Abstract:
A method of forming poly-silicon thin film transistors is described. An amorphous silicon thin film transistor is formed on a substrate, and then the Infrared (IR) heating process is used. A gate metal and source/drain metal are heated rapidly, and conduct heat energy to an amorphous silicon layer. Next, crystallization occurs in the amorphous silicon layer to form poly-silicon. Therefore a poly-silicon thin film transistor is produced.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method of forming a poly-silicon thin film transistor, and more particularly, to a method of changing an amorphous silicon thin film transistor directly into a poly-silicon thin film transistor.  
       BACKGROUND OF THE INVENTION  
       [0002]     Thin film transistors (TFT) have generally been used as devices for active matrix liquid crystal displays (AMLCD). The thin film transistors usually have two types of amorphous silicon (a-Si) and poly-silicon (poly-Si) for different silicon films. The mobility of an a-Si TFT is only about 0.5-1 cm 2 /V-s, but a poly-Si TFT has much higher mobility for better crystal character and fewer crystal defects in poly-Si. Therefore, displays fabricated from poly-Si TFT have advantages of high response speed, high resolution, and integrated driver circuits.  
         [0003]     There are some drawbacks such as lower product yield, complex processes, and high process cost existing in the poly-Si TFT fabrication. In contrast, a-Si TFTs are mainly fabricated for displays with a lower process cost and well-developed techniques.  
         [0004]     Owing to the bottleneck of crystallization techniques, the poly-Si TFT is still not the market mainstream; the conventional methods for fabricating poly-Si films are solid phase crystallization (SPC), excimer laser annealing (ELA), and metal induced lateral crystallization (MILC). SPC is not applicable to flat panel display fabrication because the upper limit temperature of a glass substrate is 650° C., while ELA has drawbacks of the high cost for laser light, process stability, and poor crystal uniformity. Besides, MILC may have problems in metal diffusion. Therefore, present poly-Si forming methods have technical problems and higher production cost for more complex processes. So the poly-Si TFT is still unable to compete with the a-Si TFT.  
         [0005]     Further, because the technical problems in poly-Si films fabrication have not been overcome, some manufacturing difficulties exist if the poly-Si TFT fabrication present is utilized to develop large-size displays.  
       SUMMARY OF THE INVENTION  
       [0006]     An objective of the present invention is to provide a method of forming poly-Si TFT for improving the flat panel display performance. The poly-Si TFT with high mobility can be produced and the superiority in a-Si TFT fabrication also can be kept at the same time. Each film material has different absorption of infrared rays (IR); a metal film with good absorption of IR and thermal stability is used as a hot plate, and the metal film serving as the hot plate absorbs heat energy from the IR by IR heating after producing an a-Si TFT. Then, the metal film transfers the heat energy to the a-Si layer, and the a-Si layer crystallizes to become a poly-Si layer, thus forming a poly-Si TFT.  
         [0007]     According to one preferred embodiment of this invention, a bottom-gate a-Si TFT with back channel etch (BCE) structural type is produced on a substrate, and then IR heating is performed. A gate metal and a source/drain (S/D) metal in the a-Si TFT absorb heat energy from the IR rapidly and transfer the heat energy to the a-Si layer; the a-Si layer is therefore induced to crystallize and become a poly-Si layer. Thus, a BCE-type a-Si TFT is transformed into a BCE-type poly-Si TFT.  
         [0008]     According to another preferred embodiment of this invention, a bottom-gate a-Si TFT with channel protect (CHP) structural type is produced on a substrate, and then IR heating is performed. A gate metal and a S/D metal in the a-Si TFT absorb heat energy from the IR rapidly and transfer the heat energy to the a-Si layer; the a-Si layer is therefore induced to crystallize and become a poly-Si layer. Thus, a CHP-type a-Si TFT is transformed into a CHP-type poly-Si TFT.  
         [0009]     According to still another preferred embodiment of this invention, a top-gate a-Si TFT is produced on a substrate, and then IR heating is performed. In the same way, a gate metal and a S/D metal in the a-Si TFT absorb heat energy from the IR rapidly and transfer the heat energy to the a-Si layer; the a-Si layer is therefore induced to crystallize and become a poly-Si layer. Thus, a top-gate a-Si TFT is transformed into a top-gate poly-Si TFT.  
         [0010]     With the application of the present invention, in addition to preserving the superiority of a-Si TFT fabrication, the poly-Si TFT with good electrical performance is also formed. Thus, the driver circuits can be integrated on a panel to reduce the cost of adding driver circuits to the a-Si TFT.  
         [0011]     Further, the poly-Si TFT devices formed by the present invention are useful for improving the liquid crystal display (LCD) performance and even employed to drive the organic light-emitting display (OLED). 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     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:  
         [0013]      FIGS. 1A-1C  are cross-sectional schematic diagrams showing the process for forming a poly-Si TFT in accordance with the first preferred embodiment of the present invention;  
         [0014]      FIGS. 2A-2D  are cross-sectional schematic diagrams showing the process for forming a poly-Si TFT in accordance with the second preferred embodiment of the present invention; and  
         [0015]      FIGS. 3A-3C  are cross-sectional schematic diagrams showing the process for forming a poly-Si TFT in accordance with the third preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     The present invention utilizes the different IR absorption of various film materials; the IR heating process is performed on an a-Si TFT product, and the film material with higher IR absorption and good thermal stability absorbs heat energy from the IR and transfers the heat energy to the a-Si layer. The a-Si layer consequently crystallizes to become a poly-Si layer. Hot plate crystallization (HPC) technique is utilized, the film material with higher IR absorption and good thermal stability in the TFT is used as a hot plate, and then the hot plate transfers heat to the a-Si layer to induce crystallization in the a-Si layer. The a-Si layer is thus changed into the poly-Si layer. Therefore, the IR heating process is merely added after the a-Si TFT fabrication processes without changing the general processes and process conditions in the a-Si TFT fabrication, and then the poly-Si TFT is directly obtained from the a-Si TFT.  
         [0017]     The metal film is the film material with higher IR absorption and good thermal stability apparently in the TFT devices, and therefore a gate metal and a source/drain (S/D) metal all can be used as the hot plates for absorbing heat energy from the IR. Then, the a-Si layer is crystallized by obtaining the heat energy transferred from the metal films; the a-Si layer is thus effectively changed into the poly-Si layer, and the poly-Si TFT is formed.  
         [0018]     The poly-Si TFT is formed by combining the present a-Si TFT fabrication processes with the present invention. According to differences in the TFT structure, there are two kinds of bottom-gate and top-gate structures. The bottom-gate structure further comprises two types of BCE and CHP structure generally used in the a-Si TFT fabrication. The method of forming the poly-Si TFT in accordance with the present invention is combined with the foregoing TFT structures respectively to produce the poly-Si TFT.  
       Embodiment 1  
       [0019]     The present invention discloses a method of forming the poly-Si TFT combined with the bottom-gate BCE structural type. Referring to  FIG. 1A , a gate metal  102  is first formed on a substrate  100  by, for example, physical vapor deposition (PVD), and is then patterned by, for example, photolithography and etching. The substrate  100  may be a glass substrate, and the gate metal  102  is a material with good electric conductivity such as chromium (Cr), molybdenum (Mo) or moly-tungsten (MoW), and the gate metal  102  is also a material with good IR absorption and thermal stability.  
         [0020]     A gate insulator  104 , an amorphous silicon (a-Si) layer  106 , and a doped a-Si layer  108  are formed in turn on the gate metal  102  and the substrate  100  simultaneously by, for example, plasma enhanced chemical vapor deposition (PECVD), and the preferred material of the gate insulator  104  is silicon nitride (SiN x ) or silicon oxide (SiO x ). Then, the a-Si layer  106  and the doped a-Si layer  108  are patterned partially by, for example, photolithography and etching to form an active layer region (not shown).  
         [0021]     Next, referring to  FIG. 1B , a S/D metal  110  is formed by, for example, PVD, and is patterned by, for example, photolithography and etching to form a data-line. The S/D metal  110  is a material with good electric conductivity such as Cr, Mo or MoW, and the S/D metal  110  is also a material with good IR absorption and thermal stability. Then, with the S/D metal  110  serving as a hard mask, the doped a-Si layer  108  between the S/D metal  110  is etched to form an opening  112  exposing a portion of the a-Si layer  106 ; the a-Si layer  106  in the opening  112  represents a channel region.  
         [0022]     The a-Si TFT is finished through the aforementioned processes; then, referring to  FIG. 1C , the heating process with an IR  114  is utilized, and the a-Si TFT is directly changed into the poly-Si TFT.  
         [0023]     The heating process with the IR  114  can utilize any IR supply method; the preferable method in the present invention is a pulsed rapid thermal processing (PRTP) technology. Because of the film materials with different IR absorption, the TFT is heated selectively by the IR  114 . The gate metal  102  and the S/D metal  110  have higher IR absorption and so absorb heat energy from the IR  114  rapidly. Therefore, the gate metal  102  and the S/D metal  110  are heated selectively to serve as the hot plates for transferring the heat energy to the doped a-Si layer  108  and the a-Si layer  106 , and then the doped a-Si layer  108  and the a-Si layer  106  are induced to crystallize to become a doped poly-Si layer  109  and a poly-Si layer  107 . The poly-Si TFT is thus formed.  
         [0024]     The highest corresponding output temperature of the heating process is preferably about 900° C. The proportion occupied by metal films in the TFT is sufficient to transfer the heat energy absorbed by the doped a-Si layer  108  and the a-Si layer  106  for forming a poly-Si. The heating process employed is a pulsed heating process, not a continuous heating, and heats the film materials in the TFT selectively. Further, a glass substrate is unable to absorb the IR  114  effectively, so the device property is not affected and there are no glass substrate deformation problems associated with the process temperature.  
       Embodiment 2  
       [0025]     The present invention discloses another method of forming the poly-Si TFT combined with the bottom-gate CHP structural type. Referring to  FIG. 2A , a gate metal  202  is first formed on a substrate  200  by, for example, PVD, and is then patterned by, for example, photolithography and etching. The substrate  200  may be a glass substrate, and the gate metal  202  is a material with good electric conductivity such as Cr, Mo or MoW; the gate metal  202  is also a material with good IR absorption and thermal stability.  
         [0026]     A gate insulator  204 , an a-Si layer  206 , and a protective layer  208  are formed in turn on the gate metal  202  and the substrate  200  simultaneously by, for example, PECVD, and the preferred material of the gate insulator  204  and the protective layer  208  is SiN x  or SiO x .  
         [0027]     Referring to  FIG. 2B , the protective layer  208  is patterned by, for example, photolithography and etching to form an etching stop layer  209  for protecting the channel region. Then, a doped a-Si layer  210  is formed on the a-Si layer  206  by, for example, PECVD. The a-Si layer  206  and the doped a-Si layer  210  are patterned partially by, for example, photolithography and etching to form an active layer region (not shown).  
         [0028]     Next, referring to  FIG. 2C , a S/D metal  212  is formed by, for example, PVD, and is patterned by, for example, photolithography and etching to form a data-line. The S/D metal  212  is a material with good electric conductivity such as Cr, Mo or MoW, and the S/D metal  110  is also a material with good IR absorption and thermal stability. Then, with the S/D metal  212  serving as a hard mask, the doped a-Si layer  210  between the S/D metal  212  is etched to form an opening  214  exposing the etching stop layer  209 . The a-Si layer  206  under the etching stop layer  209  represents the channel region. Because the etching stop layer  209  protects the a-Si layer  206 , the etching for the doped a-Si layer  210  stops at the etching stop layer  209  and avoids damaging the a-Si layer  206  under the etching stop layer  209 .  
         [0029]     The a-Si TFT is finished through the aforementioned processes. With reference to  FIG. 2D , the heating process with an IR  216  is utilized, and the a-Si TFT is directly changed into a poly-Si TFT.  
         [0030]     The heating process with the IR  216  in the present invention is also preferably PRTP technology, and the TFT is heated selectively by the rapid heating process. The highest corresponding output temperature of the heating process is preferably about 900° C. With the gate metal  202  and the S/D metal  212  serving as hot plates to absorb heat energy from the IR  216  rapidly and then transfer the heat energy to the doped a-Si layer  210  and the a-Si layer  206 , and the doped a-Si layer  210  and the a-Si layer  206  are induced to crystallize and become a doped poly-Si layer  211  and a poly-Si layer  207 . The poly-Si TFT is thus formed.  
       Embodiment 3  
       [0031]     The present invention discloses further another method of forming the poly-Si TFT combined with the top-gate structural type. Referring to  FIG. 3A , a buffer layer  302  is first formed on a substrate  300  by, for example, PECVD. The substrate  300  may be a glass substrate, and the buffer layer  302  may be a SiO x  layer. Then, an a-Si layer  304  is formed on the buffer layer  302  by, for example, PECVD.  
         [0032]     Next, a gate insulator  306  is formed on the a-Si layer  304  by, for example, PECVD; the preferred material of the gate insulator  306  is SiO x . A gate metal  308  is formed by, for example, PVD, and is then patterned by, for example, photolithography and etching. The gate metal  308  is a material with good electric conductivity such as Cr, Mo or MoW, and the gate metal  308  is also a material with good IR absorption and thermal stability.  
         [0033]     Then, referring to  FIG. 3B , an ion-implantation is performed, with the gate metal  308  serving as a mask, and the a-Si layer  304  on two sides of the gate-metal  308  is implanted with ions to define a S/D region. A dielectric interlayer  310  is deposited by, for example, PECVD, and is then patterned by, for example, photolithography and etching to form contact holes  311  which exposes the S/D region. The preferred material of the dielectric interlayer  310  is SiN x  or SiO x .  
         [0034]     Finally, a S/D metal  312  is formed by, for example, PVD, and is then patterned by, for example, photolithography and etching to form a data line. The S/D metal  312  is on the dielectric interlayer  310  and in the contact holes  311  to contact the a-Si layer  304  in the S/D region. The S/D metal  312  is a material with good electric conductivity such as Cr, Mo or MoW, and the S/D metal  312  is also a material with good IR absorption and thermal stability.  
         [0035]     The a-Si TFT is finished through the aforementioned processes. With reference to  FIG. 3C , the heating process with an IR  314  is utilized, and the a-Si TFT is directly changed into the poly-Si TFT.  
         [0036]     The heating process with the IR  314  in the present invention is also preferably a PRTP technology, and the TFT is heated selectively by the rapid heating process. The highest corresponding output temperature of the heating proves is preferably about 900° C. With the gate metal  308  and the S/D metal  312  serving as hot plates to absorb heat energy from the IR  314  rapidly and then transfer the heat energy to the a-Si layer  304 , and the a-Si layer  304  is induced to crystallize and become a poly-Si layer  205 . The poly-Si TFT is thus formed.  
         [0037]     With the foregoing embodiments, the only addition to the general a-Si TFT fabrication processes is the IR heating. The metal film materials in the TFT then absorb heat energy from the IR and transfer the heat energy to the a-Si layer, and the a-Si layer is induced to crystallize and become a poly-Si layer. Therefore, the a-Si TFT can be changed directly into a poly-Si TFT by employing the IR heating. The poly-Si TFT formed by the present invention can keep the advantages of the a-Si TFT fabrication and have a better electrical performance at the same time.  
         [0038]     In addition to the LCD, the present invention also can be employed to fabricate a poly-Si TFT for driving the OLED, and then the product performance is improved greatly.  
         [0039]     The present invention is not limited employed in TFT fabrication for flat panel display; other poly-Si TFT devices also can be fabricated by using the present invention to improve product efficiency. While the present invention has been disclosed with reference to the preferred embodiments of the present invention, it should not be considered as limited thereby. Various possible modifications and alterations by one skilled in the art can be included within the spirit and scope of the present invention, the scope of the invention is determined by the claims that follow.