Patent Publication Number: US-2002000614-A1

Title: Coplanar gate-source-drain Poly-TFT and method for fabricating the same

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a “Poly Thin Film Transistor” (hereinafter referred as Poly-TFT) and its manufacturing method, and particularly to a coplanar source-drain-gate Poly-TFT and the manufacturing method for fabricating the same.  
       [0003] 2. Description of the Related Art  
       [0004] The regular Poly-TFTs are classified by the “bottom gate structure” as shown in FIG. 10 (A) and the “top gate structure” as shown in FIG. 10 (B). A metal wire of gate is provided on the bottom side of a Poly-TFT with the bottom gate structure, and a metal wire of gate is provided on the top side of a Poly-TFT with the top gate structure.  
       [0005] As shown in FIG. 10 (A), a metal wire of gate  110  is formed on a substrate  100  in a Poly-TFT with the bottom gate structure, and then an insulating layer  120  is formed above the metal wire of gate. Afterward, a TFT layer  130  is formed above the insulating layer  120  and the excimer laser annealing is used to activate and to polycrystallize the TFT layer  130 . Furthermore, a protection layer  140  is also formed above the TFT layer  130 , and both a signal electrode  150  and an image electrode  151  must be formed to connect the TFT layer  130 .  
       [0006] As shown in the cross-sectional view in FIG. 10 (A), the metal wire of gate  110  is the only one formed on the substrate  100 . Thus, after the TFT layer  130  being formed, concave surfaces will be generated on the TFT layer  130  above both sides of the metal wire of gate  110 . Such concave surfaces will result in the effect of heat concentration during the processes of excimer laser annealing and activation afterward so that the characteristics of the TFT are greatly affected.  
       [0007] Besides, the Poly-TFT with top gate structure will generate relatively high photo leakage current while the Poly-TFT with bottom gate structure will also generate photo leakage current. On the other hand, for reducing the off-state leakage current, usually it is necessary to add a structure of “lightly doping drain” (hereinafter referred as LDD) onto the TFT. And, for forming the LDD structures, an additional photo mask is needed to perform the exposure of the areas of the LDD structures.  
       [0008] Furthermore, whether the TFT with top gate structure or the TFT with bottom gate structure will have a relatively high “capacitance of gate-source” (CGS) because the respectively upper and lower locations of the gate and the source of the TFT generate a relatively large corresponding area. Consequently, the characteristics of the TFT are greatly affected.  
       SUMMARY OF THE INVENTION  
       [0009] In view of problems mentioned above, the present invention is to provide a coplanar gate-source-drain Poly-TFT and the method for fabricating the same.  
       [0010] Another object of the invention is to provide a coplanar gate-source-drain Poly-TFT wherein a LDD structure is formed without additional photo mask and yet with the full self-alignment on the Poly-TFT, and to provide the method for fabricating the same.  
       [0011] One other object of the invention is to provide a coplanar gate-source-drain Poly-TFT wherein the TFT layer is relatively flat, and to provide the method for fabricating the same.  
       [0012] One of the objects of the invention is to provide a coplanar gate-source-drain Poly-TFT wherein the gate-source capacitance is relatively low, and to provide the method for fabricating the same.  
       [0013] In order to achieve the objects mentioned above, the fabricating method of the coplanar gate-source-drain Poly-TFT according to the invention includes the steps as follow.  
       [0014] A. A layer of metal is formed upon a transparent substrate. The metal layer consists of the respective metal wires of source, drain and gate wherein the respective gaps exist in between the metal wires of source and gate as well as in between the metal wires of drain and gate.  
       [0015] B. An insulating layer is applied to cover the metal wire of gate.  
       [0016] C. A layer of amorphous semiconductor is formed across and upon the insulating layer with both ends contacting the metal wires of source and drain respectively.  
       [0017] D. The layer of amorphous semiconductor is crystallized to become the layer of polycrystalline semiconductor.  
       [0018] E. The impurity ions of high concentration are applied for doping the areas on the layer of polycrystalline semiconductor where are in contact of the metal wires of source and drain.  
       [0019] F. The layer of polycrystalline semiconductor and the areas doped with impurity ions are activated.  
       [0020] Prior to activating the layer of polycrystalline semiconductor mentioned above, the method of fabricating the coplanar gate-source-drain Poly-TFT according to the invention further includes the steps as follow.  
       [0021] G. A layer of photo-resist is formed upon the layer of polycrystalline semiconductor.  
       [0022] H. The photo-resist layer is exposed from the bottom side of the transparent substrate, and then the exposed areas of the photo-resist layer are etched.  
       [0023] I. The impurity ions of low concentration is applied for doping the areas on the layer of polycrystalline semiconductor with respect above the gaps mentioned above so that the areas of the LDD are formed.  
       [0024] The coplanar gate-source-drain Poly-TFT according to the invention includes forming the metal layer upon the transparent substrate that includes the respective metal wires of source, gate and drain. Wherein there are respective gaps in between the metal wires of source and gate, and in between the metal wires of drain and gate. The Poly-TFT according to the invention further includes an insulating layer covering the metal wire of gate. And, the Poly-TFT according to the invention further includes a layer of amorphous semiconductor across the insulating layer with both ends contacting the metal wires of source and drain respectively. Moreover, the impurity ions of high concentration are applied for doping the side areas on the layer of polycrystalline semiconductor in contact with the metal wires of source and drain. And, the impurity ions of low concentration are applied for doping the areas on the layer of polycrystalline semiconductor with respect above the gaps. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0025] These and other objects and advantages of the present invention will become apparent with reference to the following description and accompanying drawings as follow.  
     [0026]FIG. 1 is a cross-sectional view of a metal layer being formed upon a transparent substrate according to the invention.  
     [0027]FIG. 2 is a cross-sectional view of an insulating layer being formed to cover a layer of the metal wire of gate shown in FIG. 1.  
     [0028]FIG. 3 is a cross-sectional view of a semiconductor layer being formed and crystallized shown in FIG. 2.  
     [0029]FIG. 4 is a cross-sectional view of the impurity ions of high concentration being applied for doping the semiconductor layer upon the metal layer of source-drain shown in FIG. 3.  
     [0030]FIG. 5 is a cross-sectional view of a photo-resist layer being formed upon the semiconductor layer and the photo-resist layer being exposed from the bottom side of the transparent substrate shown in FIG. 4.  
     [0031]FIG. 6 is a cross-sectional view of the impurity ions of low concentration being applied for doping the areas on the semiconductor layer with respect above the gaps in between the source and gate poles, and in between the drain and gate poles shown in FIG. 5.  
     [0032]FIG. 7 is a cross-sectional view for the removal of the photo-resist layer and the activation of the layer of polycrystalline semiconductor shown in FIG. 6.  
     [0033]FIG. 8 is a cross-sectional view of a protection layer being formed upon the layer of polycrystalline semiconductor as shown in FIG. 7.  
     [0034]FIG. 9 is a structural schematic diagram of the Poly-TFT with its source, gate and drain on a same surface according to the invention.  
     [0035]FIG. 10 is a cross-sectional view of a Poly-TFT structure according to the prior art, in which (A) shows the bottom gate structure while (B) shows the top gate structure. 
    
    
     DETAIL DESCRIPTION OF THE INVENTION  
     [0036] The preferred aspects of embodiments of a coplanar gate-source-drain “Poly Thin Film Transistor” (hereinafter referred as Poly-TFT) and the method for fabricating the same according to the present invention is illustrated with reference of the accompanying drawings as follows.  
     [0037] As shown in FIG. 1 through FIG. 8, the coplanar gate-source-drain Poly-TFT according to the present invention is to have the source, drain and gate formed simultaneously upon a substrate. And, the “lightly doping drain” (hereinafter referred as LDD) structures are formed as exposed from the bottom side of the substrate in the coplanar gate-source-drain Poly-TFT according to the invention. The steps are as follow.  
     [0038] Step 1: as shown in FIG. 1, a metal wire of source  21 , a metal wire of gate  22 , and a metal wire of drain  23  are simultaneously formed upon a transparent substrate  10 . And, a gap  24  is formed in between the metal wire of source  21  and the metal wire of gate  22  while a gap  25  is formed in between the metal wire of gate  22  and the metal wire of drain  23 . Such gaps can be used as the areas for the LDD structures. The width of gaps  24  and  25  is 1.5 mm˜2.5 mm, and the width of the metal wire of gate  22  is 6 mm˜7 mm. The widths of the metal wires and gaps are not restricted within the ranges shown herewith.  
     [0039] Step 2: as shown in FIG. 2, an insulating layer  30  (referred to FIG. 9) is applied to cover the metal wire of gate  22 . The insulating layer  30  also fills up the gap  24  in between the metal wire of source  21  and the metal wire of gate  22  as well as the gap  25  in between the metal wire of gate  22  and the metal wire of drain  23 .  
     [0040] Step 3: as shown in FIG. 3, a layer of amorphous semiconductor  40  is formed across and upon the insulating layer  30  with both ends contacting the metal wire of source  21  and the metal wire of drain  23  respectively. The width of the layer of amorphous semiconductor is determined in accordance with the characteristics of the transistor. Afterwards, a regular annealing process, such as the excimer laser annealing, is used to transform the layer of amorphous semiconductor  40  into a layer of polycrystalline semiconductor  40 . Since the insulating layer  30  already fills up the gaps  24 ,  25  while the thickness of such insulating layer  30  is relatively thin, the semiconductor layer  40  across and upon the insulating layer  30  will be relatively flat.  
     [0041] Step 4: as shown in FIG. 4, a first photo-resist layer  50  is formed with a photo mask (not shown) upon the layer of polycrystalline semiconductor  40 . Then, the impurity ions (N+) of high concentration is applied for doping the areas on the layer of polycrystalline semiconductor  40  with respect above the metal wire of source  21  and the metal wire of drain  23  that a source pole  41  and a drain pole  42  are formed respectively. The photo-resist figures on the photo-resist layer  50  are removed afterwards.  
     [0042] Step 5: as shown in FIG. 5, a second photo-resist layer  60  is formed upon the layer of polycrystalline semiconductor  40 . Then, the second photo-resist layer  60  is exposed from the bottom side of the substrate  10  with the metal layer  10  on the substrate  10  as the photo mask. The direction of exposure is the X-direction shown in FIG. 5.  
     [0043] Step 6: as shown in FIG. 6, the exposed areas  61 ,  62  of the second photo-resist layer  60  are etched. And, the impurity ions of low concentration are applied for doping the areas on the layer of polycrystalline semiconductor  40  with respect to the area  61  and area  62  so that the LDD structures are formed by this step.  
     [0044] Step 7: as shown in FIG. 7, the second photo-resist layer  60  is removed, and then the layer of polycrystalline semiconductor  40  is put in the activation process.  
     [0045] Step 8: as shown in FIG. 8, a protection layer  70  is applied to cover the layer of polycrystalline semiconductor  40  in order to protect the Poly-TFT.  
     [0046] According to steps mentioned above, a Poly-TFT with the LDD structures can be fabricated. Certainly, if it is not necessary to have the LDD structures formed upon the Poly-TFT, both the step 5 and the step 6 are saved. Specifically, after both the application of impurity ions of high concentration and the removal of the photo-resist being completed, the manufacturing sequence can jump to step 7 for activating the layer of polycrystalline semiconductor  40 .  
     [0047]FIG. 9 is a structural schematic diagram of the coplanar gate-source-drain Poly-TFT according to the invention. As shown in FIG. 9, the metal wire of source  21 , the metal of drain  23  and the metal wire of gate  22  are formed upon an substrate  10  (referred to FIG. 1). An insulating layer  30  is applied to cover the metal wire of gate  22  in order to prevent the contact of a semiconductor layer and the metal wire of gate  22 . A semiconductor layer  40  is formed across and upon the insulating layer  30 . The semiconductor layer  40  is divided into a source pole  41  and a drain pole  42  which are doped with the impurity ions of high concentration, the LDD structures  43  and  44  which are doped with the impurity ions of low concentration, and the channel area  45 . Certainly, a protection layer can be applied upon the semiconductor layer  40  in order to protect the Poly-TFT.  
     [0048] As shown in the schematic diagram of FIG. 9, the shapes of the metal wire of source  21  and the metal wire of drain  23  are not restricted to the forms as shown. If the Poly-TFT is employed in the LCD (liquid crystal display), the metal wire of drain  23  is connected to the pixel electrode and the metal wire of source  21  is connected to the data line for conduction. And, the metal wire of gate  22  is connected to the scan line.  
     [0049] It should be understood that various alternatives to the structures described herein may be employed in practicing the present invention. It is intended that the following claims define the invention and that the structure within the scope of these claims and their equivalents be covered thereby.  
     EFFECT OF THE INVENTION  
     [0050] Referring to the fabricating method of a coplanar gate-source-drain Poly-TFT according to the present invention, the metal layers of source, drain and gate are formed on a same surface. Although an insulating layer is applied to cover the metal layer of gate, due to the relatively thin thickness of the insulating layer, the semiconductor layer across and upon the insulating layer is relative flat. By means of the aforesaid structure, the relatively good effects can be obtained from the annealing and the activation.  
     [0051] Furthermore, since the metal wires of source, drain and gate are formed on the same surface, the metal wires of source, drain and gate are used to shade the light beam coming from bottom side of the substrate so as to reduce the photo leakage current of the Poly-TFT effectively. And, if the formation of the LDD structures is necessary, the figures of the metal wires can be used as a photo mask for the exposure from the bottom side of the substrate since the gaps are formed in between the source-gate poles and in between the drain-gate poles. Thus, one photo mask is saved, and the gaps are used for the fully self-alignment on the areas being formed with the LDD structures.  
     [0052] Besides, since the metal wires of source, drain and gate are simultaneously formed on the substrate  10 , another additional step is not necessary to form the metal wires of source and drain that the manufacturing process of TFT is simplified. Meanwhile, since the metal wires of source, drain and gate are formed on the same surface, the corresponding area between the source and the gate is relatively reduced, and consequently the capacitance of gate-source is relatively lowered. Therefore, the characteristics of the Poly-TFT are improved.  
     [0053] While a coplanar gate-source-drain Poly-TFT and the method for fabricating the same according to the present invention has been described with reference to an illustrative embodiment, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiment, as well as other embodiments of the invention, will be apparent to persons skilled in the art with reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments. For example, although a regular annealing process is used in step 3 in order to transform the layer of amorphous semiconductor into a layer of polycrystalline semiconductor, it is allowable to perform such annealing process together with the activation process in step 7 in order to simply the manufacturing processes.