JPH04116867A - Switching element - Google Patents

Abstract

PURPOSE:To stably realize switching characteristics having storing properties in an MIM element, by backing a metal thin film formed on a plane substrate with a solid retaining substrate, transferring the plane substrate surface to the metal thin film surface, and forming a substratum electrode having smoothness wherein height difference of uneveness is smaller than or equal to a specified value. CONSTITUTION:On a plane substrate 11 having smoothness, a film of gold (Au) is formed by a vacuum evaporation method, and a substratum electrode layer 12 is formed. An adhesion layer 13 is spread on the layer 12, and a solid retaining substrate 14 is stuck on the adhesion layer 13. The plane substrate 11 is exfoliated from the substratum electrode layer 12, thereby obtaining a plane electrode substrate constituted of the solid retaining substrate 14, the adhesion layer 13 and a substratum electrode layer 12. The height difference of uneveness of the electrode surface is smaller than or equal to 1nm. Said plane electrode substrate is used as carrier, and a monomolecular film 15 is accumulated on the substratum electrode layer 12 by an LB method. On the film 15, an upper electrode 16 is formed by vacuum evaporation method while the temperature of the plane electrode substrate is kept lower than or equal to a room temperature. A metal coating film 17 is formed by vacuum evaporation of gold (Au) on the upper electrode 16.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a switching element having a memory function.

[Prior art] Recently, interest in molecular electronics, which seeks to apply the functionality of organic molecules to electronic devices, has been increasing. Research is becoming more active. LBli is made by laminating organic molecules one molecular layer at a time1.The film thickness can be controlled in units of molecular length, and it is possible to form a -like, homogeneous ultra-thin film, so it is widely used as an insulating film. Attempts have been made. For example, a tunnel junction element with a metal-insulator-metal (MIM) structure [G.

L. Larkinset, al., “Thin Solid.
Films J (Thin Solid Films
) Volume 99 (1983)] and Metals, Insulators, and Semiconductors (
MIS) structure light emitting device [G.

Electronics Letters J, Robertset, al.
rs) Vol. 20, p. 489 (1984)
Written by “Electronics Letters J”
onics Let-ters) Volume 20, 838
Page (1984)].

Conventionally, the above studies have focused on fatty acid LB membranes, which are relatively easy to handle. However, recently, organic materials have been created one after another that overcome heat resistance and mechanical strength, which were previously thought to be inferior. In fact, the present inventors have already developed an LB film using these materials into an element with a sandwich structure sandwiched from both sides by conductive materials such as metals (generally called an MIM structure or MIM
By creating a device (called an element) and observing and measuring its material properties and electrical properties, we discovered a completely new switching phenomenon in electrical conduction (Japanese Patent Laid-Open No. 63-96956). .

Definitely! When the i-film is formed by the Langmuir-Prodgett method, an insulating film can be formed by accumulating a monomolecular film according to the shape of the base electrode, and there is no problem of non-uniform thickness of the insulating film.

[The invention tries to solve ill! ] On the other hand, the base electrode of the MIM element has conventionally been formed using a vacuum evaporation method or a sputtering method. However, the metal thin films formed by these methods are polycrystalline films, and it is extremely difficult to obtain smoothness with a height difference of 5 nm or less on the surface of the thin film. MIM on a base electrode like this
When an element is constructed, even if LBli is used, the thickness of the insulating layer tends to be non-uniform, and when a voltage is applied to the element, the electric field in the insulating layer tends to become non-uniform. For this reason,
This results in variations in the characteristics of each element, which hinders improvements in element design.

Further, especially when the thickness of the insulating film is thin, element damage due to dielectric breakdown or the like is likely to occur from a portion to which a strong electric field is applied.

In other words, although the instability of the device caused by the non-uniform thickness of the insulating film has been solved by LBM, the instability caused by the shape of the underlying electrode, that is, the presence of etches around the protrusions and depressions on the electrode surface, causes instability. The instability of the device due to electric field concentration remained unresolved. Specifically, strong local electric fields are likely to occur near the edges around the convex and concave portions of the surface of the base electrode, and when a switching element is driven for a long time, the life of the element may be shortened due to damage to the element due to dielectric breakdown, etc. Satisfactory results were often not obtained.

In addition, in order to obtain a smooth electrode surface, there are methods such as forming a thin film by epitaxial growth of metal, but there are major restrictions on the substrate and metal material that can be used.
It could not be easily used in IM devices.

[i! Means and Effects for Solving the Problem] The present inventors have conducted extensive studies in view of the above problems, and as a result, have arrived at the present invention.

That is, the present invention provides a switching element that has an organic insulator having a periodic layer structure between a pair of electrodes and has a memory property for switching characteristics, wherein the organic insulator has a periodic layer structure. The switching element is characterized in that the body is formed on a base electrode with a height difference of 1 nm or less in surface irregularities.

The present invention will be explained in detail below.

The organic insulator having a periodic layer structure is, for example, LB
Examples include organic monomolecular films or organic monomolecular cumulative films formed by the MBE method, and organic thin films formed by the MBE method. An organic monomolecular film or an organic monomolecular cumulative film formed by a method is preferable. Specifically, a squarylium-bis-6-octylazulene monolayer, a monomolecular cumulative film, a polyimide monomolecular film or a monomolecular cumulative film, a phthalocyanine monomolecular film or a monomolecular cumulative film, etc. may be mentioned. The thickness of the organic insulator is preferably lnm-1100n.

The pair of electrodes is an upper electrode and a base electrode, and the switching element of the present invention is characterized in that an electrode having a height difference of 1 nm or less in unevenness on the electrode surface is used as the base electrode.

The method for forming the base electrode will be described below. The base electrode is formed by transferring the surface of the smooth substrate. That is, a metal thin film formed on a smooth substrate having a smooth surface is backed with a solid support substrate, and by peeling off the smooth substrate, the smooth substrate surface is transferred to the metal thin film surface, and the height difference of unevenness is 1.
A base electrode having smoothness of less than nm is obtained. The method for forming the metal thin film at this time may be a conventionally known thin film forming technique such as a vacuum evaporation method or a sputtering method.

The smooth substrate may be made of any material as long as it has excellent smoothness, but a substrate with a surface unevenness having a height difference of 1 nm or less is preferable. Materials that provide such a smooth surface include, for example, the cleavage plane of a single crystal substrate such as mica, float glass,
Examples include #7059 fusion, fused silica, etc. whose main surface is formed by melting, and preferably a cleaved surface of a single crystal substrate such as mica.

The base electrode material is preferably a material that has high conductivity and also has poor adhesion to the smooth substrate. For example, metals such as Au, Ag, Pt, Pd, and Au-P
Examples include alloys such as Pt-Pd and Pt-Pd, but noble metals and noble metal alloys are preferred. Further, a multilayer film containing these metals and alloys may be used.

When backing a solid support substrate, it is convenient to use a suitable adhesive layer, but depending on the material used, strong adhesive force can be obtained by eutectic bonding, which is directly bonded to the substrate. The adhesive layer is preferably solvent-free and does not have volumetric shrinkage.
For example, insulating adhesives such as epoxy resin-based and α-cyanoacrylate-based adhesives, and conductive adhesives such as Evotec Silver Series are preferred. Moreover, when directly bonding, an adhesive layer is not necessary.

As a solid support substrate, if an adhesive layer is used, metal,
Any material such as glass, ceramics, or plastic material may be used, but if the supporting substrate is directly bonded to the electrode, it is preferable to use a relatively smooth material. It is also possible to form a thick metal layer by electroforming and use it as a support substrate.

Next, as the upper electrode material, the same material as the base electrode described above can be used, and the same type of metal as the base electrode may be used, or a different metal may be used.

Further, in the switching element of the present invention, a second insulating layer separate from the organic insulator may be formed (the second insulating layer can limit the current-carrying area between the upper and base electrodes. 2
As an insulating layer, for example, SiO□+S f s N4
, Al2x Os, etc. can be used, and the thickness is 1100n.
~500 nm is preferred.

As for the forming method, conventionally known thin film forming techniques are sufficient.

[Example] Hereinafter, the present invention will be specifically explained according to Examples.

Toif Glue 1 Gold (A
u) was formed into a film to form the base electrode layer 12. (Figure 1 (
b)) The base electrode layer 12 is made of a smooth substrate with a deposition rate of 10 persons/sec and an ultimate pressure of 2×10-' while keeping the temperature of the substrate at room temperature.
Torr.

The test was conducted under the condition that the film thickness was 2000. Next, the adhesive layer 13 (
Hi-Super 5 (epoxy resin system) manufactured by Cemedine is applied onto the base electrode layer 12, and the solid support substrate 14 is attached onto the adhesive layer 13. FIG. 1(d) The solid support substrate 14
The bonding was carried out under the conditions of a pressure of 5 kg/cm'', a temperature of 23° C., and a curing time of 24 hours. Thereafter, the smooth substrate 11 was peeled off from the base electrode layer 12, and the solid support substrate 14, adhesive layer 13, and base electrode layer were bonded. A smooth electrode substrate consisting of 12 was obtained. (
FIG. 1(e) LB is placed on the base electrode layer 12 using the smooth electrode substrate as a carrier.
A squarylium-bis-6-octylazulene (SOAZ) monomolecular film 15 was deposited by the method. (FIG. 1(f)) The details are described below.

A chloroform solution in which 5OAZ was dissolved at a concentration of 0.2 mg/mJ was developed on an aqueous phase consisting of pure water at a water temperature of 20 °C,
A monomolecular film was formed on the water surface.

Waiting for the solvent to evaporate, the surface pressure of the monomolecular film is reduced to 20
mN/m, and while keeping this constant, the smooth electrode substrate was moved at a speed of 10 mm/m in the direction across the water surface.
After gently dipping for 1 minute, gently pull it out at 5 mm/minute.
Two layers, a monolayer, were stacked in a Y shape. By repeating the above operation an appropriate number of times, 2.4.8, 12,
Seven types of monomolecular films of 20, 30, and 40 layers were accumulated in a Y shape.

Next, an upper electrode 16 (1500 electrodes) made of aluminum (8β) is formed on the film surface 15 by vacuum evaporation while keeping the smooth electrode substrate temperature below room temperature, and then gold (A
u) (500 people) was formed on the upper electrode 16 by vacuum evaporation to form the metal coating 17. (FIG. 1(g)) The size of the upper electrode 16 was 1 mmφ.

When we measured the current characteristics (IV characteristics) when voltage was applied between the upper and lower electrodes of the sample prepared as described above, we found that
The same memory-like switching characteristics as those disclosed by the present inventors in Japanese Patent Application Laid-Open No. 63-96956 were observed.

(FIG. 5) Such an element could maintain either a low-resistance ON state or a high-resistance OFF state even when no voltage was applied, and its storage stability was extremely good. Moreover, the resistance ratio in both states was about 6 digits or more.

An alternating current voltage was applied to such a device to cause it to continuously transition between the 0N10FF states for a long period of time, and the stability of the ON10FF transition was increased compared to a conventional device with large surface irregularities on the base electrode. Particularly, the effect is remarkable and stable switching characteristics are now exhibited in a device consisting of two 5OAZ layers, which had hitherto had unstable switching characteristics. Furthermore, element damage, which is thought to be caused by localized electric field concentration, is extremely unlikely (((), and the lifespan is extended (()).

Gold (A
u) was formed into a film to form the base electrode layer 22. (Figure 2 (
b)) The base electrode layer 22 is made of a smooth substrate with a temperature of 400°C, a deposition rate of 10 people/sec, and an ultimate pressure of 2XI.
The test was carried out under the conditions of O-' Torr and film thickness of 1.0 μm. Next, the silicon wafer is used as the solid support substrate 24 and heated with a heater, and the smooth substrate 2 is heated while maintaining the temperature at a constant temperature.
By lightly rubbing the surface of the base electrode layer 22 formed on the solid support substrate 24, the base electrode layer 22 and the solid support substrate 24 were eutectic bonded.

(Fig. 2 (C)) Such bonding is performed by increasing the temperature of the solid support substrate to 400°C.
℃, a pressure of 2 kg/cm", and a holding time of 1 minute. After that, the smooth substrate 21 was peeled off from the base electrode layer 22 to obtain a smooth electrode substrate consisting of the solid support substrate 24 and the base electrode layer 22. (FIG. 2(d)) LB was formed on the base electrode layer 22 using the smooth electrode substrate as a carrier.
A polyimide monomolecular cumulative film 25 was formed by the method. (FIG. 2(e)) The details are described below. Polyamic acid (molecular weight approximately 200,000) shown in formula (1) was dissolved in N,N-dimethylacetamide solvent (monomer equivalent concentration I
After 10-"M), a separately prepared IX 10-"M solution of N,N-dimethylhexadecylamine in the same solvent was mixed at a ratio of 1=2 (V/V) to form a polyamic compound represented by formula (2). An acid hexadecylamine salt solution was prepared.

A two-layer Y-type monomolecular cumulative film was created. By repeating this operation, eight types of cumulative films of 4, 8, 12, 18, 24, 30° and 42.60 layers were created.

Next, heat treatment is performed on the substrate at 300°C for 10 minutes,
When polyamic acid hexadecylamine salt is imidized, the solution is developed on an aqueous phase consisting of pure water with a water temperature of 20 ° C.
A monomolecular film was formed on the water surface. After removing the solvent, reduce the surface pressure to 2
The smooth electrode substrate was moved at a speed of 5 m in the direction across the water surface while keeping the zero surface pressure raised to 5 m N/m constant.
m/min followed by 5 m
It was gently pulled up at a speed of m/min to obtain a polyimide monomolecular cumulative film.

A second insulating layer 28 is formed on the polyimide monomolecular cumulative film 25.
As a result, SiO□ was deposited to a thickness of 3,000 yen using a vacuum evaporation method. Next, positive resist material (trade name AZ1370)
) was applied with a spinner to a film thickness of 1.2 μm. After prebaking this, exposure, development, and post baking are performed. Thereafter, etching was performed with a solution of HF:NH4F=1:7 to butteren the SiO*, and then the resist was removed by acetone ultrasonic treatment, DMF ultrasonic treatment, and pure water cleaning, and baking was performed.

Through the above steps, a structure was obtained in which exposed portions of the polyimide monomolecular cumulative film 25 having various diameters from 1 mmφ to 10 μmφ were formed, and the other portions were covered with the SiO2 film 28. (Fig. 2 (g)) Then, as shown in Fig. 2 (h), an upper electrode 26 (5000 electrodes) of aluminum (Aβ) is formed to cover the exposed part of the polyimide monomolecular cumulative film 25 by vacuum evaporation. did.

When a voltage was applied between the upper and lower electrodes of the sample prepared as described above, the current characteristics (IV characteristics) were measured.
Memory switching characteristics similar to those of Example 1 were observed, and an 0N10FF ratio of about 6 digits was obtained. Furthermore, it was confirmed that the stability of continuous ON10 F F transitions over long periods of time when AC voltage is applied is also improved compared to conventional switching elements having base electrodes with large surface irregularities. In particular, stable switching characteristics were obtained even for 4-layer and 8-layer samples, which had previously shown only unstable switching characteristics. Furthermore, element damage, which is thought to be caused by localized electric field concentration, is extremely unlikely.
The device life was also significantly extended.

Rough and cleaned fused quartz is used as the solid support substrate 34, and gold (Au) is used as the solid support substrate 34.
A film is formed on the solid support substrate 34 by a vacuum evaporation method,
A base electrode layer 32 was formed.

The base electrode layer 32 maintains the temperature of the solid support substrate at room temperature, the deposition rate is 10 persons/sec, and the ultimate pressure is 2 x 10-'.
Torr, film thickness 500 film thickness 5 tons 00 (b)) Next, a mica plate cleaved in the atmosphere was used as a smooth substrate 31, and a base electrode 3
Place it on top of 2 and press. This pressing was carried out in a nitrogen atmosphere at a pressure of 10 kg/am'' and a temperature of 500° C. for 1 hour. A smooth electrode substrate consisting of a base electrode layer 32 and a solid support substrate 34 was obtained (FIG. 3(d)) Next, a polyimide monomolecular cumulative film 35 was formed on the base electrode layer 32 in the same manner as in Example 2. (Figure 3(e)) A photosensitive polyimide (trade name PL-1200) was coated on the polyimide monomolecular cumulative film.Subsequently, prebaking, exposure, development, and curing were performed in the same manner as in Example 2. 1m
An exposed portion of the polyimide monomolecular cumulative film 35 having a diameter of mφ to 10 μmφ was formed. At this time, the thickness of the polyimide film 35 used for the pattern formation was set to 3,000 layers. (FIG. 3(g)) Aluminum (Aβ) was deposited to a thickness of 5000 on the entire surface of the polyimide film by vacuum evaporation. (Figure 3 (
h)) Next, a positive resist (trademark OMR-83) was applied with a spinner to a film thickness of 1.2 μm, exposed, developed and post-baked, and then H,PO4:HNOs
Etch Aβ into a desired pattern using a solution of :CH3CO0H:H20=16:1:2:1. The resist was removed from the substrate by acetone ultrasonic treatment, DMF ultrasonic treatment, and pure water cleaning, and the upper electrode 36 was formed. The width of the upper electrode 36 is larger than each open pattern formed on the polyimide film 35, and covers the exposed portion of the polyimide monomolecular cumulative film 35. (Figure 3(i))
When we measured the current characteristics (IV characteristics) when a voltage was applied between the upper and lower electrodes of the sample prepared as described above, we observed a memory switching characteristic similar to that in Example 1, and 6 orders of magnitude. In addition, the stability of continuous ON10 F F transitions for long periods of time when an AC voltage is applied is also higher than that of conventional switching elements having base electrodes with large surface irregularities. As in Example 2, it was confirmed that the increase was observed in all elements.

Shigoto A A mica plate is cleaved in the air to form a smooth substrate 41, and gold-palladium (Au-P) is deposited on the smooth substrate 41 by vacuum evaporation.
d) to form a base electrode layer 42. (Figure 4(b)
)) The base electrode layer 42 is made of a smooth substrate with a temperature of room temperature, a deposition rate of 10 persons/sec, and an ultimate pressure of 2×10−'.
The test was carried out under the conditions of Torr and film thickness of 1000 people. Subsequently, nickel (Ni) is formed on the base electrode layer 42 by electroforming to form a solid support substrate 44. Such electroforming uses a Watts bath to maintain the temperature at 50'C, and the current density is 0.06A/a.
m", electroforming time 2 hours, thickness 1100
I got LL. (FIG. 4(C)) Next, the smooth substrate 41 was peeled off from the underlying electrode layer 42 to obtain a smooth electrode substrate consisting of the solid support substrate 44 and the underlying electrode layer 42. (Figure 4(d)
) Subsequently, in the same manner as in Example 1, a 5OAZ monomolecular cumulative film 4 was formed.
5, an upper electrode 46, and a metal coating 47 were formed. (FIGS. 4(e) and (f)) As with Example 1, good switching characteristics were obtained in this element as well.

[Effects of the Invention] As explained above, by using an electrode substrate having a smooth surface with surface irregularities of 1 nm or less as a base electrode of an MIM element, it is possible to obtain a material having the periodic layer structure disclosed by the present inventors. It has become possible to more stably exhibit switching characteristics having memory properties in an MIM element using an organic thin film as an insulating layer, especially in a region where the thickness of the organic insulating layer is thin. Also, the resistance ratio between two memory states, i.e., O
N10 F As the F ratio becomes larger than before, O
Even when the transition between N10F and F is made to occur continuously for a long time, element damage, which is thought to be caused by local electric field concentration, becomes less likely to occur, making it possible to significantly extend the element life.

Furthermore, since such a smooth electrode substrate is formed by transferring the surface shape of a smooth substrate, there are fewer restrictions on electrode materials and substrates than in conventional methods, and it can be easily formed, making it suitable for application to MIM devices. has become easier.

[Brief explanation of the drawing]

1 to 4 show the manufacturing process of the device in each example, and FIG. 5 shows the current-voltage characteristics (1-V characteristics) obtained with the MIM device of the present invention. 11.21, 31.41 Smooth substrate 12.22, 32.42 Base electrode layer 13 Adhesive layer 14.24, 34.44 Solid support substrate 15.25,
35.45 Organic Insulating Layer 16.26.・36.46
Upper electrode 17.47 Metal coating 28.38 Second insulating layer

Claims (9)

[Claims] (1) A switching element having an organic insulator having a periodic layer structure between a pair of electrodes and having a memory property for switching characteristics, the periodic layer structure comprising:
1. A switching element comprising: an organic insulator formed on a base electrode having a surface unevenness having a height difference of 1 nm or less. (2) The switching element according to claim 1, wherein the base electrode surface is formed by transferring the surface shape of a smooth substrate. (3) The switching element according to claim 1, wherein the base electrode material is a noble metal. (4) The switching element according to claim 1, wherein the base electrode material is a noble metal alloy. (5) The switching element according to claim 1, wherein the base electrode material is a multilayer film containing a noble metal or a noble metal alloy. (6) The switching element according to claim 1, wherein the smooth substrate is a cleaved crystal substrate. (7) The switching element according to claim 1, wherein the main surface of the smooth substrate is formed by melting. (8) The switching element according to claim 1, wherein the organic insulator having a periodic layer structure is formed by the Langmuir-Blodgett method. (9) The switching element according to claim 1, further comprising a second insulating layer separate from the organic insulator having a periodic layer structure between the pair of electrodes.