Patent Publication Number: US-2006011914-A1

Title: Novel conductive elements for thin film transistors used in a flat panel display

Description:
CLAIM OF PRIORITY  
      This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application for  THIN FILM TRANSISTOR AND FLAT PANEL DISPLAY COMPRISING THE SAME  earlier filed in the Korean Intellectual Property Office on 12 Mar. 2003 and there duly assigned Serial No. 2003-15357.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a flat panel display thin film transistors. More particularly, the present invention relates to a novel structure for electrodes of the thin film transistors that do not degrade the semiconductor material of the thin film transistors in the display.  
      2. Description of the Related Art  
      A thin film transistor (hereinafter TFT) is a device of which a source electrode and a drain electrode can be electrically connected through a channel formed in a semiconductor layer which physically connects the source and drain electrodes according to a voltage applied to a gate electrode. The TFT is mainly used in an active matrix flat panel display such as an electroluminescent display and a liquid crystal display. The TFT serves to independently drive sub-pixels in a flat panel display.  
      A source electrode and a gate electrode formed on a TFT panel of a flat panel display are connected to driving circuits arranged on sides of the flat panel display through conductive lines. Generally, the source electrode, the drain electrode and the conductive lines electrically connected to the source and drain electrodes are at the same time formed with the same structure using the same material for the sake of simplifying a manufacturing process. Hereinafter, the source electrode, the drain electrode, and the conductive lines electrically connected thereto are simply referred to as “S/D electrodes and lead lines”.  
      The S/D electrodes and lead lines may be made of a chromium (Cr) based metal or a molybdenum (Mo) based metal such as Mo and MoW. However, due to a relatively high resistance, these metals are relatively impractical for forming the S/D electrodes and lead lines for use in a large flat panel display. Recently, attention has been paid to aluminum (Al) as a material for the S/D electrodes and lead lines. However, use of pure Al has a problem in that the aluminum diffuses toward and into a semiconductor layer during a heat treatment process that generally occurs subsequent to formation of the source electrode and the drain electrode. When the aluminum diffuses into the semiconductor layer, the TFT does not function properly.  
      These problems may worsen by a heat treatment process subsequent to formation of a metal electrode, and conductive lines electrically connected thereto. For example, the contact annealing process after source and drain metal sputtering is necessary in TFT fabrication, and the temperature needed to anneal can be higher than 300° C. When pure aluminum is used in the source and the drain electrodes and a high temperature anneal follows electrode formation, aluminum can diffuse into the semiconductor layer of a TFT and pose a negative effect on the electrical characteristics of the TFT.  
      U.S. patent application Publication No. 2002/0085157 to Tanaka et al (hereinafter Tanaka &#39;157) discloses electrodes made of Al. Each of the electrodes has a structure of titanium nitride (TiN)/Al, TiN/Ti/Al, or TiN/Al/Ti. Advantages of such a structure include reduction of an electrical connection resistance between the electrodes and terminals connected to the electrodes and suppression of generation of Al hillocks often formed during a heat treatment process subsequent to the formation of the electrodes. However, this Tanaka &#39;157 fails to discuss the existence of and a solution to the problem of aluminum from a pure aluminum electrode from diffusing into a semiconductor layer of a transistor during a heat treatment process.  
      Furthermore, in a case where the conductive lines which are connected to the source and drain electrodes have a three-layer structure of Ti/pure Al/Ti, TiAl 3  may be generated at an interface between the pure Al layer and the Ti layer by a heat treatment process. The TiAl 3  may increase the resistance of the conductive lines. For this reason, in a case where a flat panel display has a large size or its pixels have small sizes, a voltage drop between driving circuits and the pixels may increase when TiAl 3  is formed. Thus, the formation of TiAl 3  causes the response speed of the pixels to decrease and causes a non-uniform distribution of an image in a large display.  
     SUMMARY OF THE INVENTION  
      It is therefore an object of the present invention to provide an improved design for S/D electrodes and lead lines for TFT&#39;s used in a flat panel display.  
      It is also an object of the present invention to provide a design for electrodes in a TFT that prevent aluminum from diffusing into a semiconductor layer during a heat treatment.  
      It is also an object of the present invention to provide a novel design for electrodes in a TFT that have a low resistivity and thus result in uniform luminance even when the display size is very large.  
      It is further an object of the present invention to provide a design for electrodes in a TFT that does not result in a structure where the electrode material reacts with the semiconductive material of the TFT when subject to heat treatment.  
      These and other objects may be achieved by an electrode structure where aluminum is used but aluminum is not used in pure form. Instead, an alloy of aluminum is used in the electrodes. The aluminum alloy layer may contain about 0.1 to 5 wt % of at least one element selected from silicon, copper, neodymiumm, platinum, and nickel. The reason why an aluminum alloy and not pure aluminum should be used is because after being subject to a heat treatment, aluminum from a pure aluminum layer will diffuse into the semiconductor layer and corrupt the electrical properties of the TFT. By using an aluminum alloy and not pure aluminum in the electrode structure, the diffusion of aluminum into the semiconductor layer during a heat treatment is prevented.  
      Other features of the electrode structure are as follows. To prevent the formation of hillocks in a heat treatment, the aluminum alloy layer is bounded by titanium. To prevent the formation of highly resistive TiAl 3  during heat treatment, a diffusion prevention layer is interposed between the aluminum alloy layer and the titanium layer. Preferably, the diffusion prevention layer is TiN or titanium nitride. Optimum TiN thickness is 300 Å. The TiN layer may have 5 to 85 wt % of nitrogen. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      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 similar components, wherein:  
       FIG. 1  is a circuit view of a TFT panel;  
       FIG. 2  is a partial plan view of a TFT panel;  
       FIG. 3  is a sectional view of an electroluminescent display having a TFT;  
       FIG. 4  is a sectional view of a liquid crystal display having a TFT;  
       FIG. 5  is a sectional view of a source and drain electrodes in a TFT;  
       FIG. 6  is a top view of a TFT of  FIG. 5  after heat treatment;  
       FIG. 7  is a sectional view of a source or drain electrode in a thin film transistor (TFT) according to one embodiment of the present invention;  
       FIG. 8  is a sectional view of a TFT after heat treatment according to the present invention using the electrodes illustrated in  FIG. 7 ;  
       FIG. 9  is a top view of the TFT of  FIG. 8  after heat treatment; and  
       FIG. 10  is a sectional view of a TFT after heat treatment according to another embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Turning now to the figures,  FIG. 1  illustrates a circuit  112  for a flat panel display having a thin film transistors (TFT&#39;s)  10  and  50 . The circuit  112  includes a first TFT  10 , a second TFT  50 , a storage capacitor  40 , and a light emission unit  60 . A first source electrode  12  in the first TFT  10  is connected to a horizontal driving circuit H through a first conductive line  20  and a first gate electrode  11  in the first TFT  10  is connected to a vertical driving circuit V through a second conductive line  30 . A first drain electrode  13  in the first TFT  10  is connected to a first capacitor electrode  41  of the storage capacitor  40  and to a second gate electrode  51  of the second TFT  50 . A second capacitor electrode  42  of the storage capacitor  40  and a second source electrode  52  of the second TFT  50  are connected to a third conductive line  70 . A second drain electrode  53  of the second TFT  50  is connected to a first pixel electrode  61  of the light emission unit  60 . A second pixel electrode  62  of the light emission unit  60  is arranged to be opposite to the first pixel electrode  61  and spaced a predetermined gap apart from the first pixel electrode  61 . Between the second pixel electrode  62  and the first pixel electrode  61  is an active layer. The active layer may be an organic material layer, an inorganic material layer, or a liquid crystal layer. This active layer is arranged between the first pixel electrode  61  and second pixel electrode  62  of the light emission unit  60  according to one of the various types of flat panel displays.  
      Turning now to  FIG. 2 ,  FIG. 2  illustrates a driving unit (one of red component, blue component, and green component that constitute one pixel) of a flat panel display provided with the first TFT  10  and the second TFT  50 .  FIG. 2  is a schematic plan view illustrating a physical structure of the circuit  112  illustrated in  FIG. 1 . For the sake of simplicity, only conductive constitutional elements are illustrated in  FIG. 2 . Therefore, nonconductive constitutional elements such as a substrate, a buffer layer various types of insulating layers, a planarization layer, a light emission layer, a liquid crystal layer, a second pixel electrode, a polarization layer, an orientation layer, and a color filter layer are omitted. These nonconductive constitutional elements are instead illustrated in  FIGS. 3 and 4 . Only constitutional elements positioned at regions represented by oblique (or slanted) lines shown in  FIG. 2  are electrically connected to each other. Other regions in  FIG. 2  that are not represented by oblique lines are electrically insluated.  
      When a voltage is applied to the first gate electrode  11 , a conductive channel is formed in a semiconductor layer  80  that connects the first source electrode  12  to the first drain electrode  13 . At this time, when charge is supplied to the first source electrode  12  through the first conductive line  20 , the charge moves into the first drain electrode  13 . Another charge is supplied into the second source electrode  52  through the third conductive line  70 . Luminance of the driving unit is determined according to the charge supplied into the second source electrode  52 . When the charge of the first drain electrode  13  is supplied to the second gate electrode  51 , the charge of the second source electrode  52  moves into the second drain electrode  53 , thereby driving the first pixel electrode  61  of the light emission unit  60 . The storage capacitor  40  serves to maintain a driving operation of the first pixel electrode  61  or to increase a driving speed. For reference, the first TFT  10  and the second TFT  50  have a similar section structure, but are different in adjoining constitutional elements.  
      An electroluminescent display  114  illustrated in  FIG. 3  includes a TFT panel, a light emission layer  87 , and a second pixel electrode  62 . The TFT panel includes a substrate  81 , a TFT  50 , a first conductive line  20 , a second conductive line  30 , and a first pixel electrode  61 . In the case of a rear emission type electroluminescent display, the substrate  81  may be made of a transparent material, for example glass, and the second pixel electrode  62  may be made of a metal material with good reflectivity. On the other hand, in the case of a front emission type electroluminescent display, the second pixel electrode  62  may be made of a transparent conductive material, for example, indium tin oxide (ITO), and the first pixel electrode  61  may be made of a metal material with good reflectivity.  
      A buffer layer  82  is formed on the whole surface of the substrate  81 . A semiconductor layer  80  is formed to a predetermined pattern on the buffer layer  82 . A first insulating layer  83  is formed on the semiconductor layer  80  and on the remaining exposed surface of the buffer layer  82  where the semiconductor layer  80  is not formed. A second gate electrode  51  is formed to a predetermined pattern on the first insulating layer  83 . A second insulating layer  84  is formed on the second gate electrode  51  and the remaining exposed surface of the first insulating layer  83  on where the second gate electrode  51  is not formed. After the formation of the second insulating layer  84 , the first and second insulating layers  83  and  84  respectively are subjected to etching such as dry etching to expose portions of the semiconductor layer  80 . The exposed portions of the semiconductor layer  80  are connected to a second source electrode  52  and a second drain electrode  53  that are formed to a predetermined pattern. After the formation of the second source and drain electrodes  52  and  53  respectively, a third insulating layer  85  is formed thereon. A portion of the third insulating layer  85  is etched to electrically connect the second drain electrode  53  and the first pixel electrode  61 . After the formation of the first pixel electrode  61  on the third insulating layer  85 , a planarization layer  86  is formed. The portion of the planarization layer  86  corresponding to the first pixel electrode  61  is etched. Then, the light emission layer  87  is formed on the first pixel electrode  61  and the second pixel electrode  62  is formed on the light emission layer  87 . In addition, encapsulation layer  89  is formed over second pixel electrode  62 .  
      The TFT  50  made up of the second source electrode  52 , the second drain electrode  53 , the second gate electrode  51  and the semiconductor layer  80 . The second source electrode  52  and the second drain electrode  53  are arranged on the same horizontal plane and are separated from each other by a predetermined gap. The second source electrode  52  and the second drain electrode  53  are each physically connected to the semiconductor layer  80 . The second gate electrode  51  is electrically insulated from the second source electrode  52 , the second drain electrode  53  and the semiconductor layer  80 . The second gate electrode  51  is positioned above the semiconductor layer  80  and between the second source electrode  52  and the second drain electrode  53 . Meanwhile, generally, a TFT is divided into a staggered type, an inverted staggered type, a coplanar type, and an inverted coplanar type according to the arrangements of the above electrodes and the semiconductor layer  80 . A coplanar type is illustrated in this embodiment of the present invention, but the present invention is not limited thereto.  
      The TFT  50  of  FIG. 3  corresponds to the second TFT  50  illustrated in  FIG. 2 . In this case, the second source electrode  52  is connected to the third conductive line  70 , the second gate electrode  51  is connected to the first drain electrode  13  of the first TFT  10 , the second drain electrode  53  is connected to the first pixel electrode  61  of light emitting unit  60 , the first source electrode  12  of the first TFT  10  is connected to the first conductive line  20 , and the first gate electrode  11  is connected to the second conductive line  30 . According to this embodiment of the present invention, the first conductive line  20  corresponds to a data line for transmitting data and the second conductive line  30  corresponds to a scan line.  
      The structure of an electroluminescent display  114  will now be described in detail with reference to  FIG. 3 . As illustrated in  FIG. 3 , an electroluminescent display  114  includes the first pixel electrode  61 , the light emission layer  87  formed on the first pixel electrode  61 , and the second pixel electrode  62  formed on the light emission layer  87 . The electroluminescent display  114  can be divided into organic and inorganic electroluminescent displays. With respect to an organic electroluminescent display, the light emission layer  87  is made up of an electron transport layer, a light emission material layer, and a hole transport layer. With respect to an inorganic electroluminescent display, insulating layers are interposed between the first pixel electrode  61  and the light emission layer  87  and between the second pixel electrode  62  and the light emission layer  87 .  
      The light emission material layer  87  of an organic electroluminescent display is made of an organic material, for example, phthalocyanine such as copper phthalocyanine (CuPc), N,N′-di(naphthalene-1-yl)-N,N′-diphenyl benzidine (NPB), tris-8-hydroxyquinoline aluminium (Alq3) or the like. When charge is supplied to the first pixel electrode  61  and the second pixel electrode  62 , holes and electrons recombine with each other to generate excitons. When the excitons are changed from an excited state to a ground state, the light emission material layer  87  emits light.  
      Regarding an inorganic electroluminescent display, an inorganic material layer between the insulating layers positioned at inner sides of the first pixel electrode  61  and second pixel electrode  62  emits light. An inorganic material for the inorganic material layer may be metal sulfide such as ZnS, SrS, and CsS. Recently, alkaline earth-based calcium sulfide such as CaCa 2 S 4  and SrCa 2 S 4 , and metal oxide are also used. Transition metal such as Mn, Ce, Th, Eu, Tm, Er, Pr, and Pb and alkaline rare earth metal may be used as light emitting core atoms that form the light emission layer  87  together with the above inorganic material. When a voltage is applied between the first pixel electrode  61  and second pixel electrode  62 , electrons are accelerated and collide with the light emitting core atoms. At this time, electrons of the light emitting core atoms are excited to a higher energy level and then fall back to a ground state. Accordingly, the inorganic material layer emits light.  
      Turning now to  FIG. 4 ,  FIG. 4  illustrates a liquid crystal display  105 . A liquid crystal display and an electroluminescent display are similar to each other in terms of the structure of a TFT panel, but are different in adjoining constitutional elements. Hereinafter, only adjoining constitutional elements of the TFT panel in a liquid crystal display will be described.  
      The liquid crystal display  105  includes a TFT panel, a first orientation layer  97 , a second substrate  102 , a second pixel electrode  62 , a second orientation layer  99 , a liquid crystal layer  98 , and a polarization layer  103 . The TFT panel comprises a first substrate  91 , a TFT  50 , a first conductive line, a second conductive line, and a first pixel electrode  61 . The first substrate  91  corresponds to the substrate of an electroluminescent display.  
      The first substrate  91  and the second substrate  102  are separately manufactured. A color filter layer  101  is formed on the lower surface of the second substrate  102 . The second pixel electrode  62  is formed on the lower surface of the color filter layer  101 . The first orientation layer  97  and the second orientation layer  99  are formed on the upper surface of the first pixel electrode  61  and the lower surface of the second pixel electrode  62 , respectively. The first and second orientation layers  97  and  99  serve to allow for a proper orientation of a liquid crystal of the liquid crystal layer  98  interposed therebetween. The polarization layer  103  is formed on each of the outer surfaces of the first and second substrates  91  and  102  respectively. A spacer  104  is used to maintain a gap between the first substrate  91  and the second substrates  102 . Reference numerals  92 ,  93 ,  94 ,  95  and  96  in  FIG. 4  represent a buffer layer, a first insulating layer, a second insulating layer, a third insulating layer and a planarization layer respectively.  
      A liquid crystal display allows light to pass through or be blocked according to the arrangement of a liquid crystal. The arrangement of the liquid crystal is determined by an electric potential difference between the first and second pixel electrodes. Light that has passed through the liquid crystal layer exhibits a color of the color filter layer  101 , thereby displaying an image.  
      Turning now to  FIGS. 5 and 6 ,  FIG. 5  illustrates a cross section of a TFT after heat treatment using electrodes having pure aluminum and  FIG. 6  is a top view of the TFT after heat treatment.  FIG. 5  illustrates a semiconductor layer  80  arranged below and connected to S/D electrodes and lead lines  52  and  53 , each of which has a three-layer structure of titanium (Ti) layer (thickness: 500 Å) 232 /pure Al layer (thickness: 4,000 Å) 231 /Ti layer (thickness: 500 Å)  233 , after heat treatment at 450° C. As illustrated in  FIGS. 5 and 6 , the Al of the pure Al layer  231  diffuses towards and into the semiconductor layer  80  when heat is applied to thereby form diffusion defect portions  52   a  and  53   a  in the semiconductor layer  80 . The reason the Al of the pure Al layer  231  can diffuse towards and into the semiconductor layer  80  even though a Ti layer  233  is interposed between the pure Al layer  231  and the semiconductor layer  80  is that the Ti layer  233  is present in the form of a very thin film and/or there exists a Ti-free zone in Ti layer  233  according to the upper surface structure of the semiconductor layer  80 . Thus, the presence of a thin titanium layer  233  between the pure aluminum layer  231  of the electrode and the semiconductor layer  80  of a TFT does not prevent aluminum from the pure aluminum layer  231  from diffusing into and destroying parts of semiconductor layer  80  when heat is applied. It is to be appreciated that the presence of a TiN diffusion layer between a titanium layer and a pure aluminum layer will not prevent aluminum from diffusing into the semiconductor layer  80  when heat is applied.  
      The resultant diffusion defect portions  52   a  and  53   a  may cause the same results as when pure aluminum is deposited directly onto the semiconductor layer  80 . Defect portions  52   a  and  53   a  can prevent formation of a normal conductive channel between the source electrode and the drain electrode of a TFT. Furthermore, defect portions  52   a  and  53   a  may result in a short between the source electrode and the drain electrode, resulting in a malfunctioning TFT. Although  FIGS. 5 and 6  illustrate the source  52  and the drain  53  electrodes of second TFT  50 , the same applies to first TFT  10 .  
      Hereinafter, the structures of S/D electrodes and lead lines will be described in detail with reference to  FIGS. 2 and 7  through  10 . According to this embodiment of the present invention, the first and second gate electrodes  11  and  53  are formed simultaneously with the second conductive line  30  using the same material. The first and second source electrodes  12  and  52 , the first and second drain electrodes  13  and  53 , the first conductive line  30 , and the third conductive line  70  are at the same time formed using the same material. Since the formation sequences and materials for these conductive constitutional elements may vary according to manufacture processes, they are not limited to those described in this embodiment of the present invention.  
      According to this embodiment of the present invention, at least one of S/D electrodes and lead lines  130  is made out of an aluminum (Al) alloy layer  131 , and titanium (Ti) layers  132  and  133  formed on the respective upper and lower surfaces of the Al alloy layer  131 . Optionally, in another embodiment illustrated in  FIG. 10 , diffusion prevention layers  138  and  139  made of titanium nitride (TiN), for example, may be interposed between the Al alloy layer  131  and the respective Ti layers  132  and/or  133 . Aluminum diffusion can be prevented during a heat treatment when an aluminum alloy as opposed to pure aluminum is used in the electrode structure.  
      Preferably, the Al alloy layer  131  is made of an alloy that contains 0.1 to 5 wt %, preferably 2 wt % of at least one element selected from silicon (Si), copper (Cu), neodymium (Nd), platinum (Pt), and nickel (Ni). It has been determined empirically that when the S/D electrodes and lead lines according to this embodiment of the present invention as illustrated in  FIG. 10  have a five layer structure of Ti layer (thickness: 250 Å) 132 /TiN layer (thickness: 250 Å) 138 /Al alloy layer (thickness: 4,000 Å) 131 /TiN layer (thickness: 250 Å) 139 /Ti layer (thickness: 250 Å) 133 , Al of the Al alloy layer  131  did not diff-use toward a semiconductor layer  80  even after a heat treatment process at 450° C. Therefore, the semiconductor layer  80  was kept clear of defect portions, as illustrated in  FIG. 9 , enabling a conduction channel  180  to form during TFT operation. These good results result from use of the Al alloy layer in the electrode structure and not using pure Al in the electrode structure. When the S/D electrodes and lead lines have a three layer structure of Ti layer (thickness: 500 Å) 132 /Al alloy layer (thickness: 4,000 Å) 131 /Ti layer (thickness: 500 Å)  133  as illustrated in  FIG. 8 , the same result was obtained. In other words, the structure of  FIG. 8 , like the structure of  FIG. 10 , produced a semiconductor layer  80  as in  FIG. 9  free from defect regions  52   a  and  53   a.    
      It is to be appreciated that the empirical results of  FIG. 9  were obtained under the same experiment conditions as the empirical results of  FIG. 6  with the exception that the pure aluminum layer in the electrode stack is replaced with an aluminum alloy layer. In other words, the results of  FIGS. 6 and 9  were obtained with all parameters held constant except for the substitution of an aluminum alloy layer  131  for the pure aluminum layer  231 .  
      It is to be appreciated that titanium layers  132  and  133  are used instead of just an aluminum alloy layer  131  as the titanium layers  132  and  133  serve to prevent the formation of aluminum hillocks during heat treatment.  
      In another embodiment, a five layer electrode stack of  FIG. 10  is employed where TiN diffusion prevention layers  138  ( 139 ) are interposed between each titanium layer  132  ( 133 ) and the aluminum alloy layer  131  to prevent the formation of unwanted TiAl 3  during a heat treatment process. TiAl 3  greatly increases the resistivity of the electrodes and the conductive lines. Therefore, TiN diffusion prevention layers  138  and  139  prevent TiA  3  from forming thus keeping the resistivity of the electrodes and the conductive lines leading to the TFT low. This is particularly important in large flat panel displays where a low resistivity of electrodes and lead lines can prevent a non-uniform pixel display distribution. Although  FIGS. 8, 9  and  10  have been discussed in conjunction with second TFT  50  having a source electrode  52  and a drain electrode  53 , the novel structures of  FIGS. 8, 9  and  10  equally apply to the first TFT  10  as well.  
      An optimum thickness of the TiN diffusion prevention layers  138  and  139  is 250 Å. If the thickness of the diffusion prevention layers are too thin, Al diffusion may occur, resulting in the formation of TiAl 3  during a heat treatment. On the other hand, if the TiN diffusion layers are too thick, the production cost becomes unnecessarily too high because of the unnecessarily thick TiN layers. Preferably, the TiN layers  138  and  139  contain 5 to 85 wt % of nitrogen.  
      In a method to make the electrode stack  130  of  FIG. 10 , the Al alloy layer  131  and the Ti layers  132  and  133  are deposited by DC-magnetron sputtering under an argon (Ar) gas atmosphere. The TiN layers  138  and  139  are deposited by reactive sputtering under a mixed gas atmosphere of Ar and nitrogen (N 2 ). Such a deposited structure is etched to a predetermined pattern for the S/D electrodes and lead lines by dry etching with high frequency-enhanced plasma.  
      It is to be appreciated that  FIGS. 5 through 10  discuss second TFT  50  and second source electrode  52  and second drain electrode  53 ,  FIGS. 5 through 10  and the concepts discussed in the discussion of  FIGS. 5 through 10  above equally apply to the first TFT  10  having first source electrode  12  and first drain electrode  13 .  
      The present invention provides a novel structure for an electrode attached to a semiconductor layer in a TFT that does not form defect regions in the semiconductor layer when exposed to a heat treatment. Furthermore, the resistivity is kept low. Other embodiments include the presence of titanium layers to prevent the formation of aluminum hillocks during heat treatment process. Further embodiments include the presence of a TiN diffusion layer between the aluminum alloy layer and the titanium layers to prevent the formation of highly resistive TiAl 3  during heat treatment. By employing the novel electrode structure of the present invention in a TFT transistor, the integrity of the transistor is maintained and the resistivity of the conductive lines and the electrodes are reduced allowing for the formation of large flat panel displays having uniform luminance between the pixels.  
      While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.