KR20070115220A - Liquid crystal display - Google Patents

Abstract

An LCD is provided to form a small unit section to which an electric field is applied to reduce a driving voltage to a great extent. A first signal line is formed on a first substrate. A second signal line intersects the first signal line. A thin film transistor is connected to the first signal line and the second signal line. A pixel electrode(191) is connected to the thin film transistor, and includes a plurality of branch electrodes(192) and a connection part(193) connecting the branch electrodes. A common electrode(131) is formed on the first substrate and overlaps the pixel electrode. A second substrate faces the first substrate. A liquid crystal layer(3) includes a plurality of liquid crystal molecules(310) interposed between the first substrate and the second substrate. The liquid crystal molecules have different optical anisotropy according to intensity of an electric field formed between the pixel electrode and the common electrode.

Description

Liquid crystal display {LIQUID CRYSTAL DISPLAY}

1 is an equivalent circuit diagram of a unit pixel in a liquid crystal display according to an exemplary embodiment of the present invention.

2 is a layout view of a liquid crystal display according to an exemplary embodiment of the present invention;

3 is a cross-sectional view of the liquid crystal display of FIG. 2 taken along line III-III;

4 is a cross-sectional view of the liquid crystal display of FIG. 2 taken along the line IV-IV'-IV ";

5 is a schematic diagram illustrating an electric field generating electrode in a liquid crystal display according to an exemplary embodiment of the present invention;

FIG. 6 is a cross-sectional view of the liquid crystal display of FIG. 5 taken along line VI-VI without an electric field; FIG.

FIG. 7 is a cross-sectional view of the liquid crystal display of FIG. 5 taken along the line VI-VI under an electric field. FIG.

<Description of the symbols for the main parts of the drawings>

110: substrate 121: gate line

124: gate electrode 131: common electrode

140: gate insulating film 154: semiconductor

161 and 165: ohmic contact 171: data line

173: source electrode 175: drain electrode

180: protective film 181, 182, 185: contact hole

191: pixel electrode 192: branch electrode

193: connecting electrode 81, 82: contact auxiliary member

The present invention relates to a liquid crystal display device.

The liquid crystal display is one of the most widely used flat panel display devices. The liquid crystal display includes a display panel on which an electric field generating electrode is formed and a liquid crystal layer interposed therebetween, and applies a voltage to the electric field generating electrode to apply an electric field to the liquid crystal layer. It generates and displays the image by determining the orientation of the liquid crystal molecules of the liquid crystal layer and controlling the polarization of the incident light.

Among them, the vertical alignment (VA) mode liquid crystal display in which the long axis of the liquid crystal molecules are arranged perpendicular to the upper and lower display panels without an electric field applied to the display panel, has a high contrast ratio.

However, due to a problem in the wide viewing angle, a form in which an electric field is applied in the same plane as a liquid crystal display device in an in-plane switching (IPS) mode and a liquid crystal display device in a fringe field switching (FFS) mode has been developed.

Meanwhile, a twisted nematic mode using nematic liquid crystal is mainly used for such a liquid crystal display. Since nematic liquid crystals have optical anisotropy, when an electric field is applied, the liquid crystal molecules rotate to change their orientation in a predetermined direction, thereby performing display.

However, the liquid crystal display including the nematic liquid crystal rotates in a state in which liquid crystal molecules are aligned in a predetermined direction when an electric field is applied, and thus the viscosity of the liquid crystal affects the response speed. As a result, it is difficult to realize high-speed response.

In addition, the liquid crystal display of the IPS mode and the liquid crystal display of the FFS mode have a large driving voltage.

Accordingly, the technical problem to be achieved by the present invention is to solve the problem, and to reduce the driving voltage while improving the response speed of the liquid crystal display.

A liquid crystal display according to an exemplary embodiment of the present invention is connected to a first substrate, a first signal line formed on the first substrate, a second signal line crossing the first signal line, the first signal line, and the second signal line. A thin film transistor, a pixel electrode connected to the thin film transistor, the pixel electrode including a plurality of branch electrodes and a connection part connecting the branch electrodes, a common electrode formed on the first substrate and overlapping the pixel electrode; A second substrate facing the first substrate, and a liquid crystal layer including a plurality of liquid crystal molecules sandwiched between the first substrate and the second substrate, wherein the liquid crystal molecules are formed between the pixel electrode and the common electrode. Optical anisotropy depends on the intensity of the electric field.

The liquid crystal molecules may exhibit optical isotropy when no electric field is formed between the pixel electrode and the common electrode, and may exhibit optical anisotropy when an electric field is formed between the pixel electrode and the common electrode.

The branch electrode may be inclined at any angle with respect to the first signal line or the second signal line.

The branch electrodes of neighboring pixels may be inclined in different directions.

The common electrode may include a transparent conductive material.

The display device may further include a common electrode line formed in parallel with the first signal line or the second signal line and connected to the common electrode.

Then, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily implement the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention.

In the drawings, the thickness of layers, films, panels, regions, etc., are exaggerated for clarity. Like parts are designated by like reference numerals throughout the specification. When a part of a layer, film, area, plate, etc. is over another part, this includes not only the part directly above the other part but also another part in the middle. On the contrary, when a part is just above another part, it means that there is no other part in the middle.

Next, a liquid crystal display according to an exemplary embodiment of the present invention will be described in detail with reference to FIG. 1.

1 is an equivalent circuit diagram of a unit pixel in a liquid crystal display according to an exemplary embodiment of the present invention.

As shown in FIG. 1, the liquid crystal display includes a plurality of signal lines Gi and Dj and a plurality of pixels connected to the plurality of signal lines Gi and Dj and arranged substantially in a matrix form when viewed in an equivalent circuit. Meanwhile, the liquid crystal display includes lower and upper display panels 100 and 200 facing each other and a liquid crystal layer 3 interposed therebetween.

The signal lines Gi and Dj include a plurality of gate lines Gi that transmit gate signals and data lines Dj that transmit data signals. The gate lines Gi are parallel to each other and extend substantially in the row direction, and the data lines Dj are parallel to each other and extend substantially in the column direction.

Each pixel includes a switching element Q connected to signal lines Gi and Dj, a liquid crystal capacitor Clc, and a storage capacitor Cst connected thereto. Holding capacitor Cst can be omitted as needed.

The switching element Q is a three-terminal element of a thin film transistor or the like provided in the lower panel 100. The control terminal and the input terminal are connected to the gate line Gi and the data line Dj, respectively, and the output terminal is It is connected to the liquid crystal capacitor Clc and the holding capacitor Cst.

The liquid crystal capacitor Clc uses the pixel electrode 191 and the common electrode 131 of the lower panel 100 as two terminals, and the liquid crystal layer 3 between the two electrodes 191 and 131 functions as a dielectric. The pixel electrode 191 is connected to the switching element Q, and the common electrode 131 is formed on the front surface of the lower panel 200 and receives the common voltage Vcom.

The storage capacitor Cst serving as an auxiliary role of the liquid crystal capacitor Clc is formed by overlapping the pixel electrode 191 and the common electrode 131 provided on the lower panel 100 with an insulator interposed therebetween.

On the other hand, in order to implement color display, each pixel uniquely displays one of the primary colors (spatial division) or each pixel alternately displays the primary colors over time (time division) so that the spatial colors of these primary colors can be displayed. In this way, the desired color can be recognized in time. Examples of the primary colors include three primary colors such as red, green, and blue. 1 illustrates that each pixel includes a color filter 230 representing one of the primary colors in an area of the upper panel 200 corresponding to the pixel electrode 191. Unlike in FIG. 1, the color filter 230 may be formed above or below the pixel electrode 191 of the lower panel 100.

Hereinafter, the structure of the liquid crystal display of FIG. 1 will be described in detail with reference to FIGS. 2 and 3.

2 is a layout view of a liquid crystal display according to an exemplary embodiment of the present invention, FIG. 3 is a cross-sectional view of the liquid crystal display of FIG. 2 taken along line III-III, and FIG. 4 is a view of the liquid crystal display of FIG. Sectional drawing cut along the line IV-IV'-IV ".

First, the lower panel 100 will be described.

A plurality of gate lines 121 and a plurality of common electrode lines 125 are formed on an insulating substrate 110 made of transparent glass or plastic.

The gate line 121 transmits a gate signal and mainly extends in a horizontal direction. Each gate line 121 includes an end portion 129 having a large area for connecting a plurality of gate electrodes 124 to another layer or an external driving circuit. A gate driving circuit (not shown) for generating a gate signal is mounted on a flexible printed circuit film (not shown) attached to the substrate 110 in the form of an integrated circuit chip, or the substrate 110. May be mounted directly on the substrate, or integrated into the substrate 110. When the gate driving circuit is integrated on the substrate 110, the gate line 121 may extend to be directly connected to the gate driving circuit.

The common electrode line 125 transmits a common voltage and extends in the horizontal direction substantially in parallel with the gate line 121. The common electrode line 125 is formed of the same layer as the gate line 121. The common electrode line 125 is positioned between two neighboring gate lines 121 and has an extension (not shown) protruding up and down to block leakage light. Can have

The gate line 121 and the common electrode line 125 may be formed of aluminum-based metal such as aluminum (Al) or aluminum alloy, silver-based metal such as silver (Ag) or silver alloy, copper-based metal such as copper (Cu) or copper alloy, or molybdenum ( It may be made of molybdenum-based metals such as Mo) or molybdenum alloy, chromium (Cr), tantalum (Ta) and titanium (Ti). However, they may have a multilayer structure including two conductive films (not shown) with different physical properties.

Side surfaces of the gate line 121 and the common electrode line 125 are inclined with respect to the surface of the substrate 110, and the inclination angle is preferably about 30 degrees to about 80 degrees.

A plurality of common electrodes 131 are formed on the substrate 110 and the common electrode line 125. The common electrode 131 is commonly connected to the common electrode line 125 to receive a common voltage from the common electrode line 125. The common electrode 131 has a rectangular shape and is arranged in a matrix to almost fill the space between the gate lines 121.

The common electrode 131 may be made of a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO).

A gate insulating layer 140 made of silicon nitride (SiNx) or silicon oxide (SiOx) is formed on the gate line 121, the common electrode line 125, and the common electrode 131. The gate insulating layer 140 prevents the gate line 121 and the common electrode 131 from being short-circuited with each other, and insulates the other conductive thin film formed thereon.

On the gate insulating layer 140, a plurality of island semiconductors 154 made of hydrogenated amorphous silicon (amorphous silicon is abbreviated as a-Si), polycrystalline silicon, or the like are formed. The island semiconductor 154 is positioned over the gate electrode 124.

A plurality of island type ohmic contacts 163 and 165 are formed on the island type semiconductor 154. The ohmic contacts 163 and 165 may be made of a material such as n + hydrogenated amorphous silicon in which n-type impurities such as phosphorus (P) are heavily doped, or may be made of silicide. The island-like ohmic contacts 163 and 165 are paired and disposed on the island-like semiconductor 154.

Side surfaces of the island-like semiconductor 154 and the ohmic contacts 163 and 165 are also inclined with respect to the surface of the substrate 110, and the inclination angle is about 30 degrees to about 80 degrees.

A plurality of data lines 171 and a plurality of drain electrodes 175 are formed on the ohmic contacts 163 and 165 and the gate insulating layer 140.

The data line 171 transmits a data signal and mainly extends in the vertical direction to cross the gate line 121. Each data line 171 includes a plurality of source electrodes 173 extending toward the gate electrode 124 and an end portion 179 having a large area for connection with another layer or an external driving circuit. A data driving circuit (not shown) for generating a data signal is mounted on a flexible printed circuit film (not shown) attached to the substrate 110, directly mounted on the substrate 110, or integrated in the substrate 110. Can be. When the data driving circuit is integrated on the substrate 110, the data line 171 may be extended to be directly connected to the data driving circuit.

The drain electrode 175 is separated from the data line 171 and faces the source electrode 173 with respect to the gate electrode 124.

The data line 171 and the drain electrode 175 are preferably made of a refractory metal such as molybdenum, chromium, tantalum, and titanium, or an alloy thereof, and include a refractory metal film (not shown) and a low resistance conductive film. It may have a multilayer structure including (not shown).

The ohmic contacts 163 and 165 exist only between the semiconductor 154 thereunder and the data line 171 and the drain electrode 175 thereon to lower the contact resistance therebetween. An extension of the semiconductor 154 positioned on the gate line 121 may soften the profile of the surface to prevent the data line 171 from being disconnected. The semiconductor 154 includes portions exposed between the source electrode 173 and the drain electrode 175 and not covered by the data line 171 and the drain electrode 175.

One gate electrode 124, one source electrode 173, and one drain electrode 175 together with the island-like semiconductor 154 form one thin film transistor (TFT), and a channel of the thin film transistor ( A channel is formed in the semiconductor 154 between the source electrode 173 and the drain electrode 175.

A passivation layer 180 is formed on the data line 171, the drain electrode 175, and the exposed semiconductor 151. The passivation layer 180 is made of an inorganic insulator, and examples of the inorganic insulator include silicon nitride and silicon oxide. However, the passivation layer 180 may have a double layer structure of the lower inorganic layer and the upper organic layer so as not to damage the exposed portion of the semiconductor 151 while maintaining excellent insulating properties of the organic layer.

In the passivation layer 180, a plurality of contact holes 182 and 185 exposing the end portion 179 and the drain electrode 175 of the data line 171 are formed, respectively, and the passivation layer 180 and the gate insulating layer are formed. A plurality of contact holes 181 exposing the end portion 129 of the gate line 121 are formed at 140.

A plurality of pixel electrodes 191 and a plurality of contact assistants 81 and 82 are formed on the passivation layer 180. They may be made of a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO).

The pixel electrode 191 overlaps the common electrode 131, and includes a plurality of branch electrodes 192 arranged in parallel with each other and a connection unit 193 connecting them in common.

The branch electrode 192 is inclined at a predetermined angle with respect to the gate line 121 or the horizontal direction, and when the voltage is applied, the liquid crystal molecules 310 positioned in the pixels adjacent to each other rotate in different directions to form the liquid crystal molecules 310. Can be arranged in multiple regions. Therefore, a wide viewing angle can be obtained and the side visibility can also be improved.

The connection part 193 includes a vertical connection part connecting both ends of the branch electrodes 192 and horizontal connection parts positioned on the upper and lower parts of the branch electrode 192.

The outer boundary of the connection unit 193 defining the boundary of the pixel electrode 191 is a rectangular shape.

When a data voltage is applied to the pixel electrode 191 and a common voltage is applied to the common electrode 131, an electric field is generated by a potential difference between the two voltages, and the electric field lines are indicated by dotted lines in FIG. 3.

The pixel electrode 191 to which the data voltage is applied generates an electric field together with the common electrode 131 to which the common voltage is applied to determine the direction of the liquid crystal molecules of the liquid crystal layer 3 positioned on the two electrodes 191 and 131. . The polarization of light passing through the liquid crystal layer varies according to the direction of the liquid crystal molecules determined as described above.

The pixel electrode 191 and the common electrode 131 form a liquid crystal capacitor using the liquid crystal layer 3 as a dielectric to maintain an applied voltage even after the thin film transistor is turned off, and these 191 and 131 also have a gate insulating layer 140. ) And the passivation layer 180 as a dielectric to enhance the voltage holding capability of the liquid crystal capacitor.

The contact auxiliary members 81 and 82 are connected to the end portion 129 of the gate line 121 and the end portion 179 of the data line 171 through the contact holes 181 and 182, respectively. The contact auxiliary members 81 and 82 compensate for and protect the adhesion between the end portion 129 of the gate line 121 and the end portion 179 of the data line 171 and the external device.

Next, the upper panel 200 will be described.

A light blocking member 220 is formed on an insulating substrate 210 made of transparent glass or plastic. The light blocking member 220 is called a black matrix and blocks light leakage between the pixel electrodes 191. The light blocking member 220 has a plurality of openings 225 facing the pixel electrode 191 and having substantially the same shape as the pixel electrode 191. The light blocking member 220 may include a portion corresponding to the gate line 121 and the data line 171 and a portion corresponding to the thin film transistor.

A plurality of color filters 230 is also formed on the substrate 210. The color filter 230 is mostly present in an area surrounded by the light blocking member 220, and may extend in the vertical direction along the column of the pixel electrodes 191. Each color filter 230 may display one of primary colors such as three primary colors of red, green, and blue.

An overcoat 250 is formed on the color filter 230 and the light blocking member 220. The overcoat 250 may be made of an organic insulating material to prevent the color filter 230 from being exposed and provide a flat surface. The overcoat 250 may be omitted.

The liquid crystal layer 3 including the plurality of liquid crystal molecules 310 is interposed between the display panels 100 and 200.

The liquid crystal molecules 310 according to the exemplary embodiment of the present invention have different optical anisotropy according to the intensity of the electric field formed between the pixel electrode 191 and the common electrode 131.

If an electric field is not formed between the pixel electrode 191 and the common electrode 131, it is optically isotropic. If an electric field is formed between the pixel electrode 191 and the common electrode 131, it is optically anisotropic. The magnitude of the optical anisotropy depends on the electric field strength.

Therefore, when the electric field is not applied between the pixel electrode 191 and the common electrode 131, the liquid crystal molecules 310 are randomly arranged without direction between the two display panels 100 and 200 and are common to the pixel electrode 191. When an electric field is applied between the electrodes 131, they are arranged along the electric force lines shown in FIG.

In general, liquid crystal molecules having optical anisotropy perform display by changing the orientation due to rotation of the liquid crystal molecules according to the application of an electric field, and the response speed is determined by the intrinsic viscosity of the liquid crystal molecules. On the other hand, the liquid crystal molecules according to the exemplary embodiment of the present invention have an intrinsic viscosity because liquid crystal molecules have an optical anisotropy or not depending on whether an electric field is applied, and an optical anisotropy varies depending on the intensity of the electric field. It has nothing to do with Therefore, a high speed response can be realized regardless of the viscosity of liquid crystal molecules.

The liquid crystal molecules 310 may be a material having a liquid crystal phase called a blue phase. The blue phase has a narrow temperature range between the isotropic phase and the cholesteric phase, and may exhibit optical isotropy when no electric field is applied and optical anisotropy when an electric field is applied.

For example, it may be the following liquid crystal material:

It may also be 4-cyano-4'-pentylbiphenyl.

In addition, the composition may be a mixture of these liquid crystal materials.

In the liquid crystal display according to the exemplary embodiment of the present invention, the branch electrodes 192 of the pixel electrodes 191 which are adjacent to each other up, down, left, and right are inclined in different directions, and thus, liquid crystals positioned in neighboring pixel areas when voltage is applied. The molecules rotate in different directions so that the liquid crystal molecules are divided into multiple regions. Therefore, a wide viewing angle can be obtained and side visibility can be improved, which will be described in more detail later with reference to the drawings.

Hereinafter, an operation of the liquid crystal display according to the exemplary embodiment of the present invention will be described in detail with reference to FIGS. 5 to 7.

5 is an enlarged layout view of the common electrode 131 and the pixel electrode 191 in the liquid crystal display according to the exemplary embodiment, and FIG. 6 is a liquid crystal display of FIG. 5 without an electric field applied thereto. Is a cross-sectional view taken along the line VI-VI, and FIG. 7 is a cross-sectional view taken along the line VI-VI of the liquid crystal display of FIG. 5 to which an electric field is applied. Here, the pixel electrode 191 indicates the branch electrode 192 illustrated in FIGS. 2 to 4.

5 to 7, a planar common electrode 131 is formed on the lower substrate 110, a gate insulating layer 140 and a passivation layer 180 are covered on the common electrode 131, and a passivation layer. On the 180, a plurality of narrow pixel electrodes 191 extend in parallel to each other in the vertical direction. The width NR of the pixel electrode 191 is smaller than the gap WR between the pixel electrodes 191.

As shown in FIG. 6, in the liquid crystal display according to the exemplary embodiment of the present invention, when no voltage is applied to the common electrode 131 and the pixel electrode 191, the liquid crystal molecules 310 are disorderly arranged without orientation.

Meanwhile, as shown in FIG. 7, when a voltage is applied to the common electrode 131 and the pixel electrode 191, an electric field is generated like a dotted line, and the liquid crystal molecules 310 are arranged along the direction of the electric field.

In this case, the unit section to which the electric field is applied is a vertical section of the wide area WR between the vertical center line C (actually a plane) of the narrow region NR on the pixel electrode 191 and the pixel electrode 191. Direction center line B (actually corresponds to a plane).

In the region from the centerline C of the narrow region NR to the centerline B of the large region WR, the vertex is at the boundary line A (actually a plane) between the narrow region NR and the large region WR. An electric field having an electric field line shape having a half ellipse shape or parabolic shape (hereinafter, referred to as a half ellipse shape for convenience) having a shape is generated. The tangent of the electric line of force is almost parallel to the substrate 10 on the boundary line A between the narrow region NR and the large region WR, and the substrate 10 at the central position of the narrow region NR and the large region WR. It is almost perpendicular to.

In addition, the center and longitudinal vertices of the ellipse are located on the boundary line A between the narrow region NR and the wide region WR, and the two vertices of the horizontal direction are positioned in the large region WR and the narrow region NR, respectively. do. At this time, the horizontal vertex located in the narrow area NR has a shorter distance from the center of the ellipse than the horizontal vertex located in the wide area WR. Thus, the ellipse does not have symmetry with respect to the boundary line A. FIG. In addition, the density of electric field lines varies with position, and the strength of the electric field varies proportionally thereto. Thus, the intensity of the electric field is greatest on the boundary line AA between the narrow region NR and the large region WR, and toward the centerline CC, BB of the narrow region NR and the large region WR, and the upper portion. It becomes smaller toward the substrate 210.

This is compared with that in the liquid crystal display having the same planar structure as in the conventional IPS structure, the unit section in which the electric field is applied is from the center line of the pole (corresponding to 'C') to the center line of the neighboring electrode (corresponding to 'C'). It is as narrow as 1/2. Therefore, the liquid crystal display according to the exemplary embodiment of the present invention reduces the driving voltage significantly since the unit interval in which the electric field is applied is 1/2 even though the distance between the electrodes is the same as that of the liquid crystal display having the same planar structure as the IPS structure. Can be.

Although the liquid crystal layer 3 having the positive dielectric anisotropy has been described in the embodiment of the present invention, the liquid crystal layer 3 may have a negative dielectric constant. In this case, since the liquid crystal molecules 310 are arranged perpendicular to the electric field formed between the pixel electrode 191 and the common electrode 131, the alignment direction of the liquid crystal molecules is in the direction of the branch electrode 192 of the pixel electrode 191. It is almost perpendicular to.

Although the preferred embodiments of the present invention have been described in detail above, those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. Accordingly, the scope of the present invention is not limited thereto, and various modifications and improvements of those skilled in the art using the basic concept of the present invention as defined in the following claims also fall within the scope of the present invention.

It is possible to improve the response speed of the liquid crystal display and to lower the driving voltage.

Claims (6)

First substrate, A first signal line formed on the first substrate, A second signal line crossing the first signal line, A thin film transistor connected to the first signal line and the second signal line, A pixel electrode connected to the thin film transistor and including a plurality of branch electrodes and a connection part connecting the branch electrodes; A common electrode formed on the first substrate and overlapping the pixel electrode; A second substrate facing the first substrate, and Liquid crystal layer comprising a plurality of liquid crystal molecules sandwiched between the first substrate and the second substrate Including, The liquid crystal molecules have different optical anisotropy according to the intensity of the electric field formed between the pixel electrode and the common electrode. Liquid crystal display. In claim 1, The liquid crystal molecule If an electric field is not formed between the pixel electrode and the common electrode, it is optically isotropic. If an electric field is formed between the pixel electrode and the common electrode, it is optically anisotropic. Liquid crystal display. In claim 1, And the branch electrode is inclined at an angle with respect to the first signal line or the second signal line. In claim 3, The branch electrodes of pixels neighboring each other are inclined in different directions. In claim 1, The common electrode includes a transparent conductive material. In claim 1, And a common electrode line formed in parallel with the first signal line or the second signal line and connected to the common electrode.

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