JPH0784281A - Active element - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a highly reliable active element having a structure in which a ferroelectric active layer is sandwiched between two electrodes, in which peeling between the electrode and the active layer is suppressed. [Structure] The active layer is composed of two layers of ferroelectric films, and one ferroelectric film is formed of a material having higher conductivity than the other ferroelectric film. With this configuration, the electric field applied to the side of the highly conductive film is reduced, and polarization inversion does not occur in this layer. In this way, the occurrence of strain in the electrodes and the active layer during the operation of the device is suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-linear element used as a switching element of a display device or the like.

[0002]

2. Description of the Related Art An active element used as a switching element of a display device or the like has a structure in which an active layer is provided between two electrode films, and in particular, an active element using a ferroelectric substance in this active layer. Has been extensively used and actively researched for the following reasons.

FIG. 4 shows the basic structure of an active element. In this active element, as shown in FIG. 4, a first electrode 22 is formed on an insulating substrate 21, and a ferroelectric film is formed as an active layer 23 so as to cover the first electrode 22. Has been done. The second electrode 24 is formed on the ferroelectric film, and the element is constituted by a portion where the first electrode 22, the active layer 23 (ferroelectric film) and the second electrode 24 intersect each other. When an electric field is applied to the active element having such a structure, electric charges are induced on the surface of the ferroelectric substance which is the active layer. The amount of electric charge induced on the surface of the ferroelectric substance per unit area is called electric displacement.

FIG. 5 shows the relationship between the electric field applied to the ferroelectric substance and the electric displacement. In the figure, the horizontal axis shows the electric field E applied to the ferroelectric substance, and the vertical axis shows the electric displacement D generated on the surface of the ferroelectric substance.

When the electric field E is applied to the ferroelectric substance and the electric field E is gradually increased from 0 and the electric displacement D is measured each time, the electric field E changes along the curve OA in the figure. Even if the electric field E is made stronger than the value, the electric displacement does not increase,
The electric displacement D reaches the saturation point A. Even if the electric field E is reduced from the saturation point A of the electric displacement D to 0, the electric displacement D becomes 0.
However, electric charges remain on the surface of the ferroelectric substance. The electric charge remaining on the surface of the ferroelectric substance is the remanent polarization P _R. When the electric field E is applied in the opposite direction from this state where the electric field E is 0, the electric field E
The electric displacement D becomes 0 only when the value of becomes (-E _C ). The value of the electric field E at this time | E _C | is called the coercive electric field E _C ,
The degree of maintaining the electric displacement D is shown. It goes further electric displacement D when going by applying an electric field E varies along the curve of -E _C B of FIG reach the reverse saturation point B. When the electric field E is reduced in the opposite direction from the saturation point B, the electric displacement D does not become 0 even when the electric field E becomes 0 at this time, and the electric charge of the opposite sign remains as the residual polarization −P _R. From the position of this graph,
When the electric field E in the first direction is added, the electric field E becomes (E _C )
The electric displacement D becomes 0 only when the value becomes. The value of the electric field E at this time is also called a coercive electric field E _C. When the electric field E is further strengthened, the electric displacement D changes along the curve E _C A in the figure and reaches the saturation point A of the electric displacement D described above. Thus, the ferroelectric exhibits a hysteresis characteristic as shown between the electric field E applied to it and the electric displacement D. Here, the electric displacement D is observed as the sum of the spontaneous polarization (ferroelectric polarization) in the crystal structure of the ferroelectric substance and the polarization (paraelectric polarization) proportional to the strength of the electric field E applied to the ferroelectric substance. The remanent polarization P _R represents the spontaneous polarization when the applied electric field E is zero. (In an actual ferroelectric substance, the spontaneous polarization in the crystal structure of the ferroelectric substance is not perfect, so the remanent polarization P _R is
It is smaller than the theoretical spontaneous polarization P _S when the applied electric field E is 0. In an active element having a ferroelectric substance having such behavior in the active layer, when an electric field higher than the coercive electric field E _C is applied to the active layer 23, the spontaneous polarization of molecules in the ferroelectric substance is inverted. When the spontaneous polarization is inverted, a polarization inversion current is generated. In the active element having the ferroelectric layer in the active layer 23, the switching operation is performed by electrically reading the polarization inversion current.

The reversal of the spontaneous polarization of this ferroelectric substance is
Once the reversal occurs, the electric field E having a magnitude equal to or larger than the coercive electric field E _{C in the} opposite direction is maintained. That is, it has a memory property. Therefore, when an active element having a ferroelectric substance in an active layer is used as a switching element of a simple matrix type liquid crystal display device by utilizing the memory property of the ferroelectric substance, it can be turned on and off. If the voltage applied to the liquid crystal is set to a voltage with the coercive electric field E _C as a boundary, no voltage is applied to the liquid crystal when it is not lit, and display without crosstalk is possible. Further, since the difference between the voltages applied to the liquid crystal when the light is turned on and when the light is not turned on is large and can be reliably maintained, the occurrence of an afterimage is almost eliminated and a good display can be obtained.

As an active element using a ferroelectric material for an active layer, lead lanthanum zirconate titanate is used as the ferroelectric material, which is applied to a switching element of a liquid crystal display device by Molec. Clyst. Li
q. Clyst. , 15, 95, (1971).

Also, a polyvinylidene fluoride-trifluoroethylene copolymer is used as an active layer and is also applied to a switching element of a liquid crystal display device as Proc.
Seeding S. I. D. 91, p18 (1991)
Have been introduced to. Those using a polymer ferroelectric such as polyvinylidene fluoride-trifluoroethylene copolymer have advantages such as stability of coercive electric field and easiness of manufacturing process of an element, and have been actively studied.

[0009]

However, in an active element using a ferroelectric substance as an active layer,
Since the ferroelectric substance has piezoelectricity, there is a problem that the volume changes due to polarization reversal and the electrode film formed in contact with the ferroelectric substance peels off during long-term use.

The present invention solves the problem of electrode film peeling in an active element using a ferroelectric as an active layer,
It is to provide a highly reliable ferroelectric active device.

[0011]

An active element of the present invention is an active element having an active layer between a first electrode and a second electrode, the active layer being formed of a ferroelectric material. An active element containing a substance for increasing the electric conductivity of the ferroelectric substance in the ferroelectric substance, whereby the above object is achieved.

[0012] Preferably, the ferroelectric contains an organic compound.

Preferably, the organic compound is polyvinylidene fluoride, polyvinyl fluoride, trifluoroethylene,
It contains at least one of tetrafluoroethylene and nylon as a monomer unit.

In one embodiment, the active layer is a first layer.
And a second ferroelectric film having a conductivity higher than that of the first ferroelectric film.

In one embodiment, the concentration of the substance that increases the conductivity of the ferroelectric substance is changed in the thickness direction of the ferroelectric substance.

[0016]

The active element of the present invention is an active element having a first electrode and a second electrode, and an active layer formed of a ferroelectric material between the two electrodes. The ferroelectric layer is formed in a single-layer structure in which a substance that increases the conductivity is contained in the surface layer, or a two-layer structure in which one ferroelectric film and another ferroelectric film containing a conductive substance are formed. To form. With this configuration, the electric field applied to the active layer is reduced in the portion where the conductivity of the active layer is high. As a result, polarization reversal does not occur in the portion where the conductivity of the active layer is large, and the volume change of the active layer is suppressed, so that the strain between the electrode and the active layer is reduced.

[0017]

EXAMPLES Examples of the present invention will be described below. The present invention is not limited to the embodiments. 1 and 2 show the structure of the active element according to this embodiment.

In this active element, as shown in FIG. 1, a first electrode 2 is formed on an insulating substrate 1, and a first ferroelectric film is formed so as to cover the first electrode 2. 4 are formed. The second ferroelectric film 4 is covered with the second ferroelectric film 4.
Of the first ferroelectric film 4 is formed.
The two-layer structure of the second ferroelectric film 5 and the second ferroelectric film 5 is the active layer 3 of this active element. The second electrode 6 is formed on the second ferroelectric film 5, and the first electrode 2, the first ferroelectric film 4, the second ferroelectric film 5 and the second electrode are formed. The part where 6 intersects together functions as an element. Such an element is manufactured as follows.

First, a thin film for the first electrode 2 is formed by using a conductive material such as tantalum (Ta) or titanium (Ti) on the surface of an insulating substrate 1 made of glass, quartz, resin, silicon, ceramics or the like. To form. As the conductive material for the first electrode 2, niobium, aluminum, molybdenum, copper, indium tin oxide (ITO) or the like can be used. As the thin film forming method, a method such as sputtering, electron beam evaporation, or thermal evaporation can be used. The thin film is patterned into a predetermined shape by a known technique such as photolithography and mask devotion, and the first electrode 2
To form.

Next, a first ferroelectric film 4 is formed so as to cover the first electrode 2. As the material of the ferroelectric film,
Inorganic ferroelectrics such as barium titanate, lead zirconate titanate, lead lanthanum zirconate titanate, polyvinylidene fluoride, polyvinylidene fluoride / trifluoroethylene copolymer, polyvinylidene fluoride / tetrafluoroethylene copolymer, Any organic ferroelectric such as polyvinylidene cyanide / ethyl acetate copolymer, polyacrylonitrile, odd-numbered nylon, ferroelectric liquid crystal, ferroelectric liquid crystal polymer, or a mixture thereof may be used. Then, a polyvinylidene fluoride / trifluoroethylene copolymer was used from the viewpoints of heat resistance of the substrate during the production process and ease of production. This polyvinylidene fluoride / trifluoroethylene copolymer was formed by spin coating with a 3% methyl ethyl ketone solution. 2000 rp
A film thickness of 200 nm was obtained by coating at a rotation speed of m.
The polyvinylidene fluoride / trifluoroethylene copolymer thin film thus formed was baked in a clean oven at 140 ° C. for 30 minutes and then cooled at a rate of 50 ° C./hour. Polyvinylidene fluoride / trifluoroethylene copolymer exhibits ferroelectricity without any special treatment,
Ferroelectricity can be forcibly expressed by applying poling or shear stress.

Next, a second ferroelectric film 5 is formed so as to cover the first ferroelectric film 4. This second ferroelectric film 5
Is based on the same polyvinylidene fluoride / trifluoroethylene copolymer as that of the first ferroelectric film 4, on which metal particles such as indium tin oxide or aluminum or TCNQ (tetra-cyano-quinodimethane) is added.
/ TTF (tetra thiaful divalene) and other conductive materials are contained. When an inorganic substance is used as the base ferroelectric substance, it is preferable to use a mixture of the above-mentioned conductive substances as a target when the inorganic substance is formed by sputtering. When the sol / gel method is used, it is preferable to use a mixture of the above-mentioned conductive substance in the sol. As a matter of course, it is very difficult to use an organic conductive material as the conductive material to be contained in terms of heat resistance. When an organic ferroelectric such as polyvinylidene fluoride or a polyvinylidene fluoride / trifluoroethylene copolymer used in this example is used as the base, the conductive substance to be contained may be either inorganic or organic. Alternatively, it may be mixed in the ferroelectric solution, coated and fired.

In this way, the active layer 3 composed of the two layers of the first ferroelectric film 4 and the second ferroelectric film 5 was formed. The active layer 3 is as shown in FIG. ,
The structure may be a single layer, and the concentration of the conductive substance contained in the base ferroelectric substance may be formed to have a gradient in the thickness direction of the ferroelectric layer. With this structure, it is possible to obtain an element that functionally has the same effect as that of the two-layer structure without peeling of layers inside the active layer 3. The manufacturing process is also simplified. The active layer 3 having the single-layer structure is manufactured as follows.

After forming the first electrode 2 on the surface of the substrate 1 in the same manner as described above, a 10 wt% ethanol solution in which TCNQ and TTF are adjusted to have a molar ratio of 1: 1 is used as a ferroelectric film to form a polyfluoride film. A vinylidene chloride / trifluoroethylene copolymer membrane was used, and TCNQ and TTF were used in a molar ratio of 1:
It was dipped in a 10 wt% ethanol solution adjusted to 1 for 24 hours to impregnate it with TCNQ / TTF which is a conductive substance. Thus, a portion 7 of the ferroelectric film having a low content of the conductive substance is formed near the first electrode 2, and a conductive substance of the ferroelectric film is formed near the second electrode 6 (or the ferroelectric substance). A portion 8 having a high content rate of is formed.

After forming the active layer 3, Ta (tantalum) or Ti (titanium) is covered to cover the active layer 3.
A conductive thin film such as
The electrode 6 is formed. As the conductive film material for the second electrode 6, the same material as that for the first electrode 2 may be used.

The active element thus manufactured was operated by applying a pulse wave of 50 V and 60 Hz, but no peeling of the electrode was observed even after 2000 hours of operation, and good operating characteristics were exhibited.

In contrast to the active element of the present example produced as described above, the active element in which the active layer 3 is formed of only the polyvinylidene fluoride / trifluoroethylene copolymer film is operated under the same conditions. , About 50
The peeling of the second electrode 6 occurs from the point of time of operation,
After operating for 2000 hours, the capacity of the device decreased to about 10% of the initial capacity.

In FIG. 3, the active element according to the present invention and the active element in which the active layer is formed of only the polyvinylidene fluoride / trifluoroethylene copolymer film are operated for 2000 hours by applying a pulse wave of 50 V and 60 Hz. The hysteresis characteristic when it is made to show is shown. In the figure, the horizontal axis represents the applied voltage E to the element, and the vertical axis represents the voltage V induced in the measuring capacitor. As shown in the figure, according to the active element of this example, a normal hysteresis curve of the ferroelectric substance described in the section of the conventional example is drawn, but in the conventional element, the hysteresis curve has a flat elliptical shape. None, the induced voltage V is small, and it can be seen that almost no normal operation is performed.

[0028]

According to the present invention, an active element having a first electrode and a second electrode, and an active layer made of a ferroelectric material between the two electrodes, even when operated for a long time, It is possible to obtain a highly reliable ferroelectric active element in which peeling of electrodes is suppressed as compared with the related art.

[Brief description of drawings]

FIG. 1 is a diagram showing an active element of this embodiment.

FIG. 2 is a diagram showing another example of the active element of the present embodiment.

FIG. 3 is a diagram showing a hysteresis characteristic of an active element of this embodiment and a hysteresis characteristic of a conventional element.

FIG. 4 is a diagram showing an example of a conventional active element.

FIG. 5 is a diagram showing a hysteresis characteristic of a ferroelectric substance.

[Explanation of symbols]

 1 substrate 2 first electrode 3 active layer 4 first ferroelectric film 5 second ferroelectric film 6 second electrode 7 part of the ferroelectric film having a low content of a conductive substance. 8 A portion of the ferroelectric film having a high content of a conductive substance.

Claims (5)

[Claims] 1. An active device having an active layer between a first electrode and a second electrode, wherein the active layer is made of a ferroelectric material, and the active material is formed in the ferroelectric material. An active element containing a substance that increases conductivity. 2. The active element according to claim 1, wherein the ferroelectric contains an organic compound. 3. The active element according to claim 2, wherein the organic compound contains at least one of polyvinylidene fluoride, polyvinyl fluoride, trifluoroethylene, tetrafluoroethylene, and nylon as a monomer unit. 4. The active layer according to claim 1, wherein the active layer has a two-layer structure of a first ferroelectric film and a second ferroelectric film having a conductivity higher than that of the first ferroelectric film. element. 5. The active element according to claim 1, wherein the concentration of the substance that increases the conductivity of the ferroelectric substance changes in the thickness direction of the ferroelectric substance.