Abstract:
According to one aspect of the present invention, the thin film transistor array substrate basically includes a gate line assembly based on an Ag alloy. The Ag alloy comprises Ag and at least one of alloy elements and the alloy elements each bearing a low melting point. The gate line assembly comprises a gate electrode and a gate line. A data line assembly crosses over the gate line assembly while being insulated from the gate line assembly. The data line assembly comprises a source electrode, a drain electrode and a data line. A semiconductor layer contacts the source electrode and the drain electrode. The semiconductor layer forms a thin film transistor together with the gate electrode, the source electrode and the drain electrode. A pixel electrode is connected to the drain electrode.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (a) Field of the Invention  
           [0002]    The present invention relates to a wiring line assembly and a method for fabricating the same and, more particularly, to a wiring line assembly used for a thin film transistor array substrate for a liquid crystal display.  
           [0003]    (b) Description of the Related Art  
           [0004]    Generally, wiring lines for a semiconductor device or a display device are used for signal transmission, and hence, it becomes important lines to keep such wiring away from signal delays as much as possible.  
           [0005]    Particularly, as a large-size high-resolution liquid crystal display develops, it becomes more important that the thin film transistor array substrate for such a liquid crystal display should involve low resistance wiring lines to minimize the signal delay. For instance, a low resistance metal such as Al or Al alloy may be used for the wiring purpose.  
           [0006]    However, the Al or Al alloy-based wiring lines bear a weak physical or chemical characteristic. The Al or Al alloy erodes easily at the contacting area, when contacting other conductive materials.  
         SUMMARY OF THE INVENTION  
         [0007]    It is an object of the present invention to provide a thin film transistor array substrate that has a wiring line assembly bearing low resistance and good adhesion characteristics.  
           [0008]    This and other objects may be achieved by a thin film transistor array substrate having a wiring line assembly with the following features.  
           [0009]    The wiring line assembly is formed with an Ag alloy. The Ag alloy comprises Ag and at least one of alloy elements. Each of the alloy elements bears a low melting point.  
           [0010]    Each of the alloy elements each bear a diffusion coefficient of 1.5E-12 cm 2 /sec or more. The alloy elements each bear a melting point of 1500K or less. The wiring line assembly is formed with an Ag alloy comprising Ag and at least one of alloy elements and the alloy elements each bearing a low melting point. The composition content ratio of the alloy elements to the Ag alloy is 20 at % or less. The alloy elements are selected from the group consisting of Li, Mg, Al, Sm, and Mn. The Ag alloy is used for reflection electrodes for a reflection type liquid crystal display.  
           [0011]    According to one aspect of the present invention, the thin film transistor array substrate basically includes a gate line assembly based on an Ag alloy. The Ag alloy comprises Ag and at least one of alloy elements and the alloy elements each bear a low melting point. The gate line assembly comprises a gate electrode and a gate line. A data line assembly crosses over the gate line assembly while being insulated from the gate line assembly. The data line assembly comprises a source electrode, a drain electrode and a data line. A semiconductor layer contacts the source electrode and the drain electrode. The semiconductor layer forms a thin film transistor together with the gate electrode, the source electrode and the drain electrode. A pixel electrode is connected to the drain electrode.  
           [0012]    Herein the data line assembly may be based on an Ag alloy. The Ag alloy for the data line assembly comprises Ag and at least one of alloy elements and the alloy elements each bear a low melting point. The alloy elements each bear a diffusion coefficient of 1.5E-12 cm 2 /sec or more. The alloy elements each bear a melting point of 1500K or less. The composition content ratio of the alloy elements to the Ag alloy is 20 at % or less. The alloy elements may be selected from the group consisting of Li, Mg, Al, Sm, and Mn.  
           [0013]    Herein, the thin film transistor array substrate further comprises an insulating substrate under the gate line assembly, a gate insulating layer covering the gate line assembly and being under the semiconductor layer, and a protective layer covering the data line assembly with a contact hole exposing the drain electrode. The drain electrode is placed on the semiconductor layer together with the source electrode and the pixel electrode is connected to the drain electrode through the contact hole. The semiconductor layer is formed of hydrogenated amorphous silicon.  
           [0014]    Herein, the thin film transistor array substrate further comprises an alloy element-oxide layer. The alloy element-oxide layer being interposed between the source electrode and the semiconductor layer and between the drain electrode and the semiconductor layer.  
           [0015]    And, the thin film transistor array substrate further comprises an insulating substrate under the semiconductor layer having a source region, a drain region and a channel region, a gate insulating layer covering the semiconductor layer and being under the gate line assembly, an inter-layered insulating layer covering the gate line assembly in which the inter-layered insulating layer and the gate insulating layer have contact holes exposing the source region and the drain region and the source and the drain electrodes are connected to the source and the drain regions through the contact holes, and a protective layer covering the data line assembly with a contact hole exposing the drain electrode in which the pixel electrode is connected to the drain electrode through the contact hole of the protective layer. The semiconductor layer is formed with poly-crystalline silicon.  
           [0016]    The thin film transistor array substrate further comprises an alloy element-oxide layer. The alloy element-oxide layer is interposed between the source electrode and the semiconductor layer and between the drain electrode and the semiconductor layer.  
           [0017]    In a method for fabricating the thin film transistor array substrate, a gate line assembly is formed on an insulating substrate with an Ag alloy. The Ag alloy comprises Ag and at least one of alloy elements and the alloy elements each bearing a low melting point. The gate line assembly comprises a gate electrode and a gate line. A gate insulating layer is formed on the substrate such that the gate insulating layer covers the gate line assembly. A semiconductor layer is formed on the gate insulating layer. A data line assembly is formed on the semiconductor layer. The data line assembly comprises a source electrode, a drain electrode and a data line. A protective layer is formed on the substrate such that the protective layer covers the data line assembly. A contact hole exposing the drain electrode is formed in the protective layer. A pixel electrode is formed on the protective layer such that the pixel electrode is connected to the drain electrode through the contact hole.  
           [0018]    Herein, The data line assembly is formed with an Ag alloy. The Ag alloy comprises Ag and at least one of alloy elements each bearing a low melting point. The gate line assembly is formed through depositing an Ag alloy layer onto the substrate through sputtering a target of the Ag alloy with an oxygen concentration of 5000 ppm or less. The Ag alloy layer is patterned through a photolithography. The protective layer is formed through heat treatment at 200° C. or more. The alloy element of the Ag alloy layer for the data line assembly reacts with a silicon oxide layer to form an alloy element-oxide layer during the process of heat treatment for forming the protective layer where the silicon oxide layer is naturally formed on the semiconductor layer.  
           [0019]    Also, in a method for fabricating the thin film transistor array substrate, a semiconductor layer is formed on an insulating substrate. A gate insulating layer is formed on the substrate such that the gate insulating layer covers the semiconductor layer. A gate line assembly is formed on the gate insulating layer with an Ag alloy. The Ag alloy comprises Ag and at least one of alloy elements and the alloy elements each bear a low melting point. The gate line assembly comprises a gate electrode and a gate line. A source and a drain region are formed through doping the semiconductor layer with impurities and while defining a channel region. An inter-layered insulating layer is formed on the substrate such that the inter-layered insulating layer covers the gate line assembly. Contact holes exposing the source and the drain regions are formed in the inter-layered insulating layer and the gate insulating layer. A data line assembly is formed on the inter-layered insulating layer. The data line assembly comprises a source electrode connected to the source region, a drain electrode connected to the drain region, and a data line. A protective layer is formed on the substrate such that the protective layer covers the data line assembly. A contact hole exposing the drain electrode is formed in the protective layer. A pixel electrode is formed on the protective layer such that the pixel electrode is connected to the drain electrode through the contact hole.  
           [0020]    Herein, the data line assembly may be formed with an Ag alloy. The Ag alloy for the data line assembly comprises Ag and at least one of alloy elements and the alloy elements each bear a low melting point. The gate line assembly is formed through depositing an Ag alloy layer onto the substrate through sputtering a target of the Ag alloy with an oxygen concentration of 5000 ppm or less. The Ag alloy layer is patterned through photolithography.  
           [0021]    The protective layer is formed through heat treatment at 200° C. or more. The alloy element of the Ag alloy layer for the data line assembly reacts with a silicon oxide layer to form an alloy element-oxide layer during the process of heat treatment for forming the protective layer where the silicon oxide layer is naturally formed on the semiconductor layer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or the similar components, wherein:  
         [0023]    [0023]FIG. 1 is a graph illustrating the adhesion test results for Ag, AgAl, AgMg and Cr;  
         [0024]    [0024]FIG. 2 is a graph illustrating the AES analysis results for a sample of an AgMg-based layer under deposition;  
         [0025]    [0025]FIG. 3 is a graph illustrating the AES analysis results for a sample of an AgMg-based layer where heat-treatment is made at 400° C. under a vacuum atmosphere;  
         [0026]    [0026]FIG. 4 is a graph illustrating the AES analysis results for a sample of an AgAl-based layer line under deposition;  
         [0027]    [0027]FIG. 5 is a graph illustrating the AES analysis results for a sample of an AgAl-based layer where heat-treatment is made at 400° C. under a vacuum atmosphere;  
         [0028]    [0028]FIGS. 6A and 6B illustrate the process of forming an oxide layer on the surface and the interface of an Ag alloy-based layer through heat treatment;  
         [0029]    [0029]FIG. 7 is a graph illustrating the available range of melting points and diffusion coefficients of the alloy content for Ag alloy;  
         [0030]    [0030]FIG. 8 is a graph illustrating the available range of melting points and enthalpies of the alloy content for Ag alloy;  
         [0031]    [0031]FIG. 9 is a plan view of a thin film transistor array substrate according to a first preferred embodiment of the present invention;  
         [0032]    [0032]FIG. 10 is a cross sectional view of the thin film transistor array substrate taken along the X-X′ line of FIG. 9;  
         [0033]    [0033]FIG. 11A illustrates the first step of fabricating the thin film transistor array substrate shown in FIG. 9;  
         [0034]    [0034]FIG. 11B is a cross sectional view of the thin film transistor array substrate taken along the XIb-XIb′ line of FIG. 11A;  
         [0035]    [0035]FIG. 12A illustrates the step of fabricating the thin film transistor array substrate shown in FIG. 9 following that illustrated in FIG. 11A;  
         [0036]    [0036]FIG. 12B is a cross sectional view of the thin film transistor array substrate taken along the XIIb-XIIb′ line of FIG. 12A;  
         [0037]    [0037]FIG. 13A illustrates the step of fabricating the thin film transistor array substrate shown in FIG. 9 following that illustrated in FIG. 12A;  
         [0038]    [0038]FIG. 13B is a cross sectional view of the thin film transistor array substrate taken along the XIIIb-XIIIb′ line of FIG. 13A;  
         [0039]    [0039]FIG. 14A illustrates the step of fabricating the thin film transistor array substrate shown in FIG. 9 following that illustrated in FIG. 13A;  
         [0040]    [0040]FIG. 14B is a cross sectional view of the thin film transistor array substrate taken along the XIVb-XIVb′ line of FIG. 14A;  
         [0041]    [0041]FIG. 15A illustrates the step of fabricating the thin film transistor array substrate shown in FIG. 9 following that illustrated in FIG. 14A;  
         [0042]    [0042]FIG. 15B is a cross sectional view of the thin film transistor array substrate taken along the XVb-XVb′ line of FIG. 15A;  
         [0043]    [0043]FIG. 16 is a plan view of a thin film transistor array substrate according to a second preferred embodiment of the present invention;  
         [0044]    [0044]FIG. 17 is a cross sectional view of the thin film transistor array substrate taken along the XVII-XVII′ line of FIG. 16;  
         [0045]    [0045]FIG. 18A illustrates the first step of fabricating the thin film transistor array substrate shown in FIG. 16;  
         [0046]    [0046]FIG. 18B is a cross sectional view of the thin film transistor array substrate taken along the XVIIIb-XVIIIb′ line of FIG. 18A;  
         [0047]    [0047]FIG. 19A illustrates the step of fabricating the thin film transistor array substrate shown in FIG. 16 following that illustrated in FIG. 18A;  
         [0048]    [0048]FIG. 19B is a cross sectional view of the thin film transistor array substrate taken along the XIXb-XIXb′ line of FIG. 19A;  
         [0049]    [0049]FIG. 20A illustrates the step of fabricating the thin film transistor array substrate shown in FIG. 16 following that illustrated in FIG. 19A;  
         [0050]    [0050]FIG. 20B is a cross sectional view of the thin film transistor array substrate taken along the XXb-XXb′ line of FIG. 20A;  
         [0051]    [0051]FIG. 21A illustrates the step of fabricating the thin film transistor array substrate shown in FIG. 16 following that illustrated in FIG. 20A;  
         [0052]    [0052]FIG. 21B is a cross sectional view of the thin film transistor array substrate taken along the XXIb-XXIb′ line of FIG. 21A;  
         [0053]    [0053]FIG. 22A illustrates the step of fabricating the thin film transistor array substrate shown in FIG. 16 following that illustrated in FIG. 21A;  
         [0054]    [0054]FIG. 22B is a cross sectional view of the thin film transistor array substrate taken along the XXIIb-XXIIb′ line of FIG. 22A;  
         [0055]    [0055]FIG. 23A illustrates the step of fabricating the thin film transistor array substrate shown in FIG. 16 following that illustrated in FIG. 22A;  
         [0056]    [0056]FIG. 23B is a cross sectional view of the thin film transistor array substrate taken along the XXIIIb-XXIIIb′ line of FIG. 23A; and  
         [0057]    [0057]FIG. 24 is a graph illustrating the on/off current characteristic of a thin film transistor for a thin film transistor array substrate according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0058]    Preferred embodiments of this invention will be explained with reference to the accompanying drawings.  
         [0059]    In a large-size high resolution liquid crystal display, it is essential to develop a low resistance metal-based wiring line assembly. For this reason, silver (Ag) bearing a lowest resistance among metal has been studied to use for the wiring line assembly.  
         [0060]    However, in order to use Ag for the wiring line assembly, suitable processing conditions should have been first developed, considering the material aspects of Ag.  
         [0061]    Pure silver exhibits poor adhesion characteristic with respect to glass. And its material characteristics is easily deteriorated under H 2 SO 4 , NaCl, or KOH. In order to solve such a problem, alloy elements may be added to Ag.  
         [0062]    [0062]FIG. 1 is a graph illustrating the adhesion test results with respect to Ag, AgAl, AgMg and Cr. A thin film having a thickness of 1 μm is deposited onto a glass substrate through sputtering, and scratched by a scratch tester to measure the adhesion.  
         [0063]    In measuring by the scratch tester, a sample is arranged to be inclined by a predetermined angle, and scratched with a diamond tip to measure acoustic emission thereof. When the sampled thin film is broken up, the waveform of the acoustic emission reaches a peak. At this time, the load functioning as a critical load represents the adhesive value between the thin film and the substrate.  
         [0064]    It can be known from the graph that the critical load of Ag is about 20N, and that of AgMg is 25N. Furthermore, AgAl and Cr exhibit no peak at the load of 35N or less. It turns out that under the application of Ag alloy to the wiring line assembly, the adhesion thereof with respect to the underlying substrate can be significantly enhanced.  
         [0065]    Table 1 indicates variation in the resistivity pursuant to the annealing of Ag, AgAl, and AgMg.  
         [0066]    AgMg exhibits a resistivity close to that of Ag. In the case of AgAl, the resistivity thereof is high because it contains much of the Al content and hence, can be sufficiently reduced through lowering the Al content.  
                                                         TABLE I                                   Temperature   Ag   AgMg (Mg: 1 at %)   AgAl (Al: 5 at %)                                        As-dep   1.8732   2.284   7.54125           200° C.   1.7136   2.08   7.356           300° C.   1.5624   1.9184   7.146           400° C.   1.666   1.9472   6.279           500° C.   1.666   1.8784   6.381           600° C.   1.666   1.7344   6.426           700° C.   11.3008   1.948   7.5                      
 
         [0067]    When an Ag alloy with alloy content such as Mg and Al at a suitable proportion is used for the wiring line assembly, the adhesion characteristic thereof with respect to the underlying substrate is improved while the resistivity thereof being close to Ag.  
         [0068]    [0068]FIGS. 2, 3,  4  and  5  illustrate the auger electron spectroscopy (AES) analysis data with respect to the layered structure of an Ag alloy-based layer, a silicon oxide layer, and a silicon layer after and before the annealing.  
         [0069]    [0069]FIG. 2 is a graph illustrating the AES analysis results for a sample of an AgMg-based layer under deposition. FIG. 3 is a graph illustrating the AES analysis results for a sample of an AgMg-based layer that is heat-treated at 400° C. under a vacuum atmosphere. FIG. 4 is a graph illustrating the AES analysis results for a sample of an AgAl-based layer under deposition. FIG. 5 is a graph illustrating the AES analysis results for a sample of an AgAl-based layer that is heat-treated at 400° C. under a vacuum atmosphere.  
         [0070]    In the graphs, the sputter etching time at the horizontal axis is established to be 8 min before the annealing, and 20 min after the annealing. This means that the measurement is made at the surface of the Ag alloy-based layer before the annealing, and to the interface between the Ag alloy-based layer and a lower layer thereof after the annealing.  
         [0071]    AgMg or AgAl is sputtered at 150° C. An oxide layer of MgO or Al 2 O 3  is formed on the surface of the Ag alloy-based layer directly after the sputtering.  
         [0072]    The Mg or Al element moves to the interface between the Ag alloy-based layer and the silicon oxide layer at 400° C. under the vacuum atmosphere, and reacts with oxygen there while forming MgO or Al 2 O 3 . Considering that the oxygen content appears to present on the surface of the Ag alloy-based layer even before the annealing, it turns out that the oxygen content is segregated onto the surface of the target layer only with the temperature of deposition.  
         [0073]    This result is due to the rapid diffusion speed as well as the surface segregation of the alloy content of Mg or Al. Particularly, Mg appears to be present only on the surface and the interface of the Ag alloy-based layer after the annealing. This means that the surface segregation effect of Mg is great.  
         [0074]    [0074]FIGS. 6A and 6B conceptually illustrate the theoretical background for the above data.  
         [0075]    When the AgMg-based layer shown in FIG. 6A is heat-treated under an oxygen atmosphere, as shown in FIG. 6B, the Mg element moves to the layer surface to thereby form an oxide layer of MgO.  
         [0076]    Furthermore, the Mg element also moves to the layer interface between the Ag alloy-based layer and the silicon oxide layer, and reacts with SiO 2  to thereby form MgO. This reaction may be expressed by the chemical formula 1. 
         2Mg+SiO 2  2MgO+Si,          H 298 =−143.7 (kal/mol)   (1) 
         [0077]    The alloy content is exhausted at the wiring line so that the wiring line bears lower resistivity close to that of pure Ag.  
         [0078]    As described above, an oxide layer of the alloy element is formed at the surface and the interface between the Ag alloy-based layer and the silicon oxide layer. The oxide layer enhances the adhesion between the Ag alloy-based layer and the neighboring layers or the substrate. Particularly, when the underlying layer is based on silicon, such an oxide layer lowers contact resistance between the Ag alloy-based layer and the silicon-based layer, and prevents diffusion of the Ag content of the Ag alloy-based layer to the silicon-based layer. That is, the Ag alloy-based layer can contact the silicon-based layer in a stable manner.  
         [0079]    Such a chemical reaction should be made at a low temperature that can be endured by glass. Furthermore, the alloy content of the Ag alloy-based layer should be exhausted so much as to sufficiently lower the resistivity thereof. In this respect, it becomes important to select alloy elements with a high diffusion speed.  
         [0080]    Considering that the diffusion speed of the alloy element is closely related to the melting point, the diffusion speed of the alloy element to the surface and the interface of the Ag alloy-based layer is computed., And the melting point thereof is estimated based on the computed diffusion speed. In this way, suitable alloy elements can be determined.  
         [0081]    Let&#39;s select an alloy element moving through an Ag-based thin film with a thickness of 0.3 μm for ten (10) minutes when the diffusion of the alloy element is made at 250° C.  
         [0082]    The characteristic diffusion length can be expressed by the equation 2.  
         {square root}( D×t )=0.3 μm   (2) 
         [0083]    where D indicates the diffusion coefficient, and t indicates the period of time.  
         [0084]    It results from the equation 2 that D=1.5E−12 cm 2 /sec. Thus, it is preferable to select an alloy element having such a diffusion coefficient value or more.  
         [0085]    Meanwhile, the particle interface diffusion controls the overall diffusion at the temperature lower than 0.5×Tm (Tm refers to the melting point of the alloy element). Thus, the equation 3 can be induced. 
           T&lt; 0.5 ×Tm    (3) 
         [0086]    where T indicates the current temperature, and Tm indicates the melting point of the alloy element.  
         [0087]    Furthermore, in the metallic material having a structure of face-centered cubic crystal(FCC), the diffusion coefficient thereof can be obtained by the equation 4.  
           D= 0.3×exp(−8.5 ×Tm/T )cm 2 /sec   (4) 
         [0088]    In consideration of such conditions, the available range of melting point and diffusion coefficient for suitable alloy elements can be determined.  
         [0089]    [0089]FIG. 7 graphically illustrates the equation 4 where T is replaced by 523K (250° C.). In the graph, the horizontal axis indicates the melting point Tm, and the vertical axis indicates the diffusion coefficient D.  
         [0090]    It can be estimated from the graph that the melting point Tm should be 1500K or less to obtain a diffusion coefficient of 1.5E-12 cm 2 /sec or more.  
         [0091]    [0091]FIG. 8 is a graph illustrating the available range of melting point and enthalpy for suitable alloy elements. In the graph, the alloy elements having a melting point lower than 1500K and an oxide formation energy greater than that of SiO 2  are exemplified.  
         [0092]    As the metallic material bearing a low melting point exhibits lower surface energy, surface segregation thereof is liable to occur in a large scale. In this connection, as shown in FIG. 8, Li, Mg, Al, Sm or Mn is turned out to be a suitable alloy element.  
         [0093]    The Ag alloy for the wiring line assembly may be based on a double-sourced alloy, a triple-sourced alloy, or a quadruple-sourced alloy selected from Ag, and Li, Mg, Al, Sm, or Mn.  
         [0094]    The composition ratio of the alloy content to the Ag alloy should be 20 at % or less in consideration of the resistivity of the Ag alloy. For instance, in the case of a quadruple-sourced alloy of Ag-a-b-c, it is preferable that the content ratio of a, b and c should be established to be 0.1 at % ≦a≦20 at %, 0.1 at % ≦b≦20 at %, 0.1 at % ≦c≦20 at %, and a+b+c≦20 at %.  
         [0095]    As described above, the Ag alloy with an alloy content of Li, Mg, Al, Sm or Mn may be used to form a low resistance wiring line assembly for a wide screen display device. In this case, the wiring line assembly can bear good contact characteristic with the neighboring layers while insuring the interface stability thereof.  
         [0096]    Furthermore, it is possible to apply such an Ag alloy-based wiring line assembly for use in a reflection type liquid crystal display.  
         [0097]    A thin film transistor array substrate with an Ag alloy-based wiring line assembly and a method for fabricating the same will be now explained with reference to the drawings.  
         [0098]    [0098]FIG. 9 is a plan view of a thin film transistor array substrate according to a first preferred embodiment of the present invention, and FIG. 10 is a cross sectional view of the thin film transistor array substrate taken along the X-X′ line of FIG. 9.  
         [0099]    A gate line assembly is formed on an insulating substrate  10  with a silver (Ag) alloy bearing a low resistance. The gate line assembly includes a gate line  22  proceeding in the horizontal direction, a gate pad  24  connected to end of the gate line  22  to receive gate signals from the outside and transmit them to the gate line  22 , and a gate electrode  26  for TFT connected to the gate line  22 .  
         [0100]    The Ag alloy for the gate line assembly is formed with a main content of Ag and an alloy content of metallic elements bearing a high diffusion coefficient and a low melting point. The alloy content may be selected from Li, Mg, Al, Sm, or Mn. It is preferable that the metallic elements for the alloy content bear a diffusion coefficient of 1.5E−12 cm 2 /sec or more, and a melting point of 1500K or less.  
         [0101]    One, two or three metallic elements may be used for the alloy content. That is, the Ag alloy for the gate line assembly may be a double-sourced alloy, a triple-sourced alloy, or a quadruple-sourced alloy. It is preferable that the composition ratio of the alloy content to the Ag alloy should be 20 at % or less.  
         [0102]    As the Ag alloy exhibits good adhesion characteristic with respect to glass, the gate line assembly tightly adheres to the substrate  10  without getting loose.  
         [0103]    A gate insulating layer  30  is formed on the substrate  10  with silicon nitride to cover the gate line assembly.  
         [0104]    An island-shaped semiconductor pattern  40  is formed on the gate insulating layer  30  over the gate electrode  24  with hydrogenated amorphous silicon. Ohmic contact patterns  55  and  56  are formed on the semiconductor pattern  40  with hydrogenated amorphous silicon where n-type impurities are doped at high concentration.  
         [0105]    A metallic oxide layer  510  is formed on the ohmic contact patterns  55  and  56 . The metallic oxide layer  510  may be formed of MgO, Al 2 O 3 , or Li 2 O depending upon the material for a data line assembly to be described later. In case AgMg or AgAl is used for the data line assembly, the metallic oxide layer  510  is formed with MgO, or Al 2 O 3 .  
         [0106]    The aforementioned data line assembly is formed on the metallic oxide layer  510  and the gate insulating layer  30  with an Ag alloy bearing a low resistance.  
         [0107]    As like in the case of the gate line assembly, the Ag alloy for the data line assembly is formed with a main content of Ag, and an alloy content of metallic elements bearing a high diffusion coefficient and a low melting point. The alloy content may be selected from Li, Mg, Al, Sm, or Mn.  
         [0108]    The data line assembly includes a data line  62  proceeding in the vertical direction while crossing over the gate line  22 , a source electrode  65  branched from the data line  62  while being extended over the metallic oxide layer  510  on the one-sided ohmic contact pattern  55 , a data pad  68  connected to the one-sided end of the data line  62  to receive picture signals from the outside, and a drain electrode  66  formed on the other-sided ohmic contact pattern  56  around the gate electrode  26  while being separated from the source electrode  65 .  
         [0109]    In this structure, the metallic oxide layer  510  is disposed between the Ag alloy-based layer for the data line assembly and the silicon-based layer for the ohmic contact patterns  55  and  56 . The metallic oxide layer  510  lowers the contact resistance between the two neighboring layers while enhancing the adhesion thereof, and prevents diffusion of the Ag content of the Ag alloy-based layer to the silicon-based layer.  
         [0110]    A protective layer  70  is formed on the data line assembly with silicon nitride.  
         [0111]    Contact holes  76  and  78  exposing the drain electrode  66  and the data pad  68  are formed at the protective layer  70 , and a contact hole  74  exposing the gate pad  24  together with the gate insulating layer  30  are also formed in the protective layer  70 .  
         [0112]    A pixel electrode  82  is formed on the protective layer  70  such that it is electrically connected to the drain electrode  66  through the contact hole  76 . Furthermore, a subsidiary gate and a subsidiary data pads  86  and  88  are formed on the protective layer  70  such that they are connected to the gate and the data pads  24  and  68  through the contact holes  74  and  78 . The pixel electrode  82 , and the subsidiary gate and data pads  86  and  88  are formed with indium tin oxide (ITO) or indium zinc oxide (IZO).  
         [0113]    A method for fabricating the thin film transistor array substrate will be now explained with reference to FIGS. 11A to  15 B.  
         [0114]    As shown in FIGS. 11A and 11B, an Ag alloy-based layer is deposited onto a substrate  10 , and patterned to thereby form a gate line assembly. The Ag alloy for the gate line assembly is formed with a main content of Ag, and an alloy content of metallic elements bearing a high diffusion coefficient and a low melting point.  
         [0115]    The gate line assembly based on the Ag alloy can bear a low resistance characteristic, and a good adhesion characteristic to the glass substrate.  
         [0116]    The temperature in fabricating a liquid crystal display is relatively low. This requires that the diffusion coefficient of the alloy element should be high. Therefore, the amount of oxygen in the Ag alloy-based layer should be reduced not to oxidize the highly oxidative alloy element. When the alloy element is oxidized in the Ag alloy-based layer, the diffusion speed of the alloy element is significantly reduced limiting the above-described effects. Therefore, in the process of depositing the Ag alloy-based layer for the gate line assembly, the oxygen in the Ag alloy target should be limited to 5000 ppm or less. Furthermore, the carbon and nitrogen should be also limited to 5000 ppm or less.  
         [0117]    A metallic oxide layer (not shown) may be formed on the gate line assembly through heat treatment at 200° C. or more under an oxygen atmosphere. In case the gate line assembly is formed of AgMg, an oxide layer of MgO is formed at the interface thereof. In case the gate line assembly is formed of AgAl, an oxide layer of Al 2 O 3  is formed at the interface thereof. In these cases, the alloy elements are diffused to the surface and the interface of the gate line assembly through the heat treatment. This reduces the resistivity of the gate line assembly, and enhances its adhesion to the substrate.  
         [0118]    The gate line assembly may be naturally heat-treated at the subsequent process of depositing an insulating layer without requiring any separate process.  
         [0119]    Thereafter, as shown in FIGS. 12A and 12B, a silicon nitride-based gate insulating layer  30 , a hydrogenated amorphous silicon-based layer  40 , and an impurities-doped hydrogenated amorphous silicon-based layer  50  are sequentially deposited onto the substrate  10 . The hydrogenated amorphous silicon-based layer  40 , and the impurities-doped hydrogenated amorphous silicon-based layer  50  are patterned through photolithography to thereby form an island-shaped semiconductor pattern  40  and an island-shaped ohmic contact pattern  50  on the gate insulating layer  30  over the gate electrode  24 .  
         [0120]    As shown in FIGS. 13A and 13B, an Ag alloy-based layer is deposited onto the ohmic contact pattern  50  and the gate insulating layer  30 , and patterned through photolithography to thereby form a data line assembly. The Ag alloy for the data line assembly is formed with a main content of Ag, and an alloy content of metallic elements bearing a high diffusion coefficient and a low melting point.  
         [0121]    The data line assembly includes a data line  62  crossing over the gate line  22 , a source electrode  65  connected to the data line  62  while being extended over the gate electrode  26 , a data pad  68  connected to one-sided end of the data line  62 , and a drain electrode  66  separated from the source electrode  65  while facing the source electrode  65  around the gate electrode  26 .  
         [0122]    Thereafter, the ohmic contact pattern  50  exposed through the source and the drain electrodes  65  and  66  is etched such that it is divided into an ohmic contact pattern  55  contacting the source electrode  65 , and an ohmic contact pattern  56  contacting the drain electrode  66 .  
         [0123]    As shown in FIGS. 14A and 14B, a protective layer  70  is deposited onto the substrate  10  with an inorganic insulating material such as silicon nitride. It is preferable that the protective layer  70  should be deposited at the temperature range of 200° C. or more.  
         [0124]    In the process of forming the protective layer  70 , a metallic oxide layer  510  may be formed between the Ag alloy-based layer for the source and drain electrodes  65  and  66 , and the silicon-based layers for the ohmic contact patterns  55  and  56  and the semiconductor pattern  40 .  
         [0125]    The metallic oxide layer  510  is based on a silicon oxide layer that is naturally formed on the silicon-based layers before forming the Ag alloy-based layer for the data line assembly.  
         [0126]    The alloy elements of the Ag alloy-based layer are diffused to the interface while making the silicon oxide layer to be a dense-structured metallic oxide layer  510 . In case the data line assembly is based on AgMg, MgO is formed at the interface thereof. In case the data line assembly is based on AgAl, Al 2 O 3  is formed at the interface thereof.  
         [0127]    The metallic oxide layer  510  lowers the contact resistance between the data line assembly and the silicon-based patterns  40 ,  55  and  56  while enhancing the adhesion therebetween. This serves to obtain a stable interface characteristic of the data line assembly. Furthermore, the alloy element is exhausted from the data line assembly so that the material content for the data line assembly bears a pure Ag characteristic, which reduces the resistivity.  
         [0128]    In the usual case, surface of the silicon-based layers  40  and  50  is rinsed using HF before the formation of the data line assembly. However, even without the rinsing process using HF, the contact characteristic between the source and the drain electrodes  65  and  66  and the semiconductor pattern  40  as well as the ohmic contact patterns  55  and  56  can be improved by way of the interposed metallic oxide layer. Alternatively, in order to obtain such effects, a silicon oxide layer with 30 Å or less may be deposited onto the silicon-based layers through chemical vapor deposition (CVD).  
         [0129]    Thereafter, as shown in FIGS. 15A and 15B, the protective layer  70  is patterned together with the gate insulating layer  30  through photolithography to thereby form contact holes  74 ,  76  and  78  exposing the gate pad  24 , the drain electrode  66 , and the data pad  68 .  
         [0130]    An ITO or IZO-based layer is deposited onto the protective layer  70 , and patterned through photolithography to thereby form a pixel electrode  82  connected to the drain electrode  66  through the contact hole  76 , and a subsidiary gate and a subsidiary data pads  86  and  88  connected to the gate and the data pads  24  and  68  through the contact holes  74  and  78 .  
         [0131]    In the above-structured thin film transistor array substrate, the gate line assembly and the data line assembly are formed with an Ag alloy bearing a low resistance so that they can be well adapted for use in a wide-screened high definition liquid crystal display. Furthermore, they can reduce the contact resistance between the neighboring layers and can improve the adhesion therebetween.  
         [0132]    The above structure can be applied to a thin film transistor array substrate using poly-crystalline silicon thin film transistors.  
         [0133]    [0133]FIG. 16 is a plan view of a thin film transistor array substrate according to a second preferred embodiment of the present invention. FIG. 17 is a cross sectional view of the thin film transistor array substrate taken along the XVII-XVII′ line of FIG. 16.  
         [0134]    An island-shaped semiconductor pattern  121  is formed on an insulating substrate  100  with poly-crystalline silicon. The semiconductor pattern  121  is formed with a channel region  122 , a source and a drain regions  125  and  126  with high concentration impurities, and LDD regions with low concentration impurities placed between the channel and the source regions  122  and  125 , and between the channel and the drain regions  122  and  126 .  
         [0135]    The LDD regions  123  and  124  are resistors that minimize leakage of current at an OFF state of the thin film transistor with a poly-crystalline semiconductor channel and components.  
         [0136]    A gate insulating layer  130  is formed on the insulating substrate  100  while covering the semiconductor pattern  121 .  
         [0137]    A gate line assembly is formed on the gate insulating layer  130  with an Ag alloy. The Ag alloy for the gate line assembly is formed of a main content of Ag, and an alloy content of metallic materials bearing a high diffusion coefficient and a low melting point. The gate line assembly includes a gate line  141 , a gate pad  142  connected to end of the gate line  141  to receive gate signals from the outside and transmit them to the gate line  141 , and a gate electrode  143  connected to the gate line  141  corresponding to the channel region  122  of the semiconductor pattern  121 .  
         [0138]    As described above, if the gate line assembly is formed of an Ag alloy, it can bear a low resistance characteristic, and a stable interface characteristic.  
         [0139]    An inter-layer insulating layer  150  based on silicon nitride covers the gate line assembly.  
         [0140]    The inter-layer insulating layer  150  and the gate insulating layer  130  are provided with a contact hole  161  exposing the source region  125 , and a contact hole  162  exposing the drain region  126 .  
         [0141]    A data line assembly is formed on the inter-layer insulating layer  150  with an Ag alloy. The Ag alloy for the data line assembly is formed with a main content of Ag, and an alloy content of metallic materials bearing a high diffusion coefficient and a low melting point. The data line assembly includes a data line  171  crossing over the gate line  141 , a data pad  172  extended from end of the data line  171 , a source electrode  173  protruded from the data line  171  while contacting the source region  125  of the semiconductor pattern  121 , and a drain electrode  174  contacting the drain region  126  of the semiconductor pattern  121  while facing the source electrode  173 .  
         [0142]    A metallic oxide layer  510  is formed between the source electrode  173  and the source region  125  of the semiconductor pattern  121 , and between the drain electrode  174  and the drain region  126  of the semiconductor pattern  121 .  
         [0143]    The metallic oxide layer  510  may be formed of MgO, Al 2 O 3 , or Li 2 O depending upon the material for the data line assembly. If the data line assembly is based on AgMg or AgAl, a metallic oxide layer would be MgO or Al 2 O 3 .  
         [0144]    The metallic oxide layer  510  is formed between the Ag alloy-based layer for the source and the drain electrodes  173  and  174  and the silicon-based layer for the semiconductor pattern  121  to lower the contact resistance between the two layers while enhancing the adhesion therebetween. Furthermore, the metallic oxide layer  510  provides a cleaner interface that prevents the Ag element of the Ag alloy-based layer from diffusing into the silicon-based layer.  
         [0145]    A protective layer  180  is formed of silicon nitride to cover the data line assembly.  
         [0146]    The protective layer  180  is provided with contact holes  191  and  192  exposing the drain electrode  174  and the data pad  172 , and a contact hole  193  exposing the gate pad  142  together with the inter-layer insulating layer  150 .  
         [0147]    A pixel electrode  201  is formed on the protective layer  180  such that is electrically connected to the drain electrode  174  through the contact hole  191 . Furthermore, a subsidiary data and subsidiary gate pads  202  and  203  are formed on the protective layer  180  such that they are connected to the data and gate pads  172  and  142  through the contact holes  192  and  193 . The pixel electrode  201 , and the subsidiary data and gate pads  202  and  203  are formed of ITO or IZO.  
         [0148]    A method for fabricating the thin film transistor array substrate will be now explained with reference to FIGS. 18A through 22B.  
         [0149]    As shown in FIGS. 18A and 18B, a poly-crystalline silicon-based layer is formed on an insulating substrate  100 , and patterned through photolithography to form an island-shaped semiconductor pattern  121 .  
         [0150]    The poly-crystalline silicon layer may be formed through depositing an amorphous silicon-based layer onto the substrate, and crystallizing the amorphous silicon by way of a silicon crystallization technique such as laser annealing, or high temperature solidification.  
         [0151]    Thereafter, as shown in FIGS. 19A and 19B, a gate insulating layer  130  is formed on the substrate  100  such that it covers the semiconductor pattern  121 . An Ag alloy-based layer is deposited onto the gate insulating layer  130 , and patterned to thereby form a gate line assembly. The Ag alloy for the gate line assembly is formed with a main content of Ag, and an alloy content of metallic elements having a high diffusion coefficient and a low melting point. The gate line assembly includes a gate line  141 , a gate pad  142 , and a gate electrode  143 .  
         [0152]    As shown in FIGS. 20A and 20B, a source and a drain regions  125  and  126  with high concentration impurities, and LDD regions  123  and  124  with low concentration impurities are formed at the semiconductor pattern  121 . The portion of the semiconductor pattern  121  overlapped by the gate electrode  143  and not doped with impurities becomes a channel region  122 .  
         [0153]    In order to form such impurity-doped regions  123 ,  124 ,  125  and  126  in the semiconductor pattern  121 , the semiconductor pattern  121  except for the channel portion is doped with n-type impurities at low concentration while using the gate electrode  143  as a mask. Then, covering with a doping mask the gate electrode  143 , the channel region  122  and the LDD regions  123  and  124  of the semiconductor pattern  121  are doped with n-type impurities at high concentration through the doping mask to dope the periphery of the semiconductor pattern  121  not blocked by the doping mask at high concentration.  
         [0154]    At this time, the periphery of the semiconductor pattern  121  doped at low concentration and re-doped at high concentration becomes to be the source and the drain regions  125  and  126  with high concentration impurities., The portions of the semiconductor pattern  121  doped at low concentration becomes the LDD regions  123  and  124  with low concentration impurities, and the portions of the semiconductor pattern  121  that is not doped becomes the channel region  122 .  
         [0155]    The order of low concentration doping process and the high concentration doping process may be reversed.  
         [0156]    Thereafter, as shown in FIGS. 21A and 21B, an inter-layered insulating layer  150  is formed on the entire surface of the substrate  100  while covering the gate line assembly. The inter-layered insulating layer  150  and the gate insulating layer  130  are patterned through photolithography to thereby form a contact hole  161  exposing the source region  125 , and a contact hole  162  exposing the drain region  126 .  
         [0157]    An Ag alloy-based layer is then deposited onto the inter-layered insulating layer  150 , and patterned to thereby form a data line assembly. The Ag alloy for the data line assembly is formed with a main content of Ag, and an alloy content of metallic elements having a high diffusion coefficient and a low melting point. The data line assembly includes a data line  171  crossing over the gate line  141 , a source electrode  173  connected to the data line  171  while contacting the source region  125  through the contact hole  161 , a data pad  172  connected to one end of the data line  141 , and a drain electrode  174  separated from the source electrode  173  while contacting the drain region  126  through the contact hole  162 .  
         [0158]    Thereafter, as shown in FIGS. 22A and 22B, a protective layer  180  is deposited onto the substrate  100  with an inorganic insulating material such as silicon nitride. It is preferable that the protective layer  180  is deposited at the temperature range of 200° C. or more.  
         [0159]    In the heat treatment process for forming the protective layer  180 , a metallic oxide layer  510  may be formed between the source electrode  173  and the semiconductor pattern  121 , and between the drain electrode  174  and the semiconductor pattern  121 . The metallic oxide layer  510  is formed due to a silicon oxide layer naturally formed on the semiconductor pattern before forming the Ag alloy-based layer for the data line assembly.  
         [0160]    In the heat treatment process for forming the protective layer  180 , the alloy elements of the Ag alloy-based layer for the data line assembly is diffused to the interface and make the silicon oxide layer to be a dense-structured metallic oxide layer  520 . If the data line assembly is based on AgMg, a metallic oxide layer of MgO is formed at the interface. If the data line assembly is based on AgAl, a metallic oxide layer of Al 2 O 3  is formed at the interface.  
         [0161]    The metallic oxide layer  510  lowers the contact resistance between the data line assembly and the semiconductor pattern  121  and enhances the adhesion therebetween.  
         [0162]    Thereafter, as shown in FIGS. 23A and 23B, the protective layer  180  is patterned through photolithography to thereby form a contact hole  191  exposing the drain electrode  174 , and a contact hole  192  exposing the data pad  172 . Furthermore, the protective layer  180  is patterned together with the inter-layer insulating layer  150  to thereby form contact holes  193  exposing the gate pads  142 .  
         [0163]    Finally, an ITO or IZO-based layer is deposited onto the protective layer  180 , and patterned through a mask to thereby form a pixel electrode connected to the drain electrode  174  through the contact hole  191 , and a subsidiary data and a subsidiary gate pads  202  and  203  connected to the data and the gate pads  172  and  142  through the contact holes  192  and  193 .  
         [0164]    In the thin film transistor array substrate according to the second preferred embodiment of the present invention, the gate line assembly and the data line assembly are formed with an Ag alloy bearing a low resistance. Consequently, such a thin film transistor array substrate can be well adapted for use in a wide-screen high definition liquid crystal display. Furthermore, the contact resistance between the neighboring layers can be reduced.  
         [0165]    [0165]FIG. 24 is a graph illustrating the electrical characteristics of a thin film transistor with Ag alloy-based electrodes.  
         [0166]    A gate electrode of 2000-4000 Å is formed with Ag alloy, and a gate insulating layer of 4500 Å is formed with SiNx. A semiconductor layer of 500 Å is formed with a−Si, and an ohmic contact layer of 2500 Å is formed with n+a−Si. Source and drain electrodes of 2000-4000 Å is formed of an Ag alloy. In this structure, drain currents as a function of gate voltages are illustrated in the graph.  
         [0167]    In the graph, the ON current is 0.1 pA when the gate voltage is −5V. The OFF current is 1.6 μA when the gate voltage is 20V. As the ratio of the ON current to the OFF current is about 10 7 , it can be known that the on/off current characteristic is good.  
         [0168]    It turns out from the experimental results that the inventive thin film transistor array substrate bears good electrical characteristic.  
         [0169]    As described above, in the inventive thin film transistor array substrate, the target wiring lines are formed with an Ag alloy bearing a low resistance. Such a thin film transistor array substrate can be well adapted for use in a wide-screen high definition display device. Furthermore, the contact resistance between the neighboring lines can be reduced while giving reliability to the resulting display device.  
         [0170]    While the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art will appreciate that various modifications and substitutions can be made thereto without departing from the spirit and scope of the present invention as set forth in the appended claims.