Abstract:
The present invention discloses a thin film transistor and a method of fabricating the same. The thin film transistor includes an insulating substrate; and a semiconductor layer, a gate insulating layer, a gate electrode, an interlayer insulator, and a source/drain electrode which are formed on the substrate, wherein the gate insulating layer is formed of a filtering oxide layer having a thickness of 1 to 20 Å.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of prior application Ser. No. 11/019,456, filed on Dec. 23, 2004, which claims priority to and the benefit of Korean Patent Application No. 2004-64046, filed Aug. 13, 2004, both of which are hereby incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thin film transistor and a method of fabricating the same and, more particularly, to a thin film transistor having excellent characteristics and a method of fabricating the same in which a dual capping layer is formed of an oxide layer that a metal catalyst is difficult to diffuse and a nitride layer that a metal catalyst is easy to diffuse, a metal catalyst layer is formed on the dual capping layer, and then a crystallization process is performed to form a polysilicon layer having a large-size grain because the metal catalyst does not easily pass through the oxide layer and so a small amount of metal catalyst contributes a crystallization, and a semiconductor layer is formed of the polysilicon layer. 
     2. Description of the Related Art 
     In thin film transistor (“TFT”) used in a display device, a semiconductor layer is formed such that an amorphous silicon layer is deposited on a transparent substrate made of a glass or a quartz and the amorphous silicon layer is subjected to a dehydrogenation treatment and then is crystallized. 
     At this time, the semiconductor layer which constitutes a source, a drain and a channel area is formed by depositing an amorphous silicon layer on a transparent substrate made of a material such as a glass using a chemical vapor deposition (“CVD”) technique. The silicon layer deposited directly on the substrate using a CVD technique is an amorphous silicon layer which contains a 12% of hydrogen and thus has a low electron mobility, and when the amorphous silicon layer having such a low electron mobility is heat-treated and crystallized into a silicon layer of crystalloid structure having a high electron mobility, the silicon layer may be damaged since hydrogen contained therein may burst. In order to prevent a burst of hydrogen which may occur during crystallization, a dehydrogenation process is carried out. The dehydrogenation process is performed such that a heat-treatment is performed in the furnace at a temperature of more than about 400° C. for tens of minutes to tens of hours. Then, the dehydrogenated amorphous silicon layer is subject to a crystallization process. 
     The crystallization technique which crystallizes an amorphous silicon layer to form a poly silicon layer includes a solid phase crystallization technique, an excimer laser crystallization technique, a metal induced crystallization (MIC) technique, and a metal induced lateral crystallization (MILC) technique. The solid phase crystallization technique is one which heat-treats and crystallizes an amorphous silicon layer for several hours to tens of hours at a temperature of less than about 700° C. which is a temperature that may transform a glass which forms a substrate of a display device on which a TFT is formed. The excimer laser crystallization process is one which scans an excimer laser to an amorphous silicon layer to be heated and crystallized at a high temperature for a very short time. 
     However, the solid phase crystallization technique has disadvantages in that a relatively lengthy processing time is required and a substrate is exposed to a high temperature for a long time and thus may be easy to transform. The excimer laser crystallization technique has also disadvantages in that a high price laser device is needed and also an extrusion may occur on a crystallized surface so that an interface characteristic between a semiconductor layer and a gate insulating layer is bad. The MIC or MILC technique has disadvantages in that a large amount of metal catalyst remains on the polysilicon layer to thereby increase a leakage current of the semiconductor layer of the TFT. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a thin film transistor having excellent characteristics and a method of fabricating the same in which after sequentially forming a filtering oxide layer and a capping layer on an amorphous silicon layer and then forming a metal catalyst layer on the capping layer, in a super gain silicon (SGS) crystallization method of crystallizing the amorphous silicon layer by diffusing the metal catalyst through the capping layer and the filtering oxide layer, among the metal catalysts which pass through the capping layer and then are diffused to an interface between the capping layer and the filtering oxide layer, a smaller amount of the metal catalyst are allowed to pass through the filtering oxide layer using a feature that the metal catalyst is not easy to diffuse in the filtering oxide layer and contribute to crystallization, so that a grain size is larger and the remaining metal catalyst is small. 
     In order to achieve the object, the present invention provides a thin film transistor, comprising: an insulating substrate; and a semiconductor layer, a gate insulating layer, a gate electrode, an interlayer insulator, and a source/drain electrode which are formed on the substrate, wherein the gate insulating layer is formed of a filtering oxide layer having a thickness of 1 to 20 Å. 
     The present invention further provides a thin film transistor, comprising: an insulating substrate; and a semiconductor layer, a gate insulating layer, a gate electrode, an interlayer insulator, and a source/drain electrode, wherein the gate insulating layer is formed of a filtering natural oxide layer. 
     The gate insulating layer has a single or dual layer of a silicon oxide layer or/and a silicon nitride layer. 
     The present invention further provides a method of fabricating a thin film transistor, comprising: preparing an insulating substrate; forming an amorphous silicon layer on the substrate; forming a filtering oxide layer and a capping layer on the amorphous silicon layer; depositing a metal catalyst on the capping layer; performing a first heat-treatment to diffuse the metal catalyst to move to an interface between the filtering oxide layer and the amorphous silicon layer through the capping layer and the filtering oxide layer; performing a second heat-treatment to crystallize the amorphous silicon layer by the diffused metal catalyst to form a polysilicon layer; removing the capping layer; patterning the filtering oxide layer and the polysilicon layer to form a semiconductor layer; forming a gate insulating layer above the substrate; and forming a gate electrode above the substrate, and forming an interlayer insulator and a source/drain electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
         FIGS. 1   a  to  1   d  are cross-sectional views illustrating a crystallization process according to the present invention; 
         FIG. 2  is a photograph illustrating a plan of a polysilicon layer according to the present invention; and 
         FIG. 3  is a cross-sectional view illustrating a process of fabricating a thin film transistor using the polysilicon layer fabricated according to the present invention 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout the specification. 
       FIGS. 1   a  to  1   d  are cross-sectional views illustrating a crystallization process according to the present invention. 
     First,  FIG. 1   a  is a cross-sectional view illustrating a process of sequentially forming a buffer layer, an amorphous silicon layer and a filtering oxide layer on a substrate. As shown in  FIG. 1   a , a buffer layer  102  is formed on an insulating substrate  101  made of a plastic or a glass using a chemical vapor deposition (“CVD”) technique or a physical vapor deposition (“PVD”) technique. The buffer layer  102  has a single-layer or dual-layer structure of a silicon oxide layer or/and silicon nitride layer. Here, the buffer layer  102  serves to prevent a diffusion of moisture or impurities which may occur on the substrate  101  or to control a heat transfer speed during a crystallization process, thereby helping a crystallization of the semiconductor layer. 
     Subsequently, an amorphous silicon layer  103  is formed on the buffer layer  102 . Typically, the amorphous silicon layer  103  is formed using a CVD technique. The amorphous silicon layer formed using a CVD technique has a gas such hydrogen which decreases an electron mobility, and thus a dehydrogenation process is performed so that hydrogen does not remain in the amorphous silicon layer. 
     Then, a filtering oxide layer  104  is formed on the amorphous silicon layer  103 . The filtering oxide layer  104  is formed of a silicon oxide layer in which a metal catalyst is not easy to diffuse. 
     The silicon oxide layer may be formed by a deposition using a CVD technique or a PVD technique, or using a thermal oxide layer or a natural oxide layer using a UV oxidation technique, an oxygen plasma technique, or a natural oxidation technique. The CVD or PVD technique is performed such that an oxide layer is deposited on the amorphous silicon layer. The UV oxidation technique, the oxygen plasma technique, or the natural oxidation technique is performed such that a UV light is irradiated to a surface of the amorphous silicon layer to form a thermal oxide layer, or a thermal oxide layer is formed by heating the substrate, or an oxygen plasma is applied to a surface of the amorphous silicon layer to form a thermal oxide layer, or the amorphous silicon layer is exposed to the air or a vacuum which contains oxygen during several seconds to tens of minutes to thereby form a natural oxide layer. 
     A preferred method of forming the filtering oxide layer is the oxygen plasma technique. The oxygen plasma technique for forming the thermal oxide layer is performed under a condition of a process power of 100 to 1,000 W, a process time of 10 to 1,000 seconds, a process pressure of 70 to 400 Pa. Another preferred method is a method that a surface of the amorphous silicon layer is exposed to the air or a vacuum which contains oxygen so that a natural oxide layer is naturally formed on a surface of the amorphous silicon layer. 
     Here, a thickness of the filtering oxide layer is in a range of 1 to 20 Å. This is because a thickness of less than 1 Å means that the filtering oxide layer hardly remains and a thickness of more than 20 Å means that a metal catalyst hardly passes through the filtering oxide layer, which makes it impossible to crystallize the amorphous silicon layer. 
       FIG. 1   b  is a cross-sectional view illustrating a process of forming a capping layer and a metal catalyst layer on the filtering oxide layer. As shown in  FIG. 1   b , a capping layer  105  is formed on the filtering oxide layer  104 . The capping layer is preferably formed of a silicon nitride layer that a metal catalyst can be diffused by a heat-treatment process and may have a dual-layer structure of a silicon nitride layer and a silicon oxide layer. The capping layer  105  is formed using a CVD or PVD technique. A thickness of the capping layer is in a range of 1 to 2,000 Å. 
     Subsequently, a metal catalyst layer  106  is formed on the capping layer. The metal catalyst layer is formed of one of Ni, Pd, Ti, Ag, Au, Al, Sn, Sb, Cu, Co, Mo, Tr, Ru, and Rh, and preferably of nickel (Ni). 
     During a MIC or MILC technique, it is required to precisely control a thickness or density of the metal catalyst layer. This is because after the crystallization process the metal catalyst may remain on a surface of a polysilicon layer, resulting in an increase of a leakage current of the TFT. However, in the present invention, there is no need of precisely controlling a thickness or density of the metal catalyst layer, that is, the metal catalyst layer may be formed at a relatively thick thickness. This is because the filtering oxide layer filters the metal catalysts which are diffused so that just a small amount of a metal catalyst contribute to a crystallization, and most of them do not pass through the filtering oxide layer so that it does not contribute to a crystallization. 
       FIG. 1   c  is a cross-sectional view illustrating a process of diffusing metal catalysts through the capping layer and the filtering oxide layer by performing a first heat-treatment process and moving metal catalysts to an interface between the filtering oxide layer and the amorphous silicon layer. As shown in  FIG. 1   c , the substrate on which the buffering layer, the amorphous silicon layer, the filtering oxide layer, the capping layer, and the metal catalyst layer are formed is subjected to a first heat-treatment process  107 , so that some of the metal catalysts of the metal catalyst layer  106  move to a surface of the amorphous silicon layer  103 . That is, among the metal catalysts  106   a  and  106   b  which are diffused through the capping layer  105  due to the first heat-treatment process, just a small amount of metal catalysts  106   b  passes through the filtering oxide layer to be diffused to a surface of the amorphous silicon layer, and most of them do not reach nor pass through the filtering oxide layer. Therefore, an amount of the metal catalyst that reaches a surface of the amorphous silicon layer depends on a diffusion blocking ability of the filtering oxide layer, and a diffusion blocking ability of the filtering oxide layer is closely related to a thickness thereof. That is, as a thickness of the filtering oxide layer is thick, an amount of the catalyst to be diffused is smaller and a size of a grain is bigger, and as a thickness of the filtering oxide layer is thin, an amount of the catalyst to be diffused is more and a size of a grain is smaller. 
     Here, the first heat-treatment process of diffusing the metal catalyst is performed at a temperature of 200 to 800° C., and uses at least one of a furnace process, a RTA process, a UV process, and a laser process. 
       FIG. 1   d  is a cross-sectional view illustrating a process of crystallizing the amorphous silicon layer due to the diffused metal catalyst by performing a second heat-treatment process  108 , thereby forming a polysilicon layer. As shown in  FIG. 1   d , if the second heat-treatment process  108  is performed, due to the metal catalysts  106   b  which pass through the capping layer and the filtering layer and are diffused to a surface of the amorphous silicon layer, the amorphous silicon layer is crystallized to form a polysilicon layer  109 . That is, the metal catalyst is combined with the amorphous silicon to form a metal silicide, and the metal silicide serves as a core of crystallization and induces crystallization of the amorphous silicon layer. 
     Here, as a crystallization technique of the present invention, a super grain silicon (“SGS”) crystallization technique is used that forms the capping layer on the amorphous silicon layer, forms the metal catalyst layer on the capping layer, heating the metal catalyst layer to diffuse the metal catalysts, and crystallizes the amorphous silicon layer by the diffused metal catalysts to form the polysilicon layer. 
     Thus, by controlling an amount of a metal which is converted to the metal silicide which is a core of the crystallization, a grain size of the polysilicon can be controlled, and a control of a grain size is determined by the metal catalyst which contributes to the crystallization. Thus, a grain size of the polysilicon can be controlled by a diffusion blocking ability of the filtering oxide layer. That is, a grain size of the polysilicon can be controlled by controlling a thickness of the filtering oxide layer. 
     In summary, as a thickness of the filtering oxide layer is thick, that is, a thickness of the filtering oxide layer is close to 20 Å, a grain size of the polysilicon layer is larger, whereas as a thickness of the filtering oxide layer is thin, that is, a thickness of the filtering oxide layer is close to 1 Å, a grain size of the polysilicon layer is smaller. 
     Here, if a thickness of the filtering oxide layer is more than 20 Å, the metal catalysts are hardly diffused, crystallization does not occur, whereas if a thickness of the filtering oxide layer is less than 1 Å, the filtering oxide layer has a meaning. Thus, it is preferred that a thickness of the filtering oxide layer is in a range of 1 to 20 Å. 
     The second heat-treatment process of  FIG. 1   d  is performed without removing the capping layer and the metal catalyst layer. However, before the second heat-treatment process, the capping layer and the metal catalyst layer may be removed. Also, the metal catalyst layer may be removed after the first heat-treatment process, and the capping layer may be removed after the second heat-treatment process. Here, the second heat-treatment process is performed at a temperature of 400 to 1,300° C. and may use one or more of a furnace process, a RTA process, a UV process, and a laser process. 
       FIG. 2  is a photograph illustrating a plan of the polysilicon layer fabricated according to the present invention. The polysilicon layer of the  FIG. 2  is the polysilicon layer fabricated by a process of  FIGS. 1   a  to  1   d . A crystallization process of the polysilicon layer is simple because there is no need for precisely controlling a thickness of the metal catalyst layer, and also it has less alien metal substance which remains on the polysilicon layer than a conventional MIC or MILC technique, whereby the polysilicon layer having excellent characteristics which is small in leakage current, large in gain size and high in electron mobility can be performed. 
       FIG. 3  is a cross-sectional view illustrating a process of fabricating a TFT using the polysilicon layer fabricated according to the present invention. As described in  FIGS. 1   a  to  1   d , a polysilicon layer crystallized by a SGS crystallization and a filtering oxide layer are formed on a substrate  201  having a buffer layer  202 , and as shown in  FIG. 3 , the polysilicon layer and the filtering oxide layer are patterned to form a semiconductor layer  203  and a filtering oxide layer pattern  204   a . At this time, the semiconductor layer has excellent leakage current in that a small amount of metal catalyst remains on the semiconductor layer due to the filtering oxide layer in comparison to that formed by different conventional crystallization techniques. 
     Thereafter, a single or dual layer of a silicon oxide layer or/and a silicon nitride layer  204   b  is formed on the substrate having the filtering oxide layer pattern and the semiconductor layer  203 . Here, the filtering oxide layer pattern  204   a  and the single or dual layer of the silicon oxide layer or/and the silicon nitride layer  204   b  serve as a gate insulating layer  204  of the TFT. 
     Subsequently, a gate electrode  205  is formed on a predetermined portion of the gate insulating layer  204 , and an interlayer insulator  206  is formed to protect a lower part structure. The interlayer insulator  206  and the gate insulating layer  204  are etched to form contact holes, and then source and drain electrodes  207  are formed to fill the contact holes, thereby completing the TFT. 
     The fabricated TFT is a TFT in which an amount of the metal catalyst is controlled by the filtering oxide layer so that a small amount of metal catalyst remains compared to the MIC or MILC technique, and the semiconductor layer has a large grain size of the polysilicon layer, and the gate insulating layer is formed of a part of the filtering oxide layer. 
     As described herein before, the TFT fabricated according to the present invention has excellent characteristics in that an amount of the metal catalyst which contributes to crystallization is controlled by the filtering oxide layer that the metal catalyst is difficult to diffuse, the polysilicon layer having a large grain size is formed by the controlled metal catalyst, and an amount of the metal catalyst which remains on the polysilicon layer is minimized.