Patent ID: 12237345

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail referring to the following embodiments. In the explanation below, the invention is mainly explained by examples of the liquid crystal display device; however, the present invention is applicable to the organic EL display device and other display devices, too.

First Embodiment

FIG.1is a plan view of a liquid crystal display device, which the present invention is applied. InFIG.1, the TFT substrate10in which TFTs and pixel electrodes are formed and the counter substrate200adhere to each other at their periphery by the sealing material150; the liquid crystal is sandwiched between the TFT substrate10and the counter substrate200. The area surrounded by the sealing material150is the display area500. The peripheral driving circuits600are formed at both sides of the display area500; a part of the peripheral driving circuits600overlaps with the sealing material150in a plan view.

InFIG.1, in the display area500, the scanning lines1extend in the lateral direction (x direction) from the peripheral driving circuits600and are arranged in the longitudinal direction (y direction). The video signal lines2extend in the longitudinal direction and are arranged in the lateral direction. Video signals are supplied to the video signal lines2from driver IC160, which is installed in the terminal area170. The pixel3is formed in the area surrounded by the scanning lines1and the video signal lines2.

InFIG.1, the TFT substrate10is made bigger than the counter substrate200; the area of the TFT substrate10that does not overlap with the counter substrate200is the terminal area170where the driver IC160is installed. In the meantime, the flexible wiring substrate is connected to the terminal area170to supply power and signals to the liquid crystal display device.

The TFTs of the oxide semiconductor are used in the liquid crystal display device ofFIG.1. The TFT of the oxide semiconductor has a feature of low leak current; therefore, it is suitable for the switching element in the pixel in the display area. On the other hand, the TFT of the poly silicon semiconductor has a high leak current, however, the mobility of carriers is high; thus, it is sometimes used for the driving TFTs in the peripheral driving circuit.

FIG.2is a plan view of the pixel in the display area500of the TFT substrate10. InFIG.2, the scanning lines1extend in the lateral direction (x direction) and are arranged in the longitudinal direction (y direction). The video signal lines2extend in the longitudinal direction and are arranged in the lateral direction. The pixel electrode26and the TFT are formed in the area surrounded by the scanning lines1and the video signal lines2. The TFT inFIG.2is a top gate type TFT.

InFIG.2, the active element (the semiconductor layer) of the TFT is formed by the oxide semiconductor13. The TFT of the oxide semiconductor13has a feature of low leak current. Among the oxide semiconductors13, optically transparent and amorphous materials are called TAOS (Transparent Amorphous Oxide Semiconductor). Examples of TAOS are indium gallium zinc oxide (IGZO), indium tin zinc oxide (ITZO), zinc oxide nitride (ZnON), and indium gallium oxide (IGO). IGZO is used as the oxide semiconductor13in the present embodiments.

InFIG.2, the gate electrode15is formed above the oxide semiconductor film13via the insulating film. The gate electrode15is a branch of the scanning line1. The channel of the TFT is formed immediately below the gate electrode15via the insulating film in the oxide semiconductor film13. One side, toward the video signal line, of the oxide semiconductor is the drain, and another side, opposite to the one side of the oxide semiconductor, is the source.

InFIG.2, the drain electrode19is connected to the oxide semiconductor film13via through hole17. The drain electrode19is a branch of the video signal line2. The source electrode20is connected to the source of the oxide semiconductor film13via through hole18. The source electrode20is connected to the pixel electrode26via the through hole22formed in the organic passivation film and the through hole25formed in the capacitance insulating film.

The pixel electrode26is stripe shaped. The common electrode23is formed in a plane shape under the pixel electrode26via the capacitance insulating film. The common electrode23is formed all over the area except the through hole22. When the video signal is applied to the pixel electrode26, a line of force is generated between the common electrode23and the pixel electrode26through the liquid crystal layer; consequently, liquid crystal molecules are rotated and thus, the transmittance in each of the pixels is changed.

In the example ofFIG.2, the pixel electrode26is constituted by one stripe since the lateral size of the pixel is small as 30 microns. When the lateral size of the pixel becomes bigger, however, the shape of the pixel electrode26becomes comb shaped, which has a slit inside.FIG.2is a structure that is used in so called IPS (In Plane Switching) mode liquid crystal display device.

FIG.3is a cross sectional view along the line A-A ofFIG.2. InFIG.3, the light shading film11is formed on the TFT substrate10, which is made of glass, etc. If a flexible display device is required, the TFT substrate10is made of resin as polyimide.

The light shading film stops the light from the backlight for the oxide semiconductor film13, which is formed above the light shading film; thus, photo current in the oxide semiconductor film13is suppressed. The light shading film11is made of metal as MoW, etc. The light shading film11can utilize the same material as the gate electrode15, which is formed later.

The under coat film12is formed over the light shading film11. The under coat film12prevents the oxide semiconductor film13from being contaminated by impurities in the TFT substrate10made of e.g. glass, as well as insulates the light shading film11from the oxide semiconductor film13. The under coat film12is generally has a two layer structure of the SiN film and the SiO film; the SiO film contacts the oxide semiconductor film13, which is made on the SiO film.

In the meantime, as to be explained in fourth embodiment, some products use the light shading film11as the bottom gate for the oxide semiconductor film13. In this case, the under coat film12works as the gate insulating film. The second under coat film can be additionally used between the light shading film11and the substrate10. In this case, the second under coat film, too, may be formed by two layers of the SiO film and the SiN film.

InFIG.3, the oxide semiconductor film13is formed on the under coat film12. IGZO is used for the oxide semiconductor film13. A thickness of the oxide semiconductor film13is 15 to 100 nm. The oxide semiconductor film13is formed by sputtering. The insulating film14of SiO is formed on the oxide semiconductor film13. Generally, the gate electrode15is formed on the insulating film14, however, in the present invention, aluminum oxide film30is formed between the gate electrode15and the insulating film14. The aluminum oxide film30has a two layer structure.

The interlayer insulating film16is formed over the gate electrode15. The interlayer insulating film16can have a one layer structure of SiO film or can have a two layer structure of SiO film and SiN film. When the two layer structure is adopted, the lower layer is the SiO film and the upper layer is the SiN film.

InFIG.3, the portion of the oxide semiconductor13immediately under the gate electrode15is the channel; the left hand side is the drain, the right hand side is the source. The drain and the source are formed by ion implantation of e.g. Ar using the gate electrode15as a mask.

InFIG.3, through holes17and18are formed in the interlayer insulating film16and the insulating film14. The drain electrode19and the drain are connected via the through hole17and the source electrode20and the source are connected via the through hole18.

InFIG.3, the organic passivation film21is formed covering the drain electrode19and the source electrode20. Since the organic passivation film21has a role as a flattening film, it is formed thick as 2 micron to 4 micron. The common electrode23is formed in plane shape on the organic passivation film21; the capacitance insulating film24is formed on the common electrode23.

The pixel electrode26is formed on the capacitance insulating film24. An example of a plan view of the pixel electrode26is shown inFIG.2. The pixel capacitance is formed between the common electrode23and the pixel electrode26through the capacitance insulating film24.

InFIG.3, the through hole22is formed in the organic passivation film21and the through hole25is formed in the capacitance insulating film24to connect the pixel electrode26to the source electrode20. The alignment film27is formed on the pixel electrode26. The alignment film27is for initial alignment of the liquid crystal molecules301; the alignment is conducted through the rubbing method or the optical alignment method using ultra violet ray. In the case of IPS mode, the optical alignment method is suitable. When video signals are applied to the pixel electrode26, a line of force as depicted by arrows inFIG.3is generated between the pixel electrode26and the common electrode23and liquid crystal molecules301are rotated; consequently, the transmittance of the liquid crystal layer300is controlled.

InFIG.3, the counter substrate200is set opposing to the TFT substrate10sandwiching the liquid crystal layer300. Generally, the counter substrate200is formed by glass; the resin like polyimide is used for the counter substrate200when a flexible display device is required. The color filter201and the black matrix202are formed on an inner side of the counter substrate200. The over coat film203is formed covering the color filter201and the black matrix202. The alignment film204is formed on the over coat film203. The alignment process for the alignment film204is the same as that of the alignment film32on the TFT substrate10.

FIG.4is a cross sectional view of the TFT and its vicinity to show the feature of this embodiment. InFIG.4, the light shading film11is formed on the substrate10; the under coat film12is formed on the light shading film11; and the oxide semiconductor film13is formed on the under coat film12. The oxide semiconductor film13is formed by IGZO. The insulating film14is formed covering the oxide semiconductor film13. The insulating film14is formed by oxide rich SiO to maintain the amount of oxygen in the oxide semiconductor film13.

The gate electrode15is formed over the insulating film14, however, the aluminum oxide film30, which has a two layer structure of the first aluminum oxide film31and the second aluminum oxide film32, is formed between the gate electrode15and the insulating film14. As will be explained later, the first aluminum oxide film31and the second aluminum oxide film32are formed to maintain the amount of oxygen in the oxide semiconductor film13.

The gate electrode15is made of either Titanium (Ti) or Aluminum (Al). In this specification, the aluminum alloy that contains aluminum as a major substance like AlSi and so forth is also expressed as aluminum. The interlayer insulating film16is formed covering the gate electrode15. The through holes17and18are formed in the interlayer insulating film16and the insulating film14to connect the oxide semiconductor film13and the drain electrode19or to connect the oxide semiconductor film13and the source electrode20.

FIG.5is an enlarged cross sectional view of the gate electrode15and its vicinity. The feature of this embodiment is to form the first aluminum oxide film31and the second aluminum oxide film32between the gate electrode15and the insulating film14. InFIG.5, at the outset, the first aluminum oxide film31is formed in a thickness of 2 nm by sputtering. The first aluminum oxide film31is made oxygen rich film formed by oxygen reactive mode sputtering, namely, sputtering in the oxygen environment. Since the oxygen reactive mode sputtering gives some damage to the insulating film14, a thickness of the first aluminum oxide film31is limited to between 2 to 5 nm, preferably 2 nm. Since deposition rate in the oxygen reactive mode sputtering is low, it takes approximately 1 minute to deposit the first aluminum oxide film in 2 nm.

Subsequently, the second aluminum film32is formed by transition mode sputtering, decreasing the amount of oxygen in the sputtering chamber. A thickness of the second aluminum oxide film32is made 5 to 10 nm. In the transition mode sputtering, the deposition rate is higher than that in the oxygen reactive mode sputtering. The aluminum oxide film formed by transition mode sputtering has lower oxygen content than that of the aluminum oxide film formed by oxygen reactive mode sputtering. In other words, when aluminum oxide is expressed by AlyOx, the value x/y is bigger in the aluminum oxide formed by the oxygen reactive mode than in the aluminum oxide formed by the transition mode.

FIG.6is a cross sectional view in the sputtering equipment. InFIG.6, the cathode101and the anode100oppose to each other with predetermined distance. The aluminum target102is set on the cathode101. The substrate10, on which the sputtering film30is deposited, is set on the anode100.

The plasma103for sputtering is formed by adding predetermined gasses and applying a voltage. The gas is oxygen (02) added Argon (Ar); the sputtering mode is determined by the amount of oxygen.

FIG.7is a graph showing the sputtering modes. InFIG.7, the abscissa is the amount of flow of oxygen. The sputtering mode is changed from the metal mode, to the transition mode, and to the oxygen reactive mode according to the amount of flow of oxygen. The amount of flow of Argon is constant in all the modes. The film formed under the oxygen reactive mode sputtering contains a large amount of oxygen; the film formed under the metal mode sputtering becomes an aluminum film or a film that contains extremely low amount of oxygen; the film formed under the transition mode sputtering contains an amount of oxygen in between. In other words, when aluminum oxide is expressed by AlyOx, the value x/y becomes bigger according the amount of flow of oxygen in the sputtering process.

The ordinate ofFIG.7is a discharge voltage. The discharge voltage is proportional to the deposition rate. Namely, the deposition rate is high in the metal mode and in the transition mode; the deposition rate is low in the oxygen reactive mode.

FIGS.8-12are cross sectional views of the process that realizes the structure ofFIG.5.FIG.8is a cross sectional view that shows the first aluminum oxide film31is formed on the insulating film14by oxygen reactive mode sputtering. A thickness of the first aluminum film31is e.g. 2 nm. Since the deposition rate in the oxygen reactive mode is low, approximately 1 minute sputtering is necessary for 2 nm deposition.

During the oxygen reactive mode sputtering, an excessive amount of oxygen can be implanted in the insulating film14, which is made of SiO, there is a chance that the insulating film14gets damage; consequently the reliability of the TFT could decrease. Therefore, a thickness of the first aluminum oxide film31, which is formed by oxygen reactive mode sputtering, is preferably 5 nm or less, and most preferably 2 nm. In addition, since particles are tend to be generated in the oxygen reactive mode sputtering, it is preferable to keep the thickness of the first aluminum oxide film31thin from this aspect too. By the way, even the thickness of the first aluminum oxide film31is as thin as 2 nm, the thickness can be measured by TEM (Transmission Electron Microscopy).

FIG.9is a cross sectional view that the second aluminum oxide film32is formed by sputtering on the first aluminum oxide film31. The second aluminum oxide film32is formed by the transition mode sputtering. Damage to the insulating film14due to oxygen is lower in the transition mode sputtering compared with that in the oxygen reactive mode sputtering. The deposition rate in the transition mode is high and a chance of generation of particles during the transition mode sputtering is low.

However, since the membrane stress of the aluminum oxide film formed by transition mode sputtering tends to be high, peeling off of the film tends to occur when the film is made thick. Therefore, a thickness of the second aluminum oxide film32is preferably 5 to 15 nm.

FIG.10is a cross sectional view that the metal, which is to be the gate electrode15, is formed by sputtering on the second aluminum oxide film32. The metal for the gate electrode15is e.g. Ti, Al, MoW or so forth. The metal, especially Ti and Al, easily absorb oxygen. Therefore, the metal extracts oxygen from the oxide semiconductor film13through the insulating film14. In this embodiment, however, the first aluminum oxide film31and the second aluminum oxide film32are formed beforehand, thus, those films can be block films to prevent extraction of oxygen from the oxide semiconductor film13.

FIG.11is a cross sectional view that the gate electrode15, the second aluminum oxide film32and the first aluminum oxide film31are patterned.FIG.12is a cross sectional view wherein after the gate electrode15is patterned, ion implantation is conducted using the gate electrode15as the mask to implant e.g. Ar in the oxide semiconductor film13, except the portion under the gate electrode15, to give conductance to the oxide semiconductor film13.

The first aluminum oxide film31and the second aluminum oxide film32exist under the gate electrode15. The second aluminum oxide film32prevents the extraction of the oxygen by the gate electrode15from the oxide semiconductor film13; the first aluminum oxide film31, which is oxygen rich film, supplies oxygen to the oxide semiconductor film13through the insulating film14.

As described above, TFTs of stable characteristics having the oxide semiconductor film13as the active layer can be realized according to the first embodiment.

Second Embodiment

FIG.13is a cross sectional view according to second embodiment.FIG.13differs fromFIG.4in that the insulating film14is formed only under the gate electrode15. Namely, the oxide rich insulating film14is formed only under the gate electrode15. In the oxide semiconductor film13, it is only channel region in which supplying of the oxygen is necessary or preventing extraction of oxygen is necessary.

The region of the oxide semiconductor film13other than the channel, namely the drain region and the source region, should be conductive; therefore, it is preferable that oxygen does not exist in this region. In the structure ofFIG.13, since oxygen rich insulating film14does not exist on the drain and the source, unnecessary oxygen is not supplied to the oxide semiconductor film13.

InFIG.13, the interlayer insulating film16contacts the drain and the source of the oxide semiconductor film13. The interlayer insulating film16can be made of SiO film, the SiO film that constitutes the interlayer insulating film16contains less oxygen than the insulating film14does; therefore, supply of oxygen to the drain and the source of the oxide semiconductor film13from the interlayer insulating film16is limited.

As described above, in the structure ofFIG.13, the first aluminum oxide31can supply oxygen to the channel of the oxide semiconductor film13for which supplying of the oxygen is necessary; the second aluminum oxide film32prevent extraction of the oxygen by the gate electrode15from the oxide semiconductor film13through the insulating film14. In addition to that, the insulating film14does not excessively supply oxygen to the drain and the source; thus, TFTs of oxide semiconductor having stable characteristics can be manufactured.

Third Embodiment

FIG.14is a cross sectional view that shows third embodiment.FIG.14differs fromFIG.13of second embodiment in that the second aluminum oxide film32is formed by oxidation in the annealing process, not the film formed by sputtering. The process is as follows: the gate electrode15is formed by aluminum; after the gate electrode15is patterned, the surface of the gate electrode15is oxidized in the annealing process to form the second aluminum oxide film32.

After the gate electrode15is patterned, the drain and the source of the oxide semiconductor13is given conductivity by driving in Ar and so forth by ion implantation; subsequently the oxide semiconductor film13is necessary to be activated by annealing. InFIG.14, the aluminum oxide formed on the surface of the aluminum during the annealing is used as the second aluminum oxide film32. This structure can also perform the same function of the subject invention as explained in first embodiment and second embodiment.

FIGS.15-18are cross sectional views of the process to constitute the structure ofFIG.14.FIG.15is a cross sectional view that the first aluminum oxide film31is formed by oxygen reactive mode sputtering on the insulating film14. Manufacturing method and thickness etc. of the first aluminum oxide film31is the same as explained in first embodiment.

FIG.16shows the aluminum film, which constitutes gate electrode15, is formed by sputtering etc. over the first aluminum oxide film31.FIG.17is a cross sectional view that the gate electrode15, the first aluminum oxide film31, the insulating film14are patterned. InFIG.17, the gate electrode15made of aluminum is formed directly on the first aluminum oxide film31. InFIG.17, after the gate electrode15is patterned, the drain and the source of the oxide semiconductor film13are given conductivity except the portion under the gate electrode15of the oxide semiconductor film13by performing ion implantation using the gate electrode15as the mask.

FIG.18is a cross sectional view, in which the gate electrode15and the oxide semiconductor film13etc. are covered by the interlayer insulating film16; then the aluminum oxide film is being generated on the surface of the gate electrode15by annealing at 250 centigrade to 350 centigrade. InFIG.18, a thickness of the second aluminum film32, formed between the gate electrode15and the first aluminum oxide film31by annealing, is approximately 2 nm.

The oxygen for the formation of the second aluminum oxide32is supplied from the first aluminum oxide film31. Since the first aluminum oxide film31is an oxygen rich film, it can supply oxygen to the gate electrode15during the annealing for the formation of the second aluminum oxide film32.FIG.19is a graph that shows the amount of released oxygen when the aluminum oxide film is heated; the amount of oxygen released from the aluminum oxide is detected by TDS (Thermal Desorption Spectrometry).

InFIG.19, the abscissa is the temperature of the substrate; the ordinate is the amount of oxygen released from the aluminum oxide, namely, intensity of O2(atomic weight is 32). InFIG.19, desorption of oxygen increases in proportion to the temperature in the region of 100 centigrade to approximately 350 centigrade.FIG.19shows a relative value; however, when the amount of oxygen in the aluminum oxide film is bigger, the amount of desorption of oxygen increases.

Back toFIG.18, the second aluminum oxide film32is formed not only between the gate electrode15and the first aluminum oxide31but also formed between the gate electrode15and the interlayer insulating film16. In this case, the oxygen is supplied from the surrounding interlayer insulating film16, which is made of SiO. Therefore, the second aluminum oxide film32covers all around the gate electrode15made of metal, thus, absorption of oxygen by the gate electrode15is suppressed.

The oxygen concentration in the second aluminum oxide film32that is formed by annealing is lower than that of the first aluminum oxide film31. Therefore, the distribution of oxygen in the aluminum oxide film30is the same as that in first embodiment and second embodiment.

By the way, when the aluminum oxide film is formed on the surface of the gate electrode by annealing, there could occur a problem of an electrical connection between the gate electrode15and the upper electrode28or other wirings as depicted inFIG.20. UnlikeFIG.2, the gate electrode15inFIG.20is supplied with gate voltage from the upper electrode28via through hole161in the insulating film16. InFIG.20, the aluminum oxide film32, which is an insulator, exists between the upper electrode28and the gate electrode15.

FIG.21is a graph that shows a contact resistance when the substrate is annealed, in the structure shown inFIG.20. InFIG.21, the abscissa is annealing temperature; the ordinate is a contact resistance between the gate electrode15and the upper electrode28in the contact hole161.

As depicted inFIG.21, when annealing temperature is 250 centigrade or more, the contact resistance between the electrode28and the gate electrode15drastically decreases and approaches to an insignificant value that is essentially no problem. The possible reason is that: since the aluminum oxide film formed by annealing is in a thickness of approximately 2 nm, the metal, constituting the gate electrode15, or the metal, constituting electrode28, diffuses into the aluminum oxide film32when annealing is conducted with high temperature; even the amount of diffusion may be small, electrical connection in the through hole can be achieved.

In general, in order to decrease the contact resistance in the through hole, the through hole is cleansed with hydro fluoride (HF). InFIG.21, circles are without HF cleansing; rhombuses are with HF cleansing.FIG.21, however, shows that raising the temperature of annealing is more effective to decrease the contact resistance in the through hole than cleansing the through hole with HF. As described above, if a problem of contact resistance due to the aluminum oxide formed by annealing occurs, the following process is effective: forming the through hole for electrical connection, forming the electrode for connection in the through hole, after that, the annealing is applied to decrease the contact resistance in the through hole.

Fourth Embodiment

In first embodiment to third embodiment, the invention is explained in the case of top gate type TFT, in which the gate electrode is set above the oxide semiconductor film13. The present invention, however, is applicable to the bottom gate type TFT, in which the gate electrode15is set beneath the oxide semiconductor film13, too. Further, the present invention is applicable to the dual gate type TFT, in which the gate electrode15exists both above and beneath the oxide semiconductor film13.

FIG.22is a cross sectional view of the pixel area in the liquid crystal display device which uses the dual gate type TFT, in which the gate electrode15is above the oxide semiconductor film13and the gate electrode41is beneath the oxide semiconductor film13.FIG.22differs fromFIG.4in first embodiment in that the bottom gate41is set beneath the oxide semiconductor film13.

InFIG.22, the second under coat film40is formed on the substrate10, which is made of glass and so forth. The second under coat film40has a plural layer structure formed by SiN film and SiO film. The bottom gate electrode (the second gate electrode)41is formed on the second under coat film40. The second gate electrode41has also a role of light shading film for the oxide semiconductor film13.

The aluminum oxide film50is formed on the second gate electrode41. This aluminum oxide film50has a same role as the aluminum oxide film30in the top gate. The aluminum oxide film50also has a two layer structure as with the aluminum oxide film30, however, the processing order of the first aluminum oxide51and the second aluminum oxide52is different.

After that, the second gate insulating film42is formed. The second gate insulating film42is expressed as the under coat film in first embodiment. In this embodiment, the second gate insulating film42is made of SiO film. The second gate insulating film42can have a two layer structure of SiO film and SiN film. After that the oxide semiconductor film13is formed. The structure above the oxide semiconductor film13is the same asFIG.3.

FIG.23is an enlarged cross sectional view of the second gate electrode41and its vicinity. InFIG.23, the second gate electrode41made of e.g. aluminum is formed on the under coat film40, which is formed on the substrate10. The aluminum oxide film50is formed on the second gate electrode41; the lower layer, which is formed first, is the second aluminum oxide film52, formed in a thickness of 5 to 10 nm by transition mode sputtering. The oxygen rich first aluminum oxide film51is formed in a thickness of 2 to 5 nm, preferably 2 nm, by oxygen reactive mode sputtering on the second aluminum oxide film52.

The manufacturing method is as follows. The metal for the second gate electrode41is formed by sputtering on the second under coat film40; the second aluminum oxide film52is formed by transition mode sputtering on the second gate electrode41; the first aluminum oxide film51is formed by oxygen reactive mode sputtering on the second aluminum oxide film52. After that the resist is coated and patterned by photo lithography, subsequently, the first aluminum oxide film51, the second aluminum oxide film52and the second gate electrode41are patterned by dry etching or wet etching.

After that, the second gate insulating film42is formed covering the first aluminum oxide film51. The second gate insulating film42is made of SiO. The second gate insulating film42can be a two layer structure of SiO film and SiN film; in this case, the SiO film is set to contact the oxide semiconductor film13. The oxide semiconductor film13is formed by sputtering and patterned. The process after that is the same as first embodiment.

The functions of the first aluminum oxide film51and the second aluminum oxide film52inFIGS.22and23are the same as the first aluminum oxide film31and the second aluminum oxide film32explained in first embodiment. As described above, the present invention can be applied to the TFTs of the bottom gate type and the dual gate type.

In first embodiment to fourth embodiment, it was explained that the two layers of aluminum films were formed between the insulating film and the gate electrode. However, even two layer structure is formed initially, a boundary between the first aluminum oxide film31and the second aluminum oxide film32may become obscure due to annealing process and so forth, which is applied later. Even this case, concentration of oxygen in the aluminum oxide film is larger at the side of the insulating layer than at the side of the gate electrode.

In first embodiment to fourth embodiment, it was explained that the gate electrode was made of metal, its major substance is aluminum. The present invention is, however, applicable to the case where the gate electrode is made of other metals that can form metal oxide at the surface, like Ti (Titanium).

In the above explanation, the present invention was explained in the case of IPS mode liquid crystal display device. The present invention is, however, applicable to other types of liquid crystal display devices. The organic EL display device also uses the oxide semiconductor TFTs. The cross sectional structure of the organic EL display device is basically the same asFIG.3up to formation of the organic passivation film21. Therefore, the present invention explained above can be applied to the organic EL display device.