BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention generally relates to a thin film transistor (TFT) and a method of forming the same. More particularly, the present invention relates to a TFT with non-transparent conductive structures under conducting wires that electrically couple with a drain/source, and a method of forming the TFT by using a mask with a slit pattern.  
         [0003]     2. Description of the Prior Art  
         [0004]     TFTs have been broadly used in contemporary electronic products such as switching on/off pixels in a liquid crystal display (LCD) panel. Hence, the topic of how to improve the structure and forming method for TFT has been popularly discussed.  
         [0005]     A well-known TFT structure, as shown in  FIG. 1 , at least includes following units over a substrate  10 : a conductive structure  11 , a first dielectric layer  12 , a first semi-conductive layer  13 , a second semi-conductive layer  14 , and a second dielectric layer  15  that is electrically coupled with a conducting wire  16  (patterned conductive structure). Herein, the conductive structure  11  is used as a gate. The first semi-conductive layer  13  is used as a channel and the second semi-conductive layer  14  can be used as a drain or a source. The first dielectric layer  12  and the second dielectric layer  15  are used as insulating and protective material. The conducting wire  16  is used in electrically coupling the drain with external circuits. However,  FIG. 1  did not illustrate conducting wires, which electrically couple the gate and the source with external circuits. Basically, the source electrically coupled with external circuit via the conducting wire in the second dielectric layer, and the gate electrically coupled with external circuit via the conducting wire through the first and the second dielectric layer.  
         [0006]     Obviously, five masks and pattern-transferred processes are essential for forming the TFT while each patterned layer requires corresponding mask and process, since the patterns of the conductive structure  11 , the first semi-conductive layer  13 , the second semi-conductive layer  14 , the second dielectric layer  15  and the conducting wire  16  are different from each other. Hence, many techniques for reducing required masks and processes have been continually brought out in order to save the material cost, to shorten the process time, and to improve the production ability.  
         [0007]     For example, in a prior art, two masks were combined to form the one for forming the second semi-conductive layer  14  and the conducting wire  16  with the same material. However, the optimum cannot be achieved by using the same material since the requirements between the conducting wire and the source/drain are different. In another prior art, two masks were combined to form the one for forming the second dielectric layer  15  and the first semi-conductive layer  13 . By doing so, the problems such as higher leakage current (I off ) and drop height may be resulted from the structure of TFT size. In still another prior art, two masks were combined to form the one for forming the first semi-conductive layer  13  and the second semi-conductive layer  14 . However, the practical application can be quite difficult since the special exposure technique is required for the half tone mask. As to these techniques mentioned above can refer to as followings: A. Van Calster et. al. “A Simplified 3-Step Fabrication Scheme for high Mobility AMLCD Panels”; K. Ono et. al. “A Simplified 4 photo-Mask Process for 34-cm Diagonal TFT-LCDs” IDRC 1995; and Chang W. H. et. al. “A TFT Manufactured by 4-Msks Process with new Photolithography” IDRC 1998.  
         [0008]     Besides, the light is transmitted to the TFT from the back of the substrate  10  while the TFT is used in a LCD panel, and the first dielectric layer  12  in a prior art is commonly the transparent material, hence the first semi-conductive layer  13 , the second semi-conductive layer  14  and the conducting wire  16  may induce the light current (such as the electron-hole pairs excited in semiconductor by the light) resulting to the problem of leakage current or noise.  
         [0009]     As mentioned above, there is much space for improving a well-known structure and forming method for TFT. Hence, these problems of how to reduce the required mask in forming TFT process and how to prevent TFT from inducing the light current need to be solved.  
       SUMMARY OF THE INVENTION  
       [0010]     According to the shortcomings mentioned in the background, the present invention provides a method for only using four masks in forming a TFT, and a TFT for preventing the light current effectively to improve the foregoing drawbacks.  
         [0011]     One characteristic of the present invention is to use the same mask to define a source, a drain and a channel, and more particularly, using a mask with a slit pattern forms a non-uniform photoresist, as well as transfers patterns of the source, of the drain, and of the channel to corresponded semi-conductive layer.  
         [0012]     Another characteristic of the present invention is that the TFT has a non-transparent conductive structure, which is at least positioned under the conducting wires of the source/drain/channel, beside and insulated with the gate. By doing so, the light transmitted from the substrate is blocked and the light current induced in the TFT is negligible. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0014]      FIG. 1  shows cross-sectional views of the well-known structure of a TFT;  
         [0015]      FIGS. 2A  to  2 H show the basic processes in accordance with the present invention for forming a TFT;  
         [0016]      FIGS. 3A  to  3 D show several assumed structures of the TFT in accordance with the present invention; and  
         [0017]      FIGS. 4A  to  4 E show another basic processes in accordance with the present invention for forming a TFT  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     One preferred embodiment of the present invention is a method for forming a TFT, at least including the basic process steps described as below.  
         [0019]     As shown in  FIG. 2A , a substrate  20  is provided and a gate  21  is formed on the substrate  20 .  
         [0020]     As shown in  FIG. 2B , a dielectric layer  22  is formed on the substrate  20  and located over the gate  21 , and then a first semi-conductive layer  23  and a second semi-conductive layer  24  is formed in proper sequence on the dielectric layer  22 .  
         [0021]     As shown in  FIG. 2C , a photoresist layer  25  is formed on the second semi-conductive layer  24 , and then a mask  251  is placed over the photoresist layer  25 . Wherein the mask  251  has a slit pattern  252 , which is just over the gate  21 , and further, aims at the center of the gate  21 .  
         [0022]     As shown in  FIG. 2D , the photoresist layer  25  is patterned to form a photoresist pattern  253 , which is just over the gate  21  (or still over the substrate  20  around the gate  21 ). Wherein the photoresist pattern  253  further includes a thin photoresist pattern formed under the slit pattern  252  and a thick photoresist pattern, due to diffraction caused by the slit pattern  252 . However, the photoresist pattern  253 , in fact, has no obvious changes in its thickness but arc changes. This illustration is just drawn to stress one characteristic of the present embodiment, did not restrict the photoresist pattern  253  to form as described above.  
         [0023]     As shown in  FIG. 2E , portions of second semi-conductive layer  24  and first semi-conductive layer  23  without the photoresist pattern  253  covering are removed, as well the dielectric layer  22  without the photoresist pattern  253  covering can be removed at the same time. The present embodiment does not restrict these details.  
         [0024]     As shown in  FIG. 2F , the whole part of the thin photoresist pattern is removed. This can be achieved by using a non-selective etching process to lower the thickness of the whole photoresist pattern  253  until the second semi-conductive layer  24  exposed or only removing the thin part of the photoresist pattern  253 . The present embodiment does not restrict these details.  
         [0025]     As shown in  FIG. 2G , the portion of second semi-conductive layer  24  without the remained photoresist pattern  253  covering is removed, and then, the remained photoresist pattern  253  is removed. However, in accordance with forming different specifications of TFT, the first semi-conductive layer  23  without the remained photoresist pattern  253  covering also can be removed (or to be lowered) after removing the portions of second semi-conductive layer  24 .  
         [0026]     As described above, the present embodiment uses the mask  251  with the slit pattern  252  to define the patterns of the first semi-conductive layer  23  and the second semi-conductive layer  24  at the same time, so that the required masks can be reduced in forming TFT process. The present embodiment uses the single mask  251  and turns the photoresist layer  25  into the non-uniform photoresist pattern  253  by applying the characteristic of a diffraction pattern, which results from slit diffraction and gradually weakens from its center to both sides. Further, the semi-conductive layers  23  and  24  are patterned by using the non-uniform photoresist pattern  253  through two process steps (one is to use the whole non-uniform photoresist pattern  253 ; the other one is to remove the thin photoresist pattern  253  and then use the remained photoresist pattern  253 ). By doing so, only using one mask in the present embodiment makes respective pattern on the second semi-conductive layer  24  and the first semi-conductive layer  23 .  
         [0027]     Comparing the present embodiment with a well-known technique using the half tone mask, the basic process flows are the same for combining the photolithography processes of two semi-conductive layers by only using one mask. However, there is no extra requirement for special exposure processes but only some modifications on the mask since the present embodiment adopts silt diffraction. Hence, it is more convenient and saves the cost.  
         [0028]     In order to form conducting wires to exchange signals with external circuits, the present embodiment, as shown in  FIG. 2H , apparently can be further carried the processes out as followings: (These process steps are not described and illustrated in detail any more since they are not the characteristics of the present embodiment).  
         [0029]     (1) After removing a portion of the second semi-conductive layer  24 , the photoresist pattern  253  is removed and then an extra dielectric layer  26  is formed over the substrate and covers the first semi-conductive layer  23  and the second semi-conductive layer  24 .  
         [0030]     (2) The extra dielectric layer  26  is patterned to form an open region to expose the portion of the second semi-conductive layer  24 . Apparently, there is no effect on the dielectric layer  21  if covered or not by the photoresist pattern  253  before since the extra dielectric layer  26  covers it now.  
         [0031]     (3) The patterned conductive structure  27  is formed on the extra dielectric layer  26  and is filled in the open region.  
         [0032]     Another preferred embodiment in accordance with the present invention is a kind of TFT, as shown in  FIG. 3A  and  FIG. 3B , at least including the basic units as followings: a conductive structure  31 , a non-transparent structure  32 , a first dielectric layer  33 , a first semi-conductive layer  34 , a second semi-conductive layer  35 , a second dielectric layer  36  and patterned semi-conductive layer  37 .  
         [0033]     In the present embodiment, the conductive structure  31  is on the substrate  30 ; the non-transparent structure  32  is on the substrate  30  and electrically insulates from the conductive structure  31  (this is, the non-transparent structure  32  needs to be insulated from the conductive structure  31  while it is made of conductors, otherwise both of them can be connected together); the first dielectric layer  33  covers the substrate  30 , the conductive structure  31  and the non-transparent structure  32 ; the first semi-conductive layer  34  is on the first dielectric layer  33 , especially, on the conductive structure  31  and the non-transparent structure  32 ; the second conductive  35  is on a portion of the first semi-conductive layer  34 , especially, on the conductive structure  31  and the non-transparent structure  32 ; the second dielectric layer  36  covers the substrate  30 , the first semi-conductive layer  34  and the second semi-conductive layer  35 , further has an open region on the second dielectric layer  36  to expose a portion of the second semi-conductive layer  35 ; the patterned semi-conductive layer  37  is on the second dielectric layer  36 , further above the non-transparent structure  32 , and is filled in the open region.  
         [0034]     Comparing both  FIG. 3A  and  FIG. 3B  with  FIG. 1 , the obvious characteristic in the present embodiment is the existence of the non-transparent structure  32 . Referring to well-known techniques, the light current may be induced in the portions of the first semi-conductive layer  13 , the second semi-conductive layer  14  and the conducting wire  16  while the light is transmitted from the substrate  10  and these portions are not over the conductive structure  11 . In accordance with the present embodiment, the non-transparent structure  32  can effectively block and reduce the light transmitted from the substrate  30  in the first semi-conductive layer  34 , the second semi-conductive layer  35  and the patterned semi-conductive layer  37 . Further, the use of the non-transparent can solve the drawbacks such as light current. Hence, there is no need to change the materials and layouts for the first dielectric layer  33 , the first semi-conductive layer  34 , the second conductive  35 , the second dielectric layer  36  and the patterned semi-conductive layer  37  in the well-known TFT.  
         [0035]     What is stressed here is that the present embodiment does not restrict the material to the non-transparent structure  32 , and it can be a conductor or a dielectric. However, in order to simplify the structure and the method of the present embodiment, the same material can be used in forming the non-transparent structure  32  and the conductive structure  31  (the gate); both the non-transparent structure  32  and the conductive structure  31  are formed at the same time while the semi-conductive layer covered on the substrate  30  is patterned (this is, adjusting the mask, which is used to form the gate, forms the non-transparent structure  32  and the conductive structure  31 ). Besides, a shorter distance taken electrically insulation into account between the non-transparent structure  32  and the conductive structure  31  is preferable in order to block the light as much as possible. For example, the TFT of the present embodiment is formed by following the method in accordance with the prior embodiment. This is, the non-transparent structure insulated form the gate is formed by the way as the gate formed, and at least positions under the remained portion of the first semi-conductive layer.  
         [0036]     Further, in order to increase the portions, which are over both the non-transparent structure  32  and the conductive structure  31 , of the first semi-conductive layer  34 , the second conductive  35  and the patterned semi-conductive layer  37  to lower the possibility of the light current occurred, as shown in  FIG. 3C  and  FIG. 3D , the present embodiment can make the portions over and along the edge of the conductive structure  31  wider than others no matter in the semi-conductive layers  34  or  35 ; the present embodiment also can make the portions over and along the edge of the non-transparent structure  32  wider than others no matter in the semi-conductive layers  34  or  35 .  
         [0037]     Still another embodiment in accordance with the present invention is the method of forming TFT, at least including the basic process steps described as below.  
         [0038]     As shown in  FIG. 4A , a substrate  40  is provided, and a conductive structure  41  and a non-transparent structure  42  are formed on the substrate  40 . Herein, there is no any restriction on the distance or location between the conductive structure  41  and the non-transparent structure  42  but electrically insulation to each other. The conductive structure  41  and the non-transparent structure  42  do not be connected together while both of them are conductors. However, both of them can be connected together, as shown in  FIG. 4B , while the non-transparent structure  42  is an insulator.  
         [0039]     As shown in  FIG. 4C , a first dielectric layer  43  is form on the substrate  40  and covers both the conductive structure  41  and the non-transparent structure  42 , and then a first semi-conductive layer  44  and a second semi-conductive layer  45  are formed in proper sequence on the first dielectric layer  43 .  
         [0040]     As shown in  FIG. 4D , the portions of the second semi-conductive layer  45  and the first semi-conductive layer  44  are removed, and the remained portions of the second semi-conductive layer  45  and the first semi-conductive layer  44  are at least over the conductive structure  41 . Of course, the present embodiment do not restrict the details of the pattern process but only requires to turn the structure shown in  FIG. 4C  into the one shown in  FIG. 4D .  
         [0041]     The first semi-conductive layer  44 , which is used as a channel to meet the TFT needs and is commonly made of semiconductor, is easy to induce the light current. Hence, the present embodiment makes the remained portion of the first semi-conductive layer  44  at least covers the portion of the non-transparent structure  42 . In addition, in order to reduce the change of the light transmitted from the substrate in the first semi-conductive layer  44 , the second conductive  45  and the patterned conductive structure  47 , the present embodiment can make the portions over and along the edge of the conductive structure  41  wider than others no matter in the semi-conductive layers  44  or  45 ; the present embodiment also can make the portions over and along the edge of the non-transparent structure  42  wider than others no matter in the semi-conductive layers  44  or  45 .  
         [0042]     As shown in  FIG. 4E , a second dielectric layer  46  is formed over the substrate  40  and covers the remained portions of the second semi-conductive layer  45  and the first semi-conductive layer  44 , and further forms an open region on the second dielectric layer  46  and exposes the portion of the second semi-conductive layer  45 . A patterned semi-conductive layer  47  is then formed on the second dielectric layer  46  and filled in the open region.  
         [0043]     One characteristic, obviously, of the present embodiment is the process shown in  FIG. 4A  to form the conductive structure  41  and the non-transparent structure  42  on the substrate. The possibility of the induced light current resulted from the light transmitted from the substrate  40  can be substantially reduced by disposing the layout of the conductive structure  41  and the non-transparent structure  42  to make the first semi-conductive layer  44  and the second semi-conductive layer  45  (even the patterned semi-conductive layer  47 ) not only over the substrate  40  but also over the non-transparent structure  42 .  
         [0044]     Although specific embodiments have been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from what is intended to be limited solely by the appended claims.
The thin film transistor has a non-transparent structure besides and insulated with the gate. Hence, the light transmitted from the substrate is blocked and the light current induced in the thin film transistor is negligible. The method uses a mask with a slit pattern to form a non-uniform photoresist. Hence, the mask could be used to pattern two conductor layers for forming source/drain/channel.