JPH077851B2 - MIM switching element - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an active element used when matrix-driving various display elements typified by liquid crystal display elements and electrochromic display elements, and in particular insulated from a lower conductor thin film. The present invention relates to an insulator thin film of a MIM switching element having a three-layer structure in which a body thin film and an upper conductor thin film are sequentially laminated.

BACKGROUND OF THE INVENTION Liquid crystal display devices have been used as display units of information equipment terminals up to now, but their display capacity is relatively small, so their application range is limited. In recent years, it has been studied to use a non-linear element as a means for increasing the display capacity. As one of the non-linear elements, MIM switching composed of a three-layer structure of a lower conductor thin film, an insulator thin film and an upper conductor thin film is used. The element is receiving attention. Also in the electrochromic display element, as in the liquid crystal display element, an increase in display capacity by matrix driving using a non-linear element is being studied.

[Problems to be solved by the invention]

As a MIM switching element, it is common to use a tantalum film as the lower conductor thin film, a tantalum oxide film as the insulator thin film, and a metal film such as a tantalum film or a chromium film as the upper conductor thin film. This will be described with reference to FIG.

2 (a) is a plan view showing a MIM element in the conventional example, and FIG. 2 (b) is a cross-sectional view showing the MIM element in the conventional example, each showing a region corresponding to one pixel. The MIM switching element in the conventional example will be described below with reference to FIG.

On the substrate 20 made of glass, a tantalum film as a material for the protective film 21 is formed to a thickness of 0.1 μm by a vacuum deposition method or a sputtering method.
m over the entire surface. After that, thermal oxidation treatment is performed in a furnace at a temperature of 500 ° C. to convert the tantalum film into a tantalum oxide film, which is used as the protective film 21. The protective film 21 made of this tantalum oxide film is
In the patterning process of the lower conductor thin film 221, the substrate 2 is formed by the etchant of the lower conductor thin film 221 in the subsequent process.
It has a role of preventing 0 from being etched. That is, if the lower conductor thin film 221 is patterned without forming the protective film 21, the substrate 20 is etched and the surface roughness of the substrate 20 becomes large, so that a fine pattern cannot be obtained.

After that, a conductor thin film 22 made of a tantalum film is formed on the entire surface with a film thickness of 0.3 μm. After that, the conductor thin film 22 is patterned by photo-etching, and the line width dimension is 20 μm.
The conductor thin film 22 is formed. The conductor thin film 22 made of this tantalum film is etched using an etching solution of hydrofluoric acid: nitric acid: water = 1: 10: 10. As shown in the plan view of FIG. 2A, the conductor thin film 22 has a protrusion shape corresponding to the lower conductor thin film 221 having a line width dimension of 4 μm, and the MIM switching element is formed in this protrusion-shaped region. .

After that, an insulator thin film 23 made of a tantalum oxide film is formed with a thickness of 0.05 μm on the surfaces of the conductor thin film 22 and the lower conductor thin film 221 other than the electrode extraction region at the outermost periphery of the substrate 20. The insulator thin film 23 made of this tantalum oxide film is formed by anodizing treatment using an oxalic acid aqueous solution as an anodizing solution. The insulator thin film 23 on the surface of the lower conductor thin film 221
Function as an insulator film for the MIM switching element.

After that, an upper conductor thin film 24 made of a chromium film is formed on the entire surface and patterned by photoetching to have a line width dimension of 4 μm. Further, a transparent conductive film is formed on the entire surface, and a pixel electrode 25 connected to the upper conductor thin film 24 is formed by photoetching. Here, as the upper conductor thin film 24, not only a metal film but also the same transparent conductive film as the pixel electrode 25 may be used.

The MIM switching element thus configured exhibits a non-linear voltage-current characteristic with respect to the voltage application from the outside, and the switching operation can be performed using this MIM switching element.

Here, a liquid crystal display element will be described as an example of the display element in the following description. When the voltage V NO for lighting the liquid crystal is applied to the substrate 20 forming the MIM switching element and the counter substrate (not shown) forming the counter electrode facing the substrate 20, MI
Since the effective resistance on the M switching element side is low and the resistance on the liquid crystal side is high, most of the applied voltage V ON is applied between the pixel electrode 25 and the counter electrode, and the liquid crystal is turned on.

On the other hand, when the voltage V OFF that does not turn on the liquid crystal is applied, the effective resistance on the MIM switching element side becomes higher than the resistance value on the liquid crystal side, so almost no voltage V OFF is applied between the pixel electrode 25 and the counter electrode. No, keep it off.

By controlling the voltage applied from the outside in this way,
The on / off of the liquid crystal can be arbitrarily controlled, and the display capacity can be significantly increased as compared with the conventional case.

[Problems to be solved by the invention]

The conventional structure using a tantalum oxide film as the insulator thin film 23
Assuming that the liquid crystal display element is driven by the MIM switching element, the capacitance C MIM of the MIM switching element is sufficiently smaller than the capacitance C LC of the liquid crystal in order to obtain good display characteristics and high duty ratio. There must be.

The capacitance C MIM of the MIM switching element is given by the following equation.

C MIM = εo · εr · s / d where εo: vacuum permittivity, εr: relative permittivity of the insulator thin film, s:
Area of MIM switching element, d: thickness of insulator thin film.

In the above equation, the relative permittivity εr of the insulator thin film has a value peculiar to the insulator thin film, and is a value of 23 to 25 when a tantalum oxide film is used as the insulator thin film.

Variable parameters in the above equation are the area s of the MIM switching element and the film thickness d of the insulator thin film. However, the film thickness d of the insulator thin film cannot be excessively increased, and the contribution of the film thickness d in the direction of decreasing the capacitance C MIM of the MIM switching element is small. As a result, the electrostatic capacitance C MIM is reduced by minimizing the area s of the MIM switching element.

In order to obtain sufficient display characteristics even if the area s of the element is reduced, the area s of the MIM switching element needs to be set to 1 / 10,000 or less of the area of the pixel electrode 25. This indicates that when the size of one dot of the liquid crystal display element is 400 μm square, the size of the MIM switching element needs to be 4 μm square or less. Therefore, it is extremely difficult to form an element on the entire surface of a 10 cm square substrate, for example, considering alignment accuracy and etching accuracy, and if possible, while maintaining the characteristics.
It is desirable to be able to increase the size of MIM switching elements.

Therefore, changing the material of the insulator thin film to reduce the relative permittivity εr of the insulator thin film is disclosed in, for example, JP-A-60-241021.
It is proposed in Japanese Patent Publication No. This publication proposes to use a polymer, polyimide or polyamide-imide formed by a screen printing method, a spin coating method or a mask vapor deposition method as the insulator thin film 23.

However, the insulating thin film 23 of the above-mentioned publication has a thickness of 0.05 μm.
It is difficult to form a pinhole with a film thickness of about m and to form a large area with good reproducibility, and the lower conductor thin film 221 and the upper conductor thin film 24 are electrically connected.

[Object of the Invention]

An object of the present invention is to solve the above problems and to provide an MI thin film having a thin film thickness and a large area with no pinhole.
It is to provide an M switching element.

[Means for solving problems]

In order to achieve the above object, the insulator thin film of the MIM switching element of the present invention is characterized by using a plasma-polymerized acetonitrile film.

[Action]

As the insulator thin film of the present invention, a plasma polymerized acetonitrile film formed by a plasma polymerization method having a small relative dielectric constant εr is used. Therefore, it is possible to obtain the MIM switching element having a large element size while satisfying the driving condition of the liquid crystal display element. Furthermore, since the plasma polymerization method is used as a means for forming the plasma-polymerized acetonitrile film, pinholes do not occur in a large area, and the insulator thin film can be obtained with good reproducibility.

〔Example〕

The configuration of the MIM switching element according to the embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing an MIM switching element according to an embodiment of the present invention.

As shown in FIG. 1, a lower conductor thin film 111 is provided on a substrate 10 made of glass, and an insulator thin film 12 is further provided on the entire surface. As the insulator thin film 12, a plasma polymerized acetonitrile film formed by a plasma polymerization method is used. Further, an upper conductor thin film 13 made of a chrome film and a pixel electrode 14 made of a transparent conductive film are provided.

Next, a manufacturing method for obtaining the configuration of the MIM switching element shown in FIG. 1 will be described.

First, a copper film is formed as a material for the conductor thin film 11 on the entire surface of the substrate 10 made of glass. After that, the conductor thin film 11 made of a copper film is patterned by photoetching to form a lower conductor thin film 111 having a width of 4 μm.

After that, an insulator thin film 12 made of a plasma-polymerized acetonitrile film formed by a plasma polymerization method is formed on the entire surface of the substrate 10 excluding the electrode extraction region. The formation of the insulator thin film 12 of this plasma-polymerized acetonitrile film is performed under the conditions of acetonitrile flow rate: 80 cm ³ / min, substrate temperature: 100 ° C, internal pressure: 0.5 Torr, radio frequency (RF) power: 100 W, processing time: 10 minutes Thus, a plasma polymerized acetonitrile film having a film thickness of 0.05 μm could be formed. The relative permittivity εr of the plasma-polymerized acetonitrile film formed by this plasma-polymerization method
Was a value of 3 to 5 as a result of measurement, and showed good rectifying property, and no pinhole was generated.

Then, a chromium film is formed on the entire surface as the upper conductor thin film 13 and patterned by photoetching to form the upper conductor thin film 13 having a width of 4 μm.

After that, a transparent conductive film is formed on the entire surface as a material for the pixel electrode 14, and is patterned by photoetching to form the pixel electrode 14.

As a result, a MIM switching element having an element area of 16 μm ² was formed, and a normal liquid crystal display element assembling process was performed to obtain a liquid crystal display panel having a plasma-polymerized acetonitrile film formed by plasma polymerization as the insulator thin film 12. It was

The liquid crystal display panel provided with this plasma-polymerized acetonitrile film had a quick response and showed good display characteristics. this is
It is considered that this is because the capacitance C MIM of the MIM switching element is reduced from 1/5 that of the conventional tantalum oxide film to 1/8, which shortens the liquid crystal charging time.

In the embodiments described above, the copper film is used as the lower conductor thin film 111 and the chromium film is used as the upper conductor thin film 13, but other metal films may be used. And the substrate
By selecting an etchant that does not etch 10, it is possible to omit the formation of a protective film, which has been conventionally required.

Furthermore, both the lower conductor thin film 111 and the upper conductor thin film 13 may use transparent conductive films. The liquid crystal display panel composed of this transparent conductive film showed characteristics equal to or better than the conventional one. This is because the specific resistance of the transparent conductive film is 2 × 10 ⁻⁴ ohm · cm, which is about half that of the conventional lower conductor thin film made of tantalum film.
It can be said that this is an effective means for making a large liquid crystal display panel. further
Since all the constituent elements of the MIM switching element are made of a transparent body, even if the element size increases, they are hardly discernible in appearance, and it can be said that this is a very preferable structure in terms of display characteristics.

〔The invention's effect〕

As described above, by using the plasma-polymerized acetonitrile film formed by plasma polymerization according to the present invention as the insulating thin film, the thin film thickness is about 0.05 μm, and pinholes are not generated in a wide area. A coating having a low relative dielectric constant can be obtained relatively easily. For this reason MIM
It is very effective in improving the characteristics of switching elements, creating a large-sized liquid crystal display panel, simplifying the process, and improving reliability. Furthermore, the display quality can be improved by forming the lower conductor thin film and the upper conductor thin film with transparent conductive films.

[Brief description of drawings]

FIG. 1 is a sectional view showing an MIM switching element according to an embodiment of the present invention, FIG. 2 is a MIM switching element according to a conventional example, FIG. 2 (a) is a plan view, and FIG. 2 (b) is a sectional view. Is. 10 …… Substrate, 12 …… Insulator thin film, 13 …… Upper conductor thin film,
14 …… Pixel electrode, 111 …… Lower conductor thin film.

Claims (1)

[Claims] 1. A MIM switching element having a three-layer structure in which a lower conductor thin film, an insulator thin film, and an upper conductor thin film are sequentially laminated on a substrate surface, wherein the insulator thin film is a plasma-polymerized acetonitrile film. Switching element.