JP2010135541A - Variable resistive element and method of manufacturing same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable resistive element which shows excellent endurance characteristics and high process resistance through easy processes, and to provide a method of manufacturing the same. <P>SOLUTION: The variable resistive element has a variable resistor 13 sandwiched between a first electrode 12 and a second electrode 14 and varies in electric resistance between both the electrodes since a voltage pulse is applied between both the electrodes, wherein the variable resistor 13 is made of oxide of a material containing transition metal and one or both of the first electrode 12 and second electrode 14 are composed of a metal oxide electrode made of metal oxide. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a variable resistance element whose resistance characteristics change when a pulse voltage is applied to both ends, and a manufacturing method thereof.

  A non-volatile memory that can retain recorded information even when the power is turned off, such as a flash memory, FeRAM, or MRAM, can be used as a recording medium for music, moving images, and sentences. Also, in recent years, the resistance value can be reduced by applying voltage or current such as PCRAM using chalcogenide or the like, RRAM using metal oxide ("RRAM" is a registered trademark of the present applicant Sharp), or CBRAM using metal sulfide. Development of a nonvolatile variable resistance element using a variable resistor that changes is underway (see, for example, Patent Document 1 and Non-Patent Document 1). In Patent Document 1, an example using tantalum oxide as a variable resistor is disclosed.

  These elements are characterized in that both rewriting and reading operations are performed by applying voltage pulses. The rewriting operation is performed, for example, by changing the resistance value by applying a pulse voltage of about 2 to 4 V to the element. The read operation is performed by applying a pulse voltage of about 1 V, for example, and detecting the magnitude of the current at the time of application.

International Publication 2008/126365 Pamphlet W.W.Zhuang et al., “Novell Colossal Magnetoresistive Thin Film Nonvolatile Resistance Random Access Memory (RRAM)”, IEDM Technical Digest, pp.193-196, December 2002 P. Knauth et al., “Electrical and defect thermodynamic properties of nanocrystalline titanium dioxide.”, Journal of Applied Physics Vol. 85, Num. 2 pp897-902, January 1999. Sung-Dong Cho et al., “Study on the amorphous Ta2O5 thin film capacitors deposited by dc magnetron reactive sputtering for multichip module applications”, Materials Science and Engineering, B67 (1999), 108-112, 1999.

  However, the variable resistance element has a problem of the number of rewrites. For example, when “tantalum oxide” as disclosed in Patent Document 1 is used as a variable resistor, the resistance value becomes unstable by rewriting hundreds to thousands of times, and the resistance is increased. There is a problem of endurance that the resistance is not increased to a desired resistance value (for example, at the time of erasing processing). This problem is also apparent in FIG. 4B described later.

  An object of the present invention is to provide a variable resistance element that exhibits good endurance characteristics with an easy process and has high process resistance, and a method of manufacturing the same.

  In order to achieve the above object, a variable resistance element according to the present invention includes a variable resistor sandwiched between a first electrode and a second electrode, and a voltage pulse applied between the electrodes to A variable resistance element in which an electric resistance between electrodes changes, wherein the variable resistor is made of an oxide of a material containing a transition metal, and one or both of the first electrode and the second electrode are It is characterized by comprising a metal oxide electrode made of a metal oxide.

  In a variable resistor composed of an oxide of a material containing a transition metal, the oxygen content fluctuates by the application of a voltage pulse in the resistance change region of the current path formed in the variable resistor, and this is a resistance change. It is thought to appear. In particular, when the resistance is increased, it is considered that the oxygen content in the variable resistor increases due to anodic oxidation in the vicinity of the electrode to which a positive voltage is applied, thereby increasing the resistance of the variable resistor.

  At this time, repeated rewriting causes oxygen in the vicinity of the resistance change region to diffuse and the amount of oxygen decreases, and as a result, it is not possible to secure a sufficient amount of oxygen to cause high resistance. It will not progress.

  Therefore, when one or both of the first electrode and the second electrode in contact with the variable resistor is made to contain oxygen as a metal oxide electrode, this electrode can also serve as an oxygen supply source to the variable resistor. Become. That is, as a result of repeated rewrite operations, even if oxygen in the vicinity of the resistance change region diffuses, oxygen is supplemented from the metal oxide electrode. As a result, the variable resistor is required to achieve high resistance. The amount of oxygen can be secured. Thereby, the endurance characteristics are improved as compared with the conventional configuration.

  In addition, according to the present invention, one or both of the electrodes in contact with the variable resistor need only be made of a metal oxide, so that the variable resistor exhibiting high endurance characteristics by a normal oxide film formation process or an oxidation process. An element can be manufactured. That is, when a variable resistance element exhibiting such a high endurance characteristic is manufactured, it does not require a specific process, so that it has high versatility and does not cause a significant increase in manufacturing cost.

  In the above configuration, an auxiliary electrode made of a metal material may be further provided in contact with the surface of the metal oxide electrode on the side not in contact with the variable resistor.

  In the above structure, the metal oxide electrode may contain excessive oxygen as compared to the stoichiometric composition of the metal oxide that is stable at room temperature.

  By doing in this way, it is possible to secure a sufficient amount of oxygen necessary for the variable resistor to transition to the high resistance state.

  The metal oxide electrode may be an oxide of a material containing at least one element of Co, Ta, Ti, Ni, Cu, and W.

  The variable resistor may be an oxide of a material containing at least one element of Ta and Ti.

  Furthermore, in the variable resistance element of the present invention, a variable resistor is sandwiched between the first electrode and the second electrode, and a voltage pulse is applied between the two electrodes, whereby the electric resistance between the two electrodes is increased. A variable resistance element that changes, wherein the variable resistor is made of an oxide of a material containing a transition metal, and the oxidation is performed in a region close to the first electrode and a region close to the second electrode. It is characterized in that the composition ratio of the products is different.

  The variable resistance element manufacturing method of the present invention includes a step of forming a metal material on a predetermined substrate to form the first electrode, and a material containing a transition metal so as to be in contact with the first electrode. Forming the variable resistor by forming a film of the oxide, and forming the metal oxide at a higher oxygen content concentration than the variable resistor so as to be in contact with the variable resistor. And forming the second electrode composed of the metal oxide electrode having a resistance value lower than that of the variable resistor in a low resistance state.

  The variable resistance element manufacturing method of the present invention includes a step of forming the first electrode composed of the metal oxide electrode by forming the metal oxide on a predetermined substrate, and the first electrode. The variable value having a resistance value higher than that of the first electrode in a low resistance state is formed by forming an oxide of a material containing a transition metal at a lower oxygen content concentration than the first electrode so as to be in contact with the first electrode. A step of forming a resistor, and a step of forming the second electrode by forming a metal material so as to be in contact with the variable resistor. Here, as the predetermined substrate, for example, a substrate in which an insulating film is formed on a silicon substrate can be used.

  At this time, the first electrode may be formed by forming a metal material on the predetermined substrate to form an auxiliary electrode, and then forming the metal oxide on the auxiliary electrode.

  In addition to the above characteristics, the metal oxide film may be formed by a sputtering method in an atmosphere containing at least oxygen.

  The variable resistance element manufacturing method of the present invention includes a step of forming a metal material on a predetermined substrate, and oxidizing the upper surface of the formed metal material film to at least the upper surface of the metal material film. Forming the first electrode composed of the metal oxide electrode, and an oxide of a material containing a transition metal with a lower oxygen concentration than the first electrode so as to be in contact with the first electrode Forming the variable resistor having a resistance value higher than that of the first electrode, and forming the second electrode by forming a metal material so as to be in contact with the variable resistor. And a step of performing.

  At this time, the first electrode may be formed by oxidizing the upper surface of the metal material film in an oxygen radical atmosphere.

  In addition to the above characteristics, the metal material film is formed of a metal material containing at least one element of Co, Ta, Ti, Ni, Cu, and W.

  The variable resistance element manufacturing method of the present invention includes a step of forming a metal material on a predetermined substrate to form the first electrode, and intermittently adjusting the oxygen concentration so as to contact the metal material. Forming the variable resistor by forming an oxide of the transition metal-containing material while being raised, and forming the second electrode by forming a metal material so as to be in contact with the variable resistor And a step of forming.

  At this time, the variable resistor may be formed by forming an oxide of a material containing the transition metal by sputtering while intermittently increasing the oxygen partial pressure in an atmosphere containing oxygen.

  In addition to the above characteristics, the oxide of the material containing the transition metal can be an oxide of Ta or Ti.

  According to the configuration of the present invention, it is possible to realize a variable resistance element that can be stably rewritten even if a sufficient number of times of rewriting is performed, and furthermore, it can be realized by an easy process.

  Embodiments of a variable resistance element according to the present invention will be described with reference to the drawings. In addition, each figure shown below is shown typically and the dimension ratio on drawing differs from an actual dimension ratio. In particular, in order to facilitate the understanding of the present invention, the length in the film thickness direction may be emphasized and shown in some cases.

[First Embodiment]
FIG. 1 is a cross-sectional view showing a schematic configuration of a variable resistance element according to the first embodiment. The variable resistance element 1 shown in FIG. 1 includes a semiconductor substrate 10, a silicon oxide film 11, a first electrode 12, a variable resistor 13, and a second electrode 14.

  The manufacturing method of the variable resistance element 1 shown in FIG. 1 is as follows.

  First, after a silicon oxide film 11 is formed on the semiconductor substrate 10, an electrode material film constituting the first electrode 12 is formed. Here, an electrode material film composed of a laminated film of a Ti film (12a) and a TiN film (12b) is formed.

  Next, a metal oxide constituting the variable resistor 13, for example, tantalum oxide, is formed to a thickness of about 14 nm by reactive sputtering. This sputtering process is performed in a mixed atmosphere of argon and oxygen using metal tantalum as a target. It has been conventionally known that the resistance value of tantalum oxide or titanium oxide strongly depends on the oxygen partial pressure during film formation (see, for example, Non-Patent Documents 2 and 3 above). In the present embodiment, the oxygen partial pressure is adjusted so that the initial resistance of the created element has a relatively high resistance value of about 1 MΩ to 100 MΩ. This is because if the initial resistance value of tantalum oxide is too low, the switching operation (reversible change in resistance value of the variable resistance element) itself can be performed, but the current generated during rewriting becomes large. This is because it is not so preferable.

  Next, an electrode material film constituting the second electrode 14 is formed on the variable resistor 13. In this embodiment, cobalt oxide, which is a metal oxide, is used as an electrode material, and a film is formed by reactive sputtering. In addition, in the following drawings including FIG. 1, it is shown that the electrode (the first electrode 12 in FIG. 1) painted with a light color is formed of a metal material, and is painted with a dark color. It shows that the electrode (second electrode 14 in FIG. 1) is formed of a metal oxide.

  FIG. 2 shows the result of evaluating cobalt oxide using an evaluation element for resistance confirmation, which is graphed with the oxygen flow rate during sputtering as the horizontal axis and the resistance value as the vertical axis. Referring to FIG. 2, the resistance value shows a peak when the oxygen flow rate is about 7% (see K1 in the graph), and the resistance value decreases when the oxygen flow rate exceeds that. This indicates that the cobalt oxide film is in a state close to the stoichiometric CoO in the vicinity of the peak resistance value. This suggests that the resistance value of cobalt oxide also strongly depends on the oxygen partial pressure during film formation, as in tantalum oxide and titanium oxide.

  In the present embodiment, based on the above points, the oxygen partial pressure is adjusted so that the resistance of the second electrode 14 itself is about 10 kΩ or less, that is, a sufficiently small resistance value compared to the initial resistance of the element. As an example, about 12% of oxygen is introduced at the time of film formation of the cobalt oxide constituting the second electrode 14, and the oxygen flow rate is adjusted so as to have excess oxygen as compared with the stoichiometric composition.

  Next, the second electrode 14 and the variable resistor 13 are collectively processed using a known photolithography method and dry etching method. Thereby, the configuration shown in FIG. 1 is obtained.

  With respect to the variable resistance element 1 manufactured in this way, a measurement needle was brought into contact with each of the first electrode 12 and the second electrode 14 using a commercially available manual prober, and the electrical characteristics were evaluated. As a method for evaluating electrical characteristics, a voltage pulse is applied to both ends of the first electrode 12 and the second electrode 14 using a commercially available pulse generator, and then the wiring is switched and the resistance value of the element is determined using a commercially available semiconductor parameter analyzer. It was measured. As the applied voltage pulse at the time of rewriting, a pulse width of 20 ns and a voltage value of 2 V (absolute value) was used, and positive and negative pulse voltages were alternately applied. In reading, a voltage of 0.5 V was applied to both ends, and a resistance value was derived from the read current value.

  FIG. 3 shows a comparison result of the endurance characteristics of the variable resistance element according to the present embodiment and the conventional variable resistance element, and is graphed with the number of rewrites as the horizontal axis and the resistance value as the vertical axis. The vertical axis and the horizontal axis are logarithmically displayed. In FIG. 3, (a) is the variable resistance element of this embodiment comprised using the cobalt oxide as the 2nd electrode 14, (b) is the conventional variable comprised using the metal material (cobalt) as the 2nd electrode. The characteristic of a resistance element is shown. In addition, the conventional variable resistance element as a comparison object whose characteristics are shown in FIG. 3B has a configuration in which only the material of the second electrode 14 is different from the variable resistance element 1 of the present embodiment. .

  FIG. 3 is a graph showing resistance values read out by performing a read operation after applying a pulse voltage to the variable resistance element a plurality of times in association with the number of times of applying the pulse voltage. As shown in FIG. 3, the variable resistance element is an element that can take a high resistance state and a low resistance state, and has a characteristic of transitioning between the two resistance states when a pulse voltage is applied. When obtaining endurance characteristics, it takes a very long time to perform a read operation every time a rewrite operation is performed. Therefore, the resistance value is not read every time the pulse voltage is applied, and the resistance value is appropriately read after repeatedly applying the pulse voltage a plurality of times.

When a cobalt electrode is employed as the second electrode 14 as shown in FIG. 3B, it is indicated that the resistance cannot be sufficiently increased when the number of rewrites exceeds a certain number (10 ⁷ times) (in the figure). (See region K2). This suggests that when the high resistance is assigned to the erasing operation, the configuration using the cobalt electrode as the second electrode 14 cannot correctly perform the erasing process when the number of times of rewriting exceeds a certain number. It is.

In contrast, as shown in FIG. 3 (a), in the present embodiment employing the cobalt oxide electrode as the second electrode 14, the number of times of rewriting 10 ¹¹ times the still high resistance state and the low resistance state beyond It is possible to recognize two states with a sufficient resistance difference. This means that even exceed 10 ¹¹ times the number of rewrites, noted, is intended to mean that correctly executes the writing process and erasing process. That is, the endurance characteristic is remarkably improved according to the configuration of the present embodiment using the cobalt oxide electrode as compared with the case where the cobalt electrode is used as the second electrode 14.

  4 to 6 are graphs in which the material used for the second electrode 14 is different from that in FIG. 3 and the read resistance value is related to the number of times of application of the pulse voltage, as in FIG. It is.

  In FIG. 4, as a configuration of the present embodiment, a configuration using a tantalum oxide electrode for the second electrode 14 is shown in FIG. 4A, and a configuration using a tantalum electrode as a comparison target is shown in FIG. In FIG. 5, as a configuration of the present embodiment, a configuration using a titanium oxide electrode as the second electrode 14 is shown in FIG. 5A, and a configuration using a titanium electrode as a comparison target is shown in FIG. In FIG. 6, as the configuration of this embodiment, a configuration using a nickel oxide electrode for the second electrode 14 is shown in FIG. 6A, and a configuration using a nickel electrode as a comparison target is shown in FIG.

  4A, 5 </ b> A, and 6 </ b> A are different in the material of the second electrode 14 from the case where the cobalt oxide electrode is used for the second electrode 14. The other methods are manufactured in the same way. That is, when the electrode film (tantalum oxide electrode, titanium oxide electrode, nickel oxide electrode) constituting the second electrode 14 is formed, the oxygen flow rate is set so as to have excess oxygen compared to the stoichiometric composition. Is adjusted to form a film. Note that, similarly to tantalum oxide, titanium oxide, or cobalt oxide, the resistance value of nickel oxide is considered to depend on the oxygen partial pressure during film formation.

According to FIG. 4B, FIG. 5B, and FIG. 6B, in the case of the conventional configuration in which a tantalum electrode, a titanium electrode, and a nickel electrode are employed as the second electrode 14, the number of rewrites is a fixed number (10 ^If it exceeds ³ to 10 ⁵ times, it is indicated that the resistance cannot be sufficiently increased (see regions K3, K4, and K5 in the figure). On the other hand, according to FIGS. 4 (a), 5 (a), and 6 (a), the second electrode 14 employs a tantalum oxide electrode, a titanium oxide electrode, and a nickel oxide electrode, respectively. If the form of cobalt as in the case of oxide electrode, can be the number of times of rewriting is two states of a low resistance state still a high resistance state more than 10 ¹¹ times, to recognize a state having a sufficient resistance difference It is.

  According to the above, by adopting a metal oxide electrode containing excessive oxygen instead of the metal electrode as the second electrode 14, the endurance characteristics are greatly increased regardless of the material constituting the metal oxide electrode. I understand. In other words, compared to the conventional configuration, this can be realized simply by configuring the material of the second electrode 14 with a metal oxide, and therefore, a process unique to the present invention is not separately required. It has the merit that it can be realized by a conventional manufacturing process.

  As shown in FIGS. 3 to 6, when a metal electrode is used as the second electrode 14, it becomes difficult to increase the resistance when the number of rewrites is increased. On the other hand, such a situation is caused by using the metal oxide electrode. As a reason why it does not occur, the present inventor has come up with the following idea.

  It is known that the resistance value of a variable resistor made of a metal oxide, such as an oxide of titanium or tantalum, varies greatly depending on the composition ratio of oxygen.

  In the variable resistance element 1 using these materials as the variable resistor 13, the oxygen content varies in the resistance change region of the current path in the variable resistor 13 due to the application of the voltage pulse, and this appears as a resistance change. it is conceivable that. In particular, when the resistance is increased, it is considered that the oxygen content in the variable resistor 13 is increased by anodic oxidation in the vicinity of the electrode to which a positive voltage is applied, thereby increasing the resistance of the variable resistor 13.

  At this time, repeated rewriting causes oxygen in the vicinity of the resistance change region to diffuse and the amount of oxygen decreases, and as a result, it is not possible to secure a sufficient amount of oxygen to cause high resistance. It will not progress.

Therefore, as in this embodiment, when the second electrode 14 in contact with the variable resistor 13 contains oxygen, the second electrode 14 also serves as an oxygen supply source to the variable resistor 13. . That is, as a result of repeated rewrite operations, even if oxygen in the vicinity of the resistance change region diffuses, oxygen is supplemented from within the metal oxide constituting the second electrode 14, so that the variable resistor 13 has a high resistance. It is possible to secure the amount of oxygen necessary to realize the conversion. As a result, in the conventional configuration, the increase in resistance could not be correctly performed with the number of rewrites of about 10 ³ to 10 ⁷ times. However, in the configuration of the present embodiment, the increase in resistance can be realized even after exceeding 10 ¹¹ times. It is thought that.

  FIG. 7 shows a comparison result of heat resistance between the variable resistance element in the present embodiment and the conventional variable resistance element. In FIG. 7, as an example, the sample of this embodiment manufactured using a tantalum oxide electrode as the second electrode 14 and the sample of the conventional configuration manufactured using a tantalum electrode are each heated at 400 ° C. for 10 minutes. The resistance value measured after returning to room temperature is compared with the resistance value before heating.

  According to FIG. 7, in the case of a sample having a conventional configuration using a tantalum electrode as the second electrode 14, the resistance value is greatly reduced by heating. In contrast, in the case of the sample of the present embodiment using a tantalum oxide electrode, it can be seen that the resistance value hardly fluctuates before and after heating and is stable. According to this, it was found that using a tantalum oxide electrode as the second electrode 14 has an effect of improving not only the endurance characteristics but also the heat resistance.

  Although not shown here, when the second electrode 14 is formed using other metal oxides (cobalt oxide, titanium oxide, nickel oxide), the heat resistance is the same as in the case of tantalum oxide. It has been found that there is an effect of improving.

  Moreover, in this embodiment mentioned above, although the tantalum oxide was utilized as the variable resistor 13, it is not limited to this, It is good also as an oxide or oxynitride of other transition metals, such as titanium. The same applies to the following embodiments.

  Furthermore, in the present embodiment, a laminated film of Ti and TiN is used as the electrode on the side that does not constitute the metal oxide electrode (here, the first electrode 12). However, the present invention is not limited to this, and is usually used as an electrode. Metals (Pt, Ta, W, Cu, Ti, etc.) may be used. The same applies to the following embodiments.

[Second Embodiment]
FIG. 8 is a cross-sectional view showing a schematic configuration of a variable resistance element according to the second embodiment (hereinafter, appropriately referred to as “this embodiment”). The variable resistance element 2 shown in FIG. 8 includes a semiconductor substrate 10, a silicon oxide film 11, a first electrode 21, a variable resistor 13, and a second electrode 22. In addition, the same code | symbol is attached | subjected about the component same as 1st Embodiment.

  The manufacturing method of the variable resistance element 2 shown in FIG. 8 is as follows.

First, after forming the silicon oxide film 11 on the semiconductor substrate 10, an electrode material film constituting the first electrode 21 is formed. Here, an electrode material film composed of a laminated film (50 nm / 10 nm) of a titanium film (21a) and a titanium oxide film (21b) was formed by sputtering. Note that when the titanium oxide film 21b is formed, the resistance value of titanium oxide changes greatly depending on the oxygen content ratio. Therefore, the oxygen flow rate is appropriately adjusted during reactive sputtering, and the film is formed thereafter. The resistance value is sufficiently smaller than the resistance value in the low resistance state of the variable resistor film. In the case of titanium, TiO ₂ is a stable oxide, but low resistance titanium oxide can be easily manufactured by reducing the oxygen ratio.

Next, on the first electrode 21 (titanium oxide film 21b constituting the upper surface thereof), a metal oxide constituting the variable resistor 13 (in this embodiment, titanium oxide) is formed to 10 nm by reactive sputtering. Film. In the titanium oxide film formation step here, the oxygen flow rate is adjusted so that the composition ratio is substantially TiO ₂ . Also in the titanium oxide, like the cobalt oxide shown in FIG. 2, the resistance value shows a peak when the oxygen flow rate during sputtering is a predetermined value. For this reason, in this film-forming process, a titanium oxide film is formed at a value close to the oxygen flow rate when the peak value is shown. As a result, even in the low resistance state, the resistance value of the variable resistor 13 is significantly larger than that of the titanium oxide film 21 b constituting the first electrode 21.

  Thereafter, an electrode material film constituting the second electrode 22 is formed on the variable resistor 13. In this embodiment, a titanium electrode is used as the electrode material. Then, the second electrode 22 and the variable resistor 13 are collectively processed using a known photolithography method and dry etching method. Thereby, the configuration shown in FIG. 8 is obtained.

  FIG. 9 shows the endurance characteristics of the variable resistance element according to the present embodiment, which is graphed with the number of rewrites as the horizontal axis and the resistance value as the vertical axis. The vertical axis and the horizontal axis are logarithmically displayed.

  When obtaining the endurance characteristics of FIG. 9, as in the first embodiment, a measurement needle is brought into contact with each of the first electrode 21 and the second electrode 22 using a commercially available manual prober on the created element. Characterization was performed. As a characteristic evaluation method, a voltage pulse is applied to both ends of the first electrode 21 and the second electrode 22 using a commercially available pulse generator, then the wiring is switched and the resistance value of the element is measured using a commercially available semiconductor parameter analyzer. did. The step of applying a pulse voltage with a pulse width of 20 ns and +2 V to the first electrode 21 side and the step of applying a pulse voltage of a pulse width of 30 ns and +3 V to the second electrode 22 side were executed alternately. In reading, a voltage of 0.5 V was applied to both ends, and a resistance value was derived from the read current value.

  Also in this embodiment, as in the first embodiment, the resistance value is not read every time the pulse voltage is applied, and the resistance value is appropriately set after repeatedly applying the pulse voltage a plurality of times. Is being read.

According to FIG. 9, and FIG. 5 showing a characteristic of the first embodiment in having the same titanium oxide electrode (a) Similarly, the number of times of rewriting 10 ¹¹ times the still high resistance state and the low resistance state beyond It is possible to recognize two states with a sufficient resistance difference. FIG. 5A shows endurance characteristics when the first electrode 12 constituting the lower electrode is a metal electrode and the second electrode 14 constituting the upper electrode is a titanium oxide electrode. On the other hand, in FIG. 9, in the first electrode 21 constituting the lower electrode, the region (21b) in contact with the variable resistor 13 is a titanium oxide electrode, and the second electrode 22 constituting the upper electrode is a metal electrode. Endurance characteristics.

  As a result, regardless of whether the variable resistor 13 is above or below the variable resistor 13, the electrode contacting the variable resistor 13 is a titanium oxide electrode, so that the first and second electrodes are made of metal electrodes. It can also be seen that the endurance characteristics are significantly improved. In the present embodiment, the results are shown only when a part (21b) of the first electrode 21 is a titanium oxide electrode. However, as in the first embodiment, other metal oxides (cobalt oxides) are used. , Tantalum oxide, nickel oxide) have the same effect.

  Also in the present embodiment, the material of the variable resistor 13 and the electrode material on the side not constituting the metal oxide electrode can be appropriately selected as in the first embodiment.

[Third Embodiment]
FIG. 10 is a cross-sectional view showing a schematic configuration of a variable resistance element according to a third embodiment (hereinafter referred to as “this embodiment” as appropriate). A variable resistance element 3 shown in FIG. 10 includes a semiconductor substrate 10, a silicon oxide film 11, a first electrode 31, a variable resistor 13, a second electrode 32, and a base electrode 35. In addition, the same code | symbol is attached | subjected about the component same as 1st or 2nd embodiment.

  The manufacturing method of the variable resistance element 3 shown in FIG. 10 is as follows.

  First, after the silicon oxide film 11 is formed on the semiconductor substrate 10, a titanium film is formed as a base electrode 35 to a thickness of about 10 nm by sputtering. Next, a metal material film (here, referred to as a cobalt film 31) constituting a part of the first electrode 31 is formed with a film thickness of about 50 nm by a sputtering method.

  Next, the surface of the cobalt film 31 is oxidized by a radical oxidation method. Specifically, the oxidation temperature is set to about 400 ° C. to 600 ° C., and the oxidation treatment is performed for about 5 to 15 minutes in an oxygen radical atmosphere. Thereby, a part of the upper surface of the cobalt film 31 is oxidized to form a cobalt oxide film 31a. Note that the unoxidized cobalt film 31b remains below the cobalt oxide film 31a, and the first electrode 31 is constituted by the cobalt film 31b and the cobalt oxide film 31a.

  Next, a metal oxide (here, tantalum oxide) is formed as a variable resistor 13 on the first electrode 31 by a reactive sputtering method to a thickness of about 14 nm. Thereafter, an electrode material film constituting the second electrode 32 is formed on the variable resistor 13. In this embodiment, a tantalum electrode is used as the electrode material. Then, the second electrode 32 and the variable resistor 13 are collectively processed using a known photolithography method and dry etching method. Thereby, the configuration shown in FIG. 10 is obtained.

  FIG. 11 shows endurance characteristics of the variable resistance element according to the present embodiment, which is graphed with the number of rewrites as the horizontal axis and the resistance value as the vertical axis. The vertical axis and the horizontal axis are logarithmically displayed.

  In order to obtain the endurance characteristic of FIG. 11, as in the first and second embodiments, the measurement needle is brought into contact with each of the first electrode 31 and the second electrode 32 using a commercially available manual prober. The characteristics were evaluated. As a characteristic evaluation method, after applying a voltage pulse to both ends of the first electrode 31 and the second electrode 32 using a commercially available pulse generator, the wiring is switched and the resistance value of the element is measured using a commercially available semiconductor parameter analyzer. did. The step of applying a pulse voltage with a pulse width of 20 ns and +2 V to +3 V on the first electrode 31 side and the step of applying a pulse voltage with a pulse width of 30 ns and +2 V to +3 V on the second electrode 32 side are repeated alternately. And executed. In reading, a voltage of 0.5 V was applied to both ends, and a resistance value was derived from the read current value.

  Also in this embodiment, as in the first and second embodiments, the resistance value is not read every time the pulse voltage is applied, and after the pulse voltage is repeatedly applied a plurality of times, The resistance value is read as appropriate.

According to FIG. 11, as in FIG. 3A showing the characteristics of the first embodiment when the same cobalt oxide electrode is provided, the high resistance state and the low resistance state are still maintained even when the number of rewrites exceeds 10 ¹¹ times. It is possible to recognize two states with a sufficient resistance difference. In the first embodiment, the metal oxide electrode is formed by forming an oxide film. However, according to the present embodiment, even when the surface of the metal material is oxidized to form the oxide electrode, It can be seen that there is an effect of improving the endurance characteristics.

  FIGS. 12 and 13 are graphs showing characteristics when the same evaluation is performed by changing the metal material constituting a part of the first electrode 31 under the same manufacturing method as in the present embodiment. The characteristics when tantalum, titanium, and nickel are used as the first electrode 31 are shown in FIGS. 12A, 12B, and 12C, respectively, and the characteristics when copper and tungsten are used are shown in FIG. ) And (b). Note that the characteristic evaluation method for each of these graphs is the same as in FIG.

  In the case of forming a film of tantalum, titanium, nickel, copper, or tungsten as a metal material constituting a part of the first electrode 31, an oxidization treatment is performed thereafter, so that the upper surface of the metal material has a tantalum oxide, Titanium oxide, nickel oxide, copper oxide, and tungsten oxide are formed. That is, each of these oxides constitutes the first electrode 31 a that is in contact with the variable resistor 13.

Figure 12 Referring to Figure 13, a high resistance state, the respective low-resistance state, although some resistance variation seen, and still resistance value of the high resistance state even after the ¹⁰¹¹ times of rewrite operation It can be seen that the resistance values in the low resistance state are different enough to be sufficiently separated and have good characteristics.

  From these results, in the same way as when forming an electrode by forming a metal oxide film, even when a part of the electrode is composed of a metal oxide film by oxidizing the surface of the electrode on the variable resistor side, the endurance The characteristics can be improved.

  In the first to third embodiments described above, one of the first electrode and the second electrode is formed of a metal oxide in a region in contact with the variable resistor, and the other electrode is a metal. It was supposed to be formed. On the other hand, both electrodes may be formed of a metal oxide in a region in contact with the variable resistor.

[Fourth Embodiment]
FIG. 14 is a cross-sectional view showing a schematic configuration of a variable resistance element according to a fourth embodiment (hereinafter referred to as “this embodiment” as appropriate). The variable resistance element 4 shown in FIG. 14 includes a semiconductor substrate 10, a silicon oxide film 11, a first electrode 12, a variable resistor 41, and a second electrode 22. In addition, the same code | symbol is attached | subjected about the component same as 1st-3rd embodiment.

  The manufacturing method of the variable resistance element 4 shown in FIG. 14 is as follows.

  First, after a silicon oxide film 11 is formed on the semiconductor substrate 10, an electrode material film constituting the first electrode 12 is formed. Here, an electrode material film composed of a laminated film of a Ti film (12a) and a TiN film (12b) is formed.

  Next, a metal oxide constituting the variable resistor 41, for example, tantalum oxide, is formed to a thickness of about 14 nm by reactive sputtering. In this sputtering step, sputtering is performed in a mixed atmosphere of argon and oxygen using metal tantalum as a target, as in the first embodiment. In this embodiment, unlike the first embodiment, the oxygen content in the tantalum oxide is changed in the film forming direction by changing the oxygen partial pressure during film formation. For example, the first 1 minute and 12 seconds from the start of film formation of tantalum oxide, a film of about 10 nm is formed with an oxygen partial pressure of 20% (41a), and then a film of 2 minutes with an oxygen partial pressure of 42% A film of about 17 nm is formed (41b). As a result, a tantalum oxide film 41 having a total film thickness of about 27 nm with different oxygen contents is formed on the first electrode 12 side and the second electrode 14 side to form a variable resistor.

  Next, an electrode material film constituting the second electrode 22 is formed on the variable resistor 41 (on 41b). In this embodiment, a titanium electrode is used as the electrode material. Then, the second electrode 22 and the variable resistor 41 are collectively processed using a known photolithography method and dry etching method. Thereby, the configuration shown in FIG. 14 is obtained.

  FIG. 15 shows the endurance characteristics of the variable resistance element according to the present embodiment, which is graphed with the number of rewrites as the horizontal axis and the resistance value as the vertical axis. The vertical axis and the horizontal axis are logarithmically displayed.

  When obtaining the endurance characteristics shown in FIG. 15, as in the first to third embodiments, the measuring element is contacted with each of the first electrode 12 and the second electrode 22 using a commercially available manual prober. The characteristics were evaluated. As a characteristic evaluation method, after applying a voltage pulse to both ends of the first electrode 12 and the second electrode 22 using a commercially available pulse generator, the wiring is switched and the resistance value of the element is measured using a commercially available semiconductor parameter analyzer. did. The step of applying a pulse voltage with a pulse width of 30 ns and +2.4 to the first electrode 12 side and the step of applying a pulse voltage of a pulse width of 20 ns and +1.8 V to the second electrode 22 side are alternately repeated. And executed. In reading, a voltage of 0.5 V was applied to both ends, and a resistance value was derived from the read current value.

  Also in the present embodiment, as in the first to third embodiments, the resistance value is not read every time the pulse voltage is applied, and after the pulse voltage is repeatedly applied a plurality of times, as appropriate. The resistance value is read.

According to FIG. 15, although some resistance fluctuation is observed in each of the high resistance state and the low resistance state, the resistance value in the high resistance state and the low resistance state are still maintained after 10 ¹⁰ rewrite operations. It can be seen that the resistance values are different enough to be sufficiently separated and have good characteristics. The variable resistance element 4 in the present embodiment is made of a metal material for both the first and second electrodes. The endurance characteristics of the variable resistance element of the conventional configuration in which both the first and second electrodes are made of a metal material (FIG. 3 (b), FIG. 4 (b), FIG. 5 (b), FIG. 6 (b) )), It can be seen that the characteristics are remarkably improved.

  The difference between the present embodiment and the conventional configuration is the variable resistor film forming method. That is, the present embodiment is characterized in that a region (41b) having a high oxygen content is provided in the variable resistor 41. That is, by manufacturing in this way, the oxygen amount necessary for increasing the resistance can still be secured in the variable resistor 41 even after repeated switching operations are performed. Thus, the endurance characteristics are considered to have improved.

  In the above embodiment, when the variable resistor is formed, the metal oxide film is formed so that the oxygen content on the upper surface side is increased, but conversely, the metal is formed so that the oxygen content on the lower surface side is increased. An oxide film may be formed. For example, it is possible to reduce the oxygen content on the upper surface side by reducing the oxygen content by, for example, exposing the surface to a reducing atmosphere after forming a metal oxide film by increasing the oxygen partial pressure in advance.

  In this embodiment, tantalum oxide is used as the material of the variable resistor 41, but other metal oxides can also be used. Also, the first and second electrodes may be made of other metal materials.

Sectional drawing which shows schematic structure of the variable resistance element of 1st Embodiment. Graph showing the relationship between the resistance value of cobalt oxide and the oxygen flow rate ratio during film formation The graph which shows the comparison result of the endurance characteristic of the variable resistance element (cobalt oxide electrode) of 1st Embodiment, and a conventional structure. The graph which shows the comparison result of the endurance characteristic of the variable resistance element (tantalum oxide electrode) of 1st Embodiment, and a conventional structure. The graph which shows the comparison result of the endurance characteristic of the variable resistance element (titanium oxide electrode) of 1st Embodiment, and a conventional structure. The graph which shows the comparison result of the endurance characteristic of the variable resistance element (nickel oxide electrode) of 1st Embodiment, and a conventional structure. The graph which shows the comparison result of the heat resistance of the variable resistance element of 1st Embodiment, and a conventional structure Sectional drawing which shows schematic structure of the variable resistive element of 2nd Embodiment. The graph which shows the endurance characteristic of the variable resistance element (titanium oxide electrode) of 2nd Embodiment Sectional drawing which shows schematic structure of the variable resistance element of 3rd Embodiment The graph which shows the endurance characteristic of the variable resistance element (cobalt oxide electrode) of 3rd Embodiment The graph which shows the endurance characteristic of the variable resistance element (tantalum, titanium, nickel oxide electrode) of 3rd Embodiment The graph which shows the endurance characteristic of the variable resistance element (copper, tungsten oxide electrode) of 3rd Embodiment Sectional drawing which shows schematic structure of the variable resistance element of 4th Embodiment The graph which shows the endurance characteristic of the variable resistance element of 4th Embodiment

Explanation of symbols

1, 2, 3, 4: Variable resistance element 11: Silicon oxide film 12, 21, 31: First electrode 12a, 21a: Ti film 12b: TiN film 21b: Titanium oxide film 13, 41: Variable resistor 14, 22 32: Second electrode 35: Base electrode

Claims (16)

A variable resistance element in which a variable resistor is sandwiched between a first electrode and a second electrode, and a voltage pulse is applied between the two electrodes to change an electrical resistance between the two electrodes;
The variable resistor is made of an oxide of a material containing a transition metal,
One or both of the first electrode and the second electrode are composed of a metal oxide electrode made of a metal oxide.   The variable resistance element according to claim 1, further comprising an auxiliary electrode made of a metal material in contact with the surface of the metal oxide electrode on the side not in contact with the variable resistor.   3. The variable resistance element according to claim 1, wherein the metal oxide electrode contains oxygen in excess as compared with a stoichiometric composition stable at room temperature of the metal oxide.   4. The metal oxide electrode according to claim 1, wherein the metal oxide electrode is an oxide of a material containing at least one element of Co, Ta, Ti, Ni, Cu, and W. 5. The variable resistance element described.   The variable resistance element according to claim 1, wherein the variable resistor is an oxide of a material containing at least one element of Ta and Ti. A variable resistance element in which a variable resistor is sandwiched between a first electrode and a second electrode, and a voltage pulse is applied between the two electrodes to change an electrical resistance between the two electrodes;
The variable resistor is made of an oxide of a material containing a transition metal, and the composition ratio of the oxide is different in a region near the first electrode and a region near the second electrode. A variable resistance element. It is a manufacturing method of the variable resistance element according to claim 1,
Forming a metal material on a predetermined substrate to form the first electrode;
Forming the variable resistor by depositing an oxide of a material containing a transition metal so as to be in contact with the first electrode;
The metal having a lower resistance value than the variable resistor in a low resistance state by forming the metal oxide film with a higher oxygen content concentration than the variable resistor so as to contact the variable resistor Forming the second electrode composed of an oxide electrode. A method of manufacturing a variable resistance element, comprising: It is a manufacturing method of the variable resistance element according to claim 1,
Forming the metal oxide on a predetermined substrate to form the first electrode composed of the metal oxide electrode;
By forming an oxide of a material containing a transition metal at a lower oxygen concentration than the first electrode so as to contact the first electrode, the resistance value is lower than that of the first electrode in a low resistance state. Forming the variable resistor having a high value;
Forming the second electrode by forming a metal material so as to come into contact with the variable resistor.   9. The first electrode is formed by forming a metal material on the predetermined substrate to form an auxiliary electrode, and then forming the metal oxide on the auxiliary electrode. The manufacturing method of the variable resistance element as described in 1 ..   The method for manufacturing a variable resistance element according to claim 7, wherein the metal oxide film is formed by sputtering in an atmosphere containing at least oxygen. It is a manufacturing method of the variable resistance element according to claim 1,
Forming a metal material on a predetermined substrate;
Forming the first electrode composed of the metal oxide electrode on at least the upper surface of the metal material film by oxidizing the upper surface of the formed metal material film;
The variable having a resistance value higher than that of the first electrode by forming an oxide of a material containing a transition metal at a lower oxygen concentration than the first electrode so as to be in contact with the first electrode. Forming a resistor;
Forming the second electrode by forming a metal material so as to come into contact with the variable resistor.   The method of manufacturing a variable resistance element according to claim 11, wherein the first electrode is formed by oxidizing an upper surface of the metal material film in an oxygen radical atmosphere.   The said metal material film is comprised with the metal material containing at least any one element among Co, Ta, Ti, Ni, Cu, and W, The any one of Claims 7-12 characterized by the above-mentioned. Of manufacturing the variable resistance element. It is a manufacturing method of the variable resistance element according to claim 6,
Forming a metal material on a predetermined substrate to form the first electrode;
Forming the variable resistor by depositing an oxide of the material containing the transition metal while intermittently increasing the oxygen concentration so as to contact the metal material;
Forming the second electrode by forming a metal material so as to come into contact with the variable resistor.   15. The variable resistor is formed by depositing an oxide of a material containing the transition metal by sputtering while intermittently increasing an oxygen partial pressure in an atmosphere containing oxygen. Of manufacturing the variable resistance element.   16. The method of manufacturing a variable resistance element according to claim 14, wherein the oxide of the material containing the transition metal is an oxide of Ta or Ti.

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