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 (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. 3 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 the 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. 4, 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. 5. Such a phenomenon is called reverse tilt. As shown in Fig. 5, 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. Prior art document Patent Abstracts of Japan, Vol. 12, No. 194 (P-713) and JP-A-62 299 944 discloses a liquid crystal light quantity modulation element wherein electrode patterns are formed on the opposite surfaces of a pair of transparent electrode substrates and a liquid crystal is sealed into the space between the substrates. The electrode patterns near the display-forming part are formed by first display electrode parts and ou-ter second peripheral electrode parts formed so as to surround the first electrode parts with a fixed gap. When voltage is impressed, the distribution of electric line of force is made uniform on the first display electrode part including the peripheral part of the first electrode part and the distortion of electric lines of force is generated in the second electrode part. Consequently, torque applied to the liquid crystal molecules is uniform over the whole surface of the first electrode part and an insufficient tilt area or a tilt reversed area does not exist on the peripheral part of the first electrode part. Consequently, an element stabilized in orientation and improved in contrast can be obtained. SUMMARY OF THE INVENTION It is an object of the present invention to provide a liquid crystal display device capable of effectively suppressing reverse tilt without reducing the aperture ratio. To solve this object, the present invention provides a liquid crystal display device as specified in claim 1. Preferred embodiments of the invention are described in the subclaims. 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. 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 an 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 an exploded perspective view of a general liquid crystal display device of an active matrix type; Fig. 4 is a pictorial view of assistance in explaining reverse tilt; and Fig. 5 is a view of assistance in explaining the effect of reverse tilt on a picture element electrode. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1 showing a liquid crystal display device of an active matrix type in a fragmentary, schematic sectional view, the liquid crystal display device comprises a first transparent substrate 1, a second transparent substrate 2 disposed opposite to the first transparent substrate 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 3 sealed in the space between the first transparent substrate 1 and the second transparent substrate 2. A plurality of transparent picture element electrodes 4 are arranged in a matrix on the inner surface of the first transparent substrate 1. A plurality of insulated gate field effect thin film transistors (TFTs) 5, i.e., switching elements, respectively for operating the picture element electrodes 4 are combined respectively with the picture element electrodes 4. Address lines respectively for selecting rows of the TFTs 5 are formed respectively along rows of the picture element electrodes 4, and signal lines for sending image signals to the selected TFTs 5 are extended respectively along the columns of the picture element electrodes 4. In Fig. 1, the address line and the signal line are represented by a bus line 6 to simplify the drawing. A transparent common electrode O is formed over the inner surface of the second transparent substrate 2. The picture element and common electrodes are formed from indium tin oxide (ITO) or the like. An orientating layer, not shown, is formed at least between the inner surface of either the first transparent substrate 1 or the second transparent substrate 2 and the liquid crystal layer 3. A control portion 8 for changing the electric field applied to the liquid crystal layer 3 is formed along the edge portion 7 of the picture element electrode 4 contiguous with the bus line 6 to suppress reverse tilt that occurs in the liquid crystal layer 3. In this embodiment, the control portion 8 has a capacitance element 9 formed along the edge portion 7. The capacitance element 9 is formed by etching, for example, a transparent silicon dioxide thin film formed over the entire inner surface of the first transparent substrate 1 by sputtering or vacuum evaporation by a photolithographic etching process. A transparent acrylic thin film may be used instead of the silicon dioxide thin film. When forming the capacitance element 9 of a transparent acrylic resin, an ultraviolet-setting acrylic resin is applied to the inner surface of the first transparent substrate 1 in a uniform, transparent acrylic resin thin film, and then the transparent acrylic resin thin film is patterned by a photographic process. Reverse tilt caused by a lateral electric field 10 is suppressed for a time in which the capacitance element 9 is saturated, so that the occurrence of reverse tilt can be delayed. A rapid, longitudinal electric field 11 is applied across the most part of the picture element electrode 4 and the common electrode O. A black mask 12 is formed on the inner surface of the second transparent substrate 2 in an area corresponding to the inner side of the capacitance element 9 to screen a disclination line. The black mask 12 may be formed on the inner surface of the first transparent substrate 1. The operation of the liquid crystal display device will be described with reference to Fig. 2. The bus line 6, i.e., the address line or the signal line, is extended along the edge portion 7 of the substantially square picture element electrode 4. Since a voltage is applied continuously to the signal line, it is highly possible that the signal line causes reverse tilt. Although a voltage is applied intermittently to the address line, it is also possible that the address line causes reverse tilt. The present invention is capable of dealing with reverse tilt caused by either the address line or the signal line. When the picture element electrode 4 is selected, a voltage is applied to the selected picture element electrode 4. Then, a portion of the liquid crystal layer 3 corresponding to the major portion 13 of the picture element electrode 4, namely, a portion of the picture element electrode 4 excluding the edge portion 7, responds to the voltage and starts darkening. This portion of the liquid crystal layer 4 is in the state of normal tilt. The response of a portion of the liquid crystal layer 3 corresponding to the edge portion 7 of the picture element electrode 4 is delayed by the capacitance element 9 by a time in which the capacitance element 9 is saturated, and the portion of the liquid crystal layer 3 corresponding to the edge portion 7 starts darkening after the capacitance element 9 has been saturated. It is possible that reverse tilt occurs in this portion of the liquid crystal layer 3 corresponding to the edge portion 7 due to the effect of the lateral electric field. If reverse tilt occurs in this portion of the liquid crystal layer 3, a disclination line 14 appears along the boundary between the edge portion 7 and the major portion 13. Since the disclination line 14 is screened by the black mask 12, the contrast of the picture element to the background is not reduced. Since the portion of the liquid crystal layer 3 corresponding to the major portion 13 turns the state of normal tilt first, and then the portion of the liquid crystal layer 3 corresponding to the edge portion 7 turns the state of reverse tilt, the expansion of the portion of the liquid crystal layer 3 in the state of reverse tilt is suppressed by the portion of the liquid crystal layer 3 which has turned the the state of normal tilt before the latter portion of the liquid crystal layer 3 turns the state of reverse tilt. Although the invention has been described in its preferred form, obviously many changes and variations are possible therein. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein without departing from the scope thereof.

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 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), and 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 an electric field to the liquid crystal layer (3), characterized in that said electric field varying means comprises a capacitance element (9) formed on the edge of the picture element electrode (4) adjacent to at least one of the bus lines (6). A liquid crystal display device as claimed in claim 1, wherein the capacitance element (9) is formed by a thin silicon dioxide film. A liquid crystal display device as claimed in claim 1, wherein a black mask (12) is formed on the inner surface of the second substrate (2) in an area corresponding to the inner side of the capacitance element (9) to screen a disclination line.

Inventors: KOIKE SATOFUMI, KOIKE, SATOFUMI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: G02F1
IPC codes: G02F1