore, in this embodiment, as shown in Figure 3, by applying a first tuning voltage and a second tuning voltage, the direction of the electric field near the surface of the channel layer can be changed, thereby changing the binding state of gas molecules to the channel layer.

[0026] Preferred first tuning voltage V T1 and the second tuning voltage V T2 The value of can be set appropriately depending on the thickness of the insulating film 14. In this embodiment, the insulating film 14 has a thickness of 285 nm, and in this case, a voltage of about −40 V (40 V) is required to make the channel layer 15 have a valence band (conduction band). Also, to confirm that the channel layer 15 switches between the valence band and the conduction band, a first tuning voltage V T1 and the second tuning voltage V T2 It is preferable to apply a voltage on both the negative and positive sides. It is more preferable to apply a voltage so that the absolute values ​​of the negative and positive voltages are the same. Also, the first tuning voltage V T1 and a second tuning voltage V T2 The application time for each voltage is from a few seconds to a few minutes.

[0027] FIG. 5 shows the gas determination system 1 when the first tuning voltage V T1 When a sweep voltage is applied to the gate electrode 13 after applying the second tuning voltage V T2 10 is a graph showing a change in the current flowing between the source electrode 11 and the drain electrode 12 when a sweep voltage is applied to the gate electrode 13 after a predetermined time. The voltage applied to the gate electrode 13 is controlled by the control unit 44 . The sweep voltage varies between a first tuning voltage and a second tuning voltage different from the first tuning voltage, and the voltage increases or decreases in this range. In this embodiment, a sweep voltage that varies linearly from -40 V to 40 V in about one minute is used, and the sweep voltage varies both positively and negatively.

[0028] In this embodiment, a first tuning voltage V is applied to the gate electrode 13 of the sensor 10 to which the gas is supplied. T1 After applying a sweep voltage to the gate electrode 13 for a predetermined time, the drain current I d (First current I d1 ) is measured. The solid curve 51 shown in FIG. 5 represents the first current I d1 In the resulting curve 51, the first current I d1 The point where the first current I is at its minimum value is called the first charge neutral point 31. d1 The gate voltage value when becomes the minimum value is called the first gate voltage. As mentioned above, the first tuning voltage V T1 to the gate electrode 13, the channel layer 15 has a valence band, which makes the gas sufficiently attracted to the channel layer 15 and makes the gas a donor.

[0029] Furthermore, in this embodiment, a second tuning voltage V is applied to the gate electrode 13 of the sensor 10 to which the gas is supplied. T2 After applying a sweep voltage to the gate electrode 13 for a predetermined time, the drain current I d (Second current I d2 ) is measured. The long dashed curve 52 shown in FIG. 5 represents the second current I d2 In the resulting curve 52, the second current I d2 The point where the second current I is at its minimum value is called the second charge neutral point 32. d2 The gate voltage value when becomes the minimum value is called the second gate voltage.

[0030] In Fig. 5, the short dashed curve 50 is located at the center of the horizontal axis between the curves 51 and 52. The current I d The point where is the minimum value is set as the center point 30.

[0031] As shown in Figure 5, the second current I d2 The curve 52 showing the characteristics of the first current I versus the sweep voltage (gate voltage Vg). d1 The shape of the curve 51 representing the characteristic of the curve 51 is shifted along the horizontal axis. In Figure 5, V CNP indicates the gate voltage value when the charge neutrality point is reached, and ΔV CNP indicates the difference between the first gate voltage and the second gate voltage.

[0032] Here, the first gate voltage at the first charge neutral point 31 and the second gate voltage at the second charge neutral point 32 are specific to each type of gas adsorbed to the channel layer 15, and the band indicating the range from the first gate voltage to the second gate voltage differs for each type of gas. This is thought to be because the bonding state between the channel layer 15 and the gas that is attracted to the channel layer 15 and functions as an acceptor or donor differs for each type of gas.

[0033] FIG. 6 shows that the band indicating the range from the first gate voltage to the second gate voltage differs depending on the type of gas. FIG. 6 shows the bands for five types of gases: CO2 (carbon dioxide), C6H6 (benzene), CO (carbon monoxide), NH3 (ammonia), and O2 (oxygen). FIG. 6 shows the charge state of the channel layer in the range of CNPD (Charge Neutrality Point Disparity: |ΔVCNP| in FIG. 5). CNPD indicates the difference between the first charge neutrality point 31 and the second charge neutrality point 32, and corresponds to the band.

[0034] In Figure 6, the vertically extending strips represent bands that indicate the range from the first gate voltage to the second gate voltage. The upper part of the strip corresponds to the first gate voltage at the second charge neutral point 32, and the lower part corresponds to the second gate voltage at the first charge neutral point 31. A center point 30 is located at the center of each vertically extending band. In each band, the upper half above the center point 30 indicates the range in which the gas acts as an acceptor, and the lower half indicates the range in which the gas acts as a donor.

[0035] As shown in Figure 6, the first gate voltage and the second gate voltage differ depending on the type of gas, and the band width and range also differ. Therefore, the type of gas can be determined by using this band data. For example, in this embodiment, band data for a plurality of known gases is acquired in advance and stored in the storage unit 6. Then, by referring to the data stored in the storage unit 6, the type of gas can be determined from the band data obtained for an unknown gas. In this way, by acquiring data on the change characteristics of the drain current corresponding to the sweep voltage after applying two tuning voltages of −40 V and 40 V, it becomes possible to determine the type of gas.

[0036] Furthermore, the band showing the range from the first gate voltage to the second gate voltage changes almost linearly with changes in gas concentration. Figure 7 shows the results of measuring the first gate voltage at the first charge neutral point 31 obtained by applying a sweep voltage after applying the first tuning voltage and the second gate voltage at the second charge neutral point 32 obtained by applying a sweep voltage after applying the second tuning voltage while varying the gas concentration. In the figure, the bar graph indicates the gate voltage value at the center point 30. The vertical line indicates the band from the first gate voltage to the second gate voltage. FIG. 7(A) shows the case where acetone was used as the gas, and FIG. 7(B) shows the case where ammonia was used, and the results are shown for varying the concentration in the range of 1 to 200 ppm.

[0037] As shown in FIG. 7, the band indicating the range from the first gate voltage to the second gate voltage changes almost linearly depending on the concentration of the gas to be determined, making it possible to determine the gas concentration using the band. For example, in this embodiment, data on bands of known gases with different concentrations is acquired in advance and stored in the storage unit 6. Then, by referring to the data in the storage unit 6, the concentration of the gas can be determined from the band data obtained for the unknown gas.

[0038] [Gas detection method] A gas determination method in gas determination system 1 will be described with reference to FIGS. FIG. 8 is a flowchart illustrating an outline of the procedure for gas determination in gas determination system 1. be. FIG. 9 is a flow chart illustrating a gas determination method in the information processing device 44. FIG. 10 shows the first tuning voltage V applied to the gate electrode. T1 , the second tuning voltage V T2 10 is a diagram showing the signal waveform of the first tuning voltage V T1 and the second tuning voltage V T2 is a step function of time.

[0039] First, as shown in Fig. 8, gas is supplied into the storage chamber 20 (S1). The inside of the storage chamber 20 is at normal pressure.

[0040] Next, UV light is irradiated from the UV light source 23 toward the sensor 10 and the inside of the storage chamber 20 for one minute (S2). UV irradiation allows gases to be efficiently adsorbed into the channel layer. This is thought to be because UV irradiation removes O2, H2O, etc. from the surface of the channel layer (cleaning effect), induces a dynamic equilibrium between the adsorption and photo-stimulated desorption of gas molecules on the surface of the channel layer, increasing the number of adsorption sites available for gas in the channel layer, and accelerates adsorption due to changes in the state of the adsorbed molecules (ionization, etc.).

[0041] Next, the sensor 10 is heated by the heating unit 26 (S3). The heating temperature is preferably 95°C or higher. In this embodiment, the sensor 10 is heated to a heating temperature of 110°C. By performing UV irradiation and heating, the first tuning voltage V T1 The first current I is obtained by applying a sweep voltage after applying the d1 and a curve 51 showing the change in the second tuning voltage V T2 The second current I is obtained by applying a sweep voltage after the first voltage. d2 The curve 52 showing the change in the temperature can be more clearly distinguished from the curve 52. Details will be described later.

[0042] Next, gas determination is performed (S4). Details of the gas determination will be explained below with reference to FIGS.

[0043] Gas determination starts with a voltage of 5 to 10 mV being applied between source electrode 11 and drain electrode 12. The voltage value applied to each electrode is controlled based on a control signal from control unit 44. The linear region of the output is used as the voltage applied between the source electrode 11 and the drain electrode 12. If the voltage applied between the source electrode 11 and the drain electrode 12 is too high or too low, noise will occur, so it is preferably set to 5 to 10 mV, at which point noise generation is suppressed.

[0044] As shown in FIGS. 9 and 10, when gas determination is started, a first tuning voltage V T1 is applied for a predetermined time (S41). In this embodiment, the first tuning voltage V T1 is applied for a few seconds to a few minutes. This allows the channel layer 15 to have a valence band, and the gas is sufficiently attracted to the channel layer 15, where the gas acts as a donor. First tuning voltage V T1 The application time is set appropriately depending on the thickness of the insulating film 14, etc. In this embodiment, it is preferably 5 seconds or more, more preferably 30 seconds or more, and preferably 120 seconds or less, and even more preferably 60 seconds or less, as long as it is a time sufficient for the channel layer 15 to have a valence band. In addition, the application time can be set appropriately to a preferred value depending on the heating temperature of the sensor 10, etc.

[0045] Next, a sweep voltage is applied to the gate electrode 13, and a first current I flows between the source electrode 11 and the drain electrode 12 during the application of the sweep voltage. d1 is measured (S42). In this embodiment, the voltage is swept with a resolution of 50 mV to 100 mV, a range of 80 V, and a sweep time of 1 minute. As shown in Fig. 10, the gate voltage is gradually changed from negative to positive, such as from -40 V to 40 V. Alternatively, the gate voltage may be gradually changed from positive to negative, such as from 40 V to -40 V. First current I vs. sweep voltage d1 The measurement results are acquired by the acquisition unit 41.

[0046] Next, based on the measurement result acquired by the acquisition unit 41, the determination unit 42 determines the first current I d1A first gate voltage, which is the gate voltage value when becomes the minimum value, is determined (S43).

[0047] Next, a second tuning voltage V T2 is applied for a predetermined time (S44). In this embodiment, the second tuning voltage V T2 is applied for a few seconds to a few minutes. As a result, the channel layer 15 has a conduction band, the gas is sufficiently attracted to the channel layer 15, and the gas functions as an acceptor. The bonding state between the channel layer 15 and the gas after the second tuning voltage is applied is different from the bonding state between the channel layer 15 and the gas after the first tuning voltage is applied. Second tuning voltage V T2 The application time is set appropriately depending on the thickness of the insulating film 14, etc. In this embodiment, it is preferably 5 seconds or more, more preferably 30 seconds or more, and preferably 120 seconds or less, and even more preferably 60 seconds or less, as long as it is a time sufficient for the channel layer 15 to have a conduction band. In addition, the application time can be set appropriately to a preferred value depending on the heating temperature of the sensor 10, etc.

[0048] Next, a sweep voltage is applied to the gate electrode 13, and a second current I flows between the source electrode 11 and the drain electrode 12 during the application of the sweep voltage. d2 is measured (S45). In this embodiment, the voltage is swept with a resolution of 50 mV to 100 mV, a range of 80 V, and a sweep time of 1 minute. As shown in Fig. 10, the gate voltage is gradually changed from negative to positive, such as from -40 V to 40 V. It is also possible to gradually change the gate voltage from positive to negative, such as from 40 V to -40 V. Second current I versus sweep voltage d2 The measurement results are acquired by the acquisition unit 41.

[0049] Next, based on the measurement result acquired by the acquisition unit 41, the determination unit 42 determines the second current I d2 A second gate voltage, which is the gate voltage value when becomes the minimum value, is determined (S46).

[0050] Next, the determination unit 42 determines the type and concentration of the gas based on the first gate voltage and the second gate voltage determined in S43 and S46, with reference to the data stored in the memory unit 6 (S47). Note that although an example in which both the type and concentration of the gas are determined has been given here, it is also possible to determine either one of them.

[0051] S43, S46, and S47 are connected to the first current I d1 and the second current I d2 This corresponds to a gas determination step in which the gas is determined based on the measurement results. In this embodiment, the first current I d1 After measuring the first current I d1 The first gate voltage V g1 This step is performed by determining the second current I d2 The second gate voltage V g2 This may be done during the step of determining

[0052] In this embodiment, UV irradiation and heating are performed to obtain a first current I d1 The curve 51 shows the change in the second current I with respect to the sweep voltage. d2 This allows for more clearly distinguishable data from curve 52, which shows the change in gas concentration. This allows for more accurate gas determination.

[0053] FIG. 11 shows the first current I while applying the first tuning voltage and sweep voltage. d1 Measure the second current I while applying the second tuning voltage and the sweep voltage. d2 The first current I for the sweep voltage was measured when this series of steps was repeated five times. d1 and the second current I with respect to the sweep voltage. d2 The results of measuring the change in

[0054] 11, the solid line is a curve 51 showing the characteristics of the drain current (first current) and gate voltage obtained when a sweep voltage is applied to the gate electrode after application of the first tuning voltage, and the dashed line is a curve 52 showing the characteristics of the drain current (second current) and gate voltage obtained when a sweep voltage is applied to the gate electrode after application of the second tuning voltage.

[0055] FIG. 11(A) shows the experimental results showing the change characteristics of the current flowing between the source electrode and the drain electrode with respect to the sweep voltage when gas determination was performed without UV light irradiation and heating. FIG. 11(B) shows the experimental results showing the change characteristics of the current flowing between the source electrode and the drain electrode with respect to the sweep voltage when gas determination was performed without UV light irradiation or heating. FIG. 11(C) shows the experimental results showing the change characteristics of the current flowing between the source electrode and the drain electrode with respect to the sweep voltage when gas determination was performed with UV light irradiation and heating.

[0056] As shown in FIG. 11A, the dashed curve 52 is roughly the same as the solid curve 51 shifted to the right along the horizontal axis in the drawing. d The difference between the first gate voltage and the second gate voltage when becomes the minimum value can be calculated.

[0057] As shown in FIG. 11(B), the dashed curve 52 is roughly the same as the solid curve 51, shifted to the right along the horizontal axis and slightly upward along the vertical axis. d The difference between the first gate voltage and the second gate voltage when becomes the minimum value can be calculated.

[0058] As shown in Figure 11(C), curve 52 shown by the dashed line is such that curve 51 shown by the solid line moves to the right along the horizontal axis in the drawing and moves upward along the vertical axis, making curve 51 and curve 52 clearly distinguishable.

[0059] As described above, in all of Figures 11(A) to (C), curve 51 showing the characteristics of the drain current and gate voltage obtained when a sweep voltage is applied to the gate electrode after application of the first tuning voltage and curve 52 showing the characteristics of the drain current and gate voltage obtained when a sweep voltage is applied to the gate electrode after application of the second tuning voltage are shifted in the horizontal axis direction, and it is possible to determine the type of gas based on the first gate voltage and the second gate voltage. 11(C), by applying UV irradiation and heating, the difference between the first gate voltage and the second gate voltage in the horizontal direction can be further increased, and the band representing the range from the first gate voltage to the second gate voltage can be made clearer, thereby further improving the accuracy of gas type determination.

[0060] As described above, the gas determination method of the present invention can accurately determine the type or concentration of a gas using a gas sensor having a field-effect transistor structure with graphene as a channel. Furthermore, since a small gas sensor can be obtained, the sensor device 2 can be miniaturized.

[0061] Furthermore, in this embodiment, the channel layer 15 is formed of a laminate of a graphene layer 16 and a porous layer 17. The porous layer 17 has higher porosity than the graphene layer 16, and therefore has high gas adsorption efficiency. Moreover, the porous layer 17 is formed of the same carbon-based material (activated carbon) as the graphene layer 16, and therefore the conductive properties of the channel layer 15 can be maintained well. This allows for improved gas detection accuracy in the channel layer 15.

[0062] FIG. 12 shows a comparison of drain current characteristics when the channel layer has a single-layer structure of a graphene layer and when the channel layer has a laminated structure of a graphene layer and a porous layer. Figures 12(A) and 12(B) show the source-drain current characteristics (dashed two-dot line) when sweeping the gate voltage in a vacuum, and the source-drain current characteristics (solid line) when sweeping the gate voltage in the presence of 3 ppm ammonia with N2 as the carrier gas. Figure 12(A) shows the current characteristics when the channel layer has a single-layer structure of graphene, and Figure 12(B) shows the current characteristics when the channel layer has a stacked structure of graphene and a porous layer.

[0063] When the channel layer had a single-layer structure of graphene layer, the shift of the charge neutral point upon introduction of the ammonia gas was approximately 1.5 V, as shown in Figure 12(A), whereas when the channel layer had a stacked structure of graphene layer and porous layer, the shift of the charge neutral point upon introduction of the ammonia gas was approximately 5 V, as shown in Figure 12(B). Therefore, the sensor 10 of this embodiment, which has the porous layer 17, can increase the sensitivity to gas compared to a sensor that does not have the porous layer 17, and can therefore detect gas with high accuracy even when the gas concentration is low.

[0064] Furthermore, according to this embodiment, since the channel layer 15 has the porous layer 17, it is possible to impart selectivity to the target gas to be detected to the channel layer 15. Figures 13(A) and (B) are graphs similar to Figure 12 showing drain current characteristics, but showing the results of detecting ammonia gas in the presence of air, using the sensor 10 having the porous layer 17 made of activated carbon. 13A shows the drain current characteristics of the sensor 10 in an air atmosphere and in an atmosphere containing 5 ppb of ammonia. A clear shift in the charge neutrality point was confirmed before and after the introduction of ammonia. 13(B) shows the drain current characteristics of the sensor 10 in an air atmosphere and in an atmosphere where 500 ppt of ammonia was mixed into the air. In this example, too, a clear shift in the charge neutral point was detected before and after the introduction of ammonia. These results are thought to be due to the fact that the pores in the porous layer 17 do not allow O2 gas, which has relatively large molecules, to pass through, but allow only ammonia gas, which has relatively small molecules, to pass through, thereby filtering the O2 gas. This confirms that gas selectivity can be achieved by using the porous layer 17, which is activated carbon.

[0065] [Sensor manufacturing method] Next, a method for manufacturing the sensor 10 of this embodiment configured as above will be described.

[0066] 14(A), a graphene layer 160 is formed by a CVD (chemical vapor deposition) method or the like on the surface of a copper foil 101. The graphene layer 160 corresponds to the graphene layer 16 in the sensor 10 shown in FIG.

[0067] Alternatively, a commercially available product in which the graphene layer 160 is laminated on the copper foil 101 may be used. In this case, the copper foil 101 may have the graphene layer 160 formed on both sides thereof. In this case, only the graphene layer 160 formed on one side of the copper foil 101 is used, and the graphene layer 160 formed on the other side of the copper foil 101 is removed from the copper foil 101 by O2 plasma etching or the like.

[0068] 14(B), a protective layer 103 is formed on the surface of the graphene layer 160 by, for example, spin coating. The protective layer 103 is for protecting the graphene layer 160 and can be omitted as necessary. The resin constituting the protective layer 103 is not particularly limited, and for example, a photosensitive resist resin can be used. In this embodiment, for example, polymethyl methacrylate (PMMA) is used as the protective layer 103.

[0069] 14(C), the laminate of the copper foil 101, the graphene layer 160, and the resist resin layer 103 is floated in a solvent 105 in a container 104 to dissolve only the copper foil 101. The solvent 105 may be, for example, ammonium peroxodisulfate.

[0070] 14(D), the graphene layer 160 floating on the solvent 105 is scooped up and placed on the substrate 110, and the graphene layer 160 and the resist resin layer 103 are dried. A silicon substrate having a silicon oxide film formed on its surface is used as the substrate 110, and the graphene layer 160 is placed on the silicon oxide film. The silicon substrate and silicon oxide film correspond to the gate electrode 13 and insulating film 14 in the sensor 10 shown in FIG. 2, respectively (see FIG. 15(A)).

[0071] 14(E), the protective layer 103 is dissolved in acetone, washed, and then annealed at a predetermined temperature to enhance adhesion between the substrate 110 and the graphene layer 160.

[0072] Next, as shown in Fig. 15(A), a resist resin layer 106 is formed on the graphene layer 160 by spin coating. Then, the resist resin layer 106 is subjected to exposure and development processes, thereby patterning the resist resin layer 106 into a predetermined shape as shown in Fig. 15(B). The exposure method for the resist resin layer 106 is not particularly limited, and an exposure method using an exposure mask, a maskless exposure method such as electron beam lithography, or the like can be used.

[0073] 15(C), the graphene layer 160 exposed from the openings in the resist resin layer 106 is removed by dry etching. The graphene layer 160 is dry-etched using, for example, O 2 plasma.

[0074] 15(D), a first metal layer 107a is formed on the surface of the resist resin layer 106 and inside the openings thereof. Thereafter, as shown in FIG. 15(E), the resist resin layer 106 is removed to pattern the first metal layer 107a (lift-off), and a pattern of the first metal layer 107a adjacent to the graphene layer 160 is formed on the substrate 110.

[0075] The first metal layer 107a may be a single layer or may have a multi-layer structure. In this embodiment, chromium (Cr) is formed as an adhesive material with a thickness of about 5 nm, and gold (Au) is formed as an electrode material with a thickness of about 70 nm on top of that. The method for forming the first metal layer 107a is not particularly limited, and may be a sputtering method or a vacuum evaporation method. In this embodiment, the first metal layer 107a is formed by electron beam evaporation.

[0076] 16(A), a first resist resin layer 108a is formed on the substrate 110, and a second resist resin layer 108b is formed thereon. The first resist resin layer 108a is made of, for example, a methyl methacrylate (MMA) film, and the second resist resin layer 108b is made of, for example, a polymethyl methacrylate (PMMA) film.

[0077] Next, as shown in FIG. 16(B), the first resist resin layer 108a and the second resist resin layer 108b are exposed and developed using electron beam lithography or the like to form a resist pattern that exposes the boundary between the graphene layer 160 and the metal layer 107. In this embodiment, an opening pattern is formed in which the opening width of the first resist resin layer 108a is larger than the opening width of the second resist resin layer 108b.

[0078] 16(C), a first metal layer 107b is formed on the surface of the second resist resin layer 108b and inside the openings thereof. Thereafter, as shown in FIG. 16(D), the first and second resist resin layers 108a and 108b are removed to pattern the second metal layer 107b (lift-off), and a pattern of the second metal layer 107b spanning the boundary between the graphene layer 160 and the first metal layer 107a is formed on the substrate 110.

[0079] The second metal layer 107b may be a single layer or may have a multi-layer structure. In this embodiment, chromium (Cr) is formed as an adhesive material with a thickness of about 5 nm, and gold (Au) is formed as an electrode material with a thickness of about 20 nm on top of that. The method for forming the second metal layer 107b is not particularly limited, and may be sputtering or vacuum evaporation. In this embodiment, the second metal layer 107b is formed by electron beam evaporation.

[0080] The first metal layer 107a and the second metal layer 107b constitute a common metal layer 107. The metal layer 107 corresponds to the source electrode 11 and the drain electrode 12 in the sensor 10 shown in FIG. 2. In this embodiment, the opening width of the first resist resin layer 108a is larger than the opening width of the second resist resin layer 108b, so the second metal layer 107b is less likely to come into contact with the side surfaces of the openings in the first resist resin layer 108a, which improves the patterning accuracy of the second metal layer 107b during lift-off.

[0081] Subsequently, as shown in FIG. 17(A), a resist resin layer 121 that covers the graphene layer 160 and the metal layer 107 is formed on the substrate 10 by spin coating or the like. In FIG. 17(A), the upper part is a cross-sectional view of the substrate 110, and the lower part is a plan view thereof (the same applies to FIGS. 17(B) to (D)).

[0082] Next, as shown in FIG. 17(B), the resist resin layer 121 is exposed and developed using electron beam lithography or the like to form a resist pattern that partially covers the graphene layer 160 located between two adjacent metal layers 107.

[0083] Subsequently, as shown in FIG. 17(C), the graphene layer 160 that is not covered with the resist resin layer 121 is removed from the substrate 110 by O 2 plasma etching. The etching conditions are not particularly limited, and in this embodiment, the gas flow rate is 20 sccm, the pressure is 6 Pa, the power is RF 20 W (13.56 MHz), and the processing time is 35 seconds. The thickness of the resist resin layer 121 after the etching process is, for example, 30 nm.

[0084] 17(D), the resist resin layer 121 is annealed in a vacuum to carbonize the resist resin layer 121. As a result, an activated carbon layer 170 made of a carbon composition that constitutes the resist resin layer 121 is formed on the graphene layer 160. The activated carbon layer 170 corresponds to the porous layer 17 in the sensor 10 shown in FIG.

[0085] As described above, in this embodiment, the resist resin layer 121, which is a mask for patterning the graphene layer 160, is carbonized to form the activated carbon layer 170 on the graphene layer 160. The annealing conditions for the resist resin layer 121 are not particularly limited, and may be, for example, an annealing temperature of 300° C. and an annealing time of 3 hours and 30 minutes.

[0086] The thickness of the activated carbon layer 170 can be adjusted by the thickness of the resist resin layer 121. For example, when pattern etching the graphene layer 160, the resist resin layer 121 may be thinned to a predetermined thickness by O2 plasma etching. Alternatively, after the activated carbon layer 170 is formed, the thickness of the activated carbon layer 170 may be adjusted by performing another etching process.

[0087] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and it goes without saying that various modifications can be made within the scope of the gist of the present invention.

[0088] For example, in the above-described embodiment, the gate electrode to which the first and second tuning voltages and the sweep voltage are applied is a common gate electrode, but this is not limiting. A gate electrode to which the sweep voltage is applied may be provided separately from the gate electrodes to which the first and second tuning voltages are applied, as long as both gate electrodes are disposed opposite the graphene layer via an insulating film.

[0089] In the above embodiment, the tuning voltage (fixed voltage) has two values, the first tuning voltage and the second tuning voltage, but it may have at least two values, or may have three or more values. By having three or more values, the amount of gas information increases, enabling more accurate gas determination.

[0090] In addition, in the above-mentioned embodiment, an example was given in which voltages are applied to the gate electrode in the order of a negative first tuning voltage (-40V in the above-mentioned embodiment), a sweep voltage, a positive second tuning voltage (40V in the above-mentioned embodiment), and a sweep voltage, but voltages may also be applied to the gate electrode in the order of a positive second tuning voltage, a sweep voltage, a negative first tuning voltage, and a sweep voltage.

[0091] The porous layer may cover the entire surface of the graphene layer or may cover only a part of the graphene layer. [Explanation of symbols]

[0092] 1...Gas detection system 4. Information processing equipment 10...Sensor (sensor device) 11...Source electrode 12...Drain electrode 13...Gate electrode 14...Insulating film 15...Channel layer 16,160...graphene layers 17...Porous layer 42...Judgment section 44...Control unit 170…Activated carbon layer

Claims

1. a gate electrode; an insulating film formed on the gate electrode; a graphene layer formed on the insulating film; and a source electrode and a drain electrode disposed on the insulating film so as to face each other with the graphene layer interposed therebetween; a conductive porous layer made of a carbon-based material formed on the graphene layer; A sensor device comprising:

2. preparing a gate electrode, an insulating film formed on the gate electrode, a graphene layer formed on the insulating film, and a source electrode and a drain electrode disposed on the insulating film so as to face each other with the graphene layer interposed therebetween; forming a resin layer on the graphene layer; carbonizing the resin layer; The method further includes a dry etching step of patterning the graphene layer into a predetermined shape, The resin layer is formed on the graphene layer as a mask for patterning the graphene layer. A method for manufacturing a sensor device.

3. a sensor device including a gate electrode, an insulating film formed on the gate electrode, a graphene layer formed on the insulating film, a source electrode and a drain electrode arranged on the insulating film so as to face each other with the graphene layer interposed therebetween, and a conductive porous layer made of a carbon-based material formed on the graphene layer; an information processing device having a control unit that controls a voltage applied to the gate electrode, and a determination unit that determines a gas adsorbed in the porous layer based on a measurement result of a current flowing between the source electrode and the drain electrode; A gas determination system comprising:

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