Display device

The purpose of the present invention is to realize the TFT of the oxide semiconductor having a superior characteristics and high reliability during the product's life. The structure of the present invention is as follows. A display device comprising: a substrate including a display area where plural pixels are formed, the pixel includes a first TFT of a first oxide semiconductor, a first gate insulating film is formed on the first oxide semiconductor, the first gate insulating film is a laminated film of a first silicon oxide film and a first aluminum oxide film, a gate electrode is formed on the first aluminum film.

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent Application JP 2017-064916 filed on Mar. 29, 2017, the content of which is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a display device comprising TFTs (Thin Film Transistor) that use oxide semiconductors.

(2) Description of the Related Art

A liquid crystal display device or an organic EL display device uses TFTs for switching elements in the pixels or for the built in driving circuits. The TFT uses one of a-Si (amorphous Silicon), poly-Si (poly Silicon) or oxide semiconductor as an active layer.

The a-Si has low mobility; consequently, there are some problems to use the a-Si in the TFTs for the peripheral driving circuits. The poly-Si has high mobility, which is suitable for the TFTs for the peripheral driving circuits; however, the poly-Si has some problems for the switching TFTs in the pixels since it has bigger leak current. The oxide semiconductor has low leak current and the mobility is higher than the mobility of the a-Si; however, it has some problems of reliability in controlling defects in the semiconductor layer.

The patent document 1 (Japanese patent laid open 2012-15436) discloses the structure that the entire of the TFT, which comprises the oxide semiconductor and gate electrode, is covered by the inorganic insulating film of e.g. aluminum oxide, titanium oxide or indium oxide.

The patent document 2 (Japanese patent laid open 2015-92638) discloses the structure to suppress the gate leak caused by the tunnel effect when the gate insulating film becomes thin. The patent document 2 discloses to use the material of high dielectric constant as e.g. hafnium oxide, tantalum oxide laminated with silicon oxide, silicon nitride or aluminum oxide, etc. for the gate insulating film.

The patent document 3 (WO 2010/041686) discloses to sandwich the channel of the oxide semiconductor by the inorganic insulating film to stabilize the characteristics of the TFT. The patent document 3 discloses to use e.g. aluminum oxide, titanium oxide or indium oxide for the inorganic insulating film.

SUMMARY OF THE INVENTION

Examples of the oxide semiconductors are: IGZO (Indium Gallium Zinc Oxide), ITZO (Indium Tin Zinc Oxide), ZnON (Zinc Oxide Nitride), IGO (Indium Gallium Oxide), and so on. Since those semiconductors are transparent, they are sometimes called TAOS (Transparent Amorphous Oxide Semiconductor). By the way, for example, The ratio of the components of IGZO is generally In:Ga:Zn=1:1:1, however, in this specification, IGZO includes that deviated from the above ratio.

The initial characteristics of the TFT using the oxide semiconductor can be controlled by the amount of oxide in the oxide semiconductor or in the insulating film that contacts with the oxide semiconductor; however, controlling the reliability is difficult. Specific problem is that defects in the insulating layer increase when the amount of oxygen increases. Therefore, conventionally, the initial characteristics and the reliability have been in a relation of trade off.

Further, there has been a problem as that: even the amount of oxygen is controlled initially, the oxygen gradually moves out from the oxide semiconductor during the product's life, consequently, the characteristics of the TFT change.

The purpose of the present invention is to realize the TFT formed by the oxide semiconductor that satisfies both of the initial characteristics and the high reliability during the product's life.

The present invention solves the above problem; the concrete measures of the present inventions are as follows:

(1) A display device comprising: a substrate including a display area where plural pixels are formed, the pixel includes a first TFT of a first oxide semiconductor, a first gate insulating film is formed on the first oxide semiconductor, the first gate insulating film is a laminated film of a first silicon oxide film and a first aluminum oxide film, a gate electrode is formed on the first aluminum film.

(2) The display device according to (1), wherein the first gate electrode is formed by a laminated film that a second oxide semiconductor is laminated by a metal.

(3) The display device according to (1), wherein an interlayer insulating film is formed covering the first gate insulating film and the first gate electrode, defect density of the first silicon oxide film is less than defect density of the interlayer insulating film, the defect density of the first silicon oxide film is 1×1018(spins/cm3) or less by ESR (Electrode Spin resonance) analysis.

(4) The display device according to (3), wherein a desorption of oxygen from the first silicon oxide film in TDS (Thermal Desorption Spectrometry) analysis, provided M/z=32, the desorption of oxygen (O2) is 1×1015(molecules/cm2) or more at the temperature of 100 to 250 centigrade.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail referring to the following embodiments.

First Embodiment

FIG. 1is a plan view of a liquid crystal display device, which is used in e.g. the cellar phone, where the present invention is applied. InFIG. 1, the TFT substrate10, in which plural pixels93are formed, and the counter substrate40are adhered by the seal material80. The liquid crystal is sandwiched between the TFT substrate10and the counter substrate40. The display area90is formed inside of the seal material80. In the display area90, the scan lines91extend in lateral direction and arranged in longitudinal direction; the video signal lines92extend in longitudinal direction and arranged in lateral direction

The pixel93is formed in the area surrounded by the scan lines91and the video signal lines92. In each of the pixels93, the pixel electrode and the TFT, which controls the signals that are supplied to the pixel electrode, are formed. The TFT substrate10is made bigger than the counter substrate40; the portion of the TFT substrate10that doesn't overlap with the counter substrate40is the terminal area. The driver IC95is installed in the terminal area; the flexible wiring substrate96is connected to the terminal area to supply signals and powers to the liquid crystal display device.

FIG. 2is cross sectional view along the line A-A ofFIG. 1. InFIG. 2, the TFT substrate10and the counter substrate40are overlapped to each other. The liquid crystal layer is omitted inFIG. 2since the thickness of the liquid crystal layer is much thinner than the thicknesses of the TFT substrate10and the counter substrate40. The portion where the TFT substrate10and the counter substrate40don't overlap is the terminal area where the driver IC95is installed and the flexible wiring substrate96is connected.

Since the liquid crystal is not self-illuminant, the back light1000is set at the rear side of the TFT substrate10. Images are formed by controlling the light from the back light1000in each of the pixels. Since the liquid crystal controls only the polarized light, the lower polarizing plate510is adhered to underneath the TFT substrate10, and the upper polarizing plate520is adhered to on the counter substrate40.

FIG. 3is a cross sectional view of the display area90of the liquid crystal display device. InFIG. 3, the TFT substrate10is formed by glass or resin. The undercoat11is formed on the TFT substrate10to prevent the semiconductor layer from being contaminated by impurities from the glass or resin. The undercoat11is a laminated film of the silicon oxide film (it may be called SiO layer herein after) and the silicon nitride film (it may be called SiN layer herein after); sometimes the aluminum oxide (it may be called AlO herein after) film is laminated, too.

The oxide semiconductor12of e.g. IGZO is formed on the undercoat11. The gate insulating film13is formed covering the oxide semiconductor12. In the present invention, as will be explained later, the gate insulating film13has a structure that aluminum oxide film is laminated on the silicon oxide film. The gate electrode14is formed on the gate insulating film13. In the present embodiment, as will be explained later, the gate electrode14is a laminated film of the second oxide semiconductor and the metal film. The metal film is preferably formed by Mo, W or alloys of those metals.

InFIG. 3, after the gate electrode14is formed, the ion implantation is applied to the oxide semiconductor12using the gate electrode14as a mask to form defects in the oxide semiconductor12to give conductivity; thus, the drain area121and the source area122are formed in the oxide semiconductor12. The interlayer insulating film15is formed covering the gate electrode14and the gate insulating film13. The interlayer insulating film15is formed by the silicon oxide film; however, it can be formed by the silicon nitride film or a laminated film of the silicon oxide film and the silicon nitride film. The through holes are made through the interlayer insulating film15and the gate insulating film13to connect the drain electrode16with the drain area121, and the source electrode17with the source area122. The drain electrode16connects with the video signal line92and the source electrode17connects with the pixel electrode21through the through hole23.

The organic passivation film18is formed covering the interlayer insulating film15, drain electrode16and the source electrode17. The organic passivation film18is made as thick as 2 μm to 4 μm since it has also a role as a flattening film. The through hole23is formed in the organic passivation film18to connect the pixel electrode21and the source electrode17of the TFT.

The common electrode19is formed in a solid plane shape on the organic passivation film18. The capacitive insulating film20of SiN is formed covering the common electrode19; the pixel electrode21is formed on the capacitive insulating film20. The capacitive insulating film20is so called because a holding capacitance is formed between the common electrode19and the pixel electrode21via the capacitive insulating film20. The alignment film22is formed covering the pixel electrode21for an initial alignment of the liquid crystal molecules. The pixel electrode is stripe shaped or comb shaped in a plan view. When the voltage is applied to the pixel electrode21, the line of force as depicted by arrows inFIG. 3is generated, whereby the liquid crystal molecules301are rotated, thus the transmittance of the light from the back light is controlled in a pixel.

InFIG. 3, the counter substrate40is set to sandwich the liquid crystal layer300with the TFT substrate10. On the inner side of the counter substrate40, the color filter41is formed corresponding to the pixel electrode21to form the color images. The black matrix42is formed between the color filters41to improve the contrast of the images. The overcoat film43is formed covering the color filter41and the black matrix42. The overcoat film43prevents that the pigments in the color filter41goes out and contaminates the liquid crystal layer300. The alignment film44is formed covering the overcoat film43.

FIG. 4is a cross sectional view of the first embodiment of the present invention. InFIG. 4, the first oxide semiconductor12formed by e.g. IGZO is fabricated on the undercoat11, which is formed by a laminated film of the SiO layer and the SiN layer. The thickness of the first oxide semiconductor12is 10 nm to 70 nm. The gate insulating film13is formed covering the first oxide semiconductor12. The gate insulating film13is a laminated film of the silicon oxide film131and the first aluminum oxide film132. The thickness of the silicon oxide film131that constitutes the gate insulating film13is 50 nm to 200 nm; the thickness of the first aluminum oxide film132, which covers the silicon oxide film131, is 1 nm to 20 nm.

InFIG. 4, the gate electrode14is formed on the first aluminum oxide film132; the gate electrode14is a laminated film of the second oxide semiconductor141and the metal layer142. The metal layer142is formed by e.g. Mo or W or alloys of those metals. The second oxide semiconductor141is formed by e.g. IGZO. The materials for the first oxide semiconductor12and the second oxide semiconductor141are not necessarily the same; however, the process becomes simpler if the same material is used. The thickness of the second oxide semiconductor141is 1 nm to 30 nm.

The characteristics of the TFT using the oxide semiconductor12is maintained by oxygen supplied from the gate insulating film13. The gate insulating film13needs to have many defects to supply oxygen to the oxide semiconductor12. The gate insulating film13having many defects, however, tends to absorb the gasses used in the process, which deteriorate the characteristics of the oxide semiconductor12.

The feature of the present invention is to use the gate insulating film13comprises the silicon oxide131having less defects, and the aluminum oxide film132which is laminated on the silicon oxide131. According to this structure, the oxygen is supplied to the first oxide semiconductor12from the aluminum oxide film132through the silicon oxide film131; thus, the characteristics of the first oxide semiconductor12can be maintained stable.

Further, the present invention uses the second oxide semiconductor141as the lower layer of the gate electrode14; thus, the oxygen is supplied to the first oxide semiconductor12of the TFT from the second oxide semiconductor141. In addition, the substrate is annealed when the second semiconductor141is made; during the annealing, the oxygen, which is emitted from the aluminum oxide film132, is supplied to the first oxide semiconductor12, which constitutes the TFT. Therefore, according to the present invention, even the silicon oxide film131of less defects is used as the gate insulating film13, the characteristics of the first oxide semiconductor12can be maintained; thus, reliability of the TFT using the oxide semiconductor12can be improved.

The required characteristics of the silicon oxide film131constituting the gate insulating film13is as follows. Firstly, the defect density is low; concretely, 1×1018(spins/cm3) or less by ESR (Electron Spin resonance) analysis. The measurement condition of the ESR is: the temperature 85K; the power of the microwave is 10 mw; the direction the magnetic field is parallel to the surface of the film; the range of the magnetic field is 317±25 mT; the modulation bandwidth is 0.5 mT; the modulation frequency is 100 kHz; the time constant is 0.03 sec.

Secondly, enough oxygen must be supplied to maintain the characteristics of the first oxide semiconductor12; concretely, in TDS (Thermal Desorption Spectrometry) analysis, provided M/z=32, the desorption of oxygen (O2) is 1×1015(molecules/cm2) or more at the temperature of 100 to 250 centigrade. The conventional gate insulating layer103could not satisfy the requirements1and2.

Thirdly, desorption of gasses other than oxygen is low. The TFT substrate goes through in various processes; thus, if the defects in the film are many, the gasses used in the process are absorbed in the defects; the absorbed gasses deteriorate the characteristics of the oxide semiconductor12. Thus, the silicon oxide film131of low defects can improve the reliability of the TFT that uses the oxide semiconductor12.

Among the gasses used in the processes, N2O is evaluated as a concrete example as follows: in TDS analysis, provided M/z=44, the desorption of N2O is 8×1013(molecules/cm2) or less at the temperature of 100 to 400 centigrade.

The above characteristics are the silicon oxide film131in a completed display device. As to the measurement of the silicon oxide film131in a completed display device, the upper layers formed over the gate insulating film13constituted by the silicon oxide film131are taken away; then, the ESR or the TDS are applied.

InFIG. 4, the undercoat11is formed by two layers of the SiN layer and the SiO layer. The lower layer is the SiN layer and the upper layer is the SiO layer. The SiN layer is indispensable since it is superior in blocking the moisture; however it can be a source of hydrogen that deoxidizes the oxide semiconductor12. Therefore, the SiO layer is laminated on the SiN layer. The SiN layer and the SiO layer are formed continuously by CVD.

Since the upper layer of the silicon oxide (SiO) directly contacts the oxide semiconductor12, the characteristics of the SiO layer must be controlled; the concrete characteristics are the same as the SiO layer of the gate insulating film13. Firstly, the defect density is low; concretely, 1×1018(spins/cm3) or less by ESR (Electrode Spin resonance) analysis. By the way, the defect density of the interlayer insulating film15is 1×1018(spins/cm3) or more by the ESR analysis. Secondly, enough oxygen must be supplied to maintain the characteristics of the first oxide semiconductor; concretely, in TDS (Thermal Desorption Spectrometry) analysis, when M/z=32, the desorption of oxygen (O2) is 1×1015(molecules/cm2) or more at the temperature of 100 to 250 centigrade. Thirdly, desorption of gasses other than oxygen is low; if N2O is evaluated as a concrete example: in TDS analysis, provided M/z=44, the desorption of N2O is 8×1013(molecules/cm2) or less at the temperature of 100 to 400 centigrade.

The measurement of the silicon oxide (SiO) layer in the undercoat11is the same as the measurement of the silicon oxide film131in the gate insulating film13; namely, the upper layers formed over the undercoat11are taken away, then, the ESR or the TDS are applied to the silicon oxide (SiO) layer in the undercoat11.

FIG. 5is a cross sectional view of the second example of the present embodiment.FIG. 5differs fromFIG. 4in that the second aluminum oxide film112is added in the undercoat11. The thickness of the second aluminum oxide film112, too, is 1 nm to 20 nm. InFIG. 5, the second aluminum oxide film112is formed on the laminated layer111of the SiO layer and the SiN layer. When the undercoat is a laminated layer of three layers of SiO/SiN/SiO, the aluminum oxide layer112can be laminated on the upper most SiO layer, or the upper most SiO layer can be substituted by the aluminum oxide layer112.

The second aluminum oxide film112not only has superior characteristics in blocking moisture and other gasses but also can be a source of the oxygen to the oxide semiconductor12; therefore, it is suitable for the undercoat for the oxide semiconductor12. On the other hand, the second aluminum oxide film112has more defects in the film compared with the silicon oxide film; therefore, there is a possibility that gasses absorbed in the defects in the second aluminum oxide film112deteriorate the oxide semiconductor12. However, the characteristics of the TFT is mainly governed by the characteristics of the oxide semiconductor12on the side facing the first gate insulating film13, thus, the defects in the second aluminum oxide film112do not raise a big problem.

FIG. 6is a cross sectional view of the third example of the present embodiment.FIG. 6differs fromFIG. 4in that the metal protective layer50is formed between the drain electrode16and the oxide semiconductor12, and between the source electrode17and the oxide semiconductor12. The drain electrode16and the source electrode17are formed in the through holes formed in the interlayer insulating film15and the gate insulating film13. The through holes are made by e.g. dry etching. Since the thickness of the first oxide semiconductor12is very thin as 10 nm to 70 nm, there is a danger that the oxide semiconductor12disappears at the through holes when the thorough holes are made in the interlayer insulating film15and the gate insulating film13.

InFIG. 6, the metal protective layer50is formed between the drain electrode16and the oxide semiconductor12, and between the source electrode17and the oxide semiconductor12; thus, the oxide semiconductor12is protected from being eliminated by the etching. The metals for the protective metal50can be the same as the metals for the video signal lines92; they can be e.g. the laminated film that Al alloy is sandwiched by Titanium. The structure ofFIG. 6realizes the TFT of the oxide semiconductor having high reliability.

The Second Embodiment

FIG. 7is a cross sectional view of the second embodiment.FIG. 7differs fromFIG. 4in that the gate insulating film13is formed only under the gate electrode14. InFIG. 7, the silicon oxide film131, which constitutes the gate insulating film13, is formed on the first oxide semiconductor12; the aluminum oxide film132is formed on the silicon oxide film131. The thickness of the silicon oxide film131and the aluminum oxide film132are the same as the ones in the first embodiment.

InFIG. 7, the silicon oxide film131and the aluminum oxide film132are eliminated except underneath the gate electrode13. The merit ofFIG. 7is as follows. The first oxide semiconductor12needs to be conductive except at the channel. For that purpose, in the structure ofFIG. 4, the ion implantation is applied to the oxide semiconductor12using the gate electrode14as a mask to form the defects in the semiconductor for conductivity.

In the structure ofFIG. 7, the oxide semiconductor12is exposed after the gate insulating film13is eliminated except under the gate electrode14. In this state, if the oxide semiconductor is exposed to e.g. silane (SiH4), the exposed portion of the oxide semiconductor is reduced and gets conductivity. Alternatively, if the exposed portion of the oxide semiconductor12is applied with the Ar plasma or N2plasma, the oxide semiconductor12gets defects, thus, the oxide semiconductor12becomes conductive at the exposed portion. Therefore, in the present embodiment, the oxide semiconductor12can get necessary characteristics without applying the ion implantation.

InFIG. 7, after the necessary conductivity is given to the oxide semiconductor12, the interlayer insulating film15is formed on the gate electrode14and the first oxide semiconductor12by the SiO layer or the SiN layer, or the lamination film of the SiO layer and the SiN layer. As explained in the first embodiment, the second aluminum oxide film112can be added in the undercoat11; the metal protective layer50can be applied on the drain area and the source area of the oxide semiconductor12. The performance of the TFT of the present embodiment is the same as the TFT of the first embodiment.

Third Embodiment

FIG. 8is a cross sectional view of the third embodiment of the present invention.FIG. 8differs fromFIG. 4of the first embodiment in that the gate electrode14is formed only by the metal; the second oxide semiconductor does not exist. In this case, the second aluminum oxide film132becomes a source of the oxygen for the first oxide semiconductor12. Therefore, the silicon oxide layer131constituting the gate insulating film13can be made to have low defects.

The aluminum oxide film132is a source of the oxygen for the oxide semiconductor12; at the same time, it can have a role to confine the oxygen in the oxide semiconductor side, therefore, in many cases, the first oxide semiconductor12can maintain excellent performance and high reliability.

In the third embodiment, too, as explained in the first embodiment, the second aluminum oxide film112can be added in the undercoat11; the metal protective layer50can be applied on the drain area and the source area of the oxide semiconductor12. The structure of the second embodiment is also applicable to the third embodiment.

Fourth Embodiment

FIG. 9is a cross sectional view of the fourth embodiment of the present invention. The ON current in the TFT of the oxide semiconductor12can be 10 times bigger than the ON current in the TFT of the a-Si; however, the ON current is not so big as the TFT of the poly-Si. The dual gate structure can be adopted in the TFT of the oxide semiconductor12to increase the ON current.

FIG. 9is a cross sectional view that shows this feature. InFIG. 9, the second gate electrode60is formed on the TFT substrate10; the second gate insulating film61is formed on the second gate electrode60; the first oxide semiconductor12is formed on the second gate insulating film61. The upper layers over the first oxide semiconductor12are the same as inFIG. 4of the first embodiment.

According to the structure ofFIG. 9, the current can flow at the upper side and at the lower side of the first oxide semiconductor12; thus, the ON current can be increased. InFIG. 9, the second gate insulating film61is a silicon oxide film; the second gate electrode60is metal of e.g. Mo or W, or alloys of those metals. The second gate insulating film61can be a laminated film of the silicon nitride film and the silicon oxide film; in this case, the silicon nitride film is the lower layer and the silicon oxide film is the upper layer.

FIG. 10is the structure that the protective layer50is added on the drain area and the source area of the oxide semiconductor12inFIG. 9. The purpose of the structure is the same as explained inFIG. 6of the first embodiment; namely, to avoid the oxide semiconductor12from disappearing when the through holes are made in the interlayer insulating film15and in the gate insulating film13.

FIG. 11is a cross sectional view of the second example of the present embodiment. InFIG. 11, the laminated film of the silicon oxide film612and the third aluminum oxide film611is made for the second gate insulating film61; the laminated film of the metal layer601and the third oxide semiconductor602is made for the second gate electrode60.

In other words, inFIG. 11, the third aluminum oxide film611is formed on the second gate electrode60; the silicon oxide612is formed on the third aluminum oxide film611. The second gate electrode60is the structure that the third oxide semiconductor602is laminated on the metal layer601formed by e.g. MoW. The thicknesses of the layers are the same as explained in the first embodiment.

FIG. 12is the structure that the third oxide semiconductor602is eliminated from the second gate electrode60in the structure ofFIG. 11. The effect of the structure ofFIG. 12is the same as explained in the third embodiment. The structures ofFIG. 11andFIG. 12can realize more reliable dual gate type TFT of the oxide semiconductor. By the way, inFIGS. 11 and 12, too, the protective layer50can be formed on the drain area and the source area of the oxide semiconductor12as depicted inFIG. 10to avoid disappearing of the first oxide semiconductor12during formation of the through holes.

The Fifth Embodiment

Since the poly-Si has high carrier mobility, a high speed TFT can be realized. On the other hand, the oxide semiconductor has a low leak current, thus, the TFT of the oxide semiconductor is suitable for the switching element. Therefore, using both the TFT of the poly-Si and the TFT of the oxide semiconductor can realize the high quality display device; e.g. the TFT of the poly-Si is used in the driving circuit while the TFT of the oxide semiconductor is used as the switching TFT in the pixel.

FIG. 13is the cross sectional view of the fifth embodiment of the present invention where the TFT of the poly-Si and the TFT of the oxide semiconductor coexist. This structure is called the hybrid structure. The TFT of the oxide semiconductor is a dual gate type. InFIG. 13, the undercoat11is formed on the TFT substrate10. The structure of the undercoat11can be the same as explained in the first embodiment.

Firstly, the TFT of the poly-Si70is formed on the undercoat11. The poly-Si70is made as that: the a-Si is formed first on the undercoat11, then the a-Si is transformed to the poly-Si by applying the excimer laser on the a-Si; subsequently, the poly-Si is patterned. The third gate insulating film71is formed covering the poly-Si70. The third gate insulating film71is formed by CVD using TEOS (Tetraethyl orthosilicate) as the material.

The second gate electrode60for the TFT of the oxide semiconductor12is formed on the third gate insulating film71; the gate electrode (the third gate electrode)72for the TFT of the poly-Si70is formed, at the same time. After that, the silicon oxide film61, which is the second gate insulating film for the TFT of the second semiconductor12, is formed covering the second gate electrode60and the third gate electrode72; subsequently, the oxide semiconductor12is formed on the silicon oxide61.

The gate insulating film13, which is made by the silicon oxide film131and the aluminum oxide film132, is formed covering the oxide semiconductor12; the gate electrode14, made by the second oxide semiconductor141and the metal142, is formed on the gate insulating film13as explained in the first embodiment. By the way, as explained in the second embodiment, the first gate insulating film13can be formed only underneath the first gate electrode14. The second oxide semiconductor141can be eliminated from the gate electrode14as explained in the third embodiment.

InFIG. 13, the drain electrodes16and the source electrodes17for the TFT of the oxide semiconductor and the TFT of the poly-Si are made simultaneously. That is to say, in precede, through holes for the drain electrode16and the source electrode17are formed simultaneously for the TFT of the oxide semiconductor and the TFT of the poly-Si.

As depicted inFIG. 13, the through holes are formed through five insulating films in the poly-Si TFT while the through holes are formed through three insulating films in the oxide semiconductor TFT. Therefore, the oxide semiconductor12is exposed to the etching media longer than the poly-Si semiconductor70is exposed to; thus, the oxide semiconductor12tends to disappear in the etching process.

In addition, the poly-Si70must be cleaned by hydrogen fluoride (HF) after the through holes are formed. At the same time, the oxide semiconductor12, too, is exposed to the hydrogen fluoride (HF); the oxide semiconductor12is easily dissolved by the hydrogen fluoride (HF).

FIG. 14is the hybrid type TFT that countermeasures this problem.FIG. 14differs fromFIG. 13in that the metal protective layer50is formed on the drain area and the source area of the oxide semiconductor12. This structure in the TFT of the oxide semiconductor12is the same asFIG. 12of the fourth embodiment.

The TFT of the oxide semiconductor in the structures ofFIG. 13andFIG. 14are dual gate type, however, the TFT of the oxide semiconductor in this embodiment can be a single gate type as explained inFIGS. 4 to 8. As described above, the hybrid type TFT of excellent performance and high reliability can be realized according the embodiment of the present invention.

Sixth Embodiment

From the first embodiment through the fifth embodiment, the present invention was explained in regard to the liquid crystal display device. The present invention can be applicable to the organic EL display device as well as to the liquid crystal display device.FIG. 15is a cross sectional view of the display area of the organic EL display device. InFIG. 15, the TFT is formed on the TFT substrate10; the organic passivation film18is formed on the TFT; the through hole is formed in the organic passivation film18, which are the same as in the liquid crystal display device.

Therefore, the structures of the TFT of the oxide semiconductor explained in the first embodiment to the fifth embodiment are applicable to the organic EL display device.

InFIG. 15, the reflection electrode30is formed on the organic insulating film18; the oxide conductive film of e.g. the ITO (Indium Tin Oxide) as the anode31is formed on the reflection electrode30. The bank32formed by organic substance like acrylic is formed covering the organic passivation film18and the anode31. In the hole of the bank32, the organic EL layer33for light emitting is formed on the anode31. The organic EL layer33is a laminated film of various layers; the thickness of the organic EL layer33is thin as several hundred nanometers in total, thus each of the layers is very thin. The bank32prevents the organic EL layer33from disconnection at the steps of the anode31and the reflection electrode30.

InFIG. 15, the upper electrode, which is cathode34, is formed over the organic EL layer33by the oxide conductive film as e.g. ITO or IZO (Indium Zinc Oxide), or by the thin metal. Since the organic EL layer33is decomposed by the moisture, the protective film35is formed by e.g. the SiN layer mainly to block external moisture.

The organic EL display device uses the reflection electrode30; therefore, the external light is reflected by the reflection electrode30, which deteriorates the visibility of the screen. The circular polarizing plate37is adhered to the display surface by the adhesive36to prevent the reflection of the external light.

As described above, the structure of the organic EL display device is the same up to forming the drain electrode16and the source electrode17of the oxide semiconductor12as the structure of the liquid crystal display device. Therefore, the present invention explained from the first embodiment through the fifth embodiment is applicable to the organic EL display device.