Abstract:
The invention provides a method for fabricating a TFT including a crystalline silicon active layer. The inventive method forms a metal offset region between the metal layer used to induce the cystallization of the active layer and the channel region of the TFT without introducing an additional process such as photoresist processing. Therefore, the inventive method improves the performance and manufacturing productivity of TFT and lower its production cost as well.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a TFT used for liquid crystal display (LCD) and organic light emitting diode (OLED), etc. More particularly, the present invention relates to a TFT including a crystalline silicon active layer providing the source, drain, and channel regions of the TFT, and to a method for fabricating a TFT including the crystalline silicon active layer.  
         BACKGROUND OF THE INVENTION  
         [0002]    Thin film transistor (TFT) used for display devices such as liquid crystal display (LCD) and organic light emitting diode (OLED) is formed by depositing a silicon layer on a transparent substrate such as a glass or quartz, forming a gate and a gate electrode on the silicon layer, implanting dopant in the source and the drain regions of the silicon layer, performing an annealing to activate the dopant, and finally forming an insulation layer thereon. An active layer constituting the source, drain, and channel regions of a TFT is formed by depositing a silicon layer on a transparent substrate such as glass by means of chemical vapor deposition (CVD) and the like. The silicon layer directly deposited on the substrate by CVD is an amorphous silicon layer, which has low electron mobility. As display devices using thin film transistors requires high operation speed and miniaturized structure, the integration degree of its driving ICs becomes higher and the aperture ratio of the pixel region becomes lower. Therefore, it is required increase the electron mobility of the silicon layer in order to form the driving circuit along with the pixel TFT and to enhance the pixel aperture ratio. For this purpose, technologies for forming a polycrystalline silicon layer having high electron mobility by crystallizing an amorphous silicon layer with thermal treatment have been in use as described below.  
           [0003]    Solid phase crystallization (SPC) method is used to anneal an amorphous silicon layer at a temperature of 600° C. or below for a few hours or tens of hours. 600° C. is the temperature causing deformation of the glass constituting the substrate. However, the SPC method has the following disadvantages. Since the SPC method requires a thermal treatment for a long time, the SPC method has low productivity. In addition, when annealing a large-sized substrate, the SPC method causes deformation of the substrate during the extended thermal treatment even at a temperature of 600° C. or below.  
           [0004]    Excimer laser crystallization (ELC) method locally generates a high temperature on the silicon layer for a very short time by scanning an excimer laser beam to instantaneously crystallize the silicon layer. However, the ELC method has the following disadvantages. The ELC method has difficulties in accurately controlling the scanning of the laser beam. In addition, since the ELC method processes only one substrate at a time, the ELC method has relatively low productivity as compared to a method wherein a plurality of substrates are processed in a furnace at one time.  
           [0005]    To overcome the aforementioned disadvantages of the conventional silicon crystallization methods, a method of inducing crystallization of an amorphous silicon layer at a low temperature about 200° C. by contacting or implanting metals such as nickel, gold, and aluminum has been proposed. This phenomenon that low-temperature crystallization of amorphous silicon is induced with metal is conventionally called as metal induced crystallization (MIC). However, this metal induced crystallization (MIC) method also has following disadvantages. If a TFT is manufactured by the MIC method, the metal component used to induce the crystallization of silicon remains in the crystallized silicon providing the active layer of the TFT. The metal component remaining in the active layer causes current leakage in the channel region of the TFT.  
           [0006]    Recently, a method of crystallizing a silicon layer by inducing crystallization of amorphous silicon in the lateral direction using a metal, which is conventionally refereed to as “metal induced lateral crystallization” (MILC), was proposed. (See S. W. Lee and S. K. Joo,  IEEE Electron Device Letter,  17(4), p. 160, 1996) In the metal induced lateral crystallization (MILC) phenomenon, metal does not directly cause the crystallization of the silicon, but the silicide generated by a chemical reaction between metal and silicon induces the crystallization of the silicon. As the cystallization proceeds, the silicide propagates in the lateral direction of the silicon inducing the sequential crystallization of the adjacent silicon region. As the metal causing this MILC, nickel and palladium or the like are known to those skilled in the art. Crystallizing a silicon layer by the MILC, a silicide containing crystallization inducing metal moves along the lateral direction as the crystallization of the silicon layer proceeds. Accordingly, little metal component is left in the silicon layer crystallized by the MILC. Therefore, the crystallized silicon layer does not adversely affect the current leakage or other characteristics of the TFT including the silicon layer. In addition, using the MILC, crystallization of silicon may be induced at a relatively low temperature of 300° C.˜500° C. Thus, a plurality of substrates can be crystallized in a furnace at one time without causing any damages to the substrates.  
           [0007]    [0007]FIG. 1A to FIG. 1D are cross-sectional views illustrating a conventional method for crystallizing a silicon active layer of TFT using the MIC and the MILC methods. Referring to FIG. 1A, an amorphous silicon layer  11  is formed on an insulation substrate  10  having a buffer layer (not shown) thereon. The amorphous silicon layer  11  is patterned by photolithography so as to form an active layer. A gate insulation layer  12  and a gate electrode  13  are formed on the active layer  11  by using conventional methods. As shown in FIG. 1B, the substrate is doped with impurity using the gate electrode  13  as a mask. Thus, a source region  11 S, a channel region  11 C and a drain region  11 D are formed in the active layer. As shown in FIG. 1C, photoresist  14  is formed to cover the gate electrode  13 , the source region  11 S and the drain region  11 D in the vicinity of the gate electrode  13 , and a metal layer  15  is deposited over the substrate  10  and the photoresist  14 . As shown in FIG. 1D, after removing the photoresist  14 , the entire substrate is annealed at a temperature of 300-500° C. As a result the source and drain regions  16  covered with the residual metal layer  15  are crystallized by the MIC caused by the metal layer  15 , and the region indicated by reference numeral  17  and consisting of the metal-offset part of the source and the drain regions and the channel region under the gate electrode  13  are respectively crystallized by the MILC propagating from the source and drain regions  16  covered with the metal layer  15   
           [0008]    The photoresist  14  is formed to cover a part of the source and the drain regions adjacent to the gate electrode  13  in order to prevent the current leakage in the channel region and the degradation of the operation characteristics of the same. If the metal layer  15  is formed to cover the entire source and drain regions, the current leakage and the degradation of the operation characteristics occur because the metal component used to cause the MIC remains in the channel region  11 C and the boundaries between the channel region and the source and the drain regions. Since the operation of the source and drain regions is not substantially affected by the residual metal component, the source and drain regions apart from the channel region by a distance over 0.01˜5 μm may be crystallized by the MIC directly caused by the MIC metal  15 . Meanwhile, the channel region and the source and the drain regions adjacent to the channel region are crystallized by MILC induced by and propagating from the MIC metal. Crystallizing only the channel region and its vicinity by MILC, the time required to crystallize the entire active layer may be significantly reduced. However, when using the process shown in FIGS. 1A to  1 D, a step of forming a photoresist layer, a step of patterning and removing the photoresist should be included in the conventional TFT fabrication process.  
         SUMMARY OF THE INVENTION  
         [0009]    Accordingly, one object of the present invention is to provide a method for manufacturing a large quantity of TFT&#39;s at a low cost by conducting annealing of a plurality of substrates utilizing MILC at a relatively low temperature compared to the SPC method and the ELC method.  
           [0010]    It is another object of the present invention to provide a TFT fabrication method according to which the metal component used to induce the MIC does not remain in the channel region of the TFT and does not comprise the processes of forming and removing the photoresist layer in contrast to the conventional TFT fabrication method using MILC.  
           [0011]    According to one feature of the invention, a method for fabricating a thin film transistor (TFT) including a silicon active layer comprising the steps of providing a substrate; depositing an amorphous silicon layer on the substrate to provide an active layer of the TFT; forming a gate insulation layer and a gate electrode on the amorphous silicon layer; doping the amorphous silicon layer with impurity to form a source region, a drain region and a channel region of the active layer and depositing a crystallization inducing metal on at least a portion of the amorphous silicon layer; and crystallizing the amorphous silicon layer crystallization by conducting thermal treatment thereof, and being characterized in that the crystallization inducing metal is offset from the channel region of the active layer using a mask which does not include a photoresist layer is provided.  
           [0012]    According to another feature of the invention, the crystallization inducing metal is offset from the channel region of the active layer using a photoresist mask which is formed by processing a photoresist layer used to pattern the gate electrode and remaining on the top of the patterned gate electrode.  
           [0013]    Additional features and advantages of the present invention will be set forth or will be apparent from below detailed description of the invention. The objectives and other advantages of the invention will be realized and attained by the scheme particularly pointed out in the written description and claims hereof as well as the appended drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The embodiments of the present invention will be explained with reference to the accompanying drawings, in which:  
         [0015]    [0015]FIG. 1A to FIG. 1D are cross-sectional views illustrating the sequence of the conventional method for fabricating a crystalline silicon TFT by using MILC;  
         [0016]    [0016]FIG. 2A to FIG. 2F are cross-sectional views illustrating the method for fabricating a crystalline silicon TFT according to a preferred embodiment of the present invention;  
         [0017]    [0017]FIG. 3A to FIG. 3E are cross-sectional views illustrating the method for fabricating a crystalline silicon TFT according to another preferred embodiment of the present invention;  
         [0018]    [0018]FIG. 4A to FIG. 4E are cross-sectional views illustrating the method for fabricating a crystalline silicon TFT according to still another preferred embodiment of the present invention;  
         [0019]    [0019]FIG. 5A to FIG. 5D are cross-sectional views illustrating the method for fabricating a crystalline silicon TFT according to still another preferred embodiment of the present invention;  
         [0020]    [0020]FIG. 6A to FIG. 6D are cross-sectional views illustrating the method for fabricating a crystalline silicon TFT according to still another preferred embodiment of the present invention; and  
         [0021]    [0021]FIG. 7A to FIG. 7E are cross-sectional views illustrating the method for fabricating a crystalline silicon TFT according to still another preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0023]    [0023]FIGS. 2A to  2 F are cross-sectional views illustrating the method for fabricating a crystalline silicon TFT by using MILC according to a first preferred embodiment of the present invention. Referring to FIG. 2A, an amorphous silicon layer  21  to provide an active layer of a TFT is formed and patterned on an insulation substrate  20 . The substrate  20  is made of insulating material such as Corning 1737 glass, quartz, silicon oxide or oxidized silicon wafer. According to needs, an optional buffer layer (not shown) may be formed on the substrate in order to prevent the diffusion of contaminants from the substrate  20 . The active layer  21  is formed by depositing amorphous silicon with a thickness in the range of 100 Å to 3,000 Å, preferably with a thickness in the range of 500 Å to 1,000 Å using PECVD, LPCVD or sputtering method. The active layer  21  includes source, drain and channel regions and may also include areas reserved for other devices and electrodes. The active layer  21  is patterned to match the size of the TFT to be fabricated.  
         [0024]    [0024]FIG. 2B illustrates a cross-section of the structure in which a gate insulation layer  22  and a gate electrode  23  are formed on the substrate  20  and the active layer  21 . As shown in FIG. 2B, the gate insulation layer  22  is formed by depositing SiO 2 , SiNx, SiOxNy or a combination thereof with a thickness in the range of 300 Å to 3,000 Å, preferably with a thickness in the range of 500 Å to 1,000 Å using various deposition methods such as PECVD, LPCVD, APCVD and ECR CVD. The gate electrode layer consisting of conductive material such as metal and doped polysilicon is formed on the gate insulation layer  22  by sputtering, heating evaporation, PECVD, LPCVD, APCVD, or ECR CVD, and it is patterned to form the gate electrode  23 . The gate electrode layer is formed with a thickness in the range of 1,000 Å to 8,000 Å, preferably with a thickness in the range of 2,000 Å to 4,000 Å. The gate insulation layer  22  and the gate electrode  23  are patterned and etched using a mask. By over-etching the gate electrode  23  during the etching process the outer edges of the gate insulation layer  22  are not covered by the gate electrode  23 .  
         [0025]    [0025]FIG. 2C is a cross-sectional view illustrating the process of doping the source region  21 S and the drain region  21 D of the active layer  21  using the gate electrode  23  as a mask. For fabricating a NMOS (N-channel metal oxide semiconductor) TFT, the active layer is doped with dopant such as PH 3 , P and As with a dose of 1E11˜1E22/cm 3  (preferably 1E15˜1E21/cm 3 ) at the energy level of 10˜200 KeV (preferably 30˜100 KeV) using ion shower doping method or ion implantation method, etc. For fabricating a PMOS (P-channel metal oxide semiconductor) TFT, the active layer is doped with dopant such as B 2 H 6 , B and BH 3  with a dose of 1E11˜1E22/cm 3  (preferably 1E14˜1E21/cm 3 ) at the energy level of 10˜200 KeV. When a lightly doped region is not formed in the active layer, the doping energy is controlled so that the active layer region cover with the exposed portion of the gate insulation layer  22  is doped with high concentration of the dopant. When necessary, the dopant injection can be performed in two stages.  
         [0026]    [0026]FIG. 2D is a cross-sectional view illustrating that a metal layer  24  is formed on the structure as illustrated in FIG. 2C. Preferably, the metal layer is formed of Ni. It should be noted that the metal layer  24  is offset from the channel region  21 C covered with the gate insulation layer  22  and the gate electrode  23 . Therefore, this process does not require additional photoresist process in order to offset the metal layer  24  from the channel region  21 C. For the metal layer, various metals such as Pd, Ti, Ag, Au, Al, Sn, Sb, Cu, Co, Cr, Mo, Tr, Ru, Rh, Cd, Pt or their combination may also be used. The crystallization inducing metal may be deposited by sputtering, heating evaporation, PECVD or ion implantation. However, sputtering method is most preferably used. The thickness of the metal layer  24  can be freely selected within a range that is adequate to induce the cystallization of the active layer. Generally, the metal layer  24  is formed with a thickness in the range of 1 Å to 10,000 Å, preferably with a thickness in the range of 10 Å to 200 Å.  
         [0027]    Then, as illustrated in FIG. 2E, thermal treatment is conducted to induce the crystallization of the active layer  21  and to active the dopant injected into the source region  21 S and the drain region  21 D at the same time. The thermal treatment can be conducted by RTA (rapid thermal annealing), ELC (excimer laser crystallization) method or furnace annealing. The RTA method heats the substrate at a temperature about 700° C.˜800° C. for a few seconds or a few minutes using a tungsten-halogen lamp or a xenon arc lamp. The  4 ELC method heats the active layer to a very high temperature for a very shot time using an excimer laser. Particularly, in a preferred embodiment of the invention, the thermal treatment is performed in a furnace at a temperature about 300° C.˜700° C. for 0.1˜50 hours, desirably for 0.5˜20 hours. During the furnace annealing, the active layer is crystallized by MIC and MILC induced by the metal deposited on the active layer. As described above with reference to FIGS. 1A to  1 D, the amorphous silicon in contact with the metal layer is crystallized by MIC and the remaining portion of the active layer is crystallized by MILC. After the thermal treatment, the metal layer  24  may be either removed by etching or remained. Since the thermal treatment condition for crystallizing the amorphous silicon layer by the MILC is similar to the annealing condition used to activate the dopant implanted in the active layer, the crystallization of the active layer and the activation of the dopant may be conducted simultaneously.  
         [0028]    Then, as illustrated in FIG. 2F, a contact insulation layer  25  is formed over the substrate to cover the active layer and the gate electrode, and contact holes are formed to expose portions of the source region and the drain region using conventional methods. Through the contact holes, contact electrodes are formed to provide electrical connection to the TFT elements.  
         [0029]    [0029]FIG. 3A to FIG. 3E are cross-sectional views illustrating the process of fabricating a crystalline silicon TFT by MILC according to another preferred embodiment of the present invention. Referring to FIG. 3A, an amorphous silicon layer  31  providing the active layer of a TFT is formed and patterned on an insulation substrate  30 . A gate insulation layer  32  and a lower gate electrode  33  and a upper gate electrode  34  are formed on the amorphous silicon layer  31 . FIG. 3B illustrates a process of conducting a high-concentration doping of the amorphous silicon layer  31  using the upper gate electrode  34  as a mask to form a source region  31 S and a drain region  31 D. For fabricating a NMOS (N-channel metal oxide semiconductor) TFT, the active layer is doped with dopant such as PH 3 , P and As with a dose of 1E11˜1E22/cm 3  (preferably 1E15˜1E21/cm 3 ) at the energy level of 10˜200 KeV (preferably 30˜100 KeV) using ion shower doping method or ion implantation method, etc. For fabricating a PMOS (P-channel metal oxide semiconductor) TFT, the active layer is doped with dopant such as, B 2 H 6 , B and BH 3  with a dose of 1E11˜1E22/cm 3  (preferably 1E14˜1E21/cm 3 ) at the energy level of 10˜200 KeV. When a lightly doped region is not formed in the active layer, the doping energy is controlled so that the active layer region cover with the exposed portion of the gate insulation layer  22  is doped with high concentration of the dopant. When necessary, the dopant injection can be performed in two stages.  
         [0030]    Then, as shown in FIG. 3C, a metal layer  35  such as Ni layer for inducing the crystallization of the amorphous silicon layer  31  using the upper gate electrode  34  as a mask. For the metal inducing the crystallization of the amorphous silicon layer  31 , Ni or Pd is preferably used. Also, other metal such as various metals such as Ti, Ag, Au, Al, Sn, Sb, Cu, Co, Cr, Mo, Tr, Ru, Rh, Cd, Pt or their combination may be used. The crystallization inducing metal may be deposited by sputtering, heating evaporation, PECVD or ion implantation. However, sputtering method is most preferably used. The thickness of the metal layer  35  can be freely selected within a range that is adequate to induce the crystallization of the active layer. Generally, the metal layer  35  is formed with a thickness in the range of 1 Å to 10,000 Å, preferably with a thickness in the range of 10 Å to 200 Å.  
         [0031]    Since the width of the upper gate electrode  34  is wider than that of the lower gate electrode  33 , the crystallization inducing metal layer  35  is not formed on the portions of the active layer masked by the upper gate electrode  34 . Thus, the crystallization inducing metal  35  becomes offset from the channel region  31 C. As described above, if the crystallization inducing metal directly contacts with the channel region  31 C without the metal offset region, the residual metal component remaining in the channel region  31 C after the crystallization by MIC and MILC tend to deteriorate the performance of the TFT. In the present invention, the width of the metal offset region is set within the range of 1,000 Å to 20,000 Å, more preferably within the range of 5,000 Å to 10,000 Å. Within the scope of the present invention, the order of the high-concentration doping process and the metal layer forming process may be reversed.  
         [0032]    After forming the crystallization inducing metal layer  35 , the upper gate electrode  34  is removed as depicted in FIG. 3D. Then, as illustrated in FIG. 3E, the active layer is crystallized by a thermal treatment, and an insulation layer and contact electrode (not shown) are formed thereon to complete the TFT fabrication. The thermal treatment of FIG. 3E may use any methods that may induce the crystallization of amorphous silicon. For example, RTA (rapid thermal annealing) or ELC (excimer laser crystallization) method may be used for the thermal treatment of the amorphous silicon active layer. The RTA method performs heating at a temperature about 700° C.˜800° C. for a few seconds or a few minutes using a tungsten-halogen lamp or a xenon arc lamp. The ELC method heats the active layer to a very high temperature for a very shot time using an excimer laser. Particularly, in the present invention, the heating process is preferably performed in a furnace at a temperature of 300° C.˜700° C. for 0.1˜50 hours, desirably for 0.1˜50 hours. Since the furnace annealing method crystallizes the amorphous silicon at a temperature lower than the deformation temperature of the glass substrate, deformation or damage of the glass substrate may be prevented. In addition, because the furnace annealing method may process a plurality of substrates at one time, it substantially enhances the productivity of the process. During the thermal treatment, the portions of the silicon layer covered with the MIC metal are crystallized by the MIC cause by the MIC source metal and the portion of the silicone layer not covered with the MIC metal is crystallized by the MILC propagated from the MIC region to which the MIC metal is applied. In addition, since the thermal treatment condition for crystallizing the amorphous silicon layer by the MIC is similar to the annealing condition used to activate the dopant implanted in the active layer, the crystallization of the active layer and the activation of the dopant may be conducted simultaneously.  
         [0033]    When a TFT is fabricated by the aforementioned method, a metal offset region can be formed without including the process of forming a photoresist pattern in order to offset the MIC metal from the channel region. The TFT fabricated according to the method of the present invention has advantages of low leakage current and stable electrical characteristics. Although the above preferred embodiments of the present invention was described for the purpose of illustration, the present invention can be also embodied as other embodiments that will be described below. The process conditions for other embodiments are the same as those of the above embodiments unless mentioned otherwise in this specification.  
         [0034]    [0034]FIG. 4A to FIG. 4B are cross-sectional views illustrating the process for fabricating a crystalline silicon TFT according to another preferred embodiment of the present invention. Referring to FIG. 4A, an amorphous silicon layer  41  providing the active layer of a TFT is formed and patterned on an insulation substrate  40 . A gate insulation layer  42  and a gate electrode  43  are formed on the amorphous silicon layer  41 . As shown in FIG. 4A, the photoresist  44  used to form the gate electrode  43  is remained on the gate electrode, the gate electrode  43  is over-etched to form an undercut below the photoresist  44 . Then, as illustrated in FIG. 4B, a crystallization inducing metal layer  45  is formed using the photoresist  44  as a mask. Since the metal layer  45  is not deposited on the portions of the amorphous silicon layer masked by the photoresist, a metal offset region can be formed around the channel region under the gate electrode  43 . Then the photoresist  44  is removed as illustrated in FIG. 4C.  
         [0035]    Then a high-density doping is performed to form a source region  41 S, a drain region  41 D and a channel region as shown in FIG. 4D. The doping energy is controlled so that the dopant is injected with a high concentration and the formation of a lightly doped region is prevented. Thereafter, the active layer is crystallized by thermal treatment and a TFT is completed according to conventional methods. Again, the order of the doping process and the metal layer forming process may be reversed.  
         [0036]    [0036]FIG. 5A to FIG. 5E are cross-sectional views illustrating the method for fabricating a crystalline silicon TFT according to another preferred embodiment of the present invention. Referring to FIG. 5A, an amorphous silicon layer  51  providing the active layer of a TFT is formed and patterned on an insulation substrate  50 . A gate insulation layer  52  and a gate electrode  53  are formed on the amorphous silicon layer  51 . After formation of the gate electrode  53  doping of the amorphous silicon layer  51  is conducted using the gate electrode  53  as a mask to form a source region  51 S, a drain region  51 D and a channel region  51 C as shown in FIG. 5B. The gate electrode  53  is anodized to form an anodic oxide layer  54  on the surface of the gate electrode  53  as shown in FIG. 5C. Then, as shown in FIG. 5C, crystallization inducing metal layer  55  is formed on the entire surface of the substrate. At this time a metal offset region is formed around the gate electrode  53  due to the anodic oxide layer  54 . Then a thermal treatment is conducted to crystallize the active layer as shown in FIG. 5D. Thereafter, a TFT is completed using conventional methods. As mentioned above, the order of the doping process and the metal layer forming process may be reversed within the scope of the present invention.  
         [0037]    [0037]FIG. 6A to FIG. 6E are cross-sectional views illustrating the method for fabricating a crystalline silicon. TFT according to another preferred embodiment of the present invention. Referring to FIG. 6A, an amorphous silicon layer  61  providing the active layer of TFT is formed and patterned on an insulation substrate  60 . A gate insulation layer  62  and the gate electrode  63  are formed on the amorphous silicon layer  61 . Referring to FIG. 6A, the photoresist  64  used to pattern the gate electrode  63  remains on the gate electrode. In this state, impurity doping is conducted using the photoresist  64  as a mask thereby a source region  61 S, a drain region  61 D and a channel region  61 C are formed in the active layer. The photoresist  64  is reflowed at a temperature higher than the hard baking temperature of the photoresist. As a result, referring to FIG. 6B, the re-flowed photoresist becomes to have a shape of a hemisphere covering the gate insulation layer  62 , the gate electrode  63  and a part of the amorphous silicon layer.  
         [0038]    Then, as shown in FIG. 6C, a crystallization inducing metal layer  65  is formed on the entire area of the substrate using the re-flowed photoresist  64  as a mask. Then, as shown in FIG. 6D, the re-flowed photoresist  64  is removed. Since the metal layer  65  was not deposited on the portions of the active layer had been covered with the re-flowed photoresist, a metal offset region can be formed around the gate electrode  63 . As mentioned above, the order of the doping process and the metal forming process may be reversed within the scope of the present invention. Thereafter, thermal treatment is performed to crystallize the active layer, and a TFT is completed using conventional methods.  
         [0039]    [0039]FIG. 7A to FIG. 7G are cross-sectional views illustrating the method for fabricating a crystalline silicon TFT according to another preferred embodiment of the present invention. As shown in FIG. 7A, an amorphous silicon layer  71  providing the active layer of a TFT is formed and patterned on an insulation substrate  70 . A gate insulation layer  72  and a gate electrode  73  are formed on the amorphous silicon layer  71 . In this state, impurity doping is conducted using the gate electrode  64  as a mask thereby a source region  61 S, a drain region  61 D and a channel region  61 C are formed in the active layer. Then, as shown in FIG. 7B, an insulation layer  74  is formed to cover the entire surface of the substrate. When an anisotropic etching is conducted on the structure of FIG. 7B, the residual portions of the insulation layer  74  remains as a “sidewall”  75  on lateral surfaces of the gate insulation layer  72  and the gate electrode  73  as shown in FIG. 7C. Generally the width of the sidewall tends to increase toward the downward direction. Using this sidewall  75  as a mask, a crystallization metal layer  76  is formed over the substrate to create a metal offset region around the channel region  71 C (FIG. 7D), and a thermal treatment for crystallizing the active layer is conducted using a method as mentioned above (FIG. 7E). Thereafter, a TFT is completed according to conventional methods.  
         [0040]    The present invention provides a method for crystallizing the active layer of TFT in a large quantity using a furnace at a relatively low temperature than RTA and ELC. Particularly, the method prevents deformation or damages to the TFT substrate because the active layer of TFT is annealed in the temperature range of 300˜700° C. lower than the deformation temperature of glass. According to the method, the annealing process for activating the dopant and the crystallizing of the active layer is conducted in a single process to simplify and expedite the TFT fabrication process.  
         [0041]    The method of the present invention forms a metal offset region without using a process of forming and removing photoresist. Thus, compared to the conventional method, the present invention may efficiently offset the MILC source metal from the channel region without using additional photoresist process.  
         [0042]    Although representative embodiments of the present invention have been disclosed for illustrative purposes, those who are skilled in the art will appreciate that various modifications, additions and substitutions are possible without departing from the scope and spirit of the present invention as defined in the accompanying claims and the equivalents thereof.