Abstract:
A method of forming a low temperature polysilicon thin film transistor includes steps of: providing a substrate; forming a polysilicon layer on the substrate; forming a gate oxide layer on the polysilicon layer; forming a photoresist pattern on the gate oxide layer; using the photoresist pattern as a mask and etching the gate oxide layer and the polysilicon layer; removing the photoresist pattern; forming a gate on the gate oxide layer; and implanting dopants to form source/drain region by using the gate as a mask.

Description:
[0001]    This application claims the benefit of Taiwan application Ser. No. 92117888, filed Jun. 30, 2003. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates in general to a method for fabricating a low temperature polysilicon thin film transistor, and more particularly to a method of fabricating a low temperature polysilicon thin film transistor without residues of photoresist and chemicals.  
           [0004]    2. Description of the Related Art  
           [0005]    Polysilicon and amorphous silicon are two commonly used materials for thin film transistors (TFTs). Low temperature polysilicon TFTs have the advantages of higher electron mobility and drive current over amorphous TFTs. Therefore, the development and improvement of low temperature polysilicon TFTs fabrication process are now of great demand. Within the field of improving the low temperature polysilicon TFTs fabrication process, residues of photoresist and chemicals during the photolithography and etching process is an important issue.  
           [0006]    [0006]FIGS. 1A to  1 J are cross-sectional views showing the sequential process steps of a conventional method for fabricating a low temperature polysilicon thin film transistor. Referring to FIG. 1A, a buffer layer  102  is first formed on a substrate  100  and a polysilicon layer  104  is then formed on the buffer layer  102 . The polysilicon layer  104  is formed by annealing amorphous silicon, using excimer laser. Next, a patterned photoresist layer  105  is formed on the polysilicon layer  104 . The polysilicon layer  104  is etched using the patterned photoresist layer  105  as a mask. The photoresist layer  105  is then removed by using chemicals, such as wet dip. The structure after wet dip is shown in FIG. 1B.  
           [0007]    Referring to FIG. 1C, a gate oxide layer  108  is formed over the polysilicon layer  104 , and a conductive layer is disposed on the gate oxide layer  108 . Photolithography and etching processes are then performed to define the gate oxide layer  108  as a patterned gate  110 . Next, referring to FIG. 1D, a photoresist layer  112  is formed on the gate  110  and on the gate oxide layer  108 . Then, a heavily doping process, with high dosage of phosphorus, is performed to form source/drain region  104   a ,  104   b  of NMOS device of a CMOS transistor and also form source/drain region  104   c ,  104   d  of a NMOS device in a pixel area.  
           [0008]    The photoresist layer  112  is then removed. Low dosage of phosphorus is then implanted into the substrate  100  to form lightly doped source/drain regions  104   m ,  104   x ,  104   n  and  104   y  of the NMOS transistor, as shown in FIG. 1E. Next, referring to FIG. 1F, a photoresist layer  114  is formed on the gate  110  and the gate oxide layer  108 . Using the photoresist layer  114  as a mask, high dosage of boron is implanted in the substrate  100  to form source/drain region  104   i ,  104   j  of a PMOS transistor.  
           [0009]    Referring to FIG. 1G, the photoresist layer  114  is removed. An interlayer dielectric  116  is formed on the gate  110  and on the gate oxide layer  108 . There are a number of openings formed within the gate oxide layer  108  and within the interlayer dielectric  116 . Then, referring to FIG. 1H, electrodes  118  are formed on the interlayer dielectric  116 . The electrodes  118  fill the openings within the gate oxide layer  108  and within the interlayer dielectric  116 . The electrodes  118  thereby electrically connect the source/drain regions  104   a ,  104   c ,  104   i ,  104   b ,  104   d  and  104   j.    
           [0010]    Referring to FIG. 11, a passivation layer  120  is formed on the electrodes  118  and the interlayer dielectric  116 . An opening is formed through the passivation layer  120  to expose the electrodes  118 . Next, a transparent electrode  122  is formed on the passivation layer  120 . The transparent electrode  122  fills the opening within the passivation layer  120  to electrically connect the electrode  118 , as shown in FIG. 1J. A structure resulting from this process is shown in FIG. 1J.  
           [0011]    In the low temperature polysilicon TFT fabricated by the foregoing conventional method, the electron mobility is restricted due to photoresist and chemicals residues left on the polysilicon layer  104 , as shown in FIG. 1B. In addition, other characteristics of the low temperature TFT, such as the value of the threshold voltage and the sub-threshold swing, can be affected by the presence of photoresist and chemicals residues remaining in the device.  
         SUMMARY OF THE INVENTION  
         [0012]    It is therefore an object of the invention to provide an improved method for fabricating a low temperature polysilicon thin film transistor without photoresist and chemicals residues.  
           [0013]    The invention achieves the above-identified objects by providing a method for fabricating a NMOS transistor and a PMOS transistor on a substrate. The method includes the steps of: forming a buffer layer on the substrate; forming a polysilicon layer on the buffer layer, wherein the polysilicon layer preferably has a thickness between about 200 angstroms and 1000 angstroms; forming a gate oxide layer on the polysilicon layer, wherein the gate oxide layer preferably has a thickness between about 500 angstroms and 1500 angstroms; forming a photoresist pattern on the gate oxide layer; etching the polysilicon layer and the gate oxide layer and using the photoresist pattern as a mask by photolithography to form a first stack structure corresponding to the NMOS transistor and to form a second stack structure corresponding to the PMOS transistor; removing the photoresist pattern; forming a gate on the gate oxide layer, wherein the gate includes molybdenum, chromium, or thallium/aluminum/thallium; forming source/drain region of the NMOS transistor by implanting first heavy dopants and using a photoresist layer covering the second stack structure and lightly doped region of the NMOS transistor as a mask, wherein the first heavy dopants include phosphorus with the preferred dosage between about 1E14 dose/cm 2  and 5E15 dose/cm 2 ; implanting dopants to form the lightly doped region of the NMOS transistor by using the gate as a mask, wherein the dopants include phosphorus with the preferred dosage between about 8E12 dose/cm 2  and 5E13 dose/cm 2 ; and implanting second heavy dopants to form source/drain region of the PMOS transistor by using a photoresist layer over the first stack structure as a mask, wherein the second heavy dopants include phosphorus with the preferred dosage between about 1E14 dose/cm 2  and 5E15 dose/cm2.  
           [0014]    Embodiments of the invention further includes: forming an interlayer dielectric on the gate oxide layer, the gate and the substrate, wherein the interlayer dielectric preferably has a thickness between about 2000 angstroms and 7000 angstroms; selectively exposing the gate, the source/drain regions of the NMOS transistor and the PMOS transistor; forming a patterned passivation layer on the interlayer dielectric and the electrodes, wherein the patterned passivation layer exposes a portion of the electrodes of the NMOS transistor in a pixel area; and forming a transparent electrode to electrically connect the exposed portion of the electrodes of the NMOS transistor exposed, wherein the transparent electrode includes indium tin oxide (ITO).  
           [0015]    Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIGS. 1A to  1 J (Prior Art) are cross-sectional views showing the sequential process steps of a conventional method for fabricating a low temperature polysilicon thin film transistor.  
         [0017]    [0017]FIGS. 2A to  2 J are cross-sectional views showing the sequential process steps of the method for fabricating a low temperature polysilicon thin film transistor in accordance with one preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    The present invention aims to provide an improved method for fabricating a low temperature polysilicon thin film transistor without residues of photoresist and chemicals.  
         [0019]    [0019]FIGS. 2A to  2 J are cross-sectional views showing the sequential process steps of the method for fabricating a low temperature polysilicon thin film transistor in accordance with one preferred embodiment of the present invention. Referring first to FIG. 2A, a buffer layer  202  and a polysilicon layer  204  are sequentially formed on a substrate  200 . The buffer layer  202  can be a silicon oxide layer or a silicon nitride layer, and the substrate  200  can be a glass substrate or a plastic substrate. The polysilicon layer  204  can be formed by annealing amorphous silicon using excimer laser. The polysilicon layer  204  can have a thickness between about 200 angstroms and 1000 angstroms.  
         [0020]    Next, a gate oxide layer  208  is formed on the polysilicon layer  204 . The gate oxide layer  208  can be made of silicon dioxide, and can have a thickness between about 500 angstroms and 1500 angstroms. Then, a patterned photoresist layer  205  is formed on the polysilicon layer  204 , and the gate oxide layer  208  and the polysilicon layer  204  are etched using the patterned photoresist layer  205  as a mask. The removal of the residues of the photoresist layer  205  on the gate oxide layer  208  is performed by using chemicals, such as wet dip. The structure after wet dip is shown in FIG. 2B. In FIG. 2B, there are three stack structures having the gate oxide layer  208  and the polysilicon layer  204 . The middle and the left ones respectively function as the NMOS device and the PMOS device of the CMOS transistor. The stack structure on the right of the three stack structures is the NMOS device in the pixel area.  
         [0021]    Compared with the conventional manufacturing process, the manufacturing process of the invention has no step between the formation of the polysilicon layer  204  and the gate oxide layer  208 . The conventional process includes steps such as forming the photoresist layer  105  on the polysilicon layer  104  and removing the photoresist layer  105  by using chemicals, which do not exist in the method of the present invention. Thus, the problem of photoresist and chemical residues between the gate oxide layer  208  and the polysilicon layer  204  is overcome by applying the process of this invention. This improvement further benefits the quality of the low temperature polysilicon TFTs.  
         [0022]    Referring to FIG. 2C, a conductive layer is disposed on the gate oxide layer  208 . Photolithography and etching processes are then performed to define the patterned conductive layer and form a gate  210 . The material of the gate  210  can be molybdenum (Mo), chromium (Cr), thallium/aluminum/thallium (Ti/Al/Ti) or their combination. Next, referring to FIG. 2D, a patterned photoresist layer  212  is formed on the gate  210  and on the gate oxide layer  208 . Then, a heavily doping process, using high dosage of phosphorus, is performed to form source/drain regions  204   a ,  204   b ,  204   c ,  204   d  of a NMOS transistor. The dosage of phosphorus implanted can be between about 1E14 dose/cm 2  and 5E15 dose/cm 2 .  
         [0023]    The photoresist layer  212  then is removed. Low dosage of phosphorus is then implanted into the substrate  200  to form a lightly doped source/drain regions  104   m ,  104   n ,  104   x ,  104   y  of the NMOS transistor, as shown in FIG. 2E. The dosage of phosphorus implanted is between about 8E12 dose/cm 2  and 5E13 dose/cm 2 . Next, referring to FIG. 2F, a photoresist layer  214  is formed over the substrate  200  on the gate  210  and the gate oxide layer  208 . Using the photoresist layer  214  as a mask, high dosage of boron is implanted in the substrate  200  to form source/drain region  204   i ,  204   j  of a PMOS transistor. The dosage of boron implanted is between about 1E14 dose/cm 2  and 5E15 dose/cm 2 .  
         [0024]    Referring to FIG. 2G, the photoresist layer  214  is removed. An interlayer dielectric  216  is formed on the gate  210  and on the gate oxide layer  208 . There are a number of openings formed within the gate oxide layer  208  and within the interlayer dielectric  216  thereon. The interlayer dielectric  216  can be made of silicon dioxide, and can have a thickness between about 2000 angstroms and 7000 angstroms. Then, referring to FIG. 2H, a conductive layer, filling the openings within the gate oxide layer  208  and the interlayer dielectric  116 , is formed on the interlayer dielectric  216 . The conductive layer forms electrodes  218  electrically connecting the gate  210 , the source/drain regions  204   a ,  204   c ,  204   i ,  204   b ,  204   d  and  204   j.    
         [0025]    Referring to FIG. 21, a passivation layer  220  is formed on the electrodes  218  and on the interlayer dielectric  216 . An opening formed within the passivation layer  220  exposes a part of the electrodes  218 . Next, a transparent electrode  222  is formed on the passivation layer  220  and fills in the opening through the passivation layer  220  to electrically connect the electrode  218 , as shown in FIG. 2J. The transparent electrode  222  can be made of a conductive transparent material such as indium tin oxide (ITO).  
         [0026]    With the abovementioned manufacturing process, the residues of photoresist and chemicals on the polysilicon layer  204  can be effectively prevented. The electron mobility of the low temperature polysilicon TFT is improved as a result of the improved smooth interface between the polysilicon layer  204  and the gate oxide layer  208 . Some other defects occurring on the devices manufactured by the conventional method, such as abnormal shifting of the threshold voltage and sub-threshold swing, are effectively prevented.  
         [0027]    While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.