JPH0667167A - Plastic substrate for liquid crystal display device - Google Patents

Abstract

PURPOSE:To obtain a high contrast reflection type liquid crystal display device by preventing the degradation of contrast caused by boundary reflection of incident light. CONSTITUTION:In the first step, a thin film 2 made of an inorganic material having 1.6 to 2.0 reflective index is formed on a surface forming a transparent electrode 4 of a plastic substrate 1. In the second step, a thin film 3 made of an inorganic material having 1.2 to 1.6 reflective index is provided on the opposite face of the surface on which the transparent electrode 4 is formed. In the third step, the transparent electrodes 4 are arranged in a matrix state on the plastic substrate 1 and a switching element is provided at each of the transparent electrode 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plastic substrate for a liquid crystal display device, and more particularly to a plastic substrate for an active matrix liquid crystal display device which can be preferably used for a liquid crystal display device such as a flat panel display for OA equipment and TV.

[0002]

2. Description of the Related Art In OA equipment terminals and liquid crystal TVs, there is a strong demand for the use of large-area liquid crystal panels. Therefore, in the active matrix system, a switch is provided for each pixel so that the voltage can be maintained. Kaisho 62-62333, 61
-See the 260219 publication). In recent years, liquid crystal panels have been actively reduced in weight and cost, and the use of plastic as a substrate has been studied (Japanese Patent Laid-Open No. 1-47769).
(See the official gazette). However, when manufacturing pixel (transparent) electrodes and switching elements on plastic, there is a step of immersing the plastic in a solution of acid, alkali, water, etc., and acid, alkali, water, etc. remain in the plastic, It causes characteristic deterioration.

In order to avoid this, it is conceivable to form a thin film made of an inorganic substance on at least one surface (the surface on which the transparent electrodes and switching elements are formed), preferably both surfaces, of the plastic substrate. As such an inorganic substance, SiO _x (x = 1.1 to 1.9) and SiN _x (x = 0.5 to 1.) from the viewpoints of adhesion to a plastic substrate and permeability of various solutions.
2) Alternatively, a laminate of the above has been proposed by the present inventors (Japanese Patent Application Nos. 2-417313 and 3).
No. 189336).

On the other hand, in portable equipment represented by a notebook personal computer, a reflection type liquid crystal display device which does not use a backlight is desired from the demand of low power consumption and thinning. Polymer-dispersed liquid crystals using a scattering mode, which has been actively researched in recent years, are expected to be particularly suitable for this application because they do not require a polarizing plate.

[0005]

However, the reflection type liquid crystal display device has a problem that the contrast is lowered due to interface reflection of incident light. FIG. 4A shows a normal TN liquid crystal, FIG.
(B) is a cross-sectional view of a conventional reflective liquid crystal display device using polymer dispersed liquid crystal. In both cases, SiO ₂ thin films are formed on both sides of the plastic substrate.

An example of the operation will be described with reference to FIG. Since the rubbing directions of the alignment films provided on the upper and lower substrates are orthogonal to each other, and the polarization direction of the polarizing plate is aligned with that, incident light is reflected by the pixels to which no voltage is applied, and the upper polarizing plate is By passing through, white is displayed (actually the color through the polarizing plate). Pixels to which voltage is applied are reflective polarizing plates (because liquid crystal molecules are aligned in the vertical direction).
Black is displayed when the reflected light polarized by 90 ° does not pass through the upper polarizing plate. The contrast ratio is represented by the ratio of the amount of reflected light. As can be seen from the figure, there are many interfaces (bonding surfaces of substances with different refractive indices) before the incident light reaches the reflective polarizing plate, so the amount of reflected light in the state where black should be displayed by interface reflection Increase and decrease contrast. An operation example will be described with reference to FIG. In a pixel to which no voltage is applied, the incident light is scattered because the liquid crystal molecules and the dye have random molecular axes, and the color of the dye (generally black) is displayed. In a pixel to which a voltage is applied, incident light is reflected by a reflection plate (generally a diffuse reflection plate) because liquid crystal molecules and dye molecular axes are aligned in the vertical direction, and white is displayed. Also in this case, interface reflection occurs before the incident light reaches the polymer dispersed liquid crystal layer, and the contrast is lowered. The purpose of the present invention is to
An object of the present invention is to provide a reflective liquid crystal display device with high contrast by solving the above-mentioned drawbacks of the prior art.

[0007]

A first aspect of the present invention is to form a thin film of an inorganic substance having a refractive index of 1.6 or more and 2.0 or less on a surface on which a transparent electrode is formed. It is on a plastic substrate for display devices. A second aspect of the present invention is the plastic substrate for a liquid crystal display device, wherein a thin film made of an inorganic material having a refractive index of 1.2 or more and 1.6 or less is formed on the surface opposite to the surface on which the transparent electrode is formed. is there. A third aspect of the present invention is a plastic substrate for an active matrix type liquid crystal display device, wherein transparent electrodes are arranged in a matrix on the plastic substrate and a switching element is provided on each transparent electrode.

As described above, in a liquid crystal display device using a plastic substrate, it is indispensable to form a thin film made of an inorganic material on at least the surface of the substrate on which the transparent electrode or the switching element is formed, preferably both surfaces.
In the present invention, by limiting this inorganic substance (specifically, the refractive index), the plastic substrate is protected and at the same time reflection at the interface is reduced.

An example of the configuration of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view of the vicinity of a substrate on the incident light side of a liquid crystal display device using a polymer dispersed liquid crystal. The interface where different substances come in contact is air / plastic, plastic / transparent electrode, transparent electrode / polymer (liquid crystal). Refractive index is about 1.0 (air), 1.6 (plastic),
Since it is 2.0 (transparent electrode) and 1.5 (liquid crystal), the surface of the plastic substrate 1 on which the transparent electrode is formed has a refractive index of 1.
It is desirable to form the thin film 2 made of an inorganic material having a thickness of 6 or more and 2.0 or less, more preferably 1.7 or more and 1.9 or less. Al ₂ O ₃ , CeF ₃ , Mg as such substances
Examples thereof include O and SiO, and SiO is particularly desirable in terms of process resistance, environment resistance, film forming property, and the like. In addition, in order to further reduce the interface reflection, or more process resistance,
Two or more thin films may be formed by stacking thin films made of other substances according to the purpose such as increasing the environmental resistance.

A thin film 3 made of an inorganic substance having a refractive index of 1.2 or more and 1.6 or less, more preferably 1.2 or more and 1.4 or less is formed on the surface of the plastic substrate opposite to the surface on which the transparent electrode 4 is formed. It is desirable to form. Ca as such a substance
F ₂ , NaF, LiF, MgF _{2 and the} like can be mentioned, but MgF ₂ is particularly preferable in terms of environment resistance, film forming property and the like. In addition,
In order to further reduce interfacial reflection, increase process resistance and environment resistance, thin films made of other substances may be laminated to form two or more thin films.

Further, as shown in FIG. 2, a thin film 2'made of an inorganic substance having a refractive index of 1.6 or more and 2.0 or less, more preferably 1.7 or more and 1.9 or less so as to cover the transparent electrode. Is more effective. Also in this case, it does not prevent the thin films made of other substances from being superposed on each other as necessary to form two or more thin films. The film thickness of these thin films is several hundred Å to several μm, preferably 1000 Å to 1 μm. The refractive index of these thin films can be measured by, for example, an ellipsometer.

FIG. 3 is a cross-sectional view in which a switching element 13 is connected to each of the transparent electrodes (pixel electrodes) 4 arranged in a matrix. Each constituent material is based on FIG. Also in this case, the formation of the thin film 2 ′ shown in FIG. 2 is not hindered. The same effect can be obtained by using TN liquid crystal as the liquid crystal layer, but it is more effective if a similar thin film is formed on the lower substrate (reflector side).

As the switching element, a thin film transistor (TFT), a MIM type element having a metal-insulator-metal structure, an MSI element (Metal-Semi-Insnlator) referred to in JP-A-61-275811, a semiconductor-insulation element. There are SIS elements having a body-semiconductor layer structure and MIMIM elements of metal-insulator-metal-insulator-metal described in JP-A-64-7577. Among them, the MIM type element using a hard carbon film as an insulator is advantageous. By using a hard carbon film, it is possible to design a device in a wide range and to obtain a thin film two-terminal device having a small variation in device characteristics, an excellent threshold voltage and a high breakdown voltage, and a high yield.

There are no particular restrictions on the plastic,
It is desirable to use heat resistant plastics such as polyethylene terephthalate (PET), polyether sulfone (PES), polyarylate, polyimide, polyether ether ketone (PEEK), polyethylene naphthalate (PEN), and polyphenylene sulfide (PPS).

A method of manufacturing a thin film two-terminal element using the plastic substrate of the present invention will be described with reference to FIG. First, a thin film made of an inorganic material having a refractive index of 1.6 or more and 2.0 or less on at least the surface of the plastic substrate on which the transparent electrode is formed, and preferably on the opposite surface of the surface on which the transparent electrode is formed. A thin film made of an inorganic material having a thickness of 1.2 or more and 1.6 or less, a sputtering method, an evaporation method, a plasma CVD method.
Method, ion plating method, coating method, etc.
Deposit several μm. ITO, ZnO: Al, In
Sputtering a transparent conductive thin film such as ₂ O ₃ or SnO ₂
A film having a thickness of several hundred to several thousand Å was formed by a method such as vapor deposition, and the pixel electrode 24 was provided by etching in a predetermined pattern.

Next, Al, Ta, Ti, Cr, Ni, C
A conductive thin film such as u, Au, Ag, W, Mo, Pt, or Ni-Cr is formed into a film having a thickness of several hundred to several thousand Å by a method such as sputtering or vapor deposition, and is patterned into a predetermined pattern by etching. To form the lower conductor 21. On this lower conductor, by a plasma CVD method or an ion beam method, a hard carbon film is formed by forming a film having a thickness of 100 to 8000Å, preferably 200 to 6000Å, more preferably 300 to 4000Å, and then etching it into a predetermined pattern. 22 was formed.

Finally, Ni, Pt, Ag, Cr, Ti,
Cu, Au, W, Mo, Ta, Ni-Cr, ITO, Z
nO: Al, In ₂ O ₃ , SnO ₂ or other conductive thin film is formed to a thickness of several hundred to several thousand Å by a method such as sputtering or vapor deposition, and then etched into a predetermined pattern to form an upper conductor.
23 was set up. As the upper conductor and the lower conductor, two or more layers of the above-mentioned various conductive thin films may be used if desired. Further, the device according to the present invention is not limited to the structure shown in FIG. 5, but the structure in which the positional relationship between the upper conductor and the lower conductor is changed, or the positional relationship between the upper conductor and the lower conductor is changed, and A structure in which the conductor also serves as the pixel electrode may be adopted.

Next, a method of forming a hard carbon film used in the present invention and its characteristics will be described (Japanese Patent Laid-Open No. 3-223723).
See Japanese Patent Application Laid-Open Nos. 3-163531 to 163533).

The hydrocarbon gas as a raw material gas is, for example, paraffin hydrocarbon such as CH ₄ , C ₂ H ₈ or C ₄ H ₁₀ or C.
A gas containing at least all hydrocarbons such as olefinic hydrocarbons such as ₂ H ₄ , diolefinic hydrocarbons, acetylene hydrocarbons, and aromatic hydrocarbons can be used. In addition to hydrocarbons, compounds such as alcohols, ketones, ethers, and esters that contain at least a carbon element can be used.

As a method of forming a hard carbon film from a raw material gas in the present invention, a film formation active species is formed through a plasma state generated by a plasma method using direct current, low frequency, high frequency or microwave. A method is preferable, but a method utilizing a magnetic field effect is more preferable for causing the deposition under a low pressure for the purpose of increasing the area, improving the uniformity, and / or forming the film at a low temperature. In addition, active species can also be formed by thermal decomposition at high temperature. Besides, it may be formed through an ionic state generated by an ionization vapor deposition method, an ion beam vapor deposition method or the like, or may be formed from neutral particles generated by a vacuum vapor deposition method or a sputtering method. However, it may be formed by a combination thereof.

In the case of plasma CVD, an example of the deposition conditions for the hard carbon film thus produced is as follows. RF output: 0.1 to 50 W / cm ² Pressure: 10 ^{−3 to} 10 Torr Deposition temperature: Room temperature to 950 ° C., preferably room temperature to 300 ° C.

By this plasma state, the source gas is decomposed into radicals and ions to react with each other, so that amorphous and microcrystalline (crystal size) composed of carbon atoms C and hydrogen atoms H are formed on the substrate. Is several 10Å to several μm)
A hard carbon film containing at least one of the above is deposited. Table 1 shows various characteristics of the hard carbon film.

[0023]

[Table 1]

Measurement method: Specific resistance (ρ): Determined from the iv characteristics of a coplanar cell. Optical band gap (Egopt): calculated absorption coefficient (alpha) from the spectral characteristics are determined from the relationship ^{(αhν) 1/2 = B} (hν'-Egopt). Amount of hydrogen in film (C _H ): Calculated by integrating the peak around 2900 cm ⁻¹ from the infrared absorption spectrum and multiplying by the absorption cross section A. That is, C _H = A · ∫α (w) / w · dw Sp ³ / Sp ² : Infrared absorption spectrum is decomposed into Gaussian functions assigned to Sp ³ and Sp ² , respectively, and calculated from the area ratio. Vickers hardness (H): By micro Vickers meter. Refractive index (n): By ellipsometer. Defect density: According to ESR.

[0025]

EXAMPLES Examples of the present invention are shown below, but the present invention is not limited thereto.

Example 1 An SiO thin film was deposited on a PET substrate by vapor deposition to form 3500 liters. Next, the thin film two-terminal element shown in FIG. 5 was produced as follows. First, ITO was deposited to a thickness of 700 Å by a sputtering method and then patterned to form a pixel electrode 24. Next, deposit 1000 Å Al by vapor deposition.
After thickly depositing, the lower conductor 21 was formed by patterning. A hard carbon film is used as an insulating film 22 on the plasma CV.
After depositing 1100Å by method D, it was patterned by dry etching. Further, Ni is deposited thereon to a thickness of 1000 Å by the EB evaporation method, and then patterned to form the upper conductor.
Formed 23. At this time, the conditions for forming the hard carbon film are as follows. Pressure: 0.035 Torr CH ₄ Flow rate: 10 SCCM RF power: 0.3 W / cm ² The reflectance from the back surface of this substrate is 5%, which is lower than that when the SiO thin film is not formed (7%).

Example 2 EB on a polyarylate substrate
A MgF ₂ thin film of 3000 liters was deposited by a vapor deposition method. Further, an SiO thin film was deposited on the opposite surface by vapor deposition to 3500Å. A thin film two-terminal element shown in FIG. 5 was produced thereon as follows. First, ITO was deposited to a thickness of 700 Å by a sputtering method and then patterned to form a pixel electrode 24. Next, Al was deposited to a thickness of 600 Å by a vapor deposition method and then patterned to form a lower conductor 21. After depositing a hard carbon film on it by 1500 Å by the plasma CVD method, Ni is EB
1000 Å was deposited by the vapor deposition method. The Ni and the hard carbon film were sequentially etched to form the upper conductor 23 and the insulating film 22. Interfacial properties (adhesion, electrical barrier, etc.) can be improved by continuously forming a hard carbon film and Ni. At this time, the conditions for forming the hard carbon film are as follows. Pressure: 0.035Torr CH ₄ Flow rate: 10SCCM RF power: 0.2W / cm ² The reflectance from the back surface of this substrate is 2%, which is lower than the case where MgF ₂ thin film and SiO thin film are not formed (7%). did.

[Embodiment 3] A thin film two-terminal element was formed in the same manner as in Embodiment 2, and then SiO 2 was formed on the surface by vapor deposition.
A thin film was deposited at 3500Å.

Example 4 1500 μl of SiO thin film was deposited on the polyarylate substrate by the vapor deposition method, and then 1000 μl of MgF ₂ thin film was deposited by the EB vapor deposition method. Further, on the opposite side, deposit 2000 liters of SiO thin film by vapor deposition method and then deposit TiO ₂ thin film on 120 liters.
0Å Accumulated. A thin film two-terminal element shown in FIG. 5 was manufactured thereon in the same manner as in Example 2. The reflectance from the back surface of this substrate was 1%, which was lower than the case where each thin film was not formed (7%).

[0030]

【The invention's effect】

(1) The plastic substrate for a liquid crystal display device of the present invention has a refractive index of at least 1.6 or more and 2.0 on the surface on which the transparent electrode is formed.
Since a thin film made of the following inorganic material is provided, it has excellent process resistance and environment resistance, and can reduce reflection at the substrate / transparent electrode interface, resulting in a reflective liquid crystal display device with high contrast. To be (2) When a thin film made of an inorganic material having a refractive index of 1.2 or more and 1.6 or less is provided on the surface opposite to the surface on which the transparent electrode is formed, reflection at the air / substrate interface can be reduced,
A display with higher contrast can be obtained. (3) Further, when a thin film made of an inorganic material having a refractive index of 1.6 or more and 2.0 or less is provided so as to cover the transparent electrode, the reflection at the transparent electrode / liquid crystal interface can be reduced and the contrast can be further improved. High display is obtained. (4) The active matrix type using a switching element is more effective because it can display a high contrast even with a large capacity. (5) When a thin film two-terminal element in which an insulating film is interposed between a first conductor and a second conductor is used as a switching element, it has advantages of lower cost and higher aperture ratio than a TFT. It is particularly preferable as an active matrix substrate for liquid crystal displays. (6) In addition, since the hard carbon film is used as the insulating film,
Needless to say, it is suitable as a more reliable liquid crystal display switching element.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of an incident side substrate of a polymer dispersion type liquid crystal display device equipped with a plastic substrate according to the present invention.

FIG. 2 is a cross-sectional view of the vicinity of an incident side substrate of a polymer dispersed liquid crystal display device equipped with a plastic substrate according to the present invention.

FIG. 3 is a cross-sectional view of an incident side substrate of a polymer dispersed liquid crystal display device equipped with a plastic substrate according to the present invention.

FIG. 4A is a sectional view of a conventional reflective liquid crystal display device using TN liquid crystal.

FIG. 4B is a sectional view of a conventional reflective liquid crystal display device using polymer dispersed liquid crystal.

FIG. 5 is a perspective view of a typical example of a thin film two-terminal element of an active matrix substrate according to the present invention.

[Explanation of symbols]

 1 Plastic Substrate 2 Inorganic Thin Film 2'Inorganic Thin Film 3 Inorganic Thin Film 4 Transparent Electrode 5 Alignment Film 6 Liquid Crystal Layer (TN) 7 Polarizing Plate 8 Reflective Polarizing Plate 9 Liquid Crystal 10 Polymer Layer 11 Dye 12 Reflecting Plate 13 Switching Element 21 Lower Conductor 22 Insulating film (hard carbon film) 23 Upper conductor 24 Pixel electrode

Claims (6)

[Claims] 1. A plastic substrate for a liquid crystal display device having a transparent electrode, wherein a thin film made of an inorganic substance having a refractive index of 1.6 or more and 2.0 or less is provided on a surface of the transparent electrode. A plastic substrate for liquid crystal displays. 2. The liquid crystal display device according to claim 1, wherein a thin film made of an inorganic material having a refractive index of 1.2 or more and 1.6 or less is provided on the surface opposite to the surface on which the transparent electrode is formed. Plastic substrate. 3. The liquid crystal display device according to claim 1, wherein a thin film made of an inorganic material having a refractive index of 1.6 or more and 2.0 or less is provided so as to cover at least the transparent electrode. Plastic substrate. 4. An active matrix type liquid crystal display device, characterized in that transparent electrodes are arranged in a matrix on the plastic substrate according to claim 1, 2 or 3 and a switching element connected to each transparent electrode is provided. Plastic substrate. 5. The thin-film two-terminal element according to claim 4, wherein the switching element connected to the transparent electrode is a thin film two-terminal element in which a hard carbon film is interposed between the first conductor and the second conductor. Plastic substrate for active matrix liquid crystal display. 6. A reflection type liquid crystal display device, characterized in that the substrate according to claim 1, 2, 3, 4 or 5 is used at least on an outside light incident side.