JPH09258219A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a liquid crystal display device having an excellent contrast characteristic and paper white characteristic by providing a surface for reflecting the light of a reflection layer with ruggedness and specifying the ratio of the area of the region having the angle below a specified value of inclination of the tangent of the ruggedness to the surface to the area of the substrates to be in a specified range. SOLUTION: The reflection type LCD 20 has a polarizing plate 22, a phase difference plate 23 and a liquid crystal cell 20a in this order. The liquid crystal cell 20a has the upper substrate 26, the lower substrate (reflection substrate) 25 and the liquid crystal layer 24 held between the upper substrate 26 and the lower substrate 25. The lower substrate 25 has plural projecting parts 32, high-polymer resin layers 33 covering these projecting parts 32, the reflection layer 34 and an oriented film 35 in this order on the surface of the transparent substrate 31 on the liquid crystal layer 24 side. The reflection plate and polarizing plate 22 of the device are disposed on the different sides of the liquid crystal layer. The surface which reflects light of the reflection layer 34 has the ruggedness and the ratio of the area of the region having <2 deg. angle of inclination of the tangent of the ruggedness to the surface to the area of the substrate is 20 to 60%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflection type liquid crystal display device for displaying an incident polarized light by reflecting the polarized light, and more particularly to a field effect birefringence mode reflection type liquid crystal display device.

[0002]

2. Description of the Related Art The most important performance required for a reflective LCD (liquid crystal display device) is how to effectively use ambient light. At present, the display mode generally used for calculators, word processors and the like is a TN method (twisted nematic) method in which two polarizing plates and a reflecting plate are combined.

However, in the system using two such polarizing plates, one of two linearly polarized light components of the elliptically polarized light (including circularly polarized light and linearly polarized light) reflected by the reflection plate which are orthogonal to each other is used. The linearly polarized light component is absorbed by the polarizing plate arranged between the reflection plate and the liquid crystal layer. Therefore,
Since there is a loss of light due to the absorption of the polarizing plate, a bright display cannot be obtained.

An ECB using one polarizing plate is used as a display mode capable of gradation display and bright display.
A liquid crystal display device of (electric field controlled birefringence) mode has been proposed (Nakamura et al .: 18th Liquid Crystal Conference 3D110).

The operating principle of this ECB mode reflective liquid crystal display device will be described with reference to FIG. Figure 1 shows EC
3A and 3B are functional model diagrams of the B-mode reflective liquid crystal display device 10, in which (a) shows a dark state and (b) shows a bright state.

ECB mode reflective liquid crystal display device 10
Includes a polarizing plate 2, a retardation plate 3, a liquid crystal layer 4, and a reflection plate 5. In the state shown in FIG. 1A, since the sum of the retardations Δn · d of the liquid crystal layer 4 and the retardation plate 3 is set to λ / 4, the light enters the polarizing plate 2 as indicated by the arrow A1.
After passing through the liquid crystal layer 4 and the retardation plate 3, the linearly polarized light that has passed through becomes a circularly polarized light in the rotation direction indicated by the arrow A2. This circularly polarized light is reflected by the reflection plate 5 and becomes circularly polarized light in the direction opposite to the direction of the arrow A2, as indicated by the arrow A3. When this reverse circularly polarized light passes through the liquid crystal layer 4 and the retardation plate 3, it becomes a linearly polarized light whose polarization direction is different by 90 ° from the linearly polarized light at the time of incidence. Therefore, since this linearly polarized light cannot pass through the polarizing plate 2, a dark state is displayed.

On the other hand, in the state shown in FIG. 1B, since the sum of the retardations Δn · d of the liquid crystal layer 4 and the retardation plate 3 is set to 0, linearly polarized light transmitted through the polarizing plate 2 is obtained. The polarization state is maintained even after passing through the liquid crystal layer 4 and the retardation plate 3. The state of polarization of this linearly polarized light does not change even when reflected by the reflector 5. Therefore, since the reflected linearly polarized light maintains the polarization direction when it is incident, it is transmitted through the polarizing plate 2. As a result, the bright state is displayed. The value of the sum of the retardations Δn · d of the liquid crystal layer 4 and the retardation film 3 is controlled by changing the retardation value of the liquid crystal layer by applying a voltage to the liquid crystal layer 4. In this way, the ECB mode reflective liquid crystal display device can realize display.

Japanese Unexamined Patent Publication (Kokai) No. 7-218906 discloses a reflection type liquid crystal display device having an improved contrast characteristic by including a reflection plate which maintains the polarization state of incident polarized light well after reflection. According to the above-mentioned publication, it is preferable to use a reflection plate having a degree of polarization of reflected light of 50% or more expressed by Stokes parameter.

[0009]

However, the conventional techniques have the following problems.

Generally, since the reflection type liquid crystal display device utilizes the ambient light incident on the liquid crystal panel, it is preferable that the reflectance of the reflection plate is as high as possible over a wide range. However, when a metal film having a mirror surface is used, there is a problem in that the reflectance is high only in the regular reflection direction, the face of an observer is reflected on the display surface, and it is very dark in other than the regular reflection direction.

On the other hand, there is a standard white plate of MgO powder as a reflector having excellent paper whiteness. However, the standard white plate is excellent in paper whiteness because it is too diffusive, but when combined with the liquid crystal layer, multiple reflection occurs and light is trapped in the liquid crystal layer, resulting in darkness or low contrast. There is.

Further, the ECB mode liquid crystal display device using the reflective substrate disclosed in Japanese Patent Laid-Open No. 7-218906 mentioned above has a high contrast because the display characteristics are not optimized, but the paper is used. There is a problem of poor whiteness.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a reflection type liquid crystal display device and a reflection plate which are excellent in contrast characteristics and paper whiteness. is there.

[0014]

A liquid crystal display device of the present invention is a liquid crystal having a pair of substrates, a liquid crystal layer sandwiched between the pair of substrates, a polarizing plate, and a reflective layer. In the display device, the reflecting plate and the polarizing plate are provided on different sides of the liquid crystal layer, and the reflecting layer has unevenness on a surface that reflects light, and an inclination of a tangent to the surface of the unevenness. The ratio of the area of the region having an angle of less than 2 ° to the area of the substrate is 20% or more and 60% or less, whereby the above object is achieved.

The liquid crystal layer is preferably driven in a field effect birefringence mode.

The difference in retardation of the liquid crystal layer due to the irregularities is preferably 40 nm or less.

The difference in thickness of the liquid crystal layer due to the unevenness is 1
It is preferably μm or less.

The reflection plate according to the present invention is a substrate for reflecting light, and the light-reflecting surface has unevenness, and the area of the area of the region where the inclination angle of the tangent to the unevenness is less than 2 °. The ratio to the area of the substrate is 20% or more and 60% or less, whereby the above object is achieved.

The reflecting substrate of the present invention has a reflecting surface having irregularities, and this reflecting surface has an inclination angle of a tangent to the irregularities of 2 degrees.
The ratio of the area of the flat portion that is less than 0 to the area of the substrate is 20% or more and 60% or less. The flat portion contributes to the improvement of contrast, and the uneven portion contributes to the improvement of paper whiteness. As a result, a reflective liquid crystal display device having excellent contrast characteristics and paper whiteness can be provided.

Further, by setting the variation of the retardation of the liquid crystal layer due to the unevenness to 40 nm or less, a practical contrast 3 can be achieved and a color display with high color purity can be performed by utilizing the interference color of the ECB mode. You can

Further, the difference in thickness of the liquid crystal layer due to the unevenness is 1 μm.
STN with a large twist angle by setting m or less
It is possible to provide a liquid crystal display device in which display quality is not deteriorated due to variations in threshold characteristics even when an oriented liquid crystal layer is used.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.

(Basic Structure of ECB Mode Reflective Liquid Crystal Display Device) A sectional view of an ECB mode reflective liquid crystal display device (reflective LCD) 20 according to the present invention is shown in FIG. In this embodiment, an STN (sparts twisted nematic) type liquid crystal display device driven by a simple matrix will be described.

The reflective LCD 20 has a polarizing plate 22, a retardation plate 23 and a liquid crystal cell 20a in this order from the light incident side. The liquid crystal cell 20 a includes an upper substrate 26, a lower substrate (reflection substrate) 25, an upper substrate 26 and a lower substrate 2.
5 and the liquid crystal layer 24 sandwiched therebetween. The upper substrate 24 has a transparent substrate 27, and a transparent electrode 28 and an alignment film 29 in this order on the surface of the transparent substrate 27 on the liquid crystal layer 24 side.
Further, the lower substrate 25 has a plurality of protrusions 32, a polymer resin layer 33 covering the protrusions 32, a reflective layer 34, and an alignment film 35 in this order on the surface of the transparent substrate 31 on the liquid crystal layer 24 side. Have in. The protrusions 32 are large protrusions 32 having different heights.
It has a and a small protrusion 32b.

In this embodiment, the reflecting layer 34 is made of Al and also functions as an electrode. The reflective layer 34 and the transparent electrode 28 are electrodes arranged in stripes, respectively, and the reflective layer 34 and the transparent electrode 28 are orthogonal to each other with the liquid layer 24 in between, and form a matrix-shaped picture element. As the liquid crystal layer 24, a liquid crystal material for STN (for example, product name ZLI4427 manufactured by Merck & Co., Inc.) is used. This reflective LC
The polarizing plate 22, the retardation film 23, the liquid crystal layer 24, and the reflection layer 34 of D20 are respectively the polarizing plate 2, the retardation film 3, and the retardation plate 3 of FIG.
It functions as the liquid crystal layer 4 and the reflector 5. The retardation film 23 can be omitted by using a liquid crystal layer whose retardation varies between λ / 4 and 0 as the liquid crystal layer 24. Since the wavelength dispersion of the reflected light can be reduced by providing the retardation plate 23, display with high color purity is possible. In addition, the retardation plate 23 compensates for the viewing angle dependence caused by the alignment of the liquid crystal molecules of the liquid crystal layer 24, so that a display with a wide viewing angle can be provided.

The material of the reflective layer 34 is not limited to Al, and other metal materials may be used. When a reflective layer having no conductivity is formed, a transparent electrode is separately formed on the upper surface of the reflective layer. You may. Further, the alignment state of the liquid crystal layer is not limited to the STN type, and a liquid crystal layer whose retardation changes by application of an electric field can be widely used.

Next, the lower substrate 25 will be described with reference to FIGS. FIG. 3 is a top view of the lower substrate 25.
In the lower substrate 25, a large number of large protrusions 31a and small protrusions 31b made of resin are formed on the transparent substrate 31, respectively. Substrates having different diameters D1 and D2 at the bottoms (the surface of the substrate 31) of the large protrusions 32a and the small protrusions 32b and the distance D3 between the protrusions are manufactured. D1 and D2 are
It is preferably in the range of about 3 μm to 20 μm. If it is smaller than about 3 μm, the processing by photolithography becomes difficult. On the other hand, if it exceeds about 20 μm, the height of the convex portion becomes high, and the influence on the cell thickness becomes large. D3 is
The area ratio of the flat portion, which will be described later, is set within an appropriate range. For example, by setting D3 to about 1/3 of D1 and D2, the flat area ratio can be about 70%. As will be described later, since the diameter of the protrusion is increased by heat-treating the protrusion, the lower limit value of D3 is smaller than the lower limit value determined by photolithography and is about 1.5 μm.

FIG. 4 is a sectional view for explaining the manufacturing process of the lower substrate 25. The thickness t1 of the transparent substrate 31 is
For example, a glass substrate having a size of 1.1 mm (manufactured by Corning Incorporated,
The product name 7059) was used. As shown in FIG.
On the glass substrate 31, as an example, manufactured by Tokyo Ohka Co., Ltd., product name O
The photosensitive material of FPR800 was added with 500 r.p.m. p. m-30
00r. p. The resin layer 41 is formed by spin coating with m. In this embodiment, 2500 r.p.m. p. The resin layer 4 having a thickness t2 of, for example, 1.5 μm is spin coated for 30 seconds.
Form one.

Next, the substrate on which the resin layer 41 is formed is set to 90.
After baking for 30 minutes in an atmosphere of ° C, as shown in Fig. 4 (b), a photomask 42 having a large number of large and small circular light shielding portions 42a formed thereon is arranged and exposed. After that, as an example, development is performed with a developing solution composed of a 2.38% solution of NMD-3 manufactured by Tokyo Ohka Co., Ltd., and as shown in FIG. Protrusion 41a
And the small protrusion 41b was formed. The large protrusions 41a and the small protrusions 41b having different heights can be realized by controlling the exposure time and the development time.

The photomask 42 has a structure in which circular light-shielding portions 42, in which the large protrusions 41a and the small protrusions 41b shown in FIG. 3 are arranged, are randomly arranged. An example in which the diameter of the light shielding portion of the photomask 42 is provided so as to correspond to the diameters of the large protrusions 41a and the small protrusions 41b is shown in FIG. 4B, but the heat treatment of FIG. It is necessary to set the mask patterns (D1, D2 and D3) in consideration of the degree of deformation due to melting in the process. Typically, the diameters D1 and D2 of the protrusions 32a and 32b are caused by the deformation (heat sag) due to the heat treatment.
Is about 10 to 20% larger than the diameter of the light shielding portion of the photomask. In addition, the protrusions having different heights are not shown in FIG.
It may be formed in one photolithography process using the photomask 42 having the light shielding portions 42a of different sizes as shown in (b), or two pieces of light shielding portions of different sizes may be formed. It may be formed by performing a photolithography process twice using a photomask. Here, D1 = about 10 μm, D2 = about 8 μm,
The case where D3 = about 2 μm is shown.

Next, the glass substrate 31 on which the large protrusions 41a and the small protrusions 41b are formed is heated at 200 ° C. for 1 hour,
As shown in FIG. 4 (d), the tops of the protrusions 41a and 41b were melted to some extent to form an arc, and a large protrusion 32a and a small protrusion 32b were formed.

Next, on the glass substrate 31 in this state, as shown in FIG. 4 (e), 1000 r. p. m-3000 r. p. Spin coat with m. In this embodiment, a suitable 2000 r.p.m. p. Spin coat with m. Thereby, the polymer resin layer 33 is
The recesses between the protrusions 32a and 32b are filled, and the surface is formed to have a relatively gentle and smooth curved surface. In this embodiment, the same resin as the photosensitive resin material is applied, but different types may be applied. The thickness t4 of the large smooth projection formed by the large projection 32a on the surface of the polymer resin layer 33 is about 1 μm, and the thickness t5 of the small smooth projection formed by the small projection 32b is about 1. It was 0.3 μm. Next, a metal thin film of aluminum, nickel, chromium, silver, gold, or the like is formed on the polymer resin layer 33 with a film thickness t3, for example, 0.01 to.
The thickness is about 1.0 μm. In this embodiment, aluminum is sputtered to form the reflective layer 34. A micrograph of the surface of the reflection substrate 60 thus obtained is shown in FIG. A photomicrograph of the reflective substrate 70 obtained by performing two photolithography steps using a photomask having a circular light-shielding portion having a diameter of about 10 μm and a photomask having a circular light-shielding portion having a diameter of about 5 μm. Is shown in FIG.

As described above, the large protrusions 32a and the small protrusions 32b are randomly arranged in a plane, and the heights of the large protrusions 32a and the small protrusions 32b are changed so that the incident light with respect to the incident light is changed. Since it is possible to reduce the flat portion that gives the same phase difference, it is possible to prevent the interference of the light reflected by the reflective layer 34 and the occurrence of an interference color or a striped pattern, and to display a uniform and high color purity. It can be performed. The pattern of the photomask 42 is not limited to the above example, and may have a shape other than a circle.

Lower substrate 25 produced as described above
Then, the following processing is performed on the upper substrate 26 manufactured by the known method. First, the upper substrate 26 and the lower substrate 25
Alignment films 29 and 35 made of a polyimide resin are formed on each of the above and baked at 220 ° C. for 1 hour. In this example, a product name of Sun Ever 150 manufactured by Nissan Kagaku Co., Ltd. was used. Next, a rubbing process for aligning the liquid crystal molecules of the liquid crystal layer 24 is performed. As a result, the final alignment films 29 and 35 are formed.

Next, a sealant for sealing between the glass substrates 31 and 27 is formed by screen-printing an adhesive sealant.

When the lower substrate 25 and the upper substrate 26 thus formed are attached to each other, spacers having a diameter of 5.5 μm are scattered on the lower substrate 25 to control the film thickness of the liquid crystal layer 24. Then, the lower substrate 25 and the upper substrate 26
Are opposed to each other and pasted together with the sealing material (for example, a spacer having a diameter of 6 μm is mixed), and then the liquid crystal material is vacuum-injected between the lower substrate 25 and the upper substrate 26 to form the liquid crystal layer 24. Form. In this embodiment, a nematic liquid crystal twisted by 240 ° between the lower substrate 25 and the upper substrate 26 (as an example, a product name ZLI44 manufactured by Merck & Co., Inc.) is used.
27) is used to form the liquid crystal layer 34.

In the reflective LCD of the present invention, the surface shape of the reflective layer 33 is controlled, as will be described later, and has excellent paper whiteness and contrast. The structure of the reflective substrate (lower substrate) according to the present invention will be described in detail below.

(Area of Flat Part of Reflective Substrate) Various reflective substrates having different surface shapes were prepared by using the above-mentioned method. The reflective liquid crystal display device was manufactured using the obtained reflective substrate, and the contrast and paper whiteness were evaluated.

The area of the flat portion was used as an index showing the surface shape of the reflective substrate. As described above, the reflector having a mirror surface realizes high contrast only in the regular reflection direction, but is inferior in paper whiteness. On the other hand, the standard white plate has excellent paper whiteness, but is dark and has low contrast. We thought that the cause of this difference in reflection characteristics was the difference in surface shape between the specular reflection plate and the standard white plate, and we thought that the difference in shape could be expressed as the area ratio of the flat portion. The mirror surface is expressed as a flat area ratio of 100%, and the standard white plate is expressed as a flat area ratio of 0%.

A method of evaluating the area ratio of the flat portion of the reflective substrate will be described with reference to FIG. FIG. 5A shows a reflective substrate 50.
5B is a diagram schematically showing the surface shape of FIG. 5B, and FIG. 5B is a diagram showing an inclination angle θ obtained from the surface profile. The following observations and evaluations are performed in a state where the reflective electrode made of Al is exposed on the surface of the reflective substrate.

First, the surface of the reflective substrate 50 is observed with an interference microscope. The surface profile 52 shown in FIG. 5B is obtained by scanning the center of the visual field of the microscope along the scanning line 51 in FIG. 5A so as to pass through the top of the convex portion on the surface of the reflective substrate 50. To be In FIG. 5B, the horizontal axis represents the position in the in-plane direction of the substrate, and the vertical axis represents the height of the unevenness. Next, a tangent line 53 is drawn on the unevenness of the surface profile 52. The angle formed by the tangent line 53 and the substrate surface (the plane defined by the two-dimensional spreading direction of the reflective substrate) is defined as the tilt angle θ. In the in-plane direction of the substrate, a tangent line is drawn with respect to the unevenness at a constant interval X ₀ , and the tilt angle θ is obtained. A point where the inclination angle θ is less than 2 ° is defined as a flat portion. The point at which the tilt angle θ is measured is typically about 0.1 μm.
Approximately 1 per m (X ₀ = approximately 0.1 μm) for one convex portion
Measure 00 points. The inclination angle θ may be measured by measuring the surface profile of one entire picture element, or by appropriately selecting a region having a standard profile and measuring only that region. In the above-described method of manufacturing the reflective substrate, since the unevenness can be formed with good reproducibility, it is not necessary to measure the surface profile of the entire one pixel by sampling the area having the standard profile. From the measurement result of the tilt angle θ thus obtained, the flat area ratio was calculated by dividing the number of measurement points at which the tilt angle θ was less than 2 ° by the total number of measurement points.

6 and 7 show specific examples of the reflective substrate.
6 (a) and 7 (a) are optical micrographs of the reflection surface of the reflection substrate, and FIGS. 6 (b) and 7 (b) are histograms showing the results of evaluating the respective surface profiles. is there. The flat substrate area ratio (gray portion in the figure) of the reflective substrate 60 in FIG. 6 was about 40%, and the flat substrate area ratio (white portion in the figure) of the reflective substrate 70 in FIG. 7 was about 12%. .
In addition, each micrograph is 166μm × 256μ
This is the result of observing a part of the picture element m.

Table 1 and FIG. 8 show the results of evaluating the above-mentioned reflective LCD 20 using the various reflective substrates obtained as described above and evaluating paper whiteness and contrast. The paper whiteness was evaluated visually in a room where a plurality of fluorescent lamps were present, and the contrast was evaluated using a power meter at the time of vertical incidence.

[0044]

[Table 1]

As is clear from Table 1, the paper whiteness decreases as the flat area ratio increases. When the flat area ratio exceeds 80%, a phenomenon in which the face of the observer appears on the display surface is seen, and the overall impression is dark. From this, it is understood that the flat area ratio is 80% or less, preferably 60% or less.

On the other hand, as is clear from FIG. 8, the contrast tends to increase as the flat area ratio increases.
Even in a reflective LCD using a substrate with a flat area ratio of 20%, which has excellent paper whiteness, the contrast 4
Is obtained. If the contrast of the reflective LCD is 2 or more, display is possible, and if the contrast is 3 or more, it can be used for practical use. As can be seen from these results, the reflective LCD using a reflective substrate having a flat area ratio of 20 to 60% is excellent in paper whiteness and is capable of high-quality display with a contrast ratio of 4 or more.

(Relationship between height difference of unevenness and retardation) When a color display device using the interference color of ECB mode is constructed, reflectance and color reproducibility (color purity) are deteriorated due to variation in retardation of the liquid crystal layer. There is a problem of doing. Therefore, conditions for obtaining a display with high contrast and color purity in a reflective substrate having irregularities (protrusions) were examined.

FIG. 9 is a diagram schematically showing a reflective LCD,
Only the surface profile 92 of the reflective substrate and the liquid crystal layer 94 are shown. The surface profile 92 is as described above.
It can be measured using an interference microscope. Since the surface profile 92 of the reflective substrate has irregularities, the thickness of the liquid crystal layer 94 is
Each has a different thickness represented by a minimum value d _T (corresponding to the apex position of the convex portion of the reflective substrate) and a maximum value d _B (corresponding to the bottom point position of the concave portion of the reflective substrate). Note that, for example, in FIG.
As in (a), when the surface profile 92 of the reflective substrate has convex portions having different heights and the liquid crystal layer has thicknesses d _T1 and d _T2 , the minimum value of these is set to the minimum value d _T. And
The effective retardation of the liquid crystal layer 94 having such variations in thickness and the reflectance of the reflective LCD using such a reflective substrate are obtained as follows.

As described above, the minimum value of the thickness of the liquid crystal layer 94 is d _T and the maximum value thereof is d _B, and the middle position of the unevenness (the thickness of the liquid crystal layer 94 is (d _T + d _B ) / 2). 95) to the auxiliary line
Is calculated, and the area divided by the auxiliary line 95 is obtained. Let S1 be the area above the auxiliary line 95 and S2 be the area below the auxiliary line 95.
The reflectance R of CD is represented by the following (Equation 1). In the following mathematical formulas, A is the amplitude of light, λ is the wavelength of light, and Δn is the birefringence of liquid crystal molecules.

[0050]

[Equation 1]

Further, the effective retardation Δ of the liquid crystal layer
nd _E is represented by the following (Equation 2).

[0052]

[Equation 2]

Further, the variation Δn (d _T −d _B ) of the retardation due to the height difference of the unevenness corresponds to the difference between the two terms on the right side of the above (Equation 2) expressed by the following (Equation 3). To).
Note that (d _T −d _B ) represents an absolute value without considering the sign and unless otherwise specified.

[0054]

(Equation 3)

As a matter of course, the method for obtaining the reflectance and the retardation described above is not limited to the reflective substrate having the surface profile shown in FIG. 9A. FIG.
The present invention can also be applied to a reflective substrate having a high density of convex portions as shown in (b) and a reflective substrate having a low density of convex portions as shown in FIG. 9 (c). Furthermore, the shape of the surface profile of the reflective substrate is not limited to that shown in FIG.
The reflectance and retardation can be determined. For example, the present invention can be applied to a reflective substrate having a rectangular surface profile as shown in FIG.

By using the method described above with reference to FIG. 4, various reflective substrates having different levels of unevenness were prepared, and using these reflective substrates, the ECB mode reflective type having the configuration shown in FIG. A liquid crystal display device was manufactured. 10 to 13 show the results of examining the relationship between the effective retardation Δnd _E given by (Equation 2) and the reflectance of the obtained ECB mode reflective LCD. In the graphs of FIGS. 10 to 13, the horizontal axis represents the wavelength of the reflected light from the reflective LCD, and the vertical axis represents the reflectance. The effective retardation Δnd _{E of the} liquid crystal layer represented by the above (Equation 2) is 330 nm (see FIG. 1).
0), 360 nm (FIG. 11), 470 nm (FIG. 12) and 530 nm (FIG. 13). Further, for each of them, the results when the variation Δn (d _T −d _B ) of the retardation due to the height difference of the unevenness is 0 nm (corresponding to a specular reflection plate), 30 nm, 40 nm and 50 nm are shown. As shown in these figures, an ECB mode color display device can be constructed by utilizing the phenomenon that wavelength dispersion (interference color) occurs in reflectance due to the difference in retardation of the liquid crystal layer.

As is clear from these figures, the effective retardation value Δnd _E of the liquid crystal layer is 330 nm, 360 n
In all of m, 470 nm and 530 nm, the variation Δn (d _T −d _B ) of the retardation due to the height difference of the unevenness.
When becomes larger, the maximum transmittance (peak) decreases and the minimum transmittance (valley) increases. Therefore, it is understood that if the height difference of the unevenness is too large, the contrast decreases. Further, it can be seen that when the height difference of the unevenness becomes large, the steepness of the wavelength dependence (wavelength dispersion) of the reflectance decreases, so that the color purity of the reflected light decreases.

In addition to the above results, the variation Δn (d _T -d _B ) in retardation due to the height difference of the unevenness is 10 nm and 20.
In addition to the results in the case of nm, the relationship between the magnitude of the variation Δn (d _T −d _B ) of the retardation due to the height difference of the unevenness and the wavelength dispersion of the reflected light is summarized and shown in FIG. FIG.
4, the effective retardation value of the liquid crystal layer is 330 nm, 3
For 60 nm, 470 nm and 530 nm,
Retardation change Δn due to unevenness height
It is a chromaticity diagram showing the wavelength dispersion of the reflected light by the (d _T -d _B). As the degree of unevenness increases, in all wavelength bands (330, 360, 470, and 530 nm),
It can be seen that the chromaticity of the reflected light is approaching the white point (the state where there is no wavelength dispersion) in the central part of the chromaticity diagram. From this, it can be seen that if the height difference of the unevenness is increased, the color purity is lowered and the color reproducibility of the color display using the interference color is lowered.

Further, FIG. 15 shows the relationship between the variation Δn (d _T −d _B ) of the retardation caused by the height difference of the unevenness and the contrast. As is clear from FIG. 15, the contrast decreases as the height difference of the unevenness increases. From this figure, in order to obtain a practical contrast 3, it is necessary that the value of the variation Δn (d _T −d _B ) of the retardation due to the height difference of the unevenness be 40 nm or less, and 35 nm or less. It can be seen that a contrast of 4 or more can be obtained. Regarding the color purity, the variation Δn (d _T −
d _B) is no practical problem if 40nm or less, it is possible to provide a display having higher color purity if 35nm or less.

(Relationship Between Height Difference of Concavity and Convexity and Threshold Characteristic) In the ECB mode liquid crystal display device shown in the above embodiment, when the twist angle of the twist alignment of the liquid crystal layer is increased, the threshold value in the voltage-reflectance characteristic is increased. Become steep. In a liquid crystal display device having a steep threshold characteristic, variations in the thickness of the liquid crystal layer result in variations in the threshold voltage, which lowers the display quality.

Therefore, conditions for application to the STN type liquid crystal display device in which the twist angle of the twist alignment of the liquid crystal layer is large were examined. The height difference | d _T −d _B | of unevenness is 0 μm, 0.5
For a 240 ° twisted STN liquid crystal display device using reflection substrates of μm and 1 μm, respectively, the ratio (d / P ₀ ) of the natural pitch P ₀ of the liquid crystal material to the cell gap (thickness of the liquid crystal layer) and the threshold value. FIG. 16 shows the result of evaluation of the relationship with the steepness α of the characteristic.

The steepness α is defined by the ratio (V ₉₀ / V ₁₀ ) of the voltage value V _{10 at} which the relative reflectance reaches 10% and the voltage value V _{90 at} which the relative reflectance reaches 90%.

For example, assuming that the reflective LCD is applied to a portable information terminal device, it is necessary to be able to perform simple matrix driving with a duty ratio of 1/240. Therefore, the steepness of the threshold characteristic is shown in the right side of FIG. It is preferable that the value is lower than the boundary region (α = range of about 1.06 to 1.07) indicated by the downward diagonal line. Further, in order to obtain a stable alignment of liquid crystal molecules, d / P ₀ has a value smaller than the boundary region (range of about 0.48 to 0.51) shown by the slanting line descending to the left in FIG. Preferably there is. It is for the following reasons. When d / P ₀ is greater than about 0.6, the 240 ° twist alignment of the liquid crystal molecules becomes unstable, and stripe domains are likely to occur, resulting in poor display quality. If d / P ₀ is smaller than about 0.42, 60 ° twist orientation is likely to be formed, which is not preferable. Therefore, even if there is some unevenness in cell thickness, the ratio (d / P ₀ ) between the natural pitch P ₀ of the liquid crystal material and the cell gap d is about 0.48 in consideration of the stability of the alignment of the liquid crystal molecules.
It is preferably in the range of 0.51.

As is clear from FIG. 16, when the height difference | d _T −d _B | of the unevenness becomes large, the steepness decreases (shifts upward in FIG. 16). Therefore, in order to realize steepness and stable orientation, it is understood that the level difference of the unevenness is about 1 μm or less, more preferably about 0.5 μm or less. Further, in the STN type liquid crystal display device corresponding to the points (Δ, ○ and ●) in FIG. 16, neither stripe domain nor 60 ° twist alignment was observed.

The above-mentioned problem of the threshold characteristic is remarkable in the STN type liquid crystal display device having a large twist angle, and is not particularly a problem in the parallel alignment mode liquid crystal display device.

[0066]

According to the present invention, a reflection type liquid crystal display device having excellent paper whiteness and high contrast is provided. Further, a reflective liquid crystal display device having high color purity and having steep threshold characteristics is provided. These reflective liquid crystal display devices are used in electronic devices such as portable information terminal devices.

[Brief description of drawings]

FIG. 1 is a functional model diagram of an ECB mode reflective liquid crystal display device.

FIG. 2 is a sectional view of an ECB mode reflective liquid crystal display device 20 according to the present invention.

3 is a top view of a lower substrate 25 of the reflective liquid crystal display device 20. FIG.

FIG. 4 is a cross-sectional view illustrating a manufacturing process of a lower substrate 25 of the reflective liquid crystal display device 20.

FIG. 5 is a diagram showing a method of evaluating a flat area ratio of a reflective substrate. (A) is a figure which shows typically the surface shape of a reflective substrate, (b) is a figure which shows the inclination angle calculated | required from a surface profile.

FIG. 6 is a diagram showing a reflective substrate. (A) is an optical microscope photograph of the reflective surface of the reflective substrate, and (b) is a histogram showing the results of evaluating the respective surface profiles.

7A and 7B are diagrams showing a reflective substrate, FIG. 7A is an optical micrograph of a reflective surface of the reflective substrate, and FIG. 7B is a histogram showing the results of evaluation of respective surface profiles.

FIG. 8 is a diagram showing a relationship between a flat area ratio and a contrast of a reflective liquid crystal display device.

FIG. 9 is a diagram schematically showing a reflective liquid crystal display device.

FIG. 10 is a graph showing the relationship between effective retardation and reflectance of a liquid crystal layer.

FIG. 11 is a graph showing the relationship between effective retardation and reflectance of a liquid crystal layer.

FIG. 12 is a graph showing a relationship between effective retardation and reflectance of a liquid crystal layer.

FIG. 13 is a graph showing the relationship between effective retardation and reflectance of a liquid crystal layer.

FIG. 14 is a chromaticity diagram showing wavelength dispersion of light reflected by a reflective substrate having unevenness with different height differences.

FIG. 15 is a graph showing the relationship between the height difference of the unevenness of the reflective substrate and the contrast.

FIG. 16: Natural pitch P _{0 of} liquid crystal material and cell gap d
FIG. 6 is a diagram showing a relationship between a ratio (d / P ₀ ) and a steepness α of a threshold characteristic.

[Explanation of reference numerals] 2 polarizing plate 3 retardation plate 4 liquid crystal layer 5 reflecting plate 10 ECB mode reflective liquid crystal display device 20 reflective LCD 20a liquid crystal cell 22 polarizing plate 23 retardation plate 24 liquid crystal layer 25 lower substrate (reflective substrate) ) 26 upper substrate 27 transparent substrate 28 transparent electrodes 29, 35 alignment film 31 transparent substrate 32 protrusions 32a large protrusions 32b small protrusions 33 polymer resin layer 34 reflective layer (reflective electrode) 41 resin layer 41a large protrusions 41b small protrusions 42 Photomask 42a Light-shielding part 50 Reflective substrate 51 Scanning line 52 Surface profile 53 Tangent line 92 Surface profile 94 Liquid crystal layer 95 Auxiliary line

Claims (5)

[Claims] 1. A liquid crystal display device comprising a pair of substrates, a liquid crystal layer sandwiched between the pair of substrates, a polarizing plate, and a reflective layer, wherein the reflective layer and the polarized light are provided. The plate is provided on a different side of the liquid crystal layer, and the reflective layer has a surface for reflecting light having irregularities, and an area of a region of the substrate in which an inclination angle of a tangent to the irregularities is less than 2 ° The ratio to the area is 20% or more and 60% or less,
Liquid crystal display. 2. The liquid crystal display device according to claim 1, wherein the liquid crystal layer is driven in a field effect birefringence mode. 3. The liquid crystal display device according to claim 1, wherein a difference in retardation of the liquid crystal layer due to the unevenness is 40 nm or less. 4. The liquid crystal display device according to claim 1, wherein a difference in thickness of the liquid crystal layer due to the unevenness is 1 μm or less. 5. A substrate for reflecting light, wherein a surface for reflecting light has irregularities, and a ratio of an area of a region in which an inclination angle of a tangent to the irregularities is less than 2 ° to an area of the substrate. Is 20% or more and 60% or less.