Liquid crystal display device

A picture element electrode is divided into sub picture element electrodes by slits extending obliquely. Further, a control electrode is formed over an auxiliary capacitance bus line. Part of the sub picture element electrodes overlaps the control electrode and is capacitively coupled to the control electrode. The control electrode and the source electrode of a TFT are connected through an interconnection. Part of the sub picture element electrodes is electrically connected to the interconnection. Further, part of the sub picture element electrodes is electrically connected to an interconnection extending from the control electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority of Japanese Patent Application No.2004-106138 filed in Mar. 31, 2004, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multi-domain vertical alignment (MVA) liquid crystal display device having a plurality of regions (domains) in which the orientations of liquid crystal molecules are different from each other. In particular, the present invention relates to a liquid crystal display device in which a picture element electrode is divided into a plurality of sub picture element electrodes.

2. Description of the Prior Art

Liquid crystal display devices have the advantages that they are thin and light compared to cathode-ray tube (CRT) displays and that they can be operative at low voltages and have low power consumption. Accordingly, liquid crystal display devices are used in various kinds of electronic devices including televisions, notebook personal computers (PCs), desktop PCs, personal digital assistants (PDAs), and mobile phones. In particular, active matrix liquid crystal display devices in which a thin film transistor (TFT) as a switching element is provided for each picture element (sub-pixel) show excellent display characteristics, which are comparable to those of CRT displays, because of high operation capabilities thereof, and therefore have come to be widely used even in fields where CRT displays have been used heretofore, such as desktop PCs and televisions.

In general, as shown inFIG. 1, a liquid crystal display device includes: two transparent substrates10and20which are placed with spacers31interposed therebetween and which are bonded together using a sealing member32; and liquid crystals30contained between the substrates10and20. On one substrate10, a picture element electrode, a TFT, and the like are formed for each picture element. On the other substrate20, color filters facing the picture element electrodes and a common electrode, which is common to the picture elements, are formed. The color filters are classified into three types of red (R), green (G), and blue (B). A color filter of any one color is placed in each picture element. Three picture elements of red (R), green (G), and blue (B) which are adjacently placed constitute one pixel.

Hereinafter, the substrate on which the picture element electrodes and the TFTs are formed is referred to as a TFT substrate, and the substrate placed to face the TFT substrate is referred to as a counter substrate. Further, the structure formed by filling the liquid crystals into the space between the TFT substrate and the counter substrate is referred to as a liquid crystal panel.

The TFT substrate10is formed to be larger than the counter substrate20by an amount corresponding to connection terminals. Polarizing plates41and42are placed on both sides of the liquid crystal panel40including the TFT substrate10and the counter substrate20, respectively. Moreover, a backlight (not shown) is placed under the liquid crystal panel40.

Heretofore, twisted nematic (TN) liquid crystal display devices have been widely used in which horizontal alignment-type liquid crystals (liquid crystals with positive dielectric anisotropy) are contained between two substrates10and20and in which the liquid crystal molecules are twisted and aligned. However, TN liquid crystal display devices have the disadvantage that viewing angle characteristics are poor and that contrast and color greatly change when the screen is viewed from an oblique direction. Accordingly, vertical alignment (VA) liquid crystal display devices and multi-domain vertical alignment (MVA) liquid crystal display devices, which have favorable viewing angle characteristics, have been developed and put into practical use.

FIGS. 2A and 2Bare cross-sectional schematic views showing one example of an MVA liquid crystal display device. A TFT substrate10and a counter substrate20are placed with spacers (not shown) interposed therebetween, and vertical alignment-type liquid crystals (liquid crystals with negative dielectric anisotropy)30are contained between these substrates10and20. On a picture element electrode12of the TFT substrate10, a plurality of bank-like protrusions13are formed as domain regulation structures. The surfaces of the picture element electrode12and the protrusions13are covered with a vertical alignment film14made of, for example, polyimide.

A plurality of bank-like protrusions23are also formed as domain regulation structures under a common electrode22of the counter substrate20. These protrusions23are placed at positions obliquely deviated from the protrusions13on the substrate10. The surfaces of the common electrode22and the protrusions23are also covered with a vertical alignment film24made of, for example, polyimide.

In the MVA liquid crystal display device, in the state where a voltage is not applied between the picture element electrode12and the common electrode22, most of the liquid crystal molecules30aare aligned perpendicular to the substrate surfaces as shown inFIG. 2A. However, the liquid crystal molecules30ain the vicinities of the protrusions13and23are aligned with directions perpendicular to the inclined surfaces of the protrusions13and23.

When a predetermined voltage is applied between the picture element electrode12and the common electrode22, the liquid crystal molecules30aare aligned with an oblique direction relative to the substrate surfaces under the influence of an electric field. In this case, as shown inFIG. 2B, the tilt directions of the liquid crystal molecules30aare different on opposite sides of each of the protrusions13and23, and so-called domain division (multi-domain) is achieved.

As shown in thisFIG. 2B, in the MVA liquid crystal display device, the tilt directions of the liquid crystal molecules30awhen a voltage is applied are different on opposite sides of each of the protrusions13and23. Accordingly, the leakage of light in oblique directions is suppressed, and excellent viewing angle characteristics can be obtained.

Although the case where domain regulation structures are protrusions has been described in the above-described example, slits provided in electrodes or dimples (grooves) in a substrate surface are used as domain regulation structures in some cases. Further, though an example in which domain regulation structures are provided on both of the TFT substrate10and the counter substrate20has been described inFIGS. 2A and 2B, domain regulation structures may be formed only on any one of the TFT substrate10and the counter substrate20.

FIG. 3shows an example in which slits12aare formed as domain regulation structures in the picture element electrode12on the TFT substrate10. Since electric flux lines occur in oblique directions in the vicinities of the edge portions of the slits12a, the tilt directions of the liquid crystal molecules30aare different on opposite sides of each slit12a. Thus, alignment division is achieved, and viewing angle characteristics are improved.

FIG. 4is a plan view showing one picture element of an actual MVA liquid crystal display device, andFIG. 5is a cross-sectional schematic view of a TFT substrate of the same liquid crystal display device.

On the TFT substrate50, a plurality of gate bus lines51extending horizontally and a plurality of data bus lines55extending vertically are placed with predetermined pitches, respectively. Each of the rectangular areas defined by the gate bus lines51and the data bus lines55is a picture element region. Further, auxiliary capacitance bus lines52which are placed parallel to the gate bus lines51and which cross the centers of the picture element regions are formed on the TFT substrate50. A first insulating film61is formed between the gate bus lines51and the data bus lines55and between the auxiliary capacitance bus lines52and the data bus lines55. The gate bus lines51and the auxiliary capacitance bus lines52are electrically isolated from the data bus lines55by the first insulating film61.

For each picture element region, a TFT54, a picture element electrode56, and an auxiliary capacitance electrode53are formed. The TFT54uses part of the gate bus line51as a gate electrode. Further, the drain electrode54dof the TFT54is connected to the data bus line55, and the source electrode54sthereof is formed at a position where the source electrode54sfaces the drain electrode54dacross the gate bus line51. Furthermore, the auxiliary capacitance electrode53is formed at a position where the auxiliary capacitance electrode53faces the auxiliary capacitance bus line52with the first insulating film61interposed therebetween.

The auxiliary capacitance electrode53, the TFT54, and the data bus line55are covered with a second insulating film62, and the picture element electrode56is placed on the second insulating film62. The picture element electrode56is made of transparent conductive material such as indium-tin oxide (ITO) or the like and electrically connected to the source electrode54sof the TFT54and the auxiliary capacitance electrode53through contact holes62aand62bformed in the second insulating film62. Further, two slits56aextending diagonally are horizontal-line symmetric, in the picture element electrode56. The surface of the picture element electrode56is covered with a vertical alignment film (not shown) made of, for example, polyimide.

On a counter substrate placed to face the TFT substrate50, a black matrix (light blocking film), color filters, and a common electrode are formed. As represented by dot-dashed lines inFIG. 4, a plurality of bank-like protrusions71bending at positions over the gate bus lines51and the auxiliary capacitance bus lines52are formed on the common electrode. The slits56aof the picture element electrodes56are placed between the protrusions71.

In the liquid crystal display device formed as described above, when a predetermined voltage is applied between the picture element electrode56and the common electrode, four domains A1, A2, A3, and A4in which the orientations of liquid crystal molecules30are different from each other are formed as shown inFIG. 6. These domains A1, A2, A3, and A4are separated by the protrusions71and the slits56aas boundaries. In the case where the slits56aand the protrusions71are formed so that the areas of the domains A1, A2, A3, and A4become approximately equal to each other, the direction dependency of viewing angle characteristics becomes small.

Incidentally, in a known MVA liquid crystal display device, the phenomenon occurs in which the screen looks whitish when viewed from an oblique direction.FIG. 7is a view showing T-V (transmittance-voltage) characteristics for the case where the screen is viewed from the front and those for the case where the screen is viewed from above in a direction of 60°, with applied voltage (V) on the horizontal axis and transmittance on the vertical axis. As shown in thisFIG. 7, in the case where a voltage slightly higher than a threshold voltage is applied to the picture element electrode (portion circled in the drawing), the transmittance when the screen is viewed from the oblique direction is higher than that when the screen is viewed from the front. Further, when the applied voltage becomes high to some extent, the transmittance when the screen is viewed from the oblique direction becomes lower than that when the screen is viewed from the front. Accordingly, differences in brightness between red, green, and blue picture elements become small when the screen is viewed from the oblique direction. As a result, the phenomenon in which the screen looks whitish occurs as described previously. This phenomenon is called discolor. Discolor occurs not only in MVA liquid crystal display devices but also in TN liquid crystal display devices.

In the specification of U.S. Pat. No. 4,840,460, a technology is proposed in which each picture element is divided into a plurality of sub picture elements and in which these sub picture elements are capacitively coupled. In such a liquid crystal display device, since a potential is divided in accordance with the capacitance ratio between the sub picture elements, different voltages can be applied to the sub picture elements, respectively. Accordingly, it appears that a plurality of regions having different thresholds of T-V characteristics exist in each picture element. In the case where a plurality of regions having different thresholds of T-V characteristics exist in each picture element as described above, the phenomenon is suppressed in which the transmittance when the screen is viewed from an oblique direction becomes higher than that when the screen is viewed from the front as shown inFIG. 7. As a result, the phenomenon (discolor) in which the screen looks whitish is also suppressed. A method in which display characteristics are improved by dividing each picture element into a plurality of sub picture elements capacitively coupled is called a halftone grayscale (HT) method by capacitive coupling.

In the specification of Japanese Patent No. 3076938 (Japanese Unexamined Patent Publication No. Hei 5(1993)-66412), a liquid crystal display device is disclosed in which each picture element electrode is divided into a plurality (four inFIG. 8) of sub picture element electrodes81ato81dand in which control electrodes82ato82dare respectively placed under the sub picture element electrodes81ato81dwith an insulating film interposed therebetween, as shown inFIG. 8. In this liquid crystal display device, the sizes of the control electrodes82ato82dare different from each other, and a display voltage is applied to the control electrodes82ato82dthrough a TFT80. Further, in order to prevent the leakage of light from between the sub picture element electrodes81ato81d, a control electrode83is also placed between the sub picture element electrodes81ato81d.

In the specification of Japanese Patent No. 3401049 (Japanese Unexamined Patent Publication No. Hei 6(1994)-332009), a liquid crystal display device is disclosed in which each picture element is divided into a plurality of sub picture elements. In this liquid crystal display device, the pre-tilted angles of liquid crystal molecules at the surfaces of the sub picture elements are made to be different from each other by, for example, changing rubbing conditions for each sub picture element.

Each of these known technologies relates to a TN liquid crystal display device.

Incidentally, in the HT method by capacitive coupling, the dividing of each picture element into a plurality of sub picture elements generates gaps between the sub picture elements, and an aperture ratio greatly decreases. In a typical TN liquid crystal display device of the normally white mode, the gaps between sub picture elements become portions in which transmittance is high. Accordingly, a black matrix for blocking light in the gaps between the sub picture elements needs to be formed on a counter substrate. However, in view of the prevention of misalignment in bonding a TFT substrate and the counter substrate together and the prevention of light leakage in oblique directions, the widths of the black matrix need to be set larger than those of the gaps between the sub picture elements by approximately 20 μm (approximately 10 μm on one side). This causes a significant decrease in the aperture ratio.

As described in the specification of Japanese Patent No. 3076938, it is also possible to control the transmittance by forming control electrodes even in the gaps between the sub picture elements. However, in this case, both the control electrodes and the sub picture element electrodes need to be formed of transparent conductive material such as ITO or the like. This requires two steps for depositing transparent conductive material films and two steps for photolithography, and causes an increase in manufacturing cost.

In a liquid crystal display device described in the specification of Japanese Patent No. 3401049, the pre-tilted angles of liquid crystal molecules are made to be different in each sub picture element by, for example, changing rubbing conditions for each sub picture element. However, dust occurring in rubbing can come to be mixed in liquid crystals to deteriorate display quality. Thus, the advantage of MVA liquid crystal display devices that rubbing is unnecessary is lost.

Moreover, in MVA liquid crystal display devices, the pre-tilted angles of liquid crystal molecules need to be stably aligned in a very narrow range of approximately 88 to 89° in order to realize alignment division. For example, if the pre-tilted angles of the liquid crystal molecules become 86° or less, light passes when a voltage is not applied, and contrast decreases; if the pretilt angles become 89.5° or more, the liquid crystal molecules do not easily tilt in predetermined directions when a voltage is applied. However, it is very difficult to control the pretilt angles of the liquid crystal molecules in a range of approximately 88 to 89° with high precision by rubbing. Further, the pretilt angles of the liquid crystal molecules after rubbing has been performed on a vertical alignment film have very poor stability and easily change in water washing and heat treatment thereafter.

SUMMARY OF THE INVENTION

In light of the above, an object of the present invention is to provide a liquid crystal display device in which discolor can be prevented to have favorable display quality, which has a high aperture ratio to enable bright display, and which is easily manufactured.

A liquid crystal display device of a first invention of the present application includes: first and second substrates placed to face each other; vertical alignment-type liquid crystals contained between the first and second substrates; a gate bus line provided on the first substrate and supplied with a scan signal; a data bus line provided on the first substrate and supplied with a display signal; a switching element and a picture element electrode which are formed in each picture element region delimited by the gate bus line and the data bus line; a control electrode provided in the picture element region on the first substrate and connected to the switching element; and a common electrode provided on the second substrate. Here, the picture element electrode is divided into a plurality of sub picture element electrodes; at least part of the plurality of sub picture element electrodes is capacitively coupled to the control electrode; and when a voltage is applied to the control electrode, a voltage according to a ratio between a capacitance formed between the sub picture element electrode capacitively coupled to the control electrode and the control electrode and a capacitance formed between the sub picture element electrode capacitively coupled to the control electrode and the common electrode with the liquid crystals interposed therebetween is applied to the sub picture element electrode capacitively coupled to the control electrode.

In the liquid crystal display device of the present invention, a liquid crystal display device (VA liquid crystal display device and MVA liquid crystal display device) of the VA mode in which vertical alignment-type liquid crystals (liquid crystals with negative dielectric anisotropy) are used and a capacitive coupling structure are combined, whereby high performance which cannot be obtained when either of them is used singly can be obtained.

FIG. 9Ais a schematic diagram showing one picture element of a liquid crystal display device in which each picture element electrode is not divided.FIG. 9Bis a schematic diagram showing one picture element of a liquid crystal display device in which each picture element electrode is divided into two.FIG. 9Cis a schematic diagram showing one picture element of a liquid crystal display device in which each picture element electrode is divided into three.FIG. 9Dis a schematic diagram showing one picture element of a liquid crystal display device in which each picture element electrode is divided into four. Further,FIG. 10is a view showing the relationship between the number of divisions of each picture element electrode, the picture element pitch, and the ratio between aperture ratios in TN liquid crystal display devices and VA liquid crystal display devices, with the picture element pitch on the horizontal axis and the ratio between aperture ratios on the vertical axis. It is noted that the ratio between aperture ratios represents the ratio between the aperture ratio of the liquid crystal display device in which each picture element electrode is not divided and that of a liquid crystal display device in which each picture element electrode is divided.

In TN liquid crystal display devices, since light passes through regions around picture element electrodes, a black matrix (light blocking film) needs to be formed. In general, since the black matrix is formed on a counter substrate, the precision with which a TFT substrate and the counter substrate are bonded together becomes a problem. Accordingly, margins in misalignment for bonding are necessary in design. That is, as represented by dashed lines inFIGS. 9A to 9D, the black matrix needs to be formed to the inside of the picture element electrodes. In general, margins in misalignment for bonding are 5 to 7 μm. InFIG. 10, the ratio between aperture ratios is calculated by assuming that margins in misalignment for bonding are 5 μm.

As can be seen fromFIG. 10, the influence of gaps between sub picture element electrodes and the influence of margins in misalignment for bonding increase as the number of divisions increases. However, in the case of n-way division, there are n−1 gaps between the sub picture element electrodes, whereas there are as many as (n+1)×2 margins in misalignment for bonding. Accordingly, the margins in misalignment for bonding greatly influence the aperture ratio. Further, the gaps between the sub picture element electrodes and margins in misalignment for bonding are fixed values reflecting the actual ability or the like of a manufacturing line. Accordingly, in TN liquid crystal display devices, the ratio between aperture ratios sharply decreases when the picture element pitch decreases.

On the other hand, in the cases of liquid crystal display devices of the VA mode, since regions around picture element electrodes do not transmit light, light blocking is unnecessary. Accordingly, even if each picture element electrode is divided, margins in misalignment for bonding are unnecessary, and a decrease in the aperture ratio is small. As can be seen fromFIG. 10, in TN liquid crystal display devices, the aperture ratio when each picture element electrode is divided sharply decreases as the picture element pitch decreases. Further, the trend thereof increases as the number of divisions increases. On the other hand, in liquid crystal display devices of the VA mode, even when the picture element pitch becomes small, a decrease in the aperture ratio is small compared to those in TN liquid crystal display devices.

In addition, as described later, a decrease in the aperture ratio can be substantially set to zero by using, as domain regulation structures of an MVA liquid crystal display device, slits for dividing picture element electrodes. That is, the combination of capacitive coupling and an MVA liquid crystal display device is a very good match, and viewing angle characteristics can be improved with a decrease in brightness due to capacitive coupling minimized.

A liquid crystal display device of a second invention of the present application includes: first and second substrates placed to face each other; liquid crystals contained between the first and second substrates; a gate bus line provided on the first substrate and supplied with a scan signal; a data bus line provided on the first substrate and supplied with a display signal; a picture element electrode which is placed in a picture element region delimited by the gate bus line and the data bus line and which is divided into a plurality of sub picture element electrodes by a slit; a switching element connected to the gate bus line and the data bus line; a control electrode connected to the switching element and capacitively coupled to at least one of the plurality of sub picture element electrodes; and a common electrode provided on the second substrate. Here, at least part of the slit dividing the picture element electrode constitutes a domain regulation structure for regulating orientations of liquid crystal molecules when a voltage is applied.

That is, in the present invention, the orientations of liquid crystal molecules are regulated using the slit for isolating the sub picture element electrodes. Accordingly, when compared to the case where a slit for isolating sub picture element electrodes and a slit for regulating the orientations of liquid crystal molecules are separately provided, the number of portions where orientation is disturbed is small, and bright display is made possible.

Moreover, in the present invention, for example, part of the plurality of sub picture element electrodes are capacitively coupled to the control electrode, and the rest of the sub picture element electrodes are connected to the switching element, whereby when a display voltage is supplied, a voltage applied to the part of the sub picture element electrodes and a voltage applied to the rest of the sub picture element electrodes are different and it appears that a plurality of regions having different thresholds of T-V characteristics exist in each picture element. This suppresses the phenomenon (discolor) in which the screen looks whitish when viewed from an oblique direction, and improves display quality.

Instead of connecting the rest of the sub picture element electrodes to the switching element, the rest of the sub picture element electrodes may be capacitively coupled to the control electrode. In this case, voltages at the sub picture element electrodes when a display voltage is supplied can be controlled by adjusting the sizes of the sub picture element electrodes and the amount of overlap between each sub picture element electrode and the control electrode.

In order to more reliably control the orientations of the liquid crystal molecules, it is preferable that a domain regulation structure (second domain regulation structure) is also formed on the second substrate. In this case, a decrease in transmittance can be suppressed by forming an interconnection for connecting the switching element and the control electrode, along the domain regulation structure on the second substrate.

Moreover, if the control electrode is formed on an auxiliary capacitance bus line with an insulating film interposed therebetween, a display voltage supplied through the switching element can be more reliably maintained using the capacitance between the control electrode and the auxiliary capacitance bus line, and capacitances can be formed in the same region two-dimensionally. Accordingly, there is no loss of the aperture ratio. Furthermore, in the case where there are a plurality of sub picture element electrodes capacitively coupled to the control electrode, if a portion capacitively coupled to part of the sub picture element electrodes and a portion capacitively coupled to the rest of the sub picture element electrodes are connected using an interconnection having a narrower width than the auxiliary capacitance bus line, a decrease in transmittance in the intersection of the control electrode and the slit can be suppressed.

Moreover, by forming a conductive pattern connected to the auxiliary capacitance bus line under an interconnection for connecting the switching element and the control electrode, a decrease in transmittance in the intersection of the interconnection and the slit can be suppressed. In addition, the leakage of light in the slit portion can be more reliably prevented by forming a conductive pattern connected to the auxiliary capacitance bus line under the slit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described based on drawings.

First Embodiment

FIG. 11is a plan view showing a liquid crystal display device of a first embodiment of the present invention, andFIG. 12is a cross-sectional schematic view thereof.

As shown inFIG. 12, the liquid crystal display device of the present embodiment includes two substrates110and120made of transparent thin plates, such as glass plates or the like, and vertical alignment-type liquid crystals (liquid crystals with negative dielectric anisotropy) contained between these substrates110and120. As shown inFIG. 11, a plurality of gate bus lines111extending horizontally and a plurality of data bus lines115extending vertically are formed on the substrate110. The pitch of the gate bus lines111is, for example, approximately 300 μm, and that of the data bus lines115is, for example, approximately 100 μm. Each of the rectangular regions defined by the gate bus lines111and the data bus lines115is a picture element region.

On the substrate110, auxiliary capacitance bus lines112, which are placed parallel to the gate bus lines111and cross the centers of the picture element regions, are formed. A first insulating film131is formed between the gate bus lines111and the data bus lines115and between the auxiliary capacitance bus lines112and the data bus lines115. The gate bus lines111and the auxiliary capacitance bus lines112are electrically isolated from the data bus lines115by the first insulating film131.

For each picture element region, a TFT114, a control electrode113, and a picture element electrode are formed on the substrate110. In the present embodiment, as shown inFIG. 11, the picture element electrode is divided into four sub picture element electrodes116ato116dby slits117aextending diagonally, being symmetric about a horizontal line.

The TFT114uses part of the gate bus line111as a gate electrode. Further, the drain electrode114dof the TFT114is connected to the data bus line115, and the source electrode114sthereof is placed at a position where the source electrode114sfaces the drain electrode114dacross the gate bus line111.

The control electrode113is placed at a position where the control electrode113faces the auxiliary capacitance bus line112with the first insulating film131interposed therebetween. The control electrode113is connected to the source electrode114sof the TFT114through an interconnection115a.

The sub picture element electrodes116ato116dare made of transparent conductive material, such as ITO or the like. The widths of the slits117aisolating the sub picture element electrodes116ato116dare, for example, 10 μm.

The sub picture element electrode116ais electrically connected to the interconnection115athrough a contact hole132a, and the sub picture element electrode116dis electrically connected to an interconnection115bextending from the control electrode113, through a contact hole132b. Each of the sub picture element electrodes116band116cis capacitively coupled to the control electrode113through a second insulating film132. The sub picture element electrodes116ato116dare covered with a vertical alignment film118made of, for example, polyimide.

On the other hand, on the surface (lower surface inFIG. 12) of the counter substrate120, a black matrix (light blocking film)121and color filters122are formed. The black matrix121is made of metal, for example, such as Cr (chromium) or the like, and placed at a position where the black matrix121faces the gate bus lines111, the auxiliary capacitance bus lines112, the data bus lines115, and the TFTs114on the TFT substrate110.

The color filters122are classified into three types of red, green, and blue. A color filter of any one color among red, green, and blue is placed in each picture element.

Under the color filters122, a common electrode123made of transparent conductive material, such as ITO or the like, is formed. Under the common electrode123, bank-like protrusions124for domain regulation are formed. As shown inFIG. 11, the protrusions124bend at positions over the gate bus lines111and the auxiliary capacitance bus lines112, and are placed at positions horizontally deviated from the slits117aof the TFT substrate110. The surfaces of the common electrode123and the protrusions124are covered with a vertical alignment film125made of, for example, polyimide.

Hereinafter, the operation of the liquid crystal display device of the present embodiment constituted as described above will be described.

When a predetermined display voltage is supplied to the data bus line115and a scan signal is supplied to the gate bus line111, the TFT114is turned on, and the display voltage is written into the sub picture element electrodes116aand116dand the control electrode113, which are connected to the source electrode114s. Further, since the sub picture element electrodes116band116care capacitively coupled to the control electrode113, voltages are also applied to the sub picture element electrodes116band116c.

However, in the present embodiment, as shown inFIG. 11, the area of the sub picture element electrode116cis smaller than that of the sub picture element electrode116b, and the amount of overlap between the sub picture element electrode116cand the control electrode113is larger. Accordingly, the voltage at the sub picture element electrode116cis higher than that at the sub picture element electrode116b. If the voltage at the sub picture element electrode116ais denoted by A, that at the sub picture element electrode116bis denoted by B, that at the sub picture element electrode116cis denoted by C, and that at the sub picture element electrode116dis denoted by D, A=D>C>B is obtained.

When voltages are applied to the sub picture element electrodes116ato116das described above, the liquid crystal molecules tilted in directions orthogonal to the directions in which the protrusion124and the slits117aextend. At this time, the tilted directions of the liquid crystal molecules are opposite on opposite sides of each of the protrusion124and the slits117a. In the present embodiment, similar to the known liquid crystal display device shown inFIG. 4, four domains in which the orientations of the liquid crystal molecules are different from each other are formed.

Incidentally, in the boundary portions between the domains, transmittance becomes low because disturbances occur in the liquid crystal molecules. Further, in the case where each picture element electrode is divided into a plurality of sub picture element electrodes by slits, transmittance in slit portions becomes low because voltages are not applied to the slit portions.

FIG. 13is a schematic diagram showing an example of an MVA liquid crystal display device in which slits84and protrusions85are provided as domain regulation structures in addition to a slit89for isolating sub picture element electrodes81ato81d. In thisFIG. 13, the slits84are formed in the sub picture element electrodes81ato81d, respectively, and the protrusions85are formed on a counter substrate. Further, control electrodes82ato82dconnected to a TFT80are formed under the sub picture element electrodes81ato81d, respectively.

In the liquid crystal display device of the present embodiment shown inFIGS. 11 and 12, the slits117aisolating the sub picture element electrodes116ato116dfunction as domain regulation structures. Accordingly, there is little decrease in transmittance compared to the case where the slits89for isolating the sub picture element electrodes81ato81dand the slits84for orientation regulation are formed separately as shown inFIG. 13, and bright display can be performed.

Moreover, in the present embodiment, different voltages are applied to the pair of sub picture element electrodes116aand116d, the sub picture element electrode116b, and the sub picture element electrode116c, respectively. Accordingly, it appears that three regions having different thresholds of T-V characteristics exist in each picture element. This makes it possible to suppress the phenomenon (discolor) in which the screen looks whitish when viewed from an oblique direction, and to obtain favorable display quality.

Next, a method of manufacturing the liquid crystal display device of the present embodiment will be described. First, a method of manufacturing the TFT substrate will be described with reference toFIG. 12.

First, a metal film made of Cr (chromium) or a metal film having a laminate structure of Al (aluminum) and Ti (titanium) is formed on the substrate110made of a glass plate or the like by, for example, physical vapor deposition (PVD). Thereafter, this metal film is patterned by photolithography, thus forming the gate bus lines111and the auxiliary capacitance bus lines112. It is noted that, in order to prevent the diffusion of impurities from the substrate110, the metal film may be formed after the surface of the substrate110has been covered with an insulating film.

Next, silicon oxide or silicon nitride is deposited on the substrate110by, for example, chemical vapor deposition (CVD), thus forming the first insulating film131for covering the gate bus lines111and the auxiliary capacitance bus lines112.

Subsequently, an amorphous silicon film (or polysilicon film) having a thickness of 20 to 100 nm and a silicon nitride film are sequentially formed on the first insulating film131by, for example, CVD. Then, the silicon nitride film is patterned by photolithography, thus forming channel protective films134for protecting the channels of the TFTs114.

Thereafter, an ohmic contact layer made of amorphous silicon into which impurities are introduced at high density is formed on the entire upper surface of the substrate110, and a metal film having a laminated structure of Ti—Al—Ti is formed thereon. Then, the metal film, the ohmic contact layer, and the amorphous silicon film are patterned by photolithography, thus determining semiconductor layers111serving as the active layers of the TFTs114, and forming the data bus lines115, the source electrodes114s, the drain electrodes114d, the control electrodes113, and the interconnections115aand115b.

Next, silicon nitride is deposited on the entire upper surface of the substrate110by, for example, CVD, thus forming the second insulating film132. Then, the contact holes132acommunicating with the interconnections115aand the contact holes132bcommunicating with the interconnections115bare formed at predetermined positions in the second insulating film132by photolithography.

Subsequently, an ITO film is formed on the entire upper surface of the substrate110by sputtering or the like. This ITO film is electrically connected to the interconnections115aand115bthrough the contact holes132aand132b. Thereafter, the ITO film is patterned by photolithography, thus forming the sub picture element electrodes116ato116d. Then, the vertical alignment film118is formed by spreading polyimide over the surfaces of the sub picture element electrodes116ato116d. Thus, manufacturing of the TFT substrate is completed.

Next, a method of manufacturing the counter substrate will be described with reference toFIG. 12.

First, a metal film made of, for example, Cr or the like is formed on the substrate120, and then, the metal film is patterned, whereby the black matrix121is formed at a position where the black matrix121faces the gate bus lines111, the auxiliary capacitance bus lines112, the data bus lines115, and the TFTs114on the TFT substrate110.

Next, the red, green, and blue color filters122are formed on the substrate120using, for example, red, green, and blue photosensitive resins. A color filter of any one color among red, green, and blue is placed in each picture element.

Thereafter, an ITO film is formed on the color filters122by sputtering, thus forming the common electrode123. Then, the bank-like protrusions124made of dielectric material are formed on the common electrode123using, for example, photoresist.

Subsequently, the vertical alignment film125is formed by spreading polyimide over the surfaces of the common electrode123and the protrusions124. Thus, the counter substrate is completed.

The TFT substrate and the counter substrate formed as described above are bonded together with spacers interposed therebetween, and the vertical alignment-type liquid crystals (liquid crystals with negative dielectric anisotropy) are filled into the space between the TFT substrate and the counter substrate. Thus, the liquid crystal display device of the present embodiment is completed.

As described above, in the liquid crystal display device of the present embodiment, the control electrodes113and the interconnections115aand115bcan be formed simultaneously with the formation of the data bus lines115, the source electrodes114s, and the drain electrodes114d. Accordingly, compared to known technologies, the liquid crystal display device of the present embodiment can be easily manufactured without an increase in the number of manufacturing steps.

Second Embodiment

FIG. 14is a plan view showing one picture element of a liquid crystal display device of a second embodiment of the present invention. InFIG. 14, the same components as those inFIG. 11are denoted by the same reference numerals, and will not be further described in detail.

In the present embodiment, as shown inFIG. 14, the control electrode113and the sub picture element electrode116care electrically connected through a contact hole132cformed in the second insulating film. Accordingly, if the voltage at the sub picture element electrode116ais denoted by A, the voltage at the sub picture element electrode116bis denoted by B, the voltage at the sub picture element electrode116cis denoted by C, and the voltage at the sub picture element electrode116dis denoted by D when a display voltage is supplied through the TFT114, A=C=D>B is obtained.

In the first embodiment shown inFIG. 11, when a sufficiently high voltage is applied as a display voltage, the liquid crystal molecules are aligned with respective predetermined directions in the sub picture element regions defined by the sub picture element electrodes116ato116d, and four regions (four domains) in which the orientations of the liquid crystal molecules are different from each other are formed, thus obtaining favorable viewing angle characteristics. However, when the voltage (display voltage) of a display signal is low, e.g., when a voltage slightly higher than a threshold of T-V characteristics is applied to the sub picture element electrodes116aand116dbut voltages lower than the threshold of T-V characteristics are applied to the sub picture element electrodes116band116c, it is considered that the number of the orientations of the liquid crystal molecules becomes two (two domains), resulting in that viewing angle characteristics are deteriorated.

On the other hand, in the present embodiment, the same voltage as that for the sub picture element electrodes116aand116dis applied to the sub picture element electrode116c. Accordingly, even when a display voltage is low, four regions (four domains) in which the orientations of the liquid crystal molecules are different from each other are formed. Thus, compared to the first embodiment, deterioration in viewing angle characteristics when brightness is low is avoided.

Moreover, the first embodiment shown inFIG. 11has the disadvantage that the voltages at the sub picture element electrodes116a,116c, and116dare changed by the parasitic capacitances between the data bus line115and the sub picture element electrodes116a,116c, and116d, whereas the present embodiment has the advantage in eliminating such a disadvantage. The reason for this will be described below.

FIG. 15is an equivalent circuit diagram of a picture element part of the liquid crystal display device of the first embodiment. In thisFIG. 15, C1denotes a capacitance formed by the sub picture element electrodes116aand116dand the common electrode, C2denotes a capacitance formed by the control electrode113and the sub picture element electrode116b, C3denotes a capacitance formed by the sub picture element electrode116band the common electrode, C4denotes a capacitance formed by the control electrode113and the sub picture element electrode116c, and C5denotes a capacitance formed by the sub picture element electrode116cand the common electrode. Further, C11denotes a parasitic capacitance between the data bus line115at the left and the sub picture element electrodes116aand116d, C12denotes a parasitic capacitance between the data bus line115at the left and the sub picture element electrode116b, C13denotes a parasitic capacitance between the data bus line115at the right and the sub picture element electrode116c, and C14denotes a parasitic capacitance between the data bus line115at the right and the sub picture element electrode116b.

In the liquid crystal display device of the first embodiment, since the left edges of the sub picture element electrodes116aand116dface the data bus line115at the left, the parasitic capacitance C11occurs between the data bus line115at the left and the sub picture element electrodes116aand116d. Further, since the right edge of the sub picture element electrode116cfaces the data bus line115at the right, the parasitic capacitance C13occurs between the sub picture element electrode116cand the data bus line115at the right.

In general, in a liquid crystal display device, a display voltage supplied to odd-numbered data bus lines and that supplied to even-numbered data bus lines are opposite in polarity. Further, the polarity of a display voltage supplied to each data bus line is reversed for every one flame. In the liquid crystal display device of the first embodiment, in the case where the liquid crystal display device is driven as described above, a display voltage supplied to the data bus line115influences the voltages at the sub picture element electrodes116a,116c, and116dthrough the parasitic capacitances C11and C13, and the voltages at the sub picture element electrodes116a,116d, and116care changed.

Incidentally, in the sub picture element electrode116b, the length of the edge facing the data bus line115at the left and the total length of the edges facing the data bus line115at the right are approximately equal to each other. Accordingly, in the case where display voltages in opposite polarities are applied to the data bus line115at the left and the data bus line115at the right, respectively, a change in voltage due to the influence of the data bus line115at the left is canceled by a change in voltage due to the influence of the data bus line115at the right. Consequently, the voltage at the sub picture element electrode116bhardly changes.

FIG. 16is an equivalent circuit diagram of a picture element part of the liquid crystal display device of the second embodiment. In thisFIG. 16, C1denotes a capacitance formed by the sub picture element electrodes116aand116dand the common electrode, C2denotes a capacitance formed by the control electrode113and the sub picture element electrode116b, C3denotes a capacitance formed by the sub picture element electrode116band the common electrode, and C5denotes a capacitance formed by the sub picture element electrode116cand the common electrode. Further, C11denotes a parasitic capacitance between the data bus line115at the left and the sub picture element electrodes116aand116d, C12denotes a parasitic capacitance between the data bus line115at the left and the sub picture element electrode116b, C13denotes a parasitic capacitance between the data bus line115at the right and the sub picture element electrode116c, and C14denotes a parasitic capacitance between the data bus line115at the right and the sub picture element electrode116b.

As shown in thisFIG. 16, in the present embodiment, the sub picture element electrodes116a,116c, and116dare electrically connected to each other. Further, as shown inFIG. 14, the total length of the left edges of the sub picture element electrodes116aand116dwhich face the data bus line at the left and the length of the right edge of the sub picture element electrode116cwhich faces the data bus line at the right are approximately equal to each other. Accordingly, in the case where display voltages of opposite polarities are applied to the data bus line at the left and the data bus line at the right, respectively, a change in voltage at the sub picture element electrodes116a,116c, and116ddue to the influence of the data bus line at the left is canceled by a change in voltage at the sub picture element electrodes116a,116c, and116ddue to the influence of the data bus line at the right. Consequently, the voltages at the sub picture element electrodes116a,116c, and116dhardly change. Thus, the liquid crystal display device of the present embodiment has the effect that display quality more favorable than that of the first embodiment can be obtained, in addition to an effect similar to that of the first embodiment.

Incidentally, in the present embodiment, if S1denotes the total area (total area of low-threshold regions) of the sub picture element electrodes116a,116c, and116dconnected to the TFT114, V1denotes a voltage applied to these sub picture element electrodes116a,116c, and116d, S2denotes the area (area of a high-threshold region) of the sub picture element electrode116bcapacitively coupled to the control electrode113, and V2denotes a voltage applied to the sub picture element electrode116b, it is preferable, for example, that S1:S2is set within a range of 1:9 to 6:4 and that V2/V1is set within a range of 0.8 to 0.59. For example, S1:S2is set to 4:6, and V2/V1is set to 0.72.

Third Embodiment

FIG. 17is a plan view showing one picture element of a liquid crystal display device of a third embodiment of the present invention. The present embodiment differs from the first embodiment in that the shapes of sub picture element electrodes delimited by slits are different, but other components are basically the same as those of the first embodiment. Accordingly, inFIG. 17, the same components as those inFIG. 11are denoted by the same reference numerals, and will not be further described in detail.

In the present embodiment, as shown inFIG. 17, a picture element electrode is divided into four sub picture element electrodes151a,151b,151c, and151dby slits150aextending obliquely and slits150bextending vertically. The sub picture element electrode151ais horizontal-line symmetric. The shape of the sub picture element electrode151aand the position thereof relative to the protrusion124are determined so that four regions (four domains) in which the orientations of the liquid crystal molecules are different from each other when a voltage is applied are formed and that the sizes of these regions become approximately equal to each other. Further, the sub picture element electrode151ais capacitively coupled to the control electrode113through the second insulating film.

The sub picture element electrode151bis also formed with a horizontal-line symmetric. The shape of the sub picture element electrode151band the position thereof relative to the protrusion124are determined so that four regions (four domains) in which the orientations of the liquid crystal molecules are different from each other when a voltage is applied are formed and that the sizes of these regions become approximately equal to each other. Further, the sub picture element electrode151bis also capacitively coupled to the control electrode113through the second insulating film. However, in the present embodiment, the sizes of the sub picture element electrodes151aand151band the amount of overlap between each sub picture element electrode151aor151band the control electrode113are set so that the voltage at the sub picture element electrode151bbecomes higher than that at the sub picture element electrode151awhen a voltage is applied to the control electrode113.

The sub picture element electrodes151cand151dare horizontal-line symmetrically placed across the auxiliary capacitance bus line112. Under these sub picture element electrodes151cand151d, interconnections153aand153bconnected to the source electrode114sof the TFT114and the control electrode113are formed. The sub picture element electrode151cis connected to the interconnection153athrough a contact hole154a, and the sub picture element electrode151dis connected to the interconnection153bthrough a contact hole154b. The shapes of these sub picture element electrodes151cand151dand the positions thereof relative to the protrusion124are also determined so that four regions (four domains) in which the orientations of the liquid crystal molecules are different from each other when a voltage is applied are formed and that the sizes of these regions become approximately equal to each other.

Compared to the liquid crystal display device of the second embodiment shown inFIG. 14, the liquid crystal display device of the present embodiment has the following advantage. Specifically, in the second embodiment, when a display voltage is supplied, the voltages at three (sub picture element electrodes116a,116c, and116d) of the four sub picture element electrodes116ato116dbecome equal to each other. Accordingly, in the liquid crystal display device of the second embodiment, only two regions having different thresholds of T-V characteristics exist in each picture element, and the effect of suppressing discolor is small compared to the first embodiment. On the other hand, in the present embodiment, when a display voltage is supplied, the voltage at the sub picture element electrode151a, the voltage at the sub picture element electrode151b, and the voltage at the sub picture element electrodes151cand151dare different from each other. That is, in the present embodiment, three regions having different thresholds of T-V characteristics exist in each picture element. Accordingly, similar to the first embodiment, the effect of suppressing discolor is large in the liquid crystal display device of the present embodiment.

Moreover, in the present embodiment, for each of the sub picture element electrode151a, the sub picture element electrode151b, and the pair of the sub picture element electrodes151cand151d, the shape or shapes and the position or positions thereof are determined so that four regions (four domains) in which the orientations of the liquid crystal molecules when a voltage is applied are different from each other are formed. Accordingly, even if a display voltage is low, viewing angle characteristics are not deteriorated, unlike the first embodiment.

Incidentally, in the present embodiment, if S1denotes the total area (total area of low-threshold regions) of the sub picture element electrodes151cand151d, S2denotes the area (area of an intermediate-threshold region) of the sub picture element electrode151b, and S3denotes the area (area of a high-threshold region) of the sub picture element electrode151a, it is preferable, for example, that the ratio S1:S2:S3between these areas is set to 1:2:7, 2:2:6, or the like.

Fourth Embodiment

FIG. 18is a plan view showing one picture element of a liquid crystal display device of a fourth embodiment of the present invention. The present embodiment differs from the third embodiment shown inFIG. 17in that the shapes of sub picture element electrodes delimited by slits are different, but other components are basically the same as those of the third embodiment. Accordingly, inFIG. 18, the same components as those inFIG. 17are denoted by the same reference numerals, and will not be further described in detail.

In the present embodiment, as shown inFIG. 18, a picture element electrode is divided into four sub picture element electrodes161a,161b,161c, and161dby slits150aand150cextending obliquely. Each of the sub picture element electrodes161aand161bis horizontal-line symmetric. The shape of each sub picture element electrode161aor161band the position thereof relative to the protrusion124are determined so that four regions (four domains) in which the orientations of the liquid crystal molecules are different from each other when a voltage is applied are formed and that the sizes of these regions become approximately equal to each other. Further, these sub picture element electrodes161aand161bare capacitively coupled to the control electrode113through the second insulating film.

The sub picture element electrodes161cand161dare horizontal-line symmetrically placed across the auxiliary capacitance bus line112. Under these sub picture element electrodes161cand161d, the interconnections153aand153bconnected to the source electrode114sof the TFT114and the control electrode113are formed. The sub picture element electrode161cis connected to the interconnection153athrough the contact hole154a, and the sub picture element electrode161dis connected to the interconnection153bthrough the contact hole154b. The shapes of these sub picture element electrodes161cand161dand the positions thereof relative to the protrusion124are also determined so that four regions (four domains) in which the orientations of the liquid crystal molecules are different from each other when a voltage is applied are formed and that the sizes of these regions become approximately equal to each other.

Compared to the liquid crystal display device of the third embodiment shown inFIG. 17, the present embodiment has the following advantage. Specifically, in the liquid crystal display device of the third embodiment, the slits150bintersect the slits150aand the protrusion124, which are domain regulation structures, at an angle of approximately 45°. In the case where such slits exist, disturbances occur in the orientations of the liquid crystal molecules, and portions with low transmittance occur as shown in the light transmission view ofFIG. 19A.

On the other hand, in the present embodiment, there are no slits intersecting the slits150aand the protrusion124, which are domain regulation structures, at an angle of approximately 45°, and the slits150care formed approximately perpendicular to the slits150a. In this case, as shown in the light transmission view ofFIG. 19B, disturbances in the liquid crystal molecules are reduced, and a decrease in transmittance is suppressed. Thus, the liquid crystal display device of the present embodiment has the effect that display brighter than that in the liquid crystal display device of the third embodiment can be performed, in addition to an effect similar to that of the third embodiment.

Fifth Embodiment

FIG. 20is a plan view showing one picture element of a liquid crystal display device of a fifth embodiment of the present invention. The present embodiment differs from the fourth embodiment shown inFIG. 18in that the pattern shapes of interconnections are different, but other components are basically the same as those of the fourth embodiment. Accordingly, inFIG. 20, the same components as those inFIG. 18are denoted by the same reference numerals, and will not be further described in detail.

An interconnection163aincludes a portion extending from the source electrode114stoward the protrusion124in an oblique direction (direction approximately perpendicular to the protrusion124), a portion extending along the protrusion124, and a portion extending parallel to the data bus line115toward the control electrode113. Further, the interconnection163ais electrically connected to the sub picture element electrode161cthrough the contact hole154aat the portion extending along the protrusion124. Meanwhile, an interconnection163bincludes a portion extending parallel to the data bus line115from the control electrode113toward the protrusion124and a portion extending along the protrusion124. Further, the interconnection163bis electrically connected to the sub picture element electrode161dthrough the contact hole154bat the tip of the portion extending along the protrusion124.

In the liquid crystal display device of the fourth embodiment shown inFIG. 18, in the portions of the interconnections153aand153bwhich are exposed through the slits150aand150b, disturbances in the orientations of the liquid crystal molecules occur under the influence of electric flux lines occurring from the interconnections153aand153b. Thus, portions with low transmittance will be resulted.

On the other hand, in the present embodiment, the interconnections163aand163bare placed along the protrusion124as described previously. The protrusion124serves as boundaries between domains, and is a portion with low transmittance originally. Accordingly, a decrease in transmittance in the portions in which the interconnections163aand163bare exposed through the slits150ccan be avoided by placing the interconnections163aand163balong the protrusion124as shown inFIG. 20. Thus, the liquid crystal display device of the present embodiment has the effect that display brighter than that in the fourth embodiment can be performed, in addition to an effect similar to that of the fourth embodiment.

Sixth Embodiment

FIG. 21is a plan view showing one picture element of a liquid crystal display device of a sixth embodiment of the present invention.FIG. 22is a schematic diagram showing the portion surrounded by the dashed-line circle A inFIG. 21under magnification, andFIG. 23is a schematic diagram showing the portion surrounded by the dashed-line circle B inFIG. 21under magnification. It is noted that, inFIG. 21, the same components as those inFIG. 20are denoted by the same reference numerals, and will not be further described in detail.

In the present embodiment, as shown inFIG. 22, an ITO film170for covering the interconnection163ais formed in the intersection of the slit150aand the interconnection163a. This ITO film170constitutes part of the sub picture element electrode161a. Thus, electric flux lines occurring from the interconnection163acan be shielded to prevent disturbances in the liquid crystal molecules by covering the exposed portion of the interconnection163awith the ITO film170.FIG. 24Ais a light transmission view for the intersection of the slit150aand the interconnection163ain the liquid crystal display device (the case where there is no ITO film170) of the fifth embodiment, andFIG. 24Bis a light transmission view for the intersection of the slit150aand the interconnection163ain the liquid crystal display device (the case where there is the ITO film170) of the present embodiment. From theseFIGS. 24A and 24B, it can be seen that, in the present embodiment, light transmittance in the intersection of the slit150aand the interconnection163ais improved compared to the fifth embodiment.

Moreover, in the present embodiment, as shown inFIG. 23, a control electrode113acapacitively coupled to the sub picture element electrode161aand a control electrode113bcapacitively coupled to the sub picture element electrode161bare connected using an interconnection113cnarrower than the auxiliary capacitance bus line112. In the control electrode113shown inFIG. 20, since the control electrode113is exposed through the slit150a, electric flux lines occurring from the control electrode113disturb the orientations of the liquid crystal molecules, and transmittance decreases as shown in the light transmission view ofFIG. 25A.

On the other hand, in the present embodiment, the edges of the interconnection113cfor connecting the control electrodes113aand113bare placed over the auxiliary capacitance bus line112. Since the auxiliary capacitance bus line112is generally held at ground potential, electric flux lines occurring from the interconnection113care absorbed by the auxiliary capacitance bus line112, and disturbances in the orientations of the liquid crystal molecules are suppressed.FIG. 25Bis a light transmission view for the intersection of the control electrode113and the slit150ain the present embodiment. FromFIGS. 25A and 25B, it can be seen that, in the present embodiment, light transmittance in the vicinity of the control electrode113is improved compared to the fifth embodiment.

Seventh Embodiment

FIG. 26is a plan view showing one picture element of a liquid crystal display device of a seventh embodiment of the present invention. The present embodiment differs from the first embodiment shown inFIG. 11in that the shapes of sub picture element electrodes delimited by slits and the pattern shapes of protrusions formed on the counter substrate are different, but other components are basically the same as those of the first embodiment. Accordingly, inFIG. 26, the same components as those inFIG. 11are denoted by the same reference numerals, and will not be further described in detail.

In the present embodiment, a picture element electrode is divided into five sub picture element electrodes181ato181eby slits180aextending obliquely. The sub picture element electrodes181aand181eare horizontal-line symmetrically placed across the auxiliary capacitance bus line112. Under the sub picture element electrode181a, a control electrode183ahaving a rectangular shape is formed with the second insulating film interposed therebetween, and the sub picture element electrode181ais capacitively coupled to the control electrode183a. This control electrode183ais electrically connected to an interconnection185aextending from the source electrode114sto the control electrode113.

Moreover, under the sub picture element electrode181e, a control electrode183bhaving a rectangular shape is formed with the second insulating film interposed therebetween, and the sub picture element electrode181eis capacitively coupled to the control electrode183b. This control electrode183bis electrically connected to an interconnection185bextending from the control electrode113. For these sub picture element electrodes181aand181e, the shapes thereof and the position of a protrusion187are determined so that four regions (two regions for each of the sub picture element electrodes181aand181e) in which the orientations of the liquid crystal molecules are different from each other when a voltage is applied are formed and that the sizes of these regions become approximately equal to each other.

The sub picture element electrodes181band181dare also horizontal-line symmetrically placed across the auxiliary capacitance bus line112. The sub picture element electrode181bis electrically connected to the interconnection185athrough a contact hole182b. Further, the sub picture element electrode181dis electrically connected to the interconnection185bthrough a contact hole182c. For these sub picture element electrodes181band181d, the shapes thereof and the position of a protrusion187are also determined so that four regions (two regions for each of the sub picture element electrodes181band181d) in which the orientations of the liquid crystal molecules are different from each other when a voltage is applied are formed and that the sizes of these regions become approximately equal to each other.

The sub picture element electrode181cis capacitively coupled to the control electrode113. For the sub picture element electrode181c, the shape thereof and the position of a protrusion187are also determined so that four regions in which the orientations of the liquid crystal molecules are different from each other when a voltage is applied are formed and that the sizes of these regions become approximately equal to each other.

In the present embodiment, if the voltage at the sub picture element electrode181ais denoted by A, that at the sub picture element electrode181bis denoted by B, that at the sub picture element electrode181cis denoted by C, and that at the sub picture element electrode181dis denoted by D when a display voltage is supplied, B=D>A=E>C is obtained.

In the present embodiment, it is possible to easily optimize the area ratio between the three regions having different thresholds of T-V characteristics. For example, as shown inFIG. 27, the total area of the sub picture element electrodes181band181dis denoted by M1, the total area of the sub picture element electrodes181aand181eis denoted by M2, and the area of the sub picture element electrode is denoted by M3. In a liquid crystal display device having a certain size, an experiment has revealed that discolor is minimized when the area ratio M1:M2:M3is 1:2:7 (M1:M2:M3=1:2:7). In this case, if the widths of the protrusions187are set to 10 μm, the widths of the slits180aare set to 10 μm, and the intervals between the protrusions and the slits are respectively set to 3 μm, 7 μm, and 25 μm for the pair of the sub picture element electrodes181band181d, the pair of the sub picture element electrodes181aand181e, and the sub picture element electrode181c, the area ratio M1:M2:M3becomes approximately 1:2:7. Thus, in the present embodiment, the area ratio between the sub picture element electrodes181ato181ecan be easily set to a predetermined ratio only by setting the intervals between the protrusions187and the slits180a.

FIG. 28is a plan view showing one picture element of a liquid crystal display device of modification1of the seventh embodiment. In this modification1, conductive patterns112aand112bextending from the auxiliary capacitance bus line112are provided under the interconnections185aand185b. Electric flux lines occurring from the interconnections185aand185bare absorbed by these conductive patterns112aand112b, and it is possible to suppress disturbances in the orientations of the liquid crystal molecules in the portions in which the interconnections185aand185bare exposed through the slits180a.

FIG. 29is a plan view showing one picture element of a liquid crystal display device of modification2of the seventh embodiment. In this modification2, conductive patterns112cconnected to the auxiliary capacitance bus line112are further formed under the slits180a. Since the portions under the slits180aare held at ground potential by these conductive patterns112c, it is possible to more reliably prevent light transmission in the portions corresponding to the slits180a.

Other Embodiments

As shown inFIG. 30, in a liquid crystal display device having a multi-gap structure in which the cell gaps of red (R), green (G), and blue (B) picture elements are different from each other, the capacitances (liquid crystal capacitances) between the common electrode and the picture element electrodes differ among each color picture element. For example, as shown inFIG. 30, in the case where the cell gap of a blue picture element B is 3.6 μm, that of a green picture element G is 4.6 μm, and that of a red picture element R is 5.6 μm, the liquid crystal capacitance of the blue picture element B is 4.6/3.6 times that of the green picture element G. As described in the first to seventh embodiments, in order to equalize voltage drops due to capacitive coupling among the red, green, and blue picture elements, it is necessary that the ratio between the liquid crystal capacitance and the capacitance to the control electrode be constant. Accordingly, it is necessary to set the ratio in terms of the amount of overlap between the control electrode and the sub picture element electrodes to the inverse ratio in terms of the cell gap. That is, as shown inFIG. 31, the area of the control electrode113G of the green picture element G is set to 5.6/4.6 times that of the control electrode113R of the red picture element R. Further, the area of the control electrode113B of the blue picture element B is set to 4.6/3.6 times that of the control electrode113G of the green picture element G.

Voltage drops due to capacitive coupling are equalized by changing the size of the control electrode for each of the red picture element R, the green picture element G, and the blue picture element B as described above, thus obtaining favorable display quality. Incidentally, instead of changing the area of the control electrode, the thickness of the insulating film between the control electrode and the sub picture element electrode may be changed for each color picture element. However, in the case where the thickness of the insulating film between the control electrode and the sub picture element electrode is changed for each color picture element, the number of manufacturing steps increases. Accordingly, changing the area of the control electrode as described above is easier.

FIG. 32shows an example in which scattering layers190are provided in portions corresponding to the sub picture elements having the lowest threshold of T-V characteristics in any one of the first to seventh embodiments. It is noted that, inFIG. 32,195aand195bdenote polarizing plates respectively placed on both sides of the liquid crystal panel.

In the case where a display voltage is low, only part of the sub picture elements in each picture element transmit light, but the other sub picture elements do not transmit light. Accordingly, in the case where the picture element size is large, the impression that the screen is harsh is made. For this reason, as shown inFIG. 32, the scattering layers190are formed in portions corresponding to the sub picture elements having the lowest threshold of T-V characteristics. The scattering layers190are formed by, for example, a method in which the surface of the substrate120is roughened by etching, or the like. This allows light to be scattered and suppresses the harshness of the screen when brightness is low. Further, there is also the effect that viewing angle characteristics when brightness is low are improved by scattering.

Incidentally, in the case where a light scattering layer is placed between the polarizing plates195aand195b, polarization is disturbed and contrast is therefore decreased. However, in the case where the scattering layers190are placed only in part of the picture elements as in the example shown in thisFIG. 32, a decrease in contrast is small, and problems do not occur in practical use.

In all of the first to seventh embodiments, the case where the present invention is applied to a transmissive liquid crystal display device has been described. However, the present invention can also be applied to a reflective liquid crystal display device or a semi-transmissive liquid crystal display.

(Area Ratio and Voltage Ratio)

In the case where capacitive coupling and a liquid crystal display device of the VA mode are combined as in the invention of the present application, there are the ranges of optimum values for the area ratio between sub picture element electrodes and the voltage ratio therebetween.FIGS. 33A to 33C,FIG. 34, andFIG. 35are views showing the results of investigating combinations of the parameters. In theseFIGS. 33Ato33C,FIG. 34, andFIG. 35, the horizontal axis represents the area ratio between sub picture element electrodes. That is, the horizontal axis represents the ratio between the area (area of a low-threshold region) of a sub picture element electrode directly coupled to a TFT and the area (area of a high-threshold region) of a sub picture element electrode capacitively coupled to a control electrode. Further, the vertical axis represents a voltage ratio, i.e., the ratio of the voltage at the sub picture element electrode capacitively coupled to the control electrode to the voltage at the sub picture element electrode directly coupled to the TFT. The voltage at the capacitively coupled sub picture element electrode is determined by capacitive voltage division.

For the above-described combinations, typical images which are easily influenced by discolor in a liquid crystal display device of the VA mode were selected, and contour graphs were created using a γ coefficient at the peak intensity level. It is noted that, in the graphs, the direction of the combination in which the γ coefficient is large, i.e., the combination in which the effect of improving discolor is larger, is indicated by an arrow.

Each ofFIGS. 33A to 33Cshows the values of the intensity levels of an image of human skin. Human skin color is a color remembered by humans. An impression on a person in the image is often judged by color, and human skin color is important. Accordingly, the image can be said to be a typical image for which the improvement of discolor is important. Further,FIG. 34shows the values of the intensity levels of an image of meat. The reproducibility of the image is important in terms of the possibility of arousing appetite, and this image is an image of lower intensity levels which is darker than that of human skin.

FromFIGS. 33A to 33CandFIG. 34, it is obvious that there is little effect of improving discolor for voltage ratios of 0.8 or more and that the voltage ratio must be 0.8 or less.

Moreover, in terms of the area ratio, in the image of meat which is at low intensity levels, the effect of improving discolor is favorable when the area ratio is 1:9 or the like. In the images of human skin, the effect of improving discolor is favorable when the area ratio is 2:8 to 5:5, but the effect starts to decrease when the area ratio is 6:4. Further, when the area ratio is 6:4, no effect of improving discolor can be obtained in the image of meat which are at low intensity levels.

FIG. 35is a view showing the relationship between transmittance and the parameters of capacitive coupling. Since sufficient voltages are not supplied to sub picture element electrodes in high-threshold regions, transmittance tends to decrease. However, as the proportion of the areas of the sub picture element electrodes in the high-threshold regions decreases, and as the voltage ratio increases and deviations of thresholds decrease, a decrease in transmittance is reduced. The direction of the arrow in the drawing is the direction of favorable parameters at which transmittance is high.

From the comparison betweenFIG. 35and each ofFIGS. 33A to 33CandFIG. 34, optimum solutions for the parameters exist in opposite directions. As conditions under which a good balance is maintained between skin, meat, and transmittance, a four to six division and a voltage ratio of approximately 0.72 are ideal.

Incidentally, for a three-way division, combinations are more complex, but the trend is the same as that of the two-way division. However, an experiment has revealed the following fact: in three picture elements of a low-threshold region, an intermediate-threshold region, and a high-threshold region, if conditions for the case where the combination of (the low-threshold region+the intermediate-threshold region) and the high-threshold region or the combination of the low-threshold region and (the intermediate-threshold region+the high-threshold region) is considered fall into the ranges of the conditions for the case of the two-way division, there is not large difference. It is noted that, in the case where each picture element electrode is divided into three regions of a low-threshold region, an intermediate-threshold region, and a high-threshold region, 1:2:7, 1:3:6, 2:2:6, and the like are ideal conditions.