Abstract:
A structure with a transistor is disclosed comprising a substrate, a gas barrier layer on the substrate, and a transistor on the gas barrier layer. The transistor can include an oxide semiconductor layer. The oxide semiconductor layers can comprise In—Ga—Zn—O. A display, such as a liquid crystal display, can have such a structure.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is related to a flexible transistor having an oxide semiconductor layer. 
   A transistor of the present invention can be used for devices such as a liquid crystal display, an organic light emitting display, a micro capsule type electrophoresis-type display, a photo-write type cholesteric liquid crystal type display, a TwistingBall type display, a toner type display, a movable film type display and an RFID (RadioFrequencyIdentification) sensor. 
   2. Description of the Related Art 
   Generally the transistor which used an amorphous silicon semi-conductor or a polysilicon semi-conductor as a driving transistor of electron device has been used. However, film formation temperature equal to or more than 200 degrees Celsius is required for manufacture of an amorphous silicon semi-conductor and polysilicon semi-conductor of high quality. Therefore, it was difficult to realize a flexible transistor with the use of an inexpensive plastic substrate. 
   The transistor which used organic semiconductor in semiconductor layer is studied energetically to realize a flexible transistor with the use of an inexpensive plastic substrate. Because organic semiconductor can be made at cold temperature, organic semiconductor can be formed on a plastic substrate. However, mobility of organic semiconductor is extremely low. In addition, degradation of organic semiconductor is easy to occur. Therefore, practical use of organic semiconductor has not yet come. 
   In such a situation, for example, a transistor having a semiconductor layer made of In—Ga—Zn—O, which showed high mobility even if it was made at room temperature, has been proposed. (K. Nomuraetal. Nature, 432, 488 (2004)) 
   According to this literature, by using In—Ga—Zn—O as a semiconductor layer, a transistor having an excellent semi-conductor of about 10 cm 2 /Vs mobility on a polyethylene terephthalate substrate could be made in room temperature. 
   However, when In—Ga—Zn—O is made by sputter method, the semiconductor characteristic is extremely sensitively influenced by oxygen concentration during formation of film. 
   For example, when In—Ga—Zn—O is deposited under a condition of 1% oxygen flow rate (argon flow rate 100%), it becomes an electric conductor of which conductivity as a film is more than 1 (S/cm). On the other hand, when In—Ga—Zn—O is layered under a condition of 50% oxygen flow rate, it becomes a insulating material of which conductivity as a film is less than 1*10 −13  (S/cm). 
   Oxygen concentration at a film formation surface of a semiconductor greatly influences the number of oxygen deficiency to be included in a semiconductor film. Therefore, in order to make a semiconductor characteristic stable, it is extremely important to control oxygen concentration at a film formation surface of a semi-conductor film. 
   When inorganic materials such as glass are used as a substrate, deoxidation from a substrate does not almost occur during deposition of a semiconductor. Therefore, control of oxygen concentration during deposition of semiconductor is easy. 
   However, when a plastic substrate is used as a substrate, deoxidation from a plastic substrate occurs. Therefore, control of oxygen concentration at a surface of semi-conductor film formation becomes very difficult. 
   In addition, water vapor contained in a plastic substrate scatters to a layer of a semi-conductor when time passes in a case of the transistor that a layer of a semi-conductor of In—Ga—Zn—O is formed on a plastic substrate. Therefore, mobility and an ON/OFF ratio of a transistor fall. 
   In the present invention, oxygen and water vapor from a plastic substrate are shielded by a gas barrier layer, and they do not go to an oxide semiconductor layer. And in the present invention, a flexible structure with a transistor of which characteristic is stable and of which mobility and on/off ratio are kept to be high is provided. 
   SUMMARY OF THE INVENTION 
   One embodiment of the present invention is disclosed. In the present invention, oxygen and water vapor from a substrate are shielded by a gas barrier layer, and they do not go to an oxide semiconductor layer. And in the present invention, a flexible structure with a transistor of which characteristic is stable and of which mobility and on/off ratio are kept high is provided. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1  ( a ), ( b ), ( c ) and ( d ) are explanatory drawings which show a cross section structure of an example of a structure with a transistor of the present invention. 
       FIGS. 2  ( a ), ( b ), ( c ) and ( d ) are explanatory drawings which show a cross section structure of an example of a structure with a transistor of the present invention. 
       FIG. 3  is an explanatory drawing which shows relationships between an oxygen transmission rate of a gas barrier material and mobility and between an oxygen transmission rate of a gas barrier material and on/off ratio, about a structure with a transistor of the present invention. 
       FIG. 4  is an explanatory drawing which shows relationships between a water-vapor transmission rate of a gas barrier material and mobility and between a water-vapor transmission rate of a gas barrier material and on/off ratio, about a structure with a transistor of the present invention. 
       FIG. 5  is a constructional example of a structure with a transistor of the present invention. 
   

   In these drawings,  1  is a plastic substrate;  2  is a gas barrier layer;  3  is a semiconductor layer;  4  is a gate insulator;  5  is a gate electrode;  6  is a source electrode; and  7  is a drain electrode. 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   For a transistor which can be used for the present invention, there are a positive stagger type (top gate, top contact), an inverted stagger type (bottom gate, top contact) and a coplanar type (bottom gate, bottom contact) as shown in  FIG. 5 . 
   A basic embodiment of the present invention is explained below. 
   In other words, a transistor having the following members is explained: 
   the oxide semiconductor layer that is formed on a plastic plate; 
   the source electrode that is arranged to be touching electrically the oxide semiconductor layer; 
   the drain electrode that is arranged to be touching electrically the oxide semiconductor layer and to be isolated by the source electrode; 
   the gate insulator (formed on the oxide semiconductor layer) that is located between the source electrode and the drain electrode when the plastic substrate is watched from right above; and 
   the gate electrode that is formed on the gate insulator. 
   At first, gas barrier layer  2  is formed on plastic substrate  1 . ( FIG. 1  ( a )) 
   As for a material of a plastic substrate, polymethyl methacrylate, polyalylate, polycarbonate, polystyrene, polyethylen sulfide, polyethersulfone, polyolefin, polyethylene terephthalate, polyethylenenaphthalate, cyclo-olefin polymer, polyether sulfone, triacetylcellulose, a polyvinyl fluoride film, ethylene-tetrafluoroethylene copolymer resin, weatherable polyethylene terephthalate, weatherable polypropylene, glass fiber-reinforced acryl resin film, glass fiber-reinforced polycarbonate, polyimide, transparent polyimide, fluorinated resin, cyclic polyolefin resin and poly acrylic resin can be used, but it is not limited to these materials. 
   These materials may be used alone, and a composite substrate having two or more kinds of materials may be used. 
   For a material of gas barrier layer  2 , inorganic oxide and inorganic nitride can be used. To be concrete, oxidation silicon, silicon nitride, aluminium oxide, magnesium fluoride, magnesium oxide, yttria can be used alone, or a mixture including two or more kind of these materials can be used, but usable materials are not limited to these materials. Oxidation silicon, silicon nitride and aluminium oxide are preferable used from the viewpoint of good gas barrier performance, inexpensiveness of manufacturing cost and transparency. 
   For film formation method of gas barrier layer  2 , vacuum evaporation method, ion plating method, sputter method, laser ablation method, plasma CVD) technique, photo-CVD, hot wire CVD method, spin coat method, dip coat method, screen printing, letterpress printing, intaglio printing, lithography and ink jet method can be used, but usable methods are not limited to these methods. 
   Gas barrier layer  2  may be opaque and may be transparent. When all of a plastic substrate, a gas barrier layer, a semiconductor layer, a gate insulator and an electrode are transparent, a transparence transistor can be realized. 
   Next, semiconductor layer  3  is formed on gas barrier layer  2 . 
   For materials of semiconductor layer  3 , zinc oxide, indium oxide, tin oxide, tungsten oxide and zinc oxide gallium indium which are oxides including one or more element among zinc, indium, tin, tungsten, magnesium and gallium can be used, but usable materials are not limited to these materials. Structures of these materials may be any of monocrystal, polycrystal, crystallite, mixed crystal of crystal/amorphous, nanocrystals embedded in amorphous and amorphous. As for the film thickness of semiconductor layer, it is preferable more than 20 nm. 
   For film formation method of semiconductor layer  3 , sputter method, pulsed laser deposition, vacuum evaporation method, CVD (Chemical Vapor Deposition) method, MBE (Molecular Beam Epitaxy) method, spin coat method, dip coat method, screen printing, letterpress printing, intaglio printing, lithography and ink jet method can be used, but sputter method, pulsed laser deposition, vacuum evaporation method and CVD (Chemical Vapor Deposition) method are preferable. 
   RF magnetron sputtering technique and DC sputter method can be used for sputter method. Heating evaporation, electron beam evaporation and ion plating method can be used for a vacuum evaporation method. Hot wire CVD method and plasma CVD technique can be used for CVD method. But usable methods are limited to these methods. 
   Next, gate insulator  4  is formed on semiconductor layer  3 . 
   For material of gate insulator  4 , silicon oxide, silicon nitride, aluminium oxide, tantalum oxide, yttria, hafnium oxide, hafnium aluminates, oxidation zirconia, titanium oxide, polyethylene terephthalate, polyethylenenaphthalate, polypropylene, cyclo-olefin polymer, polyamide, polyether sulfone, polymethyl methacrylate, polycarbonate, polyalylate, polyethylen, polystyrol, teflon (registered mark), ebonite, epoxy and lacquer tree can be used. 
   For a manufacturing method of gate insulator  4 , vacuum evaporation method, ion plating method, sputter method, laser ablation method, plasma CVD (Chemical Vapor Deposition) method, photo-CVD, hot wire CVD method, spin coat method, dip coat method, screen printing, letterpress printing, intaglio printing, lithography and ink jet method can be used. 
   It is desirable that conductivity of gate insulator  4  is equal to or less than 10 −12  S/cm to control a leak current between metal electrodes (gate electrode  5 , source electrode  6  and drain electrode  7 ). In addition, it is desirable for thickness of gate insulator  4  to be 50 nm-2 μm. 
   In addition, if material of gate insulator  4  and material of gas barrier layer  2  are same, an inexpensive transistor can be made. 
   Next, electrodes (gate electrode  5 , source electrode  6  and drain electrode  7 ) are formed, and a transistor is obtained. ( FIG. 1  ( d )) 
   For materials of electrodes (gate electrode  5 , source electrode  6  and drain electrode  7 ), gold, platinum, silver, palladium, copper, aluminium, nichrome, chromium, titanium, indium, indium oxide, zinc oxide, tin oxide, cadmium oxide and gallium oxide can be used. 
   When transparent conducting oxide such as indium oxide, zinc oxide and tin oxide is used, it is desirable that conductivity of a metal electrode is increased by mixing dopant. For example, it is desirable that zinc oxide is retracted by mixing gallium, aluminium, boron or the like, and that tin oxide is retracted by mixing fluorine, antimony or the like, and that indium oxide is retracted by mixing tin, zinc, titanium, cerium, hafnium, zirconia or the like. 
   In addition, for increasing production efficiency, it is desirable that only dopant concentration is increased by using a electrode material which is same material as a mother material of a oxide semiconductor. 
   Film thickness of metal electrode has to be equal to or more than 15 nm at least. 
   In addition, materials of a gate electrode, a source electrode and a drain electrode may be identical or may be different from each other. 
   For a manufacturing method of electrodes (gate electrode  5 , source electrode  6  and drain electrode  7 ), vacuum evaporation method, ion plating method, sputter method, laser ablation method, plasma CVD technique, photo-CVD, hot wire CVD method, spin coat method, dip coat method, screen printing, letterpress printing, intaglio printing, lithography and ink jet method can be used. 
   In addition, gas barrier layer  2  may be formed on both sides of plastic substrate  1 . ( FIG. 2  ( d )) 
   In the present invention, oxygen and water vapor from a plastic substrate are shielded by a gas barrier layer, and they do not go to an oxide semiconductor layer. And in the present invention, a flexible structure with a transistor of which characteristic is stable and of which mobility and on/off ratio are kept to be high is provided. 
   EXAMPLES 
   At first, aluminium oxide of different thickness was deposited on polyethylene terephthalate (PET) of 100 μm thickness by continuous roll-to-roll evaporation method using electron beam thermal process. Then, gas barrier property (water vapor transmission rate, oxygen transmission rate) was measured. In addition, to raise gas barrier property, aluminium oxide was deposited on both sides of PET film in Example 1, 2, 11, 12 and 13. (In addition, aluminium oxide was not deposited on PET film in Comparative Example.) 
   A semiconductor layer having film thickness of 40 nm was made on the PET with deposited aluminium oxide by RF magnetron sputtering technique [sputtering gas: Ar 19.4 SCCM, oxygen 0.6 SCCM (oxygen flow rate ratio 3%)] using InGaZnO 4  as a sputter target. 
   A gate insulator having film thickness of 170 nm comprising Y 2 O 3  was made on one part of this semiconductor layer by RF magnetron sputtering technique [sputtering gas: Ar 1 SCCM, oxygen 20 SCCM] using Y 2 O 3  as a sputter target. 
   Next, sputter was performed on this gate insulator and semiconductor layer by using 10% Sn doped In 2 O 3  as a sputter target [sputtering gas: Ar 20 SCCM, oxygen 1 SCCM]. Then, a source electrode, a drain electrode and a gate electrode were made by photo-lithography method, where three areas of a source electrode, drain electrode and gate electrode were separated from each other, and a transistor was obtained. 
   As for the transistor, the channel-length was 50 μm and the channel width was 200 μm. 
   A manufacture condition was summarized in table 1. 
   
     
       
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
               Temperature 
               Working 
               Gas flow 
                 
               Film 
             
             
                 
                 
               of a substrate 
               pressure 
               rate 
               Input power 
               thickness 
             
             
                 
               target 
               (° C.) 
               (Pa) 
               (SCCM) 
               (W) 
               (nm) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               Semiconductor 
               InGaZnO 4   
               40 
               0.5 
               Ar: 19.4 
               20 
               50 
             
             
               layer 
                 
                 
                 
               O 2 : 0.6 
             
             
               Gate insulator 
               Y 2 O 3   
               40 
               0.5 
               Ar: 1 
               30 
               150 
             
             
                 
                 
                 
                 
               O 2 : 020 
             
             
               Source electrode 
               In 2 O 3   
               40 
               0.5 
               Ar: 20 
               20 
               100 
             
             
               Drain electrode 
               (Sn 10% 
                 
                 
               O 2 : 1 
             
             
               Gate electrode 
               doped) 
             
             
                 
             
           
        
       
     
   
   After manufacturing samples, in measuring property of the transistor, an analyzer of parameters of a semiconductor (model name: Keithley 4200, a product of Keithley co.) was used, and mobility and on/off ratio was calculated based on transfer characteristic. The measured results are shown in  FIGS. 3 and 4 , and Table 2. 
   &lt;Result of Example&gt; 
   When an oxygen transmission rate of a gas barrier material of the transistor was equal to or less than 3.5 cc/m 2 /day/atm and water-vapor transmission of that is equal to or less than 3 g/m 2 /day, good characteristic was shown, that is, ON/OFF ratio was equal to or more than 107 and mobility was equal to or more than 4 cm 2 /Vs. 
   &lt;Result of Comparative Example&gt; 
   In addition, as for the transistor without a gas barrier layer, ON/OFF ratio was less than 10 7  and mobility was less than 3 cm 2 /Vs. Those values were not suitable for use of a transistor. 
   In addition, oxygen transmission rate of the PET on which aluminium oxide was deposited was measured under the conditions of temperature 30 degrees Celsius and humidity 70% RH by using an oxygen transmission rate measuring apparatus (a model name: OXTRAN made in Mocon company). 
   In addition, water-vapor transmission of the PET which aluminium oxide was deposited was measured under the conditions of temperature 40 degrees Celsius and humidity 90% RH by using a water-vapor transmission measuring apparatus (a model name: PERMATRAN made in Mocon company). 
   Oxygen Transmission Rate 
                                                                                 Al 2 O 3     side                       thickness   having   O 2 TR   Mobility   On/off           (nm)   Al 2 O 3     (cc/m 2 /day)   (cm 2 /Vs)   ratio                                    Example 1   52   Both sides   0.03   5.6   7.1 × 10 7         Example 2   50   Both sides   0.06   5.8   1.2 × 10 8         Example 3   47   Both sides   1.1   6.2   7.5 × 10 7         Example 4   42   Both sides   0.4   5.8   6.8 × 10 7         Example 5   47   One side   0.9   6   1.1 × 10 8         Example 6   42   One side   1.4   5.4   6.3 × 10 7         Example 7   37   One side   2.1   5.1   7.8 × 10 7         Example 8   33   One side   2.4   4.9   7.7 × 10 7         Example 9   27   One side   2.8   4.4   7.5 × 10 7         Example   22   One side   3.2   5.1   2.5 × 10 7         10       Reference   17   One side   5.1   0.9   6.8 × 10 6         Example 1       Reference   12   One side   6.9   0.7   4.2 × 10 5         Example 2                    
Water Vapor Transmission Ratio
 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
                 
                 
             
             
                 
               Al 2 O 3   
               side 
                 
                 
                 
             
             
                 
               thickness 
               having 
               WVTR 
               Mobility 
               On/off 
             
             
                 
               (nm) 
               Al 2 O 3   
               (g/m 2 /day) 
               (cm 2 /Vs) 
               ratio 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               Example 11 
               48 
               Both sides 
               0.02 
               5.6 
               6.1 × 10 7   
             
             
               Example 12 
               45 
               Both sides 
               0.05 
               5.5 
                 1 × 10 8   
             
             
               Example 13 
               40 
               Both sides 
               0.1 
               5.8 
               7.5 × 10 7   
             
             
               Example 14 
               50 
               One side 
               0.5 
               6 
                 8 × 10 7   
             
             
               Example 15 
               40 
               One side 
               1 
               5.5 
               7.3 × 10 7   
             
             
               Example 16 
               30 
               One side 
               1.3 
               5.7 
                 6 × 10 7   
             
             
               Example 17 
               20 
               One side 
               1.9 
               5.4 
                 7 × 10 7   
             
             
               Example 18 
               15 
               One side 
               2.3 
               5.2 
               8.2 × 10 7   
             
             
               Example 19 
               10 
               One side 
               2.6 
               5 
               4.8 × 10 7   
             
             
               Reference 
               6 
               One side 
               3.5 
               2.6 
                 6 × 10 6   
             
             
               Example 3 
             
             
               Reference 
               4 
               One side 
               5 
               1.9 
                 9 × 10 5   
             
             
               Example 4 
             
             
               Reference 
               2 
               One side 
               7 
               0.8 
               4.9 × 10 5   
             
             
               Example 5 
             
             
                 
             
           
        
       
     
   
   Comparative Example 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
                 
                 
             
             
                 
               Al 2 O 3   
                 
                 
                 
                 
             
             
                 
               thickness 
               O 2 TR 
               WVTR 
               Mobility 
               On/off 
             
             
                 
               (nm) 
               (cc/m 2 /day) 
               (g/m 2 /day) 
               (cm 2 /Vs) 
               ratio 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               Comparative 
               No gas 
               19 
               7.5 
               0.2 
               3 × 10 5   
             
             
               Example 
               barrier 
             
             
                 
               layer