Liquid crystal display device

Abstract: A liquid crystal display device comprises: a first substrate; (1) picture element electrodes (4) formed on the inner surface of the first substrate (1) and arranged in a matrix; switching elements (5) formed on the inner surface of the first substrate (1) and combined respectively with the picture element electrodes; (4) address lines (6) formed on the inner surface of the first substrate (1) so as to extend respectively along the rows of the picture element electrodes; (4) signal lines (6) formed on the inner surface of the first substrate (1) so as to extend respectively along the columns of the picture element electrodes; (4) a second substrate (2) disposed opposite to the first substrate (1) so that a space of a predetermined thickness is formed between the first and second substrates; (1,2) a common electrode (0) formed over the inner surface of the second substrate; (2) a liquid crystal layer (3) sealed in the space; and an orienting layer formed at least between the inner surface of either the first substrate (1) or the second substrate (2) and the liquid crystal layer (3). The liquid crystal display device is provided with a reverse tilt suppressing means (8) for suppressing the reverse tilt of a portion of the liquid crystal layer corresponding to the edge portion (7) of each picture element electrode (4).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a liquid crystal display device of an active matrix type provided with a matrix of picture element electrodes and switching devices and, more specifically, to a liquid crystal display device of an active matrix type incorporating a structure capable of suppressing the inverse orientation of the liquid crystal molecules or reverse tilt that occurs along the edges of the picture element electrodes. Description of the Related Art The construction of a general liquid crystal display device of an active matrix type will be described with reference to Fig. 4 to facilitate understanding the present invention. A transmissive type liquid crystal display device comprises two polarizers 110 and 111, and a liquid crystal panel sandwiched between a pair of polarizer 110 and 111, and comprising a first glass substrate 101, a second glass substrate 102 disposed opposite to the first glass substrate 101, and a liquid crystal 103 sealed in the space between the two glass substrates 101 and 102. Formed on the inner surface of the first glass substrate 101 are signal lines 104, address lines 105 extending in a direction perpendicular to the signal lines 104, thin film transistors (TFTs) 106, i.e., switching devices, arranged in a matrix in areas demarcated by the signal lines 104 and the address lines 105, and picture element electrodes 107 arranged in a matrix in the areas demarcated by the signal lines 104 and the address lines 105. Formed on the inner surface of the second glass substrate 102 are a common electrode 108, and R, G and B color filters 109. When driving the liquid crystal display panel, selection signals having a pulse width corresponding to a horizontal scanning period is applied sequentially to the address lines 105. In a period where one of the address lines 105 is selected, sampled image signals are held on the signal lines 104, and then, image signals are written in the corresponding picture element electrodes 107. The image signal written in each picture element electrode 107 is held for a period corresponding to one field, and then a signal of the opposite polarity is written in the picture element electrode 107 for the next field. Thus, the liquid crystal 103 is driven by alternate signals. Electric fields created between the signal lines 104 and the picture element electrodes 107 and between the address lines 105 and the picture element electrodes 107 disturb the molecular orientation of the liquid crystal. Such irregular molecular orientation of the liquid crystal is called reverse tilt. Referring to Fig. 5, the signal line 104 or the address line 105, and the picture element electrode 107 are formed on the first glass substrate 101, and the surface of the first glass substrate 101 is coated with a alignment layer 112. A lateral electric field substantially parallel to the surface of the first glass substrate 101 is created between the signal line 104 (the address line 105) and the picture element electrode 107 and the orientation of the molecules 113 of the liquid crystal is changed forcibly by the electric field. Consequently, strain is induced therein and elastic energy is concentrated. Since it is possible that the interaction of the molecules 113 of the liquid crystal induces strain energy in the picture element electrode 107, an abnormal domain 114 of an orientation different from that of most part of the picture element electrode 107 is formed in the picture element electrode 107 as shown in Fig. 6. Such a phenomenon is called reverse tilt. As shown in Fig. 6, the abnormal domain 114 is formed by reverse tilt along the edges of the picture element electrode 107 extending along the signal line 104 (or the address line 105). A disclination occurs along the boundary between the abnormal domain 114 and the normal domain 115 and an emission line appears along the disclination line. If a picture element is displayed in black against the while background, the contrast ratio of the device is reduced because the emission line increases the transmissivity of the black picture element. Various reverse tilt suppressing means have been proposed, none of which, however, has practically been effective. For example, a reverse tilt suppressing means disclosed in Japanese Patent Laid-open (Kokai) No. Hei 1-266512 removes beforehand a region of a picture element electrode in which reverse tilt is liable to occur or covers a region of a picture element electrode in which reverse tilt is liable to occur with a black mask. Such a reverse tilt suppressing means, however, reduces the aperture ratio of the device because the effective area of the picture element electrode is reduced. Patent Abstracts of Japan, vol. 12, no. 194 (P-713) 7 June 1988 discloses a liquid crystal light quantity modulation element comprising a display electrode part. An outer electrode part is provided adjacent to the edge portions of the display electrode part in order to suppress any deterioration of electric lines of force caused by the edge portions of the display electrode part themselves. No hint is given to shield the display electrode part from being influenced by bus lines located adjacent to an edge portion of the display electrode part. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a liquid crystal display device capable of effectively suppressing reverse tilt caused by the influence of bus lines located adjacent to an edge portion of a display electrode without reducing the aperture ratio. The liquid crystal display device of an active matrix type is used widely as a display screen for personal computers, word processors, television sets and viewfinders. Since the miniaturized liquid crystal display device has minute picture elements and the bus lines and the picture element electrodes are arranged closely, the adverse effect of reverse tilt is significant. Reverse tilt causes disclination lines to appear to reduce the contrast of picture elements to the background. Capable of suppressing reverse tilt without sacrificing the aperture ratio, the present invention is effectively applicable particularly to a miniaturized high-definition liquid crystal display device having minute picture elements. A liquid crystal display device according to the present invention comprises the features of appended claim 1. The reverse tilt suppressing means comprises a control portion for making the electric field applied to a portion of the liquid crystal layer extending along the edge of the picture element electrode different from the electric field applied to a portion of the liquid crystal layer extending over the central portion of the picture element electrode. The control portion cancels the effect of a lateral electric field that causes reverse tilt to suppress reverse tilt or to limit a region in which reverse tilt occurs. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings, in which: Fig. 1 is a fragmentary, schematic sectional view of a liquid crystal display device in a first embodiment according to the present invention; Fig. 2 is a typical view of assistance in explaining the operation of the liquid crystal display device of Fig. 1; Fig. 3 is a fragmentary, schematic sectional view of a liquid crystal display device in a second embodiment according to the present invention; Fig. 4 is an exploded perspective view of a general liquid crystal display device of an active matrix type; Fig. 5 is a pictorial view of assistance in explaining reverse tilt; and Fig. 6 is a view of assistance in explaining the effect of reverse tilt on a picture element electrode. First Embodiment A liquid crystal display device in a first embodiment according to the present invention is shown in Fig. 1. Referring to Fig. 1, the liquid crystal display device comprises a first transparent substrate 1, a second transparent substrate 2 disposed opposite to the first transparent substrate 1 so as to form a space of a predetermined thickness between the first transparent substrate 1 and the second transparent substrate 2, and a liquid crystal layer seal in the space between the first transparent substrate 1 and the second transparent substrate 2. A plurality of transparent picture element electrodes 4 are formed in a matrix on the inner surface of the first transparent substrate 1, and a plurality of TFTs 5 for operating the picture element electrodes 4 are combined, respectively, with the picture element electrodes 4. Address lines are formed respectively along the rows of the picture element electrodes 4, and signal lines are formed respectively along the columns of the picture element electrodes 4. A common electrode O is formed over the inner surface of the second transparent substrate 2. Orientation layers 16 are formed respectively over the respective inner surfaces of the first transparent substrate 1 and the second transparent substrate 2. A control part 8 for changing an electric field applied to the liquid crystal layer 3 has a step 15 formed under the edge portion 7 of the picture element electrode 4. The step 15 is formed by patterning, for example, a silicon dioxide thin film. Since the edge portion 7 of the picture element electrode 4 is raised by the step 15, the thickness d1 of a portion of the liquid crystal layer 3 corresponding to the edge portion 7 of the picture element electrode 4 is smaller than the thickness d2 of a portion of the liquid crystal layer 3 corresponding to the major portion 13 of the picture element electrode 4. The operation of the liquid crystal display device in the first embodiment will be described with reference to Fig. 2. When the picture element electrode 4 is selected and a voltage is applied thereto, a portion of the liquid crystal layer 3 corresponding to the edge portion 7 starts darkening first because the thickness dl of the portion is smaller than the thickness d2 of a portion of the same corresponding to the major portion of the picture element electrode 4. The portion of the liquid crystal layer 3 corresponding to the edge portion 7 is in the state of normal tilt because the thickness dl of the portion of the liquid crystal layer 3 corresponding to the edge portion 7 is relatively small and hence the intensity of a longitudinal electric field 11 applied to the same portion of the liquid crystal layer 3 is high enough to cancel a lateral electric field 10 that causes reverse tilt. Subsequently, a portion of the liquid crystal layer 3 corresponding to the major portion 13 of the picture element electrode 4 and being in the state of normal tilt responds to the voltage and starts darkening. Since both the portion corresponding to the edge portion 7 of the picture element electrode 4 and the portion corresponding to the major portion 13 of the same are in the state of normal tilt, no disclination line appears. Accordingly, the liquid crystal display device need not be provided with any black mask and hence the picture element electrode has a large aperture ratio. Second Embodiment A liquid crystal display device in a second embodiment according to the present invention will be described hereinafter with reference to Fig. 3, in which parts like or corresponding to those of the liquid crystal display device in the first embodiment are denoted by the same reference characters and the description thereof will be omitted. The liquid crystal display device in the second embodiment is different from that in the first embodiment in that the former liquid crystal display device is provided with a step 17 formed on the inner surface of the second transparent substrate 2 at a position corresponding to the edge portion 7 of the picture element electrode 4 formed on the inner surface of the first transparent substrate 1 and covered with the common electrode O. Accordingly, the thickness dl of a portion of the liquid crystal layer 3 corresponding to the edge portion 7 of the picture element electrode 4 is smaller than the thickness d2 of a portion of the same corresponding to the major portion 13 of the picture element electrode 4. The step 17 is formed by patterning a film of an ultraviolet-setting acrylic resin. The material forming the step 17 need not necessarily be limited to the ultraviolet-setting acrylic resin or silicon dioxide; the step may be formed of any material capable of forming a transparent film and of being readily patterned.

Claims: A liquid crystal display device comprising: a first substrate (1); a plurality of picture elements arranged in a matrix on the inner surface of the first substrate (1), each element comprising a picture element electrode (4) and a switching element associated with the picture element electrode (4); bus lines (6) located adjacent to each picture element electrode (4), said bus lines (6) comprising address lines associated with the rows of the picture element electrodes (4) and signal lines associated with the columns of the picture element electrodes (4); a second substrate (2) spaced apart from the first substrate (1) and having an electrode (0) on the inner surface thereof; a liquid crystal layer (3) sealed between the first and second substrates (1,2), characterized by reverse tilt suppressing means for suppressing the reverse tilt of a portion of the liquid crystal layer (3) corresponding to the edge portion of the picture element electrode (4), wherein said reverse tilt suppressing means comprises means for varying the electric field applied to the liquid crystal layer (3), said electric field varying means comprising a step portion (15,17) so that the thickness of the liquid crystal layer (3) corresponding to the edge of the picture element electrode (4) is smaller than that of the liquid crystal layer (3) corresponding to the picture element electrode (4) other than at the edge. A liquid crystal display device as claimed in claim 1, wherein said step portion (15) is formed under an edge portion of each picture element electrode (4). A liquid crystal display device as claimed in claim 1, wherein said step portion (17) is provided at a position on the inner surface of the second substrate (2) corresponding to the edge of the picture element electrode (4). A liquid crystal display device as claimed in claim 1, wherein said step portion (15,17) is provided adjacent to signal lines and/or address lines.

Inventors: KOIKE SATOFUMI, KOIKE, SATOFUMI, Koike, Satofumi, c/o Sony Corporation
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: G02F1
IPC codes: G02F1