JP5517065B2 - Switching element and switch array - Google Patents

Description

  The present invention relates to a switching element using a polymer electrolyte membrane that can be used as a resistance random access memory element (RRAM) and a switch array configured using such a switching element.

Conventionally, various proposals have been made on switching elements using electrolyte membranes. For example, Non-Patent Document 1 describes that when a voltage is applied to Cu ₂ S sandwiched between copper and a platinum electrode, the resistance changes due to the movement of copper ions. However, Cu ₂ S has a problem that it is not suitable for a flexible element because it has poor mechanical strength and is brittle.

Non-Patent Document 2 describes that when a voltage is applied to a counter electrode of Ag ₂ S and platinum having a nanometer-order gap, silver is deposited between the electrodes due to an oxidation-reduction reaction, and the resistance changes. ing. However, since Ag ₂ S has the same properties as Cu ₂ S, the problem of lack of flexibility has not been solved.

Furthermore, Non-Patent Document 3 describes that when a voltage is applied in a structure in which both sides of a TiO ₂ film are sandwiched between platinum electrodes, a resistance change is caused by the movement of oxygen ions. However, the problem that the configuration described here lacks flexibility has not been solved.

  The RRAM element using the above-mentioned inorganic material is manufactured through complicated manufacturing processes such as formation of an electrolyte film in a vacuum apparatus, patterning of an electrode by electron beam exposure, and an etching process. I have a problem of mass consumption.

  The subject of this invention is solving the said problem and providing a flexible switching element.

Here, the polymer of the polymer electrolyte material may be polyethylene glycol, polyethylene oxide, polyvinyl pyrrolidone, polyacrylonitrile, polyvinylidene fluoride, or polymethyl methacrylate.
The electrolyte salt of the polymer electrolyte material may be a silver electrolyte salt, a copper electrolyte salt, a magnesium electrolyte salt, an iron electrolyte salt, a nickel electrolyte salt, a cobalt electrolyte salt, or a manganese electrolyte salt.
Further, at least a part of the first electrode may be formed of silver, copper, magnesium, iron, nickel, cobalt, or manganese.
The second electrode may be platinum, gold, iridium, tungsten, or aluminum.
The thickness of the polymer electrolyte material may be 1 μm or less.
Further, by adjusting the concentration of the electrolyte salt of the polymer electrolyte material, the on-voltage that transitions from the high-resistance state to the low-resistance state and the off-voltage that transitions from the low-resistance state to the high-resistance state may be controlled.
Further, this switching element may be volatile so as to maintain a low resistance state even when the applied voltage is zero.
The switching element may be volatile so as not to maintain a low resistance state when the applied voltage is zero.
The switching element may be formed on a plastic substrate such as polyethylene terephthalate, polyethylene naphthalate, or polyimide.
According to another aspect of the present invention, a switch array in which at least two or more switching elements in which at least one of the first electrode and the second electrode is electrically connected to a common line are arranged on a substrate are arranged. Is given.
Here, the substrate may be made of a flexible material.

  According to the present invention, a flexible switching element can be manufactured. In particular, inorganic materials lacked flexibility, the manufacturing process became high temperature, and the manufacturing cost was high. The present invention avoids the disadvantages of this inorganic material, and does not require patterning or etching with a photoresist or the like, and can be produced at low cost by a printing technique such as inkjet.

  Furthermore, since a switch speed of 50 Hz or more can be obtained, it can be applied to a drive circuit for a paper display or a flat panel display.

The conceptual diagram of the structure of the switching element of this invention. The conceptual diagram of the operation mechanism of the switching element of this invention. The conceptual diagram of the structure of the switch array of this invention is shown. The graph which shows an example of the IV curve of the switching element produced in the Example. The graph which shows the salt concentration dependence of the on-voltage and off-voltage of the switching element produced in the Example. The graph which shows an example of on / off ratio of the switching operation produced in the Example. The graph which shows an example of the holding | maintenance tolerance of the ON state of the switching element produced in the Example. The graph which shows an example of the pulse application voltage response of the switching element produced in the Example.

  For example, as shown in FIG. 1, the switching element of the present invention includes an adhesion layer 12, a lower metal electrode 13, a polymer electrolyte membrane 14 having a thickness of 500 nm or less, and an upper metal electrode 15 on a substrate 11 made of glass or plastic. It has a laminated structure. In the example shown in FIG. 1, titanium is used as the adhesion layer 12, platinum is used as the inactive lower metal electrode 13, and silver is used as the electrochemically active metal electrode 15. Here, the electrochemically active metal is a metal that can be dissolved into the polymer electrolyte by an anodic oxidation reaction when a voltage is applied (specifically, silver, copper, magnesium, iron, nickel, cobalt, manganese, etc.) It is. The electrode 15 may contain another metal, or this electrochemically active metal may be provided only on the side of the electrode 15 that contacts the polymer electrolyte membrane 14. The electrochemically inactive metal is a metal that does not dissolve in the polymer electrolyte even when a voltage is applied. Further, when plastic is used as the material of the substrate 11, it is of course not limited to this, but for example, polyethylene terephthalate, polyethylene naphthalate, or polyimide can be used.

In FIG. 1, as the polymer electrolyte membrane 14, a polymer mixed with an electrolyte salt of several percent by weight is used. The manufacturing method of the electrolyte membrane is as follows. First, a required amount of a polymer and an electrolyte salt are mixed in methanol or pure water, and the mixture is stirred with a stirrer for 12 hours or more to prepare an electrolytic solution. After several μl of this electrolytic solution is dropped on the platinum electrode, an electrolyte membrane is formed by heating at 120 ° C. for 12 hours or more in a vacuum dryer. Here, the electrolyte salt is an electrochemically active metal salt used for the metal electrode 15, for example, a silver electrolyte salt such as AgClO ₄ or AgNO ₃ , a copper electrolyte salt such as Cu (ClO ₄ ) ₂ , Magnesium electrolyte salt such as Mg (NO ₃ ) ₂ , iron electrolyte salt such as Fe (NO ₃ ) ₂ , nickel electrolyte salt such as Ni (NO ₃ ) ₂ , cobalt electrolyte salt such as Co (NO ₃ ) ₂ , or Mn Manganese electrolyte salts such as (NO ₃ ) ₂ can be used.
Examples of the polymer that can be used here include, but are not limited to, polyethylene glycol, polyethylene oxide, polyvinyl pyrrolidone, polyacrylonitrile, polyvinylidene fluoride, and polymethyl methacrylate. In general, this polymer preferably satisfies the following conditions.
1) The glass transition temperature is lower than room temperature The ionic conductivity in the polymer is due to the “softness” of the polymer. In other words, ions move through the polymer with thermal vibration. In general, the state of a polymer changes depending on the temperature, and as the temperature rises, it takes a glass (completely solid) state → a state of stretching like a rubber → a liquid state. The first glass → rubber transition is defined as the glass transition temperature, and the rubber → liquid transition is defined as the melting point. Since charges (both ions and electrons) do not move at all in the glass state, the use temperature needs to be higher than the glass transition temperature in order to exhibit ionic conductivity. Conversely, when considering operation at room temperature, the glass transition temperature must be at least lower than room temperature. More preferably, the glass transition temperature should be 0 ° C. or lower. On the other hand, regarding the melting point, it is difficult to become a liquid at the temperature at which it is actually operated.
2) The average molecular weight is 10,000 or more Note that the preferred molecular weight is determined by the coating method of the polymer electrolyte (for example, ink jet printing technology), and therefore generally has to be within a specific value range. I can't say that.
2) Solubility in water and organic solvents To make a polymer film, a polymer and an electrolyte salt are dissolved in a liquid material and dropped onto a substrate. Therefore, it is desirable that the polymer and the electrolyte salt have solubility in water and an organic solvent.

  Next, a method for manufacturing the entire element structure will be described. First, the titanium adhesion layer 12 and the platinum electrode 13 are formed on the substrate 11 by sputtering using a metal mask. Thereafter, the polymer electrolyte membrane 14 is formed by the above method. Finally, the silver electrode 5 is formed by vacuum vapor deposition using a metal mask. The silver electrode can be formed by sputtering, but the device characteristics are deteriorated because the polymer film is damaged. It was also found that when the temperature during electrode formation is high, silver diffuses into the polymer electrolyte membrane, resulting in low initial resistance between the electrodes. As a result of experiments and examinations by the present inventors, it has been found that vacuum deposition or electron beam deposition is best performed while cooling the substrate with cooling water.

  The mode of operation of the switching element of the present invention is shown in FIG. 2 taking a silver / silver ion conductive polymer electrolyte / platinum structure as an example. As shown in FIG. 2A, when a positive voltage is applied to the silver electrode 21, the silver ions (cations) 24 in the electrolyte membrane are transferred to the platinum electrode 22 and the anions 25 are transferred to the silver electrode 21. It moves via thermal vibration of the molecule 23. At this time, silver atoms are ionized by the anodic oxidation reaction at the silver / electrolyte interface, and these silver ions 24 also move to the platinum electrode 22.

  Due to the movement of the silver ions 24, the silver ion concentration on the platinum electrode 22 reaches supersaturation, and a large number of silver heterogeneous nucleation occurs on the platinum electrode 22. The silver nucleus grows preferentially toward the silver electrode 21 while taking in the silver ions 24, and eventually one of the silver nuclei reaches the silver electrode, and as shown in FIG. The filament 27 is formed and the element is in a low resistance state.

  In this low resistance state, when a negative voltage is applied to the silver electrode 21, the surface of the filament 27 is dissolved by an oxidation reaction. As a result, as shown in FIG. 2C, the filament 27 is cut at the thinnest portion, and the element transitions to a high resistance state.

  In this high resistance state, when a positive voltage is applied to the silver electrode 21, silver ions 24 dissolved in the base polymer 23 gather at the tip of the cut filament 27, and the filament is formed by heterogeneous nucleation. It is reconstructed (FIG. 2 (d)).

  Thereafter, when positive and negative voltages are applied, the element takes a low resistance state and a high resistance state while repeating the construction and dissolution of the silver filament 27. In this way, in the present invention, the resistance change between the electrodes can be repeatedly realized by controlling the movement of the metal ions in the polymer electrolyte and the oxidation-reduction reaction by the applied voltage. Conventionally, it has been described in Patent Document 1 that a resistance change occurs in a similar element structure using a solid electrolyte of an inorganic material. However, the solid electrolyte of an inorganic material is not flexible and has limited applications. As described above. It has not been known that a switch operation is performed with a polymer material, and the inventors have discovered it for the first time.

  FIG. 3 shows a configuration example of a switch array which is another embodiment of the present invention. On a plastic substrate 31 such as polyethylene terephthalate, polyethylene naphthalate, or polyimide, a lower metal electrode 32, a polymer electrolyte membrane 33, and an upper metal electrode 34 are formed with a crossbar structure. The upper (row) and lower (column) metal electrodes are shared, and the upper and lower electrodes can be electrically connected by applying a voltage to a specific row and column. In this way, it is possible to easily construct a flexible switch array using the mechanical flexibility of the polymer.

  The present invention will be described more specifically with reference to the following examples. Materials, usage fees, ratios, processing procedures, substrate types, and the like shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Accordingly, the scope of the present invention is not limited to the following examples.

(Element fabrication)
As a polymer electrolyte, an electrolyte solution is prepared in polyethylene oxide (PEO) and silver perchlorate (AgClO ₄ ) at different salt concentrations from 1 to 6% by weight (wt%), and the above method is used to perform the switching having the structure shown in FIG. An element was produced.

(Measurement of current-voltage characteristics)
The current-voltage curve (IV curve) of the produced switching element was measured. Typical results for devices with salt concentrations of 2, 4, 5, and 6 wt% are shown in FIG. FIG. 4A is an IV curve of an element having a salt concentration of 2 wt%. When a positive voltage is applied to the silver electrode, the element changes sharply from a high resistance (off) state to a low resistance (on) state. This low resistance state is maintained even when the applied voltage is returned to 0V. That is, a non-volatile switch operation is shown. Next, when a negative voltage is applied, the state rapidly returns from the on state to the off state. This on-off was confirmed to be repeatable. FIG. 4B is an IV curve with a salt concentration of 4 wt%. The on-voltage that transitions from the off-state to the on-state increases, but the nonvolatile operation is maintained. FIG. 4C is an IV curve with a salt concentration of 5 wt%. The on-voltage further increases, and the element transitions to the off state before the applied voltage returns to 0V. That is, it shows a volatile operation. FIG. 4D is an IV curve with a salt concentration of 6 wt%. At this concentration, switching action no longer takes place.

  FIG. 5 shows the observed on and off voltages as a function of salt concentration. As the salt concentration increases, the on-voltage increases while the off-voltage decreases. It was also found that at 5 wt%, volatile switch operation occurs. This suggests that the on / off voltage value and non-volatile / volatile behavior can be controlled by adjusting the salt concentration.

FIG. 6 shows an example of a resistance-voltage curve of an element having a salt concentration of 3 wt%. It was confirmed that the resistance change due to voltage application, that is, the on / off ratio was as high as about 10 ⁵ .

(Check memory operation)
The memory characteristics of the switching element were measured. After applying a positive voltage to the silver electrode to turn on the device, the duration of this on state was measured. An example of the result is shown in FIG. 0.05 V was applied to the device for 5 seconds at 1 minute intervals, and the current at that time was measured. Otherwise, no voltage is applied to the element. As a result of this measurement, it was confirmed that the ON state was maintained for at least one week when no voltage was applied. Furthermore, it was confirmed that the OFF state also has a retention resistance of one week or longer. These results indicate that the switching element has sufficient characteristics as a memory element.

(Switch speed measurement)
In order to evaluate the switching speed of the switching element, the response current of pulse voltage application was measured. An example of the result is shown in FIG. The write pulse for turning on is + 1V, the erase pulse for turning off is -1V, the read pulse for checking the element state is ± 0.2V, and each pulse width is set to 8 milliseconds. did. The switch speed was confirmed to be at least 0.1 milliseconds or less. This indicates that application to a paper display and flat panel display circuit driven at 50 Hz is also possible.

  As described in detail above, the present invention has been completely attempted to use a polymer as a material for such an electrolyte in a switching element that causes formation and cutting of a metal filament in an electrolyte between electrodes. The present invention provides a switching element having no configuration and a switch array using the switching element. Because of the high flexibility according to the present invention, applications that were impossible with conventional switching elements using inorganic electrolytes are possible. Moreover, since writing can be performed at a low voltage and the writing result lasts for a long time, a wide range of applications such as a paper display and a flat panel display circuit are expected.

11 Substrate 12 Adhesion layers 13 and 32 Lower metal electrodes 14 and 33 Polymer electrolyte membranes 15 and 34 Upper metal electrode 21 Silver electrode 22 Platinum electrode 23 Polymer 24 Cation (metal ion)
25 Anion 26 Precipitated metal 27 Metal filament 31 Plastic substrate

WO2003 / 028124

Appl. Phys. Lett. 82 (2003) 3032 Nature 433 (2005) 47 Nature Nanotech. 3 (2008) 429

Claims (10)

A polymer organic electrolyte material having ionic conductivity;
A metal salt present in the polymer organic electrolyte material;
A first electrode containing the same metal as the metal salt in the polymer organic electrolyte material;
Providing a second electrode made of a second metal having a lower electrochemical activity than the metal in the first electrode ,
A switching element used at a temperature higher than the glass transition temperature of the polymer organic electrolyte material and lower than its melting point . The switching element according to claim 1, wherein the polymer of the polymer organic electrolyte material is any one of polyethylene glycol, polyethylene oxide, polyvinyl pyrrolidone, polyacrylonitrile, polyvinylidene fluoride, and polymethyl methacrylate. The switching according to claim 1 or 2, wherein the electrolyte salt of the polymer organic electrolyte material is a silver electrolyte salt, a copper electrolyte salt, a magnesium electrolyte salt, an iron electrolyte salt, a nickel electrolyte salt, a cobalt electrolyte salt, or a manganese electrolyte salt. element.   The switching element according to claim 3, wherein at least a part of the first electrode is formed of silver, copper, magnesium, iron, nickel, cobalt, or manganese.   The switching element according to claim 1, wherein the second electrode is any one of platinum, gold, iridium, tungsten, and aluminum. The switching element according to claim 1, wherein the polymer organic electrolyte material has a thickness of 1 μm or less. By controlling the concentration of the electrolyte salt of the polymer organic electrolyte material, the on-voltage that transitions from the high-resistance state to the low-resistance state and the off-voltage that transitions from the low-resistance state to the high-resistance state are controlled. 7. The switching element according to any one of 6. The switching element according to claim 1, which is formed on a plastic substrate such as polyethylene terephthalate, polyethylene naphthalate, or polyimide . 9. A switch array in which at least two switching elements according to claim 1 are arranged on a substrate, wherein at least one of the first electrode and the second electrode is electrically connected to a common line. . The switch array according to claim 9, wherein the substrate is made of a flexible material.