Patent Publication Number: US-6665035-B2

Title: Method for assembling a multi-domain liquid crystal display device having field affecting electrode

Description:
This application is a Continuation of application Ser. No. 09/357,247 filed on Jul. 20, 1999 now U.S. Pat. No. 6,449,025. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display device (LCD), and more particularly to a multi-domain liquid crystal display device capable distorting electric field and shielding light. 
     2. Description of the Related Art 
     Recently, a LCD has been proposed where the liquid crystal is not aligned and the liquid crystal is driven by side electrodes insulated from pixel electrodes. FIG. 1 is a sectional view of pixel unit of the conventional LCDs. 
     Regarding conventional LCDs, a plurality of gate bus lines arranged in a first direction on a first substrate and a plurality of data bus lines arranged in a second direction on the first substrate divide the first substrate into a plurality of pixel regions. 
     A thin film transistor (TFT) applies image signal delivered from the data bus line to a pixel electrode  13  on a passivation layer  4 . The TFT is formed on each pixel region and comprises a gate electrode, a gate insulator, a semiconductor layer, a source electrode, and a drain electrode, etc. 
     Side electrode  15  is formed surrounding the pixel electrode  13  on the gate insulator, thereon passivation layer  4  is formed over the whole first substrate, and a part of pixel electrode  13  overlaps side electrode  15 . 
     Alternatively, pixel electrode  13  is formed on the gate insulator and passivation layer  4  is formed over the whole first substrate. Afterward, side electrode  15  is formed as overlapping a part of pixel electrode  13 . Moreover, it is possible to pattern the pixel electrode  13  by etching and dividing the pixel region. 
     On a second substrate, a common electrode  17  is formed and together with pixel electrode.  13  applies electric field to a liquid crystal layer. Side electrode  15  and open area (slit)  19  distort the electric field applied to the liquid crystal layer. Then, liquid crystal molecules are variously driven in a unit pixel. Thus, when voltage is applied to the LCD, dielectric energy due to the distorted electric field arranges the liquid crystal directors in a needed position. 
     In the LCDs, however, open area  19  in common electrode  17  or pixel electrode  13  is necessary, and the liquid crystal molecules could be driven stably when the open area is wider. If the electrodes do not have a slit or the width of the slit is narrow, the electric field distortion needed to divide the pixel region becomes weak. Then, when voltage over a threshold voltage, V th , is applied, the time needed to stabilize the liquid crystal directors increases. 
     In particular, the response time can be over 100 msec. At this time, disclination occurs from the area where the liquid crystal directors are parallel with a transmittance axis of the polarizer, which results in a decrease in brightness. Further, according to the surface state of LCDs, the liquid crystal texture has an irregular structure. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a LCD that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide a LCD having a wide viewing angle from a multi-domain effect. 
     Another object of the present invention is to provide a LCD having high brightness by stable arrangement of liquid crystal molecules. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, a multi-domain liquid crystal display device comprises first and second substrates facing each other; a liquid crystal layer between the first and second substrates; a plurality of gate bus lines arranged in a first direction on the first substrate and a plurality of data bus lines arranged in a second direction on the first substrate to define a pixel region; a thin film transistor positioned at a crossing area of the data bus line and the gate bus line and comprising a gate electrode, a semiconductor layer, and source/drain electrodes; a pixel electrode in the pixel region; a field-affecting electrode on the second substrate; a color filter layer on the field affecting electrode; and a common electrode on the color filter layer. 
     In another aspect, the multi-domain liquid crystal display of the present invention comprises first and second substrates facing each other; a liquid crystal layer between the first and second substrates; a plurality of gate bus lines arranged in a first direction on the first substrate and a plurality of data bus lines arranged in a second direction on the first substrate to define a pixel region; a thin film transistor positioned at a crossing area of the data bus line and the gate bus line and comprising a gate electrode, a semiconductor layer, and source/drain electrodes; a pixel electrode in the pixel region; a subsidiary electrode on a the second substrate; a color filter layer on the subsidiary electrode; a common electrode on the color filter layer; and an alignment layer on at least one of the first and second substrates. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of the specification, illustrates embodiments of the invention and together with description serve to explain the principles of the invention. 
     In the drawings: 
     FIG. 1 is sectional view of the liquid crystal display device in the related art; 
     FIGS. 2A to  2 E are plan views of the multi-domain liquid crystal display devices according to the embodiments of the present invention; 
     FIGS. 3A to  3 H are more plan views of the multi-domain liquid crystal display devices according to the embodiments of the present invention; 
     FIGS. 4A to  4 D are sectional views of the multi-domain liquid crystal display devices according to the first, second, third, and fourth embodiments of the present invention; 
     FIG. 5 is a plan view of the multi-domain liquid crystal display device according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the multi-domain liquid crystal display device of the present invention is explained in detail by accompanying the drawings. 
     FIGS. 2A to  2 E and FIGS. 3A to  3 H are plan views of the multi-domain liquid crystal display devices according to the embodiments of the present invention, and FIGS. 4A to  4 D are sectional views of the multi-domain liquid crystal display devices according to the first, second, third, and fourth embodiments of the present invention. 
     Referring to the figures, the multi-domain liquid crystal display device of the present invention comprises first and second substrates  31 ,  33 , a plurality of gate bus lines  1  arranged in a first direction on first substrate  31  and a plurality of data bus lines  3  arranged in a second direction on the first substrate, a thin film transistor (TFT), a passivation layer  37 , and a pixel electrode  13 . 
     Data bus lines  3  and gate bus lines  1  divide the first substrate into a plurality of pixel regions. The TFT comprises a gate electrode  11 , a gate insulator  35 , a semiconductor layer  5 , an ohmic contact layer  6 , and source/drain electrodes  7 ,  9 . Passivation layer  37  is formed on the whole first substrate  31 . Pixel electrode  13  is coupled to drain electrode  9  and overlapped the TFT and/or data bus lines  3 , and gate bus lines  1  on passivation layer  37 . 
     The present invention comprises a subsidiary or auxiliary electrode  27 . The subsidiary electrode  27  distorts electric field. The subsidiary electrode  27  may also shields light from leaking at gate bus lines  1 , data bus lines  3 , and the TFT. A color filter layer  23  is on the subsidiary electrode  27 , and a common electrode  17  is on the color filter layer  23 . A liquid crystal layer is between first and second substrates  31 ,  33 . 
     In the LCD, a passivation layer  37  may be formed to prevent a short between subsidiary electrode  27  and common electrode  17 . The passivation layer  37  also prevents a decrease in the reliability due to the reaction of color filter layer  23  and the liquid crystal layer. Moreover, referring to FIG. 4B, an overcoat layer may be formed between the color filter layer  23  and the common, electrode  17 . 
     Subsidiary electrode  27  is formed preferably from a material having good conductivity, such as Cr. A voltage supply such as an operational amplifier may be used to provide amplified signals to apply voltages. 
     For example, for a driving voltage of  5 V, pixel electrode  13  is applied with |0˜5|V, common electrode  17 , V com , is at 0V, and subsidiary electrode  27  is applied with |V com −α|V. Here, α is preferably in a range of 0&lt;α&lt;V op  (operating voltage), and more preferably in a range of 0&lt;α&lt;V th  (threshold voltage). 
     If α is a large value in the given range, the potential difference between common electrode  17  and subsidiary electrode  27  is large correspondingly and the distortion effect of the electric field formed in the LCD is increased. 
     As an another example, for a driving voltage of 10V, pixel electrode  13  is applied with 0˜10V, common electrode  17 , V com , is at 5V, and subsidiary electrode  27  is applied with |V com −α|V. Here, α is preferably in a range of |5−α|&gt;0. 
     If α is a large value in the given range, the potential difference between common electrode  17  and subsidiary electrode  27  is large correspondingly and the distortion effect of the electric field formed in the LCD is increased. 
     To manufacture the LCD, in each pixel region on the first substrate  31 , a TFT is formed comprising gate electrode  11 , gate insulator  35 , semiconductor layer  5 , ohmic contact layer  6  and source/drain electrodes  7 ,  9 . A plurality of gate bus lines  1  and a plurality of data bus lines  3  are formed to divide the first substrate  31  into a plurality of pixel regions. 
     Gate electrode  11  and gate bus lines  1  are formed from depositing by sputtering and patterning a metal such as Al, Mo, Cr, Ta or Al alloy. Alternatively, it is possible to form the gate electrode and gate bus line as a double layer, the double layer is formed from different materials. Gate insulator  35  is formed from depositing by PECVD and patterning an insulating material such as SiN x  or SiO x . 
     Semiconductor layer  5  and the ohmic contact layer  6  are formed from depositing by PECVD (Plasma Enhancement Chemical Vapor Deposition) and patterning amorphous silicon (a-Si) and doped amorphous silicon (n +  a-Si). Also, gate insulator  35 , amorphous silicon (a-Si), and doped amorphous silicon (n +  a-Si) are formed by PECVD and patterned. So, gate insulator  35 , semiconductor layer  5 , and ohmic contact layer  6  are formed. 
     Data bus line  3  and source/drain electrodes  7 ,  9  are formed by sputtering and patterning a metal such as Al, Mo, Cr, Ta, Al alloy, etc. Alternatively, it is possible to form the data bus line  3  and source/drain electrodes  7 ,  9  as a double layer, the double layer is formed from different materials. 
     Passivation layer  37  is formed with a material such as BCB (BenzoCycloButene), acrylic resin, polyimide-based material, SiN x  or SiO x . Pixel electrode  13  is formed by sputtering and patterning a metal such as ITO (indium tin oxide). A contact hole  39  is formed to connect the pixel electrode  13  to the drain electrode  9  and storage electrode (not shown in the figure) by opening and patterning a part of the passivation layer  37  on drain electrode  9 . 
     In the pixel electrode  13  or the passivation layer  37 , an electric field inducing window  51  like a slit or hole is formed to distort the electric field (refer to the FIGS. 4A and.  4 B) The electric field inducing window  51  in FIGS. 4C and 4D may extend to the substrate  31 , especially if the gate insulator  35  is performed by patterning shallowly or deeply. 
     Alternatively, the hole  51  (FIG. 4B) in the passivation layer  37  may be a recess that does not extend all the way to the gate insulator  35 , and the hole  51  (FIG. 4C) in the passivation layer  37  and the gate insulator  35  may be a recess that does not extend all the way to the substrate  31 . Moreover, source/drain electrodes  7 ,  9  are electrically connected to pixel electrode  13  through contact hole  39 . 
     On second substrate  33 , subsidiary electrode  27  is formed from depositing by sputtering and etching a metal. Color filter layer  23  is formed with R, G, B (Red, Green, Blue) elements repeatedly on each pixel. Subsidiary electrode  27  includes a metal having a good conductivity, and preferably Cr. 
     Overcoat layer  29  is formed on color filter layer  23 , preferably from resin. Common electrode  17  is formed from depositing ITO by sputtering and patterning. The liquid crystal layer is formed by injecting liquid crystal between first and second substrates  31 ,  33 . The liquid crystal layer comprises liquid crystal molecules having positive or negative dielectric anisotropy, and chiral dopants may be added thereto. 
     In pixel electrode  13 , a multi-domain effect can be obtained by forming electric field inducing window  51  like a slit or hole. Slit is made by etching pixel electrode  13 , and hole is made by etching a portion of the passivation layer or gate insulator and depositing the pixel electrode  13 . 
     Referring to FIGS. 2A to  2 E, a two-domain is obtained by dividing each pixel horizontally, vertically, and/or diagonally, respectively. Referring to FIGS. 3A to  3 H, a multi-domain is obtained by dividing each pixel into four domains such as “+”, “x”, or double “Y” shapes. Other shapes are contemplated by the present invention for achieving multi-domain. 
     It is also possible to form a slit in common electrode  17  on the second substrate  33 , such as shown in U.S. patent application Ser. No. 09/256,180, entitled “A MULTI-DOMAIN LIQUID CRYSTAL DISPLAY DEVICE” by Seong Moh SEO et al., and filed concurrently with this application, which is hereby incorporated by reference. 
     To apply voltage (V com ) to the subsidiary electrode  27 , Ag-dotting part is formed in each corner of driving area on first substrate  31 , electric field is applied with second substrate  33 , and the liquid crystal molecules are driven by the potential difference. A voltage (V com ) is applied to subsidiary electrode  27  by connecting the Ag-dotting part to the subsidiary electrode  27 , which is accomplished simultaneously by forming the subsidiary electrode  27 . 
     On at least one substrate, a compensation film  29  is formed with polymer. The compensation film  29  is a negative uniaxial film, which has one optical axis, and compensates the phase difference of the direction according to viewing-angle. Hence, it is possible to compensate effectively the right-left viewing-angle by widening the area without gray inversion, increasing contrast ratio in an inclined direction, and forming one pixel to multi-domain. 
     In the present multi-domain liquid crystal display device, it is possible to form a negative biaxial film as the compensation film, which has two optical axes and wider viewing-angle characteristics as compared with the negative uniaxial film. The compensation film  29  could be formed on both substrates or on one of them. 
     After forming the compensation film, polarizer (not shown in the figure) is formed on at least one substrate. At this time, the compensation film and polarizer are preferably composed as one. 
     FIG. 5 is a plan view of the multi-domain liquid crystal display device according to another embodiment of the present invention. 
     In the multi-domain LCD of the present invention, the aperture ratio is enhanced by an optimum structure design of a “n-line” thin film transistor (U.S. Pat. No. 5,694,185) so as to reduce power consumption, increase luminance, and lower reflection, thus improving contrast ratio. Aperture ratio is increased by forming the TFT above the gate line and providing a “n-line” TFT. The parasitic capacitor, occurring between the gate bus line  1  and the drain electrode  9 , can be reduced when a TFT-having the same channel length as the symmetrical TFT structure is manufactured due to effect of channel length extension. 
     Furthermore, in the present LCD, an alignment layer (not shown in the figure) is formed over the whole first and/or second substrates. The alignment layer includes a material such as polyamide or polyimide based materials, PVA (polyvinylalcohol), polyamic acid or SiO 2 . When rubbing is used to determine an alignment direction, it should be possible to apply any material suitable for the rubbing treatment. 
     Moreover, it is possible to form the alignment layer with a photosensitive material such as PVCN (polyvinylcinnamate), PSCN (polysiloxanecinnamate), and CelCN (cellulosecinnamate) based materials. Any material suitable for the photo-aligning treatment may be used. Irradiating light once on the alignment layer determines the alignment or pretilt direction and the pretilt angle. The light used in the photo-alignment is preferably a light in a range of ultraviolet light, and any of non-polarized light, linearly polarized light, and partially polarized light can be used. 
     In the rubbing or photo-alignment treatment, it is possible to apply one or both of the first and second substrates. Different aligning-treatment may be applied on each substrate. 
     From the aligning-treatment, a multi-domain LCD is formed with at least two domains and LC molecules of the LC layer are aligned differently one another on each domain. That is, the multi-domain is obtained by dividing each pixel into four domains such as in a “+” or “x” shape, or dividing each pixel horizontally, vertically, and/or diagonally, and differently alignment-treating or forming alignment directions on each domain and on each substrate. 
     It is possible to have at least one domain of the divided domains unaligned. It is also possible to have all domains unaligned. 
     It is possible to apply the LCD of the present invention to any alignment and mode including, for example, (1) a homogeneous alignment where liquid crystal molecules in the liquid crystal layer are aligned homogeneously to surfaces of the first and second substrates, (2) a homeotropic alignment where liquid crystal molecules in the liquid crystal layer are aligned homeotropically to surfaces of the first and second substrates, (3) a tilted alignment where liquid crystal molecules in the liquid crystal layer are aligned tiltedly to surfaces of the first and second substrates, (4) a twisted alignment where liquid crystal molecules in the liquid crystal layer are aligned twistedly to surfaces of the first and second substrates, and (5) a hybrid alignment where liquid crystal molecules in the liquid crystal layer are aligned homogeneously to surface of one substrate between the first and second substrates and are aligned homeotropically to surface of the other substrate. 
     Consequently, since the subsidiary electrode of the present invention is formed to distort electric field and to shield light on the upper substrate, it is possible to simplify the electrode patterning process and to ensure good aperture ratio. The distortion effect of electric field is increased by controlling the voltage of the subsidiary electrode, thus obtaining a wide-viewing angle. 
     Also, with aligning-treatment, a rapid response time and a stable LC structure can be obtained by the pretilt and anchoring energy produced. Moreover, the disclination is removed to improve brightness. 
     It will be apparent to those skilled in the art that various modifications can be made in the multi-domain liquid crystal display device of the present invention without departing from the sprit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.