Patent Description:
Hydrogen energy has been recently gathering attention as a solution to environmental problems. As specific measures therefore, fuel cell vehicles using hydrogen as fuel have been starting to be sold. Accordingly, the development of infrastructures including hydrogen stations is also gathering attention. In order to realize a hydrogen energy-based society, it is urgently necessary to develop laws, for example, reconsider safety standards. Further, the importance of hydrogen gas sensing devices is also increasing in the safety aspect in terms of tangible factors.

As the types of the hydrogen gas sensing devices, types such as a contact combustion type, a semiconductor type, and a gas heat conduction type are used. The contact combustion type is a type using catalytic combustion heat obtained by a catalyst (Pt, Pd, and the like) of combustible gas. The semiconductor type is a type using the change in electrical conductivity caused by gas adsorption on a metal oxide semiconductor surface. The gas heat conduction type is a type using the difference in heat conduction between target gas and standard gas.

In addition, in a gas sensing device of the related art, a heating heater is installed adjacent to a gas detecting element in order to increase the detection sensitivity of the gas detecting element (for example, PTL <NUM>, PTL <NUM>, and NPL <NUM> to NPL <NUM>). In the gas sensing device, the ambient temperature of the gas detecting element is normally maintained to be <NUM> or more at the time of measurement by the heating heater.

<NPL> discloses a <NUM>-dimensional metal/oxide/metal nanofilm composed of an NbO<NUM> layer, <NUM> thick, with upright-standing Nb<NUM>O<NUM> nanocolumns which work as semiconducting nano-channels, assembled between two parallel metal electrodes, whose resistivity measured along the columns is greatly impacted by the surface and interface reactions.

However, as in the gas sensing device of the related art, when the gas detecting element is heated to <NUM> or more, an electricity consumption of about <NUM> mW is necessary at minimum. Therefore, when the gas sensing device is used in a constant ON state, the electricity consumption becomes extremely high. In addition, in order to sense the hydrogen gas, hydrogen atoms generated when the hydrogen gas is decomposed at an electrode surface need to be diffused in the electrode and reach a gas-sensitive body. At this time, the gas sensing speed becomes faster when the speed at which the hydrogen atoms reach the gas-sensitive body layer is fast. Therefore, the improvement of the gas sensing speed in a low temperature operation is a challenge.

This disclosure has been made to solve the abovementioned problem, and an object thereof is to provide a gas sensor capable of sensing gas containing hydrogen atoms with low electricity consumption and at high speed, and the like.

In accordance with an aspect of the present disclosure, there is provided a gas sensor according to claim <NUM>.

In accordance with another aspect of the present disclosure, there is provided a fuel cell vehicle as defined in claim <NUM>.

According to this disclosure, the gas sensor capable of sensing the gas containing hydrogen atoms with low electricity consumption and at high speed, and the like can be provided.

First, the underlying knowledge of this disclosure is described.

As the result of intensive studies, the inventors of the present disclosure have found that the gas sensor of the related art has the following problems.

As the hydrogen gas sensing device, the contact combustion type, the semiconductor type, and the gas heat conduction type have been hitherto mainly used in Japan. The contact combustion type is a sensing device using catalytic combustion heat obtained by a catalyst (Pt, Pd, and the like) of combustible gas. In the contact combustion-type hydrogen gas sensing device, as the concentration of the hydrogen gas rises, the element temperature rises due to combustion, and the resistance increases. The sensor output is linear with respect to the gas concentration, but there is a problem in gas selectivity.

The semiconductor-type hydrogen gas sensing device uses the change in electrical conductivity caused by gas adsorption on a metal oxide semiconductor surface. In the semiconductor-type hydrogen gas sensing device, the sensor output is logarithmic with respect to the gas concentration, and hence the sensitivity is high even in low-concentration regions.

The gas heat conduction-type hydrogen gas sensing device uses the difference in heat conduction between target gas and standard gas. In the gas heat conduction-type hydrogen gas sensing device, the heat conductivity of the hydrogen gas is higher than other combustible gas. Therefore, the gas heat conduction-type hydrogen gas sensing device is suitable for sensing especially in high-concentration regions.

A hydrogen gas sensing device having a MIM structure obtained by laminating an insulating film and a metal film described in PTL <NUM> is classified as the semiconductor type. In the hydrogen gas sensing device having a MIM structure, an insulating film obtained by adding a predetermined amount of palladium and glass to tantalum pentoxide (Ta<NUM>O<NUM>) is used as the gas-sensitive insulating film, and Pt is used as the upper and lower sandwiching metal electrodes. However, there is no description on the detailed mechanism. Thus, description can be made as follows when a phenomenon similar to that of the mechanism described in NPL <NUM> relating to a gas sensing device (Pt-Ta<NUM>O<NUM>-Si) using a MIS structure is assumed to be caused.

When gas containing hydrogen gas comes into contact with a surface of Pt that is a catalytic metal, for example, the hydrogen gas is decomposed into hydrogen atoms by the catalysis of Pt. Further, the decomposed hydrogen atoms are diffused in a Pt electrode and reach tantalum pentoxide (Ta<NUM>O<NUM>) that is the gas-sensitive body. The hydrogen atoms that have reached tantalum pentoxide (Ta<NUM>O<NUM>) reduce tantalum pentoxide (Ta<NUM>O<NUM>) and are oxidized in accordance with the chemical reaction formula below, thereby becoming water.

Ta<NUM>O<NUM>+2xH →xH<NUM>O+Ta<NUM>O<NUM>-x.

At this time, it is conceived that the flow of the current is facilitated because of an oxygen defect in tantalum pentoxide formed by the hydrogen atom taking an oxygen atom from tantalum pentoxide (Ta<NUM>O<NUM>) in the gas-sensitive insulating film.

Meanwhile, when the gas containing hydrogen gas is removed from the Pt surface, it is conceived that a reversed process in accordance with the following chemical reaction formula:.

xH<NUM>O+Ta<NUM>O<NUM>-x →Ta<NUM>O<NUM>+2xH.

occurs, the oxygen defect in tantalum pentoxide is removed, and it becomes difficult for the current to flow. By the mechanism as above, it is conceived that the MIM structure in which an insulating film obtained by adding a predetermined amount of palladium and glass to tantalum pentoxide (Ta<NUM>O<NUM>) is used as the gas-sensitive insulating film and Pt is used as the upper and lower metal electrodes sandwiching the gas-sensitive insulating film functions as a gas sensing device that senses the gas containing hydrogen atoms.

Now, when the hydrogen atoms are dissociated by Pt that is a catalytic metal, the rate of the hydrogen atoms to be dissociated from molecules containing the hydrogen atoms by the catalysis is proportional to the temperature rise. In other words, it is conceived that the detection sensitivity of the gas increases as the gas detecting element temperature rises. In the gas sensing device of the related art, in order to improve the detection sensitivity of the gas containing hydrogen atoms, the gas detecting element at the time of measurement is heated to <NUM> or more.

For example, in the gas sensing device having a MIM structure described in PTL <NUM>, the gas detecting element temperature is raised to <NUM> by applying a predetermined voltage to a heating heater provided adjacent to the gas detecting element.

Not only in the gas sensing device having a MIM structure but also in the gas sensing device having a MIS structure using the catalysis of metal, a heating heater is installed adjacent to the gas detecting element and is normally used by maintaining the ambient temperature at <NUM> or more. For example, in the gas sensing device described in NPL <NUM>, a temperature of <NUM> or more is needed when the MIS structure is used as a diode. In addition, a gas sensing device described in NPL <NUM> using a MIS structure as a transistor is operated while setting the ambient temperature of the gas detecting element to <NUM>.

In addition, in a contact combustion-type gas sensing device described in NPL <NUM> using the catalysis of metal, the gas detecting element is heated to from <NUM> to <NUM> at the time of operation.

Further, in a hot-wire semiconductor-type gas sensing device and a gas heat conduction-type gas sensing device described in NPL <NUM> that do not use the catalysis of metal, the gas detecting element is heated to <NUM> or more for both types.

However, when the gas detecting element is heated to <NUM> or more, an electricity consumption of about <NUM> mW is necessary at minimum. Therefore, when the gas sensing device is used in a constant ON state, the electricity consumption becomes extremely high. In addition, in order to sense the hydrogen gas, hydrogen atoms generated by decomposing the hydrogen gas at an electrode surface need to be diffused in the electrode and reach the gas-sensitive body. At this time, the gas sensing speed of the gas sensing device improves as the speed at which the hydrogen atoms reach the gas-sensitive body becomes faster. Therefore, the improvement of the gas sensing speed in a low temperature operation is a challenge for the gas sensing device.

Thus, in this disclosure, a gas sensor capable of sensing gas containing hydrogen atoms with low electricity consumption and at high speed is realized by a gas sensor having the following configuration.

That is, the gas sensor according to this embodiment is a gas sensor as follows. In the gas sensor, a first electrode and a second electrode are arranged on a substrate in an opposed manner so as to form a gap, and at least a part of a gas-sensitive body layer formed by metal oxide is exposed via the gap. Further, the gas-sensitive body layer is in contact with the second electrode and includes a local region having a degree of oxygen deficiency larger than that of the metal oxide layer in the gas-sensitive body layer. Further, the resistance of the metal oxide decreases when gas molecules containing hydrogen atoms are detected. The second electrode causes catalysis that dissociates the hydrogen atoms from the gas molecules containing the hydrogen atoms. The resistance of the metal oxide layer decreases as the hydrogen atoms are dissociated from the gas molecules in a part of the second electrode in contact with the local region and the dissociated hydrogen atoms are bound to oxygen atoms in the local region of the metal oxide layer.

In a configuration in which at least one of the first electrode and the second electrode is in contact with the local region having a degree of oxygen deficiency larger than that of the metal oxide layer, the current flowing between the first electrode and the second electrode is concentrated on the local region having a larger degree of oxygen deficiency. As a result, the temperature of the local region can be raised with less current. When the temperature of the local region rises, the temperature of the surface of the second electrode also rises. In the second electrode that causes catalysis, the rate of the dissociation of the hydrogen atoms from the gas molecules containing the hydrogen atoms increases in accordance with the temperature rise.

Hereinafter, certain exemplary embodiments of the present disclosure are described in greater detail with reference to the accompanying Drawings.

In the drawings, elements regarding substantially identical structures, processing, and effects are assigned with a same reference sign, and explanation of such substantially identical elements is not repeated. Numerical values, materials, film forming methods, and the like described in the following embodiments are merely examples for explaining the embodiments according to the present disclosure in more detail, and are not intended to limit the present disclosure. Connection relationships between constituent elements described in the following embodiments are merely examples for explaining the embodiments according to the present disclosure in more detail, and connection relationships for achieving the functions of the present disclosure are not limited to the described relationships. Furthermore, the present disclosure is defined by the claims. Therefore, among the constituent elements in the following embodiments, constituent elements that are not described in independent claims that show the most generic concept of the present disclosure are described as elements constituting more desirable configurations, although such constituent elements are not necessarily required to achieve the object of the present disclosure.

<FIG> is a cross-sectional view illustrating an example of the configuration of gas sensor <NUM> according to Embodiment <NUM>.

Gas sensor <NUM> according to this embodiment includes substrate <NUM>, interlayer insulating film <NUM> formed on substrate <NUM>, gas-sensitive insulating film <NUM> that is a metal oxide layer formed on interlayer insulating film <NUM>, first electrode <NUM>, second electrode <NUM>, and gap <NUM> formed by arranging first electrode <NUM> and second electrode <NUM> to be opposed to each other. First electrode <NUM> is formed on gas-sensitive insulating film <NUM>, and second electrode <NUM> is formed on gas-sensitive insulating film <NUM> so as to have gap <NUM> between first electrode <NUM> and second electrode <NUM>, and further disposed on the first electrode (<NUM>). In addition, in gap <NUM> formed by arranging first electrode <NUM> and second electrode <NUM> to be opposed to each other, a part of gas-sensitive insulating film <NUM> is exposed.

Note that, as described below, first electrode <NUM> and second electrode <NUM> are not limited to be formed on gas-sensitive insulating film <NUM>, and may be formed above substrate <NUM> at the same layer level as gas-sensitive insulating film <NUM>.

Gas-sensitive insulating film <NUM> is a layer in which the resistance reversibly changes on the basis of an electrical signal provided between first electrode <NUM> and second electrode <NUM>. For example, gas-sensitive insulating film <NUM> is a layer that reversibly transitions to a high-resistance state and a low-resistance state in accordance with a voltage difference provided between first electrode <NUM> and second electrode <NUM>.

Now, gas-sensitive insulating film <NUM> is arranged in contact with second electrode <NUM>, and has local region <NUM> that is not in contact with first electrode <NUM> in the region of gap <NUM> formed by arranging first electrode <NUM> and second electrode <NUM> to be opposed to each other. In local region <NUM>, the degree of oxygen deficiency reversibly changes in accordance with the application of an electrical pulse provided between first electrode <NUM> and second electrode <NUM>. It is conceived that local region <NUM> includes filaments (conductive paths) formed by oxygen defection sites.

It is conceived that, in the resistance change phenomenon in gas-sensitive insulating film <NUM>, the resistance changes as the degree of oxygen deficiency in the filaments in very small local region <NUM> changes when an oxidation-reduction reaction occurs in local region <NUM>.

Note that, in this disclosure, the "degree of oxygen deficiency" is a rate of deficient oxygen with respect to the amount of oxygen forming the oxide having a stoichiometric composition (when there are a plurality of stoichiometric compositions, the stoichiometric composition of which resistance is the highest out of those stoichiometric compositions) in metal oxide. The metal oxide having a stoichiometric composition is more stable and has a higher resistance than metal oxide having other compositions.

For example, when the metal is tantalum (Ta), the oxide having the stoichiometric composition according to the abovementioned definition is Ta<NUM>O<NUM>, and hence can be expressed as TaO<NUM>. The degree of oxygen deficiency of TaO<NUM> is <NUM>%, and the degree of oxygen deficiency of TaO<NUM> is the degree of oxygen deficiency = (<NUM>-<NUM>)/<NUM> = <NUM>%. In addition, metal oxide with excess oxygen has a degree of oxygen deficiency that is a negative value. Note that, in this disclosure, unless otherwise noted, the degree of oxygen deficiency is described so as to include positive values, <NUM>, and negative values.

The oxide having a small degree of oxygen deficiency has a high resistance because the oxide is closer to the oxide having a stoichiometric composition, and the oxide having a large degree of oxygen deficiency has a low resistance because the oxide is closer to the metal forming the oxide.

The "oxygen content" is a ratio of the oxygen atoms to the total number of atoms. For example, the oxygen content of Ta<NUM>O<NUM> is a ratio (O/(Ta+O)) of the oxygen atoms to the total number of atoms, and is <NUM> atm%. Therefore, the oxygen content of the oxygen deficient-type tantalum oxide is larger than <NUM> and is smaller than <NUM> atm%.

Gas-sensitive insulating film <NUM> has local region <NUM>. The degree of oxygen deficiency of local region <NUM> is larger than the degree of oxygen deficiency of gas-sensitive insulating film <NUM>.

Local region <NUM> is formed in gas-sensitive insulating film <NUM> by applying an initial breakdown voltage across first electrode <NUM> and second electrode <NUM>. Now, the initial breakdown voltage is a voltage of which absolute value is larger than that of an application voltage applied across first electrode <NUM> and second electrode <NUM> in order to cause the state to reversibly transition to a high-resistance state and a low-resistance state. Note that, as the initial breakdown voltage, a voltage lower than the application voltage for causing the state to reversibly transition to the high-resistance state and the low-resistance state described above may be repeatedly applied or applied for a predetermined amount of time. By the initial breakdown, local region <NUM> in contact with second electrode <NUM> and first electrode <NUM> is formed.

In this disclosure, a local region means a region out of gas-sensitive insulating film <NUM> through which the current dominantly flows when voltage is applied across first electrode <NUM> and second electrode <NUM>. Note that local region <NUM> means a region including a set of a plurality of filaments (conductive paths) formed in gas-sensitive insulating film <NUM>. That is, the resistance change in gas-sensitive insulating film <NUM> is realized through local region <NUM>. Therefore, when a drive voltage is applied to gas-sensitive insulating film <NUM>, the current dominantly flows through local region <NUM> including the filaments.

In addition, local region <NUM> is a resistance change layer that reversibly transitions to a high-resistance state and a low-resistance state on the basis of the voltage difference provided between first electrode <NUM> and second electrode <NUM>. Local region <NUM> may be in either of the high-resistance state and the low-resistance state. In both states, the state changes to a state with a lower resistance when the gas molecules containing the hydrogen atoms reach a place near second electrode <NUM> that causes catalysis. Therefore, the gas molecules containing the hydrogen atoms can be sensed.

However, the rate of the resistance change that occurs when the gas molecules containing the hydrogen atoms reach the place near second electrode <NUM> that causes catalysis becomes larger when local region <NUM> is maintained in a high-resistance state as compared to when local region <NUM> is maintained in a low-resistance state. Therefore, it is preferred that the local region be maintained in a high-resistance state.

The size of local region <NUM> may be small, and is a size by which one end thereof does not come into contact with first electrode <NUM>. Local region <NUM> has a size by which the filaments necessary for at least causing the current to flow can be secured. The output value of the current flowing through gas-sensitive insulating film <NUM> differs depending on the size and the resistance state of local region <NUM>.

The formation of the filaments in local region <NUM> can be described with use of a percolation model described in PTL <NUM>. Now, it is assumed that the filaments are formed when the oxygen defection sites in local region <NUM> are connected. The percolation model is a model based on a theory that the probability of the connection of the oxygen defection sites (hereinafter simply referred to as a defection site) and the like being formed increases when the density of the defection sites and the like exceeds a certain threshold value given a random distribution of the defection sites and the like in local region <NUM>. Note that the metal oxide is herein formed by a metal ion and an oxygen ion. Further, the "defection" means that oxygen is lacking from the stoichiometric composition in the metal oxide. Further, "the density of the defection sites" also corresponds to the degree of oxygen deficiency. In other words, when the degree of oxygen deficiency increases, the density of the defection sites increases as well.

Local region <NUM> may be formed in only one place or in a plurality of places in gas-sensitive insulating film <NUM> of gas sensor <NUM>. The number of local regions <NUM> formed in gas-sensitive insulating film <NUM> can be checked by an electron beam absorbed current (EBAC) analysis, for example.

In order to place gas sensor <NUM> in a state in which the gas containing hydrogen atoms can be sensed, the resistance of gas-sensitive insulating film <NUM> of gas sensor <NUM> is set to a predetermined resistance by applying a voltage satisfying a predetermined condition across first electrode <NUM> and second electrode <NUM> by an external power supply. The resistance of gas-sensitive insulating film <NUM> of gas sensor <NUM> reversibly increases or decreases in accordance with the voltage difference in the voltage applied across first electrode <NUM> and second electrode <NUM>. For example, when a pulse voltage of a predetermined polarity of which amplitude is larger than that of a predetermined threshold value voltage is applied, the resistance of gas-sensitive insulating film <NUM> increases or decreases. The voltage as above may be hereinafter referred to as a "voltage for writing". Meanwhile, when a pulse voltage of which amplitude is smaller than that of the threshold value voltage is applied, the resistance of gas-sensitive insulating film <NUM> does not change. The voltage as above may be hereinafter referred to as a "voltage for reading".

Gas-sensitive insulating film <NUM> is formed by an oxygen deficient metal oxide. More specifically, the metal oxide is a transition metal oxide. At least one of transition metal such as tantalum (Ta), hafnium (Hf), titanium (Ti), zirconium (Zr), niobium (Nb), tungsten (W), nickel (Ni), and iron (Fe) and aluminum (Al) may be selected as the mother metal of the transition metal oxide. The transition metal can be placed in a plurality of oxidation states, and hence different resistance states can be realized by the oxidation-reduction reaction. Now, the oxygen deficient metal oxide refers to metal oxide of which oxygen content amount (atom ratio: the rate of the number of oxygen atoms with respect to the total number of atoms) is smaller than that of the composition of metal oxide (typically, an insulator) having a stoichiometric composition, and many thereof normally behave in a semiconducting manner. By using the oxygen deficient metal oxide for gas-sensitive insulating film <NUM>, a stable resistance change operation with excellent reproducibility can be realized in gas sensor <NUM>.

For example, when hafnium oxide is used as the metal oxide forming gas-sensitive insulating film <NUM>, the resistance of gas-sensitive insulating film <NUM> can be changed in a stable manner when x is <NUM> or more when the composition is HfOx. In this case, the film thickness of the metal oxide may be from <NUM> to <NUM>.

In addition, when zirconium oxide is used as the metal oxide forming gas-sensitive insulating film <NUM>, the resistance of gas-sensitive insulating film <NUM> can be changed in a stable manner when x is <NUM> or more when the composition is ZrOx. In this case, the film thickness of the metal oxide may be from <NUM> to <NUM>. In addition, when tantalum oxide is used as the metal oxide forming gas-sensitive insulating film <NUM>, the resistance of gas-sensitive insulating film <NUM> can be changed in a stable manner when x is <NUM> or more when the composition is TaOx. The composition of the metal oxide layer can be measured with use of a Rutherford backscattering method.

The material of first electrode <NUM> and second electrode <NUM> is selected from, for example, platinum (Pt), iridium (Ir), palladium (Pd), silver (Ag), nickel (Ni), tungsten (W), copper (Cu), aluminum (Al), tantalum (Ta), titanium (Ti), titanium nitride (TiN), tantalum nitride (TaN), titanium nitride aluminum (TiAlN), and the like.

The second electrode <NUM> is formed by a material that causes catalysis that dissociates the hydrogen atoms from the gas molecules containing the hydrogen atoms such as at least one of platinum (Pt), iridium (Ir), and palladium (Pd), for example. In addition, first electrode <NUM> is formed by a material of which standard electrode potential is lower than that of the metal forming first metal oxide such as tungsten (W), nickel (Ni), tantalum (Ta), titanium (Ti), aluminum (Al), tantalum nitride (TaN), and titanium nitride (TiN), for example. The standard electrode potential shows a characteristic in which oxidize becomes more difficult as the value thereof becomes higher.

In addition, a silicon single crystal substrate or a semiconductor substrate, for example, can be used as substrate <NUM>, but substrate <NUM> is not limited thereto. Gas-sensitive insulating film <NUM> can be formed in a relatively low substrate temperature, and hence gas-sensitive insulating film <NUM> can be formed on a resin material and the like, for example.

In addition, gas sensor <NUM> may further include load elements electrically connected to gas-sensitive insulating film <NUM>, for example, a fixed resistor, a transistor, or a diode.

Next, an example of a manufacturing method of gas sensor <NUM> according to this embodiment is described with reference to <FIG>.

First, as illustrated in <FIG>, interlayer insulating film <NUM> having a thickness of <NUM> is formed on substrate <NUM> that is single crystal silicon, for example, by thermal oxidation. Then, an oxygen deficient oxide layer is formed on interlayer insulating film <NUM> by reactive sputtering using a Ta target, for example. As a result, gas-sensitive insulating film <NUM> is formed.

Next, as illustrated in <FIG>, a Ti thin film having a thickness of <NUM>, for example, is formed on gas-sensitive insulating film <NUM> as first electrode <NUM> by electron beam vapor deposition. Then, although not shown, processing is performed on first electrode <NUM> so as to obtain a predetermined shape by a photolithography process. Note that, the photolithography process can be omitted by using a metal mask when first electrode <NUM> is formed.

Then, as illustrated in <FIG>, second electrode <NUM> is formed by an oblique vapor deposition process. In the oblique vapor deposition process, when a material that forms second electrode <NUM> is vapor-deposited from the direction of angle θ with respect to substrate <NUM> as illustrated in <FIG>, the pattern of first electrode <NUM> having height H forms a shadow, and gap <NUM> having width G is formed. A Pt thin film having a thickness of <NUM>, for example, is formed on gas-sensitive insulating film <NUM> and first electrode <NUM> as second electrode <NUM>.

Lastly, as illustrated in <FIG>, local region <NUM> illustrated in <FIG> is formed in gas-sensitive insulating film <NUM> by applying the initial breakdown voltage across first electrode <NUM> and second electrode <NUM>, thereby completing gas sensor <NUM>.

An example of a range of the voltage for forming local region <NUM> is described below with reference to <FIG>.

Gas sensor <NUM> that is a sample used in the measurement in <FIG> sets the film thickness of first electrode <NUM> to <NUM> and sets the film thickness of second electrode <NUM> to <NUM>. The pattern size of both electrodes is <NUM>×<NUM>.

The film thickness of gas-sensitive insulating film <NUM> is <NUM>, for example. As gas-sensitive insulating film <NUM>, TaOy (y=<NUM>) that is oxide having a film thickness of <NUM> is used. In gas sensor <NUM> as above, when the voltage for reading (for example, <NUM> V) is applied across the electrodes, the initial resistance is from about <NUM><NUM> Ω to about <NUM><NUM> Ω.

As shown in <FIG>, when the resistance of gas sensor <NUM> is an initial resistance (A value, for example, from <NUM><NUM> Ω to <NUM><NUM> Ω that is higher than resistance (high resistance) HR in a high-resistance state. The value is an example, and the initial resistance is not limited to the value), the resistance state is changed to resistance (low resistance) LR in a low-resistance state by applying the initial breakdown voltage across the electrodes (S101). Then, when two types of voltage pulses of which pulse width is <NUM> ns and polarities are different from each other, that is, a positive voltage pulse and a negative voltage pulse, for example, are alternately applied between first electrode <NUM> and second electrode <NUM> of gas sensor <NUM> as the voltage for writing, the resistance of gas-sensitive insulating film <NUM> changes as shown in <FIG>.

That is, when a positive voltage pulse (pulse width of <NUM> ns) is applied between first electrode <NUM> and second electrode <NUM> as the voltage for writing, the resistance of gas-sensitive insulating film <NUM> increases to high resistance HR from low resistance LR (S102). Meanwhile, when a negative voltage pulse (pulse width of <NUM> ns) is applied between first electrode <NUM> and second electrode <NUM> as the voltage for writing, the resistance of gas-sensitive insulating film <NUM> decreases to low resistance LR from high resistance HR (S103). In other words, the metal oxide layer forming gas-sensitive insulating film <NUM> reversibly transitions to a high-resistance state and a low-resistance state on the basis of the voltage applied across first electrode <NUM> and second electrode <NUM>. Note that the polarity of the voltage pulse is "positive" when the potential of second electrode <NUM> is high with respect to the potential of first electrode <NUM>, and is "negative" when the potential of second electrode <NUM> is low with respect to the potential of first electrode <NUM>.

An evaluation example of a resistance change characteristic of the hydrogen-containing gas of gas sensor <NUM> formed as above is described.

<FIG> is block diagram illustrating an example of gas evaluation system <NUM> used for the evaluation of gas sensor <NUM>. Gas evaluation system <NUM> illustrated in <FIG> includes sealed container <NUM> that stores therein gas sensor <NUM>, power supply <NUM>, and current measuring instrument <NUM>. Sealed container <NUM> is connected to hydrogen gas cylinder <NUM> and argon gas cylinder <NUM> via introduction valves <NUM> and <NUM>, respectively, and is able to discharge the internal gas via exhaust valve <NUM>. In gas sensor <NUM>, power supply <NUM> is a power supply circuit that constantly applies a predetermined voltage across first electrode <NUM> and second electrode <NUM>. Current measuring instrument <NUM> is a measurement circuit that measures the current flowing through gas-sensitive insulating film <NUM> when a predetermined voltage is applied across first electrode <NUM> and second electrode <NUM> in gas sensor <NUM>.

<FIG> is a graph showing an evaluation example of gas sensor <NUM>. The horizontal axis indicates the time (a. ) and the vertical axis indicates the current (a. ) flowing between first electrode <NUM> and second electrode <NUM>. In the evaluation experiment, first, nitrogen gas is introduced into sealed container <NUM> in which gas sensor <NUM> is placed. Then, argon gas is switched to hydrogen gas. Then, the hydrogen gas is further switched to nitrogen gas.

<FIG> shows the result at this time, and the horizontal axis indicates three periods in which the former argon introduction (Step S201), the hydrogen introduction (Step S202), and the latter argon introduction (Step S203) are performed. It can be seen that the current starts to increase after the introduced gas is switched from argon gas to hydrogen gas. In addition, it can be seen that the current starts to decrease after the introduced gas is switched from hydrogen gas to argon gas.

In this evaluation example, gas sensor <NUM> in which local region <NUM> is set to a high-resistance state by applying a predetermined voltage (potential difference) across first electrode <NUM> and second electrode <NUM> in advance is used. In the monitoring operation of the hydrogen-containing gas, a gas sensing voltage of <NUM> V is applied across first electrode <NUM> and second electrode <NUM>, and the current of from <NUM>µA to <NUM>µA flows between first electrode <NUM> and second electrode <NUM> in a state in which the hydrogen gas is detected. Therefore, according to gas sensor <NUM>, it can be seen that the hydrogen-containing gas can be monitored with extremely small electricity consumption of from <NUM> mW to <NUM> mW.

From the result above, the detection mechanism of the hydrogen gas in gas sensor <NUM> is assumed to be as follows.

When the hydrogen-containing gas comes into contact with second electrode <NUM>, the hydrogen atoms are dissociated from the hydrogen-containing gas by the catalysis caused by second electrode <NUM>. In order to maintain an equilibrium state, the dissociated hydrogen atoms are diffused in second electrode <NUM> and reach local region <NUM> in the region of gap <NUM>.

By the hydrogen atoms, a reduction reaction occurs in very small local region <NUM>, and the degree of oxygen deficiency in local region <NUM> increases. As a result, the connection of the filaments in local region <NUM> becomes easier, and the resistance of local region <NUM> decreases. As a result, it is conceived that the current flowing between first electrode <NUM> and second electrode <NUM> increases.

Conversely, when there is no hydrogen-containing gas near second electrode <NUM>, the dissociated hydrogen atoms become hydrogen molecules near the surface of second electrode <NUM> in order to maintain an equilibrium state, and leaves the surface of second electrode <NUM> to the outside.

Accordingly, the hydrogen atoms that have formed the water molecules in local region <NUM> returns to second electrode <NUM> by a reduction reaction. Meanwhile, the oxygen that has formed the water molecules becomes bound to the oxygen defect, thereby reducing the degree of oxygen deficiency.

As a result, it is conceived that it becomes difficult for the filaments in local region <NUM> to be connected, and the resistance increases. As a result, the current flowing between first electrode <NUM> and second electrode <NUM> decreases.

In addition, it is conceived that, in the abovementioned operation, the detectable gas is not limited to hydrogen gas, and the abovementioned operation occurs for various types of hydrogen-containing gas such as methane and alcohol, for example.

As described above, according to gas sensor <NUM> according to this embodiment, a detecting element with excellent power-saving properties capable of generating heat with only the current for sensing the resistance state and detecting the hydrogen-containing gas without heating with a separate heater can be obtained.

Now, the effect of the material of second electrode <NUM> is examined in order to check the mechanism of gas sensor <NUM>. Specifically, gas sensor <NUM> in which the material of second electrode <NUM> is Pt that causes catalysis with respect to hydrogen and gas sensor <NUM> in which the material of second electrode <NUM> is TiN that does not cause catalysis with respect to hydrogen are generated. Then, the change in resistance is measured when hydrogen/argon gas of which hydrogen concentration is <NUM>% is introduced for each of a case in which second electrode <NUM> is Pt and a case in which second electrode <NUM> is TiN. The measurement results at this time are shown in <FIG> is a graph showing the resistance measurement result after the gas is introduced when second electrode <NUM> of gas sensor <NUM> is Pt. <FIG> is a graph showing the resistance measurement result after the gas is introduced when second electrode <NUM> of gas sensor <NUM> is TiN.

In the resistance measurement, gas sensor <NUM> is arranged in a sealed container, and measurement is performed by introducing air atmosphere (Air) into the sealed container from time point <NUM> to time point <NUM>, introducing argon atmosphere (Ar) into the sealed container from time point <NUM> to time point <NUM>, introducing hydrogen/argon gas (Ar-H<NUM>) in addition to argon atmosphere (Ar) into the sealed container from time point <NUM> to time point <NUM>, introducing argon atmosphere (Ar) into the sealed container again from time point <NUM>,<NUM> to time point <NUM>,<NUM>, and performing vacuum drawing (Vac) of the sealed container from time point <NUM>,<NUM> to time point <NUM>,<NUM>.

As shown in <FIG>, when the material of second electrode <NUM> is Pt that causes catalysis, the resistance of gas sensor <NUM> decreases by about three digits when the hydrogen/argon gas is introduced to the argon atmosphere (the period of Ar+Ar-H<NUM> shown in <FIG>). Further, by completely removing hydrogen gas by performing vacuum drawing on sealed container <NUM> (the period of Vac shown in <FIG>), the resistance of gas sensor <NUM> recovers to the original value again. Meanwhile, as shown in <FIG>, when the material of second electrode <NUM> is TiN that does not cause catalysis, a change in the resistance of gas sensor <NUM> cannot be seen even when the hydrogen/argon gas is introduced into the sealed container of the argon atmosphere (the period of Ar+Ar-H<NUM> shown in <FIG>).

From those results, it is conceived that a material that causes catalysis is necessary for second electrode <NUM>, and the hydrogen gas is dissociated into hydrogen atoms when the catalysis occurs on the electrode surface.

In addition, the relationship between the reaction time of gas sensor <NUM> and the gas flow rate of the hydrogen/argon gas to be introduced is examined. <FIG> is a graph showing the measurement result of the gas introduction flow rate and the sensor output current of gas sensor <NUM>. Note that the shaded parts in <FIG> indicate a time zone in which the hydrogen gas is introduced, and the introduced gas flow rate is the value described on the upper part of <FIG>.

As illustrated in <FIG>, the output current of gas sensor <NUM> greatly increases by introducing the hydrogen/argon gas into the sealed container, and the output current of gas sensor <NUM> decreases by stopping the hydrogen/argon gas introduction. Further, it can be understood from <FIG> that the rising of the current measurement value becomes faster in accordance with the increase in the gas flow rate of the hydrogen/argon gas to be introduced. With use of the relationship, the hydrogen gas concentration can be measured by measuring the time until the current reaches a predetermined current from the introduction of the hydrogen/argon gas.

Next, the effect of the resistance change of gas sensor <NUM> due to the forming state is examined. <FIG> is a graph showing the measurement result of the gas introduction flow rate and the resistance in accordance with whether there is forming of gas sensor <NUM>. Note that the shaded parts in <FIG> indicate time zones in which the hydrogen gas is introduced, and the introduced gas flow rate in each time zone is the value described on the upper portion of <FIG>.

As indicated by solid line R1 in <FIG>, in gas sensor <NUM> with forming, the resistance decreases by three digits by the introduction of the hydrogen/argon gas. In addition, as with the result in <FIG>, the time until the state reaches a low-resistance state becomes shorter as the gas flow rate of the hydrogen/argon gas increases. The resistance decreases by the introduction of the hydrogen/argon gas also in gas sensor <NUM> without the forming process indicated by dotted line R2 in <FIG>, but the change does not go further than a change by about one digit. This is assumed to be caused by the state of the local region being different from the case with the forming process and the oxygen lacking density being low in the local region of the gas sensor without the forming.

From the results, the gas detection sensitivity of gas sensor <NUM> can be adjusted by controlling the state of the local region, that is, the oxygen lacking density in the local region by the forming process.

Further, the dependency of the gas sensing voltage is examined for gas sensor <NUM> in which second electrode <NUM> is formed by Pt. <FIG> are graphs showing the measurement result of the gas sensing voltage and the output current of gas sensor <NUM>. <FIG> each show the output current when gas sensing voltage VREAD is <NUM> mV, <NUM> mV, and <NUM> mV, respectively. Note that <FIG> alternately repeat the period in which the hydrogen/argon gas of <NUM>/min is introduced (the period of Ar+H<NUM> shown in <FIG>) and the period in which air is introduced (the period of Air shown in <FIG>) every <NUM>.

As illustrated in <FIG>, when gas sensing voltage VREAD is <NUM> mV, the resistance decreases and a current of <NUM> nA is sensed when the hydrogen/argon gas is introduced to gas sensor <NUM>. Further, when gas sensing voltage VREAD is reduced to <NUM> mV and <NUM> mV, the detected current decreases to <NUM>/<NUM> and <NUM>/<NUM>, respectively, in accordance with the decrease in voltage as shown in <FIG>. The gas sensing voltage has little effect on the rising situation of the current after the hydrogen/argon gas is introduced. Therefore, in gas sensor <NUM>, gas sensing voltage VREAD can be reduced to a voltage of at least about <NUM> mV. In other words, the electricity consumption does not necessarily need to be about <NUM> mW as in the gas sensing device of the related art when the gas detecting element is heated to <NUM> or more, and energy conservation and long life can be expected at the same time.

As described above, in gas sensor <NUM>, the presence of gas can be sensed with a low gas sensing voltage and at high speed. Therefore, the gas containing hydrogen atoms can be sensed with low electricity consumption and at high speed.

<FIG> is a cross-sectional view illustrating a configuration example of gas sensor <NUM> according to Embodiment <NUM>. Only the differences from gas sensor <NUM> according to Embodiment <NUM> are described below.

Gas sensor <NUM> illustrated in <FIG> includes substrate <NUM>, interlayer insulating film <NUM>, gas-sensitive insulating film <NUM>, first electrode <NUM>, second electrode <NUM>, and gap <NUM>. In addition, insulator layer <NUM> is formed so as to cover first electrode <NUM>, second electrode <NUM>, and gas-sensitive insulating film <NUM> in gap <NUM>. Note that insulator layer <NUM> only needs to be formed so as to at least cover gas-sensitive insulating film <NUM> in gap <NUM>.

Insulator layer <NUM> has a function of selectively causing hydrogen gas to pass therethrough (in other words, a function of causing the hydrogen gas to easily pass therethrough but not causing gas other than the hydrogen gas to easily pass therethrough). As a result, gas-sensitive insulating film <NUM> does not directly come into contact with gas, and hence it is effective in terms of improving the reliability of the sensor operation. Note that insulator layer <NUM> is formed by a silicon oxide film, for example.

The function of insulator layer <NUM> for selectively causing hydrogen gas to pass therethrough depends on the film thickness of insulator layer <NUM>. For example, when insulator layer <NUM> is a silicon oxide film and the film thickness of insulator layer <NUM> is too thin, the electrons in second electrode <NUM> may be transmitted through insulator layer <NUM> to leak out, and then interact with molecules from the outside, which may cause the adsorption of the molecules, the dissociation of the hydrogen atoms from the molecules, or the like. Then, it is hard to say that the passing of gas other than the hydrogen gas is suppressed. Therefore, the film thickness of insulator layer <NUM> is preferred to be a film thickness by which the number of hydrogen molecules necessary for changing the resistance of the metal oxide layer are transmitted within a predetermined amount of time.

The thickness of insulator layer <NUM> for not causing gas other than the hydrogen gas to pass therethrough may be <NUM>, for example, as described below. The lower limit of the thickness of insulator layer <NUM> may be <NUM>, for example, on the basis of the disclosure of NPL <NUM>.

<FIG> is a cross-sectional view of structure body <NUM> having a double gate-silicon on insulator (DG-SOI) structure described in NPL <NUM>. Structure body <NUM> is contemplated as a structure body obtained by forming silicon oxide films <NUM> and <NUM> on upper and lower main surfaces of silicon substrate <NUM>, and further depositing polysilicon films <NUM> and <NUM> on exposed surfaces of silicon oxide films <NUM> and <NUM>. For the purpose of calculation, a thickness ts of silicon substrate <NUM> and a thickness tg of each polysilicon film are <NUM>, and a thickness tox of each of silicon oxide films <NUM> and <NUM> is controlled.

<FIG> shows a result obtained by calculating the existence probability (Ps1 to Ps4) of the electrons in silicon substrate <NUM> by changing the thickness tox of each of silicon oxide films <NUM> and <NUM> in structure body <NUM>. In <FIG>, Ps1 to Ps4 correspond to the energy levels of the electrons in silicon substrate <NUM> in the DG-SOI structure illustrated in <FIG>, and indicate the existence probability of the electrons on the orbits thereof. In any of Ps1 to Ps4, when tox becomes <NUM> or less, the existence probability of the electrons in silicon substrate <NUM> becomes significantly smaller than <NUM>. This means that the electrons in silicon substrate <NUM> are passing through silicon oxide films <NUM> and <NUM> and are leaking out to polysilicon films <NUM> and <NUM>. In any of Ps1 to Ps4, when tox is <NUM> or more, the existence probability of the electrons in silicon substrate <NUM> becomes about <NUM>. This means that the electrons in silicon substrate <NUM> cannot pass through silicon oxide films <NUM> and <NUM> and do not leak out to polysilicon films <NUM> and <NUM>.

From the calculation result, the electrons cannot substantially pass through a silicon oxide film of which film thickness tox is <NUM> or more. Therefore, by depositing a silicon oxide film having a thickness of <NUM> or more on second electrode <NUM>, the electrons in second electrode <NUM> can be prevented from interacting with the molecules existing on the outside. As a result, external gas is not adsorbed by the surface of second electrode <NUM>, and the hydrogen atoms are not dissociated from the molecules containing the hydrogen atoms at second electrode <NUM> that causes catalysis.

Note that it is not always true that it is better when the thickness of the silicon oxide film is thicker. When silicon oxide film is too thick, it requires time for gas-sensitive insulating film <NUM> to cause resistance change by the hydrogen molecules that have passed through the silicon oxide film and reached second electrode <NUM>. Therefore, there is an upper limit to the thickness of the silicon oxide film in order to realize a desired response time (for example, within one second as a standard desired for the gas sensor used for a fuel cell vehicle).

In gas sensor <NUM>, the number of hydrogen molecules that need to reach second electrode <NUM> in order to cause gas-sensitive insulating film <NUM> set to be in a high-resistance state to transition to a low-resistance state depends on the material and the size of gas sensor <NUM>. In a specific example of the gas sensor examined by the inventors of the present disclosure, the number is <NUM>,<NUM>. In other words, in the gas sensor, at least <NUM>,<NUM> hydrogen molecules are required to pass through the silicon oxide film and reach second electrode <NUM> within one second.

When hydrogen gas having a hydrogen molecule density of N<NUM> exists on the silicon oxide film surface, and the number of the hydrogen molecules transmitted through the silicon oxide film in t second is expressed by n, n is obtained by the following expression. [Expression <NUM>] <MAT>.

<FIG> is a graph showing the result obtained by calculating the relationship between the number of hydrogen molecules transmitted through the silicon oxide film in one second and the film thickness of the silicon oxide film when hydrogen molecule density N<NUM> is <NUM>% on the basis of Expression <NUM>. The dotted line indicates <NUM>,<NUM> that is an example of the number of hydrogen molecules necessary for the resistance change of gas-sensitive insulating film <NUM>. It can be seen from <FIG> that <NUM>,<NUM> hydrogen molecules necessary for the resistance change reach the surface of second electrode <NUM> within one second when the thickness of silicon oxide film is <NUM> or less.

As described above, by causing the thickness tox of the silicon oxide film to be within a desired range, the leaking of the hydrogen molecules to polysilicon films <NUM> and <NUM> can be suppressed. In addition, <NUM>,<NUM> hydrogen molecules necessary for the resistance change can be caused to reach the surface of second electrode <NUM> within a desired response time. As an example, the thickness tox of the silicon oxide film may be <NUM> or more and <NUM> or less for the gas sensor used for the fuel cell vehicle. As a result, the leaking of the hydrogen molecules to polysilicon films <NUM> and <NUM> can be suppressed. In addition, <NUM>,<NUM> hydrogen molecules necessary for the resistance change can be caused to reach the surface of second electrode <NUM> within a desired response time (within one second for the gas sensor used for the fuel cell vehicle).

<FIG>, and <FIG> are cross-sectional views illustrating configuration examples of gas sensors according to Embodiment <NUM>. <FIG> shows an exemplary gas sensor which is not according to the present invention. Only parts of gas sensors <NUM>, <NUM>, <NUM>, and <NUM> according to this embodiment that are different from gas sensor <NUM> according to Embodiment <NUM> are described below.

In gas sensor <NUM> illustrated in <FIG>, on interlayer insulating film <NUM> formed on substrate <NUM>, gas-sensitive insulating film <NUM> is formed in a region that is larger than first electrode <NUM> and second electrode <NUM>. Note that substrate <NUM> and interlayer insulating film <NUM> are similar to substrate <NUM> and interlayer insulating film <NUM> of gas sensor <NUM> illustrated in Embodiment <NUM>, and hence the description thereof is omitted.

As a result, only first electrode <NUM> and second electrode <NUM> of the minimum width need to be arranged on gas-sensitive insulating film <NUM>. Therefore, local region <NUM> can be efficiently formed on gas-sensitive insulating film <NUM>. Therefore, gas sensor <NUM> can be formed by efficiently arranging first electrode <NUM> and second electrode <NUM>.

Gas sensor <NUM> illustrated in <FIG> has a structure in which, on interlayer insulating film <NUM> formed on substrate <NUM>, gas-sensitive insulating film <NUM> is in contact with an end surface of at least one of first electrode <NUM> and second electrode <NUM>. In other words, one of first electrode <NUM> and second electrode <NUM> is arranged at the same layer level as gas-sensitive insulating film <NUM>, and the other is arranged on gas-sensitive insulating film <NUM>. Further, in planar view, a region in which gas-sensitive insulating film <NUM> is exposed is provided between a region in which first electrode <NUM> is arranged and a region in which second electrode <NUM> is arranged. Note that substrate <NUM> and interlayer insulating film <NUM> are similar to substrate <NUM> and interlayer insulating film <NUM> of gas sensor <NUM> described in Embodiment <NUM>, and hence the description thereof is omitted.

As a result, local region <NUM> is formed between an end surface of first electrode <NUM> or second electrode <NUM> arranged at the same layer level as gas-sensitive insulating film <NUM> and second electrode <NUM> or first electrode <NUM> arranged on gas-sensitive insulating film <NUM>. In addition, the surface of gas-sensitive insulating film <NUM> other than the part in which local region <NUM> is desired to be formed is covered with first electrode <NUM> or second electrode <NUM>. Therefore, the reaction of gas-sensitive insulating film <NUM> with other gas in parts other than the part in which local region <NUM> is formed can be suppressed.

Gas sensor <NUM> illustrated in <FIG> has a structure in which, on interlayer insulating film <NUM> formed on substrate <NUM>, gas-sensitive insulating film <NUM> in contact with end surfaces of both of first electrode <NUM> and second electrode <NUM>. In other words, first electrode <NUM> and second electrode <NUM> are arranged at the same layer level as gas-sensitive insulating film <NUM>. Further, a region in which gas-sensitive insulating film <NUM> is exposed is provided between a region in which first electrode <NUM> is arranged and a region in which second electrode <NUM> is arranged in planar view. Note that substrate <NUM> and interlayer insulating film <NUM> are similar to substrate <NUM> and interlayer insulating film <NUM> of gas sensor <NUM> described in Embodiment <NUM>, and hence the description thereof is omitted.

As a result, by applying voltage between the end surface of first electrode <NUM> and the end surface of second electrode <NUM> in gas-sensitive insulating film <NUM> exposed between the region in which first electrode <NUM> is arranged and the region in which second electrode <NUM> is arranged, local region <NUM> can be easily formed in gas-sensitive insulating film <NUM>. In addition, the surface of gas-sensitive insulating film <NUM> other than the part in which local region <NUM> is desired to be formed is covered with first electrode <NUM> or second electrode <NUM>. Therefore, the reaction of gas-sensitive insulating film <NUM> with other gas in parts other than the part in which local region <NUM> is formed can be suppressed.

Gas sensor <NUM> illustrated in <FIG> is not part of the claimed invention and includes two layers, that is, first oxide layer 603A and second oxide layer 603B as the gas-sensitive insulating film on interlayer insulating film <NUM> formed on substrate <NUM>. Note that the gas-sensitive insulating film may be formed by two or more layers. Substrate <NUM> and interlayer insulating film <NUM> are similar to substrate <NUM> and interlayer insulating film <NUM> of gas sensor <NUM> described in Embodiment <NUM>, and hence the description thereof is omitted.

First oxide layer 603A and second oxide layer 603B are laminated on a part of interlayer insulating film <NUM> in the stated order. In addition, first electrode <NUM> is formed on interlayer insulating film <NUM> so as to be continuous to first oxide layer 603A. In other words, an end portion of first oxide layer 603A is connected with an end portion of first electrode <NUM>. In addition, second electrode <NUM> is formed on at least a part of second oxide layer 603B.

Further, local region <NUM> arranged in contact with second electrode <NUM> and not in contact with first electrode <NUM> is included in first oxide layer 603A and second oxide layer 603B.

At least a part of local region <NUM> is formed in second oxide layer 603B, and the degree of oxygen deficiency of local region <NUM> reversibly changes in accordance with the application of an electrical pulse. It is conceived that local region <NUM> includes a filament formed by oxygen defection sites.

In other words, gas-sensitive insulating film <NUM> has a laminated structure at least including first oxide layer 603A including first metal oxide and second oxide layer 603B including second metal oxide. Further, first oxide layer 603A is arranged between first electrode <NUM> and second oxide layer 603B, and second oxide layer 603B is arranged between first oxide layer 603A and second electrode <NUM>. The thickness of second oxide layer 603B may be thinner than the thickness of first oxide layer 603A. In this case, a structure in which local region <NUM> is not in contact with first electrode <NUM> described below can be easily formed. The resistance of second oxide layer 603B is higher than the resistance of first oxide layer 603A, and hence most voltage applied to gas-sensitive insulating film <NUM> is applied to second oxide layer 603B.

In addition, in this disclosure, when the metal forming first oxide layer 603A and second oxide layer 603B is the same, a term "oxygen content" may be used in place of the "degree of oxygen deficiency". The expression "the oxygen content is high" corresponds to the expression "the degree of oxygen deficiency is small", and the expression "the oxygen content is low" corresponds to the expression "the degree of oxygen deficiency is large". However, gas-sensitive insulating film <NUM> according to this embodiment is not limited to the case where the metal forming first oxide layer 603A and second oxide layer 603B is the same, and the metal may be different. That is, first oxide layer 603A and second oxide layer 603B may be different metal oxides.

When a first metal forming first oxide layer 603A and a second metal forming second oxide layer 603B are the same, the oxygen content is in a corresponding relationship with the degree of oxygen deficiency. That is, when the oxygen content of the second metal oxide is larger than the oxygen content of the first metal oxide, the degree of oxygen deficiency of the first metal oxide is smaller than the degree of oxygen deficiency of the second metal oxide. The degree of oxygen deficiency of local region <NUM> is larger than the degree of oxygen deficiency of second oxide layer 603B, and is different from the degree of oxygen deficiency of first oxide layer 603A.

As described above, local region <NUM> is formed in at least one of first oxide layer 603A and second oxide layer 603B by applying an initial breakdown voltage across first electrode <NUM> and second electrode <NUM>. For example, in gas sensor <NUM>, it is conceived that the current easily flows through a region in which the distance between first electrode <NUM> and second electrode <NUM> is short. Therefore, by the initial breakdown, local region <NUM> is formed in a region that is in contact with second electrode <NUM>, passes through second oxide layer 603B, partially enters first oxide layer 603A, and is not in contact with first electrode <NUM>.

Fuel cell vehicle <NUM> according to Embodiment <NUM> includes any of the gas sensors described in Embodiment <NUM> to Embodiment <NUM> described above. Fuel cell vehicle <NUM> detects the hydrogen gas in the vehicle with the gas sensor.

<FIG> is a side view illustrating a configuration example of fuel cell vehicle <NUM> according to this embodiment.

Fuel cell vehicle <NUM> includes passenger compartment <NUM>, luggage compartment <NUM>, gas tank compartment <NUM>, fuel tank <NUM>, gas sensor <NUM>, piping <NUM>, fuel cell compartment <NUM>, fuel cell <NUM>, gas sensor <NUM>, motor compartment <NUM>, and motor <NUM>.

Fuel tank <NUM> is provided in gas tank compartment <NUM>, and retains hydrogen gas as the fuel gas. Gas sensor <NUM> detects fuel gas leakage in gas tank compartment <NUM>.

Fuel cell <NUM> is formed as a fuel cell stack by stacking cells each including a fuel electrode, an air electrode, and an electrolyte as a base unit. Fuel cell <NUM> is provided in fuel cell compartment <NUM>. The hydrogen gas in fuel tank <NUM> is transmitted into fuel cell <NUM> in fuel cell compartment <NUM> through piping <NUM>, and electricity is generated by causing the hydrogen gas and oxygen gas in the atmosphere to react with each other in fuel cell <NUM>. Gas sensor <NUM> detects the leakage of hydrogen gas in fuel cell compartment <NUM>.

Motor <NUM> is provided in motor compartment <NUM>, rotates with the electricity generated by fuel cell <NUM>, and causes fuel cell vehicle <NUM> to travel.

As described above, in the gas sensor according to this disclosure, hydrogen gas can be detected with extremely small electricity consumption of about <NUM> mW, for example. Therefore, the leakage of hydrogen gas can be constantly monitored without greatly increasing the standby power of the fuel cell vehicle by utilizing the excellent power-saving properties of the gas sensor.

For example, a predetermined voltage may be constantly applied to gas sensors <NUM> and <NUM> regardless of the operation state of an ignition key in fuel cell vehicle <NUM>, and it may be determined whether there is hydrogen gas on the outside of fuel tank <NUM> in gas tank compartment <NUM> and the outside of fuel cell <NUM> in fuel cell compartment <NUM> on the basis of the current amount flowing through gas sensors <NUM> and <NUM>.

As a result, for example, it is already determined whether there is leakage of hydrogen gas at the time point at which the operation of the ignition key is received, and hence the starting time of the fuel cell vehicle can be shortened as compared to when the gas sensor is driven in order to determine whether there is leakage of hydrogen gas after the operation of the ignition key is received. In addition, safety can be improved by continuing the monitoring of the leakage of hydrogen gas after the travel of the fuel cell vehicle, for example, after storing the fuel cell vehicle in a garage.

Although the gas sensor and the fuel cell vehicle according to the aspects of the present disclosure have been described based on the above embodiments, the present disclosure is not limited to the embodiments. Those skilled in the art will be readily appreciated that various modifications and combinations of the constituent elements and functions in the embodiments are possible within the scope of the appended claims without materially departing from the novel teachings and advantages of the present disclosure.

For example, the abovementioned gas sensor may further include a measurement circuit that measures the current flowing through the sensitive insulating film when a predetermined voltage is applied across the first electrode and the second electrode. In addition, a power supply circuit that constantly applies a predetermined voltage across the first electrode and the second electrode may be further included.

According to the configuration as above, a highly-convenient gas sensor can be obtained as a module part including a measurement circuit and a power supply circuit.

Claim 1:
A gas sensor, comprising:
a gas-sensitive body layer (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) disposed above a substrate (<NUM>) and including a metal oxide layer;
a first electrode (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>), formed by a material of which standard electrode potential is lower than that of the metal forming said metal oxide, disposed at a same layer level as a layer level of the gas-sensitive body layer above the substrate or disposed on the gas-sensitive body layer ; and
a second electrode (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>), comprising a material that causes catalysis of hydrogen gas into hydrogen atoms, disposed at a same layer level as a layer level of the gas-sensitive body layer above the substrate or disposed on the gas-sensitive body layer, and further disposed on the first electrode, the second electrode being apart from the first electrode by a gap (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>), wherein:
the gas-sensitive body layer has a resistance change characteristic that reversibly transitions to a high-resistance state and a low-resistance state on basis of a voltage applied across the first electrode and the second electrode;
at least a part of the gas-sensitive body layer is exposed via the gap ; and
the gas-sensitive body layer has a resistance that decreases when gas containing a hydrogen atom is in contact with the second electrode.