Abstract:
A method of fabricating an organic thin film transistor is provided. The method includes forming a source, a drain and a gate on a substrate and forming a dielectric layer to isolate the gate from the source and isolate the gate from the drain. An organic active material layer is formed on the substrate to fill a channel region between the source and the drain and cover the source and the drain. A barrier material layer is formed on the organic active material layer. Thereafter, the barrier material layer and the organic active material layer are patterned to form a barrier layer and an organic active layer and expose the source and the drain.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 97140161, filed on Oct. 20, 2008. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thin film transistor (TFT) and method of fabricating the same, and more particularly, to an organic thin film transistor (OTFT) and method of fabricating the same. 
     2. Description of Related Art 
     Along maturation of technology, lighter, thinner, portable and flexible displays such as electronic paper have caught attention of many people, and many large companies have participated in their development OTFTs utilize organic molecular materials to develop TFTs suitable for electronic products. OTFTs have greatest advantages of being able to be fabricated under low temperatures, having simple processes, being able to be made in large areas, low fabricating costs, and being able to maintain transistor element properties even when panels are bent to achieve effects of normal imaging quality. Applications as such may accelerate realization of flexible electronic products such as displays. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of fabricating an OTFT, which comprises forming a source, a drain and a gate on a substrate, and forming a dielectric layer to isolate the gate from the source and the drain. An organic active material layer is formed on the substrate to fill a channel region between the source and the drain and cover the source and the drain. A barrier material layer is formed on the organic active material layer. Thereafter, the barrier material layer and the organic active material layer are patterned to form a barrier layer and an organic active layer; exposing the source and the drain. 
     The present invention further provides an OTFT, which comprises a gate, a source, a drain, a dielectric layer, an organic active layer and a barrier layer. The gate is disposed on a substrate; the source and the drain are disposed on two sides of the gate. The dielectric layer is disposed on the substrate to isolate the gate from the source and the drain. The organic active layer is disposed on a channel region between the source and the drain, corresponding to the gate. The barrier layer is disposed on the organic active layer. 
     In order to make the aforementioned and other objects, features and advantages of the present invention more comprehensible, several embodiments accompanied with figures are described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIGS. 1A to 1D  are cross-sectional views illustrating a method of fabricating an OTFT of a top gate structure according to an embodiment of the present invention. 
         FIGS. 2A to 2D  are cross-sectional views illustrating a method of fabricating an OTFT of a bottom gate structure according to another embodiment of the present invention. 
         FIG. 3A  is an electrical characteristic view showing an OTFT of a top gate structure formed by patterning an organic active layer using a method according to the embodiment of the present invention. 
         FIG. 3B  is an electrical characteristic view showing an OTFT of a top gate structure formed by not patterning an organic active layer according to the prior art. 
         FIG. 4A  is an electrical characteristic view showing an OTFT of a bottom gate structure formed by patterning an organic active layer using a method according to the embodiment of the present invention. 
         FIG. 4B  is an electrical relationship view showing an OTFT of a bottom gate structure formed by not patterning an organic active layer according to the prior art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A method of fabricating an OTFT of the present invention uses photolithographic and etching processes to pattern an organic active material layer. Most or all of the organic active material layer covering a source and a drain is removed, so that an organic active layer only is formed on a channel region between the source and the drain. In the present invention, before performing the photolithographic process to the organic active material layer, a barrier layer is formed on the organic active material layer. Since the organic active material layer avoids being destroyed by a subsequent process of removing a photoresist layer with protection of the barrier layer, the formed OTFT has excellent device characteristics. The following uses a method of fabricating an OTFT of a top gate structure and a method of fabricating an OTFT of a bottom gate structure as examples to illustrate the present invention, but the present invention is not limited to the examples. 
       FIGS. 1A to 1D  are cross-sectional views illustrating a method of fabricating an OTFT of a top gate structure according to an embodiment of the present invention. 
     Referring to  FIG. 1A , in the method of fabricating the OTFT according to the present embodiment, a source  14  and a drain  16  (or a source  16  and a drain  14 ) are formed on a substrate. A channel region  12  is formed between the source  14  and the drain  16  (or the source  16  and the drain  14 ). The substrate  10  may be a rigid substrate or a flexible substrate. A material of the rigid substrate is, for example, glass, quartz or silicon wafer. A material of the flexible substrate is, for example, plastic such as acrylic, metal foil or paper. The source  14  and the drain  16  (or the source  16  and the drain  14 ) are formed by, for example, forming a conductive material layer and patterning the conductive material layer. A material of the conductive material layer is, for example, a metal such as aluminum, copper, molybdenum, chromium or alloys of above. A method of forming the conductive material layer comprises performing a physical vapor deposition process such as a sputtering process or an evaporation process. According to another embodiment, a method of forming the source  14  and the drain  16  (or the source  16  and the drain  14 ) may comprise directly forming a patterned conductive layer, for example fabricating with a conductive inkjet printing method or other printing or transfer printing technologies. 
     Then, an organic active material layer  18  is formed on the substrate  10 . A material of the organic active material layer  18  is, for example, an organic material including an N-type or P-type organic small molecule, an organic polymer, or a mixture of an organic small molecule and an organic polymer. A material of the organic small molecule is, for example, pentacene. A material of the organic polymer is, for example, poly-(3-hexylthiophene) (P3HT) or polyacrylic acid (PAA). A method of forming the organic active material layer  18  is a solution process such as a spin coating method, a die coating method and a roll coating method. 
     Afterwards, referring to  FIG. 1B , a barrier material layer  20  is formed on the organic active material layer  18 . A material of the barrier material layer  20  includes a dielectric material. The dielectric material includes poly-4-vinylphenol (PVP), polyvinyl alcohol (PVA) or parylene. The material of the barrier material layer  20  and a material of a dielectric layer  24  (as shown on  FIG. 1D ) subsequently formed may be same or different. A method of forming the barrier material layer  20  is a solution process such as a spin coating method, a die coating method and a roll coating method. The method of forming the barrier material layer  20  comprises, for example, dissolving the dielectric material in a first solvent, and further coating the dielectric material dissolved in the first solvent on the organic active material layer  18 , wherein the organic active material layer  18  is indissolvable in the first solvent. If the method of forming the organic active material layer  18  comprises dissolving the organic material in a second solvent, and further filling the channel region  12  between the source  14  and the drain  16  (or the source  16  and the drain  14 ) and covering the source  14  and the drain  16  (or the source  16  and the drain  14 ) with the organic material dissolved in the second solvent by coating, the second solvent and the first solvent have different polarities. By forming the barrier material layer  20  using a coating method, the formed barrier material layer  20  has excellent uniformity, and the coating method may be performed under low temperatures, for example under 150 degrees centigrade. Hence, compared with conventional inorganic processes, the coating method is extremely suitable for fabricating TFTs using a flexible substrate. 
     Afterwards, a patterned photosensitive layer  22  is formed on the barrier material layer  20 . A method of forming the patterned photosensitive layer  22  comprises, for example, forming a photosensitive material layer, then patterning the photosensitive material layer by exposure and development to form the photosensitive layer  22 . A material of the photosensitive material layer includes a UV-curable material, which may be a positive type or a negative type. The photosensitive layer  22  is a photoresist layer, for example. A thickness of the photosensitive layer  22  is, for example, 0.1 to 1 micrometer. Since the photosensitive layer  22  is patterned by an exposure method, the photosensitive layer  22  is accurately aligned and thus fine patterns are formed. 
     Then, referring to  FIGS. 1B and 1C , using the patterned photosensitive layer  22  as a mask, the etching process is performed to remove a part of the barrier material layer  20  and a part of the organic active material layer  18  (shown on  FIG. 1B ). Thereby, a barrier layer  20   a  and an organic active layer  18   a  are formed in the channel region  12  between the source  14  and the drain  16  (or the source  16  and the drain  14 ) and surfaces of the source  14  and the drain  16  (or the source  16  and the drain  14 ) are exposed (shown on  FIG. 1C ). Next, the patterned photosensitive layer  22  is further removed. A method of removing the part of the barrier material layer  20  and the part of the organic active material layer  18  and the patterned photosensitive layer  22  comprises, for example, performing the etching process using oxygen plasma. 
     Afterwards, referring to  FIG. 1D , the dielectric layer  24  is formed on the substrate, covering upper surfaces of the source  14 , the drain  16  (or the source  16 , the drain  14 )) and the barrier layer  20   a  and sidewalls of the barrier layer  20   a  and the organic active layer  18   a . The material of the dielectric layer  24  includes an inorganic dielectric material or an organic dielectric material. The inorganic dielectric material is, for example, silicon oxide or silicon nitride. The organic dielectric material is, for example, poly(4-vinyl phenol) (PVP) or parylene. A method of forming the dielectric layer  24  comprises, for example, a chemical vapor deposition method, a printing method, a spin coating method, an ink-jet method, a dip coating method, an evaporation method or other physical deposition methods. Since the surface of the organic active layer  18   a  is covered by the barrier layer  20   a  and the side of the organic active layer  18   a  is covered by the dielectric layer  24 , sidewall leakage problems caused by the organic active layer  18   a  being exposed in air and being destroyed by moisture or oxygen are avoided. 
     Then, a gate  26  is formed on the dielectric layer  24  between the source  14  and the drain  16  (or the source  16  and the drain  14 ), completing fabricating an OTFT  100 A of the top gate structure. A method of forming the gate  26  comprises, for example, forming a gate material layer, and patterning the gate material layer using photolithographic and etching processes. A material of the gate material layer includes a metal or doped polysilicon. The metal is, for example, aluminum, copper, molybdenum, chromium or alloys of above. A method of forming the gate material layer comprises, for example, a physical vapor deposition process or a chemical vapor deposition process. The physical vapor deposition process is, for example, a sputtering process or an evaporation process. According to another embodiment, the method of forming the gate  26  may comprise directly forming a patterned conductive layer, for example with an inkjet printing method or other printing or transfer printing technologies. Then, an interlayer dielectric layer (not shown) may be formed on the substrate  10  to cover the gate  26  and the dielectric layer  24 . 
       FIGS. 2A to 2D  are cross-sectional views illustrating a method of fabricating an OTFT of a bottom gate structure according to another embodiment of the present invention. Materials and method of formation of components of the OTFT of the following are similar to that according to the above embodiment, and are not repeatedly described for simplification. 
     First, referring to  FIG. 2A , the gate  26  is formed on the substrate  10 , and then the dielectric layer  24  is formed on the substrate  10  to cover the gate  26 . 
     Then, referring to  FIG. 2B , the source  14  and the drain  16  (or the source  16  and the drain  14 ) are formed on the dielectric layer  24  at the two sides of the gate  26 . Next, the organic active material layer  18  is formed on the substrate  10 . 
     Then, referring to  FIG. 2C , the barrier material layer  20  is formed on the organic active material layer  18 . Next, the patterned photosensitive layer  22  is formed on the barrier material layer  20 . The thickness of the photosensitive layer  22  is not specifically limited, which is for example, 0.5 to 5 micrometers. 
     Afterwards, referring to  FIGS. 2C and 2D , using the patterned photosensitive layer  22  as the mask, the part of the barrier material layer  20  and the part of the organic active material layer  18  are removed to form the barrier layer  20   a  and the organic active layer  18   a  in the channel region  12  between the source  14  and the drain  16  (or the source  16  and the drain  14 ), thereby exposing the surfaces of the source  14  and the drain  16  (or the source  16  and the drain  14 ). Next, the patterned photosensitive layer  22  may be selectively removed, meaning that the patterned photosensitive layer  22  can be removed or preserved, completing an OTFT  100 B of the bottom gate structure. 
     Afterwards, the interlayer dielectric layer (not shown) is formed on the substrate  10  to cover the surfaces of the source  14 , the drain  16  (or the source  16 , the drain  14 ) and the barrier layer  20   a  and the sidewalls of the barrier layer  20   a  and the organic active layer  18   a.    
       FIGS. 3A and 3B  are electrical characteristic views showing OTFTs of top gate structures formed by and not by patterning organic active layers using methods according to the embodiments of the present invention and the prior art respectively.  FIGS. 4A and 4B  are electrical characteristic views showing OTFTs of bottom gate structures formed by and not by patterning organic active layers using methods according to the embodiments of the present invention and the prior art respectively. Relative results are shown as in Table 1. 
     
       
         
               
               
             
               
               
               
             
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 OTFT 
               
             
          
           
               
                   
                 Top gate structure 
                 Bottom gate structure 
               
             
          
           
               
                   
                 Organic active layer 
               
             
          
           
               
                   
                 Patterned 
                 Unpatterned 
                 Patterned 
                 Unpatterned 
               
               
                   
                   
               
             
          
           
               
                 Line width W/Space 
                 200/20 
                 200/20 
                 200/100 
                 200/100 
               
               
                 L (micrometer/ 
               
               
                 micrometer) 
               
               
                 Mobility rate 
                 2.5 × 10 −3   
                 3.6 × 10 −3   
                 1.1 × 10 −2   
                 5.9 × 10 −3   
               
               
                 (cm 2 /V · second) 
               
               
                 On/off current ratio 
                 1.3 × 10 5   
                 1.04 × 10 2   
                 1.2 × 10 4   
                 4.3 × 10 2   
               
               
                 Threshold voltage 
                 8.2 
                 −2.86 
                 −13.38 
                 −6.35 
               
               
                 (V) 
               
               
                   
               
             
          
         
       
     
     Referring to the above Table, the OTFT formed according to the embodiments of the present invention has excellent functional characteristics. 
     In summary, the method of the present invention uses the photolithographic and etching processes to pattern the organic active layer. Hence, the very fine patterns can be defined, so that the organic active layer solely covers the channel region, and does not cover or only slightly covers parts of the source and the drain region. Moreover, since it is the dielectric layer that directly covers the source and the drain, problems of components on non-channel regions having undesirable optical characteristics due to the organic active layer being unpatterned are improved. Furthermore, since the surface of the organic active layer is covered by the barrier layer and the sidewall of the organic active layer is covered by the dielectric layer, the sidewall leakage problems caused by the organic active layer being exposed in air and being destroyed by moisture or oxygen are avoided. In addition, the method according to the embodiments of the present invention may be completed under temperatures lower than 150 degrees centigrade. Hence, the components have excellent functional characteristics and reliability of the components is enhanced. Additionally, the processes of the OTFT disclosed according to the embodiments of the present invention are simple and may be completed using coating processes and in large areas. Therefore, costs of using expensive machinery are reduced, and costs of materials of the processes and costs the processes are also reduced. The method of the present invention is therefore suitable for being used in mass production. 
     Although the present invention has been described with reference to the above embodiments, it will be apparent to one of the ordinary skill in the art that modifications to the described embodiment may be made without departing from the spirit of the invention. Accordingly, the scope of the invention will be defined by the attached claims not by the above detailed description.