JP2013138265A - Switching element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching element in which a yield and an element characteristic are improved.SOLUTION: When forming a switching element comprising a laminated structure of an ion conducting layer and an ion supply layer, and an electrode pair for applying voltage to the laminated structure, the ion supply layer is formed as a layer comprising copper or silver, and metal which can form alloy with the copper.

Description

  The present invention relates to a switching element utilizing an electrochemical reaction and an element manufacturing method thereof.

  Patent Document 1 discloses a switching element using an electrochemical reaction, and Patent Document 2 discloses a storage device configured using the switching element.

  In the case of a semiconductor switch, the resistance increases with the miniaturization of the element. However, in the case of a switching element using the above-described electrochemical reaction, ON / OFF is switched by nano-sized metal bridge. Independent of size.

  A conceptual configuration and operation of the switching element disclosed in Patent Document 1 will be described with reference to FIG. In the configuration of the switching element, the ion supply layer 13 and the solid electrolyte layer 14 are laminated in this order on the first electrode 11, and the second electrode 12 is formed on the solid electrolyte layer 14.

  When the voltage applied between the ion supply layer and the second electrode exceeds the threshold voltage with the second electrode as negative, the operation of the switching element disclosed in Patent Document 2 The electrical resistance between the two electrodes decreases and transitions to the on state. Conversely, when the voltage applied between the ion supply layer and the second electrode exceeds the threshold voltage with the second electrode as positive, the electrical resistance between the first electrode layer and the second electrode layer increases. , Transition to the off state. Even when a voltage equal to or lower than the threshold voltage is applied or the power supply is removed, the on state and the off state are maintained.

  As a model of the generation mechanism of the first current-voltage characteristics, when a voltage equal to or lower than the negative threshold is applied to the switching element that is off as the first voltage pulse, metal ions are transferred from the ion supply layer 13 to the solid electrolyte layer 14. Is supplied, and the conductivity increases (on state). Further, in the on state, conduction by electrons also contributes to the electric conduction of the solid electrolyte layer 16, and therefore it is estimated that the on resistance of the solid electrolyte switching element 10 according to the present invention becomes extremely small.

  Next, when a voltage of a positive threshold value or more is applied as the second voltage pulse, the metal ions in the solid electrolyte layer move to the ion supply layer side, and the vicinity of the interface between the second electrode and the solid electrolyte layer This results in a layer lacking metal ions. Since this ion-deficient layer has a low electric conductivity, it is estimated that the electric conductivity of the solid electrolyte switching element becomes small and transitions to the off state again.

  In the ON state, when the first electrode 11 is grounded and a negative voltage is applied to the second electrode 12, the metal of the ion supply layer 13 becomes a metal ion and moves to the ion conductive layer 13. Then, metal ions in the ion conductive layer 14 are deposited as a metal on the surface of the second electrode 12, and a metal dendrite that connects the first electrode 11 and the second electrode 12 is formed by the deposited metal. The metal dendrite is a metal deposit in which metal ions in the ion conductive layer 14 are deposited. It is considered that the switch is turned on by electrically connecting the first electrode 11 and the second electrode 12 with a metal dendrite.

  On the other hand, when the first electrode 11 is grounded and a positive voltage is applied to the second electrode 12 in the ON state, the metal dendrite is dissolved in the ion conductive layer 13 and part of the metal dendrite is cut off. Thereby, the electrical connection between the first electrode 11 and the second electrode 12 is cut, and the switch is turned off. It should be noted that the electrical characteristics change from the stage before the electrical connection is completely broken, such as the resistance between the first electrode 11 and the second electrode 12 is increased, or the capacitance between the electrodes is changed. The connection is lost. The material of the second electrode 12 is preferably one that does not supply metal ions into the ion conductive layer when a voltage is applied. In order to switch from the off state to the on state, a negative voltage may be applied to the second electrode 12 again.

  Furthermore, it is disclosed that when an ion supply layer material is used as the first electrode material, it is not necessary to provide an ion supply layer.

  Furthermore, the material of the solid electrolyte layer is preferably an ion conductive material, and the ion supply layer is preferably a material that supplies ions to the ion conductive material. Examples of the material for the solid electrolyte layer include copper sulfide, chromium sulfide, silver sulfide, titanium sulfide, tungsten sulfide, nickel sulfide, tantalum sulfide, molybdenum sulfide, zinc sulfide, germanium-antimony-tellurium compound, and arsenic-tellurium-germanium-silicon compound. Silver or copper is disclosed as a material for the ion supply layer.

International Publication No. 2003/094227 International Publication No. 2003/028124

A switching element composed of a solid electrolyte layer made of an ion conductive material (hereinafter referred to as an ion conductive layer) and an ion supply layer is turned on by metal bridging, and the ON resistance does not depend on the size. Compared to the on-resistance of the transistor, the element area can be reduced. However, switching elements using copper for the ion supply layer and copper sulfide (Cu ₂ S) for the ion conductive layer may have a high on-resistance immediately after manufacture.

  When copper, which is the same material as the ion supply layer, is used as the first electrode and copper sulfide is used for the ion conductive layer, surface roughness is observed in the copper / copper sulfide portion of the one having high on-resistance immediately after manufacture. As a result of inspecting the cross section, a cavity was generated in the first electrode.

  The ion conductive layer used in these switching elements has a property of oxidizing the electrode metal by an electrochemical reaction. For this reason, the electrodes are corroded, and the yield of the elements is reduced or the characteristics are deteriorated. As described above, when the conventional switching element is made of copper for the first electrode and copper sulfide for the ion conductive layer, surface roughness may occur in the copper / copper sulfide portion. This is because the copper of the first electrode is oxidized, becomes copper ions, and is absorbed by copper sulfide, which is the ion conductive layer, and a cavity is generated in the first electrode.

  The present invention has been made to solve the problems that occur in the conventional device fabrication process as described above, and aims to improve the yield and characteristics of the present switching device.

  In order to solve the above problems, the present invention provides an ion conductive layer, an ion supply layer provided in contact with the ion conductive layer, a second electrode provided in contact with the ion conductive layer, and the ion A switching element comprising a first electrode provided in contact with a supply layer, wherein the ion supply layer includes copper or silver and a metal capable of forming an alloy with copper. It is an element.

  The metal capable of forming an alloy with copper is preferably aluminum or titanium, and the addition ratio of the metal capable of forming an alloy with copper is 0.1% by weight or more and 5% by weight with respect to the total amount of the metal. The following is more preferable.

  Furthermore, it is preferable that the ion conductive layer and the ion supply layer are heat-treated at 350 ° C. or more for 30 minutes or more in a state where the first electrode is provided on the ion supply layer. .

Furthermore, the manufacturing method of the switching element of the present invention includes an ion conductive layer, an ion supply layer provided in contact with the ion conductive layer, a second electrode provided in contact with the ion conductive layer, and the ion A first electrode provided in contact with a supply layer, wherein the ion supply layer includes copper or silver and a metal capable of forming an alloy with copper, and a method for manufacturing a switching element,
Forming an ion conductive layer on the ion supply layer;
Forming a second electrode on the ion conductive layer;
Have
Between the step of forming the ion supply layer and the step of forming the ion conductive layer, the ion supply layer has a heat treatment step of 350 ° C. or higher for 30 minutes or longer.

  According to the present invention, corrosion of the ion supply layer is suppressed, and the yield and characteristics of the switching element can be improved.

It is a cross-sectional schematic diagram which shows one structural example of the conventional switching element. It is typical sectional drawing which shows one structural example of the switching element of this invention. It is a figure which shows an example of the manufacturing method of the switching element of this invention. It is a figure which shows an example of the manufacturing method of the switching element of this invention. It is an electron micrograph which shows the surface structure of the switching element produced by the Example and the comparative example. It is a graph which shows the electrical property of the switching element produced by the Example and the comparative example.

  The inventors observed that voids (vacancies) generated in the ion supply layer were observed immediately after forming the ion conductive layer on the ion supply layer. It was speculated that there was a problem with the cleaning process.

  Copper sulfide or silver sulfide used for the ion conduction layer has the property of oxidizing copper or silver used for the ion supply layer. Therefore, after the ion supply layer is formed, the ion supply layer and the ion sulfide are washed. From the difference in ionization tendency of the conductive layer, it is estimated that copper ions were deposited.

  This phenomenon is presumed to occur similarly in a switching element using silver / silver sulfide.

  On the other hand, in this invention, generation | occurrence | production of said problem can be suppressed by adding the metal which can form an alloy with copper to copper. That is, it is expected that copper electromigration due to electron flow is suppressed. Furthermore, suppression of copper movement is considered to lead to suppression of corrosion of the first electrode. This effect is considered to be obtained similarly when silver is used. The present inventors add titanium or aluminum to copper or silver, which is a material of the ion supply layer, and after forming the ion supply layer, before forming the ion conductive layer, enlarge the crystal agglomerates of copper or silver. We have found that this phenomenon can be improved by baking at high temperatures.

  Titanium or aluminum is preferably 0.1 to 5% by weight. The high temperature baking is performed in an inert atmosphere such as nitrogen, helium, or argon (nitrogen, atmosphere) at a temperature of 350 ° C. or higher, preferably 300 ° C. or higher and 400 ° C. or lower, for 30 minutes or longer, preferably 30 minutes or longer and within 1 hour. be able to. When the temperature and time are insufficient, crystallization does not occur. When the temperature and time are excessive, diffusion of copper or silver to the substrate surface occurs due to surface migration.

  FIG. 2 is a schematic cross-sectional view showing a configuration example of the switching element according to the present invention. In this switching element, the first electrode 21 also serves as an ion supply layer. Note that an ion supply layer may be provided at a position in contact with the ion conductive layer, and a first electrode for voltage application made of a material different from that of the ion supply layer may be separately provided in contact with the ion supply layer.

  In the illustrated switching element, a silicon oxide film 26 having a thickness of 300 nm formed on a silicon substrate 25 is formed, and a first electrode 21 made of copper having a thickness of 100 nm is formed on the silicon oxide film. An ion conductive layer 23 made of copper sulfide having a thickness of 35 nm is formed on the electrode. A silicon-based insulating film 24 is formed so as to surround the first electrode 21 and the ion conductive layer 23. As the insulating film 24, a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or a silicon oxide insulating film formed using an organic silicon material having a low dielectric constant, for example, HSQ (hydrogen silsesquioxane) is used. ) A membrane can be used. In this embodiment, a 200 nm thick HSQ film is used. HSQ patterning technology is described in Journal of Vacuum Science Technology B, Vol. 16, No. 1, pages 69-76, 1998.

  A second electrode 22 made of platinum having a thickness of 100 nm is formed through an opening formed in the insulating film 24.

  The silicon oxide film 26 can be formed by a normal thermal oxidation method or a CVD method.

  Next, an example of the manufacturing method will be described using the schematic process cross-sectional views of FIGS.

  A silicon oxide film 26 having a thickness of 300 nm is formed on the silicon substrate 25 by using a thermal oxidation method (see FIG. 3A). Next, a photoresist is spin-coated on the silicon substrate 25, and an opening for forming the first electrode 21 that also functions as an ion supply layer is formed using the photoresist layer 29 by a normal photolithography method. To do. The size of the opening was 3 μm.

  Next, a metal film 21 ′ having a thickness of 100 nm is formed (see FIG. 3B). The metal film is made of copper or silver and a metal capable of forming an alloy with copper (preferably aluminum or titanium). Specifically, a metal film 21 ′ was formed on the silicon substrate 25 using a target prepared by adjusting a desired component ratio in advance by a sputtering method. By using the sputtering method, the metal film 21 ′ reflecting the composition of the target can be formed. The relationship between the photoresist 29 and the metal film 21 ′ will be described with reference to the enlarged view of FIG. When the metal film 21 ′ is formed using the sputtering method, the metal film 21 ′ is mainly formed on the substrate in a direction parallel to the target. However, since the step coverage is good, the metal film 21 ′ is also formed on the resist side surface. Usually, it is difficult to remove the photoresist, which may cause burrs around the electrode. Therefore, after applying a lift-off layer (Shipley LOL2000) before applying the photoresist, baking was performed on a hot plate at 150 ° C. for 2 minutes, and then a normal photolithography process was performed. This lift-off layer is soluble in an alkaline developer. For this reason, it is etched together with the photoresist during the photoresist development process, developed on a pattern receding from the opening of the photoresist, creating a cavity below the periphery of the opening of the photoresist, and the photoresist has a vertical structure. Due to this eaves structure, the metal film 21 'is not formed under the photoresist.

  When the photoresist is removed in this state, the metal film 21 ′ deposited on the photoresist is removed except for the metal film 21 ′ deposited in the opening. This method is called a lift-off method.

  The lift-off method was used to remove the portion other than the portion formed in the opening of the metal film 21 ′ together with the photoresist to form the first electrode 21 (see FIG. 3 (4)).

  Thereafter, annealing was performed at 350 ° C. for 30 minutes in a nitrogen atmosphere. At the time of introducing the substrate into the annealing furnace, the substrate was introduced into the high temperature portion after the substitution of residual oxygen at the time of introduction in the nitrogen atmosphere for 10 minutes at the sample introduction portion. After annealing, the substrate temperature was cooled for 20 minutes in a nitrogen atmosphere, and the substrate was taken out in a state where the substrate temperature was 80 ° C. or lower.

  Next, the ion conductive layer 23 is formed using the lift-off method in the same manner as the first electrode. Specifically, first, a photoresist film (not shown) is formed on the silicon substrate 25, and an opening for forming the ion conductive layer 23 is formed in the photoresist film using a normal photoresist method. It is formed on the photoresist film by using a photolithography method. It is necessary to prevent the first electrode from protruding from the opening, and the shape is smaller than that of the first electrode. The size of the opening was 2 μm. The exposed surface of the first electrode 21 is cleaned with oxygen plasma to remove organic substances such as a resist residue, and then the oxide film on the surface of the first electrode exposed in the opening is diluted with dilute hydrofluoric acid (49 seconds). % Hydrofluoric acid: water = 1: 100) and removed. Thereafter, copper sulfide having a film thickness of 35 nm was vapor-deposited by a laser ablation vapor deposition method, and then the photoresist was removed to form an ion conductive layer 23 (see FIG. 4 (1)). The laser used was a KrF excimer laser with an output of 1 W.

  The laser ablation vapor deposition method is a vapor deposition method in which a vapor deposition source is irradiated with a laser beam to evaporate the evaporation source. As is well known, a laser has a very high energy density, and when the laser beam is condensed and irradiated onto a material, a rapid temperature rise occurs locally at the place where the laser beam is irradiated.

  The rapid temperature rise causes the material to liquefy and vaporize rapidly, but the outermost surface of the target becomes lower than the inside due to radiation cooling and the heat of vaporization of the material. With explosive volume expansion inside the higher temperature, the material becomes clusters and ions and pops out with an angular distribution that is perpendicular to the surface. At this time, the raw material that has jumped out is exposed to the laser beam, so that it is re-excited as the temperature rises abruptly and becomes thermal plasma.

The advantages of laser ablation deposition are:
(1) Difficult to occur in composition.
(2) Since light having a very large power density is used, even a material having a high melting point can be easily thinned if it is a material that absorbs light.
(3) Since the influence of vapor pressure is small unlike other physical film formation methods, the atmospheric gas pressure in the reaction system can be increased.
(4) Since no resistance heating method or filament for electron beam is used, the contamination of the thin film is small.
(5) Since the ablation particles reach the substrate as a group in a short time, the thin film can be grown in pulses.

  Subsequently, HSQ (hydrogen silsesquioxane) containing silicon oxide as a main component was spin-coated to form an insulating film 24 with a thickness of 150 nm (see FIG. 4B). An opening for forming the second electrode 22 was formed in the insulating film 24 (see FIG. 4C). The opening needs to prevent the ion conductive layer 23 from protruding from the opening 27 and has a shape smaller than that of the ion conductive layer 23.

  The openings were formed by performing electron beam exposure on the insulating film (HSQ film) 24 using an electron beam exposure method and patterning. The size of the opening was 200 nm. Since the HSQ film patterning method is disclosed in, for example, Journal of Vacuum Science Technology B, Vol. 16, No. 1, pages 69-76 (1998), detailed description thereof is omitted here.

  Then, the 2nd electrode 22 which consists of a platinum layer (100 nm) was formed by the lift-off method similarly to the case where the electrode 21 was produced. Sputtering was used to form the platinum layer. Since the sputtering method is formed isotropically, a metal film is also formed on the side surface of the opening 27, and wiring breakage due to a step does not occur (see FIG. 4 (4)).

  For the sake of simplicity, a barrier layer made of Ti / TiN provided to prevent the metal material from diffusing into the insulating film is omitted. As the barrier layer, when the first electrode 21 is manufactured in FIG. 4A, a barrier layer made of a Ti / TIN film is formed on the silicon oxide film 26 by sputtering before forming copper. Thus, diffusion of copper metal molecules into the silicon oxide film 26 due to heat treatment or the like can be suppressed.

  Furthermore, in the above example, platinum is used as the second electrode 22, but the second electrode 22 is not limited to platinum as long as it is an electrode material that does not elute metal ions into the ion conductive layer. For example, tungsten, tantalum, titanium, or the like may be used.

  Although the HSQ film is used as the insulating film 24, the HSQ film is known as an insulating film having a low dielectric constant (relative dielectric constant is about 2 to 3), and is used as an interlayer insulating film of an LSI. The reason why a low dielectric constant is preferred as an LSI material is that wiring delay can be reduced by reducing electrostatic coupling between wiring layers. This is because in the structure of this embodiment, the electrostatic coupling between the ion conductive layer 23 and the second electrode 22 can be reduced, and the signal delay at each electrode can be suppressed.

Hereinafter, copper or silver is used for the first electrode (ion supply layer) material, copper sulfide (Cu ₂ S) or silver sulfide (Ag ₂ S) is used for the material of the ion conductive layer, and copper is used. Other metals were included. In each Example, after forming an ion conductive layer on the first electrode, the ion conductive layer was observed from the ion conductive layer side using SEM in the state of FIG. “%” Is based on weight.

<Example 1>
Copper-aluminum added with 1.0% by weight of aluminum (Al) was used as the material for the ion supply layer, and copper sulfide was used as the material for the ion conductive layer.

<Example 2>
Copper-titanium added with 1.0% by weight of titanium (Ti) was used as the material for the ion supply layer, and copper sulfide was used as the material for the ion conductive layer.

<Comparative Example 1>
It was manufactured by the same manufacturing method as in the examples except that pure copper (purity 99.99%) was used as the material for the ion supply layer.

  FIG. 5 shows electron micrographs of the surface of copper sulfide in the devices obtained in the examples and comparative examples.

  5A is a photograph of Comparative Example 1 of pure copper / copper sulfide, and FIG. 5B is a photograph of Example 1 of copper (99 wt%): aluminum (1.0 wt%), FIG. 5C is a photograph of Example 2 of copper (99% by weight): titanium (1.0% by weight).

  In FIG. 5, a portion surrounded by a dotted line is a region where copper sulfide is formed, and a region that appears black indicates a void.

  In FIG. 5A, voids are formed over a wide area in the copper in the copper sulfide deposition portion. On the other hand, the sample of Example 1 in which 99% copper: 1% aluminum in FIG. 5B has a void portion of 5% or less, and in 99% copper: 1% titanium in FIG. 30% or less.

  Next, the electrical characteristics of the completed switching element will be described. FIG. 6 shows the switching characteristics of the manufactured switching element.

  FIG. 6A shows the switching characteristics of pure copper / copper sulfide comparative example 1, and FIG. 6B shows the switching characteristics of example 1 of copper (99% by weight): aluminum (1.0%). FIG. 6 (c) shows the switching characteristics of Example 2 of copper (99% by weight): titanium (1.0%).

  In the measurement of the switching characteristics, the first electrode 21 is fixed at 0V, the negative voltage is applied to the second electrode 22 from 0V to -0.5V and then returned to 0V, and then the second electrode 22 is applied with the positive voltage from 0V to 0.3V. Apply and return to 0V. These were repeated twice. A, B, C, and D in the figure indicate the order of application. The current is limited to 1 mA by the measuring device. The switching element shifts to an ON state when a negative voltage is applied to the second electrode 22, and thereafter shifts to an OFF state when a positive voltage is applied.

  As shown in FIG. 6A, in the switching element using the copper electrode for the first electrode 21, the first negative voltage is applied (solid line A in FIG. 6A) to -ON at 0.3V. Migrated. However, a clear OFF characteristic is not observed in the subsequent application of a positive voltage (solid line B in FIG. 6A). Further, the switching characteristics were not observed in the second voltage application (solid lines C and D in FIG. 6A). This is because the supply of copper ions from the first electrode (ion supply layer) 21 into the ion conductive layer 23 is insufficient.

  On the other hand, the switching element of 99% copper: 1% aluminum in FIG. 6B and 99% copper: 1% titanium in FIG. 6C shows clear ON characteristics and OFF characteristics. In particular, an element using 99% copper: 1% aluminum for the first electrode 21 has stable characteristics and shows the best characteristics. By adding 1% of aluminum to the copper of the first electrode 21, it was possible to effectively suppress the outflow of copper from the first electrode 21 during the manufacturing process. Thereby, the yield and characteristics of the switching element can be improved.

11, 21 First electrode 12, 22 Second electrode 13, 23 Ion conductive layer 24 Insulating layer 25 Silicon substrate 26 Silicon oxide film 29 Resist 41 Copper / copper sulfide 42 Copper: Aluminum / copper sulfide 43 Copper: Titanium / copper sulfide

Claims (7)

An ion conductive layer; an ion supply layer provided in contact with the ion conductive layer; a second electrode provided in contact with the ion conductive layer; and a first electrode provided in contact with the ion supply layer. A switching element comprising:
The switching element, wherein the ion supply layer includes copper or silver and a metal capable of forming an alloy with copper. The switching element according to claim 1, wherein the metal capable of forming an alloy with copper is aluminum or titanium. The content of the metal that can form an alloy with copper relative to the total amount of the copper or silver and the metal that can form an alloy with copper is 0.1 wt% or more and 5 wt% or less. The switching element according to claim 1 or 2. The switching element according to claim 1, wherein the ion supply layer is subjected to a heat treatment at 350 ° C. or more for 30 minutes or more. An ion conductive layer; an ion supply layer provided in contact with the ion conductive layer; a second electrode provided in contact with the ion conductive layer; and a first electrode provided in contact with the ion supply layer. And the ion supply layer includes copper or silver and a metal capable of forming an alloy with copper, and a method for manufacturing a switching element,
Forming an ion conductive layer on the ion supply layer;
Forming a second electrode on the ion conductive layer;
Have
A method for manufacturing a switching element, comprising: a heat treatment step of 350 ° C. or more for 30 minutes or more between the step of forming the ion supply layer and the step of forming the ion conductive layer. . 6. The method of manufacturing a switching element according to claim 5, wherein the metal capable of forming an alloy with copper is aluminum or titanium. The content of the metal that can form an alloy with copper relative to the total amount of the copper and the metal that can form an alloy with copper is 0.1 wt% or more and 5 wt% or less. A method for manufacturing the switching element according to 5 or 6.