JP2013222784A - Resistance change type nonvolatile memory and method of manufacturing the same - Google Patents

Abstract

A variable resistance nonvolatile memory having an excellent switching performance using an inexpensive aluminum substrate as a material is provided.
SOLUTION: Porous alumina is produced by anodizing an aluminum plate, and the insulation film in that portion is made to have a low resistance, so that it is composed of three layers of aluminum layer / anodized alumina layer / aluminum layer. A planar memory element is formed, and a highly integrated resistance change memory with little variation in switching voltage is formed. Therefore, based on the pulse voltage application plating method, an aluminum metal plate is anodized in a sulfuric acid aqueous solution, and lattice defects and metal ions are introduced into the oxide to produce porous alumina. An element is formed, and an intersection portion of a hexagonal lattice adjacently defined by nanoholes of the porous alumina layer formed in alignment is formed as a filament.
[Selection] Figure 13

Description

  The present invention relates to a variable resistance nonvolatile memory formed by anodizing aluminum and a manufacturing method thereof.

  Various types of resistance change memory have been studied, and those using anodized aluminum can be used by the National Institute for Materials Science, as shown in the following non-patent literature. Have made suggestions. In addition, as shown in the following Patent Documents 1 and 2, various patent applications have been filed, and each of these is a template of each nanohole of the porous alumina layer formed in alignment, and a resistance change material is placed inside the nanohole. To embed.

  However, in such a resistance change type memory, it is a problem to suppress the fluctuation width of the switching voltage, and the fluctuation width is set to 2 which is a value obtained by dividing the maximum value of the switching voltage by the minimum value in consideration of setting of a threshold value or the like. It is a desirable condition that it should be less than double.

  Resistance change memory using the switching phenomenon of porous alumina, http: // www. nims. go. jp / AAA_ReRAM / TI2008. pdf # search = 'resistance change memory using the switching phenomenon of porous alumina'

JP 2005-256102 A JP 2005-236003 A

  For example, iridium or platinum can be used for the electrode part as a terminal material that realizes such conditions, but these materials are expensive and are currently not suitable for practical use. Yes.

In view of these conventional situations, the inventor of the present application,
(1) Restriction of molding process area constituting each switching part,
(2) Electric field concentration in the switching part,
(3) Ease of chemical treatment in oxide film production,
From these three approaches, we challenged the challenge of suppressing the fluctuation width of the switching voltage in a resistance change type memory using anodized alumina.

  That is, the present invention pays attention to aluminum, which is an inexpensive material, and has excellent switching performance using an aluminum thin film formed on a metal electrode on an insulating substrate or an aluminum oxide oxidized on the aluminum substrate itself as an insulating film. Another object is to provide a variable resistance nonvolatile memory.

  The manufacturing method of the variable resistance nonvolatile memory according to the present invention includes aluminium containing lattice defects and metal ions introduced into porous alumina previously prepared by an anodic oxidation method in an aqueous sulfuric acid solution based on a pulse voltage application plating method. A hexagonal lattice that forms a planar memory element composed of three layers of a metal electrode layer / an anodized alumina layer / a lower metal electrode layer including aluminum and that is adjacently partitioned by nanoholes in the porous alumina layer formed in alignment It is characterized in that the intersecting point part of is formed as a filament. The variable resistance nonvolatile memory of the present invention is manufactured by the above method.

  According to the present invention, when anodized alumina is selected as the insulating film of the resistance change memory, it is possible to provide a resistance change nonvolatile memory excellent in switching performance by realizing a reduction in resistance of the insulating film. become.

It is a figure which shows the structure of ReRAM typically. It is a figure which shows the IV curve of ReRAM shown in FIG. It is a figure which similarly shows switching speed. It is a figure which shows that the switching phenomenon of ReRAM originates in destruction of an insulating film. It is a figure which shows the typical IV curve of ReRAM. It is a figure which shows the form which can select several filaments at random. It is a figure which shows the example of reduction of the path | pass number of a filament from a normal state (A). It is a figure (A) showing an aluminum anodic oxide film treated with oxalic acid and its SEM image, and a figure (B) showing a dimple structure of AAO treated with sulfuric acid and its SEM image. It is a figure which shows the data of the IV characteristic of oxalic acid AAO and sulfuric acid AAO. It is a figure which shows the formation area | region of the filament for suppression of switching voltage fluctuation | variation, and the additional electrochemical process to an oxide film. It is a figure shown in the detail of sample preparation and experiment. It is a figure which shows the SEM image of AAO processed electrochemically. It is a figure which shows the characteristic of the IV curve of two samples. It is a figure shown about the detail of a switching voltage. It is a figure which shows the outline | summary of the numerical characteristic regarding AAO as produced | generated and AAO processed electrochemically. It is a figure which shows the IV curve of the trial which tried to suppress the fluctuation | variation of a switching voltage by the electrochemical process which induces a reduction of a contact area and leakage current using AAO. It is a conceptual diagram which shows the procedure of the pulse electric field plating method as an electrochemical process. It is a figure for showing the preparation device model of the sample in the embodiment of the present invention. It is a flowchart which shows the sample preparation procedure in the model shown in FIG. It is a figure which shows the example of filament formation using porous alumina.

Embodiments of the present invention will be described below with reference to the drawings.
The inventor of the present application, as described above, as a material of the resistance change memory,
(1) Restriction of area constituting each switching part,
(2) Electric field concentration in the switching part,
(3) Ease of chemical treatment in oxide film production,
From these three approaches, we decided to make a resistance change memory with anodized alumina, which is an inexpensive material, and tried how to make it possible to suppress the fluctuation width of the switching voltage.

  As a result, based on the pulse voltage application plating method, lattice defects and metal ions are introduced into the oxide in an aqueous sulfuric acid solution, thereby miniaturizing the switching portion (for example, about 70 nm), electric field concentration, and switching voltage fluctuation width. The respective results for the above approach of suppression were obtained, and the target resistance change memory could be manufactured.

  That is, porous alumina is produced by anodizing an aluminum plate, and the resistance of the insulating film at that portion is reduced, so that an upper metal electrode layer containing aluminum / an anodized alumina layer / a lower metal electrode layer containing aluminum A planar memory device composed of three layers was formed, and a highly integrated resistance change memory with little variation in switching voltage was successfully formed.

The reason for selecting porous alumina in the present invention is as follows.
From the research results known so far, what can be said about the resistance change memory is that the crystallinity is good, and if a thick insulating film is created, the switching voltage will be different for each switching and its reproducibility will be poor At the same time, the number of switching times (memory life) is also significantly reduced. Based on this fact, in order to suppress the switching voltage and improve the number of switching times, a film having poor crystallinity may be formed. The inventor of the present application has selected porous alumina, which is an amorphous material, as the solution. Porous alumina is a film formed by electrochemical oxidation in a solution of several degrees Celsius, so the resulting thin film is originally amorphous. Furthermore, since oxidation proceeds while electricity flows in the film, it is easy to improve the film quality by selecting various solutions for electrochemical treatment. For this reason, the idea was to create a resistance change memory using porous alumina.

  In the following, as an embodiment of the present invention, an additional electrochemical treatment of switching characteristics of ReRAM (Resistive Switching Memory: hereinafter also referred to as a resistance variable nonvolatile memory) using anodized aluminum will be described.

  The variable resistance nonvolatile memory is expected as a new generation of memory due to excellent characteristics such as a simple structure, high-speed switching, and non-volatility. FIG. 1 schematically shows the structure of the ReRAM. An insulating oxide thin film 1 (hereinafter also referred to as an insulating film 1) is sandwiched between upper and lower electrodes 2 and 3. When a voltage is applied to this structure, the current gradually increases as shown in FIG. 2, and greatly increases above the threshold voltage. This increase in current means that the insulating state has transitioned to the metallic state. This low resistance state is maintained in that state even when the voltage is zero volts. In this embodiment, the lower electrode 3 is an Al substrate unless otherwise specified.

  In the negative voltage region, the reverse phenomenon occurs. That is, the metal state changes to an insulating state at a threshold voltage or higher. In this sense, the variable resistance nonvolatile memory can be said to be a memory that can store the state of the material. Moreover, as shown in FIG. 3, the switching speed is excellent and is several nanoseconds. This switching phenomenon is observed in many types of oxide thin films. That is, it can have many options.

  FIG. 4 shows that the switching phenomenon of ReRAM is caused by the breakdown of the insulating film 1. The ReRAM in the state (A) before the voltage is applied is in an insulating state, and when a voltage exceeding the threshold value is applied, the insulating film 1 is broken, a large number of conductive filaments 4 are generated, and the insulating film 1 is It becomes conductive. By applying the reverse voltage, the filament 4 is broken around the interface (the lower electrode 3 in the figure) (the portion indicated by the arrow 5 surrounded by a dotted line). Hereinafter, the destruction is referred to as soft breakdown.

  FIG. 5 is a typical IV curve of ReRAM. The current is plotted on a logarithmic scale. As shown by dotted ellipses and arrows in the figure, a large voltage fluctuation is observed around the threshold voltage. A decrease in reproducibility is a serious problem because it hinders practical use. Regarding the reason for the fluctuation of the switching voltage, the present inventor considers the cause as follows based on the filament model. That is, when a voltage exceeding the threshold is applied, many filaments having different resistance values and lengths are produced by soft breakdown. In all switching, some filaments can be selected randomly, as shown in FIG.

FIG. 7 shows an example of reducing the number of filament passes from the normal state (A).
(1) First, the formation region is limited. That is, geometrically reduce the type and number of filaments (B).
(2) Concentrate the electric field. The electrodes 2 and 3 are in a state having protrusions 2a and 3a (C).
(3) Electrochemical treatment of the oxide film is performed to reduce the insulation resistance of the oxide film (D).

  Regarding the method shown in FIG. 7, the inventor of the present application tried (1) and (2). Anodized alumina oxide was used due to geometric constraints. This is because the anodized alumina oxide has nanoholes, and its size and density can be easily controlled. The size depends on the electrolyte solution. A typical nanohole size is about 40 nm when anodized with oxalic acid and about several tens of nm when sulfuric acid is used.

  The inventor of the present application created two different nano-dimple structures by removing the oxide layer and oxidized the surface of the dimple aluminum in air. A dimple structure with aluminum protrusions is suitable for checking both the contact area and the effect of limiting electric field concentration. FIG. 8 shows an aluminum anodic oxide film treated with oxalic acid (A) (hereinafter referred to as oxalic acid AAO. AAO means an aluminum anodic oxide film), and AAO treated with sulfuric acid (A) ( Hereinafter, it is a diagram showing a dimple structure of sulfuric acid AAO) and its SEM image, and hexagonal lattice dimples (that is, nanoholes) can be seen. The area of the contact portions 6 and 7 with respect to the upper electrode 2 is larger in (A) than in (B), and is about 70 nm for the oxalic acid sample and about 50 nm for the sulfuric acid sample. In the figure, 8 is a naturally generated oxide film, 9 is a protrusion formed on the Al substrate 3 (lower electrode), and 10 is a nanohole.

  FIG. 9 is a diagram showing IV characteristic data of oxalic acid AAO and sulfuric acid AAO. In the sample of oxalic acid AAO that gives a large contact area, a relatively wide variation is observed in the switching voltage, and the IV window (portion shown in the figure as a void) is small. On the other hand, in the sample of AAO sulfate, the IV window is greatly opened and the contact area is narrowed. It can be said that a small contact area is preferable.

  FIG. 10 shows the formation region of the filament for suppressing the switching voltage fluctuation and the additional electrochemical treatment to the oxide film, and (A) shows the cross-sectional structure of the as-produced AAO (as grown AAO) sample. (B) shows leakage current introduction and resistivity reduction of an electrochemically treated AAO sample (ET AAO). In order to obtain a nanodimple structure as shown in the drawing, several procedures have been conventionally required. However, in the present invention, a simple method using an AAO thin film is adopted. AAO can limit the contact area with the filament formation region or electrode due to many nanoholes. Electrochemical treatment is expected to induce leakage current, and the resistivity of the insulating film is lowered. That is, the present invention intends to suppress the fluctuation of the switching voltage by the above method. In the figure, 11 is the AAO that has been generated, and 12 is the filament path.

FIG. 11 is a diagram showing in detail the preparation of a sample in which pores are regularly arranged and the treatment for reducing the resistance of porous alumina. This figure shows a two-step anodic oxidation procedure for creating regularly arranged pores.
(1) First, the first anodic oxidation of the Al sheet material 13 is performed using oxalic acid,
(2) AAO11 is removed using a mixture of phosphoric acid and chromic acid,
(3) The second anodic oxidation of the Al sheet material is performed again using oxalic acid to obtain AAO as it is produced,
(4) Apply a DC bias voltage in an appropriate chemical solution to perform an electrochemical treatment to obtain an electrochemically treated AAO;
(5) An electrode (TE) is attached to the upper surface.

  More specifically, first, the cleaned Al plate is anodized with oxalic acid, and then the porous alumina is chemically removed. At this stage, nano-sized dents are obtained. In order to create the target nanohole, the second stage anodizing treatment is performed with the same acid, but the above-described AAO sample immediately after generation can be obtained in these three steps. In order to reduce the resistivity, an electrochemical treatment is required for the AAO sample immediately after production. For this treatment, a DC bias voltage is applied in an appropriate chemical solution. Thereby, a low-resistance sample can be obtained. IV measurement is performed at room temperature by the two-terminal method. The maximum current is set to be limited to 1 milliampere so that the device is not completely destroyed. Here, a monopolar operation is adopted instead of the normal bipolar operation.

  FIG. 12 shows SEM images of electrochemically treated AAO, where (A) shows a cross section and (B) shows an image of the top surface. The thickness of the porous layer (the layer in which nanoholes are formed in the insulating oxide thin film 1) is 260 nm. A nonmagnetic aluminum plate is used for the lower electrode. As shown in the image (B) on the upper surface, the width of the contact area is 70 nm. In the figure, reference numeral 10 denotes a nanohole.

  FIG. 13 shows the characteristics of the IV curves of the two samples, the upper figure shows the result of the SET process and the lower figure shows the result of the RESET process. Here, “SET” is defined as a transition from an insulating state to a metal state, and “RESET” is defined to mean the opposite of “SET”. As-produced AAO samples do not find any improvement in SET voltage variation and the SET region is still wide. The threshold voltage is also relatively high, as if reflecting the thickness of the sample (about 260 nm). RESET is not as favorable as the result of SET. In other words, it must be said that reproducibility is poor. Contrary to expectations, no improvement was seen in the as-produced sample. This may be an indication that the oxide film is too thick and that the contact area limitation was not sufficiently small relative to the filament size. In addition, the figure on the right side in the figure shows the results shown by the electrochemical treatment sample. It can be seen that large windows are open on both sides of the SET and RESET IV curves. When the SET voltage is observed, it can be seen that the fluctuation is suppressed. The ratio between the maximum switching value and the minimum switching value is about 2. The RESET IV curve also looks favorable. That is, it can be said that electrochemical treatment is very preferable for improving reproducibility.

  FIG. 14 is a diagram showing details of the switching voltage. With reference to this figure, the IV curve is quantitatively evaluated. First, the durability of both samples with respect to switching repeats is 38 times for as-produced AAO and 100 times for ET AAO. Low resistance ET AAO has shown favorable results. In FIG. 14, the switching dependence on the switching voltage is shown for both SET and RESET. The set voltage of AAO as generated shows an increasing tendency with respect to the number of repetitions. This means that the filament formation becomes more difficult as the number of repetitions increases.

  In the as-generated AAO, the SET voltage is distributed between 2 and 16 volts. The RESET voltage is in the range of 0.2 to 1 volt. On the other hand, the electrochemical sample has a switching voltage with a small absolute value, and the distribution width of the SET voltage and the RESET voltage is narrow. The two diagrams on the lower side of FIG. 14 are histograms showing cycle dependency. In the case of as-produced samples, no statistical distribution can be found due to poor durability. In order to evaluate the distribution width, the ratio between the maximum and minimum switching voltage values defined by these figures is calculated. For AAO as generated, the ratio of SET voltage to RESET voltage is 5 and 8. On the other hand, AAO subjected to electrochemical treatment shows a clear distribution of both voltages. This voltage distribution appears to be composed of several distribution functions. However, narrow and sharp peaks may indicate that there are only a few types of filaments in the film material. The ratio between the SET voltage and the RESET voltage is 2, which is the same value as described above.

  FIG. 15 shows an outline of numerical characteristics relating to AAO as generated and electrochemically treated AAO. In the case of oxalic acid anodization, the contact area is about 70 nm. The as-produced sample shows a poor on / off ratio at a voltage of 0.1V. The ratio of the SET voltage to the RESET voltage between the maximum value and the minimum value is not very good. Its values are 8 and 5. The durability of switching is 38, reflecting such bad properties. Contrary to the expectation of the inventor of the present application, the restriction of the filament forming region using the hole of the name of porous alumina was not sufficient to suppress the switching fluctuation. On the other hand, electrochemically treated AAO shows better improvement results. That is, the on / off ratio is 150, the ratio of the SET voltage to the RESET voltage is 2, and the number of times of switching exceeds 100.

  In conclusion, as shown in FIG. 16, the inventor of the present application tried to suppress the fluctuation of the switching voltage by using AAO to reduce the contact area and electrochemical processing that induces leakage current. Only the limitation of the above did not show a very good reduction effect. The reproducibility of the as-generated AAO switching was not good (the contact area could still be large relative to the filament size). However, ET AAO that has undergone additional electrochemical treatment on the as-produced AAO can improve the properties related to the fluctuation range of the switching voltage.

Hereinafter, a sample manufacturing procedure and the like in the embodiment of the present invention will be described.
FIG. 17 is a conceptual diagram showing a procedure of a pulse electric field plating method as an electrochemical treatment. The procedure in the form of this figure is:
(1) An Ni plate is arranged on the plus electrode 21 and an Al plate with a porous alumina film is arranged on the minus electrode 22 (the arrangement of the electrodes is opposite to the anodic oxidation).
(2) These are immersed in the electrolyte solution 24 in the container 23 such as a beaker. The temperature of the electrolyte solution 24 is 56 ° C., for example. And it stirs with the magnetic stirrer 25. FIG. The electrolyte solution 24 contains the following components and uses distilled water to a total volume of, for example, 0.3 liters.
Electrolyte solution 1: nickel sulfate 1.54 mol / liter + nickel chloride (25 g) + boric acid (10.5 g)
Electrolyte solution 2: Cobalt sulfate 1.54 mol / liter + Cobalt chloride (24 g) + Boric acid (14 g)
(3) A voltage is applied by the pulse power supply 26 to perform electrochemical treatment. For example, the applied voltage is 3 V, the current is 60 mA / cm ² , the pulse width is 60 milliseconds, the off time is 940 milliseconds, and the voltage application time is 5 minutes.
(4) Then, the Al plate used as the negative electrode 22 is washed with distilled water and completed as a sample.

  FIG. 18 is a diagram for illustrating a sample creation apparatus model according to the embodiment of the present invention. In the figure, 30 is a magnetic stirrer, 31 is a tap water container, 32 is a beaker, 33 is a thermometer, 34 is a throw-in cooler, 35 is a temperature controller for the thermometer and the throw-in cooler, and 36 is a throw-in cooler. It is a controller part. Then, an Al plate is disposed as the plus electrode 37 and a Pt plate is disposed as the minus electrode 38, and a voltage is applied from the power source 39 by being immersed in the electrolyte solution in the beaker 32.

FIG. 19 is a flowchart showing a sample preparation procedure in the model shown in FIG.
First, the electrode plate is degreased with acetone (step S1), and electropolishing (the solution used is, for example, 60 wt% perchloric acid: ethanol = 1: 4) (step S2) and the first step of anodic oxidation is performed. (Step S3). Next, alumina etching (the solution used is, for example, 6 wt% phosphoric acid, 1.8 wt% chromic acid) is performed (step S4), and anodic oxidation of the two-step anodic oxidation is performed (step S5). Next, pore widening (PW) processing (for example, using 0.5 wt% phosphoric acid) is performed (step S6). Then, residual substrate etching (5 wt% mercury chloride) is performed (step S7), and an annealing process is performed (step S8).

The sample preparation conditions and sample evaluation are collectively described as follows. That is, the anodizing condition is
Raw material: 99.5% Al sheet Electrolyte solution: Oxalic acid
: Sulfuric acid
:phosphoric acid
:citric acid
: Chromic acid Solution concentration: 0.05 to 0.5 mol / l
Oxidation time: 1-50 hours Solution temperature: 3-30 ° C
Applied voltage: 10-40V
It is.

  FIG. 20 is a view showing an example of filament formation using porous alumina. In this example, the three improvements described above are applied to porous alumina. The sample 1 shown in FIG. 20A restricts the formation space of the filament 4 and the nanohole 10 restricts the formation space of the filament 4. Sample 2 was obtained by reducing the resistance of porous alumina, which was subjected to electrochemical treatment and introduced oxygen vacancies 40 and metal ions 41 to promote the formation of filaments 4 of the same kind.

  The present invention is not limited to the embodiments described above, and many variations are possible by those having ordinary knowledge in the art within the technical idea of the present invention.

The present invention can be applied to USB memory, SD memory, and the like.

1: Insulating oxide thin film 2: Electrode (upper electrode)
2a: Projection 3: Electrode (lower electrode)
4: Filament 6: Contact portion 10: Nanohole 13: Al sheet material 21: Positive electrode 22: Negative electrode 23: Container 24: Electrolyte solution 25: Magnetic stirrer 26: Pulse power source 30: Magnetic stirrer 31: Tap water container 32: Beaker 33: Thermometer 34: Throw-in cooler 35: Temperature controller 36: Controller part 37 of throw-in cooler 37: Positive electrode 38: Negative electrode 39: Power supply 40: Oxygen hole 41: Metal ion

Claims (3)

Based on pulse voltage application plating method, lattice defects and metal ions are introduced into porous alumina prepared in advance by anodic oxidation method, and it is composed of metal electrode layer containing aluminum / anodized alumina layer / lower metal electrode layer containing aluminum. Forming a planar memory device,
A method of manufacturing a variable resistance nonvolatile memory, characterized in that an intersection of hexagonal lattices adjacently partitioned by nanoholes of the porous alumina layer formed in alignment is formed as a filament. The method of manufacturing a variable resistance nonvolatile memory according to claim 1,
The method of manufacturing a variable resistance nonvolatile memory, characterized in that the formation of the filament involves a process of introducing an oxygen vacancy or metal ions by performing an electrochemical treatment after forming a porous alumina layer.   A variable resistance nonvolatile memory produced by using the variable resistance nonvolatile memory manufacturing method according to claim 1.