Patent Publication Number: US-2022223707-A1

Title: Semiconductor device

Description:
The present application is a continuation application of International Application No. PCT/JP2020/035034, filed on Sep. 16, 2020, which claims priority to Japanese Patent Application No. 2019-181794, filed on Oct. 2, 2019. The contents of these applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a semiconductor device including a display device and an optical sensor device using a TFT made of an oxide semiconductor. 
     (2) Description of the Related Art 
     A TFT (Thin Film Transistor) including an oxide semiconductor has larger OFF resistance compared with a TFT including a poly silicon semiconductor, and has a higher mobility compared with a TFT including a a-Si (Amorphous silicon) semiconductor; therefore, it can be used in a display device such as a liquid crystal display device and an organic EL display device and so forth, or in semiconductor devices as sensors and so forth. 
     In a semiconductor device using a TFT, an aluminum (Al) wiring is often used for a drain electrode, a source electrode of a TFT, and a video signal line, a scanning line, and the like. This is because the wiring resistance can be reduced. However, the Al wiring tends to be broken due to electromigration or stress migration. In order to prevent this, Patent document 1 discloses to form titanium nitride (TiN) around an Al wiring. 
     In addition, in the above-described semiconductor device, ITO (Indium Tin Oxide) film which is a transparent metal oxide conductive film is used together with an Al wiring. When the ITO film and Al wiring are directly connected, Al deprives the oxygen of ITO and the Al wiring and the ITO film cannot electrically connect. In order to prevent this, in the comparative example of Patent Document 2, an Al wiring has a three layer structure of Ti, Al, and TiN. 
     PRIOR ART REFERENCE 
     Patent Document 
     
         
         [Patent document 1] Japanese patent application publication Hei 6-291119 
         [Patent document 2] Japanese patent application publication 2012-43821 
       
    
     SUMMARY OF THE INVENTION 
     When oxygen is removed, an oxide semiconductor becomes metallized and becomes conductive. Further, in a TFT including an oxide semiconductor, when oxygen is removed from a channel region, a TFT becomes conductive, and the operation as a TFT becomes impossible. 
     On the other hand, in a semiconductor device using a TFT, a metal is used for a gate electrode, a drain electrode, a source electrode, and the like of a TFT. Metals have the property of depriving oxygen. In the bottom-gate oxide semiconductor TFT, a gate insulating film is present on the gate electrode, and an oxide semiconductor film is provided on the gate insulating film. In such a configuration, a phenomenon occurs in which the gate electrode, which is a metal, deprives the oxide semiconductor of oxygen via the gate insulating film, and the oxide semiconductor TFT does not operate. 
     An object of the present invention is to prevent a phenomenon in which an oxide semiconductor TFT does not operate, in particular, when oxygen is removed from a channel region of the oxide semiconductor. 
     The present invention solves the above problems, and the main specific means thereof are as follows. 
     (1) A semiconductor device having a TFT, in which a gate insulating film is formed on a gate electrode, and an oxide semiconductor film is formed on the gate insulating film; the oxide semiconductor film including a channel region, a drain region, and a source region; in which a metal nitride film is formed on a top surface of the gate electrode in an opposing portion to the channel region in a plan view; and the metal nitride film is not formed at a part of the top surface of the gate electrode.
 
(2) A semiconductor device having a TFT, in which a gate insulating film is formed on a gate electrode, and an oxide semiconductor film is formed on the gate insulating film; the oxide semiconductor film includes a channel region, a drain region, and a source region; in which a metal oxide film is formed on a top surface of the gate electrode in an opposing portion to the channel region in a plan view, and the metal oxide film is not formed at a part of the top surface of the gate electrode.
 
(3) A semiconductor device having a TFT, in which a gate insulating film is formed on a gate electrode, and an oxide semiconductor film is formed on the gate insulating film; the oxide semiconductor film includes a channel region, a drain region, and a source region; in which an insulating metal oxide film is formed on a top surface of the gate electrode in an opposing portion to the channel region in a plan view, and the metal oxide film is not formed at a part of the top surface of the gate electrode.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a liquid crystal display device; 
         FIG. 2  is a plan view of the display area of a liquid crystal display device; 
         FIG. 3  is a cross sectional view of the display area of a liquid crystal display device; 
         FIG. 4  is a cross sectional view of the structure of a TFT and its vicinity according to a first example of embodiment 1; 
         FIG. 5  is an example of a cross-sectional view of the gate electrode according to embodiment 1; 
         FIG. 6  is a cross sectional view of the structure of a TFT and its vicinity according to a second example of embodiment 1; 
         FIG. 7  is a cross sectional view of the structure of a TFT and its vicinity according to a third example of embodiment 1; 
         FIG. 8  is a cross sectional view of the structure of a TFT and its vicinity according to a fourth example of embodiment 1; 
         FIG. 9  is a cross sectional view of the structure of a TFT and its vicinity according to a first example of embodiment 2; 
         FIG. 10  is a cross sectional view of the structure of a TFT and its vicinity according to a second example of embodiment 2; 
         FIG. 11  is a cross sectional view of the structure of a TFT and its vicinity according to a third example of embodiment 2; 
         FIG. 12  is a cross sectional view of the structure of a TFT and its vicinity according to a fourth example of embodiment 2; 
         FIG. 13  is a cross sectional view of the structure of a TFT and its vicinity according to a first example of embodiment 3; 
         FIG. 14  is a cross sectional view of the structure of a TFT and its vicinity according to a second example of embodiment 3; 
         FIG. 15  is a cross sectional view of the structure of a TFT and its vicinity according to a third example of embodiment 3; 
         FIG. 16  is a cross sectional view of the structure of a TFT and its vicinity according to a fourth example of embodiment 3; 
         FIG. 17  is a cross sectional view of the structure of a TFT and its vicinity according to a first example of embodiment 4; 
         FIG. 18  is a cross sectional view of the structure of a TFT and its vicinity according to a second example of embodiment 4; 
         FIG. 19  is a cross sectional view of a TFT and its vicinity according to a comparative example; 
         FIG. 20  is a detailed cross sectional view of the comparative example; and 
         FIG. 21  is another detailed cross sectional view of the comparative example. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the contents of the present invention will be described in detail by taking a liquid crystal display device as an example.  FIG. 1  is a plan view of an exemplary liquid crystal display device to which the present invention is applied. In  FIG. 1 , the TFT substrate  100  and the counter substrate  200  are bonded together by a sealant  16 , and a liquid crystal layer is sandwiched between the TFT substrate  100  and the counter substrate  200 . A display region  14  is formed at a portion where the TFT substrate  100  and the counter substrate  200  overlap each other. 
     In the display region  14  of the TFT substrate  100 , scanning lines  11  extend in the horizontal direction (x-direction) and are arranged in the vertical direction (y-direction). Further, the video signal lines  12  extend in the vertical direction and are arranged in the horizontal direction. A region surrounded by the scanning line  11  and the video signal line  12  becomes a pixel  13 .
 
The TFT substrate  100  is formed larger than the counter substrate  200 , and a portion where the TFT substrate  100  does not overlap with the counter substrate  200  is a terminal region  15 . A flexible wiring board  17  is connected to the terminal region  15 . A driver IC for driving a liquid crystal display device is mounted on a flexible wiring board  17 .
 
     Since the liquid crystal does not emit light, a backlight is disposed on the back surface of the TFT substrate  100 . A liquid crystal display panel forms an image by controlling light from a backlight for each pixel. The flexible wiring board  17  is bent on the back surface of the backlight so as to reduce the overall outer shape of the liquid crystal display device. 
     In the liquid crystal display device of the present invention, a TFT using an oxide semiconductor, which has a small leakage current, is used in the display region  14 . For example, a scanning line driving circuit is formed in a frame portion near a sealing material, and a TFT using a polysilicon semiconductor, which has a high mobility, is often used for the scanning line driving circuit, but a TFT using an oxide semiconductor can also be used. 
       FIG. 2  is a plan view of a pixel in a display region.  FIG. 2  is a liquid crystal display device of a system called FFS (Fringe Field Switching) in an IPS (In Plan Switching) system. In  FIG. 2 , a bottom-gate TFT including an oxide semiconductor  106  is used. Since the oxide semiconductor TFT has a small leakage current, it is suitable as a switching TFT. 
     In  FIG. 2 , the scanning lines  11  extend in the horizontal direction (x-direction) and are arranged in the vertical direction (y-direction). Further, the video signal lines  12  extend in the vertical direction and are arranged in the horizontal direction. A pixel electrode  116  is formed in an area surrounded by the scanning line  11  and the video signal line  12 . In  FIG. 2 , an oxide semiconductor TFT including an oxide semiconductor  106  is formed between a video signal line  12  and a pixel electrode  116 . In an oxide semiconductor TFT, a video signal line  12  constitutes a drain electrode  107 , and a scanning line  11  branches to form a gate electrode  104  of an oxide semiconductor TFT. The source electrode  108  of the oxide semiconductor TFT extends toward the pixel electrode  116  and is connected to the pixel electrode  116  via the through hole  112 . 
     The pixel electrode  116  is formed in a comb shape. The pixel electrode  116  has a slit  1161 . Below the pixel electrode  116 , a common electrode  113  is formed in a planar shape with a capacitance insulating film interposed therebetween. The common electrode  113  is formed in common to each pixel. When a video signal is supplied to the pixel electrode  116 , an electric line of force passing through the liquid crystal layer is generated between the pixel electrode  116  and the common electrode  113 , and an image is formed by rotating the liquid crystal molecules. 
       FIG. 3  is an example of a cross-sectional view of the liquid crystal display device corresponding to  FIG. 2 . In  FIG. 3 , a bottom-gate TFT including an oxide semiconductor film  106  is used. Since the oxide semiconductor TFT has a small leakage current, it is suitable as a switching TFT. 
     Examples of the oxide semiconductor include IGZO (Indium Gallium Zinc Oxide), ITZO (Indium Tin Zinc Oxide), ZnON (Zinc Oxide Nitride), IGO (Indium Gallium Oxide), and so forth. In this embodiment, IGZO is used as an oxide semiconductor. 
     In  FIG. 3 , a polyimide substrate  100  is formed on a glass substrate  90 . At the end of the process, when the glass substrate  90  is peeled off from the polyimide substrate  100 , the liquid crystal display device becomes a flexible liquid crystal display device. On a polyimide substrate  100 , a base film constituted from three layers of, a first base film  101  formed of silicon oxide (SiO), a second base film  102  made of silicon nitride (SiN), and a third base film  103  made of silicon oxide (SiO), is formed. 
     A gate electrode  104  is formed on the third base film  103 . The gate electrode  104  has a stacked structure of Ti and Al. A gate insulating film  105  is formed of SiO covering the gate electrode  104 , and an oxide semiconductor film  106  is formed over the gate insulating film  105 . A drain electrode  107  is stacked on one end of the oxide semiconductor film  106 , and a source electrode  108  is stacked on the other end of the oxide semiconductor  106 . Each the drain electrode  107  and the source electrode  108  is formed of metal or an alloy. 
     A first interlayer insulating film  109  made of SiO is formed so as to cover the oxide semiconductor  106 , the drain electrode  107 , and the source electrode  108 , and a second interlayer insulating film  110  made of SiN is formed thereon. The first interlayer insulating film  109  is formed of SiO to supply oxygen from SiO to the channel region of the oxide semiconductor  106 . 
     An organic passivation film  111  is formed of an acrylic resin, for example, on the second interlayer insulating film  110 . Since the organic passivation film  111  serves as a planarization film, it is formed as thick as about 2 μm. In the organic passivation film  111 , a through hole  112  is formed in order to establish conduction between the source electrode  108  and the pixel electrode  116 . 
     A common electrode  113  is formed in a planar shape on an organic passivation film  111 , a capacitor insulating film  114  is formed thereon, and a pixel electrode  116  is formed thereon. In order to establish conduction between the pixel electrode  116  and the source electrode  108 , a through hole  115  is formed in the through hole  112  of the organic passivation film  111  in the capacitance insulating film  114 . An alignment film  117  for initial alignment of liquid crystal molecules is formed covering the pixel electrode  116 . 
     A counter substrate  200  is disposed opposite to the pixel electrode  116  and the like with the liquid crystal layer  300  interposed therebetween. A color filter  201  and a black matrix  202  are formed inside the counter substrate  200 . The black matrix  202  covers the TFT and the through hole  112  and prevents light leakage. An overcoat film  203  is formed covering a color filter  201  and a black matrix  202 , and an alignment film  204  is formed thereon. 
     In  FIG. 3 , when a voltage is applied to the pixel electrode  116 , electric lines of force passing through the liquid crystal layer  300  are generated, thereby rotating the liquid crystal molecules  301  and changing the light transmittance of the liquid crystal layer  300 . An image is formed by changing the light transmittance of the liquid crystal layer  300  for each pixel. 
     As shown in  FIG. 3 , a gate electrode  104  formed of metal faces the oxide semiconductor  106  with the gate insulating film  105  interposed therebetween. Since the gate electrode  104  is made of metal, oxygen is extracted from the oxide semiconductor  106  through the gate insulating film  105 . Then, the resistance of the oxide semiconductor  106  decreases, and the TFT does not operate normally. 
       FIG. 19  is a cross-sectional view of the vicinity of the TFT as a comparative example to cope with this problem.  FIG. 19  is a sectional view of the vicinity of the TFT in which the oxide semiconductor  106  is used, and the right side is a capacitance wiring  120  and a capacitance electrode  122 , which are formed simultaneously with the TFT, to form a capacitance. 
     The layer configuration of  FIG. 19  is as described in  FIG. 3 .  FIG. 19  differs from  FIG. 3  in that a first gate insulating film  1051  formed of SiN and a second gate insulating film  1052  formed of SiO exist as the gate insulating film  105  between the gate electrode  104  and the oxide semiconductor film  106 . The SiO film  1052  is in contact with the oxide semiconductor film  106 . 
       FIG. 20  is a cross-sectional view showing an operation of the first gate insulating film  1051  formed of SiN in  FIG. 19 . The SiN film  1051  has a property of blocking oxygen. In  FIG. 20 , the gate electrode  104  formed of metal tends to attract oxygen from the SiO film as the second gate insulating film  1052  or the oxide semiconductor film  106 . As shown in  FIG. 20 , it is possible to prevent oxygen from being removed from the oxide semiconductor  106  by blocking oxygen migration by the SiN film serving as the first gate insulating film  1051 . The mark X indicated by an arrow in  FIG. 20  indicates that oxygen is blocked by the SiN film  1051 . 
       FIG. 21  is a cross-sectional view showing a problem of the configuration of  FIG. 19 . The SiN film constituting the first gate insulating film  1051  blocks oxygen; however, it releases hydrogen. When this hydrogen reaches the oxide semiconductor film  106 , the oxide semiconductor is reduced, that is, oxygen is removed, so that the oxide semiconductor film  106  becomes conductive. 
     Therefore, in the configuration of  FIG. 19 , sufficient countermeasures are not taken. Embodiments 1 to 4 described below provide a structure that prevents oxygen from being deprived from the oxide semiconductor film  106  while preventing such problems. 
     Embodiment 1 
       FIG. 4  is a cross-sectional view showing a first example of embodiment 1. The layer configuration of  FIG. 4  is similar to that described in  FIG. 3 . As shown in  FIG. 5 , the gate electrode  104  has a structure in which an Al film  1042  is sandwiched between a Ti film which is a base metal  1041  and a Ti film which is a cap metal  1043 . The feature of  FIG. 4  and  FIG. 5  is that titanium nitride (TiN) which is a metal nitride film  10  is formed on the gate electrode  104 . A gate insulating film  105  made of a SiO film is formed on the titanium nitride film  10 , and an oxide semiconductor film  106  is formed over the gate insulating film  105 . 
     Although the titanium nitride film  10  is formed by sputtering, it can be continuously performed in the same chamber as the sputtering of the Ti film which is the cap metal  1043 . In other words, after forming the Ti film  1043  by sputtering, a TiN film  10  can be formed by reactive sputtering by introducing nitrogen gas. 
     A film thicknesses in  FIG. 5  are, for example, as follows. A thickness of the base metal is  1041  is 50 nm, a thickness of the Al film  1042  is 300 nm, and a thickness of the cap metal  1043  is 50 nm. A thickness of the TiN film  10  is, for example, 10 nm, but may be about 5 to 30 nm. A thickness of the gate insulating film  105  formed of SiO is, for example, 300 to 500 nm, and a thickness of the oxide semiconductor  106  is, for example, 50 nm. By the way, the base metal  1041  in  FIG. 5  may be omitted. 
     In  FIG. 4 , the titanium nitride film  10  formed on the gate electrode  104  prevents oxygen from being absorbed into the gate electrode  104  from the oxide semiconductor  106 . Accordingly, changing of characteristics of the oxide semiconductor TFT can be avoided. 
     In  FIG. 4 , the titanium nitride film  10  is not formed on the entire upper surface of the gate electrode  104 , but there are holes in the titanium nitride film  10  on the surface of the gate electrode  104 . This hole is used to form an intermediate resistance region (also referred to as an LDD region)  1062  in the oxide semiconductor film  106 . That is, in the portion where the hole is formed, oxygen is extracted from the oxide semiconductor film  106 , so that the resistance of the oxide semiconductor  106  is reduced in this portion, and the intermediate resistance region  1062  is formed in this portion of the oxide semiconductor film  106 . The intermediate resistance region  1062  suppresses generation of hot carriers and stabilizes characteristics of the oxide semiconductor TFT. 
     In the regions  1063  and  1064  in which the drain electrode  107  or the source electrode  108  is laminated on the oxide semiconductor  106 , since a large amount of oxygen is removed from the oxide semiconductors  1063  and  1064  by the drain electrode  107  or the source electrode  108  which is a metal, regions of the oxide semiconductors  1063  and  1064  have conductivity. On the other hand, a high resistance is maintained in the channel region  1061  of the oxide semiconductor since oxygen is maintained in the channel region  1061  due to presence of the titanium nitride film  10 ; thus, characteristics of the TFT can be maintained. Therefore, reliability of the oxide semiconductor TFT can be maintained. 
     On the right side of  FIG. 4 , a titanium oxide film  10  is also formed on the capacitor wiring  120  formed simultaneously with the gate electrode  104 . The titanium oxide film  10  is, however, conductive, thus, conduction between the capacitor wiring  120  and the capacitor electrode  122  is not impaired. 
       FIG. 6  is a cross-sectional view showing a second example of embodiment 1.  FIG. 6  is different from  FIG. 4  in that the titanium nitride film  10  formed on the gate electrode  104  is formed only in a portion corresponding to the channel region  1061  of the oxide semiconductor  106 . In other words, in the channel region  1061  of the oxide semiconductor  106 , oxygen is prevented from being removed due to the titanium nitride film  10 , so that high resistance can be maintained. However, since oxygen is extracted from the oxide semiconductor  106  corresponding to the portion where the titanium nitride film  10  is not formed on the gate electrode  104 ; consequently, a resistance of the oxide semiconductor  106  decreases. Incidentally, in a region of the oxide semiconductor  106  in which the drain electrode  107  and the source electrode  108  are stacked, i e., the drain region  1063  and the source region  1064 , oxygen is extracted by the drain electrode  107  and the source electrode  108  in a large amount, thus a resistance of the drain region  1063  and a resistance of the source region  1064  decrease greatly. On the other hand, in a region between the channel region  1061  and a region the drain region  1063  or a region between the channel region  1061  and the source region  1064 , oxygen is extracted only by gate electrode  104  via gate insulating film  105 ; thus a resistance of the oxide semiconductor  106  does not decrease in large amount compared with the drain region  1063  and the source region  1064 . Therefore, an intermediate resistance region (LDD region) is formed also in the configuration of  FIG. 6 . Accordingly, in the configuration of  FIG. 6 , an oxide semiconductor TFT with stable characteristics can be formed. 
       FIG. 7  is a cross-sectional view showing a third example of embodiment 1.  FIG. 7  is different from  FIG. 4  in embodiment 1 in that a titanium nitride film  10  is also formed on the side surface of the gate electrode  104 . Thus, absorption of oxygen from the oxide semiconductor  106  by the gate electrode  104  can be more efficiently prevented. 
     In order to form the titanium nitride film  10  on the side surface of the gate electrode  104 , it is preferable that the taper of the side surface of the gate electrode  104  is not steep. For this purpose, the taper angle θ of the side surface of the gate electrode  104  is preferably between 40 and 60 degrees. 
       FIG. 8  is a cross-sectional view showing a fourth example of embodiment 1.  FIG. 8  is different from  FIG. 6  in the second example in that a titanium nitride film  10  is also formed on the side surface of the gate electrode  104 . Thus, absorption of oxygen from the oxide semiconductor  106  by the gate electrode  104  can be more efficiently prevented. Other configurations of  FIG. 8  are similar to those described with reference to  FIGS. 6 and 7 .
 
Although, in the above description, titanium nitride is used as the metal nitride film  10 , the metal nitride film  10  is not limited thereto. For example, tantalum nitride (TaNx) or the like can be used.
 
     Embodiment 2 
     The structure of embodiment 2 is different from that of the embodiment 1 in that the substrate of the liquid crystal display device is not a polyimide substrate  100  but a glass substrate  90 .  FIG. 9  is a cross-sectional view showing a first example of embodiment 2.  FIG. 9  is different from  FIG. 4  of embodiment 1 in that the polyimide substrate and the first to third base films do not exist, and a gate electrode  104  is directly formed on the glass substrate  90 . 
     Generally, a non-alkali glass is used as the glass substrate  90 . The influence of impurities from the glass substrate  90  on the oxide semiconductor  106  is stopped by the gate electrode  104 , which is metal. However, if there is a possibility that the influence of impurities from the glass substrate  90  remains, the base films of  101 ,  102  and  103  as described in  FIG. 3  may be formed. Since the other layer structure in  FIG. 9  is similar to that in  FIG. 4 , description thereof will be omitted. 
       FIG. 10  is a cross-sectional view showing a second example of embodiment 2.  FIG. 10  is different from  FIG. 6  in embodiment 1 in that the polyimide substrate and the first to third base films do not exist, and a gate electrode  104  is directly formed on the glass substrate  90 . Since this difference is the same as described in  FIG. 9 , a description thereof will be omitted. 
       FIG. 11  is a cross-sectional view showing a third example of embodiment 2.  FIG. 11  is different from  FIG. 7  of embodiment 1 in that the polyimide substrate and the first to third base films do not exist, and a gate electrode  104  is directly formed on the glass substrate  90 . Since this difference is the same as described in  FIG. 9 , a description thereof is omitted. 
       FIG. 12  is a cross-sectional view showing a fourth example of embodiment 2.  FIG. 12  is different from  FIG. 8  of embodiment 1 in that the polyimide substrate and the first to third base films do not exist, and a gate electrode  104  is directly formed on the glass substrate  90 . Since this difference is the same as described in  FIG. 9 , a description thereof is omitted. 
     Embodiment 3 
       FIG. 13  is a cross-sectional view showing a first example of embodiment 3.  FIG. 13  is different from  FIG. 4  of embodiment 1 in that a metal oxide film  20  is formed on the gate electrode  104  instead of a metal nitride film. The metal oxide film  20  can also prevent the gate electrode  104  from extracting oxygen from the oxide semiconductor  106 . In other words, the action of the metal oxide film  20  is the same as that of the metal nitride film in  FIG. 4  of embodiment 1. 
     Examples of the type of the metal oxide  20  include various oxide semiconductors described above, metal oxide conductors such as ITO, and insulating metal oxides such as alumina (AlOx). Examples of other metal oxide conductors include AZO (Aluminum doped Zinc Oxide) and IZO (Indium Zinc Oxide). As in the case of the metal nitride film, the thickness of the metal oxide film  20  is preferably 5 to 30 nm. 
     Since the metal oxide may be an insulator, a through hole is formed in a portion where the capacitor wiring  120  is connected to the capacitor electrode  121  in the metal oxide film  20  formed on the capacitor wiring  120  in  FIG. 13 . Other configurations and operations of  FIG. 13  are similar to those described in  FIG. 4  of embodiment 1, and therefore, a description thereof is omitted. 
       FIG. 14  is a cross-sectional view showing a second example of embodiment 3.  FIG. 14  differs from  FIG. 6  of embodiment 1 in that a metal oxide film  20  is formed instead of a metal nitride film. Since the action of the metal oxide film  20  in  FIG. 14  is the same as that of the metal nitride film  20  in  FIG. 6 , therefore, a description thereof is omitted. 
       FIG. 15  is a cross-sectional view showing a third example of embodiment 3. In  FIG. 15 , a side surface of the gate electrode  104  is covered with a metal oxide film  20 .  FIG. 15  differs from  FIG. 7  of embodiment 1 in that a metal oxide film  20  is formed instead of a metal nitride film. Since the action of the metal oxide film  20  in  FIG. 15  is the same as that of the metal nitride film  10  in  FIG. 7 , a description thereof is omitted. 
       FIG. 16  is a cross-sectional view showing a fourth example of embodiment 3. In  FIG. 6 , the side surface of the gate electrode  104  is covered with a metal oxide film  20 .  FIG. 16  is different from  FIG. 8  of embodiment 1 in that the metal oxide film  20  is formed instead of the metal nitride  10 . Since the action of the metal oxide film  20  in  FIG. 16  is the same as that of the metal nitride film  10  in  FIG. 8 , a description thereof is omitted. 
     Embodiment 4 
       FIG. 17  is a cross-sectional view showing a first example of embodiment 4. In  FIG. 17 , a metal oxide film  20  which is an insulating film is formed on the gate electrode  104 . As the metal oxide film  20  which is an insulating film, for example, an alumina (AlOx) film can be mentioned. The thickness of the alumina (AlOx) film  30  is 5 to 30 nm as in the case of the metal nitride film in embodiment 1. 
     The action of the alumina (AlOx) film  30  is similar to that of the metal nitride film described in  FIG. 4  or  FIG. 7  of embodiment 1. Since the alumina (AlOx) film  30  is an insulating film, it can cover not only the gate electrode  104  or the capacitor wiring  120  but also the entire surface of the substrate. Thus, the alumina (AlOx) film  30  can be made to act as a block film for blocking impurities from the glass substrate  90 , the polyimide substrate  100 , and the like. 
       FIG. 18  is a cross-sectional view showing a second example of embodiment 4.  FIG. 18  differs from  FIG. 12  of embodiment 2 in that an alumina (AlOx) film  30  is formed on the gate electrode  104  instead of a metal nitride film, and an alumina (AlOx) film  30  is formed not only on the gate electrode  104  and the capacitor wiring  120  but also on the entire surface of the substrate. The action of the alumina (AlOx) film  30  is the same as that described in  FIG. 17  and the like, and therefore, a description thereof is omitted. 
     As described above, according to the present invention, it is possible to effectively prevent oxygen from being lost from the oxide semiconductor film and to form a stable oxide semiconductor TFT. 
     Note that, although an example in which an oxide semiconductor is applied to a liquid crystal display device has been described above, the present invention can be applied to an apparatus including another display device such as an organic EL display device, a two dimensional light sensor, or the like.