Abstract:
A method of fabricating polycrystalline silicon layer of TFT is provided. The method includes sequentially forming an insulating layer, a first amorphous silicon layer, and a cap layer on a substrate. A laser annealing is performed to transform the first amorphous silicon layer to a first polycrystalline silicon layer, wherein at least one hole is formed in the amorphous silicon layer during the laser annealing process. Thereafter, the cap layer is removed. A portion of the insulating layer exposed within the hole is removed to form a second opening. A second amorphous silicon layer is formed over the first polycrystalline silicon layer filling the second opening. Finally a second annealing is performed to transform the second amorphous silicon layer to a second polycrystalline silicon layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the priority benefit of Taiwan application serial no. 92120193, filed Jul. 24, 2003.  
       BACKGROUND OF INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to a method of fabricating Thin Film Transistor Liquid Crystal Display (TFT-LCD), and more particularly, relates to a method of fabricating a polysilicon film of TFT array in a TFT-LCD thereof.  
         [0004]     2. Description of the Related Art  
         [0005]     An ordinary active TFT LCD array is generally categorized into polysilicon TFT and amorphous silicon TFT based materials used for making the TFT LCD, where a polysilicon (poly-Si) TFT being capable of integrating driving circuit thus provides a higher opening rate and lower fabrication cost than a corresponding amorphous silicon (a-Si) TFT. Another reason that polysilicon TFT technology is greatly promoted is that poly-Si TFT significantly reduces device feature size so that high image resolution can be achieved. In order to mass-produce polysilicon TFT-LCD, three primary conditions are low temperature (about 450 to 550° C.) process, low-temperature filming technology for high quality gate-insulator layer, and broad ion-implantation.  
         [0006]     In view of the cost of a glass substrate, low temperature thin film process is adopted where Solid Phase Crystallization (SPC) is introduced thereby, yet the active temperature not only tends to be relatively higher than expected, which is around 600° C., but also causes degraded crystallization. Thus Excimer Laser Crystallization (ELC) or Excimer Laser Annealing (ELA) process that is applied to the foregoing low-temperature TFT process is developed, wherein an a-Si thin film is fused by laser scanning and is crystallized to poly-Si thin film.  
         [0007]     Providing process temperature lower than 450° C. in ELC and providing higher electron mobility and lower current leakage than SPC in forming an amorphous silicon thin film, a less expensive glass substrate is introduced so as to reduce fabrication cost whereas better TFT device characteristic is obtained thereby.  
         [0008]     Referring to  FIG. 1A , a substrate  100  is provided. A first insulating layer  102  is formed on the substrate  100 . Next, a photolithography etching is performed to form a first opening  104  in the first insulating layer  102 . In the sub-micron technology, the photolithography technology is not applicable to the present micro TFT field, because the threshold feature of the first opening  104  using photolithography technique is about 1 micrometer, which is relatively large compared to the threshold crystal feature size for TFT thin film.  
         [0009]     Attempts to resolve the issue is illustrated with reference to  FIG. 1B . A second insulating layer  106  is further formed over the first insulating layer  102  and the first opening  104 . The deposition of the second insulating layer  106  further shrinks the first opening  104  to a second opening  108  to satisfy the feature size requirement for polysilicon TFT crystallization.  
         [0010]     Referring to  FIG. 1C , an a-Si layer  110  is formed over the second insulating layer  106 . Next, fuse and liquefy the a-Si layer  110  by an Excimer Laser  112 .  
         [0011]     Finally, referring to  FIG. 1D , the fused liquefied silicon undergoes crystallization from the second opening  108  to transform the a-Si layer  110  into a poly-Si layer  114 , which is suitable for forming source/drain and channel of a TFT therein.  
         [0012]     However, problems in the foregoing process do exist, as described below.  
         [0013]     The forming of the first opening  104  in the foregoing process requires a mask process and an additional deposition step of forming the second insulating layer  106  adjusting to the size of the first opening  104 , and therefore not only complication but also lowers throughput results.  
         [0014]     Moreover, the scheme of depositing the second insulating layer  106  for adjusting to the size of the second opening  108  requires precise control of the process conditions, thus narrowing the processing tolerance window.  
       SUMMARY OF INVENTION  
       [0015]     According to foregoing issues, one object of the present invention is to provide a method of fabricating a poly-Si thin film, wherein the steps of complicated photolithography exposure, extra deposition procedure, etc. can be excluded, and an opening with proper deep sub-micron dimensions can be formed.  
         [0016]     Another object of the present invention is to provide a method of fabricating a poly-Si film, wherein an opening having a size sufficient for poly-Si thin film crystallization can be formed without precise control of process conditions, and thereby increasing the process window allowing greater process condition tolerance.  
         [0017]     The present invention provides a method of fabricating a poly-Si layer, wherein a substrate is provided, an insulating layer, a first a-Si layer, and a cap layer are sequentially formed over the substrate. A first laser annealing is performed for transforming the first a-Si layer into a first poly-Si layer having at least one hole. Next, the cap layer is removed, and then a portion of the insulating layer within the hole is removed to form a first opening in the insulating layer, and the first opening and the insulating layer form a second opening. Subsequently, a second a-Si layer is formed over the first a-Si layer and the second opening, wherein the second a-Si layer has a recess over the second opening. Finally, the resulting structure is subjected to a second laser annealing, wherein an unfused portion of the second a-Si layer at a bottom of the second opening serves as a seed for crystal growth during the crystallization, thus the second a-Si layer is transformed into a second poly-Si layer.  
         [0018]     The present invention provides another method of fabricating a poly-Si thin film. A substrate is provided. An insulating layer, a first a-Si layer, and a cap layer are sequentially formed over the substrate. A first laser annealing is performed to transform the first a-Si layer into a first poly-Si layer having at least a first hole. Afterwards, the cap layer is removed, removing a portion of the insulating layer exposed within the first hole to form a first opening in the insulating layer, and the first hole and the first opening define a second opening. Then a dielectric layer is formed over the first poly-Si layer and the second opening, and a second a-Si layer is formed over the dielectric layer, wherein the second a-Si layer has a recess over the second opening. Finally, the resulting structure is subjected to a second annealing, wherein a portion of the second a-Si layer within the recess serves as the seed for crystal growth during the crystallization, so that the second a-Si layer is transformed into a second poly-Si layer.  
         [0019]     The present invention provides another method of fabricating a poly-Si thin film. A substrate is provided. An insulating layer, a first a-Si layer, and a cap layer are formed sequentially over the substrate. Thereafter the resulting structure is subjected to a first annealing wherein the first a-Si layer is transformed into a first poly-Si layer having at least a first hole. Next, the cap layer is removed, and then a portion of the insulating layer exposed within the first hole is removed to form a first opening in the insulating layer, and the first hole and the first opening form a second opening. Then a dielectric layer having a second hole is formed over the first poly-Si layer and the second opening, wherein the second hole is formed within the second opening. Next, a second a-Si layer is formed over the dielectric layer. Finally, the resulting structure is subjected to a second laser annealing. A portion of the second a-Si layer over the second hole is subjected to a higher temperature than other portion of the second a-Si layer relative to the second hole, and crystallization lasts longer, so that the second a-Si layer is transformed into a second poly-Si layer.  
         [0020]     According to the foregoing description, it is noted that a proper deep sub-micron hole in the insulating layer is formed by sequentially forming an insulating layer, a a-Si layer and a cap layer over the substrate and then performing a laser annealing process without performing any photolithography and etching. Accordingly, process steps such as light exposure, photolithography and additional deposition as described above for forming an opening having a deep sub-micron feature can be effectively excluded. Thus, the throughput can also be effectively increased.  
         [0021]     Moreover, the method of the present invention can be implemented without precisely controlling the process conditions by forming the cap layer, the a-Si layer, the insulating layer or laser annealing process. Thus the method of the present invention has a broader process tolerance compared to the conventional process described above. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0022]      FIGS. 1A  to  1 D show the cross sectional views illustrating the progression of the process according to a conventional method of fabricating a polysilicon (poly-Si) thin film.  
         [0023]      FIGS. 2A  to  2 E show the cross sectional viewsillustrating the progression of the process of a method of fabricating a poly-Si thin film according to a first embodiment of the present invention.  
         [0024]      FIGS. 3A  to  3 F show the cross sectional viewsillustrating the progression of the process of a method of fabricating a poly-Si thin film according to a second embodiment of the present invention.  
         [0025]      FIGS. 4A  to  4 F show the cross sectional viewsillustrating the progression of the process of a method of fabricating a poly-Si thin film according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]     First Embodiment  
         [0027]     Referring to  FIGS. 2A  to  2 E, show the cross sectional viewsillustrating the progression of the process of a method of fabricating a polysilicon (poly-Si) thin film according to the first embodiment of the present invention.  
         [0028]     Referring to  FIG. 2A , a substrate  200  is provided, wherein the material of the substrate  200  includes a silicon wafer, a glass substrate or a plastic substrate, for example. An insulating layer  202  is formed over the substrate  200 , wherein the insulating layer  202  includes silicon dioxidecan be formed by performing a conventional deposition process such as Low Pressure Chemical Vapor Deposition (LPVCD), Plasma Enhanced Chemical Vapor Deposition (PECVD) or sputtering. Thereafter a first a-Si layer  204 , which can be formed by performing a conventional process such as LPVCD, PECVD or sputtering, is formed over the insulating layer  202 . Further, a cap layer  206  is formed over the first a-Si layer  204 , wherein the material of the cap layer  206  includes a silicon dioxide, for example, wherein the cap layer  206  may be formed by performing a conventional deposition process such as LPCVD, PECVD, or sputtering. Afterwards, the resulting structure is subjected to a first laser annealing  208 , for example, an excimer laser may be used to perform the first laser annealing  208 , so as to fuse the first a-Si layer  204 . The energy density of the excimer laser is about 50 to 500 mJ/cm 2 .  
         [0029]     Referring to  FIG. 2B , a first poly-Si layer  210  is formed transformed from the first a-Si layer  204  through crystallization. In addition, a plurality of holes are randomly formed in the first poly-Si layer  210 , however, in the  FIG. 2B , only one hole  212  is shown for illustration purpose.  
         [0030]     According to the foregoing procedures, the reasons why the hole  212  is formed in the first poly-Si layer  210  is not exactly known but it is most likely due to a cohesion force of poly-Si being stronger than an adhesion force between the cap layer and the first poly-Si layer  210 . The first poly-Si layer  210  shrinks inwardly to form the holes  212  as the first a-Si layer  204  is transformed into the first poly-Si layer  210 . Additionally, each of the holes  212  has the feature of a proper deep sub-micron dimension for back-end crystallization.  
         [0031]     Referring to  FIG. 2C , the cap layer  206  is removed by performing a wet etching or an anisotropic dry etching. Thereafter, a portion of the insulating layer  202  exposed within the hole  212  is removed to form a first opening  214 , wherein the step of removing the portion of the insulating layer  202  exposed within the first opening  214  can be carried out by performing a wet etching, for example. The width of the first opening  214  is smaller than about 0.5 micron for further crystallization. The hole  212  and the first opening  214  form a second opening  216 .  
         [0032]     Referring to  FIG. 2D , a second a-Si layer  218  is formed over the first poly-Si layer  210  and the second opening  216 , wherein the second a-Si layer  218  is deposited by performing LPCVD, PECVD, or sputtering, for example, wherein the second a-Si layer  218  includes a recess  220  neighboring with the second opening  216 . The resulting structure is subjected to a second laser annealing  222 , for example, using an excimer laser to irradiate the second a-Si layer  218  with an energy density of about 50 to 500 mJ/cm 2  so as to fuse the second a-Si layer  218  and the first poly-Si layer  210 . According to the second opening  216 , an unfused portion of the second a-Si layer  218  serves as a seed for crystallization, wherein the unfused portion of the second a-Si layer  218  is at the bottom of the second opening  216 .  
         [0033]     Finally, referring to  FIG. 2E , a second poly-Si layer  224  is transformed from a fused portion of the second a-Si layer  218  and the first poly-Si layer  210  crystal growing in a lateral direction  226 .  
         [0034]     Second Embodiment  
         [0035]     Referring to the  FIGS. 3A  to  3 F, show the cross sectional viewsillustrating the progression of the process of a method of fabricating a poly-Si film according to a second embodiment of the present invention.  
         [0036]     Referring to  FIG. 3A , a substrate  300  is provided, wherein the material of the substrate  300  includes, for example, a silicon wafer, a glass or a plastic. An insulating layer  302  is formed over the substrate  300 , wherein the material of the insulating layer  302  includes, for example, a silicon dioxide, and the insulating layer  302  can be formed by, for example, performing a conventional deposition process such as a LPVCD, a PECVD or a sputtering. Thereafter, a first a-Si layer  304  is formed over the insulating layer  302 , by performing, for example, a LPCVD, PECVD or sputtering process.  
         [0037]     Further, a cap layer  306  is formed over the first a-Si layer  304 , wherein the material of the cap layer  306  includestemptemp, for example, silicon dioxide, and the cap layer  306  can be formed by, for example, performing a conventional deposition process such as LPCVD, PECVD or sputtering. The resulting structure is then subjected to a first laser annealing  308 , for example, performing an excimer laser annealing to fuse the first a-Si layer  304 . The energy density of the excimer laser is about 50 to 500 mJ/cm 2 .  
         [0038]     Referring to  FIG. 3B , a first poly-Si layer  310  is formed transformed from the first a-Si layer  304  through the fusion and crystallization. Moreover, as described in the first embodiment, as the first a-Si layer  304  is transformed to the first poly-Si layer  310 , a plurality of holes  312  are randomly formed in the first poly-Si layer  310 , however only a single hole  312  is shown in  FIG. 3B  for illustration purpose.  
         [0039]     Referring to  FIG. 3C , the cap layer  306  is removed, wherein the step of removing the cap layer  306  is accomplished by, for example, performing a wet etching using hydrofluoric acid or an anisotropic dry etching. Thereafter, a portion of the insulating layer  302  exposed within the hole  312  is removed to form a first opening  314 , wherein the portion of the insulating layer  302  exposed within the first opening  314  can be removed by, for example, performing a wet etching. The first opening  314  has a width smaller than about 0.5 micron for further crystallization. The hole  312  and the first opening  314  constitute a second opening  316 .  
         [0040]     Referring to  FIG. 3D , a dielectric layer  317  is formed over the first poly-Si layer  310  and the second opening  316 , wherein the dielectric layer  317  can be formed by, for example, performing a conventional process such as either LPCVD, PECVD or sputtering, wherein the dielectric layer  317  includes a recess  320  neighboring with the second opening  316 .  
         [0041]     Referring to  FIG. 3E , a second a-Si layer  318  is formed over the dielectric layer  317 , wherein the second a-Si layer  318  is formed by, for example, performing with a conventional deposition process such as a LPCVD, a PECVD, or a sputtering process. Thereafter, the resulting structure is subjected to a second laser annealing  322  by performing, for example, an excimer laser annealing, to irradiate the second a-Si layer  318 . The energy density of the excimer laser is about 50 to 500 mJ/cm 2 .  
         [0042]     Finally, referring to  FIG. 3F , a second poly-Si layer  324  is formed transformed from a fused portion of the second a-Si layer  318  crystal growing in a lateral direction  326 , wherein an unfused portion of the second a-Si layer  318  neighboring with the recess  320  serves as a seed for crystallization.  
         [0043]     Referring to the  FIGS. 4A  to  4 F, show the cross-sectional views illustrating the progression of the process of the method of fabricating a poly-Si film according to a third embodiment of the present invention.  
         [0044]     Referring to  FIG. 4A , a substrate  400  is provided, wherein the material of the substrate  400  includes, for example, silicon wafer, glass, or plastic. An insulating layer  402  is formed over the substrate  400 , wherein the material of the insulating layer  402  includes, for example, silicon dioxide, and wherein the insulating layer  402  can be formed by performing conventional deposition methods such as LPVCD, PECVD, or sputtering. Thereafter a first a-Si  404  is formed over the insulating layer  402 , which can be formed by performing LPCVD, PECVD or sputtering method, for example.  
         [0045]     Next, a cap layer  406  is formed over the first a-Si layer  404 , wherein the material of the cap layer  406  includes, for example, silicon dioxide, and wherein the cap layer  406  can be formed by performing conventional deposition methods such as LPCVD, PECVD or sputtering method. Thereafter, the resulting structure is subject to a first laser annealing  408  by performing, for example, an excimer laser, so as to fuse the first a-Si layer  404 . The energy density of the excimer laser is about 50 to 500 mJ/cm 2 .  
         [0046]     Referring to  FIG. 4B , a first poly-Si layer  410  is formed from the first a-Si layer  404  through fusion and crystallization. Moreover, a plurality of first holes  412  are randomly formed in the first poly-Si layer  410 , however, in the  FIG. 4B , only one first hole  412  is shown for illustration purpose.  
         [0047]     Further, referring to  FIG. 4C , the cap layer  406  is removed, wherein the method for removing the cap layer  406  is accomplished by performing a wet etching using hydrofluoric acid or anisotropic dry etching. Thereafter, a portion of the insulating layer  402  within the first hole  412  is removed to form a first opening  414 , wherein the portion of the insulating layer  402  is removed by performing a wet etching, for example. The first opening  414  formed by the foregoing method has a width smaller than about 0.5 micron for further crystallization. The first opening  412  and the first opening  414  constitute a second opening  416 .  
         [0048]     Next, referring to  FIG. 4D , a dielectric layer  417  is formed over the first poly-Si layer  410  and the second opening  416 , wherein the dielectric layer  417  can be formed by performing LPCVD, PECVD or sputtering, for example. A second hole  420  is formed as an air space in the dielectric layer  417 , wherein the second hole  420  is neighboring with the second opening  416 .  
         [0049]     Furthermore, referring to  FIG. 4E , a second a-Si layer  418  is formed over the dielectric layer  417 , wherein the second a-Si layer  418  is formed by performing LPCVD, PECVD or sputtering, for example. Thereafter, the resulting structure is subjected to a second laser annealing  422  by performing an excimer laser annealing for example, to irradiate and fuse the second a-Si layer  418 . The energy density of the excimer laser is about 50 to 500 mJ/cm 2 .  
         [0050]     Finally, referring to  FIG. 4F , a second poly-Si layer  424  is transformed from the second a-Si layer  418  through fusion and crystallization. When the second laser annealing  422  is performed, a portion of the second a-Si layer  418  over the second hole  420  is subjected to a higher temperature than other portion of the second a-Si player  418  relative to the second hole  420  because the thermal conductivity is poor around the second hole  420 . A lateral crystallization progress from a region with lowest temperature (not shown) along the direction  426  is performed, wherein the lateral crystallization lasts longer around the second hole  420 .  
         [0051]     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 covers modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.