Liquid crystal display

Abstract: A liquid crystal display including first and second translucent substrates (2,3) opposed to each other, first and second transparent electrodes (5,6) formed on opposite faces of the first and second substrates (2,3) respectively, orientation films (11,12) formed on the opposite faces so as to cover the first and second transparent electrodes (5,6), a liquid crystal (4) disposed in a space between the orientation film (11,12), the first electrode (5) being divided into a plurality of dot electrodes arranged in the row and column directions, and the second electrode (6) being divided into a plurality of stripe electrodes arranged opposite to the dot electrodes of the first electrode (5) in the row direction, a first conductive film (7) for serially connecting the dot electrodes in each column, and a second conductive film (8) provided along the longitudinal edge of the stripe electrode, wherein the first and second substrates (2,3) are positionally set such that the second conductive film (8) is opposed to concave portions formed between the dot electrodes in the row direction.

Description: FIELD OF THE INVENTION The present invention relates to a liquid crystal display using a ferroelectric liquid crystal, for example. DESCRIPTION OF THE RELATED ART With the development of information processing devices including computers as main item, display means have played an important part as means for transmitting information. A typical example of conventional display means is a CRT (cathode ray tube). Recently, a liquid crystal display has widely been utilized for the merit that its structure can be miniaturized and power consumption can be reduced. Furthermore, it is greatly desired that a liquid crystal display device having a larger display area and a larger capacity should be realized. A liquid crystal display using a ferroelectric liquid crystal has the most potential to realize such desire. A chiral smectic liquid crystal is normally used for the ferroelectric liquid crystal. In general, a ferroelectric chiral smectic phase takes a helical structure in molecular configuration. When being put into a thin cell, the helical structure is affected and comes loose by an interface. Consequently, with reference to Figs. 10 (a), (b) and (c), liquid crystal molecules 18 form two domains, that is, a domain inclined from a smectic layer normal 17 by an angle of +  19 and a domain inclined in the symmetrically opposite direction by an angle of -  20 as shown in Fig. 10 (a). When a voltage is applied, there dominates one domain in which all molecules in spontaneous polarization are uniform, i.e., a molecular orientation becomes constant as a whole as shown in Fig. 10 (b). When a voltage having reversed polarity is applied, there dominates another domain in which all molecular orientations are constant in the symmetrically opposite direction to that of Fig. 10 (b) as shown in Fig. 10 (c). Thus, according to the voltage applied or reversely applied, an optical axis is changed or controlled. Consequently, a liquid crystal display can essentially be structured by addition of a polarizing plate. Referring continually to the above-mentioned liquid crystal display, when an electric field is turned off, the molecular orientation set up before the turning off is held afterwards by the orientation regulating power of the interface as shown in Fig. 10 (d). Consequently, good memory effect can be obtained. In the case where a multiplex driving display having a high duty is desired to be carried out, the memory effect as noted is very effective. There will be described a typical prior art with reference to Fig. 8. A pair of translucent or transparent substrates 21 and 22 each has stripe-shaped transparent electrodes 23 and 24 which are provided on opposite faces of the substrates 21 and 22 respectively. And in setting up these translucent substrates 21 and 22 opposite to each other, the transparent electrodes 23 and 24 are provided to form a matrix electrode structure. Orientation films 27 and 28 are layered on the transparent electrodes 23 and 24 respectively with insulating films 25 and 26 inbetween. A ferroelectric liquid crystal 29 is filled between the translucent substrates 21 and 22. Polarizing plates 30 and 31 are provided on the outer surfaces of the translucent substrates 21 and 22. Thus, a ferroelectric liquid crystal display 32 is formed. When the liquid crystal display of the prior art is driven, a voltage is applied to one end of the transparent electrode. In this case, when the electric resistance of the transparent electrode is great, the voltage is attenuated sharply on the other end of the transparent electrode. As a result, the voltage to be applied to a liquid crystal layer is made uneven. Consequently, the display characteristics of the liquid crystal display are made non-uniform. To eliminate the above-mentioned drawback, there has been adopted a measure shown in Fig. 9 in which low-resistance conductive films 33 and 34 are provided in contact with the transparent electrodes in their longitudinal directions so that the electric resistance of the transparent electrodes is reduced. The low-resistance conductive films 33 and 34 are made of a metal or the like of which electric resistance is smaller than that of the transparent electrodes. Referring to another problem included in a ferroelectric liquid crystal display according to the prior art, it is that the memory state in a picture element is inverted. That is, when an electric field is applied as shown in Fig. 10 (d), a switch is turned on as shown in Fig. 10 (b) and the electric field is then removed, the state shown in Fig. 10 (d) is returned instead of staying at Fig. 10 (b). There has been described that this phenomenon occurs owing to a reverse electric field which is generated by the spontaneous polarization of the ferroelectric liquid crystal [Yoshida et al; Collection of Papers Presented to 13th Liquid Crystal Symposium, 2Z15 (1987)]. As the result of detailed studies, it has been found that the memory inversion is easily caused in the edge portion of the electrode (Japanese Unexamined Patent Publication No. H1-179915). It has been reported that low-resistance metallic wiring 33, 34 shown in Fig. 9 is effective in preventing the memory inversion. (Japanese Unexamined Patent Publication Nos. H1-179915, H1-280724 and the like). In that case, however, a metal or the like wiring provided on the transparent electrode causes undesirable narrowing of the transparent portion or area of the electrode. Consequently, the active or utilizable aperture of the display is lowered. Further in this art, when the film thickness of the metal or the like is increased so that a line width is reduced in order to prevent the active aperture from being lowered, there is increased a possibility that electric contact is caused between upper and lower translucent substrates. Consequently, display grade is impaired. This is not an important problem for a twisted nematic (TN) liquid crystal display having a comparatively great cell thickness, but this is serious for a ferroelectric liquid crystal display which needs such a thin cell thickness as 0.5 to 4 µ m, preferably 2 µ m or less. Stated in summary, in the liquid crystal display according to the prior art, the electrode resistance should be reduced by the low-resistance conductive film such as a metal or the like in order to make the display characteristics uniform. In addition, the active aperture is lowered in preventing the memory inversion. Consequently there is an increased possibility of electrical contact occurring between the upper and lower transparent substrates. As a result, the quality of the display is lowered. EP-A-0 062 289, on which the preamble of claim 1 is based, discloses a liquid crystal display comprising: first and second translucent substrates opposed to each other; first and second transparent electrodes formed on opposed faces of a respective one of the first and second substrates, the first electrode being formed of a plurality of dot electrodes arranged in row and column directions and the second electrode being formed of a plurality of stripe electrodes arranged opposite to the dot electrodes of the first electrode in the row direction; first and second orientation films formed on the opposed faces of the first and second substrates so as to cover the first and second transparent electrodes respectively; a liquid crystal disposed in a space between the orientation films; and a plurality of first conductive films, each first conductive film being arranged to serially connect the dot electrodes in a respective one of the columns. In the display of EP-A-0 062 289 the first conductive film is relatively thick, and is placed above the first electrodes. This display therefore suffers from the disadvantages outlined above. According to the present invention there is provided a liquid crystal display comprising: first and second translucent substrates opposed to each other; first and second transparent electrodes formed on opposed faces of a respective one of the first and second substrates, the first electrode being formed of a plurality of dot electrodes arranged in row and column directions and the second electrode being formed of a plurality of stripe electrodes arranged opposite to the dot electrodes of the first electrode in the row direction; first and second orientation films formed on the opposed faces of the first and second substrates so as to cover the first and second transparent electrodes respectively; a liquid crystal disposed in a space between the orientation films; and a plurality of first conductive films, each first conductive film being arranged to serially connect the dot electrodes along a respective one of the columns, the liquid crystal display being characterised in that it further comprises a plurality of second conductive films, each second conductive film being provided along a longitudinal edge of a respective one of the stripe electrodes; and in that the first and second substrates are positioned relative to one another such that each second conductive film is opposed to a concave portion which is formed between adjacent rows of dot electrodes and which extends in the row direction. The present invention also provides a liquid crystal display comprising: first and second translucent substrates opposed to each other; first and second transparent electrodes formed on opposed faces of a respective one of the first and second substrates, the first electrode being formed of a plurality of dot electrodes arranged in row and column directions; first and second orientation films formed on the opposed faces of the first and second substrates so as to cover the first and second transparent electrodes respectively; a liquid crystal disposed in a space between the orientation films; and a plurality of first conductive films, each first conductive film being arranged to serially connect the dot electrodes of the first electrode in a respective one of the rows; the liquid crystal display being characterised in that the second electrode is formed of a plurality of dot electrodes arranged in row and column directions; in that it further comprises a plurality of second conductive films, each second conductive film being arranged to serially connect the dot electrodes of the second electrode in a respective one of the columns; and in that the first and second substrates are positioned relative to one another such that the dot electrodes of the first electrode are opposed to the dot electrodes of the second electrode, each first conductive film being opposed to a concave portion which is formed between adjacent rows of dot electrodes of the second electrode and which extends in the row direction and each second conductive film being opposed to a concave portion which is formed between adjacent columns of dot electrodes of the first electrode and which extends in the column direction. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a section view of a liquid crystal display according to a first embodiment of the present invention; Figure 2 is a section view of a liquid crystal display according to a second embodiment of the present invention; Figures 3 (a) to (f) are views showing the first half steps of forming a dot electrode and a low-resistance conductive film in the liquid crystal display according to the second embodiment of the present invention; Figure 4 is a perspective view showing the schematic structure of a substrate according to a third embodiment of the present invention; Figure 5 is a perspective view showing the schematic structure of a substrate according to a fourth embodiment of the present invention; Figure 6 is a perspective view showing the schematic structure of a substrate according to a fifth embodiment of the present invention; Figure 7 is a perspective view showing the schematic structure of a substrate according to a sixth embodiment of the present invention; Figure 8 is a section view of a liquid crystal display according to the prior art; Figure 9 is a section view of another liquid crystal display according to the prior art; and Figures 10 (a) to (d) are views showing the behaviors of ferroelectric liquid crystal molecules. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Preferred embodiments of the present invention will be described in detail, by way of example with reference to the drawings. The present invention should not be construed as being limited by the following embodiments. Embodiment 1 Fig. 1 is a section view showing the schematic structure of a liquid crystal display according to a first embodiment of the present invention. In Fig. 1, a ferroelectric liquid crystal display 1 comprises a pair of translucent substrates 2 and 3, dot electrodes 5, stripe transparent electrodes 6, low-resistance conductive films 7 and 8, orientation films 11 and 12, a ferroelectric liquid crystal 4, and polarizing plates 13 and 14. The translucent substrates 2 and 3 are provided opposite to each other. The dot electrodes 5 are transparent electrodes which are formed in a matrix on a surface of the translucent substrate 2 opposite the translucent substrate 3. The stripe transparent electrodes 6 are formed in a row direction at equal spaces on a surface of the translucent substrate 3 opposite to the translucent substrate 2. The low-resistance conductive films 7 are provided along side edges on the surfaces of the dot electrodes 5 in the column direction and electrically connect the dot electrodes 5 respectively. The low-resistance conductive films 8 are provided along and in contact with side edges on the surfaces of the stripe transparent electrodes 6. The orientation films 11 and 12 cover the dot electrodes 5 and the stripe transparent electrodes 6 through insulating films 9 and 10, respectively. The ferroelectric liquid crystal 4 is disposed between the orientation films 11 and 12. The polarizing plates 13 and 14 are provided on an outer surface of the translucent substrates 2 and 3, respectively. The translucent substrates 2 and 3 are provided opposite to each other such that the low-resistance conductive films 8 on the stripe transparent electrodes 6 are positioned above trench portions between the dot electrodes 5. A typical example of the translucent substrates 2 and 3 is glass. The dot electrode 5 and the stripe transparent electrodes are formed of an indium tin oxide (ITO) film. Preferably, the thickness of the ITO film is 2000 Å. It is preferred that the low-resistance conductive films 7 and 8 are typically formed by depositing molybdenum with the use of a sputtering method, and have a thickness of 2200 Å and a width of 5 µ m. The dot electrode 5 is a square having a side length of 55 µ m. A space between the dot electrodes 5 is 10 µ m Embodiment 2 Fig. 2 is a section view showing the schematic structure of a liquid crystal display according to a second embodiment of the present invention. In Fig. 2, a ferroelectric liquid crystal display 1a comprises translucent substrates 2 and 3 such as glass. A ferroelectric liquid crystal 4 is disposed between the orientation films 11 and 12. Dot electrodes 5a and 5b are provided as transparent electrodes on opposite faces of the translucent substrates 2 and 3, respectively. The dot electrodes 5a and 5b are both arranged in a matrix. Low-resistance conductive films 7 and 8 are formed so as to connect the edges of the dot electrodes 5a and 5b for each row and each column. On each low-resistance conductive film 7 and 8 are sequentially provided an insulating film 9 and 10 and an orientation film 11 and 12. The translucent substrates 2 and 3 are laminated as shown by an arrow in Fig. 2 such that the concave portions of the low-resistance conductive films 7 and 8 are opposed to each other in the outside direction of the translucent substrates 2 and 3, the low-resistance conductive films 7 are opposed to concave portions formed between the dot electrodes 5b in the row direction and the low-resistance conductive films 8 are opposed to concave portions formed between the dot electrodes 5a in the column direction. Polarizing plates 13 and 14 are provided as crossed nicols on the outer surfaces of the translucent substrates 2 and 3. ITO and molybdenum are deposited as the dot electrodes 5a and 5b and the low-resistance conductive films 7 and 8 on the translucent substrates 2 and 3 by sputtering respectively, and are then etched so as to have a configuration shown in Fig. 2. Figs. 3 (a) to (f) are views showing steps of forming the dot electrodes 5a and 5b and the low-resistance conductive films 7 and 8 on the translucent substrates 2 and 3 in the liquid crystal display 1a according to the second embodiment. Only the case of the translucent substrate 2 is shown in Figs. 3 (a) to (f). As shown in Fig. 3 (a), an ITO film 5c is formed as a dot electrode 5 at a thickness of 2000 Å on a translucent substrate 2 by sputtering. A photoresist 16 is applied on the ITO film 5c. Then, temporary burning is carried out at a temperature of 90°C. Exposure is carried out by a high pressure mercury lamp with the use of a photomask 35 having a shape shown in Fig. 3 (a) so as to perform development. Thereafter, regular burning is carried out at a temperature of 120°C. Consequently, a photoresist is formed as shown in Fig. 3 (b). Subsequently, etching is carried out for 4 minutes at a temperature of 35°C by using a solution containing oxalic acid in an amount of 47 %, so that squares shown in Fig. 3 (c) are obtained by patterning. As shown in Fig. 3 (d), a molybdenum film 7a is formed at a thickness of 2200 Å by sputtering. Then, the photoresist 16 is applied on the molybdenum film 7a in similar to the case of the ITO film 5c. Then, temporary burning is carried out. Exposure is carried out by means of a photomask 36 shown in Fig. 3 (e). Development and regular burning are sequentially carried out. Thereafter, etching is carried out for 3 minutes at a temperature of 25°C by using a solution containing in an amount of 25 % a mixture of H₃PO₄ and HNO₃ at a ratio of 5 to 1. Consequently, a configuration shown in Fig. 3 (f) is obtained by patterning. In this case, the side length of a dot electrode 5a is 55 µ m, a space between the dot electrodes 5a is 10 µ m and the line width of a low-resistance conductive film 7 is 5 µ m. SiO₂ (trade name: OCD TYPE-II manufactured by Tokyo Ohka Kogyo Co., Ltd.) is applied on the above-mentioned configuration by a spinner. Then, burning is carried out to form an insulating film 9. A solution containing methacresol in an amount of 1 % of a Nylon 6 (manufactured by TORAY Co., Ltd.) is applied on the insulating film 9 by the spinner. Thereafter, burning is carried out to form an orientation film 11. These steps are not shown in Figs. 3 (a) to (f). Wrapping is applied on the orientation film 11 of the translucent substrate 2 thus formed and the orientation film 12 of a translucent substrate 3 processed in similar to Figs. 3 (a) to (f) so as to become parallel or antiparallel. As shown by an arrow in Fig. 2, the low-resistance conductive films 7 and 8 on the dot electrodes 5a and 5b are aligned opposite to trench portions between the dot electrodes 5a and 5b in the row and column directions so as to have a cell thickness of 1.8 µ m. Then, a ferroelectric liquid crystal 4 is injected and sealed. Thereafter, polarizers 13 and 14 are provided on the outer surfaces of the ferroelectric liquid crystal 4. Consequently, a liquid crystal display 1a shown in Fig. 2 is formed. Referring to the above-mentioned liquid crystal display, the dot electrodes 5a and 5b are electrically connected to each other by the low-resistance conductive films 7 and 8 comprised of molybdenum of which electric resistance is smaller than that of the transparent electrode. Consequently, an image can be displayed by selectively applying a voltage in similar to a display using a conventional stripe electrode. A voltage to be applied is not attenuated on the dot electrodes 5a and 5b. A portion in which the low-resistance conductive films are opposed to each other has a long distance. Consequently, electric contact is not caused between upper and lower substrates. In addition, memory inversion is not caused on electrode edges. Thus, good display characteristics can be obtained all over a picture. Embodiment 3 Fig. 4 is a perspective view showing the schematic structure of a substrate according to a third embodiment of the present invention. A ferroelectric liquid crystal display according to the present embodiment is the same as in Embodiment 2 of Fig. 2 except that a low-resistance conductive film 7a comprised of molybdenum is formed at a thickness of 1100 Å on a transparent electrode so as to have a shape shown in Fig. 4. It is sufficient that the position of a photomask is shifted to manufacture the low-resistance conductive film 7a in Fig. 3 (e). More specifically, the low-resistance conductive film 7a comes in contact with the side walls of dot electrodes 5 in trench portions therebetween. Thus, the low-resistance conductive film is formed over picture and non-picture element portions. Consequently, even if the line width and thickness of the low-resistance conductive film on the dot (picture element) electrode are made smaller than those of Embodiment 2, good display characteristics can be obtained in similar to Embodiment 2. Furthermore, the line width is reduced so that the active aperture of the display can be increased. Consequently, display having a higher contrast can be obtained. Embodiment 4 Fig. 5 is a perspective view showing the schematic structure of a substrate according to a fourth embodiment of the present invention. In Fig. 5, a low-resistance conductive film 7 is provided in a trench portion between dot electrodes 5. Portions between columns or rows are completely isolated by an insulating film 15 such that the low-resistance conductive film 7 electrically comes in contact with the dot electrodes 5 in a column or row on one side and does not come in contact with the dot electrodes 5 on the other side. According to the present embodiment, a Ta₂O₅ film is formed at a thickness of 2000 Å by sputtering. Then, a resist pattern is formed by photolithography in similar to Embodiments 2 and 3. Thereafter, plasma etching is carried out by using CF₄ as a gas. Thus, the insulating film 15 is formed. Molybdenum is deposited at a thickness of 500 Å on the insulating film 15 so as to form the low-resistance conductive film 7 in similar to Embodiments 2 and 3. A cell is produced on the same conditions as in Embodiments 2 and 3 except for the foregoing. In this case, it is preferred that the insulating film 15 comprised of Ta₂O₅ has a thickness of 2000 Å or more such that the low-resistance conductive film 7 comprised of molybdenum does not electrically come in contact with the dot electrode 5 comprised of ITO. The low-resistance conductive film 7 covers a non-electrode portion so that it has a line width of 11 µ m. Consequently, it is sufficient that the low-resistance conductive film 7 has a thickness of 500 Å. According to the present embodiment, good display characteristics can be obtained in similar to Embodiments 2 and 3. Furthermore, the low-resistance conductive film is formed so as to completely intercept the light in a non-picture element portion. Consequently, display in the non-picture element portion can be made uniform. Thus, display having a higher contrast can be obtained. Embodiment 5 Fig. 6 is a perspective view showing the schematic structure of a substrate according to a fifth embodiment of the present invention. In the present embodiment, a low-resistance conductive film 7 is provided by using an insulating film 15 in similar to Embodiment 4. The insulating film 15 is formed so as to almost cover the upper faces of dot electrodes 5 and to fill up trench portions between the dot electrodes 5. The low-resistance conductive film 7 comes in contact with the upper faces of the dot electrodes 5 which are not covered by the insulating film 15 in the row or column direction. Consequently, the dot electrodes 5 are electrically connected to each other. The structure of the present embodiment is the same as that of Embodiment 2 except for the structure of the substrate. The liquid crystal display has display characteristics having a good contrast in similar to Embodiment 2. Embodiment 6 Fig. 7 is a perspective view showing the schematic structure of a substrate according to a sixth embodiment of the present invention. In Fig. 7, a low-resistance conductive film 7 is formed on a translucent substrate. Dot electrodes 5 are formed so as to electrically come in contact with the low-resistance conductive film 7 through an insulating film 15 at a column or row on one side. The structure of the present embodiment is the same as that of Embodiment 4 except for the foregoing. According to the liquid crystal display of the present embodiment, display characteristics having a good contrast can be obtained in similar to Embodiments 4 and 5. According to Embodiments 1 to 6, molybdenum is used for the low-resistance conductive film. Even if aluminum, titanium, tantalum or the like is used in place of molybdenum, the same results can be obtained. In addition, even if Al₂O₃, ZnO, Y₂O₃, SiO₂, SiNx or the like is used for the insulating film 15 in place of Ta₂O₅, the same results can be obtained. According to the present invention, the low-resistance conductive films are provided so as to electrically connect a plurality of dot electrodes arranged in a matrix in the row or column direction. Consequently, the low-resistance conductive film formed between the adjacent dot electrodes is positioned lower by the thickness of the dot electrode than the height of the low-resistance conductive film on the dot electrode. A cell is produced such that the low-resistance conductive film between the adjacent dot electrodes is opposed to the low-resistance conductive film on the opposite translucent substrate. Consequently, there can be reduced a possibility that the electric contact between the translucent substrates is caused. Furthermore, the thickness of the low-resistance conductive film can be increased. In addition, the width of the low-resistance conductive film can be reduced. Thus, the active aperture can be prevented from being decreased sharply.

Claims: A liquid crystal display (1) comprising: first and second translucent substrates (2,3) opposed to each other; first and second transparent electrodes (5, 6) formed on opposed faces of a respective one of the first and second substrates (2,3), the first electrode (5) being formed of a plurality of dot electrodes arranged in row and column directions and the second electrode (6) being formed of a plurality of stripe electrodes arranged opposite to the dot electrodes of the first electrode (5) in the row direction; first and second orientation films (11, 12) formed on the opposed faces of the first and second substrates (2,3) so as to cover the first and second transparent electrodes (5,6) respectively; a liquid crystal (4) disposed in a space between the orientation films (11,12); and a plurality of first conductive films (7), each first conductive film being arranged to serially connect the dot electrodes in a respective one of the columns; the liquid crystal display being characterised in that it further comprises a plurality of second conductive films (8), each second conductive film being provided along a longitudinal edge of a respective one of the stripe electrodes; and in that the first and second substrates (2,3) are positioned relative to one another such that each second conductive film (8) is opposed to a concave portion which is formed between adjacent rows of dot electrodes and which extends in the row direction. A liquid crystal display (1) comprising: first and second translucent substrates (2, 3) opposed to each other; first and second transparent electrodes (5a,5b) formed on opposed faces of a respective one of the first and second substrates (2,3), the first electrode (5a) being formed of a plurality of dot electrodes arranged in row and column directions; first and second orientation films (11,12) formed on the opposed faces of the first and second substrates (2,3) so as to cover the first and second transparent electrodes (5a, 5b) respectively; a liquid crystal (4) disposed in a space between the orientation films (11,12); and a plurality of first conductive films (7), each first conductive film being arranged to serially connect the dot electrodes of the first electrode in a respective one of the rows; the liquid crystal display being characterised in that the second electrode (5b) is formed of a plurality of dot electrodes arranged in row and column directions; in that it further comprises a plurality of second conductive films (8), each second conductive film being arranged to serially connect the dot electrodes of the second electrode in a respective one of the columns; and in that the first and second substrates (2,3) are positioned relative to one another such that the dot electrodes of the first electrode are opposed to the dot electrodes of the second electrode, each first conductive film (7) being opposed to a concave portion which is formed between adjacent rows of dot electrodes of the second electrode and which extends in the row direction and each second conductive film (8) being opposed to a concave portion which is formed between adjacent columns of dot electrodes of the first electrode and which extends in the column direction.

Inventors: ITOH NOBUYUKI, ITOH, NOBUYUKI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: G02F1
IPC codes: G02F1