Patent Description:
Patent Literature (PTL) <NUM> and PTL <NUM> disclose a gas sensor that detects gas molecules containing hydrogen atoms.

The conventional techniques, however, have a problem of low performance in detecting low-concentrated hydrogen in particular.

In view of the above, the present disclosure provides a hydrogen sensor, a hydrogen detection method, and a hydrogen detection device whose performance in detecting low-concentrated hydrogen is improved.

The invention is a hydrogen detection method as defined by claim <NUM>. The hydrogen detection method according to an aspect of the present invention is a hydrogen detection method in a hydrogen sensor, the hydrogen sensor including: a first electrode which is planar; a second electrode which is planar, faces the first electrode, and includes an exposed portion; a metal oxide layer which is sandwiched between a surface of the first electrode and a surface of the second electrode facing each other, and has a resistance that changes due to hydrogen; and two terminals connected to the second electrode with the exposed portion being interposed therebetween in plan view of the second electrode, the hydrogen detection method including: passing a current through the exposed portion by applying a voltage between the two terminals; and detecting a gas containing hydrogen atoms by detecting a decrease in a resistance value between the two terminals.

The invention is a hydrogen detection device as defined by claim <NUM>. The hydrogen detection device according to an aspect of the present invention includes: a first electrode which is planar; a second electrode which is planar, faces the first electrode, and includes an exposed portion; a metal oxide layer which is sandwiched between a surface of the first electrode and a surface of the second electrode facing each other, and has a resistance that changes due to hydrogen; two terminals connected to the second electrode; and a drive circuit that, in a state of passing a current through the exposed portion by applying a voltage between the two terminals, detects a gas containing hydrogen atoms by detecting a decrease in a resistance value between the two terminals.

A hydrogen detection method, and a hydrogen detection device according to the present disclosure can improve the performance in detecting low-concentrated hydrogen.

Hereinafter, embodiments are specifically described with reference to the drawings.

Note that each of the embodiments described below shows a general or specific example. The numerical values, shapes, materials, constituent elements, the arrangement and connection of the constituent elements, steps, the processing order of the steps etc. illustrated in the embodiments described below are mere examples, and are not intended to limit the present disclosure.

<FIG> is a cross-sectional view illustrating a configuration example of hydrogen sensor <NUM> according to Embodiment <NUM>. <FIG> is a top view illustrating a configuration example of hydrogen sensor <NUM> according to Embodiment <NUM>. Note that <FIG> illustrates a schematic cross section along line IA-IA of <FIG>, viewed in the arrow direction.

As illustrated in <FIG> and <FIG>, key components of hydrogen sensor <NUM> include first electrode <NUM>, metal oxide layer <NUM>, second electrode <NUM>, first terminal <NUM>, second terminal <NUM>, and third terminal <NUM>. The key components of hydrogen sensor <NUM> are covered by insulating film <NUM>, insulating films 107a through 107c, and insulating films 109a and 109b. These insulating films, however, have openings 106a, 111a, 112a, and 113a.

First electrode <NUM> is a planar electrode and has two surfaces. Of the two surfaces of first electrode <NUM>, one surface (i.e., the upper surface in <FIG>) is in contact with metal oxide layer <NUM>, and the other surface (i.e., the lower surface in <FIG>) is in contact with insulating film 107b and via <NUM>. In <FIG>, first electrode <NUM> is in a rectangular shape of the same size as that of second electrode <NUM>. First electrode <NUM> may include, for example, a material having a standard electrode potential lower than that of metals forming metal oxides, such as tungsten, nickel, tantalum, titanium, aluminum, tantalum nitride, and titanium nitride. The higher the value of the standard electrode potential is, the more resistant to oxidation the material is. First electrode <NUM> in <FIG> is formed with, for example, tantalum nitride (TaN) or titanium nitride (TiN), or laminations thereof.

Metal oxide layer <NUM> is sandwiched between a surface of first electrode <NUM> and a surface of second electrode <NUM> facing each other, is formed with a metal oxide serving as a gas-sensitive resistance film, and has a resistance value that reversibly changes according to the presence and absence of a hydrogen-containing gas in a gas in contact with second electrode <NUM>. It is sufficient so long as metal oxide layer <NUM> has a property that its resistance is changed by hydrogen. Metal oxide layer <NUM> is formed with an oxygen-deficient metal oxide. As the base metal of metal oxide layer <NUM>, at least one of the following may be selected: aluminum (Al) and transition metals such as tantalum (Ta), hafnium (Hf), titanium (Ti), zirconium (Zr), niobium (Nb), tungsten (W), nickel (Ni), and iron (Fe). Since transition metals can take on plural oxidation states, different resistance states can be realized through redox reactions. Here, the "degree of oxygen deficiency" of a metal oxide is the ratio of deficiency of oxygen in the metal oxide to the amount of oxygen in an oxide having a stoichiometric composition composed of the same elements as those of the metal oxide. Here, the oxygen deficiency is a value obtained by subtracting the amount of oxygen in the metal oxide from the amount of oxygen in the metal oxide having a stoichiometric composition. If there can be two or more metal oxides having stoichiometric compositions composed of the same elements as those of the metal oxide, the degree of oxygen deficiency of the metal oxide is defined based on one of the two or more metal oxides having stoichiometric compositions that has the highest resistance value. Metal oxides having stoichiometric compositions are more stable and higher in resistance value than metal oxides having other compositions. For example, when the base metal of metal oxide layer <NUM> is tantalum (Ta), the oxide having a stoichiometric composition as defined above is Ta<NUM>O<NUM>, so metal oxide layer <NUM> 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 (<NUM> - <NUM>) / <NUM> - <NUM>%. The degree of oxygen deficiency of a metal oxide with excess oxygen is a negative value. Note that in the present disclosure, the degree of oxygen deficiency can take a positive value, <NUM>, or a negative value unless otherwise noted. An oxide with a low degree of oxygen deficiency has a high resistance value because it is closer to an oxide having a stoichiometric composition, whereas an oxide with a high degree of oxygen deficiency has a low resistance value because it is closer to the metal included in the oxide.

Metal oxide layer <NUM> illustrated in <FIG> includes: first layer 104a in contact with first electrode <NUM>; second layer 104b in contact with first layer 104a and second electrode <NUM>; and isolation layer 104i. The degree of oxygen deficiency of second layer 104b is lower than that of first layer 104a. For example, first layer 104a is TaOx. Second layer 104b is Ta<NUM>O<NUM> whose degree of oxygen deficiency is lower than that of first layer 104a. Metal oxide layer <NUM> includes isolation layer 104i at the perimeter in plan view of first electrode <NUM>.

Here, plan view means viewing hydrogen sensor <NUM> according to the present invention from a viewpoint in the layer-stacking direction in <FIG>; in other words, viewing from a viewpoint in the direction normal to any of the surfaces of, for example, first electrode <NUM> and second electrode <NUM> that are planar. For example, plan view refers to viewing the top surface of hydrogen sensor <NUM> illustrated in <FIG>.

The resistance state of such metal oxide layer <NUM> is that the resistance value decreases according to a hydrogen-containing gas that comes into contact with second electrode <NUM>. In detail, when a hydrogen-containing gas is present in a detection-target gas, hydrogen atoms are dissociated from the hydrogen-containing gas in second electrode <NUM>. The dissociated hydrogen atoms enter metal oxide layer <NUM> and form impurity levels. In particular, the dissociated hydrogen atoms concentrate in the vicinity of the interface with the second electrode, making the apparent thickness of second layer 104b smaller. As a result, the resistance value of metal oxide layer <NUM> decreases.

Second electrode <NUM> is a planar electrode with hydrogen dissociability, and has two surfaces. Of the two surfaces of second electrode <NUM>, one surface (i.e., the lower surface in <FIG>) is in contact with metal oxide layer <NUM>, and the other surface (i.e., the upper surface in <FIG>) is in contact with metal layer <NUM> and the outside air. Second electrode <NUM> has, in aperture 106a, exposed portion 106e that is exposed to the outside air. Second electrode <NUM> is formed with, for example, a material that catalyzes dissociation of hydrogen atoms from gas molecules having hydrogen atoms, such as platinum (Pt), iridium (Ir), palladium (Pd), or nickel (Ni), or an alloy containing at least one of these. It is assumed that second electrode <NUM> in <FIG> is platinum (Pt). Two terminals, namely first terminal <NUM> and second terminal <NUM>, are connected to second electrode <NUM>.

First terminal <NUM> is connected to second electrode <NUM> through via <NUM>.

Second terminal <NUM> is connected to second electrode <NUM> through via <NUM>. First terminal <NUM> and second terminal <NUM> are connected, via openings 111a and 112a, respectively, to an external drive circuit that drives hydrogen sensor <NUM>.

As illustrated in <FIG>, first terminal <NUM> and second terminal <NUM> are disposed with exposed portion 106e being interposed therebetween in plan view of second electrode <NUM>. With first terminal <NUM> and second terminal <NUM> disposed in this manner, application of a predetermined voltage between first terminal <NUM> and second terminal <NUM> causes passage of current through exposed portion 106e of second electrode <NUM>, that is, causes current to flow through exposed portion 106e. The passage of current through exposed portion 106e of second electrode <NUM> is considered to activate the hydrogen dissociation by exposed portion 106e. Note that the predetermined voltage may be voltages that are opposite to each other in polarity.

In hydrogen sensor <NUM>, the resistance value between first terminal <NUM> and second terminal <NUM> changes when gas molecules containing hydrogen atoms come into contact with exposed portion 106e during the passage of a current through exposed portion 106e. By the above-described drive circuit detecting this change in the resistance value, gas molecules containing hydrogen atoms are detected.

Third terminal <NUM> is connected to first electrode <NUM> via opening 113a, via <NUM>, wiring <NUM>, and via <NUM>. Third terminal <NUM> is connected, via opening 113a, to the external drive circuit that drives hydrogen sensor <NUM>. In hydrogen sensor <NUM>, the resistance between first electrode <NUM> and second electrode <NUM> changes when gas molecules containing hydrogen atoms come into contact with exposed portion 106e during the passage of a current through exposed portion 106e. In other words, in hydrogen sensor <NUM>, the resistance value between first terminal <NUM> or second terminal <NUM> and third terminal <NUM> changes when gas molecules containing hydrogen atoms come into contact with exposed portion 106e during the passage of a current through exposed portion 106e. Gas molecules containing hydrogen atoms are detected also through detection, by the above-described drive circuit, of the change in the resistance value.

Note that insulating film <NUM>, insulating films 107a through 107c, and insulating films 109a and 109b that cover the key components of hydrogen sensor <NUM> are formed with a silicon oxide film, a silicon nitride film, etc..

Metal layer <NUM> is formed on the upper surface of second electrode <NUM> except for opening 106a. Metal layer <NUM> includes, for example, TiAlN as the material, and is formed as an etching stopper for forming vias <NUM>, but is not essential.

The laminate of first electrode <NUM>, metal oxide layer <NUM>, and second electrode <NUM> is a structure that can be used as a storage element of resistance random access memory (ReRAM). The storage element of the resistance random access memory is a digital storage element which uses two of possible states that metal oxide layer <NUM> can take, i.e., a high-resistance state and a low-resistance state. Hydrogen sensor <NUM> according to the present disclosure uses the high-resistance state among the possible states of metal oxide layer <NUM>.

<FIG> illustrates an example of metal oxide layer <NUM> having a two-layer configuration with first layer 104a that includes TaOx as the material and second layer 104b that includes, as the material, Ta<NUM>O<NUM> whose degree of oxygen deficiency is low. However, metal oxide layer <NUM> may have a one-layer configuration having, as the material, TaOx or Ta<NUM>O<NUM> whose degree of oxygen deficiency is low.

Next, a hydrogen detection method performed using hydrogen sensor <NUM> and a hydrogen detection device including hydrogen sensor <NUM> are described.

<FIG> is a block diagram illustrating a configuration example of hydrogen detection device <NUM> including hydrogen sensor <NUM> and drive circuit <NUM> that performs a hydrogen detection method according to Embodiment <NUM>. In <FIG>, hydrogen detection device <NUM> includes drive circuit <NUM> and hydrogen sensor <NUM>. Drive circuit <NUM> is connected to hydrogen sensor <NUM> via at least three wires connected to first terminal <NUM>, second terminal <NUM>, and third terminal <NUM> of hydrogen sensor <NUM>. Drive circuit <NUM> is a microcomputer including, for example, a CPU, ROM, and RAM. At least three wires are connected to ports of the microcomputer.

<FIG> is a flowchart illustrating a hydrogen detection method performed by drive circuit <NUM> using hydrogen sensor <NUM>. In <FIG>, drive circuit <NUM> first starts passing current between first terminal <NUM> and second terminal <NUM> (S1). In other words, drive circuit <NUM> applies a predetermined voltage between first terminal <NUM> and second terminal <NUM>. For example, voltages that are opposite to each other in polarity, e.g., +V1 and -V1, are applied to first terminal <NUM> and second terminal <NUM>. The current that passes through exposed portion 106e of second electrode <NUM> as a result of the voltage application is sufficient so long as it is in a range from several milliamperes to several tens of milliamperes, for example.

Next, drive circuit <NUM> measures resistance value Rh between first terminal <NUM> and second terminal <NUM> (S2), and further measures resistance value Rv between first terminal <NUM> or second terminal <NUM> and third terminal <NUM> (S3). Furthermore, drive circuit <NUM> determines whether measured resistance value Rh is less than threshold th1, and determines whether measured resistance value Rv is less than threshold th2 (S4). If at least one of resistance values Rh and Rv is determined to be less than the threshold, it is determined as "hydrogen present" (S5), and if not, it is determined as "no hydrogen" (S6).

Drive circuit <NUM> may repeat steps S2 through S6 at a constant cycle of several hundreds of milliseconds to several seconds, for example.

Note that <FIG> illustrates an example of constantly passing a current between first terminal <NUM> and second terminal <NUM>; however, a current may be passed only during the processing of steps S2 and S3.

In step S5, it may be determined as "hydrogen present" if resistance value Rh and resistance value Rv are both determined to be less than the respective thresholds.

One of steps S2 and S3 may be omitted, and only one of resistance values Rh and Rv may be used for the determination.

Next, operations of hydrogen sensor <NUM> according to Embodiment <NUM> are described using experimental data.

<FIG> illustrates experimental data on a hydrogen sensor of a comparative example. Compared to hydrogen sensor <NUM> according to Embodiment <NUM>, the hydrogen sensor of the comparative example has a configuration which does not include second terminal <NUM> or the same configuration as hydrogen sensor <NUM> of Embodiment <NUM> except that first terminal <NUM> and second terminal <NUM> are short-circuited. In <FIG>, the horizontal axis represents time. The vertical axis represents current i3 between third terminal <NUM> and first terminal <NUM>, that is, current i3 flowing between first electrode <NUM> and second electrode <NUM>. The measurement conditions are as follows: a voltage of typically <NUM> V is applied between third terminal <NUM> and first terminal <NUM>, and a voltage of -<NUM> V is applied therebetween for <NUM> milliseconds for every second. Also, during the time period from <NUM> to <NUM> in seconds, exposed portion 106e of second electrode <NUM> is in contact with a gas containing <NUM>% of hydrogen. During the time period from <NUM> to <NUM> in seconds, exposed portion 106e of second electrode <NUM> is in contact with a gas containing <NUM>% of hydrogen. During the time period from <NUM> to <NUM> in seconds, exposed portion 106e of second electrode <NUM> is in contact with a gas containing <NUM>% of hydrogen.

With the hydrogen sensor of the comparative example under these measurement conditions, current i3 was constant regardless of the presence or absence of hydrogen as illustrated in <FIG>. In other words, the hydrogen sensor of the comparative example did not react to the gas containing <NUM>% of hydrogen and could not detect hydrogen.

<FIG> illustrates an experimental result of hydrogen sensor <NUM> according to Embodiment <NUM>. The horizontal axis of <FIG> represents the same time axis as in <FIG>. The vertical axis represents current i3 between third terminal <NUM> and first terminal <NUM>, that is, current i3 flowing between first electrode <NUM> and second electrode <NUM>. The measurement conditions are different from those in <FIG> in that a condition of passing a current between first terminal <NUM> and second terminal <NUM> is added. In other words, in <FIG>, a current of about <NUM> mA is applied between first terminal <NUM> and second terminal <NUM>.

With hydrogen sensor <NUM> according to Embodiment <NUM> under these measurement conditions, current i3 increases in the time period from <NUM> to <NUM> in seconds as compared to the other time periods. In other words, in the time period from <NUM> to <NUM> in seconds, hydrogen atoms are dissociated from the gas that has come into contact with exposed portion 106e of second electrode <NUM>, and the dissociated hydrogen atoms enter metal oxide layer <NUM> and form impurity levels, causing a decrease in the resistance value of metal oxide layer <NUM>. As a result, current i3 increases in the time period from <NUM> to <NUM> in seconds. Furthermore, in the time period from <NUM> to <NUM> in seconds, current i3 decreases as compared to the previous time period. According to <FIG>, the current between first terminal <NUM> and third terminal <NUM> increases and decreases in response to the presence and absence of hydrogen. It can be understood that the hydrogen detection performance of hydrogen sensor <NUM> is improved as compared to that of the hydrogen sensor illustrated in <FIG>.

<FIG> illustrates an experimental result of hydrogen sensor <NUM> according to Embodiment <NUM>. The horizontal axis of <FIG> represents the same time axis as in <FIG>. The vertical axis of <FIG> is different from that of <FIG>, and represents current i1 between first terminal <NUM> and second terminal <NUM>. The measurement conditions are the same as those in <FIG>. However, it is assumed that, for example, voltages that are opposite to each other in polarity, such as +<NUM> V and -<NUM> V, are applied to first terminal <NUM> and second terminal <NUM> and a current of about <NUM> mA is passed through first terminal <NUM> and second terminal <NUM>. Note that the value of the current passed is determined based on the resistance value of second electrode <NUM>.

With hydrogen sensor <NUM> according to Embodiment <NUM> under these measurement conditions, current i1 increases in the time period from <NUM> to <NUM> in seconds as compared to the other time periods. Furthermore, in the time period from <NUM> to <NUM> in seconds, current i1 decreases as compared to the previous time period. According to <FIG>, the current between first terminal <NUM> and second terminal <NUM> increases and decreases in response to the presence and absence of hydrogen.

Comparison between <FIG> and <FIG> and <FIG> shows that the passage of a current between first terminal <NUM> and second terminal <NUM> leads to an improvement in the hydrogen detection capability. Moreover, as a result of exposed portion 106e coming into contact with hydrogen, the resistance between first terminal <NUM> and third terminal <NUM> decreases, and the resistance between first terminal <NUM> and second terminal <NUM> also decreases. In other words, hydrogen detection is possible in two ways. That is to say, one way of hydrogen detection is through a change in the resistance between first terminal <NUM> and second terminal <NUM>, and another way of hydrogen detection is through a change in the resistance between first terminal <NUM> and third terminal <NUM>.

As described above, hydrogen sensor <NUM> according to Embodiment <NUM> includes: first electrode <NUM> which is planar; second electrode <NUM> which is planar, faces first electrode <NUM>, and includes exposed portion 106e; metal oxide layer <NUM> which is sandwiched between a surface of first electrode <NUM> and a surface of second electrode <NUM> facing each other, and has a resistance that changes due to hydrogen; and first terminal <NUM> and second terminal <NUM> as two terminals connected to second electrode <NUM>.

With this, since hydrogen sensor <NUM> includes two terminals for passing a current through second electrode <NUM>, passage of a current can lead to an improvement in the hydrogen detection performance.

Here, the two terminals, i.e., first terminal <NUM> and second terminal <NUM>, may be positioned with exposed portion 106e being interposed therebetween in plan view of second electrode <NUM> which is planar.

With this, it is possible to pass a current through exposed portion 106e that comes into contact with a gas, and the hydrogen detection performance can be efficiently improved.

Here, a current may be passed through exposed portion 106e by applying predetermined voltages to two terminals <NUM> and <NUM>.

Here, voltages opposite to each other in polarity may be applied to the two terminals as predetermined voltages.

With this, the voltage applied to the central part of exposed portion 106e can be made substantially <NUM> V, and the hydrogen detection performance can be efficiently improved.

Here, in hydrogen sensor <NUM>, a resistance between first electrode <NUM> and second electrode <NUM> may change when gas molecules containing hydrogen atoms come into contact with exposed portion 106e during passage of a current through exposed portion 106e.

With this, hydrogen can be detected through a change in the resistance between first electrode <NUM> and second electrode <NUM>.

Here, in hydrogen sensor <NUM>, a resistance between the two terminals may change when gas molecules containing hydrogen atoms come into contact with exposed portion 106e during passage of a current through exposed portion 106e.

With this, hydrogen can be detected through a change in the resistance between the two terminals, i.e., first terminal <NUM> and second terminal <NUM>.

Here, hydrogen sensor <NUM> may include: a first via which is connected to, of two main surfaces of first electrode <NUM>, a main surface farther from metal oxide layer <NUM>, and overlaps with exposed portion 106e in plan view of second electrode <NUM> which is planar; and a connection terminal (i.e., third terminal <NUM>) connected to the first via.

Here, the two terminals may be connected to second electrode <NUM> via two second vias connected to second electrode <NUM>, and the first via may be located at a central position between the two second vias.

With this configuration, first terminal <NUM> and second terminal <NUM> are disposed approximately symmetrically about exposed portion 106e through which a current is passed, and thus, a key current path between first electrode <NUM> and second electrode <NUM> can be formed at the central part of exposed portion 106e. As a result, the hydrogen detection performance can be improved.

Here, metal oxide layer <NUM> may include (i) first layer 104a in contact with first electrode <NUM> and (ii) second layer 104b in contact with first layer 104a and second electrode <NUM>, and second layer 104b may have a degree of oxygen deficiency lower than a degree of oxygen deficiency of first layer 104a.

With this, the gas sensitivity of second layer 104b for the hydrogen atoms dissociated by second electrode <NUM> can be enhanced.

A hydrogen detection method according to Embodiment <NUM> is a hydrogen detection method in a hydrogen sensor, the hydrogen sensor including: first electrode <NUM> which is planar; second electrode <NUM> which is planar, faces first electrode <NUM>, and includes exposed portion 106e; metal oxide layer <NUM> which is sandwiched between a surface of first electrode <NUM> and a surface of second electrode <NUM> facing each other, and has a resistance that changes due to hydrogen; and two terminals (i.e., first terminal <NUM> and second terminal <NUM>) connected to second electrode <NUM> with exposed portion 106e being interposed therebetween in plan view of second electrode <NUM> which is planar, the hydrogen detection method including: passing a current through exposed portion 106e by applying a voltage between the two terminals; and detecting a gas containing hydrogen atoms detecting a decrease in a resistance value between first terminal <NUM> and second terminal <NUM> and, optionally, by detecting a decrease in resistance value between first electrode <NUM> and second electrode <NUM>.

With this, passage of a current between the two terminals, i.e., first terminal <NUM> and second terminal <NUM>, can lead to an improvement in the hydrogen detection performance.

Hydrogen detection device <NUM> according to Embodiment <NUM> includes: first electrode <NUM> which is planar; second electrode <NUM> which is planar, faces first electrode <NUM>, and includes exposed portion 106e; metal oxide layer <NUM> which is sandwiched between a surface of first electrode <NUM> and a surface of second electrode <NUM>, and has a resistance that changes due to hydrogen; two terminals (i.e., first terminal <NUM> and second terminal <NUM>) connected to second electrode <NUM>; and drive circuit <NUM> that, in a state of passing a current through exposed portion 106e by applying a voltage between the two terminals, detects a gas containing hydrogen atoms by detecting a decrease in a resistance value between the two terminals and, optionally, by detecting a decrease in resistance value between first electrode <NUM> and second electrode <NUM>.

In Embodiment <NUM>, a configuration example of hydrogen sensor <NUM> is described. Hydrogen sensor <NUM> according to Embodiment <NUM> includes, in addition to the elements of hydrogen sensor <NUM> according to Embodiment <NUM>, a local region called a filament inside metal oxide layer <NUM>. With hydrogen sensor <NUM> that includes the local region, the hydrogen detection performance can be further enhanced and the reaction speed in the hydrogen detection can be further increased.

<FIG> is a cross-sectional view illustrating a configuration example of hydrogen sensor <NUM> according to Embodiment <NUM>. Hydrogen sensor <NUM> in <FIG> is different from hydrogen sensor <NUM> in <FIG> in that local region <NUM> is added. Hereinafter, different aspects are mainly described, and overlapping descriptions of the same aspects are avoided.

Local region <NUM> is a region which is not in contact with first electrode <NUM> but is in contact with second electrode <NUM>, and whose degree of oxygen deficiency is higher than that of metal oxide layer <NUM> surrounding local region <NUM>. Local region <NUM> is a region in which current flows more easily than in metal oxide layer <NUM>. That is to say, local region <NUM> is a minute region that includes a filament (a conductive path) formed by oxygen vacancies. In addition, local region <NUM> is formed at approximately the central part of exposed portion 106e in plan view of second electrode <NUM>. Local region <NUM> or the filament is formed by a process called forming. In the forming process, a pulse that serves as an electrical stress is applied between second electrode <NUM> and first electrode <NUM>. Local region <NUM> can be formed with dependence on the magnitude and duration of the pulse.

<FIG> illustrates experimental data on a hydrogen sensor of a comparative example not forming part of the invention. Compared to hydrogen sensor <NUM> according to Embodiment <NUM>, the hydrogen sensor of the comparative example has a configuration which does not include second terminal <NUM> or the same configuration as hydrogen sensor <NUM> of Embodiment <NUM> except that first terminal <NUM> and second terminal <NUM> are short-circuited. The measurement conditions in <FIG> are the same as those in <FIG>.

Under the same measurement conditions as in <FIG>, the hydrogen sensor of the comparative example reacted to hydrogen but the increase over time of current i3 was gradual, as illustrated in <FIG>.

<FIG> illustrates an experimental result of hydrogen sensor <NUM> according to Embodiment <NUM>. The measurement conditions in <FIG> are the same as those in <FIG>.

In hydrogen sensor <NUM> according to Embodiment <NUM>, current i3 increased in the time period from <NUM> to <NUM> in seconds as compared to the other time periods, and the increase over time of current i3 was rapid as illustrated in <FIG>. In other words, in the time period from <NUM> to <NUM> in seconds, hydrogen atoms are dissociated more quickly from the gas that has come into contact with exposed portion 106e of second electrode <NUM>, and the dissociated hydrogen atoms enter metal oxide layer <NUM> and form impurity levels, causing a decrease in the resistance value of metal oxide layer <NUM>. As a result, current i3 rapidly increases in the time period from <NUM> to <NUM> in seconds. Furthermore, in the time period from <NUM> to <NUM> in seconds, current i3 decreases as compared to the previous time period. According to <FIG>, the current between first terminal <NUM> and third terminal <NUM> increases and decreases in response to the presence and absence of hydrogen. The amount of change in <FIG> is greater than that in <FIG>.

As described above, hydrogen sensor <NUM> according to Embodiment <NUM> includes a local region which is located inside metal oxide layer <NUM>, is in contact with second electrode <NUM>, and has a degree of oxygen deficiency higher than a degree of oxygen deficiency of metal oxide layer <NUM>.

With this, the hydrogen detection performance can be improved, and the response speed in the hydrogen detection can be increased.

Hydrogen sensor <NUM> includes a local region which is located inside metal oxide layer <NUM>, is in contact with second electrode <NUM>, and is a region in which current flows more easily than in metal oxide layer <NUM>.

With this, too, the hydrogen detection performance can be improved, and the response speed in the hydrogen detection can be increased.

In Embodiment <NUM>, a configuration example of hydrogen sensor <NUM> is described. Hydrogen sensor <NUM> according to Embodiment <NUM> is different from hydrogen sensor <NUM> according to Embodiment <NUM> in that metal oxide layer <NUM> has a three-layer configuration. With this configuration example, the hydrogen detection performance can be further enhanced, and the response speed in the hydrogen detection can be further increased.

<FIG> is a cross-sectional view illustrating a configuration example of hydrogen sensor <NUM> according to Embodiment <NUM>. Hydrogen sensor <NUM> in <FIG> is different from hydrogen sensor <NUM> in <FIG> in that third layer 104c is added in metal oxide layer <NUM>. Hereinafter, different aspects are mainly described, and overlapping descriptions of the same aspects are avoided.

Third layer 104c is in contact with second layer 104b and second electrode <NUM>. The degree of oxygen deficiency of third layer 104c is higher than that of second layer 104b. For example, third layer 104c includes, as the material, TaOx or Ta<NUM>O<NUM> whose degree of oxygen deficiency is higher than that of second layer 104b which includes Ta<NUM>O<NUM> as the material. The degree of oxygen deficiency of third layer 104c is lower than that of first layer 104a.

<FIG> illustrates an example of metal oxide layer <NUM> having a three-layer configuration with first layer 104a that includes TaOx as the material, second layer 104b that includes, as the material, Ta<NUM>O<NUM> whose degree of oxygen deficiency is low, and third layer 104c that includes, as the material, Ta<NUM>O<NUM> whose degree of oxygen deficiency is higher than that of second layer 104b. However, metal oxide layer <NUM> may have a two-layer configuration with Ta<NUM>O<NUM> whose degree of oxygen deficiency is low and TaOx or Ta<NUM>O<NUM> whose degree of oxygen deficiency is higher than that of the former TazOs.

With the hydrogen sensor of the comparative example under the same measurement conditions as those in <FIG>, current i3 was constant regardless of the presence or absence of hydrogen as illustrated in <FIG>. In other words, the hydrogen sensor of the comparative example did not react to the gas containing <NUM>% of hydrogen and could not detect hydrogen.

In hydrogen sensor <NUM> according to Embodiment <NUM>, current i3 increases in the time period from <NUM> to <NUM> in seconds as compared to the other time periods as illustrated in <FIG>. Furthermore, in the time period from <NUM> to <NUM> in seconds, current i3 decreases as compared to the previous time period. According to <FIG>, current i3 between first terminal <NUM> and third terminal <NUM> increases and decreases in response to the presence and absence of hydrogen. Also, in <FIG>, the response speed in the hydrogen detection is faster than that in <FIG>.

As described above, metal oxide layer <NUM> according to Embodiment <NUM> includes (i) first layer 104a in contact with first electrode <NUM>, (ii) second layer 104b in contact with first layer 104a, and (iii) third layer 104c in contact with second layer 104b and second electrode <NUM>, and third layer 104c has a degree of oxygen deficiency higher than a degree of oxygen deficiency of second layer 104b.

With this, the gas sensitivity of third layer 104c for the hydrogen atoms dissociated by second electrode <NUM> can be further enhanced.

The degree of oxygen deficiency of third layer 104c may be lower than that of first layer 104a.

Needless to say, the hydrogen detection device and hydrogen detection method illustrated in <FIG> and <FIG> can be implemented likewise using hydrogen sensors <NUM> according to Embodiments <NUM> and <NUM>.

Note that in the example described in each embodiment, second electrode <NUM> is connected to two terminals, i.e., first terminal <NUM> and second terminal <NUM>, but the total number of terminals connected to second electrode <NUM> is not limited to two, and may be three or more. When second electrode <NUM> has three or more terminals, it is sufficient so long as at least one of the three or more terminals is equivalent to first terminal <NUM> and at least one of the three or more terminals is equivalent to second terminal <NUM>.

Although the hydrogen detection method, and hydrogen detection device according to one or more aspects have been described based on embodiments, the present invention is not limited to such embodiments. Various modifications to the embodiments that are conceivable to those skilled in the art, as well as forms resulting from combinations of constituent elements of different embodiments may be included within the scope as defined by the independent claims.

Claim 1:
A hydrogen detection device (<NUM>) comprising:
a first electrode (<NUM>) which is planar;
a second electrode (<NUM>) which is planar, faces the first electrode (<NUM>);
a metal oxide layer (<NUM>) which is sandwiched between a surface of the first electrode (<NUM>) and a surface of the second electrode (<NUM>) facing each other, and has a resistance that changes due to hydrogen;
two terminals (<NUM>, <NUM>) connected to the second electrode (<NUM>); and
a drive circuit (<NUM>);
characterized in that
the second electrode (<NUM>) further comprises an exposed portion (106e) being interposed between the two terminals (<NUM>, <NUM>) connected to the second electrode (<NUM>) in plan view of the second electrode (<NUM>) and
the drive circuit (<NUM>) that, in a state of passing a current through the exposed portion (106e) by applying a voltage between the two terminals (<NUM>, <NUM>), detects a gas containing hydrogen atoms by detecting a decrease in a resistance value (Rh) between the two terminals (<NUM>, <NUM>).