Abstract:
To provide a semiconductor device restraining high frequency impedance and restraining deterioration of a semiconductor layer, a gate wiring  26  is extended while meandering and intersects with a substantially straight line portion of a semiconductor layer  02  by a plurality of times thereby providing a plurality of gates.

Description:
This is a divisional of U.S. application Ser. No. 08/965,907, filed Nov. 7, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention disclosed in the specification relates to a semiconductor device constituted by using a thin film transistor having a plurality of gate electrodes. Further, the present invention relates to a method of fabricating the semiconductor device. 
     2. Description of Related Art 
     In recent years, there has been increased thin film transistors using polycrystal silicon films in semiconductor layers. According to a thin film transistor using a polycrystal silicon film, high speed operation that is faster than operation of a thin film transistor using an amorphous silicon film by two digits or more can be performed since a mobility thereof is large. 
     Therefore, there poses a problem of hot carrier effect in which hot carriers generated in a channel jump into a gate insulating film and deteriorate a threshold voltage or a mutual conductance. 
     Conventionally, in respect of the above-described problem, there has been known a thin film transistor of a multi gate type alleviating the hot carrier effect by providing a plurality of gate electrodes and weakening an electric field applied on a single gate. 
     FIG. 5 shows an example where a thin film transistor of a conventional double gate type is used as a switching element of a pixel matrix portion of a liquid crystal display device. 
     As shown by FIG. 5, a semiconductor layer  02  and a source wiring  25  form a contact at a source electrode  22 . Further, the semiconductor layer  02  is extended while meandering and intersects with a gate wiring  26  at regions  21  and  21 ′. Further, the semiconductor layer  02  and a pixel electrode  24  form a contact at a drain electrode  23 . Portions of the gate wiring at the intersected regions  21  and  21 ′ function as gate electrodes. 
     As is an apparent from FIG. 5, the conventional multi gate type thin film transistor is constituted by the gate wiring  26  in a substantially straight line shape and the meandering semiconductor layer  02 . 
     By adopting such a constitution, a distance between the source and the drain is prolonged and therefore, the ON resistance is increased. Further, the resistance of a semiconductor layer is generally larger than that of a metal conductor and therefore, in respect of the semiconductor layer  02  meandering as shown by FIG. 5, the high frequency impedance of the meandering portion is increased which gives rise to deterioration of the element. 
     It is the object of the present invention disclosed in the specification to resolve the above-described problem. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention disclosed in the specification, there is provided a semiconductor device, wherein a meandering gate wiring traverses a substantially straight line portion of a semiconductor layer of a thin film transistor by a plurality of times thereby providing a plurality of gates. 
     According to other aspect of the present invention, there is provided a semiconductor device, wherein a switching element of a pixel matrix portion of a liquid crystal display device is a multi gate type thin film transistor in which a meandering gate wiring traverses a substantially straight line portion of a semiconductor layer by a plurality of times thereby providing a plurality of gates. 
     Further, the above-described gate wiring is featured in comprising a metal having the resistance smaller than the resistance of the semiconductor layer. 
     According to other aspect of the present invention, there is provided a method of fabricating a semiconductor device including a step of forming a semiconductor layer having a substantially straight line portion on a substrate, a step of forming a gate insulating film and a metal conductive film above the semiconductor layer, a step of pattering the metal conductive film into a gate wiring, a step of doping impurities to the semiconductor layer with the gate wiring as a mask and a step of irradiating a laser beam wherein the gate wiring meanders and intersects the substantially straight line portion of the semiconductor layer by a plurality of times. 
     According to the present invention, the gate comprising a metal having small resistance is meandered and therefore, an increase in impedance in the resistance imposed on the meandering portion is small. Further, there is no meandering portion in the semiconductor layer and therefore, deterioration by heat generation or the like can be restrained. 
     Further, the distance between the source and the drain is shortened compared with that in the conventional example and therefore, the ON resistance can be reduced and the thin film transistor having high mobility can be fabricated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a pixel matrix portion constituted by using the present invention; 
     FIGS. 2A,  2 B,  2 C and  2 D are views showing steps of fabricating a pixel matrix portion constituted by using the present invention; 
     FIG. 3 is a top view of a pixel matrix portion constituted by using the present invention; 
     FIG. 4 is a sectional view of a pixel matrix portion constituted by using the present invention; and 
     FIG. 5 is a top view of a conventional pixel matrix portion. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an example of using a thin film transistor according to the present invention in a switching element of a pixel matrix portion of a liquid crystal display device. 
     A contact is formed by a source wiring  25  and a semiconductor layer  02  at a source electrode  22 . Further, the semiconductor layer  02  which is extended in a substantially straight line shape and a gate wiring  26  intersect with each other at portions  21  and  21 ′. 
     Further, a contact is formed by the semiconductor layer  02  and a pixel electrode  24  at a drain electrode  23 . A field effect transistor is constituted by forming gate electrodes at the regions  21  and  21 ′ where the meandering gate electrode  26  intersects with the semiconductor layer  02 . 
     FIGS.  2 (A),  2 (B),  2 (C) and  2 (D) show steps of fabricating the semiconductor device shown by FIG.  1 . 
     First, a semiconductor layer  02  is formed on an insulating substrate  01 . Next, a gate insulating film  10  is formed on the semiconductor layer and on top of the gate insulating film  10 , a metal conductive film  20  for constituting a gate wiring is formed all over the face. Further, the surface of the metal conductive film  20  is oxidized by an anodic oxidation process thereby forming an upper anodically oxidized film  11 . 
     Further, a resist mask is coated on the upper anodically oxidized film  11  and patterning is performed in a form of the gate wiring thereby providing portions  30  and  30 ′. In this way, a state of FIG.  2 (A) is produced. Further, the gate wiring is formed by etching the upper anodically oxidized film  11  and portions of the metal conductive film  20  which are not covered by the resist mask. Successively, the anodic oxidation process is performed again on the gate wiring by which porous anodically oxidized films  12  and  12 ′ and anodically oxidized films  13  and  13 ′ having a dense film quality are formed. At the same time, gate electrodes  21  and  21 ′ are formed. 
     Under this state, impurities are added by which a source region  04 , a drain region  05  and an impurity added region  06  are formed. In this way, a state shown by FIG.  2 (B) is produced. Next, the porous anodically oxidized films  12  and  12 ′ are removed. In this way, a state shown by FIG.  2 (C) is produced. That is, there is formed a thin film transistor of a double gate type having a semiconductor layer in which channel regions  03  and  03 ′ are formed below the respective gate electrodes  21  and  21 ′ and offset regions  07  and  07 ′ are formed contiguous to the channel regions. 
     Thereafter, a first interlayer insulating film  15  is formed all over the face, contact holes are opened at the source region  04  and the drain region  05  and a source electrode  22  and a drain electrode  23  are formed. Incidentally, the source electrode is constituted by a portion of a source wiring. Further, a second interlayer insulating film  16  is formed and a contact hole for the drain electrode  23  and a pixel electrode  24  is formed. Next, the pixel electrode  24  is formed. In this way, as shown by FIG.  2 (D), the thin film transistor that is a switching element of an active matrix device. 
     EMBODIMENT 1 
     FIGS.  2 (A),  2 (B),  2 (C) and  2 (D) show fabrication steps of the embodiment. According to the embodiment, there are shown steps of fabricating on a glass substrate a thin film transistor that is a switching element of a pixel matrix portion. 
     Although in this embodiment, a glass substrate is used as a substrate, a quartz substrate, a semiconductor substrate having an insulating surface may be used in place of the glass substrate. 
     First, an underlayer protective film, not illustrated, is formed on a glass substrate  01 . The underlayer protective film contributes to prevention of diffusion of impurities from the substrate, relaxation of thermal stress of the thin film transistor and relaxation of internal stress. According to the embodiment, a silicon oxide film is formed by a film thickness of 2000 ∪ as the underlayer protective film through a sputtering process. 
     Next, a semiconductor layer is formed. In the embodiment, a silicon film is used as the semiconductor. Although a plasma CVD (Chemical Vapor Deposition) process, a low pressure thermal CVD process and the like may be used as methods of fabricating a silicon film, in this embodiment, an amorphous silicon film of I type (intrinsic or substantially intrinsic electric conductiveness) is formed by a plasma CVD process. The film thickness of the silicon film falls in a range of 300 through 1000 ∪ and according to the embodiment, the silicon film is formed by a film thickness of 500 ∪. 
     Further, according to the embodiment, the amorphous silicon film is crystallized to constitute polycrystals. Although as crystallization processes, there are a thermal crystallization process and a crystallization process using a laser beam, according to the embodiment, the crystallization is performed by using a laser beam. Naturally, when an amorphous silicon is used as a semiconductor layer, the step of crystallization is not needed. 
     Patterning is performed on the polycrystal silicon film formed as described above, by which a pattern designated by numeral  02  of FIG.  2 (A) is formed. Next, a gate insulating film  10  is formed on the semiconductor layer  02 . A silicon oxide film, a silicon nitride film, a silicon oxinitride film or a laminated film of these or the like is used as the gate insulating film  10 . In this embodiment, a silicon oxinitride film is formed by the plasma CVD process using TEOS (tetraethylorthosilicate) and N 2 O as raw materials. The film thickness of the gate insulating film falls in a range of 500 through 2000 ∪ and according to the embodiment, the gate insulating film is formed with a film thickness of 1200 ∪. 
     Next, an aluminum film having a film thickness of 3000 through 10000 ∪, or 4000 ∪ in this embodiment is formed on the gate insulating film  10  by a sputtering process as a metal conductive film  20 . The metal conductive film  20  constitutes a gate wiring in later steps. 
     In forming the aluminum film, an aluminum alloy target including a substance of silicon, scandium or the like by 0.1 through 5.0 weight % is used. In this embodiment, the aluminum film is formed by using a target including 0.2 weight % of scandium. 
     Scandium is included to restrain formation of projections referred to as hillocks or whiskers by abnormal growth of aluminum in a later thermal step at 100° C. or higher. 
     Although in the embodiment, an aluminum alloy is used for the metal conductive film, a Cr film, a Ti film, a Ta film, a MoTa film or a laminated film of these may be used. 
     Next, an anodic oxidation using tartaric acid as an electrolyte is performed with the aluminum film  20  as an anode. 
     By this step, an upper anodically oxidized film  11  is formed on the surface of the aluminum film  20  with a film thickness of 100 ∪. 
     The upper anodically oxidized film  11  functions to restrain occurrence of hillocks or whiskers in later steps. Further, the anodically oxidized film also functions to prevent shortcircuit in the vertical direction between the metal conductive film for constituting a gate wiring and a wiring arranged thereabove. 
     Next, a resist mask is coated on the anodically oxidized film  11 . Further, the resist mask is patterned in a form of a gate wiring. In this way, resist patterns  30  and  30 ′ are formed. 
     When a state shown by FIG.  2 (A) is produced in this way, the anodically oxidized film  11  and the aluminum film  20  are etched into the shape of a gate wiring by using the resist masks  30  and  30 ′. 
     Successively, anodic oxidization is performed with the gate wiring as an anode. In this case, oxalic acid is used for an electrolytic solution under the following conditions. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Voltage 
                 8 V 
               
               
                   
                 Temperature 
                 30° C. 
               
               
                   
                 Time 
                 40 minutes 
               
               
                   
                   
               
             
          
         
       
     
     In this way, the porous anodically oxidized films designated by notations  12  and  12 ′ of FIG.  2 (B) are formed. In the anodic oxidation, the porous anodically oxidized film is not formed at the upper portion of the gate wiring since the upper anodically oxidized films  11  and  11 ′ remain thereabove. 
     Next, further anodic oxidation is performed. In the anodic oxidation, tartaric acid is used for an electrolytic solution under the following condition. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Voltage 
                 100 V 
               
               
                   
                 Temperature 
                 10° C. 
               
               
                   
                 Time 
                 45 minutes 
               
               
                   
                   
               
             
          
         
       
     
     In this way, anodically oxidized films  13  and  13 ′ having a dense film quality are formed between side walls of the gate electrodes and the porous anodically oxidized films  12  and  12 ′. Thus, gate electrodes  21  and  21 ′ are formed at portions where the gate wiring intersects with the semiconductor layer. 
     The dense anodically oxidized films  13  and  13 ′ are formed to prevent aluminum constituting the gate wiring from being etched by an etchant in removing the porous anodically oxidized films  12  and  12 ′. 
     Next, impurities are doped with the gate electrodes  21  and  21 ′ formed with the anodically oxidized films as masks. As processes of doping impurities, there are a plasma doping process and an ion implantation process. In this embodiment, impurities are doped by a plasma doping process that is also adaptable to a large area. 
     In the embodiment, a thin film transistor having an N type conductiveness is formed by doping phosphorus as an impurity. A thin film transistor having a P type conductiveness may be fabricated by doping boron in place of phosphorus. 
     In this way, as shown by FIG.  2 (B), a source region  04 , a drain region  05  and a N type region  06  added with impurities are formed. At the same time, I type regions  08  and  08 ′ which have not been added with impurities due to presence of the gate electrodes  21  and  21 ′ remain. 
     Next, the porous anodically oxidized films  12  and  12 ′ are removed. The etching is performed by the wet etching process using an etchant mixed with acetic acid, nitric acid, phosphoric acid and water. Further, the source region  04 , the drain region  05  and the N type region  06  which become amorphous by being doped with phosphor, are irradiated with a laser beam. 
     By irradiating the laser beam, the regions  04 ,  05  and  06  doped with impurities are activated and crystallized. In this embodiment, an excimer laser is used as a laser beam. 
     In this way, a state as shown by FIG.  2 (C) is produced. In this case, regions below the gate electrodes  21  and  21 ′ constituting the I type layers, become channel regions  03  and  03 ′. Further, the I type regions which are made to remain by presence of the porous anodically oxidized films  12  and  12 ′ and the dense anodically oxidized films  13  and  13 ′, become offset regions  07  and  07 ′. 
     Next, a silicon nitride film is formed as a first interlayer insulating film  15  by a film thickness of 3000 ∪ through a plasma CVD process. Although in the embodiment, the silicon nitride film is used for the first interlayer insulating film  15 , a silicon oxide film, a silicon oxinitride film or a laminated film of these may be used. 
     Further, contact holes are perforated at the source region  04  and the drain region  05 . Then, a source electrode  22  and a drain electrode  23  are formed. Incidentally, a source wiring is extended from the source electrode. 
     In this embodiment, the source wiring and the drain electrode are formed by forming three layers film of a titanium film, an aluminum film and a titanium film by a sputtering process and patterning the three layers film. 
     Next, a second interlayer insulating film  16  is formed. In this embodiment, a laminated film of a silicon nitride film and a polyimide film is used as the second interlayer insulating film  16 . Further, a contact hole is formed at the drain electrode  23  and a pixel electrode  24  is formed as shown by FIG.  2 (D). 
     In this embodiment, the pixel electrode  24  is formed by forming an ITO (Indium Tin Oxide) film by a thickness of 1000 Å through a sputtering process and patterning it. Finally, the thin film transistor is hydrogenerated and the wiring is sintered by performing a heating treatment in a hydrogen atmosphere at 350° C. 
     In this way, a pixel matrix portion of a liquid crystal display device is finished as shown by FIG.  2 (D). Although in the embodiment, only the offset regions are formed, lightly doped impurity regions may be formed by adding impurities at a low concentration after removing the porous anodically oxidized films  12  and  12 ′. 
     EMBODIMENT 2 
     This embodiment is featured in using a thin film transistor of a triple gate type at a pixel matrix portion as shown by the constitution of FIG.  3 . 
     That is, a source wiring  25  and the semiconductor layer  02  constitute a contact at a portion of the source electrode  22  and the pixel electrode  24  and the semiconductor layer  02  constitute a contact at a portion of the drain electrode  23 . Further, regions  21 ,  21 ′ and  21 ″ where the meandered and extended gate wiring  26  intersects with the semiconductor layer  02 , function as gate electrodes. 
     The fabrication steps of the embodiment are performed by changing the pattern of the resist mask in the step of patterning the gate wiring of Embodiment 1. 
     EMBODIMENT 3 
     The embodiment is an example of using a thin film transistor having the constitution of a bottom gate. FIG. 4 shows the sectional view. 
     That is, gate electrodes  21  and  21 ′ are present on top of the glass substrate  01 , on which the gate insulating film  10  and the semiconductor layer are arranged. The semiconductor layer includes the source region  04 , the drain region  05  and the impurity added region  06  which are added with impurities and the I type channel regions  03  and  03 ′. 
     Further, the first interlayer insulating film  15  is present thereon and the source electrode  22  in contact with the source region  04  and the drain electrode  23  forming a contact along with the drain region are placed. Further, the second interlayer insulating film  16  is present thereon and the pixel electrode  24  and the drain electrode  23  forms a contact. Although not shown in the drawing, offset regions or lightly doped regions may be formed depending upon the desired characteristics of the transistors. 
     By using the present invention disclosed in the specification, a semiconductor device having small ON resistance and small power consumption can be provided. Further, a semiconductor device having inconsiderable deterioration of a semiconductor layer and high reliability can be provided.