Abstract:
A method of fabricating a thin film transistor (TFT) includes first providing a strip-shaped silicon island which is a thin film region with a predetermined long side and short side. Next, the strip-shaped silicon island is subject to an ion implantation to form a first ion doping region and a second ion doping region. The first and second ion doping regions, respectively used as the source and the drain of the TFT, are located at two sides along the long side of the island and substantially perpendicular to the gate. A gate is formed over the strip-shaped silicon island and the first and second ion doping regions, wherein the gate is substantially parallel to the direction of the short side.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of and claims priority benefit of an application Ser. No. 11/561,898, filed on Nov. 21, 2006, now pending, which claims the priority benefit of Taiwan application serial no. 95107552, filed on Mar. 7, 2006. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a thin film transistor structure. More particularly, the present invention relates to an arrangement of a gate structure of a thin film transistor. 
     2. Description of Related Art 
     One of various techniques for fabricating thin film transistors is a lateral grain growth technique named heat retaining layer enhanced crystallization (HREC).  FIG. 1  is a schematic cross-sectional view of the technique. As shown in  FIG. 1 , a strip-shaped amorphous silicon island pattern  102  is formed on a substrate  100 , and a heat retaining layer (HRL)  104  is then formed thereon. Thereafter, the strip-shaped amorphous silicon island layer is subject to a laser annealing procedure, such that the strip-shaped amorphous silicon island layer is melted and induced to produce a super lateral grain grown crystalline silicon. The method takes the advantage of the characteristic that the HRL is capable of partially absorbing a particular laser spectrum. Thus, after a part of laser energy is absorbed by the HRL, a continuous supplementary heating effect is exerted on the amorphous silicon, thereby facilitating reducing the cooling speed of the melted silicon during laser crystallization. Accordingly, a longer lateral grain growth is achieved. 
       FIG. 2  is a top view of a thin film transistor (TFT) fabricated by the conventional definition. First, a silicon island  10  is formed by using the aforementioned manner. As shown in  FIG. 2 , the crystalline silicon island formed through laser annealing has a longitudinal primary grain boundary P in the middle, and secondary grain boundaries, also referred to as a lateral grain boundary, being substantially perpendicular to the longitudinal primary grain boundary P. During the fabrication of the transistor, the electric current is designed to flow in a direction parallel to the grain boundary in order to make the current flowing smoothly. Therefore, as shown in  FIG. 2 , a gate  20  is located over the longitudinal primary grain boundary P, while source/drain regions  24  are located at the left and right sides of the primary grain boundary P. In addition, a gate contact  22  is further electrically connected to the gate  20 . With this structure, after the transistor is turned on, the current I flows in a direction parallel to the grain boundary, and is only blocked by the primary grain boundary P in the midway. As such, high carrier mobility and superior device quality can be obtained. 
     Although such a design can provide better device properties, the active region of the amorphous silicon must be patterned first in the technique of using the heat retaining layer enhanced crystallization (H-REC) to obtain the lateral grown polysilicon. The width W of the amorphous silicon island is limited to be greater than the channel length L due to the above step, such that the direction of the lateral grain growth is parallel to the direction of the current flow, and high carrier mobility and control of the grain position can be obtained. In other words, the conventional method of defining the channel length L by a gate region and defining the channel width W by the dimension of the active region limits the device dimension, i.e., the width is larger than the length. Therefore, the device dimension has only few variations in circuit design. 
     In summary, the TFT manufactured by the conventional technique described above is limited to that the width W thereof must be larger than the channel length L, which results in low design freedom of device dimensions. Furthermore, if the width of the channel region of component is intended to be smaller than the length, an additional mask and photolithography process must be added. Thus, the process cost is increased due to the expensive mask and the complex process procedures. 
     Accordingly, it has become an urgent matter, under the current grain growth method, how to enhance the design freedom of device dimension so as to increase the adaptability, and how to change the device dimension without adding additional masks so as to reduce the process cost. 
     SUMMARY OF THE INVENTION 
     In view of the above, the present invention is directed to a TFT structure and a method of fabricating the same, for increasing the design freedom of the device dimension without increasing the manufacturing cost. 
     The present invention is further directed to a TFT structure and a method of fabricating the same, for improving the low yield due to drift caused by the photolithographic and etching process, so that the process tolerance is enhanced and thereby the yield is increased. 
     The present invention is further directed to a TFT structure and a method of fabricating the same, for increasing the design freedom of the device dimension without increasing the manufacturing cost, and further enhancing the device reliability. 
     In view of the above, the present invention provides a thin film transistor (TFT) structure, which includes a strip-shaped silicon island, a gate, a first ion doping region and a second ion doping region. The strip-shaped island is a thin film region with a predetermined long side and short side, and has a plurality of lateral grain boundaries substantially parallel to the direction of the short side of the silicon island. The gate is located over the longitudinal silicon island and substantially parallel to the lateral grain boundaries. The first and the second ion doping regions, used as a source and a drain regions of the TFT, are located at two sides along the long side of the strip-shaped silicon island and substantially perpendicular to the gate. 
     In the TFT structure described above, the strip-shaped silicon island can be, for example, a polysilicon island. The polysilicon island can further include a primary grain boundary located at the center of the strip-shaped silicon island and is substantially perpendicular to the plurality of lateral grain boundaries. The long side of the gate is substantially perpendicular to the primary grain boundary. 
     According to an embodiment of the present invention, the aforementioned TFT structure can be formed as a dual-gate structure. That is, the aforementioned TFT structure can further include a third ion doping region located in the strip-shaped silicon island and below the gate, and the third ion doping region is substantially parallel to the first and second ion doping regions. In addition, the third ion doping region can be substantially located at a position consistent with the primary grain boundary. 
     According to an embodiment of the present invention, in order to further enhance the reliability of the TFT, the first and second ion doping regions are respectively located on a portion of both sides of the strip-shaped silicon island, and the first and the second ion doping region are misaligned with each other. 
     According to an embodiment of the present invention, the TFT structure can further include a gate contact, a source contact and a drain contact, which are respectively connected to the gate, the first ion doping region and the second ion doping region. 
     Additionally, the present invention also provides a method of fabricating a TFT. A strip-shaped silicon island is provided, which is a thin film region with a predetermined long side and short side, or further has a plurality of lateral grain boundaries substantially parallel to the short side. Then the strip-shaped silicon island is subject to an ion implantation to form a first ion doping region and a second ion doping region. The first and second ion doping regions, respectively used as the source and the drain of the TFT, are located at two sides along the long side of the island and substantially perpendicular to the gate. A gate is formed over the strip-shaped silicon island and the first and second ion doping regions, and the gate is substantially parallel to the lateral grain boundaries. 
     According to an embodiment of the present invention, the strip-shaped silicon island can further includes a primary grain boundary, and the primary grain boundary is located at the center of the longitudinal silicon island and is substantially perpendicular to the plurality grain boundaries. When using a polysilicon thin film, the direction of the long side of the gate is substantially perpendicular to the primary grain boundary. 
     The aforementioned method of fabricating a TFT can further include forming a third ion doping region located in the strip-shaped silicon island and below the gate, in which the third ion doping region is substantially parallel to the first and second ion doping regions. The third ion doping region can be substantially formed at a position consistent with the primary grain boundary. 
     In the aforementioned method of fabricating a TFT, the first and second ion doping regions are respectively formed on a portion of both sides of the strip-shaped silicon island, and the first and second ion doping regions are misaligned with each other. 
     The aforementioned method of fabricating a TFT can further comprise forming a gate contact, a source contact and a drain contact respectively electrically connected to the gate, the first ion doping region and the second ion doping region. 
     With the TFT structure and the method of fabricating the same described above, a problem existing in the conventional crystallization technique that an active region must be patterned in advance for the crystallization and an additional mask must be added to etch the device so as to satisfy the requirement for the device having a length greater than the width, is thus solved. Therefore, the present invention utilizes the arrangement of the gate to define the channel width of the components, and the size of the active region (the strip-shaped silicon island) is the channel length of the device. Accordingly, the limitations to the device dimension can be eliminated without increasing the number of the masks in the process, thus the number of the masks can be saved and the variability and reliability of the device designs can also be enhanced. 
     In order to the make the aforementioned and other features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of the crystallization of silicon grown by using the heat retaining layer. 
         FIG. 2  is a top view of a transistor of the conventional art. 
         FIG. 3  is a top view of a transistor according to an embodiment of the present invention. 
         FIG. 4  is a top view of a variation of the transistor in  FIG. 3 . 
         FIG. 5  is a top view of a transistor according to another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The concept of the present invention is to redefine and design the arrangement of the gate to allow the design of device dimension to have larger freedom. 
       FIG. 3  is a top view of a transistor according to an embodiment of the present invention. First, a strip-shaped silicon island  10  is formed in the manner described above. As shown in  FIG. 3 , the strip-shaped silicon island formed by laser annealing is a thin film region with predetermined long side and short side. The strip-shaped silicon island  10  can be a polysilicon island, and the polysilicon island  10  is used as an example below. The polysilicon island  10  has a longitudinal primary grain boundary P at the center and lateral grain boundaries also named secondary grain boundaries substantially perpendicular to the longitudinal primary grain boundary P. The steps above are the same as those in the conventional art. 
     Thereafter, a gate  30  is formed over the primary grain boundary P and has the long side substantially perpendicular to the primary grain boundary P. A gate contact  32  is electrically connected to the gate  30  and located at one end of the gate  30  to provide a control signal to the gate  30  for controlling the on/off of the transistor. 
     The ion doping regions  44 ,  48 , used as the source/drain regions of the transistor, are respectively located at two sides of the silicon island  10 , i.e., two sides of the primary grain boundary P, and are respectively electrically connected to the source and drain contacts  42 ,  46 . 
     With the structure as shown in  FIG. 3 , when the transistor is turned on, the current remains in the direction substantially parallel to the grain boundaries, and is only blocked by the primary grain boundary P in the midway. However, since the location of the gate  30  is revolved by 90 degrees, i.e., substantially perpendicular to the extension of the primary grain boundary P, the length L of channel region below the gate  30  and between the source and drain regions  44 ,  48  becomes larger than the width W. 
     Therefore, it is apparent from the above embodiment that by readjusting the location of the gate  30 , a structure with the length L of channel region larger than the width W can be obtained according to the present invention without limitation of the conventional structure that the width W must be larger than the length L. Namely, in the present invention, the location of the gate  30  and the short side of the gate are used to define the effective width W of the channel region of the component, and the size of the silicon island  10  and the pitch between the ion doping regions  44 ,  48  are used to define the effective length L of the channel region of component. With the defining method, the device dimension is increased and the variability of the circuit design is enhanced. Additionally, in this structure an additional mask is not necessary for redefining the width and length of the channel region, thus reducing the overall manufacturing cost. 
     In addition, according to the structure provided by the present invention, errors of photolithographic draft and pattern shift can be reduced, making it feasible to fabricate devices on a flexible substrate. Description is made hereafter with the comparison between  FIG. 2  and  FIG. 3 . In the conventional structure as shown in  FIG. 2 , a gate  20  is located along the extension of the primary grain boundary P, and the source/drain regions  24  are located at two sides of the gate  20 . The region covered by the gate is the channel region. During the patterning, the gate  20  cannot cover the source/drain regions  24  if a position shifting of the gate  20  exists, such that the channel region contains intrinsic polysilicon with high impedance. Hence, the gate must be precisely patterned according to the conventional structure. On the contrary, according to the structure of the present invention shown in  FIG. 3 , even though a position shifting occurs during patterning the gate  30  and the gate  30  is shifted upward or downward, the final gate structure  30  is still across the source and drain regions  44  and  48 , without affecting the portion of channel region. Therefore, the transistor structure of the present invention can improve the yield. 
       FIG. 4  is a top view of a variation of the transistor as shown in  FIG. 3 . The difference between the structures of  FIG. 4  and  FIG. 3  lies in that the ion doping regions  44 ′ and  48 ′ of the source and drain regions in  FIG. 4  are respectively only formed on a portion of both sides of the silicon island  10  instead of the region of whole sides. The source/drain doping regions  44 ′ and  48 ′ are respectively located on a portion of the two sides of the polysilicon island  10  and are misaligned with each other. The structure and the dimension characteristics (the width and length of the channel) in  FIG. 4  are essentially the same as those in  FIG. 3 , and the difference lies in the sizes of the source/drain doping regions. 
     With the structure of  FIG. 3 , a leakage current for example I′ shown in  FIG. 4  might occur in some circumstance. However, with the structure of  FIG. 4 , the current leakage can be prevented because the source/drain doping region  48 ′ on the left of  FIG. 4  is not doped on the whole left side of the silicon island  10 . Similarly, only a portion of the right side region of the silicon island  10  in  FIG. 4  forms the source/drain doping region  44 ′. With the structure, the source/drain doping region away from the gate  30  is formed on one side of the silicon island  10 , so the occurrence of current leakage is further reduced. 
       FIG. 5  is a top view of a transistor according to another embodiment of the present invention.  FIG. 5  shows a design structure of forming a so-called dual-gate device by adding a doping region. As shown in  FIG. 5 , one doping region  34  is further formed over the primary grain boundary P of the poly-silicon  10 , serving as the third source/drain doping region. A dual-gate device structure is formed accompanied with the gate region  30  above. The L 1  and L 2  in  FIG. 5  respectively indicate two effective channel lengths. 
     With the structure described above, since the current flowing through the channel region completely passes the grain boundary only without being blocked by the primary grain boundary P after the transistor is turned on, the device reliability is further enhanced. A modification of the structure in  FIG. 3  is illustrated in  FIG. 5 , which becomes a dual-gate structure. Also, the device in  FIG. 4  can be modified to a dual-gate structure in the same manner. The corresponding drawing can be derived for those skilled in the art based on the modification in  FIG. 5 , and will not be provided herein. 
     Furthermore, although the HRC technique is employed to fabricate the polysilicon thin films in the above embodiment, the present invention is not limited thereto. For example, the device structure provided by the present invention, i.e., the rearrangement of the gate, can also be applied to the lateral grain growth mechanism. During the grain growth, the lateral grain growth mechanism does not first form a primary grain boundary P at the middle, but the grain are laterally grown from one side of the pre-patterned amorphous silicon island to the other, such that the crystalline orientation of the polysilicon island finally assumes a lateral grain boundary. When the present invention is implemented, the long side of the gate is only disposed along the crystalline orientation. 
     In view of the above, the limitation that the short side of the silicon island must be smaller than the long side to generate a crystallization in which the lateral grain growth direction is parallel to the current direction can be eliminated by changing the arrangement of the gate and using a new definition to define the width and length of the channel. Further, the width of the channel can be made smaller than the length without adding additional masks, thus reducing the manufacturing cost and number of masks. Since the position of the gate is changed, the problem resulting from the shift of the photolithographic process in the conventional structure is reduced. 
     Therefore, the present invention employs a novel arrangement manner of the gate and define manner of the device dimension, and uses the original number of masks and the arrangement of the gate. In this manner, the limitation to the device dimension is eliminated, and thereby the application of the technique is increased. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.