Patent Application
20100201932
The present application claims priority from Japanese application serial No. 2009-27309, filed on Feb. 9, 2009, the content of which is hereby incorporated by reference into this application.
1. Field of the Invention
The present invention relates to a display device, and more particularly to a relatively small lateral-electric-field liquid crystal display device which possesses an excellent viewing angle characteristic.
2. Background Art
In a liquid crystal display device, a TFT substrate on which pixel electrodes, thin film transistors (TFT) and the like are formed in a matrix array and a counter substrate which faces the TFT substrate in an opposed manner and forms color filters and the like on portions thereof corresponding to the pixel electrodes formed on the TFT substrate are arranged. Further, liquid crystal is sandwiched between the TFT substrate and the counter substrate. An image is formed by controlling optical transmissivity of liquid crystal molecules for every pixel.
A viewing angle characteristic is important in the liquid crystal display device. Here, the viewing angle characteristic is a phenomenon where brightness is changed or chromaticity is changed between a case where a screen is viewed from a front side and a case where the screen is viewed in the oblique direction. An IPS (In Plane Switching) liquid crystal display device which operates liquid crystal molecules using an electric field generated in the horizontal direction possesses an excellent viewing angle characteristic.
In the liquid crystal display device, a pixel region is formed of an ITO (Indium Tin Oxide) film which constitutes a transparent electrode. When a thickness of the ITO film becomes large, there may be a case where the display device cannot perform a completely white display due to a spectral characteristic of the ITO film. Further, for example, incase of a liquid crystal display device which uses STN (Super Twisted Nematic) liquid crystal and does not use TFTs as switching elements, scanning electrodes arranged in a stripe shape are formed on one substrate, video signal electrodes arranged in a stripe shape are formed on another substrate, and pixels are formed at intersections of the scanning electrodes and the video signal electrodes. In this case, when a thickness of an ITO film which constitutes the scanning electrodes or the video signal electrodes is large, these electrodes can be observed from the outside so that image quality is deteriorated.
On the other hand, ITO is metal oxide and is chemically stable and hence, ITO is used as a material for forming terminal electrodes at a terminal area. An ITO film which is used as a terminal electrode plays a role of protecting the terminal area and hence, the ITO film is required to have a predetermined thickness. However, when the thickness of the ITO film which is used as the electrode in a display region becomes large, the above-mentioned drawbacks arise.
JP-A-11-64870 (patent document 1) discloses an STN liquid crystal display device having the following constitution to make a film thickness of an ITO film at a terminal area large and to make the thickness of the ITO film in a display region small. That is, the manufacture of the STN liquid crystal display device includes a photolithography step of patterning an ITO film, and a photolithography step of decreasing a thickness of the ITO film in a display region. Due to such a constitution, the thickness of the ITO film in the display region can be set smaller than the film thickness of the ITO film at the terminal area.
A TFT liquid crystal display device which uses a TFT as a switching element in each pixel has the structure and the manner of operation completely different from those of the STN liquid crystal display device. Further, also in the TFT liquid crystal display device, the usual TN liquid crystal display device, the VA liquid crystal display device and the IPS liquid crystal display device completely differ from each other with respect to the structure and the manner of operation.
In the IPS liquid crystal display device, a comb-teeth-shaped pixel electrode or a counter electrode is formed of an ITO film for every pixel. When a size of the pixel becomes small along with the miniaturization of the liquid crystal display device, a width, a pitch and the like of the pixel electrodes or the counter electrodes also become small. In the IPS liquid crystal display device, to impart initial alignment to liquid crystal, an alignment film is formed on pixel electrodes and the like, and rubbing is applied to the alignment film in a specified direction thus determining the initial alignment direction of liquid crystal.
In this case, when a thickness of the ITO film is large, for example, there arises a phenomenon that rubbing is not sufficiently applied to an alignment film arranged in a valley between the comb-teeth-shaped electrode and the comb-teeth-shaped electrode. When rubbing is not sufficiently applied to the alignment film, there arises a phenomenon that liquid crystal molecules do not return to the initial alignment direction after being twisted or the like due to an electric field. This phenomenon appears, on a screen of the liquid crystal display device, as a phenomenon in which a screen of a previous frame remains, that is, image retention phenomenon. This image retention extremely deteriorates image quality.
It is an object of the present invention to provide an IPS liquid crystal display device which can suppress such an image retention phenomenon.
The present invention has been made to overcome the above-mentioned drawbacks, and the specific constitutions of the present invention are as follows.
(1) According to one aspect of the present invention, there is provided a liquid crystal display device having a liquid crystal display panel, the liquid crystal display panel which includes: a TFT substrate where pixels each of which includes a comb-teeth-shaped first electrode, a planar second electrode and a TFT are formed in a matrix array so as to form a display region, and a terminal area which includes terminal electrodes is formed outside the display region; a counter substrate on which color filters are formed; and a liquid crystal layer which is sandwiched between the TFT substrate and the counter substrate, wherein the first electrode is arranged above the second electrode with an insulation film sandwiched therebetween, the first electrode and the second electrode are formed of an ITO film, the first electrode is connected with the TFT via the ITO film formed in a through hole, and the terminal electrodes are formed of the ITO film, and a thickness of the ITO film for forming the first electrode is smaller than a thickness of the ITO film formed in the through hole and a thickness of the ITO film formed on the terminal area.
(2) In the liquid crystal display device having the above-mentioned constitution (1), the first electrode is a pixel electrode, and the second electrode is a counter electrode.
(3) In the liquid crystal display device having the above-mentioned constitution (1), the first electrode is a counter electrode, and the second electrode is a pixel electrode.
(4) In the liquid crystal display device having any one of the above-mentioned constitutions (1) to (3), a thickness of the ITO film for forming the first electrode is half or less of a thickness of the ITO film formed in the through hole and also is half or less of a thickness of the ITO film which constitutes the terminal electrodes.
(5) According to another aspect of the present invention, there is provided a liquid crystal display device having a liquid crystal display panel, the liquid crystal display panel which includes: a TFT substrate where pixels each of which includes a comb-teeth-shaped pixel electrode and a TFT are formed in a matrix array so as to form a display region, and a terminal area which includes terminal electrodes is formed outside the display region; a counter substrate on which color filters are formed; and a liquid crystal layer which is sandwiched between the TFT substrate and the counter substrate, wherein the pixel electrode is formed of an ITO film, the first electrode is connected with the TFT via the ITO film formed in a through hole, and the terminal electrodes are formed of the ITO film, and a thickness of the ITO film for forming the pixel electrode is smaller than a thickness of the ITO film formed in the through hole and a thickness of the ITO film formed on the terminal area.
(6) In the liquid crystal display device having the above-mentioned constitution (5), a thickness of the ITO film for forming the pixel electrode is half or less of a thickness of the ITO film formed in the through hole and also is half or less of a thickness of the ITO film which constitutes the terminal electrodes.
(7) According to still another aspect of the present invention, there is provided a manufacturing method of a liquid crystal display device having a liquid crystal display panel, the liquid crystal display panel which includes: a TFT substrate where pixels each of which includes a comb-teeth-shaped first electrode, a planar second electrode and a TFT are formed in a matrix array so as to form a display region, and the first electrode being arranged above the second electrode with an insulation film sandwiched therebetween, the first electrode being formed of an ITO film and the first electrode and the TFT being connected with each other via an ITO film formed in a through hole, and a terminal area which includes terminal electrodes formed of an ITO film is formed outside the display region; a counter substrate on which color filters are formed; and a liquid crystal layer which is sandwiched between the TFT substrate and the counter substrate, the manufacturing method of a liquid crystal display device including the steps of: simultaneously forming the ITO film for forming the first electrodes, the ITO film formed at the through holes and the ITO film for forming the terminal electrodes; applying half exposure to portions of a photo resist corresponding to the pixel electrodes in performing patterning by applying the photo resist on the ITO films which are formed simultaneously and by exposing the photo resist; developing the photo resist and etching the ITO film using the photo resist; etching back the photo resist so as to remove the photo resist formed on the pixel electrodes and leaving the photo resist formed at the through holes and at the terminal electrodes, etching the pixel electrodes so as to make the ITO film for forming the pixel electrodes small; and removing the photo resist formed at the through holes and at the terminal electrodes.
(8) In the manufacturing method of the liquid crystal display device having the above-mentioned constitution (7), the photo resist is etched back by an oxygen asher.
According to the present invention, the film thickness of the ITO film which constitutes the pixel electrodes is set smaller than the film thickness of the ITO film formed at the through holes and the film thickness of the ITO film formed at the terminal areas. Accordingly, it is possible to acquire a liquid crystal display device which can sufficiently perform rubbing in the vicinity of the pixel electrodes thus having no image retention while ensuring the reliability of the through hole portions and the reliability of the terminal areas.
Further, according to the present invention, in the photolithography step, by applying the half exposure to portions where the pixel electrodes are formed and the full exposure to other portions, it is possible to form the thin ITO film at the pixel electrodes and the thick ITO film at the through holes and terminal areas without increasing the number of photolithography steps.
A gate insulation film 102 made of SiN is formed so as to cover the gate electrodes 101. A semiconductor layer 103 which is formed of an a−Si film is formed over the gate insulation film 102 at a position where the semiconductor layer 103 faces the gate electrode 101 in an opposed manner. The a−Si film is formed by a plasma CVD method. The a−Si film forms a channel portion of a TFT, and a source electrode 104 and a drain electrode 105 are formed on the a−Si film in a state that the channel portion is sandwiched between the source electrode 104 and the drain electrode 105. Here, an n+Si layer not shown in the drawing is formed between the a−Si film and the source electrode 104 and between the a−Si film and the drain electrode 105. The n+Si layer is provided for establishing an ohmic contact between the semiconductor layer and the source electrode 104 and between the semiconductor layer and the drain electrode 105.
A video signal line functions also as the source electrode 104, and the drain electrode 105 is connected to a pixel electrode 110. The source electrode 104 and the drain electrode 105 are formed on the same layer simultaneously. In this embodiment, the source electrode 104 or the drain electrode 105 is made of an MoCr alloy. When it is necessary to lower electric resistance of the source electrode 104 or the drain electrode 105, for example, the source electrode 104 or the drain electrode 105 adopts the electrode structure where an AlNd alloy layer is sandwiched between MoCr alloy layers.
An inorganic passivation film 106 made of SiN is formed so as to cover the TFTs. The inorganic passivation film 106 is provided for protecting, particularly, the channel portions of the TFTs from impurities. An organic passivation film 107 is formed on the inorganic passivation film 106. Besides a role of protecting the TFTs, the organic passivation film 107 also has a surface leveling function so that the organic passivation film 107 has a large thickness. That is, the thickness of the organic passivation film 107 is set to 1 μm to 4 μm.
The organic passivation film 107 is formed using a photosensitive acrylic resin, a silicon resin, a polyimide resin or the like as a material. It is necessary to form through holes in the organic passivation film 107 at positions where the pixel electrodes 110 and the drain electrodes 105 are connected with each other. Since the organic passivation film 107 has photosensitivity, it is possible to form the through holes by exposing and developing the organic passivation film 107 per se without using a photoresist.
A counter electrode 108 is formed on the organic passivation film 107. The counter electrode 108 is formed such that a transparent conductive film made of ITO (Indium Tin Oxide) is formed on the whole display region by sputtering. That is, the counter electrode 108 is formed in a planar shape. After forming the counter electrode 108 on the whole surface of the display region by sputtering, the counter electrode 108 is removed by etching only at through hole portions where the pixel electrode 110 and the drain electrode 105 are made conductive with each other.
An upper insulation film 109 made of SiN is formed so as to cover the counter electrode 108. After upper electrodes are formed, the through holes are formed in the upper electrodes by etching. Using the upper insulation film 109 as a photo resist, the through holes 111 are formed in the inorganic passivation film 106 by etching. Thereafter, an ITO film for forming the pixel electrodes 110 is formed by sputtering so as to cover the upper insulation film 109 and the through holes 111. By patterning the ITO film which is formed by sputtering, the pixel electrodes 110 are formed. The ITO film for forming the pixel electrodes 110 is also adhered to the through holes 111. The drain electrode 105 which extends from the TFT and the pixel electrode 110 are made conductive with each other via the through hole 111, and a video signal is supplied to the pixel electrode 110.
The pixel electrode 110 is formed of comb-teeth-shaped electrodes. A slit 112 is formed between comb-tooth-shaped electrodes. A reference voltage is applied to the counter electrode 108 and a voltage in response to a video signal is applied to the pixel electrode 110. When the voltage is applied to the pixel electrode 110, as shown in
In the display area shown in
A size of the pixel is small and hence, a width w of the comb-tooth-shaped electrode and a distance g between the comb-teeth-shaped electrodes are also small. For example, the width w of the comb-tooth-shaped electrode is approximately 4 μm, and the distance g between the comb-tooth-shaped electrodes is approximately 4 μm. The distance between the comb-tooth-shaped electrodes is small in this manner and hence, there arises a drawback that rubbing is not applied to a region defined between the comb-tooth-shaped electrodes sufficiently as described later.
In the examples shown in
In
An overcoat film 203 is formed so as to cover the color filters 201 and the black matrix 202. Surfaces of the color filters 201 and the black matrix 202 are uneven and hence, the overcoat film 203 is provided for surface leveling. The alignment film 113 which determines the initial alignment of liquid crystal is formed on the overcoat film 203. In
As shown in
As shown in
Rubbing is an operation where a cloth-like article rubs the alignment film 113 formed on the TFT substrate 100 or on the counter substrate 200. The fiber 150 which constitutes the cloth is shown in
As shown in
Accordingly, as shown in
When a voltage is applied to the pixel electrode 110 in an initial alignment state, liquid crystal molecules are twisted in the direction of an electric field, while when the application of the voltage to the pixel electrode 110 is stopped, liquid crystal molecules return to the initial alignment state. In such a case, when the alignment film 113 is not rubbed sufficiently, even after the application of voltage to the pixel electrode 110 is stopped, liquid crystal molecules hardly return to the initial alignment direction. Accordingly, an image displayed on a preceding frame remains and this phenomenon is observed as image retention. The generation of the image retention remarkably deteriorates image quality and hence, it is necessary to prevent the generation of the image retention.
As has been explained heretofore, the generation of the image retention is caused by the insufficient rubbing of the alignment film 113 disposed between the comb-tooth-shaped electrodes. This is because that the fiber 150 of the rubbing cloth cannot intrude into the gap defined between the comb-tooth-shaped electrodes. To allow the fiber 150 of the rubbing cloth to sufficiently intrude into the gap between the comb-tooth-shaped electrodes, it is considered effective to increase the distance between the comb-tooth-shaped electrodes or to decrease a film thickness of the comb-tooth-shaped electrode.
For ensuring predetermined resolution, it is necessary to decrease a size of the pixel and hence, the reduction of a size of the pixel electrode 110 is limited. Accordingly, it is difficult to increase the distance between the comb-teeth-shaped electrodes. On the other hand, the role of the pixel electrode 110 is to generate an electric field in a liquid crystal layer and hence, there arises no problem even when the resistance of the pixel electrode 110 is increased.
Accordingly, from a viewpoint of the electric resistance, there is no problem in decreasing a film thickness of the ITO film which forms the pixel electrode 110. Although the film thickness of the ITO film which forms the current pixel electrode 110 is approximately 77 μm, even when the film thickness of the ITO film is decreased to approximately 50 μm or less or is further decreased to approximately 30 μm or less or approximately 10 μm or less, it is possible to operate the liquid crystal display device without causing any problem.
However, with respect to the ITO film at the through hole portions 111 and the ITO film used for forming the terminal electrodes 400 of the terminal area 401 which are formed simultaneously with the formation of the ITO film at the pixel electrodes 110, it is necessary to maintain the film thickness of the ITO film at the through hole portions 111 and the film thickness of the ITO film used in the terminal electrodes 400 to approximately 77 μm. The through hole portion 111 has an uneven surface. Accordingly, to prevent the conduction failure attributed to a broken step in the through hole portion 111, it is necessary for the ITO film to ensure a predetermined thickness. Further, when the ITO film cannot ensure a predetermined film thickness at the terminal electrode 400 in the terminal area 401, the ITO film cannot maintain a role of a protective film.
In this manner, it is necessary to form the thin ITO film at the pixel electrode 110 and the thick ITO film at the through hole portions 111 and at the terminal area 401 while forming these ITO films simultaneously. The ITO film may be formed separately among the pixel electrode 110, the through hole portion 111 and the terminal area 401. In this case, however, the number of photolithography steps is increased and this becomes one of factors which push up a manufacturing cost.
By performing the process explained hereinafter, the present invention can form the thin ITO film at the pixel electrodes 110 and the thick ITO film at the through hole portions 111 and the terminal area 401 without increasing the number of photolithography steps. A process which can provide the above-mentioned structure is explained by the embodiments described hereinafter.
Further, in
In
In
In
In the present invention, a half exposure technique is used in exposing the photo resist 120. That is, in the exposure of the photo resist 120 at the through hole portions 111 and at the terminal areas 401, a stripe-shaped or dot-shaped pattern is formed on the mask so as to decrease an exposure amount to the photo resist 120 at these portions 111, 401 compared to the an exposure amount to the photo resist 120 at the pixel-electrode-110 portions. When an exposure amount is small, a photo reaction of the photo resist 120 does not progress sufficiently and hence, when the photo resist 120 is developed, a film thickness of a half exposure portion of the photo resist 120 becomes large.
Thereafter, etching-back (half ashing) is applied to the photo resist 120 using an oxygen asher with respect to the TFT substrate 1 in a state shown in
Thereafter, light etching is applied to the ITO film in a state shown in
By performing the light etching of the ITO film in
After forming the ITO films which differ in film thickness as shown in
In the explanation made heretofore, the explanation has been made with respect to the IPS liquid crystal display device where the counter electrode 108 is formed of a planar matted film and the comb-teeth-shaped pixel electrode 110 is arranged on the counter electrode 108 by way of upper electrode. However, the present invention is also applicable to a liquid crystal display device having the opposite constitution where the pixel electrode 110 is formed of a planar matted film and the comb-teeth-shaped counter electrode 108 is arranged on the pixel electrode 110 by way of an insulation film.
Further, different from the above-mentioned IPS electrode structure explained heretofore, the present invention is also applicable to the IPS electrode structure where the comb-teeth-shaped pixel electrode 110 is arranged on the comb-teeth-shaped counter electrode 108 by way of the insulation film.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-027309 | Feb 2009 | JP | national |
