Liquid crystal display device and method for its production

Abstract: In a liquid crystal display device of a lateral electric field driving system, coloring problem in display is prevented in all the white, intermediate tone and the black display mode. In this liquid crystal display device: one of an incident side polarizing plate (5) and an exit side polarizing plate (9) has its polarizing axis substantially coincide with the direction of the orientation of a liquid crystal layer (10); an optical compensating layer (8) is interposed between at least one of transparent substrates (1,6) and the incident side polarizing plate (5) or the exit side polarizing plate (9) both disposed outside the at least one of the transparent substrates (1,6), the optical compensating layer (8) having a refractive index anisotropy with the other polarity other than that in the refractive index of the liquid crystal layer (10); and, the optical compensating layer (8) has the direction of its anisotropy axis substantially coincide with the direction of the orientation of the liquid crystal layer (10), provided that the orientation direction includes a predetermined pre-tilt angle.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a liquid crystal display device and a method for its production, and more particularly to an active matrix type liquid crystal display device using a lateral electric field driving system and a method for the production thereof. 2. Description of the Related Art: Since an active matrix type liquid crystal display device (AMLCD) using thin film transistors (TFTs) of field effect type used as switching devices for its pixels is excellent in image quality, AMLCD has been widely used as a monitor of a portable computer or of a desktop computer of a space-saving type. Recently, particularly, in order to improve the characteristics of view angle, a display method using a lateral electric field has been proposed. In this proposed method, for example, as disclosed in Japanese Patent Laid-Open No. Hei 6-160878, a pixel electrode and its counter electrode are formed in parallel to each other on the same substrate. In driving the thus formed liquid display device, a voltage is applied between these electrodes to establish an electric field in parallel to the substrates, so that the direction of an anisotropic axis (i.e., director) of the liquid crystal is varied in a plane parallel to a surface of the substrate, whereby a transmission luminous energy is controlled. Due to this, it is possible for such liquid crystal display device to obtain a pixel capable of having substantially the constant brightness even when viewed from a view point varying in a wide view angle. In contrast with a twisted nematic (TN) system, there is no fear that the liquid crystal director is so energized as to deviate from a surface of the substrate, which causes the relationship between the transmission luminous energy and the applied voltage to considerably vary, the relationship being obtained when the liquid crystal display device is viewed in a direction parallel to the liquid crystal director. In general, a conventional liquid crystal display device such as one disclosed in the above Japanese Patent Laid-Open No. Hei 6-160878 has a construction in which: the liquid crystal director is homogeneously oriented in a predetermined direction substantially parallel to the substrate (homogeneous orientation); as shown in Fig. 11, the liquid crystal layer is sandwiched between a pair of polarizing plates so as to have their polarizing axes crossed with each other at right angles; and, the polarizing axis of one of the polarizing plates is coincident with the direction of orientation of the liquid crystal director. Due to such construction, it is possible to steadily lower the black level in brightness by displaying the black when no voltage is applied and by rotating the liquid crystal director upon application of a voltage to display the white. Further, in order to improve the liquid crystal device in coloring problem occurring when the device displaying the white is viewed from an oblique direction, there is proposed a construction in which each of the pixel electrode and its counter electrode is bent into a V-shaped form in each of the pixels, as shown in Fig. 12 (for example, see "O Plus E, January issue for 1997, page 102). In this construction, when a voltage is applied between these electrodes, two electric fields different in direction are established to cause the liquid crystal molecule to twist in two directions. In its white display mode, when the liquid crystal display device is viewed from an oblique direction C, a region 1 in which the liquid crystal director is oriented to a direction "A" assumes red color. On the other hand, a region 2 in which the liquid crystal director is oriented to a direction B assumes blue color. Since these colors in such two regions are compensated with each other, it is possible to considerably relieve the coloring problem appearing when the liquid crystal device displaying the white is viewed from the oblique direction. Further, in Japanese Patent Laid-Open No. Hei9-80424, there is proposed a method in which: in the liquid crystal display device of a lateral electric field system, in order to prevent the coloring problem from occurring when a display screen of the liquid crystal display device in its white display mode is viewed from an oblique direction, a double refraction medium is interposed between the opposite substrates or the like and the polarizing plates. In this proposed method, when a maximum driving voltage is applied, i.e., in the white display mode, an anisotropic axis of the double refraction medium coincides in direction with each of the liquid crystal director and the polarizing axes of the polarizing plates. A problem to be resolved by the present invention is as follows: In the former of the conventional methods, when the liquid crystal display device is in its intermediate tone display mode or in its white display mode, since the regions 1 and 2 having been twisted in two different directions are compensated with each other, it is possible to considerably prevent the coloring problem from occurring when the liquid crystal display device is viewed from an oblique direction. However, in the black display mode, when the liquid crystal display device is viewed from the oblique direction, in the strictest sense of the word, the direction of the liquid crystal director is not coincident with the direction of the polarizing axis, which results in a retardation, i.e., delay in phase. Due to this, when the liquid crystal display device displaying the black is viewed in the oblique direction, a reddish or a bluish coloring problem occur, depending on the direction from which the display is viewed. Further, in the conventional method disclosed in Japanese Patent Laid-Open No. Hei9-80424, in the black display mode, the anisotropic axis of the double refraction medium is different in direction from the polarizing axis of the polarizing plate. Due to this, it is impossible for the conventional method to prevent the coloring problem from occurring in the black display mode. A liquid crystal display device as specified in the preamble of claim 1 is already known from US-A-5 576 867. Furthermore, it is known from the articles of S. Aratani et al. in Jpn. J. Appl. Phys., Vol 36 (15 Jan 1997), pp. L27 - L29, and of H. Wakemoto et al. in SID 97 Digest (13 May 1997), pp. 929 - 932 to suppress color-shift in in-plane switching liquid crystal display device by provision of a V-shaped pixel and counter electrode pattern. SUMMARY OF THE INVENTION Under such circumstances, the present invention was made. Consequently, it is an object of the present invention to provide an active matrix type liquid crystal display device using a lateral electric field driving system, which is capable of preventing a coloring problem in any of a white display mode, an intermediate tone display mode and a black display mode. According to a first aspect of the present invention, the above object of the present invention is accomplished by providing a liquid crystal display device as defined in claim 1. According to a second aspect of the present invention, the above object of the present invention is accomplished by providing: The liquid crystal display device as set forth in the first aspect of the present invention, wherein: a product of ΔnL and dL is substantially equal to a product of ΔnF and dF, ΔnL is a difference in the refractive index anisotropy of the liquid crystal layer, dL is the thickness of the liquid crystal layer, ΔnF is a difference in the refractive index anisotropy of the optical compensating layer, and dF is the thickness of the optical compensating layer. According to a third aspect of the present invention, the above object of the present invention is accomplished by providing: The liquid crystal display device as set forth in any one of the first or second aspect of the present invention, wherein: at least formed on the first transparent substrate are: a plurality of scan lines (11); a plurality of signal lines (14) extending in a direction substantially perpendicular to the scan lines (11) ; and, a plurality of switching elements disposed in the vicinities of the intersections of the scan lines (11) and the signal lines (14), each of the switching elements being connected with the pixel electrode. According to a fourth aspect of the present invention, the above object of the present invention is accomplished by providing a method of producing a liquid crystal display device as defined in claim 4. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which: Fig. 1(a) is a plan view of an essential part of the liquid crystal display device of an embodiment of the present invention; Fig. 1 (b) is a cross-sectional view of the essential part of the liquid crystal display device, taken along the line A-A of Fig. 1(a); Fig. 2 is a cross-sectional view of the essential part of the liquid crystal display device, taken along the line B-B of Fig. 1(a); Fig. 3 (a) is a plan view of an essential part of the active matrix type substrate in the liquid crystal display device of the present invention, illustrating the method of producing the liquid crystal display device; Fig. 3 (b) is a cross-sectional view of the essential part of the substrate, taken along the line A-A of Fig. 3(a); Fig. 3 (c) is a cross-sectional view of the essential part of the substrate, taken along the line B-B of Fig. 3(a); Fig. 3 (d) is a cross-sectional view of the essential part of the substrate, taken along the line C-C of Fig. 3(a); Fig. 4 (a) is a plan view of an essential part of the active matrix type substrate in the liquid crystal display device of the present invention, illustrating the method of the production of the liquid crystal display device; Fig. 4 (b) is a cross-sectional view of the essential part of the substrate, taken along the line A'-A' of Fig. 4(a); Fig. 4 (c) is a cross-sectional view of the essential part of the substrate, taken along the line B'-B' of Fig. 4(a); Fig. 4 (d) is a cross-sectional view of the essential part of the substrate, taken along the line C'-C' of Fig. 4(a); Fig. 5(a) is a cross-sectional view of the transparent insulation counter substrate, illustrating the method of the production of the liquid crystal display device of the present invention; Fig. 5(b) is a cross-sectional view of the orientation film of the counter substrate shown in Fig. 5(a); Fig. 6 is a graph showing the relationship between the voltage (V), which is applied to both the pixel electrode and its counter electrode, and the transmission factor; Figs. 7(a) and 7(b) are graphs showing the relationship between the wavelength and the transmission factor, illustrating the effect of the optical compensation layer in the liquid crystal display device of the present invention; Fig. 8 is a cross-sectional view of an essential part of the liquid crystal display device of another embodiment of the present invention; Figs. 9(a), 9(b) and 9(c) are schematic views illustrating the operation of the liquid crystal display device, more particularly, illustrating the relationships among the liquid crystal director, the absorption axis of the polarizing plate, and the anisotropic axis of the optical compensation layer, illustrating the operation of the liquid crystal display device of the present invention; Fig. 10 is a schematic diagram showing the difference in refractive index anisotropy in case that the liquid crystal display device is viewed from an oblique direction, illustrating the operation of the liquid crystal display device of the present invention; Fig. 11 is a plan view (No. 1) of the conventional liquid crystal display device; and Fig. 12 is a plan view (No. 2) of the conventional liquid crystal display device. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinbelow, the preferred embodiments of the present invention will be described with reference to the accompanying drawings. In an embodiment of a liquid crystal display device of the present invention, as shown in Fig. 1(a), a plurality of scan lines 11 and a plurality of signal lines 14 are disposed in a matrix orientation on one of transparent insulation substrates of the liquid crystal display device. In the vicinity of the intersection of each of the scan lines 11 and each of the signal lines 14, there is disposed a field effect type TFT having a construction in which: an island-shaped amorphous silicon layer serves as a semiconductor layer 13; and, the scan line serves as a gate electrode. In the above construction, regions surrounded by the scan lines 11and the signal lines 14 define pixel regions. In each of these pixel regions, each of the counter electrodes 12 and each of the pixel electrodes 15 are disposed in parallel to each other in a manner such that each of them is bent in center so as to be formed into a substantially V-shaped form in parallel to each other, as shown in Fig. 2. Each of the pixel electrodes 15 is connected with a source electrode of the TFT, and the signal line 14 is connected with a drain electrode of the TFT. In this case, the scan line 11 also serves as a gate electrode of the TFT. In operation, when the scan line 11 is energized, a potential of the signal line 14 is written, so that an ac voltage is applied between the signal line 14 and the counter electrode 12 having been kept at a predetermined potential due to a counter electrode bus line12a. The liquid crystal layer 10 sandwiched between a pair of the transparent insulation substrates 1, 6 through orientation films 4, 7 is oriented in the rubbing direction of the orientation films 4, 7 until the electric field is established. When angles formed between the orientation direction and the directions of the electric fields in the above two regions are expressed by the reference characters α1 and α2, as shown in Fig. 1(a), the following equation is established: α1 = α2 Namely, α1 is equal to α2. When a voltage is applied between these electrodes 12, 15, two electric fields different in direction from each other are established since each of the electrodes is bent in its central portion. Due to the presence of these electric fields, the liquid crystal in a left half of the pixel rotates counterclockwise, while the liquid crystal in a right half of the same pixel rotates clockwise. Further, as shown in Fig. 1(b), disposed outside the pair of the transparent insulation substrates 1, 6 are a pair of polarizing plates 5, 9. In addition, an optical compensation layer 8 is sandwiched between the transparent insulation substrate 6 and the polarizing plate 9. The optical compensation layer 8 has a uniaxial refractive index anisotropy. The anisotropic axis of the compensation layer 8 is so oriented as to be parallel to the orientation direction of the liquid crystal director 10a which is free from any electric field and includes a pre-tilt angle. Here, when a difference in refractive index anisotropy of the optical compensation layer 8 is expressed by ΔnF, the thickness of the same layer 8 is expressed by dF, a difference in refractive index anisotropy of the liquid crystal layer 10 is expressed by ΔnL, and the thickness of the same layer 10 is expressed by dL, the following equation is established: ΔnF × dF = ΔnL × dL The absorption axis (which is perpendicular to the polarizing axis) of the incident side polarizing plate 5 is so oriented as to be parallel to the rubbing direction. On the other hand, the absorption axis of the exit side polarizing plate 9 is so oriented as to be perpendicular to the absorption axis of the incident side polarizing plate 5. Due to the above construction, it is possible to prevent a coloring problem inherent in retardation from occurring in the liquid crystal layer 10 when the layer 10 is in its black display mode and is viewed from an oblique direction. which may improve the black display mode in image quality. On the other hand, since the optical compensation layer 8 has the projection of its refractive index anisotropoic axis aligned with the polarizing axis of the exit side polarizing plate 9, there is no fear that a retardation due to the presence of the optical compensation layer 8 occurs. Consequently, when the liquid crystal display device is viewed from its front side, the optical compensation layer 8 does not adversely affect the front display properties of the liquid crystal display device at all. Further, a retardation occurring when the liquid crystal display device is viewed from an oblique direction is slight in itself, and may cancels a slight retardation occurring when the liquid crystal display device in its black display mode is viewed from an oblique direction. Consequently, in the white display mode and the intermediate tone display mode, since a retardation occurring when the liquid crystal layer is viewed from an oblique direction is sufficiently slight, such retardation substantially does not adversely affect the view angle properties of the liquid crystal layer 10 in its white and intermediate tone display modes. As described above, in the white and intermediate tone display modes, since the liquid crystal rotates counterclockwise in the left half of the pixel and rotates clockwise in the right half of the same pixel, it is possible for these halves of the pixel to compensate with each other, which enables the liquid crystal display device to be substantially free from the coloring problem inherent in the In-Plane Switching (IPS) when viewed from an oblique direction. Now, referring to Figs. 3 to 5, a method of producing the liquid crystal display device having the above construction will be described. The active matrix type substrate 101 (Fig. 1) having the above construction is produced as follows: namely, As shown in Figs. 3(a) to 3(d), a chromium film having a thickness of approximately 150 nm is deposited to the transparent insulation substrate 1, and then subjected to a patterning process comprising photolithography and etching steps, so that both the counter electrode bus line12a and the scan lines 11 are formed. The counter electrode bus line 12a laterally extends in each of an upper and a lower portion of the pixel. Also formed through the above process are V-shaped counter electrodes 12 connected with the counter electrode bus line12a in a central area of the pixel. After that, as shown in Figs. 4(a) to 4(d), further deposited to the transparent insulation substrate 1 in sequence are: a gate insulation film 2 constructed of a silicon nitride film having a thickness of approximately 400 nm; a semiconductor layer 13 constructed of an amorphous silicon film having the thickness of approximately 350 nm; and, a contact layer 13a constructed of an n-type amorphous silicon film having a thickness of approximately 300 nm, wherein these components are so patterned as to assume island shapes so that a TFT portion is formed. In this case, the semiconductor layer 13 is formed on the scan line 11 which also serves as a gate electrode of the TFT. Subsequent to the above, the chromium film having a thickness of approximately 150 nm is deposited to this semiconductor layer 13, and then subjected to a patterning process comprising photolithography and etching steps, so that the pixel electrode 15 assuming a substantially V-shaped form is formed in the central area of the pixel so as to extend in parallel to the counter electrodes 12. Also formed togther with this pixel electrode 15 is a signal line 14, which is positioned in each of opposite side areas of the pixel and intersects with the scan line 11. After that, a protective insulation film 3 constructed of a silicon nitride film having a thickness of approximately 200 nm is deposited to the signal lines 14. Then, in order to permit peripheral terminals to be connected with individual bus lines, the insulation layer is removed. In this embodiment, the V-shaped pixel electrode 15 and its counter electrodes 12 are spaced apart from each other by a distance of approximately 10 µm so as to extend in parallel to each other. Each of these electrodes 12, 14 has a width of approximately 3 µm. Subsequent to the above, as shown in Figs. 5 (a) and 5(b), a color filter is formed in each of the pixels on the transparent insulation substrate 6. Then, a counter substrate 102 is formed. After that, orientation films 4 and 7 are applied to the above active matrix type substrate 101 and the counter substrate 102 including the color filter at every pixel, respectively. The thus applied orientation films 4, 7 are then subjected to a rubbing process which is performed in parallel to a direction in which the scan line 11 extends. These substrates are bonded to each other in a condition in which they are spaced apart from each other to form therebetween a cell gap dLC, and have their peripheral portions fixed by means of sealing members. This cell gap is then filled with liquid crystal substance, which is injected and sealed therein. The cell gap dLC has a width of approximately 4.5 µm. The injected liquid crystal substance has a difference ΔnL in refractive index anisotropy of 0.067. At this time, as shown in Fig. 1, the liquid crystal director 10a is homogeneously oriented in parallel to the rubbing direction. In this case, the liquid crystal molecule has a predetermined pre-tilt angle relative to the surface of the substrate. Further, the optical compensation layer 8 is applied to an outer side of the counter substrate in the color filter side. The optical compensation layer 8 has a negative uniaxial refractive index anisotropy, and has the direction of its refractive index anisotropy including the pre-tilt angle be parallel to the liquid crystal director 10a. Further, a product of: the difference ΔnF in refractive index anisotropy of the optical compensating layer 8; and, the thickness dF of the same layer 8 is equal to ΔnLC × dLC, i.e, equal to approximately 302 nm. Applied to the outermost opposite surfaces of the above assemply of the liquid crystal display device are the incident side polarizing plate 5 and the exit side polarizing plate 9. At this time, the absorption axis of the incident side polarizing plate 5 extends in parallel to the projection of the liquid crystal director 10a projected onto the surface of the substrate. On the other hand, the absorption axis of the exit side polarizing plate 9 extends in parallel to the surface of the substrate, and is perpendicular to the absorption axis of the incident side polarizing plate 5. Fig. 6 is a graph showing the V-T characteristics, i.e., the relationship between the voltage (V), which is applied to both the pixel electrode and its counter electrode, and the transmission factor, the graph being obtained when the liquid crystal display device of this embodiment is viewed from the front side of the device. In this graph: the ordinate shows the transmission factor (%) expressed in a linear scale; and, the abscissa shows the applied voltage (V) expressed also in a linear scale. As is clear from this graph, it was found that an excellent display is obtained in a contrast of at least 100:1. Figs. 7(a) is a graph showing the relationship between the wavelength and the transmission factor, illustrating the effect of the optical compensation layer in the liquid crystal display device of the present invention in comparison with the conventional type of the active matrix liquid crystal display device having no optical compensation layer 9, with respect to the spectrum in the transmission factor obtained when the display device is in its black display mode and is viewed from a direction which forms an angle of 60 degrees with the normal to the substrate and forms an angle of 45 degrees with the rubbing direction of Fig. 1(a). In the graph shown in Fig. 7(a): the ordinate shows the transmission factor (%) expressed in a linear scale; and, the abscissa shows the wavelength (nm) expressed also in a linear scale. As is in the above graph, Figs. 7(b) is a graph showing the relationship between the wavelength and the transmission factor, illustrating the effect of the optical compensation layer in the liquid crystal display device of the present invention in comparison with the conventional type of the active matrix liquid crystal display device having no optical compensation layer, with respect to the spectrum in the transmission factor obtained when the display device is in its black display mode and is viewed from a direction which forms an angle of 60 degrees with the normal to the substrate and forms an angle of 135 degrees with the rubbing direction of Fig. 1(a). In the graph shown in Fig. 7(b): the ordinate shows the transmission factor (%) expressed in a linear scale; and, the abscissa shows the wavelength (nm) expressed also in a linear scale. In the conventional liquid crystal display device described above, when the substrate of the display device is viewed from an oblique direction, the red is larger in transmission factor than the blue so that the reddish coloring problem occurs. In this condition, when the substrate is viewed from a direction perpendicular to the above oblique direction, now the red is smaller in transmission factor than the red so that the bluish coloring problem occurs. In contrast with this, in the active matrix type liquid crystal display device of this embodiment of the present invention, the spectrum in transmission factor is substantially flat, which realizes an excellent display substantially free from any coloring problem. Further, in this embodiment of the present invention, since the left half of the pixel region rotates counterclockwise while the right half of the same pixel region rotates clockwise, these halves of the pixel region are compensated with each other, which enables both the intermediate tone display and the white display to be free from any coloring problem so that the display device of the present invention may have excellent display characteristics. Though the embodiment of the present invention has been described in the above with reference to the accompanying drawings, it should be understood that the present invention is not limited in scope to its embodiment only. For example, in this embodiment of the present invention, as described above, the polarizing axis of the exit side polarizing plate 9 extends in parallel to the projection component of the liquid crystal director 10a projected onto the substrate surface, and the polarizing axis of the incident side polarizing plate 5 is perpendicular to the polarizing axis of the exit side polarizing plate 9. However, it is also possible for the display device of the present invention to have a construction in which the polarizing axis of the incident side polarizing plate 5 extends extends in parallel to the projection component of the liquid crystal director 10a projected onto the substrate surface, and the polarizing axis of the exit side polarizing plate 9 is perpendicular to the polarizing axis of the incident side polarizing plate 5. Further, in the embodiment of the present invention described above, though the optical compensation layer 8 is interposed between the exit side polarizing plate 9 and the transparent insulation substrate 6, it is also possible to interpose the optical compensation layer 8 between the incident side polarizing plate 5 and the transparent insulation substrate 1, as shown in Fig. 8. Further, it is also possible to interpose the optical compensation layer 8 between the exit side polarizing plate 9 and the transparent insulation substrate 6 and between the incident side polarizing plate 5 and the transparent insulation substrate 1 in a manner such that the sum of the optical compensation layers 8 interposed therebetween is equal in retardation to the liquid crystal layer. Further, in the embodiment of the present invention, though the rubbing direction is parallel to the scan line 11 and the V-shaped form is used, it is also possible to have the rubbing direction be parallel to the signal line 14 and the V-shaped form inverted. Even when the inverted V-shaped form is used, the optical compensation layer 8 has its anisotropic axis so oriented as to be parallel to the orientation direction of the liquid crystal director 10a which is free from any electric field and includes the pre-tilt angle. In the present invention having the above construction, the liquid crystal layer is homogeneously oriented in a predetermined direction in the black display mode in a manner such that one of the first and the second polarizing plate has its polarizing axis be substantially parallel to the orientation direction of the liquid crystal layer. Consequently, when the display device is viewed in its black display mode from its front side, as shown in Fig. 9(a), since the absorption axis of the incident side polarizing plate (which axis is perpendicular to the polarizing axis) is parallel to the direction of the liquid crystal director (i.e, anisotropic axis), the light passed through the incident side polarizing plate is subjected to retardation in phase due to the presence of the refractive index anisotropy of the liquid crystal layer. In contrast with this, when the liquid crystal molecule has a predetermined pre-tilt angle and viewed from an oblique direction, as shown in Fig. 9(b), the liquid crystal director differs in direction from the absorption axis of the polarizing plate, which causes a retardation in the liquid crystal layer. As described above, when the retardation occurs in the liquid crystal layer, the amount of the transmissive light varies depending on the relationship between: a product of a difference in refractive index between the ordinary light and the extraordinary light appearing when the light passes through the liquid crystal layer and a traveling distance of the light passing though the liquid crystal layer; and, the wavelength. Due to this, in the black display mode, when leakage of the light occurs in the oblique viewing field, such leakage of the light causes the coloring problem. In this liquid crystal display device of the present invention, the optical compensation layer has the uniaxial refractive index anisotropy which is different in polarity from the refractive index anisotropy of the liquid crystal layer. This optical compensation layer has the direction of its anisotropic axis be parallel to the orientation direction of the liquid crystal layer, i.e., parallel to the polarizing axis of the polarizing plate. In other words, since the direction of the anisotropic axis of this optical compensation layer and the direction of the polarizing axis are the same in an azimuth angle  shown in Fig. 10, any retardation does not occur when the display device is viewed from its front side. Consequently, the optical compensation layer does not affect at all the display characteristics of the device viewed from its front side. On the other hand, since the direction of the anisotropic axis of the optical compensation layer is parallel to the orientation direction of the liquid crystal layer including the pre-tilt angle, the direction of the anisotropic axis of the optical compensation layer is coincident with the optical axis (i.e., anisotropic axis) of the liquid crystal layer even when the liquid crystal display device is viewed from any direction, as shown in Fig. 9(C) illustrating this relationship. Consequently, a retardation occurring in the liquid crystal layer after the light passed through the incident side polarizing plate is always offset by the optical compensation layer having the refractive index anisotropy which is different in polarity from that of the liquid crystal layer. Due to this, it is possible to prevent any coloring problem from occurring when the display device is viewed from an oblique direction in its black display mode. Particularly, when a difference in refractive index anisotropy of the liquid crystal layer is expressed by ΔnL, the thickness of the same layer is expressed by dL, a difference in refractive index anisotropy of the optical compensation layer is expressed by ΔnF, and the thickness of the same layer is expressed by dF, it is possible to realize the most effective prevention of the coloring problem by substantially establishing the following equation: ΔnL × dL = ΔnF × dF In its white display mode, the liquid crystal has been twisted approximately by an angle of 45 degrees based on its initial orientation direction. In this mode, when the liquid crystal is viewed from its front side, a difference in refractive index anisotropy between the ordinary light and the extraordinary light is equal to when a difference ΔnL in refractive index anisotropy inherent in the liquid crystal. Consequently, when the thickness of the liquid crystal layer is expressed by dL, the retardation between the ordinary light and the extraordinary light is expressed by the equation: Δ nL × dL. Due to this, when the liquid crystal display device is viewed from the oblique direction, the following occurs: namely As shown in Fig. 10, when the substrate of the display device is viewed from a direction perpendicular to the liquid crystal director, a difference Δn in refractive index between the ordinary light and the extraordinary light is equal to a difference between the major axis and the minor axis of an ellipse corresponding to a cross-section of a spheroid of refractive index a major axis of which is parallel to the liquid crystal director, the cross-section being taken along the wavefront of the light. In this case, since the light beam intersects the major axis of the spheroid, the thus obtained cross-section includes the major axis of the spheroid so that the difference Δn in refractive index is always equal to the difference ΔnL in refractive index anisotropy. Consequently, when the angle  formed between the light beam and the normal of the substrate increases, the retardation Δn × L increases as the length L of optical path of the light passing through the liquid crystal. Due to this, the more the wavelength increases, the more the transmission facter increases in tendency. In the white display mode of the display device viewed from its front side, the color on the display becomes reddish as the angle  increases. On the other hand, when the display device is viewed from the same direction as that of the liquid crystal director, the difference Δn in refractive index between the ordinary light and the extraordinary light decreases as as the angle  increases, because the effective length of the major axis decreases while the minor axis keeps its length constant. In this case, the retardation decreases as the angle  increases, even when the length of the optical path increases as the angle  increases. Due to this, the display on the device becomes bluish as the angle  increases. As described above, as the result of division the electric field into two parts different from each other in direction, in the white diplay mode of the display device, when the device is viewed from an oblique direction C, the pixel's region having the liquid crystal director oriented only in a direction "A" shows a reddish color, while the pixel's region having the liquid crystal director oriented only in a direction "B" shows a bluish color. Since these pixel's regions are compensated with each other, it is possible to considerably resolve the coloring problem occurring when the display device is viewed from the oblique direction. It is thus apparent that the present invention is not limited to the above embodiments but may be changed and modified without departing from the scope of the invention as defined by the claims.

Claims: A liquid crystal display device comprising: a first and a second transparent substrate (1, 6) oppositely disposed and spaced apart from each other through a cell gap; a liquid crystal layer (10) which has a refractive index anisotropy with one polarity and is homogeneously oriented in parallel to a surface of said transparent substrates (1, 6); a first and a second polarizing plate (5, 9) having their polarizing axes crossed with each other at right angles, said first and said second polarizing plate (5, 9) being disposed outside said first and second transparent substrate (1, 6); and an optical compensating layer (8) interposed between at least one of said first and said second transparent substrates (1, 6) and one of said first and said second polarzing plates (5, 9), wherein a pixel electrode (15) and its counter electrode (12) opposite thereto are formed for every pixel on said first transparent substrate (1), such that a voltage applied between said pixel electrode (15) and said counter electrode (12) establishes an electric field in parallel to said surface of said first (transparent substrate (1, 6), whereby said liquid crystal layer (10) is controlled in its orientation, characterized in that the pixel electrode (15) and the counter electrodes (12) are arranged such that each of said pixels is provided with a plurality of regions formed between said pixel electrode (15) and said counter electrode (12), said regions being different from each other in the direction of the electric field; an angle, which is formed between the direction of said electric field and that of the orientation of said liquid crystal layer (10) free from said electric field, is substantially constant in any of said regions; said liquid crystal layer (10) of said regions permits its liquid crystal molecules to rotate in opposite directions to each other when said electric field is established; one of said first and said second polarizing plate (5, 9) has its polarizing axis substantially coincident with the orientation of said liquid crystal layer (10); said optical compensating layer (8) has a refractive index anisotropy with the polarity opposite to that of said liquid crystal; and said optical compensating layer (8) has the direction of its anisotropy axis substantially coincide with the direction of the orientation of said liquid crystal layer (10), including a possible predetermined pre-tilt angle of the liquid crystal molecule. The liquid crystal display device according to claim 1, characterized in that : a product of ΔnL and dL is substantially equal to a product of ΔnF and dF, where ΔnL is a difference in refractive index anisotropy of said liquid crystal layer (10), dL is the thickness of said liquid crystal layer (10), ΔnF is a difference in refractive index anisotropy of said optical compensating layer (8), and dF is the thickness of said optical compensating layer (8). The liquid crystal display device according to any one of claims 1 or 2, characterized in that : at least formed on said first transparent substrate (1) are: a plurality of scan lines (11) ; a plurality of signal lines (14) extending in a direction substantially perpendicular to said scan lines (11); and, a plurality of switching elements disposed in the vicinities of the intersections of said scan lines (11) and said signal lines (14), each of said switching elements being connected with said pixel electrode (15). A method of producing a liquid crystal display device, comprising the steps of: forming a pixel electrode (15) and its counter electrode (12) on a surface of a first transparent substrate (1) for every pixel, such that each of said pixels is provided with a plurality of regions formed between said pixel electrode (15) and said counter electrode (12), said regions being different from each other in direction of an electric field, wherein an angle, which is formed between the direction of said electric field and that of the orientation of a liquid crystal layer (10) free from said electric field, is substantially constant in any of said regions, and said liquid crystal layer (10) of said regions permits its liquid crystal molecules to rotate in opposite directions to each other when said electric field is established; disposing a second transparent substrate (6) opposite to said first transparent substrate (1) so as to form a cell gap therebetween, in which cell gap the liquid crystal layer (10) having a refractive index anisotropy with one polarity is formed and homogeneously oriented in a direction substantially parallel to a surface of said first transparent substrate (1, 6); forming an optical compensating layer (8) outside one of said first and said second transparent substrates (1, 6) in a manner such that said optical compensating layer (8) has the direction of its anisotropy axis substantially coincident with the direction of the orientation of said liquid crystal layer (10), including a possible predetermined pre-tilt angle thereof, said optical compensating layer (8) having a refractive index anisotropy of the polarity opposite to that of said liquid crystal; and forming a first and a second polarizing plate (5, 9) having their polarizing axes crossed with each other at right angles, said first and said second polarizing plates (5, 9) being disposed outside said compensating layer (8) and outside said first and said second transparent substrates (1, 6), one of said first and said second polarizing plate (5, 9) having its polarizing axis substantially coincident with the direction of the orientation of said liquid crystal layer (10).

Inventors: MATSUMOTO KIMIKAZU, NISHIDA SHINICHI, WATANABE MAKOTO, MATSUMOTO, KIMIKAZU, NISHIDA, SHINICHI, WATANABE, MAKOTO
Applicants: NEC LCD TECHNOLOGIES LTD, NEC LCD TECHNOLOGIES, LTD.
Application year: 1999
Main classification: G02F1
IPC codes: G02F1