Display device

The purpose of the present invention is to form both LTPS TFT and semiconductor TFT in a same substrate. The feature of the display device to realize the above purpose is that: a display device having a display area containing a pixel comprising: the pixel includes a first TFT having an oxide semiconductor, a gate insulating film is formed on the oxide semiconductor, a first gate electrode is formed on the gate insulating film, a first source/drain electrode formed by a metal or an alloy contacts a source or a drain of the semiconductor the first gate electrode and the first source/drain electrode are formed by the same material.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent Application JP 2016-184101 filed on Sep. 21, 2016, 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 of hybrid structure, which includes both of a TFT of poly-silicon and a TFT of oxide semiconductor.

(2) Description of the Related Art

A liquid crystal display device comprises a TFT substrate where thin film transistors (TFT) and pixel electrodes are formed and a counter substrate opposing to the TFT substrate, wherein a liquid crystal layer is sandwiched between the TFT substrate and the counter substrate. Images are formed by controlling the transmittance of lights in each of pixels. On the other hand, an organic EL display device forms color images by an organic light emitting layer and a TFT formed in individual pixels. An organic EL display has a merit for a thin display compared to a liquid crystal display device since an organic EL display device doesn't need a backlight.

LTPS (Low Temperature Poly-Si) is suitable for a TFT in a driving circuit. On the contrary, an oxide semiconductor has a high OFF resistance, thus gives a TFT of low OFF current.

Japanese patent laid open 2013-175718 and Japanese patent laid open 2011-54812 disclose a TFT having an oxide semiconductor. Japanese patent laid open 2013-175718 discloses forming a metal oxide on a semiconductor, which constitutes channel, and to use the metal oxide as a gate insulating film. Japanese patent laid open 2011-54812 discloses to use a metal oxide or a semiconductor layer as a sacrificing layer for channel etching in a bottom gate type TFT having an oxide semiconductor.

SUMMARY OF THE INVENTION

A switching TFT in a pixel needs to have low leak current. A TFT of oxide semiconductor can make a low leak current TFT. An oxide semiconductor, which is amorphous and optically transparent, is called TAOS (Transparent Amorphous Oxide Semiconductor). TAOS includes IGZO (Indium Gallium Zinc Oxide), ITZO (Indium Tin Zinc Oxide), ZnON (Zinc Oxide Nitride), IGO (Indium Gallium Oxide), and so on. Herein after, an oxide semiconductor is represented by TAOS. TAOS has a character that a mobility of carriers is low, thus, sometimes it is difficult to form a driving circuit by the TFT of TAOS built in the display device. Herein after, a term of TAOS is also used as a term of TFT including TAOS in this specification.

On the other hand the TFT of LTPS has high carrier mobility, thus a driving circuit can be formed by the TFT of LTPS. Herein after, a term of LTPS is also used as a term of TFT including LTPS. Since the TFT of LTPS has a character that a leak current is comparatively high, a serially connected two TFTs of LTPS is used for a switching TFT.

Consequently, it is reasonable to use the TAOS for a switching element and to use the LTPS for a driving circuit. However, since LTPS and TAOS are different materials, there is a task to solve to form the LTPS and the TAOS on a same substrate. Namely, when forming a source electrode and a drain electrode in the LTPS TFT, it is necessary to clean a surface of the LTPS by hydrofluoric acid (HF) to remove surface oxide. Hydrofluoric acid (HF), however, dissolves TAOS, thus, a same process is not applicable for the LTPS and the TAOS.

A purpose of the present invention is to solve the above problem, and enables to form the LTPS TFT and the TAOS TFT on a same substrate by a common process.

The present invention solves the above problem; concrete structures are as follows:(1) A display device having a display area containing a pixel comprising: the pixel includes a first TFT having an oxide semiconductor, a gate insulating film is formed on the oxide semiconductor, a first gate electrode is formed on the gate insulating film, a first source/drain electrode formed by a metal or an alloy contacts a source or a drain of the semiconductor the first gate electrode and the first source/drain electrode are formed by the same material.(2) The display device according to (1), wherein the gate insulating film covers the oxide semiconductor, the first source/drain electrode contacts the source or the drain of the oxide semiconductor through a through hole formed in the gate insulating film.(3) The display device according to (1), wherein the gate insulating film is formed in island shape to cover a channel portion of the first TFT, the gate insulating layer doesn't exist between the first source/drain electrode and the oxide semiconductor.(4) The display device according to (1), wherein a driving circuit is formed outside of the display area, the driving circuit includes a second TFT having a Poly-Si semiconductor.(5) A display device having a display area containing a pixel comprising: the pixel includes a first TFT having an oxide semiconductor, a gate insulating film is formed on the oxide semiconductor, a first gate electrode is formed on the gate insulating film, a first source/drain electrode formed by a metal or an alloy contacts a source or a drain of the semiconductor a first insulating film is formed to cover the oxide semiconductor, the gate electrode, and the first source/drain electrode, a second source/drain electrode connects with the first source/drain electrode through a through hole formed in the first insulating film, the first source/drain electrode exists at a bottom of the through hole formed in the first insulating film.(6) The display device according to claim (5), wherein the gate insulating film is formed to cover the oxide semiconductor, the first source/drain electrode connects with a source or a drain of the first TFT through a through hole formed in the gate insulating film.(7) The display device according to (5), wherein the gate insulating film is formed in island shape to cover a channel of the first TFT, the gate insulating film doesn't exists between the first source/drain electrode and the oxide semiconductor.(8) The display device according to claim (5), wherein a driving circuit is formed outside of the display area, the driving circuit includes a second TFT having Poly-Si semiconductor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

First Embodiment

At the outset, the case is explained when the present invention is applied to a liquid crystal display device.FIG. 1is a plan view of the liquid crystal display device1where the present invention is applied.FIG. 2is a cross sectional view along A-A ofFIG. 1. InFIG. 1andFIG. 2, the TFT substrate100and the counter substrate200are opposed to each other and a liquid crystal layer is sandwiched between the TFT substrate100and the counter substrate200. The lower polarizing plate130is attached underneath the TFT substrate100: the upper polarizing plate230is attached on the counter substrate200. An assembly of the TFT substrate100, the counter substrate200, the lower polarizing plate130, and the upper polarizing plate230is called the liquid crystal display panel500.

The TFT substrate100is bigger than the counter substrate200, a portion of the TFT substrate100where the counter substrate200doesn't overlap is the terminal area150. The driver IC170that supply video signals is installed on the terminal area150. The flexible circuit substrate160is connected to the terminal area160. The back light400is set at a back of the liquid crystal panel500since the liquid crystal display panel500doesn't emit light.

The liquid crystal display device1can be divided into the display area10and the peripheral area20as depicted inFIG. 1. Many pixels are formed in the display area10and each of the pixels has a switching TFT. A driving circuit, which drives scanning lines or video signal lines, is formed in the peripheral area20.

A TFT that is used in a pixel must have low leak current. TAOS, which has low leak current, is suitable for a TFT in a pixel, while Poly-Si (Poly-Silicon), which has high mobility, is suitable for a driving circuit. Since the LTPS is often used as the Poly-Si in a liquid crystal display device, “Poly-Si” is sometimes described as “LTPS” in this specification. In a process of the LTPS TFT, through holes must be made in insulating layers, which covers the LTPS, to connect the LTPS with a drain electrode or a source electrode; in addition, cleaning of a surface of the LTPS by hydrofluoric acid (HF) at the through holes is necessary to remove oxide from the surface of the LTPS.

However, when the same process is applied to the TAOS TFT, the TAOS is dissolved by hydrofluoric acid (HF) at through holes, as a result, the TAOS TFT cannot be realized. Therefore, this problem must be solved to realize to form both the LTPS TFT and the TAOS TFT on the same substrate. The structure that solves this problem is explained referring toFIGS. 3-8.

FIG. 3is an equivalent circuit of a liquid crystal display device. InFIG. 3, the scanning lines11extend in lateral direction and arranged in longitudinal direction in a display area. The video signal lines extend in longitudinal direction and arranged in lateral direction. A pixel is formed in an area surrounded by the scanning lines11and the video signal lines12. Each pixel has a switching TFT of the TAOS and a liquid crystal layer; and a storage capacitance is formed in parallel to the liquid crystal layer. The liquid crystal is driven by a pixel electrode and a common electrode. A pixel electrode connects with a source of the TFT; the common electrode connects with the common line13. Generally, a fixed voltage is supplied to the common line13from the common line driving circuit23.

InFIG. 3, the scanning line driving circuit21is set at left side of the display area. This scanning line driving circuit21is formed by TFTs of Poly-Si and is built in the TFT substrate. The video signal line driving circuit23is formed at lower side (longitudinally lower side inFIG. 3) of the display area.

Poly-Si TFTs are used in a peripheral circuit, while TAOS TFTs are used in the display area. The display area and the peripheral area are formed simultaneously. In this case, in the Poly-Si TFT, the poly-Si at through holes must be cleaned by hydrofluoric acid (HF) before forming source/drain electrode, however, at the same time TAOS at through holes are cleaned by hydrofluoric acid (HF). The problem is that the TAOS is dissolved by the hydrofluoric acid (HF), as a result, the TAOS disappears at the through holes.

FIG. 4is a plan view of the TAOS TFT in each pixel. InFIG. 4, a pixel is formed in an area surrounded by the video signal lines12and the scanning lines11, a TFT using the TAOS107is formed in the pixel. The gate insulating film108is formed to cover a part of the TAOS107. The gate electrode109, diverged from the scanning line11, is set on the gate insulating film108. A part of the TAOS107, which overlaps with the gate electrode109, is a channel.

The source electrode110or the drain electrode110is formed at both sides of the TAOS. Herein after, the source electrode110and the drain electrode110may be called source/drain electrode110. One of the source/drain electrodes110connects with the video signal line12and another of the source/drain electrodes110connects with the pixel electrode. InFIG. 4, the source/drain electrodes110and the video signal line12are connected via through hole.

FIG. 5is a plan view, which shows another example of the TAOS TFT in each pixel.FIG. 5is different fromFIG. 4in that a diverged video signal line12forms one of the source/drain electrodes110, and the gate electrode109connects with the scanning line11via a through hole. A problem, which arises when the Poly-Si TFT and the TAOS TFT are formed on the same substrate, is common inFIG. 4andFIG. 5. Herein after, the present invention is explained referring toFIG. 4.

FIG. 6is cross sectional structures of the TAOS TFT in a various manufacturing processes according to the present invention.FIG. 6shows only the layers constituting the TAOS TFT, and other layers are omitted. A ofFIG. 6shows: the TAOS107, formed on the TFT substrate100of e.g. glass, is patterned and covered by the gate insulating film108. B ofFIG. 6is a cross sectional view that shows the gate insulating film108is patterned.

C ofFIG. 6is a cross sectional view that shows the metal film (METAL) is formed on the gate insulating film108and the TAOS107. The metal film is formed e.g. by a laminated film of Ti—Al Alloy-Ti. D ofFIG. 6is a cross sectional drawing that shows the metal film is patterned to form the source/drain electrodes110and the gate electrode109.

The feature ofFIG. 6is to form the source/drain electrodes110and the gate electrode109simultaneously by the same metal. Thanks to the structure, dissolving of the TAOS by the hydrofluoric acid (HF) at the thorough holes can be avoided during cleaning the through holes, which are formed in an insulting layer that covers the gate electrode109and the source/drain electrodes110. The reason is that the through holes are formed on the source/drain electrodes110on the TAOS semiconductor.

FIGS. 7 and 8are detailed cross sections showing a structure and a manufacturing process thereof according to the present invention. InFIGS. 7 and 8, the left hand figure is a Poly-Si TFT used in a driving circuit formed in peripheral area, while the right hand figure is a TAOS TFT used in pixel area.

InFIG. 7, the undercoat101is formed on the TFT substrate100formed by glass or resin. The undercoat101is to block impurities from the TFT substrate100such as glass; the undercoat101is generally formed by SiO, SiN, or a laminated film of SiO layer and SiN layer. In this specification, a notation of AB (e.g. SiO) means a compound formed by A and B; however, it doesn't necessarily mean A and B are the same amount; even a basic ratio of elements exists, according to process conditions, a ratio between A and B becomes different from the basic ratio.

The semiconductor layer102of Poly-Si for the Poly-Si TFT is formed on the undercoat101. The first gate insulating film103is formed covering the Poly-Si semiconductor layer102; the first gate electrode104is formed on the first gate insulating film103. The gate electrode104is formed by e.g. a lamination film of Ti/Al alloy-Ti, or MoW alloy. The light blocking layer1041is simultaneously formed at a place where the TAOS TFT is formed, in a plan view, in the display area to suppress a photo current in the TAOS107.

The first interlayer insulating film105by SiN is formed on the first gate electrode104; then the second interlayer insulating film106is formed on the first interlayer insulating film105. The TAOS107is formed on the second interlayer insulating film106. The second gate insulating film108is formed on the TAOS107, and the second gate insulating film108is patterned. It is preferable to form the second interlayer insulating film106and the second gate insulating film108by SiO so as to sandwich the TAOS107. The reason is that SiN discharges hydrogen, which deteriorates characteristics of the TAOS107.

After that, the metal layer is formed on the second gate insulating film108and the TAOS107; then the metal layer is patterned to form the second gate electrode109and source/drain electrodes110simultaneously.

After that, as depicted inFIG. 8, the third interlayer insulating film111is formed on the second gate electrode109and the source/drain electrodes110, then, the fourth interlayer insulating film112is formed on the third interlayer insulating film111. It is preferable to form the third interlayer insulating film111by SiO and to form the fourth interlayer insulating film112by SiN. It is possible to form the fourth interlayer insulating film112directly on the second gate electrode109and the source/drain electrodes110without forming the third interlayer insulating film111.

After that, at the Poly-Si TFT, through holes113are formed in the first interlayer insulating film105through the fourth interlayer insulating film112, while, at the TAOS TFT, through holes114are formed in the third interlayer insulating film111and the fourth interlayer insulating film112. Through holes113and through holes114are formed simultaneously. Those through holes are made by dry etching with CF (CF4) based gas or with CHF (CHF3) based gas.

The through holes113are made through 5 layers while the through holes114are made through 2 layers; however, in the through holes114, the source/drain electrodes110can be an etching stopper, thus, simultaneous forming of the through holes113and the through holes114is possible.

After forming the through holes113and the through holes114, cleaning of the through holes by hydrofluoric acid (HF) is necessary. The through holes114are also cleaned by hydrofluoric acid (HF), however, the source/drain electrodes110can be an etching stopper, thus, the hydrofluoric acid (HF) doesn't contact to the TAOS107. Therefore, TAOS is not dissolved by the hydrofluoric acid (HF).

As described above, according to the present invention, the Poly-Si TFT and the TAOS TFT can be formed on the same substrate and by a common process. Therefore, it is possible to form TFTs of less leak current in the display area, and to form TFTs of high speed in the peripheral driving circuit.

Second Embodiment

FIGS. 9-15are figures of the structures and manufacturing processes according to embodiment 2 of the present invention. InFIGS. 9-15, the left hand figures are a cross sectional views, while the right hand figures are plan views. InFIGS. 9-15, layers not relating to the TAOS are omitted.

A and B ofFIG. 9are figures that TAOS107is formed on the substrate100and patterned in island shape. C and D ofFIG. 9are figures that the gate insulating film108is formed on the TAOS107and the through holes1081are made in the gate insulating film108. E and F ofFIG. 9are figures that the metal film is formed on the gate insulating film108and the TAOS107, and the metal film108is patterned.

As emphasized by the circles in E ofFIG. 9, it is important there is an area that the TAOS107doesn't overlap with the source/drain electrodes110, that is to say, there is an exposed area in the TAOS. After the ion implantation, which is explained later, the source/drain electrode110and a channel of the TAOS107are electrically connected through this exposed area of the TAOS. In E and F ofFIG. 9, the source/drain electrodes110extract oxygen from the TAOS107in areas1072where the TAOS107contacts with the source/drain electrodes110; thus, the TAOS becomes conductive in these areas.

G ofFIG. 9is a figure that the ion implantation (I/I) is made on the structure of E ofFIG. 9, thus a doped area1071is formed in the TAOS107. In the ion implantation (I/I), ion can be either one of B (Boron), P (Phosphor), Ar (Argon) etc. however, it is not necessary to limit kinds of ion. The purpose of the ion implantation (I/I) is to destroy the basic structure of the TAOS107, thus, to give the TAOS107conductivity; therefore, kinds of ion can be chosen widely.

G ofFIG. 9, the area indicated by1071is a conductive area of the TAOA107. The area1072where the source/drain electrode110and TAOS107contact each other is also a conductive area of the TAOS107. Thus, a drain and a source are formed sandwiching the cannel107of the TAOS.

In H ofFIG. 9, the area of the TAOS107that is not covered by the gate electrode109and the source/drain electrode110is conductive since ion is doped in this area. ON current flows through the area1071of H ofFIG. 9to the source/drain electrode110. In other words, the circled portion in E ofFIG. 9is a route where the current flows to the source/drain electrode110.

FIG. 10is a second example of a structure of the TAOS TFT in embodiment 2. F and H ofFIG. 10are different from F and H ofFIG. 9in that a width of the source/drain electrodes10is narrower in F and H ofFIG. 10than that of F and H ofFIG. 9. InFIG. 10, TAOS107around the source/drain electrode110becomes conductive after the ion implantation (I/I), however, a route where the current flows to the source/drain electrode110is the same asFIG. 9. That is to say, the current flows to the source/drain electrodes110through the circled portions E ofFIG. 10.

FIG. 11is a third example of a structure of TAOS TFT in embodiment 2. F and H ofFIG. 11are different from F and H ofFIG. 10in that the exposed portions1071of TAOS107are located at outer sides of the source/drain electrode110, namely, exposed portions1071inFIG. 11are located in more remote areas from the gate electrode109compared to in the case ofFIG. 10. That is to say the circled portions in E ofFIG. 11are routes for the current to the source/drain electrodes110. The circled portions of F inFIG. 11cannot be routes of the current to the source/drain electrodes110. The structure ofFIG. 11is used in some specific layouts; however, performance of TFTs is the same betweenFIG. 10andFIG. 11.

FIG. 12is a fourth example of a structure of the TAOS TFT in embodiment 2.FIG. 12differs fromFIG. 10in that the through hole1081formed in the gate insulating layer108extends beyond the outer edge of TAOS the107.FIG. 11further differs fromFIG. 10in that exposed portions of TAOS107are located at outer sides of the source/drain electrode110, which are more remote areas from the gate electrode109.

In F ofFIG. 12, exposed areas of the TAOS107can be routes for the current to the source/drain electrodes110, however, the circled portions in F ofFIG. 12cannot be routes for the current to the source/drain electrodes110. The structure ofFIG. 12is used in some specific layouts; however, performance of TFTs is the same betweenFIG. 10andFIG. 12.

FIG. 13is a fifth example of a structure of the TAOS TFT in embodiment 2.FIG. 13differs fromFIG. 9in that the through hole1081formed in the gate insulating layer108extends beyond the outer edge of the TAOS107. In E ofFIG. 13, routes of the current to the source/drain electrodes are exposed areas of the TAOS107, which are the same as E ofFIG. 9. The structure ofFIG. 13is used in some specific layouts; however, performance of TFTs is essentially the same betweenFIG. 9andFIG. 13.

FIG. 14is a sixth example of a structure of the TAOS TFT in embodiment 2.FIG. 14differs fromFIG. 13in that exposed areas of the TAOS in the through holes1081of the gate insulating film108extend along a channel direction of the TAOS TFT. The circled portions in F ofFIG. 14are routes of the current to the source/drain electrodes110. The structure ofFIG. 14is used in some specific layouts; however, a performance of TFTs is essentially the same betweenFIG. 13andFIG. 14.

FIG. 15is a seventh example of a structure of the TAOS TFT in embodiment 2.FIG. 15differs fromFIG. 13in that a width of the source/drain electrodes110is narrow, thus, exposed areas of the TAOS exist at both sides of the source/drain electrode110in the through hole1081in the gate insulating film108. Other structures are the same as explained inFIG. 13.

A common feature throughFIGS. 9-15is that, in H of each ofFIGS. 9-15, an exposed area of the TAOS107adjacent to the source/drain electrode110forms a route of the current to the source/drain electrode110.

Third Embodiment

FIGS. 16-22are figures of the structures and manufacturing processes according to embodiment 3 of the present invention. InFIGS. 16-22, the left hand figures are cross sectional views, while the right hand figures are plan views. InFIGS. 16-22, layers not relating to the TAOS are omitted.

A and B ofFIG. 16are figures that the TAOS107is formed on the substrate100and patterned in island shape. C and D ofFIG. 16are figures that the gate insulating film108is formed on the TAOS107and is patterned in island shape.

E and F ofFIG. 16are figures that the gate electrode109is formed on the gate insulating film108; and the source/drain electrodes110are directly formed on TAOS107at both sides of the TAOS107. A portion of the TAOS107, where the source/drain electrodes110contacts, is conductive since oxygen is extracted from the TAOS107by the metal or the alloy, which constitutes the source/drain electrode110.

After that, as depicted in G ofFIG. 16, the TAOS107, which is not covered by the gate electrode109or by the source/drain electrode110, becomes conductive by the ion implantation (I/I). The reason is the same as explained inFIG. 9. Consequently, as depicted in H ofFIG. 16, an area of the TAOS107around the source/drain electrode110becomes conductive. ON current flows into the source/drain electrode110from all around the source/drain electrode110inFIG. 16; thus, ON resistance can be made low. By the way, the circled portions in E ofFIG. 16are areas that become conductive after the ion implantation (I/I).

FIG. 17is a second example of a structure of the TAOS TFT in embodiment 3.FIG. 17differs fromFIG. 16in that, as depicted in E-F ofFIG. 17, a part of the source/drain electrode110is formed on a part of the gate insulating film108. Therefore, the current cannot flow into the source/drain electrode at the circled portions in E ofFIG. 17; however, the current can flow into the source/drain electrode110at the circled portions in F ofFIG. 17. Namely, after the ion implantation, the current can flow into the source/drain electrode110from the TAOS107at three sides of the source/drain electrode110. The structure ofFIG. 17is used in some specific layouts; however, ON resistance doesn't change so much and performance of TFTs is actually the same betweenFIG. 16andFIG. 17.

FIG. 18is a third example of a structure of the TAOS TFT in embodiment 3.FIG. 18differs fromFIG. 16in that, as depicted in E-H ofFIG. 18, a width of the source/drain electrode110is wider than a width of TAOS107; consequently, edges of the TAOS107are covered by the source/drain electrode110inFIG. 18. The width means a width in a direction of channel width. Therefore, the current flows into the source/drain electrode110at one side of the source/drain electrode110after the ion implantation depicted in G ofFIG. 18. Namely, the current flows into the source/drain electrode110at the circled portions in E ofFIG. 18. The structure ofFIG. 18is used in some specific layouts; however, ON resistance doesn't change so much and performance of TFTs is actually the same betweenFIG. 16andFIG. 18.

FIG. 19is a fourth example of a structure of the TAOS TFT in embodiment 3.FIG. 19differs fromFIG. 16in that, as depicted in E-H ofFIG. 19, a width of the source/drain electrode110is wider than a width of TAOS107. The width means a width in a direction of channel width. The structure ofFIG. 19is used in some specific layouts; however, ON resistance doesn't change so much and performance of TFTs is actually the same betweenFIG. 16andFIG. 19.

FIG. 20is a fifth example of a structure of the TAOS TFT in embodiment 3.FIG. 20differs fromFIG. 16in that, as depicted in E-H ofFIG. 20, edges of the source/drain electrodes110extend beyond edges of TAOS107. Therefore, after the ion implantation, as depicted in G ofFIG. 20, the current can flow into the source/drain electrode110from TAOS107at three sides of the source/drain electrode110. The circled portions in E ofFIG. 20are the routes that the current flows into the source/drain electrodes110after the ion implantation (I/I). The structure ofFIG. 20is used in some specific layouts; however, ON resistance doesn't change so much and performance of TFTs is actually the same betweenFIG. 16andFIG. 20.

FIG. 21is a sixth example of a structure of the TAOS TFT in embodiment 3.FIG. 21differs fromFIG. 20in that, as depicted in E-H ofFIG. 21, a part of the source/drain electrodes110is formed on a part of the gate insulating film108. Therefore, after the ion implantation (I/I), as depicted in G ofFIG. 21, ON current flows into the source/drain electrode110from two sides of the source/drain electrode110. The circles in F ofFIG. 21are areas where the current can flow into the source/drain electrodes110.

On the other hand, the current cannot flow to the source/drain electrodes110at the circled portions in E ofFIG. 21. The reason is that conductivity doesn't reveal in the TAOS107at an area where the source/drain electrode110overlaps the gate insulating film108. The structure ofFIG. 21is used in some specific layouts; however, ON resistance doesn't change so much and performance of TFTs is actually the same betweenFIG. 16andFIG. 21.

FIG. 22is a seventh example of a structure of TAOS TFT in embodiment 3.FIG. 22differs fromFIG. 18in that, as depicted in E-H ofFIG. 22, the source/drain electrode110has a projection1101, and the projection1101is formed on a part of the gate insulating film108. The circled portions in E ofFIG. 22cannot be a route of the current, however, the circled portions in F ofFIG. 22can be routes for the current to flow into the source/drain electrodes110after the ion implantation. The structure ofFIG. 22is used in some specific layouts; however, ON resistance doesn't change so much and performance of TFTs is actually the same betweenFIG. 16andFIG. 22.

A common feature throughFIGS. 16-22is that, in H of each ofFIGS. 16-22, an exposed area of TAOS107adjacent to the source/drain electrode110forms a route for the current to the source/drain electrode110.

Fourth Embodiment

FIG. 23is a cross sectional view that the TAOS, explained in embodiments 1-3, is used in a display area of a liquid crystal display device. The TFT array layer120is formed on the TFT substrate100. The TFT array layer120has the TAOS TFT array layer structure depicted by e.g.FIG. 8. The organic passivation film116is formed on the TFT array layer120.

FIG. 23is an IPS type liquid crystal display device where the common electrode121of a planar shape is formed on the organic passivation film116. The capacitive insulating layer122is formed on the common electrode121and the pixel electrode123is formed on the capacitive insulating layer122. The pixel electrode123is comb like shaped or stripe like shaped. The alignment layer124, for intimal alignment of the liquid crystal molecules301, is formed on the pixel electrode123.

When a video signal is applied to the pixel electrode123, then a potential difference is formed between the pixel electrode123and the common electrode121, a line of force is generated as depicted by an arrow inFIG. 12, which rotates the liquid crystal molecules301and thus a transmittance of the liquid crystal layer300is controlled, consequently images are formed.

InFIG. 23, the counter substrate200is set above the TFT substrate100to sandwich the liquid crystal layer300. The color filter201and the black matrix202are formed on the counter substrate200. The overcoat film203is formed to cover the color filter201and the black matrix202; the alignment film204is formed on the overcoat film203.

In a liquid crystal display device, when a video signal is applied to the pixel electrode123, a voltage is retained for one frame period by a storage capacitance formed between the pixel electrode123and the common electrode122sandwiching the capacitive insulating layer122. In this case, if a leak in the TFT is large, the voltage of the pixel electrode123changes, thus, flickers occur; consequently, images are deteriorated. According to the present invention, a leak of the TFT can be made low, thus, a liquid crystal display device having high quality images is realized.

Fifth Embodiment

A combination of the LTPS TFTs and the TAOS TFTs explained in embodiments 1-3 can be applied to an organic EL display device.FIG. 24is a plan view of an organic EL display device2. The organic EL display device2ofFIG. 24has the display area10and the peripheral circuit area20. Organic EL driving TFTs and switching TFTs are formed in the display area10. A TAOS TFT, which has low leak current, is preferable for a TFT formed in the display area. A peripheral circuit is formed by mainly Poly-Si TFTs in the peripheral circuit area20.

InFIG. 24, the polarizing plate220for preventing reflection is adhered to the display area10. Since an organic EL display device has a reflection electrode, the polarizing plate220is used to prevent a reflection of external light. The terminal area150is formed outside of the display area10. The driver IC170is installed on the terminal area150. The flexible wiring circuit plate160, which supplies power or signals to the organic EL display, is connected to the terminal area150.

FIG. 25is a cross sectional view ofFIG. 24along B-B line. The display element layer210, which includes organic EL layers, is formed on the TFT substrate100. The display element layer210is formed corresponding to the display area10ofFIG. 24. Since an organic EL substance is decomposed by moisture, the protecting layer214made by SiN is formed to cover the display element layer214. The terminal area150is formed outside of the display element layer210; the driver IC170is installed on the terminal area150and the flexible wiring circuit plate160is connected to the terminal area150.

FIG. 26is a cross sectional view of the display area of the organic EL display device. The TFT array layer120is formed on the TFT substrate100. The TFT array layer120includes the layer structure of TFT ofFIG. 8; the organic passivation film116is formed on the TFT array layer120.

The lower electrode211is formed on the organic passivation film116inFIG. 26. The lower electrode211is a laminated structure of metal or alloy constituting a reflection electrode and a transparent conductive film constituting an anode. The organic EL layer212is formed on the lower electrode211. The organic EL layer212includes e.g. an electron injection layer, an electron transport layer, a light emitting layer, a hole transport layer, and a hole injection layer.

The upper electrode213, which works as a cathode, is formed on the organic EL layer212. The upper layer213is formed by a transparent conductive film such as IZO (Indium Zinc Oxide) or ITO (Indium Tin Oxide). The upper electrode213can also be made by thin film of metal as Silver or metal alloys. From the TFT array layer to the upper electrode213constitute the display element layer210. The protecting layer214made by such as SiN is formed on the upper electrode213; the polarizing plate220is adhered to the protecting layer214by the adhesive221to prevent a reflection of external light.

Several kinds of TFTs such as driving TFTs or switching TFTs are formed in the TFT array layer120. According to the present invention, the LTPS TFTs and the TAOS TFTs can be formed by a common process, thus, various combinations of the LTPS TFTs and the TAOS TFTs are possible; consequently, an organic EL display device having high quality images and low power consumption can be realized.

In the above embodiments, the TAOS TFTs are used in the display area and the LTPS TFTs are used in the peripheral driving circuit; however, the TAOS TFTs can be added in the peripheral driving circuit or the LTPS TFTs can be added in the display area.

In the above embodiments, TFTs are explained as a top gate type where a gate electrode is above a semiconductor layer; however, the present invention is applicable to TFTs of a bottom gate type where a gate electrode is beneath a semiconductor layer.