Abstract:
A TFT substrate with reduced pixel defect rate is presented. The TFT substrate includes a pixel electrode, a negative line to apply a reverse voltage to the pixel electrode, and a recovery transistor including a drain electrode overlapping a part of the negative line with a insulating layer disposed between the negative line and the drain electrode. A contact hole is formed on the negative line and the drain electrode, and a bridge electrode connects the negative line and the drain electrode through the contact hole. 
     The thin film transistor substrate and a display apparatus presented herein protect a data line assembly metal layer and decrease pixel defect. An improved reverse voltage efficiency is applied to a pixel electrode to protect a drain electrode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a continuation application of U.S. patent application Ser. No. 11/403,172 filed on Apr. 11, 2006, now U.S. Pat. No. 7,470,950, which claims the benefit of Korean Patent Application No. 2005-0043059 filed on May 23, 2005 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a thin film transistor substrate and a display apparatus having the same, and more particularly to a thin film transistor substrate and a display apparatus using the thin film transistor substrate and having a pixel electrode on which a light emitting layer is formed. 
   2. Description of the Related Art 
   Today, OLED (organic light emitting diode) is a popular type of flat panel display that is especially appreciated for its low voltage-driving, light weight, slim shape, wide angular field, and quick response. OLED displays are generally classified into two categories depending on the driving method: a passive matrix and an active matrix. The passive matrix OLED is simple in its manufacturing process but has the disadvantage of dramatically increased power consumption with increases in the size of the display and the resolution. Due to this disadvantage, the use of passive matrix is somewhat limited to a small-sized display apparatuses. While the active matrix OLED is more useful for large displays and high resolution, its manufacturing process is more complicated. 
   A plurality of TFT transistors are provided on the OLED substrate to drive and an anode electrode forming a pixel and a cathode functioning as a reference voltage are formed on the TFT. When a voltage is applied to the two electrodes, an exciton is formed by combination of a hole and an electron. The exciton emits light in a light emitting layer interposed between the two electrodes. The OLED displays images by adjusting this emitted light. 
   A plurality of TFTs are formed on the OLED substrate. A switching transistor connected to a data line and a driving transistor connected to a voltage supply line form a pixel. A data line assembly metal layer is deposited and formed into a source electrode and a drain electrode. A plurality of contact holes are formed to connect wires when a plurality of TFTs are formed. However, these contact holes can be problematic when an etchant flows through the contact hole during processing and causes wire lifting. 
   Wire lifting poses a problem especially around a boundary area where a gate line assembly metal layer overlaps the data line assembly metal layer due to a height difference between the metal layers. A thin protective layer is formed at the boundary area. When the contact hole is formed on the thin protective layer, the wire lifting is largely induced by the etchant and it becomes difficult to apply a normal voltage to the pixel. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an aspect of the present invention to provide a thin film transistor substrate and a display apparatus having the same that protect a data line assembly metal layer and decrease pixel defect. 
   The thin film transistor substrate and a display apparatus having the same include an improved reverse voltage efficiency applied to a pixel electrode as a drain electrode is protected. 
   In one aspect, the invention is a TFT substrate including a pixel electrode; a negative line to apply a reverse voltage to the pixel electrode; a recovery transistor comprising a drain electrode overlapping a part of the negative line with an insulating layer disposed between the negative line and the drain electrode; a contact hole formed on the negative line and the drain electrode; and a bridge electrode connecting the negative line and the drain electrode through the contact hole. 
   The contact hole may not overlap a boundary area between the negative line and the drain electrode. 
   There may be multiple contact holes. 
   The area of a base of the contact hole disposed on the drain electrode may be approximately 50 μm 2 ˜100 μm 2 . The area of the base of the contact hole disposed on the negative line is approximately 40 μm 2 ˜70 μm 2 . 
   According to the embodiment of the present invention, the contact hole has an approximately rectangular base, a short side of the rectangle is parallel to a width direction of the negative line, and a length of the short side is approximately one-third of the width of the negative line. 
   The bridge electrode may be made of ITO, IZO, a-ITO or a-IZO. 
   The TFT substrate may further include a recovery line to apply a recovery-on voltage to the recovery transistor. 
   The TFT substrate may further include a gate line; a data line extending substantially perpendicularly to the gate line; and a switching transistor disposed near where the gate line and the data line overlap. The switching transistor applies a data voltage. 
   The negative line may be provided on the same layer as the gate line. 
   The TFT substrate may also include a voltage supply line applying a driving voltage to the pixel electrode and a driving transistor applying a voltage that is about equal to the difference between the data voltage and the driving voltage to the pixel electrode. 
   The voltage supply line may be provided parallel to the data line, and the data line and the voltage supply line may be alternately provided between the adjacent pixel electrodes. 
   The data line may be provided in pairs between the adjacent two pixel electrodes. 
   The voltage supply line may be provided on the same layer as the data line. 
   In another aspect, the invention is a display apparatus including a pixel electrode; a negative line to apply a reverse voltage to the pixel electrode; a recovery transistor comprising a drain electrode overlapping a part of the negative line; a contact hole formed on the negative line and the drain electrode; a bridge electrode connecting the negative line and the drain electrode through the contact hole; and a light emitting layer provided on the pixel electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and/or other aspects and advantages of the present invention will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which: 
       FIG. 1  is a plan view of a TFT substrate according to a first embodiment of the present invention; 
       FIG. 2  is a sectional view of  FIG. 1 , taken along line II-II; 
       FIGS. 3   a  through  3   d  are views describing a manufacturing method of the TFT substrate according to the first embodiment; 
       FIG. 4  is an equivalent circuit diagram of a pixel according to the first embodiment of the present invention; 
       FIG. 5  is a sectional view of a display apparatus according to the first embodiment of the present invention; and 
       FIG. 6  is a sectional view of a TFT substrate according to a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Reference will now be made in detail to exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings wherein like reference numerals refer to like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
   A first embodiment according to the present invention will be described with reference to  FIGS. 1 through 5 . 
     FIG. 1  is a plan view of a TFT substrate according to a first embodiment of the present invention. A TFT substrate used for an OLED (organic light emitting diode) will be explained as an example in the embodiment but may be used for any other display apparatus. 
   As shown in  FIG. 1 , the TFT substrate includes a gate line assembly metal layer  110 ,  120 ,  130 ,  140 ,  310 ,  410 ,  510 ; a data line assembly metal layer  210 ,  220 ,  320 ,  420 ,  520 ; a switching transistor  300 ; a driving transistor  400 ; a recovery transistor  500 ; and a pixel electrode  600 . The gate line assembly metal layer (which is used to form a gate line  110 , a recovery line  120 , a negative  130 , a driving voltage applying line and gate electrodes  140 ,  310 ,  410 ,  510 ); the data line assembly metal layer (which is used to form a voltage supply line  210 , data line  220 , and drain electrodes  320 ,  420 ,  520 ); and a transparent electrode substance layer forming the pixel electrode  600  and bridge electrodes  235 ,  355 ,  555 ,  565  are provided as different layers. 
   The gate line assembly metal layer includes the gate line  110  extending in a first direction; the recovery line  120  formed parallel to the gate line  110  and carrying a recovery-on voltage; the negative line  130  to apply a reverse voltage to the pixel electrode  600 ; the driving voltage applying line  140  to apply a driving voltage to a voltage supply line  210 ; and gate electrodes  310 ,  410 ,  510  of transistors  300 ,  400 ,  500 , respectively. 
   The data line assembly metal layer includes the voltage supply line  210  that extends substantially perpendicular to the gate line  110 ; the data line  220  to which a data voltage is applied; drain electrodes  320 ,  420 ,  520  of transistors  300 ,  400 ,  500 , respectively; and source electrodes  330 ,  430 ,  530 . 
   The voltage supply line  210  and the data line  220  are provided parallel to each other and extend substantially perpendicular to the gate line  110 , thereby forming a pixel area in a matrix configuration. The pixel electrode  600  is formed in each pixel area and the voltage supply line  210  and the data line  220  are alternately arranged between the two pixel electrodes  600 . That is, the voltage supply line  210  and the data line  220  are disposed between the adjacent pixel electrodes  600  in a second direction that is substantially perpendicular to the first direction. 
   Generally, one pixel electrode  600  is connected to each one of the voltage supply line  210  and the data line  220 . However, in the embodiment shown, two pixel electrodes  600  share one voltage supply line  210 . Namely, the two adjacent pixel electrodes  600  arranged on the voltage supply line  210  receive the driving voltage from the same voltage supply line  210 . 
   This shared-voltage-supply line configuration simplifies the manufacturing process by decreasing the number of a lines and voltage-application points. Hence, an electro magnetic interference is improved. Furthermore, since an area of the pixel electrode  600  increases as the number of the line decreases, the aperture ratio is improved. 
   The driving voltage applying line  140  and the voltage supply line  210  are electrically connected through a first bridge electrode  235  connecting a contacting hole  230   a ,  230   b  exposing the driving voltage applying line  140  and a contacting hole  230   c  exposing the voltage supply line  210 . The first bridge electrode  235  and a second through a fourth bridge electrodes  355 ,  555 ,  565 , which are described below, are usually made of a transparent conductive substance such as ITO (indium tin oxide), IZO (indium zinc oxide), a-ITO (amorphous-indium tin oxide), a-IZO (amorphous-indium zinc oxide) and the like. 
   The data lines  220   a ,  220   b  (collectively referred to as “data line  220 ”) are arranged in a pair between the pixel electrodes  600 . Each of the two data lines  220   a ,  220   b  are indirectly connected to the adjacent pixel electrode  600  and apply the data voltage to the pixel electrode  600 . The data lines  220   a ,  220   b  are not limited to being arranged like in the embodiment. For example, they may be arranged between the adjacent pixel electrodes  600  as one data line. However, when planning the layout of the data line  220 , it should be taken into consideration that the switching transistor  300  connected to the data line  220   a ,  220   b  and the driving transistor  400  connected to the voltage supply line  210  are electrically connected. 
   The transistors  300 ,  400 ,  500  will be described with reference to  FIGS. 2 and 3   a  through  3   d .  FIG. 2  is a sectional view of  FIG. 1  taken along line II-II and shows the recovery transistor  500  connected to other electrodes through four contacting holes  560   a ,  560   b ,  550   a ,  550   b  and the bridge electrode  555 ,  565 . The recovery transistor  500 , the switching transistor  300 , and the driving transistor  400  have a similar basic configuration except for the contact hole  550   a ,  560   b.    
   The gate line assembly metal layer  130 ,  410 ,  510  is formed on the substrate substance  10 . The gate line assembly  130 ,  410 ,  510  may be made of a metal in a single-layer or in a multi-layer. The gate line assembly  130 ,  410 ,  510  includes the gate electrode  410  of the driving transistor  400  branching from the gate line  110 , the gate electrode  510  of the recovery transistor  500  forming a part of the recovery line  120 , and the negative line  130 . 
   A gate insulating layer  20  made of SiNx (silicon nitride) and the like covers the gate line assembly metal layer  130 ,  410 ,  510 . 
   A semiconductor layer  540  made of a semiconductor such as amorphous silicon and the like is formed on the gate insulating layer  20  of the gate electrode  510  of the recovery transistor  500 . On the semiconductor layer  540  are formed an ohmic contact layer  542  made of n+ hydrogenated amorphous silicon highly-doped with silicide or n-type dopant. 
   The data line assembly metal layer, which includes a drain electrode  520  and a source electrode  530 , is formed on the ohmic contact layer  542  and the gate insulating layer  30 . The data line assembly metal layer may also be made of a metal in a single-layer or in a multi-layer structure. The data line assembly metal layer includes the source electrode  530  connected to the gate electrode  410  of the driving transistor  400  and the drain electrode  520  separated from the source electrode  530  and formed on the ohmic contact layer  542  on the recovery line  120 . The drain electrode  520  overlaps the negative line  130 . In brief, the drain electrode  520  of the recovery transistor  500  overlaps the negative line  130 , and the source electrode  530  of the recovery transistor  500  overlaps the gate electrode  410  of the driving transistor  400 . 
   A protective layer  30  made of silicon nitride, an a—Si:C:O layer or an a—Si:O:F layer deposited by the PECVD method, an acryl organic insulating layer, etc. is formed on the data line assembly metal layer (which includes the drain and source electrodes  520 ,  530 ). The semiconductor layer  540  is not covered by the protective layer  30 . On the protective layer  30  are formed the gate electrode  410  of the driving transistor  400 , the source electrode  530 , the drain electrode  520 , and the contact hole  550   a ,  550   b ,  560   a ,  560   b  exposing the negative line  130 . 
   The third bridge electrode  555  and the fourth bridge electrode  565  are formed on the protective layer  30 . The third bridge electrode  555  and the fourth bridge electrode  565  are made of transparent conductive substance ITO, IZO, a-ITO, a-IZO, etc. 
   Conventionally, only one contact hole that passes through the drain electrode  520  of the recovery transistor  500  and the negative line  130  is formed. Likewise, when the contact hole is formed on a boundary area  522  where the data line assembly metal layer like the drain electrode  520  and the gate line assembly metal layer like the negative line  130  overlap, the protective layer is made thinner than in other places due to a height difference between the two layers in the area. 
   In a conventional OLED display, an etchant, used when the bridge electrode is formed, flows into the boundary area  522  and causes the bridge electrode (e.g.,  555 ) to be lifted. However, in this embodiment, the lifting is avoided because there is no contact hole in the boundary area  522 , and the protective layer  30  is formed on the boundary area  522  where the drain electrode  520  and the negative line  130  overlap. Each of the contact holes  550   a ,  550   b  has enough area for the reverse voltage applied from the negative line  130  to be transmitted to the recovery transistor  500 . The area of the base of the contact hole  550   a  disposed on the negative line  130  is preferably about 40 μm 2 ˜70 μm 2  and an area of the base of the contact hole  550   b  disposed on the drain electrode  520  is preferably about 50 μm 2 ˜100 μm 2 . The contact hole  550   a  has a nearly rectangular shape with two short sides d 2 , d 4  and two long sides d 1 , d 3 . The short sides d 2 , d 4  of the rectangle are formed parallel to a width of the negative line  130  measured in the second direction. The length of the short sides d 2 , d 4  is preferably about one-third of the width of the negative line  130 . When the width of the negative line  130  is about 12 μm and the contact hole  550   a  is formed in a rectangular shape on the drain electrode  520 , the long side d 1  may be 15 μm˜20 μm and the short side d 2  may be 4 μm. Furthermore, as for the contact hole  550   b  disposed on the negative line  130 , the long side d 3  is preferably 14 μm˜16 μm and the short side d 4  is preferably 4 μm. 
   Provided that the contact holes  550   a ,  550   b  have a predetermined area for the base and remain outside the boundary area  522 , they may be formed in any suitable shapes and numbers. 
     FIGS. 3   a  through  3   d  shows a manufacturing method of the TFT substrate, more particularly, of the recovery transistor. 
   First, the gate line assembly substance is deposited on the substrate  10  and patterned by a photolithography using a mask, to thereby form the gate line assembly metal layer that includes the gate electrode  410  of the driving transistor  400 , the gate electrode  510  of the recovery transistor  500 , and the negative line  130 . Then, the gate insulating layer  20  is deposited on the gate line assembly metal layer. As shown in  FIG. 3   b , the semiconductor layer  540  and the ohmic contact layer  542  are orderly deposited on the gate electrode  510  and patterned by photolithography using a mask, thereby forming the semiconductor layer  540  and the ohmic contact layer  542  as islands on the gate insulating layer  20  on the gate electrode  510 . 
   Next, as shown in  FIG. 3   c , after the data line assembly material is deposited, it is patterned by photolithography using a mask to form the data line assembly metal layer including the source electrode  530  across to the recovery line  120  and the drain electrode  520  separated from the source electrode  530 . Thereafter, the portion of the ohmic contact layer  542  not overlapped by the data line assembly layer is etched, thereby exposing the semiconductor layer  540 . Subsequently, as shown in  FIG. 3   d , the protective layer  30  is formed. The protective layer  30  is formed by the PECVD method using silicon source gas and nitrogen source gas. When the protective layer  30  is etched by an etching gas including hydrochloric acid, the contact hole  560   a  exposes at least a part of the gate electrode  410  of the driving transistor  400 , the contact hole  560   b  exposes at least a part of the source electrode  530 , the contact hole  550   a  exposes at least a part of the drain electrode  520 , and the contact hole  550   b  exposes at least a part of the negative line  130 . 
   Finally, the transparent conductive substance such as ITO, IZO, a-ITO, or a-IZO is deposited on the protective layer  30 . As shown in  FIG. 2 , the transparent conductive substance is patterned by photolithography using a mask to form the fourth bridge electrode  565  and the third bridge electrode  555 . The third bridge electrode  555  connects the drain electrode  520  to the negative line  130 . The fourth bridge electrode  565  connects the gate electrode  410  of the driving transistor  400  to the source electrode  530 . A Cr etchant or an IZO etchant may be used according to the transparent conductive substance when the bridge electrodes  555 ,  565  are formed. 
   The switching transistor  300  includes the gate electrode  310  forming a part of the gate line  110 , the drain electrode  320  branching from the voltage supply line  220   a ,  220   b , the source electrode  330  separated from the drain electrode  320 , and the semiconductor layer  340  formed between the drain electrode  320  and the source electrode  330 . 
   The driving transistor  400  includes the gate electrode  410  formed under the voltage supply line  210 , the drain electrode  420  formed in a part of the driving voltage line  210  and formed on the gate electrode  410 , the source electrode  430  separated from the drain electrode  420  and expanded to the pixel area, and the semiconductor layer  440  extending in the second direction between the drain electrode  420  and the source electrode  430  along the voltage supply line  210 . The contact hole  450  (see  FIG. 5 ) is formed in order to connect the pixel electrode  600  to the source electrode  430 . 
   The pixel electrode  600  applied with an image signal from the source electrode  430  is formed on the protective layer  30 . The pixel electrode  600  is physically and electrically connected to the source electrode  430  through the contact hole  450  and receives the image signal. 
   A partition is formed on the pixel electrode  600  in order to separate a radiating layer (not shown) from the pixel electrode  600 . The pixel electrode  600  is aligned with an anode electrode that receives the image signal. 
   The contact hole  610  is for storage capacitor (Cst) and is electrically connected to the gate electrode  410  of the driving transistor  400  with the pixel electrode  600 . The storage capacitor (Cst) stores a voltage corresponding to the difference between the data voltage and the driving voltage and regularly maintains the stored voltage during a frame. 
     FIG. 4  is an equivalent circuit diagram of a pixel according to the first embodiment of the present invention. A signal transmitting process will be described with reference to  FIG. 4 . 
   First, a gate-on voltage applied to the gate line  110  is transmitted to the gate electrode  310  of the switching transistor  300 . In response to the gate-on voltage, the data voltage applied from the data line  220   a ,  220   b  flows out to the source electrode  330  through the drain electrode  320 . The source electrode  330  of the switching transistor  300  is electrically connected to the gate electrode  410  of the driving transistor  400  through the contact hole  350   a ,  350   b  and the second bridge electrode  355 . 
   The driving transistor  400  controls current of a light emitting layer (see  FIG. 5 ) with the data voltage applied to the gate electrode  410  and the driving voltage applied form the driving supply line. The current of the light emitting layer is proportional to the difference between the data voltage applied from the gate electrode  410  and the driving voltage applied from the drain electrode  420 . 
   The capacitor (Cst) stores the voltage corresponding to the difference between the data voltage and the driving voltage and regularly maintains the data voltage applied to the gate electrode  410  of the driving transistor  400  and the electric current applied to the pixel electrode  600  during a frame. 
   The recovery transistor  500  applies a reverse voltage to the driving transistor  400  so that a residual electric current is not accumulated in the driving transistor  400 . Since the drain electrode  420  of the driving transistor  400  is applied with the regular driving voltage, residual electric current may accumulate in the driving transistor  400 , preventing proper transmission of the image signal to the pixel electrode  600 . To prevent this problem, a reverse voltage is applied to the driving transistor  400 , thereby discharging the accumulated electric current. Namely, the electric current passing through the driving transistor  400  is applied with a reverse bias voltage. 
   The gate-on voltage is applied to the gate line  110 , and the recovery-on voltage is applied to the recovery line  120 . When the recovery transistor is turned on by the recovery-on voltage, the reverse voltage applied from the negative line  130  is transmitted to the gate electrode  410  of the driving transistor  400  through the drain electrode  520  and the source electrode  530 . 
     FIG. 5  is a sectional view of a display apparatus according to the first embodiment. The display apparatus includes the partition  40  formed on the TFT  400  and separating the pixel area, the light emitting layer  700  formed on the pixel electrode  600 , and the cathode electrode  800  formed on the surface of the substrate. The pixel electrode  600  is connected to the drain electrode of the TFT  400  through the contact hole  450  formed on the protective layer  30 . The pixel electrode  600  acts as an anode and provides holes (positive charge carriers) to the light emitting layer  700 . 
   The partition  40  made of the organic substance is formed between the pixel areas. The partition  40  prevents the pixel electrodes from being short-circuited and separates the pixel electrodes from one another. 
   The cathode electrode  800  is provided on the light emitting layer  700  and is usually made of an opaque substance such as aluminum, silver, etc. The cathode electrode  800  is made of a metal having a low work function and/or a transparent conductive substance. The metal with a low work function allows an electron to easily transfer into the light emitting layer  700 . Although light may exit in either/both directions of the substrate  10  depending on the desired application, light exits in the direction of the substrate  10  in the embodiment shown. 
   Although not shown in  FIG. 5 , OLED may further include a protective layer to protect the cathode electrode  800  and a paper bag to prevent moisture and air from infiltrating the light emitting layer  700 . 
     FIG. 6  is a sectional view of a TFT substrate according to a second embodiment of the present invention.  FIG. 6  has the same configuration as the above-described embodiments except for the TFT substrate and the contact hole in  FIG. 2 . Thus, the description of the configuration will not be repeated. 
   Three of contact holes  550   a   1 ,  550   a   2 ,  550   a   3  are formed on a drain electrode  520  of a recovery transistor  500  and two of contacts holes  550   b   1 ,  550   b   2  are formed on a negative line  130 . A third bridge electrode  556  is formed on the contact holes  550   a   1 ,  550   a   2 ,  550   a   3 ,  550   b   1 ,  550   b   2 . 
   As compared with  FIG. 2 , multiple contact holes are disposed on the drain electrode  520  and the negative line  130 . In this case, the contact holes  550   a   1 ,  550   a   2 ,  550   a   3  on the drain electrode  520  may have a rectangular base with the long sides being about 4 μm˜5 μm and the short sides being about 4 μm. The contact holes  550   b   1 ,  550   b   2  may have a rectangular base with the long sides being about 5 μm˜6 μm and the short sides being about 4 μm. However, these sizes are just examples and the contact holes may have any suitable size unless they overlap a boundary of the drain electrode  520 . 
   Although a few embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.