Patent ID: 12242158

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described in detail below, referring to embodiments.

First Embodiment

FIG.1is a plan view of a liquid crystal display device employing IPS technology used in the present invention, illustrating a pixel structure. There are various types of the IPS technology. In one type, a common electrode is formed in a plane, a comb-teeth pixel electrode is arranged on the common electrode with an insulating film arranged therebetween, and liquid crystal molecules are rotated by an electric field generated between the pixel electrode and the common electrode. This type can achieve a relatively large transmittance and is therefore currently the main stream.

InFIG.1, scanning lines10extend in a transverse direction and are arranged in a longitudinal direction with a predetermined pitch. The longitudinal pitch of the scanning lines10corresponds to the longitudinal size of a pixel. Video signal lines20extend in the longitudinal direction and are arranged in the transverse direction with a predetermined pitch. The transverse pitch of the video signal lines20corresponds to the transverse size of the pixel.

In the pixel, a stripe pixel electrode111extends in the longitudinal direction. InFIG.1, the pixel electrode has a form of a single stripe because the pixel pitch is as small as 30 μm or less. However, when the pixel pitch is large, a comb-teeth electrode having a slit is formed as the pixel electrode.

A video signal is supplied to the pixel electrode111from a corresponding video signal line20via a through hole and a TFT. InFIG.1, the video signal line and a semiconductor layer103are connected via the through hole120. The semiconductor layer103extends below the video signal line20, passes below the scanning line10, is then bent, passes below the scanning line10again, and is connected to a contact electrode107via a through hole140. The contact electrode107is connected to the pixel electrode via through holes130and131. In a portion where the semiconductor layer103passes below the scanning line10, the TFT is formed. In this case, the scanning line10also serves as a gate electrode. Therefore, inFIG.1, two TFTs are formed from the video signal line20to the pixel electrode11, that is, a so-called double-gate method is applied.

InFIG.1, a direction of an alignment axis115formed in an alignment film is at an angle of θ with respect to the extending direction of the pixel electrode111. The reason why the angle θ is formed is to define a direction of rotation of liquid crystal molecules when an electric field is applied to the pixel electrode111. θ is from about 5° to about 15°, preferably from 7° to 10°. Note that in some cases the direction of the alignment axis115is along the longitudinal direction inFIG.1and the extending direction of the pixel electrode111is inclined by θ.FIG.1shows a case where dielectric anisotropy of the liquid crystal molecules is positive. The angle of the alignment axis in the case where the dielectric anisotropy of the liquid crystal molecules is negative is an angle rotated from that inFIG.1by 90°.

InFIG.1, a common electrode is formed on an entire surface except for the through hole and its nearby portion. The common electrode109on the upper side of the scanning line and that on the lower side of the scanning line inFIG.1are connected via a common-electrode bridge1091. The existence of the common-electrode bridge1091is a problem in order to increase the resolution and reduce the pixel pitch.

FIG.2is a cross-sectional view, taken along line A-A inFIG.1. The TFT inFIG.2is a so-called top gate TFT using LTPS (Low Temperature Poli-Si) as a semiconductor material. Meanwhile, in the case of using a-Si as the semiconductor material, a so-called bottom gate TFT is frequently used. The following description is made referring to the case of using the top gate TFT as an example, but the present invention can be also applied to the case of using the bottom gate TFT.

InFIG.2, a first underlying film101of SiN and a second underlying film102of SiO2are formed on a glass substrate100by CVD (Chemical Vapor Deposition). The first underlying film101and the second underlying film102have a role of preventing impurities from the glass substrate100from contaminating the semiconductor layer103.

The semiconductor layer103is formed on the second underlying film102. This semiconductor layer103is obtained by depositing an a-Si film on the second underlying film102by CVD and then converting it to a poly-Si film by laser annealing. This poly-Si film is patterned by photolithography.

A gate insulating film104is formed on the semiconductor layer103. This gate insulating film104is an SiO2film deposited by using TEOS (Tetraethoxysilane). This film is also deposited by CVD. A gate electrode105is formed on the gate insulating film104. The gate electrode105is formed by the scanning line10. The gate electrode105is formed by a MoW film, for example. When the resistance of the gate electrode105or the scanning line10has to be reduced, Al alloy is used.

Then, an interlayer insulating film106is formed of SiO2or SiN to cover the gate electrode105. The interlayer insulating film106is formed for insulating the gate electrode105and the contact electrode107from each other. The semiconductor layer103is connected to the video signal line20via the through hole120formed in the gate insulating film104and the interlayer insulating film106. Also, a through hole140is formed in the interlayer insulating film106and the gate insulating film104for connecting a source portion S of the TFT to the contact electrode107. The through hole120and the through hole140in the interlayer insulating film106and the gate insulating film104are formed simultaneously.

The contact electrode107is formed on the interlayer insulating film106. The semiconductor layer103extends below the video signal line20and passes below the scanning line10, that is, the gate electrode105twice, as illustrated inFIGS.1and2. In the portion where the semiconductor layer103passes below the scanning line10, the TFTs are formed. In other words, the source portion S and a drain portion D of the TFT are formed to sandwich the gate electrode105therebetween, when seen in a plan view. The contact electrode107is connected to the semiconductor layer103via the through hole140formed in the interlayer insulating film106and the gate insulating film104.

The contact electrode107and the video signal line20are formed simultaneously in the same layer. For the contact electrode107and the video signal line20, AlSi alloy is used, for example, for achieving a small resistance. In this case, a structure sandwiching AlSi alloy between a barrier layer of MoW, for example, and a cap layer is employed, because AlSi alloy may cause hillocks or diffusion of Al to another layer.

An organic passivation film108is formed to cover the contact electrode107, the video signal line20, and the interlayer insulating film106. The organic passivation film108is formed of photosensitive acrylic resin. Other than the acrylic resin, silicone resin, epoxy resin, and polyimide resin can be used for the organic passivation film108, for example. The organic passivation film108serves as a flattening film and is therefore formed to be thick. The thickness of the organic passivation film108is from 1 μm to 4 μm. In most cases, the thickness of the organic passivation film108is from about 2 μm to about 3 μm.

The through hole130and the through hole131are respectively formed in the organic passivation film108and a capacity insulating film110described later for achieving electric continuity between the pixel electrode111and the contact electrode107. Photosensitive resin is used for the organic passivation film108. The photosensitive resin is applied and is then exposed with light. After the light exposure, only a portion exposed with light is dissolved by a specific developing agent. That is, the use of the photosensitive resin can eliminate formation of photoresist. After the through hole130is formed in the organic passivation film108, firing is performed at about 230° C., so that the organic passivation film108is completed.

Subsequently, a transparent conductive film forming the common electrode109, e.g., an ITO (Indium Tin Oxide) film, is formed by sputtering, and is patterned so that the ITO film is removed from the through hole130and a portion around the through hole130. The common electrode109can be formed in a plane to be common to the pixels. However, the common electrode109has to be formed in a portion other than the through hole130. Therefore, in the case of reducing the pixel pitch, the common-electrode bridge1091inFIG.1is a problem.

In a second embodiment of the present invention, a connection ITO40is formed to cover the through hole130simultaneously with the common electrode109, as illustrated inFIG.9. The reason for this is to ensure a margin for achieving contact between the contact electrode107and the pixel electrode. In this case, it is necessary to insulate the connection ITO40and the common electrode109from each other.

Returning toFIG.2, SiN forming the capacity insulating film110is deposited by CVD on the entire surface. Then, the through hole131is formed in the capacity insulating film110for achieving electric continuity between the contact electrode107and the pixel electrode111in the through hole130.

Then, an ITO film is formed by sputtering and is patterned to form the pixel electrode111. An example of the planar shape of the pixel electrode111is shown inFIG.1. An alignment film material is applied onto the pixel electrode111by flexography or ink jet printing, for example, and is fired, so that the alignment film112is formed. As an alignment process of the alignment film112, rubbing or optical alignment using polarized ultraviolet rays is used.

When a voltage is applied across the pixel electrode111and the common electrode109, lines of electric force are generated as illustrated inFIG.2. This electric field rotates liquid crystal molecules301, and controls the amount of light passing through a liquid crystal layer300on a pixel-by-pixel basis. In this manner, an image is formed.

InFIG.2, a counter substrate200is arranged with the liquid crystal layer300sandwiched between the counter substrate200and the TFT substrate. Inside the counter substrate200is formed a color filter201. As the color filter201, any one of red, green, and blue color filters is formed for every pixel, so that a color image is formed. A black matrix202is formed between the color filter201and the adjacent color filter201to improve a contrast of the image. The black matrix202also serves as a light shielding film of the TFT that prevents a photocurrent from flowing in the TFT.

An overcoat film203is formed to cover the color filter201and the black matrix202. Because the surface of the color filter201and the black matrix202is uneven, the overcoat film203smoothens the surface. An alignment film112is formed on the overcoat film203to define an initial alignment of the liquid crystal molecules. As the alignment process of the alignment film112, rubbing or optical alignment is used as in the alignment film112on the TFT substrate100side.

The above-described structure is merely an example. For example, an inorganic passivation film of SiN or the like is formed between the contact electrodes107and the video signal lines20in the TFT substrate100in some products.

FIG.3is an enlarged plan view of the through hole130and its nearby portion inFIG.1. The pixel electrode is omitted inFIG.3. InFIG.3, a region where the common electrode109is not formed is provided around the through hole130to have a rectangular hole shape. Thus, the common electrode109on the upper side of the through hole130and that on the lower side are connected by the common-electrode bridge1091. When the pixel pitch is reduced, the existence of the common-electrode bridge1091becomes a problem. More specifically, because ITO forming the common electrode109has a large resistivity, the width of the common-electrode bridge1091has to be larger than those of the video signal line20and the semiconductor layer103. Therefore, the existence of the common-electrode bridge1091is a problem, in particular, in the case of reducing the horizontal pixel pitch.

FIG.4is a plan view of the pixel in the case where the present embodiment is applied.FIG.4is different fromFIG.1in a method of connecting the upper common electrode109and the lower common electrode109inFIG.4. InFIG.4, the common electrodes109extend in the transverse direction in stripes on the upper and lower sides of the through hole130. A common metal wiring30extends above the common electrode109in the longitudinal direction to cover the video signal line20. The common metal wiring30is used for reducing the resistance of the common electrode109.

InFIG.4, the upper common electrode109and the lower common electrode109are electrically connected by the common metal wiring30. That is, the common metal wiring30serves as the bridge1091between the upper common electrode109and the lower common electrode109. The common metal wiring30is formed of a metal, for example, MoCr alloy, MoW alloy, or Al alloy, and therefore has a smaller resistance than ITO. Thus, the width of the wiring can be made smaller. In other words, the upper common electrode109and the lower common electrode109can be connected by the common metal wiring30having the smaller width. A larger feature inFIG.4is that the common metal wiring30as the bridge1091is arranged for every other video signal line20. This arrangement can further reduce the horizontal pixel pitch. The other structure inFIG.4is the same as that inFIG.1and therefore the description thereof is omitted.

FIG.5is a cross-sectional view, taken along line B-B inFIG.4, and illustrates a cross-section of a portion where the common metal wiring30serves as the bridge1091between the common electrodes109inFIG.4.FIG.5is different fromFIG.2in that the common metal wiring30extends on the organic passivation film108on the left side in a region covering the video signal line20, and is connected to the common electrode109. Other portions inFIG.5are the same as those inFIG.2and therefore the description thereof is omitted.

FIG.6is an enlarged plan view of the through hole130and its nearby portion inFIG.4. The pixel electrode is omitted inFIG.6. InFIG.6, the common electrode109on the upper side of the through hole130and that on the lower side of the through hole130are connected by the common metal wiring30. The common metal wiring30is formed to cover every other video signal line20. Consequently, the pixel pitch d2inFIG.6is smaller than the pixel pitch d1inFIG.3. That is, the structure inFIG.6can be applied to a higher-resolution screen.

FIG.7is a cross-sectional view illustrating another form of the present invention, corresponding to the cross-section taken along line B-B inFIG.4.FIG.7is different fromFIG.5in that the bridge1091connecting the common electrodes109in the region covering the video signal line20has a multilayer structure of the ITO film109forming the common electrode and the common metal wiring30. Due to the multilayer structure, the resistance of the bridge1091can be slightly reduced, as compared with the resistance in the case ofFIG.5. Further, due to the multilayer structure, tolerance for disconnection of the bridge1091can be increased. Note that patterning of the common electrode109is performed by photolithography and is not burden in the process.

In the present embodiment described above, the bridge1091connecting the common electrodes109is formed by the common metal wiring30, and the common metal wiring30is formed to correspond to every other video signal line20. However, in a product in which the resistance of the common electrode109is not a big problem, the bridge1091can be formed by ITO forming the common electrode109to correspond to every other video signal line20without using the common metal wiring30. Also in this case, reduction of the pixel pitch can be achieved by the existence of the region including no bridge1091.

Second Embodiment

FIG.8is a plan view of the pixel to which the present invention is applied, illustrating the through hole130and its nearby portion. The pixel electrode is omitted inFIG.8. The plan view of the entire pixel in the present embodiment is the same asFIG.1, and the cross-sectional view is the same asFIG.2.FIG.8is different fromFIG.3in a connection ITO40formed in the portion of the through hole130, and the diameter and location of the through hole131formed in the capacity insulating film110.

In order to form the through hole131in the capacity insulating film110only at the bottom of the through hole130formed in the organic passivation film108, it is necessary to make the diameter of the through hole130large, which is disadvantageous for reducing the pixel pitch. In the present embodiment, the connection ITO40formed simultaneously with the common electrode109is used, thereby offering freedom of designing the location and shape of the through hole131formed in the capacity insulating film110. Due to this, the diameter of the through hole130formed in the organic passivation film108can be reduced.

InFIG.8, the connection ITO40is formed to cover the through hole130. The connection ITO40is formed simultaneously with the common electrode109. Therefore, there is no process burden. However, the connection ITO40has to be insulated from the common electrode109, because the connection ITO40is connected to the pixel electrode. The capacity insulating film110of SiN is formed to cover the connection ITO40and the common electrode109, and the through hole131is formed in the capacity insulating film110. InFIG.8, the through hole131is formed not only at the bottom of the through hole130but also on the side face of the through hole130and a part of an upper surface of a portion around the through hole130. Therefore, even in the case where the through hole130is small, the through hole131can be easily formed.

FIG.9is a cross-sectional view, taken along line C-C inFIG.8. InFIG.9, the connection ITO40is formed to cover the through hole130in the organic passivation film108. The capacity insulating film110is formed to cover the connection ITO40, and the through hole131is formed in the capacity insulating film110. In this through hole131, the connection ITO40is exposed and is to be connected to the pixel electrode. As illustrated inFIG.9, the through hole131in the capacity insulating film110can be formed to be large in the present embodiment, even in the case where the through hole130formed in the organic passivation film108is small. Therefore, reliability of connection can be improved.

However, the connection ITO40has to be insulated from the common electrode109. Because the connection ITO40and the common electrode109are formed in the same layer, a gap g1between the connection ITO40and the common electrode109has to be sufficiently large when the bridge1091connecting the upper common electrode109and the lower common electrode109, illustrated inFIG.8, is formed by the same ITO film as that for the common electrode109. Thus, there is a limit on reduction of the pixel pitch.

In the present embodiment, connection between the upper common electrode109and the lower common electrode109is achieved by the common metal wiring30, as illustrated inFIG.10. The common metal wiring30is arranged to correspond to every other video signal line. On a side where no common metal wiring30is provided, insulation between the connection ITO40and the common electrode109or the common metal wiring30is not a problem. Therefore, inFIG.10, it is only necessary to pay attention to a gap g2on this side.

Meanwhile, on a side where the common metal wiring30is provided inFIG.10, the gap g1between the connection ITO40and the common metal wiring30has to be ensured. Therefore, a horizontal center position of the connection ITO40is shifted to the side where no common metal wiring30is provided from the center position of the pixel. Due to this shifting, the transverse diameter of the pixel can be reduced and therefore the pixel pitch can be reduced.

In other words, in the present embodiment, the diameter of the through hole130formed in the organic passivation film108can be reduced. Also, the pixel pitch can be further reduced by shifting the center of the connection ITO40from the center of the pixel, that is, the center between the video signal lines20.

As in the first embodiment, the bridge1091connecting the upper common electrode109and the lower common electrode109can have a multilayer structure of ITO forming the common electrode109and the common metal wiring30or a structure including only ITO forming the common electrode109.

Third Embodiment

FIG.11is a plan view of the pixel according to a third embodiment, illustrating the through hole130and its nearby portion. The basic structure and the cross-section of the pixel are similar to those illustrated inFIGS.1and2. A feature inFIG.11is that one of a red pixel, a green pixel, and a blue pixel has a larger horizontal diameter than those of others. The reason is to handle different requests for a white tone from customers, for example. InFIG.11, the diameter of the blue pixel is larger than those of other pixels. That is, B>R=G inFIG.11. However, the red pixel or the green pixel is larger in some cases.

InFIG.11, the common electrodes109extend in stripes in the horizontal direction on upper and lower sides of the through hole130. While the upper common electrode109and the lower common electrode109are connected by the common metal wiring30as the bridge1091, the common metal wiring30for the bridge1091is mainly provided only in the blue pixel having the larger width. The structure of the through hole130inFIG.11is the same as that described referring toFIGS.8to10. In the through holes130arranged on both sides of the common metal wiring30inFIG.11, the horizontal center of the connection ITO40is arranged in a direction away from the common metal wiring30. The reason for this is the same as that described in the second embodiment.

With the structure inFIG.11, the common metal wiring30for the bridge1091is formed in a portion corresponding to the pixel having the larger pixel width, the bridge1091is not formed in other portions, and the connection ITO40is formed, so that the diameter of the through hole130can be reduced. Therefore, the pixel pitch can be reduced.

Also, inFIG.11, the bridge1091formed in the portion corresponding to the wider pixel can have a multilayer structure of the common metal wiring30and ITO forming the common electrode109or a structure including only ITO forming the common electrode109, other than the structure including only the common metal wiring30, as in the first embodiment.

Although the bridge electrode is formed only in the portion corresponding to the pixel having the larger pixel width inFIG.11, the present embodiment is not limited thereto. Even in the case where the red, green, and blue pixels have the same pixel width, the common metal wiring30for bridge connection can be provided for every three video signal lines20. Also in this case, the center of the connection ITO40is formed to go away from the bridge1091, thereby further enhancing the effect of reduction of the pixel pitch.

In the present embodiment described above, the structure is described in the case where the connection ITO40is formed in the portion of the through hole130. However, according to the present embodiment, the pixel pitch can be reduced as a whole by forming the bridge1091mainly for the wider pixel, even in a structure including no connection ITO40.

Fourth Embodiment

It is necessary to define a gap between a TFT substrate and a counter substrate in a liquid crystal display device. The gap between the TFT substrate100and the counter substrate200is generally defined by a columnar spacer.FIG.12illustrates an example in which the gap between the TFT substrate100and the counter substrate200is defined by the columnar spacer50in the present embodiment. The columnar spacer50formed in the counter substrate200defines the gap between the TFT substrate100and the counter substrate200. The columnar spacer50is formed in the counter substrate200simultaneously with the overcoat film203. Another difference betweenFIGS.12and2is that the common electrode109or the bridge1091does not exist in a portion of the TFT substrate100that is to come into contact with the columnar spacer50. The other structure inFIG.12is the same as that inFIG.2.

A tip of the columnar spacer50comes into contact with the alignment film112formed in the TFT substrate100. However, when the alignment film112is scraped by this contact, scraped chips cause bright spots. Such scraping can easily occur, especially in the case where a surface opposed to the columnar spacer50, with which the tip of the columnar spacer50is to come into contact, is uneven as illustrated inFIG.13.FIG.13illustrates a case where the tip of the columnar spacer50comes into contact with an end portion of the bridge1091. In a region including a step of the bridge1091, that is, a region A inFIG.13, the alignment film112can be easily scraped. This bridge1091is the ITO film formed simultaneously with the common electrode109in some cases and is the common metal wiring30in other cases.

FIG.14is a plan view of the through hole130and its nearby portion, illustrating a feature of the present embodiment. InFIG.14, the pixel electrode is omitted. InFIG.14, the columnar spacer50comes into contact with the TFT substrate100in a portion where the common metal wiring30as the bridge1091connecting the upper common electrode109and the lower common electrode109or the ITO film formed simultaneously with the common electrode109does not exist. With this structure, the step illustrated inFIG.13can be eliminated at the tip of the columnar spacer50and it is therefore possible to prevent the alignment film112from being scraped. The other structure inFIG.14is the same as that inFIG.6and therefore the description thereof is omitted.

The structure of the present embodiment can be applied to the structure of the second embodiment illustrated inFIG.10and the structure of the third embodiment illustrated inFIG.11, for example. In other words, the tip of the columnar spacer50can come into contact with a portion of the TFT substrate100above the video signal line20, in which the ITO film formed simultaneously with the common electrode109or the common metal wiring30is not formed.