US20100127266A1 - Thin film transistor, method for manufacturing same, display device, and method for manufacturing same - Google Patents

Abstract

A thin film transistor includes: an insulating layer; a gate electrode provided on the insulating layer; a gate insulating film provided on the gate electrode; a semiconductor layer provided on the gate insulating film, the semiconductor layer being formed of oxide; a source electrode and a drain electrode provided on the semiconductor layer; and a channel protecting layer provided between the source and drain electrodes and the semiconductor layer. The source electrode is opposed to one end of the gate electrode. The drain electrode is opposed to another end of the gate electrode. The another end is opposite to the one end. The drain electrode is apart from the source electrode. The channel protecting layer covers at least a part of a side face of a part of the semiconductor layer. The part of the semiconductor layer is not covered with the source electrode and the drain electrode above the gate electrode.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2008-295853, filed on Nov. 19, 2008; the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• The invention relates to a thin film transistor, a method for manufacturing the same, a display device, and a method for manufacturing the same.
• 2. Background Art
• Thin film transistors (TFTs) are widely used for liquid crystal display devices, organic EL display devices, and the like.
• Amorphous silicon TFTs used for large-size liquid crystal display devices have a mobility of approximately 1 cm²/Vs and they can be formed uniformly with a large area at low cost. Further improvements in size and resolution are required these days. Furthermore, an active matrix organic EL display devices which require a large driving current have been developed, and a novel active material of low cost, high uniformity, high reliability, and high mobility is required.
• In the development circumstances mentioned above, oxide semiconductors receive attention as a material usable for the channel layer of TFT in these days.
• For example, TFTs which have a transparent conductive oxide thin film containing ZnO as a main component for the channel layer are developed actively. The thin film mentioned above can be film-formed with a large area at a relatively low temperature, and achieve a higher mobility than amorphous silicon. For example, JP-A 2004-103957 (Kokai) discloses a TFT using In—Ga—Zn—O-based amorphous oxide. Since the thin film mentioned above can be film-formed at a low temperature and is transparent in the visible range, it is considered that a flexible and transparent TFT can be formed on a substrate of plastic or film. Furthermore, approximately ten times as high field-effect mobility as amorphous silicon is obtained.
• On the other hand, it is reported that, for example, the electric conductivity of oxide semiconductor changes with oxygen concentration during sputtering film-formation (for example, see Applied Physics Letters, 90, 192101 (2007)). Thus, in regard to oxide semiconductors, electrical characteristics are very sensitive to the concentration of contained oxygen, and the oxygen concentration changes due to heat treatment to cause deteriorated characteristics, for example. This is a major factor that prevents TFTs using an oxide semiconductor from their practical use.
SUMMARY OF THE INVENTION
• According to an aspect of the invention, there is provided a thin film transistor including: an insulating layer; a gate electrode provided on the insulating layer; a gate insulating film provided on the gate electrode; a semiconductor layer provided on the gate insulating film, the semiconductor layer being formed of oxide; a source electrode and a drain electrode provided on the semiconductor layer, the source electrode being opposed to one end of the gate electrode, the drain electrode being opposed to another end of the gate electrode, the another end being opposite to the one end, the drain electrode being apart from the source electrode; and a channel protecting layer provided between the source and drain electrodes and the semiconductor layer, the channel protecting layer covering at least a part of a side face of a part of the semiconductor layer, the part of the semiconductor layer being not covered with the source electrode and the drain electrode above the gate electrode.
• According to another aspect of the invention, there is provided a method for manufacturing a thin film transistor, the thin film transistor including: a substrate; a gate electrode provided on the substrate; a gate insulating film provided on the gate electrode; a semiconductor layer provided on the gate insulating film, the semiconductor layer being formed of oxide; a source electrode and a drain electrode provided on the semiconductor layer, the source electrode being opposed to one end of the gate electrode, the drain electrode being opposed to another end of the gate electrode, the another end being opposite to the one end, the drain electrode being apart from the source electrode; and a channel protecting layer provided between the source and drain electrodes and the semiconductor layer, the method including: forming the gate electrode on the substrate; forming the gate insulating film on the gate electrode; forming the semiconductor layer on the gate insulating film; forming the channel protecting layer covering at least a part of a side face of the semiconductor layer above the gate electrode; performing a heat treatment on the semiconductor layer and the channel protecting layer at a temperature not less than 160° C.; and forming the source electrode and the drain electrode on the semiconductor layer and the channel protecting layer after the performing the heat treatment.
• According to another aspect of the invention, there is provided a display device including: a thin film transistor including: an insulating layer; a gate electrode provided on the insulating layer; a gate insulating film provided on the gate electrode; a semiconductor layer provided on the gate electrode, the semiconductor layer being formed of oxide; a source electrode and a drain electrode provided on the semiconductor layer, the source electrode being opposed to one end of the gate electrode, the drain electrode being opposed to another end of the gate electrode, the another end being opposite to the one end, the drain electrode being apart from the source electrode; and a channel protecting layer provided between the source and drain electrodes and the semiconductor layer, the channel protecting layer covering at least a part of a side face of a part of the semiconductor layer, the part of the semiconductor layer being not covered with the source electrode and the drain electrode above the gate electrode; a pixel electrode connected to one of the source electrode and the drain electrode, the pixel electrode being formed of the oxide and having a lower electric resistance than the semiconductor layer; and an optical element producing at least one of a change in optical characteristics and light emitting by an electric signal provided to the pixel electrode.
• According to another aspect of the invention, there is provided a method for manufacturing a display device, the display device including: a thin film transistor including: a substrate; a gate electrode provide on the substrate; a gate insulating film provide on the gate electrode; a semiconductor layer provided on the gate insulating film, the semiconductor layer being formed of oxide; a source electrode and a drain electrode provided on the semiconductor layer, the source electrode being opposed to one end of the gate electrode, the drain electrode being opposed to another end of the gate electrode, the another end being opposite to the one end, the drain electrode being apart from the source electrode ; and a channel protecting layer provided between the source and drain electrodes and the semiconductor layer; a pixel electrode connected to one of the source electrode and the drain electrode; and an optical element producing at least one of a change in optical characteristics and light emitting by an electric signal provided to the pixel electrode, the method including: forming the gate electrode on the substrate; forming the gate insulating film on the gate electrode; forming a layer of the oxide on the gate insulating film; forming the channel protecting layer covering at least a part of a side face of the layer of the oxide above the gate electrode and exposing the layer of the oxide in a third region, the pixel electrode being to be formed in the third region; performing a heat treatment on the layer of the oxide and the channel protecting layer at a temperature not less than 160° C. to decrease an electric resistance of the layer of the oxide not covered with the channel protecting layer to form the pixel electrode; and forming the source electrode and the drain electrode on the semiconductor layer and the channel protecting layer after the performing the heat treatment.
• According to another aspect of the invention, there is provided a method for manufacturing a display device, the display device including: a thin film transistor including: a substrate; a gate electrode provided on the substrate; a gate insulating film provided on the gate electrode; a semiconductor layer provided on the gate insulating film, the semiconductor layer being formed of oxide; a source electrode and a drain electrode provided on the semiconductor layer, the source electrode being opposed to one end of the gate electrode, the drain electrode being opposed to another end of the gate electrode, the another end being opposite to the one end, the drain electrode being apart from the source electrode; and a channel protecting layer provided between the source and drain electrodes and the semiconductor layer; a pixel electrode connected to one of the source electrode and the drain electrode; and an optical element producing at least one of a change in optical characteristics and light emitting by an electric signal provided to the pixel electrode, the method including: forming the gate electrode on the substrate; forming the gate insulating film on the gate electrode; performing surface processing selectively changing smoothness of a surface of the gate insulating film; and forming a layer of the oxide on the gate insulating film.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIGS. 1A to 1D are schematic cross-sectional views illustrating the configuration of a thin film transistor according to a first embodiment of the invention;
• FIGS. 2A to 2C are sequential schematic cross-sectional views illustrating a method for manufacturing a thin film transistor according to a first example of the invention;
• FIGS. 3A and 3B are sequential schematic cross-sectional views continuing fromFIG. 2C;
• FIG. 4 is a graph illustrating characteristics of the thin film transistor according to the first example of the invention;
• FIGS. 5A to 5D are schematic cross-sectional views illustrating the configuration of a thin film transistor of a first comparative example;
• FIG. 6 is a graph illustrating characteristics of the thin film transistor of the first comparative example;
• FIGS. 7A to 7C are schematic cross-sectional views illustrating the configuration of a thin film transistor of a second comparative example;
• FIGS. 8A to 8D are schematic views illustrating the configuration of the thin film transistor used for the experiments and experimental results;
• FIGS. 9A to 9D are schematic planar views each illustrating the configuration of a thin film transistor of a modification example according to the first embodiment of the invention;
• FIGS. 10A to 10D are sequential schematic cross-sectional views illustrating a method for manufacturing a thin film transistor according to a second example of the invention;
• FIGS. 11A to 11C are sequential schematic cross-sectional views illustrating a method for manufacturing a thin film transistor according to a third example of the invention;
• FIGS. 12A and 12B are sequential schematic cross-sectional views continuing fromFIG. 11C;
• FIG. 13 is a schematic cross-sectional view illustrating the configuration of a display device according to a second embodiment of the invention;
• FIGS. 14A to 14C are sequential schematic cross-sectional views illustrating a method for manufacturing a display device according to a fourth example of the invention;
• FIGS. 15A and 15B are sequential schematic cross-sectional views continuing fromFIG. 14C;
• FIGS. 16A to 16D are sequential schematic cross-sectional views illustrating a method for manufacturing a display device according to a fifth example of the invention;
• FIGS. 17A and 17B are circuit diagrams illustrating equivalent circuits of other display devices according to the second embodiment of the invention;
• FIG. 18 is a circuit diagram illustrating an equivalent circuit of another display device according to the second embodiment of the invention;
• FIG. 19 is a flow chart illustrating a method for manufacturing a thin film transistor according to a third embodiment of the invention;
• FIG. 20 is a flow chart illustrating a method for manufacturing a display device according to a fourth embodiment of the invention; and
• FIG. 21 is a flow chart illustrating another method for manufacturing a display device according to the fourth embodiment of the t invention.
DETAILED DESCRIPTION OF THE INVENTION
• Embodiments of the invention will now be described with reference to the drawings.
• The drawings are schematic or conceptual; and the relationships between the thickness and width of portions, the proportions of sizes among portions, and the like are not necessarily the same as the actual values thereof. Further, the dimensions and proportions may be illustrated differently among drawings, even for identical portions.
• In the specification and drawings of the application, components similar to those described in regard to a drawing thereinabove are marked with the same reference signs, and a detailed description is omitted as appropriate.
First Embodiment
• FIGS. 1A to 1D are schematic cross-sectional views illustrating the configuration of a thin film transistor according to a first embodiment of the invention.
• That is,FIG. 1A is a schematic plan view,FIG. 1B is an omitted schematic plan view, andFIG. 1C andFIG. 1D are cross-sectional views taken along line A-A′ and line B-B′ ofFIG. 1A, respectively.
• As illustrated inFIGS. 1A to 1D, athin film transistor11 according to the first embodiment of the invention includes: agate electrode120 provided on amajor face111 of an insulatinglayer110; asemiconductor layer140 provided above thegate electrode120 via agate insulating film130 and formed of oxide; achannel protecting layer150 provided on thesemiconductor layer140; and asource electrode161 and adrain electrode162 provided away from each other so as to cover part of thesemiconductor layer140 and part of thechannel protecting layer150.
• InFIG. 1A, the insulatinglayer110 and thegate insulating film130 are omitted, and inFIG. 1B, the insulatinglayer110, thegate insulating film130, thesource electrode161, and thedrain electrode162 are omitted.
• As illustrated inFIG. 1B, thesemiconductor layer140 includes asource contact region141 and adrain contact region142 that are electrically connected to thesource electrode161 and thedrain electrode162, respectively. Thesource contact region141 and thedrain contact region142 are provided away from each other. Thesource electrode161 is opposed to one end of thegate electrode120. Thedrain electrode162 is opposed to another end of thegate electrode120. The another end is opposite to the one end. Thedrain electrode162 is apart from thesource electrode161.
• Thechannel protecting layer150 is provided so as to cover aside face140sand anupper face140uof thesemiconductor layer140 except thesource contact region141 and thedrain contact region142. However, as described below, thechannel protecting layer150 may be provided so as to cover theupper face140uand at least part of theside face140sof thesemiconductor layer140.
• That is, thethin film transistor11 includes: the insulatinglayer110; thegate electrode120 provided on the insulatinglayer110; thegate insulating film130 provided on thegate electrode120; thesemiconductor layer140 provided on thegate insulating film130, thesemiconductor layer140 being made of oxide; thesource electrode161 and thedrain electrode162 provided on thesemiconductor layer140, thesource electrode161 being opposed to one end of thegate electrode120, thedrain electrode162 being opposed to another end of thegate electrode120, the another end being opposite to the one end, thedrain electrode162 being apart from thesource electrode161; and thechannel protecting layer150 provided between the source and drainelectrodes161,162 and thesemiconductor layer140, thechannel protecting layer150 covering at least a part of theside face140sof thesemiconductor layer140 not covered with thesource electrode161 and thedrain electrode162 above thegate electrode120.
• The insulatinglayer110 is provided on a substrate, for example. At this time, a transparent glass substrate, for example, may be used for the substrate. However, it is not limited thereto, but a plastic substrate, a substrate with a color filter, and a non-transparent substrate of silicon, stainless steel, etc., for example, may be used. Further, if the substrate has insulating properties, the substrate itself may be used as the insulatinglayer110. In the following, descriptions are given with the case taken up where the insulatinglayer110 is an insulative substrate.
• For thegate electrode120, a high melting point metal such as MoW, Ta, and W may be used, for example. Further, Al alloys containing Al as a main component which are submitted with hillock preventions, or stacked films of Al and a high melting point metal may be used. However, the invention is not limited thereto, but an optional conductive material may be used for thegate electrode120.
• For thegate insulating film130, silicon oxide (SiO_x) may be used, for example. However, the invention is not limited thereto, but an optional insulating film of silicon nitride (SiN_x), silicon oxynitride, etc. may be used. Further, stacked films of them may be used.
• For thesemiconductor layer140, an amorphous oxide semiconductor of In—Ga—Zn—O type and the like may be used, for example. Thesemiconductor layer140 made of this amorphous oxide semiconductor is formed by reactive sputtering, for example. When the amorphous oxide semiconductor layer is observed by using, for example, a transmission electron microscope or X-ray diffraction, no diffraction pattern and the like are observed. In addition to the above, an optional oxide semiconductor containing, for example, Zn or the like may be used for thesemiconductor layer140.
• Thus, thesemiconductor layer140 may include an amorphous oxide which contains Zn. The amorphous oxide may further contain at least one of In and Ga.
• Approximately 10 nm (nanometers) are enough for the thickness of thesemiconductor layer140 in order to ensure electrical characteristics. Specifically, thesemiconductor layer140 may have a thickness of approximately 10 nm to 100 nm.
• For thechannel protecting layer150, a material, such as silicon oxide, having higher resistance against acids than thesemiconductor layer140 is used. However, the invention is not limited thereto, but an optional insulating material containing oxygen may be used. For example, alumina, silicon oxynitride, and the like also may be used. Further, stacked films of these films may be used.
• For thesource electrode161 and thedrain electrode162, an optional conductive material may be used. Further, for example, optional conductive stacked films such as Ti/Al/Ti and Mo/Al/Mo may be used. In this specific example, stacked films of aMo film166, anAl film167, and aMo film168 are used for thesource electrode161 and thedrain electrode162.
• In the above, thesource contact region141 and thedrain contact region142 may be mutually exchanged. That is, thesource electrode161 and thedrain electrode162 may be mutually exchanged.
• In order to maintain reliability of thethin film transistor11, a passivation film illustratively made of an insulating material such as SiN_xand the like is formed so as to cover the entire structure illustrated in the drawings. However, the passivation film is omitted in the drawings. Further, an insulating layer of an organic resin etc. for planarization and, depending on circumstances, an insulating layer of a colored organic resin etc. such as a color filter are formed thereon. However, these films are omitted in the drawings.
• Thus, in the case where a thin film transistor is generally used for display devices of active matrix type for TFT-LCDs or organic EL, and the like, the passivation film is formed, and at this time the thin film transistor receives a heat treatment at a temperature of 150° C. or higher, for example. Further, when forming the passivation film by using, for example, PE-CVD (plasma enhanced chemical vapor deposition), the heating temperature is approximately 250° C.
• At this time, thesemiconductor layer140 is heated during formation of the passivation layer mentioned above. Here, in thethin film transistor11 according to this embodiment, thesource contact region141 and thedrain contact region142 of thesemiconductor layer140 are covered with thesource electrode161 and thedrain electrode162, respectively. Further, in a region of thesemiconductor layer140 not covered with these electrodes, theupper face140uand theside face140sof thesemiconductor layer140 are covered with thechannel protecting layer150. This suppresses variation in oxygen concentration at thesemiconductor layer140 when the heat treatment mentioned above is performed, and characteristics are not changed.
• Thus, thethin film transistor11 according to this embodiment can provide a thin film transistor based on an oxide semiconductor which suppresses variation in oxygen concentration caused by heat treatment to suppress characteristic variation.
First Example
• Athin film transistor11a(not illustrated) according to a first example of this embodiment has the structure illustrated inFIGS. 1A to 1D. A method for manufacturing thethin film transistor11aof the first example will now be described.
• FIGS. 2A to 2C are sequential schematic cross-sectional views illustrating a method for manufacturing the thin film transistor according to the first example of the invention.
• FIGS. 3A and 3B are sequential schematic cross-sectional views continuing fromFIG. 2C.
• In each ofFIGS. 2A to 3B, the drawing on the left side is a cross-sectional view corresponding to the cross-section taken along line A-A′ ofFIG. 1A, and the drawing on the right side is a cross-sectional view corresponding to the cross-section taken along line C-C′ ofFIG. 1A. In these drawings, a contact unit is also illustrated in addition to the portion of the thin film transistor.
• As illustrated inFIG. 2A, first, anAl film121fand anMo film122fwere film-formed with thicknesses of 100 nm and 30 nm, respectively, on themajor face111 of aglass substrate110g(substrate110g) that is the insulatinglayer110 by sputtering, and were patterned into a prescribed pattern to form agate electrode120. Photolithography was used for the patterning, and a mixed acid of phosphoric acid, acetic acid, nitric acid, and water was used for the etching. At this time, acontact unit123 of thegate electrode120 of the thin film transistor was also formed simultaneously. Thecontact unit123 is also formed of stacked films of theAl film121fand theMo film122f.
• Next, as illustrated inFIG. 2B, an SiO₂film130fwas deposited with a thickness of 200 nm as agate insulating film130, by PE-CVD using TEOS (tetraethyl orthosilicate) as raw material. At this time, the film-formation temperature was 350° C.
• Further, on thegate insulating film130, anoxide layer140f(a layer of oxide) made of In—Ga—Zn—O oxide which forms asemiconductor layer140 was film-formed with a thickness of 30 nm by reactive DC sputtering. At this time, the ratio of oxygen was 5% to argon. Then, theoxide layer140fwas patterned into a prescribed pattern across thegate electrode120 by using oxalic acid of 2%, and thus thesemiconductor layer140 was formed.
• Further, as illustrated inFIG. 2C, an SiO₂film150fwhich forms achannel protecting layer150 was deposited with a thickness of 200 nm by PE-CVD of TEOS. At this time, the film-formation gas was a mixed gas of O₂and TEOS, and the film-formation temperature was 350° C. After that, the SiO₂film150fwas patterned into a prescribed pattern covering theside face140sand theupper face140uof thesemiconductor layer140 except regions that form later thesource contact region141 and thedrain contact region142, and thus thechannel protecting layer150 was formed.
• At this time, mask exposure and back face exposure using thegate electrode120 as a mask were used in combination for lithography at the time of patterning of the SiO₂film150f.The etching was performed by RIE (reactive ion etching) using CF₄gas.
• After that, annealing was performed at 350° C. for one hour under an air atmosphere to remove damages in thesemiconductor layer140 due to the PE-CVD process for film-formation of the SiO₂film150f.
• Then, as illustrated inFIG. 3A, acontact hole123hwas formed in the SiO₂film130fthat forms thegate insulating film130 in thecontact unit123 for extracting thegate electrode120. That is, the SiO₂film130fof thegate insulating film130 was etched into a prescribed shape by using buffered hydrofluoric acid.
• Then, as illustrated inFIG. 3B, theMo film166, theAl film167, and the Mo film168 (not illustrated) that form thesource electrode161 and thedrain electrode162 were film-formed with thicknesses of 10 nm, 300 nm, and 50 nm, respectively, by sputtering to formstacked films160f,and thestacked films160fwere patterned into a prescribed pattern by using a mixed acid to form thesource electrode161 and thedrain electrode162. Thereby, the configuration of thethin film transistor11ais completed. Further, thestacked films160fof theMo film166, theAl film167, and theMo film168 are buried in thecontact hole123h,and are patterned into a prescribed shape to form thecontact unit123.
• After that, in order to remove element damages during the processes, annealing is performed in a clean oven at 230° C. for approximately one hour. Thereby, thethin film transistor11aaccording to this example is completed.
• FIG. 4 is a graph illustrating characteristics of the thin film transistor according to the first example of the invention.
• That is, the drawing illustrates characteristics in the case where a heat treatment is performed after thethin film transistor11aaccording to the first example is completed and the heat treatment conditions are changed. A solid line A1 illustrates the initial characteristics without the heat treatment, a broken line A2 illustrates the characteristics after a heat treatment of 160° C. under an Ar atmosphere, and an alternate long and short dash line A3 illustrates the characteristics after a heat treatment of 230° C. under an Ar atmosphere. In the drawing, the horizontal axis represents the gate voltage Vg, and the vertical axis represents the drain current Id.
• As illustrated inFIG. 4, in thethin film transistor11a,a high ON/OFF ratio is obtained in each case of the initial characteristics without the heat treatment (solid line A1), the 160° C. heat treatment (broken line A2), and the 230° C. heat treatment (alternate long and short dash line A3), and further, characteristics are little changed by the heat treatment. These are effects of covering thesemiconductor layer140 with thechannel protecting layer150, thesource electrode161, and thedrain electrode162, and thereby suppressing change in the concentration of oxygen contained in thesemiconductor layer140 due to the heat treatment. Thereby, stable operation is achieved.
First Comparative Example
• FIGS. 5A to 5D are schematic cross-sectional views illustrating the configuration of a thin film transistor of a first comparative example.
• That is,FIG. 5A is a schematic plan view,FIG. 5B is an omitted schematic plan view, andFIG. 5C andFIG. 5D are cross-sectional views taken along line A-A′ and line B-B′ ofFIG. 5A, respectively.
• As illustrated inFIGS. 5A to 5D, in athin film transistor91 of the first comparative example, thechannel protecting layer150 is provided so as to cover theupper face140uof thesemiconductor layer140 but to expose theside face140s.The others are like thethin film transistor11 according to this embodiment and therefore a description is omitted. That is, thethin film transistor91 of the first comparative example has a structure in which theside face140sof thesemiconductor layer140 is not covered with thechannel protecting layer150 in thethin film transistor11 according to the first embodiment and thethin film transistor11aof the first example.
• The configuration of thisthin film transistor91 is general in the case where a silicon-based semiconductor such as amorphous silicon and polysilicon is used instead of an oxide semiconductor as thesemiconductor layer140.
• Thethin film transistor91 is fabricated as follows.
• TheAl film121fand theMo film122fthat form thegate electrode120 are film-formed with thicknesses of 100 nm and 30 nm, respectively, on themajor face111 of the insulatinglayer110 such as a glass substrate by sputtering, and are patterned into a prescribed pattern. Photolithography is used for the patterning, and a mixed acid of phosphoric acid, acetic acid, nitric acid, and water is used for the etching.
• After that, the SiO₂film130fis deposited with a thickness of 200 nm as thegate insulating film130 by PE-CVD of TEOS. Further, theoxide layer140fmade of In—Ga—Zn—O oxide is film-formed with a thickness of 30 nm on thegate insulating film130 as thesemiconductor layer140 by reactive DC sputtering. The ratio of oxygen is 5% to argon.
• Then, the SiO₂film150fis deposited with a thickness of 200 nm as thechannel protecting layer150 by PE-CVD of TEOS. After that, this SiO₂film150fis patterned into a prescribed pattern. At this time, mask exposure and back face exposure using thegate electrode120 as a mask were used in combination for photolithography for patterning of the SiO₂film150f.The etching was performed by RIE using CF₄gas.
• Then, theoxide layer140fis patterned into a prescribed pattern by using oxalic acid of 2%. After that, in order to cause thesemiconductor layer140 to recover from damages during PE-CVD, annealing is performed at 350° C. for one hour under an air atmosphere. After that, in order to form a contact hole (not illustrated) for extracting thegate electrode120, the exposedgate insulating film130 is etching-removed by using buffered hydrofluoric acid. Further, theMo film166, theAl film167, and theMo film168 that form thesource electrode161 and thedrain electrode162 are film-formed with thicknesses of 10 nm, 30 nm, and 50 nm, respectively, by sputtering, and are patterned into a prescribed pattern by using the mixed acid mentioned above. After that, in order to cause thesemiconductor layer140 to recover from damages during the processes, annealing is performed at 230° C. for one hour under an air atmosphere.
• In thethin film transistor91 having such a configuration, characteristics vary greatly during a heat treatment of subsequent passivation film formation, and causing a practical problem.
• FIG. 6 is a graph illustrating characteristics of the thin film transistor of the first comparative example.
• That is, the drawing illustrates characteristics in the case where a heat treatment is performed after thethin film transistor91 of the first comparative example is completed, and the heat treatment conditions are changed. A solid line A1 illustrates the initial characteristics without the heat treatment, a broken line A2 illustrates the characteristics after the heat treatment of 160° C. under an Ar atmosphere, and an alternate long and short dash line A3 illustrates the characteristics after the heat treatment of 230° C. under an Ar atmosphere. In the drawing, the horizontal axis represents the gate voltage Vg, and the vertical axis represents the drain current Id.
• As illustrated inFIG. 6, in thethin film transistor91, a high ON/OFF ratio is obtained in the case of the initial characteristics without the heat treatment (solid line A1). However, in the case of the heat treatment of 160° C. (broken line A2), the Id-Vg characteristic curved line shifts in the direction of low Vg, and there is a convex A2a on the Id-Vg characteristic curved line. Further, in the case of the heat treatment of 230° C. (alternate long and short dash line A3), the ON/OFF ratios are very low, and mostly the ON state is presented (conduction state).
• This is because, in thethin film transistor91, theside face140sof thesemiconductor layer140 is not covered with thechannel protecting layer150, and therefore the oxygen concentration at the surface of theside face140sof thesemiconductor layer140 changes due to the heat treatment. Specifically, oxygen contained in thesemiconductor layer140 leaves thesemiconductor layer140, which makes thesemiconductor layer140 low resistive.
Second Comparative Example
• FIGS. 7A to 7C are schematic cross-sectional views illustrating the configuration of a thin film transistor of a second comparative example.
• That is,FIG. 7A is a schematic plan view, andFIG. 7B andFIG. 7C are cross-sectional views taken along line A-A′ and line B-B′ ofFIG. 7A, respectively.
• As illustrated inFIGS. 7A to 7C, thethin film transistor92 of the second comparative example does not include thechannel protecting layer150. That is, thethin film transistor92 has a back channel cut structure. Thethin film transistor92 is fabricated as follows.
• TheAl film121fand theMo film122fthat form thegate electrode120 are film-formed with thicknesses of 100 nm and 30 nm, respectively, on themajor face111 of the insulatinglayer110 such as a glass substrate by sputtering, and are patterned into a prescribed pattern. Photolithography is used for the patterning, and a mixed acid of phosphoric acid, acetic acid, nitric acid, and water is used for the etching.
• Then, the SiO₂film130fis deposited with a thickness of 200 nm as thegate insulating film130 by PE-CVD of TEOS. Further, on thegate insulating film130, theoxide layer140fmade of In—Ga—Zn—O oxide is film-formed with a thickness of 30 nm as thesemiconductor layer140 by reactive DC sputtering. The ratio of oxygen is 5% to argon. Then, theoxide layer140fis patterned into a prescribed pattern by using oxalic acid of 2%, and thus thesemiconductor layer140 is formed
• After that, in order to form a contact hole (not illustrated) for extracting thegate electrode120, the exposedgate insulating film130 is etching-removed by using buffered hydrofluoric acid. Further, theMo film166, theAl film167, and theMo film168 that form thesource electrode161 and thedrain electrode162 are film-formed with thicknesses of 10 nm, 300 nm, and 50 nm, respectively, by sputtering, and are patterned into a prescribed pattern by using the mixed acid mentioned above. After that, in order to cause thesemiconductor layer140 to recover from damages during the processes, annealing is performed at 230° C. for one hour under an air atmosphere.
• In thethin film transistor92 having such a configuration, since thechannel protecting layer150 is not provided, theupper face140uand theside face140sof thesemiconductor layer140 are exposed. Accordingly, characteristics vary greatly due to a heat treatment for subsequent formation of the passivation film and the like. For example, characteristics deteriorate due to the heat treatment more significantly than the characteristics of thethin film transistor91 illustrated inFIG. 6, and causing a greater practical problem. This is because oxygen contained in thesemiconductor layer140 leaves the exposedupper face140uand the exposedside face140sof thesemiconductor layer140, and thereby thesemiconductor layer140 becomes low resistive.
• In the case where a thin film transistor is generally used for display devices of active matrix type for TFT-LCDs or organic EL, and the like, it is necessary to form the passivation film in order to improve reliability. However, in thethin film transistors91 and92 of the first and second comparative examples mentioned above, electrical characteristics deteriorate by being exposed to an inert atmosphere or a vacuum at a temperature of 150° C. or higher. This is because oxygen at the exposed portion of the surface of theoxide layer140fmade of In—Ga—Zn—O oxide leaves theoxide layer140f,and thereby thesemiconductor layer140 becomes low resistive. In general, in the case where the passivation film is formed by using PE-CVD, it is necessary to perform a heat treatment of approximately 200° C. in view of the barrier properties of the passivation film. However, in thethin film transistors91 and92, characteristics deteriorate if such a heat treatment is performed.
• At this time, for example, performing a heat treatment at 350° C. or higher in an oxidizing atmosphere enables to recover characteristics from the deterioration. However, if such a high temperature treatment is performed, a reaction occurs between the In—Ga—Zn—O oxide of thesemiconductor layer140, and thesource electrode161 and thedrain electrode162, for example, which results in deteriorated characteristics. Further, if Al is used for thesource electrode161 and thedrain electrode162, a hillock occurs at these electrodes and the problem occurs that this hillock damages the passivation film, for example. Thus, employing the heat treatment mentioned above is difficult practically.
• In contrast, in thethin film transistor11 according to this embodiment and thethin film transistor11aaccording to the first example, theupper face140uand theside face140sof thesemiconductor layer140 are covered with thechannel protecting layer150, thesource electrode161 and thedrain electrode162. This suppresses change of the concentration of oxygen contained in thesemiconductor layer140 due to a heat treatment. Thus, stable operation can be achieved even if the heat treatment during formation of the passivation film and the like is performed, as illustrated inFIGS. 2A to 2C.
• As described above, in the thin film transistor using an oxide semiconductor, theside face140sas well as theupper face140uof thesemiconductor layer140 is covered with thechannel protecting layer150, and thereby good characteristics resistant to the heat treatment are obtained. The structure of thethin film transistor11 according to this embodiment and the effects thereof have been found on the bases of the following experimental results.
• The inventors forecasted that, in thethin film transistor92 of the second comparative example which does not use thechannel protecting layer150, characteristics would deteriorate due to the heat treatment. That is, in a configuration where thechannel protecting layer150 is not employed, thesemiconductor layer140 is easily damaged in various kinds of processes during the manufacturing processes, and also in a thin film transistor using a semiconductor layer of amorphous silicon, polysilicon, or the like, for example, characteristics deteriorate according to circumstances. Therefore, the inventors presumed that thesemiconductor layer140 would be damaged more easily in the case of using an oxide semiconductor in which characteristics vary easily.
• On the other hand, there is no problem in the case where, for example, not an oxide semiconductor but a silicon-based semiconductor such as amorphous silicon and polysilicon is used as thesemiconductor layer140 in the configuration of thethin film transistor91 of the first comparative example illustrated inFIGS. 5A to 5D. That is, in amorphous silicon, polysilicon, and the like, although hydrogen on the surface leaves due to the heat treatment on some occasions, the portion where hydrogen has left becomes high resistive, and therefore there is generally no problem. Accordingly, even if the side face of thesemiconductor layer140 is exposed, significant change in characteristics due to the heat treatment does not occur.
• Therefore, the inventors expected that, also in the case where an oxide semiconductor is used as thesemiconductor layer140, characteristic variation would be suppressed to a level of no practical problem by providing thechannel protecting layer150 on theupper face140uof thesemiconductor layer140. However, as previously illustrated inFIG. 6, actually, characteristics varied greatly due to the heat treatment in thethin film transistor91 of the first comparative example.
• In this regard, the inventors carried out experiments on the relationships between the disposition of thechannel protecting layer150 and thesemiconductor layer140, and electrical characteristics.
• FIGS. 8A to 8D are schematic views illustrating the configuration of the thin film transistor used for the experiments and experimental results. That is,FIG. 8A is a schematic plan view illustrating the configuration of the thin film transistor used for the experiments,FIG. 8B is an equivalent circuit diagram illustrating electrical characteristics of the thin film transistor, andFIGS. 8C and 8D are graphs illustrating results of characteristic measurement when the heat treatment conditions are changed. InFIGS. 8C and 8D, the horizontal axis represents the gate voltage Vg, the vertical axis ofFIG. 8C represents the drain current Id on a logarithmic scale, and the vertical axis ofFIG. 8D represents the drain current Id on a regular interval scale. InFIG. 8C andFIG. 8D, a solid line A1 illustrates the initial characteristics without the heat treatment and a broken line A2 illustrates the characteristics after the heat treatment of 160° C. under an Ar atmosphere. A dotted line A4 inFIG. 8D illustrates virtual characteristics created by extending the characteristics of the broken line A2 in the region of low gate voltage Vg to the region of high gate voltage Vg.
• As illustrated inFIG. 8A, in athin film transistor93 used for the experiments, thesemiconductor layer140 has a larger planar shape than thechannel protecting layer150. That is, in the region above thegate electrode120, theupper face140uof thesemiconductor layer140 corresponding to the channel region where thesource electrode161 and thedrain electrode162 are opposed to each other is covered with thechannel protecting layer150. However, theupper face140uof thesemiconductor layer140 other than the channel region and theside face140sare not covered with thechannel protecting layer150.
• Thethin film transistor93 having such a structure showed the characteristics illustrated inFIG. 8C andFIG. 8D.
• That is, as illustrated inFIG. 8C, in thethin film transistor93, a high ON/OFF ratio is obtained in the case of the initial characteristics without the heat treatment (solid line A1). However, in the case of the heat treatment of 160° C. (broken line A2), the ON/OFF ratios are very low, and mostly the ON state is presented (conduction state). That is, the ON/OFF ratios further deteriorate than the characteristics of the heat treatment of 160° C. (broken line A2) of thethin film transistor91 illustrated inFIG. 6. It is considered that this is because thethin film transistor93 has a larger area of thesemiconductor layer140 not covered with thechannel protecting layer150 than thethin film transistor91.
• When the characteristics ofFIG. 8C are seen on a regular interval scale as illustrated inFIG. 8D, the characteristics of the heat treatment of 160° C. (broken line A2) have a form similar to the initial characteristics without the heat treatment (solid line A1). That is, in both the solid line A1 and the broken line A2, the drain current Id rises rapidly at gate voltages of approximately −4 V or higher. However, in regard to the broken line A2, a large current illustrated with the dotted line A4 flows even when the gate voltage is lower than approximately −4 V, and it was assumed that this current (dotted line A4) and the current rapidly increasing at gate voltages of approximately −4 V or higher would be combined to become the characteristics of the broken line A2.
• From these characteristics, it was assumed that the characteristics of the broken line A2 of thethin film transistor93 after the heat treatment of 160° C. would be characteristics of a configuration in which elements having mutually different characteristics are connected in parallel.
• That is, as illustrated inFIG. 8A, it is considered that the characteristics of the broken line A2 are combined characteristics of: the characteristics at a channel sectioncurrent path145cthat is covered with thechannel protecting layer150 in a region where thesource electrode161 and thedrain electrode162 are opposed to each other; and the characteristics at a surrounding sectioncurrent path145ssuch as theside face140sof thesemiconductor layer140 not covered with thechannel protecting layer150.
• That is, as illustrated inFIG. 8B, thethin film transistor93 can be regarded as a configuration in which achannel section transistor93acorresponding to the channel sectioncurrent path145cand a surroundingsection transistor93bcorresponding to the surrounding sectioncurrent path145sare connected in parallel. It is considered that thechannel section transistor93ahas the characteristics of the solid line A1 illustrated inFIG. 8D. On the other hand, it is considered that the surroundingsection transistor93bhas characteristics similar to the characteristics of the dotted line A4 illustrated inFIG. 8D.
• Consequently, it is considered that the broken line A2 presents characteristics resulting from the parallel connection of thechannel section transistor93aand the surrounding section transistor94b,that is, characteristics resulting from the combination of the solid line A1 and the dotted line A4.
• Thus, it was found that characteristics deteriorate due to the heat treatment when thesemiconductor layer140 is not covered with thechannel protecting layer150. Specifically, it is considered that oxygen leaves the exposed portion of thesemiconductor layer140, thereby the resistance of thesemiconductor layer140 decreases, and thus characteristics change.
• Therefore, since thechannel protecting layer150 is not provided in thethin film transistor92 of the second comparative example, the surface of thesemiconductor140 is exposed in a large area of theupper face140uand theside face140sof thesemiconductor layer140, and thus characteristics vary greatly due to heat treatment.
• Further, in thethin film transistor91 of the first comparative example, thesemiconductor layer140 is not covered with thechannel protecting layer150 on theside face140s,and it is assumed that characteristics changed at this portion due to the heat treatment. For example, in the case of the broken line A2 illustrated inFIG. 6, the convex A2a occurs as previously described, and this also suggests that the characteristics of the broken line A2 are characteristics resulting from the parallel connection of thechannel section transistor93aand the surroundingsection transistor93b.
• As described above, it was found that, in the configuration of thethin film transistor91 of the first comparative example illustrated inFIGS. 5A to 5D, though there is no problem in the case where a semiconductor such as amorphous silicon and polysilicon is used as thesemiconductor layer140, there is a problem in the case where an oxide semiconductor of which electrical characteristics greatly depend on the amount of oxygen is used. That is, in the case where an oxide semiconductor is used, if theside face140sof thesemiconductor layer140 is exposed, the amount of oxygen varies at the portion due to, for example, the heat treatment and consequently characteristics deteriorate significantly. While in conventional thin film transistors using a semiconductor such as amorphous silicon and polysilicon, it has not been necessary to cover theside face140sas well as theupper face140uof thesemiconductor layer140 with thechannel protecting layer150 such a configuration is necessary peculiarly for thin film transistors using thesemiconductor layer140 made of an oxide semiconductor.
• Here, in thethin film transistor11 according to this embodiment and thethin film transistor11aof the first example, thechannel protecting layer150 is a layer provided between thesemiconductor layer140, and thesource electrode161 and thedrain electrode162. Thechannel protecting layer150 covers at least part of theupper face140uof thesemiconductor layer140. Furthermore, at least part of thechannel protecting layer150 is covered with thesource electrode161 and thedrain electrode162.
• Since thechannel protecting layer150 is provided in order to protect thesemiconductor layer140, it is formed after forming thesemiconductor layer140. Further, thechannel protecting layer150 is formed before forming thesource electrode161 and thedrain electrode162. As previously described in regard toFIG. 2C toFIG. 3B, this is for the purpose of allowing to perform high temperature annealing at 350° C. for one hour under an air atmosphere and the like, for example, in order to cause thesemiconductor layer140 to recover from damages resulting from film-forming thechannel protecting layer150 on thesemiconductor layer140 by, for example, PE-CVD.
• For example, if thesemiconductor layer140 is formed, after that, a film that forms thesource electrode161 and thedrain electrode162 is formed, and subsequently thechannel protecting layer150 is formed, a high temperature treatment for recovery from damages during formation of thechannel protecting layer150 is performed also on thesource electrode161 and thedrain electrode162. If such a high temperature treatment is performed on thesource electrode161 and thedrain electrode162, characteristic deterioration and/or a hillock caused by a reaction between thesemiconductor layer140, and thesource electrode161 and thedrain electrode162 occur as described previously. Thus, this process cannot be employed practically.
• Therefore, in thethin film transistor11 according to this embodiment and thethin film transistor11aof the first example, thechannel protecting layer150 is provided between thesemiconductor layer140, and thesource electrode161 and thedrain electrode162 in order to enable high temperature treatment for recovery from damages at the time of forming thechannel protecting layer150 on thesemiconductor layer140.
• In thethin film transistor11 according to this embodiment and thethin film transistor11aof the first example, thechannel protecting layer150 is provided so as to cover theupper face140uand theside face140sof thesemiconductor layer140 not covered with thesource electrode161 and thedrain electrode162. However, embodiments of the invention are not limited thereto. That is, it is sufficient that thechannel protecting layer150 is provided so as to cut off at least part of a current path that forms the surroundingsection transistor93bdescribed in regard toFIGS. 8A to 8D, and thus various modifications are possible.
• FIGS. 9A to 9D are schematic planar views each illustrating the configuration of a thin film transistor of a modification example according to the first embodiment of the invention.
• In these drawings, the insulatinglayer110 and thegate insulating film130 are omitted. As illustrated inFIG. 9A, in thethin film transistor12 of the modification example according to this embodiment, thesemiconductor layer140 includes a region exposed to the outside of thesource electrode161 and thedrain electrode162. However, end portions of thesemiconductor layer140 in the channel direction (gate length direction) of the channel region where thesource electrode161 and thedrain electrode162 are opposed to each other are covered with thechannel protecting layer150.
• That is, in this case, theside face140sof thesemiconductor layer140 which is placed between: anextension line161pof a side of thesource electrode161 opposed to thedrain electrode162; and anextension line162pof a side of thedrain electrode162 opposed to thesource electrode161 is covered with thechannel protecting layer150. Accordingly, in avicinity region146sof theside face140sbetween theextension line161pand theextension line162p,theside face140sis high resistive with no affect by the heat treatment.
• Therefore, the surrounding sectioncurrent path145sis cut off in thevicinity region146sof theside face140sbetween theextension line161pand theextension line162p.Thereby, even if a heat treatment for formation of the passivation film and the like is performed, thethin film transistor12 provides a thin film transistor using an oxide semiconductor which suppresses variation in oxide concentration caused by the heat treatment to suppress characteristic variation.
• Further, as illustrated inFIG. 9B, in a thin film transistor of another modification example according to this embodiment, thesemiconductor layer140 includes a region exposed to the outside of thesource electrode161 and thedrain electrode162. Further, thesemiconductor layer140 includes a concave which pulls back to the inside in a region that is placed between: theextension line161pof a side of thesource electrode161 opposed to thedrain electrode162; and theextension line162pof a side of thedrain electrode162 opposed to thesource electrode161. Also in this case, end portions of thesemiconductor layer140 in the channel direction (gate length direction) in the channel region where thesource electrode161 and thedrain electrode162 are opposed to each other are covered with thechannel protecting layer150.
• That is, also in this case, theside face140sof thesemiconductor layer140 which is placed between: theextension line161pof a side of thesource electrode161 opposed to thedrain electrode162; and theextension line162pof a side of thedrain electrode162 opposed to thesource electrode161 is covered with thechannel protecting layer150.
• Therefore, in thevicinity region146sof theside face140sbetween theextension line161pand theextension line162p,theside face140sis high resistive with no affect by the heat treatment. Accordingly, the surrounding sectioncurrent path145sis cut off in thevicinity region146sof theside face140sbetween theextension line161pand theextension line162p.Thereby, even if a heat treatment for formation of the passivation film and the like is performed, thethin film transistor13 provides a thin film transistor using an oxide semiconductor which suppresses variation in oxide concentration caused by heat treatment to suppress characteristic variation.
• Thus, it is sufficient that thechannel protecting layer150 is provided so as to cover theside face140sand theupper face140uof thesemiconductor layer140 which are placed between: theextension line161pof a side of thesource electrode161 opposed to thedrain electrode162; and theextension line162pof a side of thedrain electrode162 opposed to thesource electrode161. Thereby, the surrounding sectioncurrent path145sis substantially cut off in thevicinity region146sof theside face140sbetween theextension line161pand theextension line162p.Therefore, even if there is a portion where theside face140sand/or theupper face140uof thesemiconductor layer140 are not covered with thesource electrode161 and thedrain electrode162, thethin film transistor13 is substantially not affected by characteristic variation of thesemiconductor layer140 due to the heat treatment because the surrounding sectioncurrent path145sis cut off.
• Further, as illustrated inFIG. 9C, also in athin film transistor13aof another modification example according to this embodiment, thesemiconductor layer140 includes a region exposed to the outside of thesource electrode161 and thedrain electrode162. Further, part of theside face140sof thesemiconductor layer140 is covered with thechannel protecting layer150 in a region that is placed above thegate electrode120 and between: theextension line161pof a side of thesource electrode161 opposed to thedrain electrode162; and theextension line162pof a side of thedrain electrode162 opposed to thesource electrode161. That is, also in this case, part of end portions of thesemiconductor layer140 in the channel direction (gate length direction) of the channel region where thesource electrode161 and thedrain electrode162 are opposed to each other are covered with thechannel protecting layer150.
• Thereby, the surrounding sectioncurrent path145sis substantially cut off at part of theside face140sbetween theextension line161pand theextension line162p.Therefore, even if there is a portion where theside face140sand/or theupper face140uof thesemiconductor layer140 are not covered with thesource electrode161 and thedrain electrode162, thethin film transistor13ais substantially not affected by characteristic variation of thesemiconductor layer140 due to the heat treatment because the surrounding sectioncurrent path145sis cut off.
• Thus, it is sufficient that thechannel protecting layer150 is provided on at least part of theside face140sof thesemiconductor layer140 so as to cut off the surrounding sectioncurrent path145sof thesemiconductor layer140.
• In the configurations illustrated inFIGS. 9A,9B and9C, regions where thesemiconductor layer140 is not covered with thesource electrode161 and thedrain electrode162 and spreads from the area corresponding to them may be etching-removed by using thesource electrode161, thedrain electrode162, and thechannel protecting layer150 as a mask.
• Further, as illustrated inFIG. 9D, in a thin film transistor of another modification example according to this embodiment, thechannel protecting layer150 is not formed in an island shape but provided with a large area so as to cover theentire semiconductor layer140, andopenings161qand162q are provided in thechannel protecting layer150 at portions where thesemiconductor layer140 contacts thesource electrode161 and thedrain electrode162.
• Thereby, in thethin film transistor14, only thechannel section transistor93acorresponding to the channel sectioncurrent path145cof the channel region where thesource electrode161 and thedrain electrode162 are opposed to each other is formed, but the surroundingsection transistor93bcorresponding to the surrounding sectioncurrent path145sis not formed. Thereby, even if a heat treatment for formation of the passivation film and the like is performed, thethin film transistor14 provides a thin film transistor using an oxide semiconductor which suppresses variation in oxygen concentration caused by heat treatment to suppress characteristic variation.
• Also in this case, theside face140sand theupper face140uof thesemiconductor layer140 which are placed between: theextension line161pof a side of thesource electrode161 opposed to thedrain electrode162; and theextension line162pof a side of thedrain electrode162 opposed to thesource electrode161 are covered with thechannel protecting layer150.
• Thus, thechannel protecting layer150 further covers theupper face140uof thesemiconductor layer140 between theextension line161pof a side of thesource electrode161 opposed to thedrain electrode162 and theextension line162pof a side of thedrain electrode162 opposed to thesource electrode161.
Second Example
• Athin film transistor15 according to a second example of the invention has a configuration similar to thethin film transistor11adescribed in regard toFIGS. 2A to 2C andFIGS. 3A and 3B. However, it is fabricated by a manufacturing method different from that for thethin film transistor11a.That is, photolithography for patterning thechannel protecting layer150 and photolithography for patterning a portion for extracting thegate electrode120 are simultaneously performed, which reduces the number of processes. A method for manufacturing a thin film transistor according to this example will now be described.
• FIGS. 10A to 10D are sequential schematic cross-sectional views illustrating a method for manufacturing a thin film transistor according to the second example of the invention.
• The drawings are cross-sectional views corresponding to the cross-section taken along line A-A′ ofFIG. 1A. Also in these drawings, a contact unit is illustrated in addition to the portion of the thin film transistor.
• First, as illustrated inFIG. 10A, theAl film121fand theMo film122fwere film-formed with thicknesses of 100 nm and 30 nm, respectively, on themajor face111 of theglass substrate110gthat is the insulatinglayer110 by sputtering, and then were patterned into a prescribed pattern to form thegate electrode120. Photolithography was used for the patterning, and a mixed acid of phosphoric acid, acetic acid, nitric acid, and water was used for the etching. At this time, thecontact unit123 of thegate electrode120 of the thin film transistor was also formed simultaneously. Thecontact unit123 is also formed of stacked films of theAl film121fand theMo film122f.
• Next, as illustrated inFIG. 10B, the SiO₂film130fwas deposited with a thickness of 200 nm as thegate insulating film130 by PE-CVD using TEOS as raw material. At this time, the film-formation temperature was 350° C.
• Further, on thegate insulating film130, theoxide layer140fmade of In—Ga—Zn—O oxide which forms thesemiconductor layer140 was film-formed with a thickness of 30 nm by reactive DC sputtering. At this time, the ratio of oxygen was 5% to argon. Then, theoxide layer140fwas patterned into a prescribed pattern across thegate electrode120 by using oxalic acid of 2%, and thus thesemiconductor layer140 was formed.
• Further, as illustrated inFIG. 10C, the SiO₂film150fthat forms thechannel protecting layer150 was deposited with a thickness of 200 nm by PE-CVD of TEOS. At this time, the film-formation gas was a mixed gas of O₂and TEOS, and the film-formation temperature was 350° C. After that, the SiO₂film150fwas patterned into a prescribed pattern covering theside face140sand theupper face140uof thesemiconductor layer140 except regions that form later thesource contact region141 and thedrain contact region142, and thus thechannel protecting layer150 was formed.
• The mask exposure and back face exposure using thegate electrode120 as a mask were performed in combination for photolithography at the time of patterning of the SiO₂film150f.The etching at this time was performed by RIE using CF₄gas.
• At this time, after removing the SiO₂film150fof thechannel protecting layer150 deposited on thecontact unit123, the SiO₂film130fof thegate insulating film130 was removed to expose the stacked films of theAl film121fand theMo film122fof thecontact unit123.
• After that, annealing was performed at 350° C. for one hour under an air atmosphere to remove damages in thesemiconductor layer140 due to the PE-CVD process for film-formation of the SiO₂film150f.
• Then, as illustrated inFIG. 10D, theMo film166, theAl film167, and theMo film168 that form thesource electrode161 and thedrain electrode162 were film-formed with thicknesses of 10 nm, 300 nm, and 50 nm, respectively, by sputtering to form the stackedfilms160f,and thestacked films160fwere patterned into a prescribed pattern by using a mixed acid to form thesource electrode161 and thedrain electrode162. Thereby, the configuration of thethin film transistor11ais completed. Further, thestacked films160fof theMo film166, theAl film167, and theMo film168 are provided on the stacked films of theAl film121fand theMo film122fthat form thecontact unit123, and thus thecontact unit123 is formed.
• After that, in order to remove element damages during the processes, annealing is performed in a clean oven at 230° C. for approximately one hour. Thus, thethin film transistor15 according to this example is obtained.
• Thus, by performing patterning of thechannel protecting layer150 and patterning of thecontact unit123 for extracting thegate electrode120 by the same process of photolithography, processes for patterning of the contact hole122hillustrated inFIG. 3A can be omitted, and the manufacturing becomes easier. Thethin film transistor15 manufactured by such a method also provides a thin film transistor using an oxide semiconductor which suppresses variation in oxygen concentration caused by the heat treatment to suppress characteristic variation.
Third Example
• A thin film transistor according to a third example of the invention is a modification to thethin film transistor11aaccording to the first example. It can shorten channel length to improve current driving capability of transistor, and reduce characteristic deterioration due to a reaction between thesource electrode161 and thedrain electrode162, and the channel.
• FIGS. 11A to 11C are sequential schematic cross-sectional views illustrating a method for manufacturing a thin film transistor according to the third example of the invention.
• FIGS. 12A and 12B are sequential schematic cross-sectional views continuing fromFIG. 11C.
• These drawings are cross-sectional views corresponding to the cross-section taken along line A-A′ ofFIG. 1A. Also in these drawings, a contact unit is illustrated in addition to the portion of the thin film transistor.
• First, as illustrated inFIG. 11A, theAl film121fand theMo film122fwere film-formed with thicknesses of 100 nm and 30 nm, respectively, on themajor face111 of theglass substrate110gthat is the insulatinglayer110 by sputtering, and then were patterned into a prescribed pattern to form thegate electrode120. Photolithography was used for the patterning, and a mixed acid of phosphoric acid, acetic acid, nitric acid, and water was used for the etching. At this time, thecontact unit123 of thegate electrode120 of the thin film transistor was also formed simultaneously. Thecontact unit123 is also formed of stacked films of theAl film121fand theMo film122f.
• Next, as illustrated inFIG. 11B, the SiO₂film130fwas deposited with a thickness of 200 nm as thegate insulating film130 by PE-CVD using TEOS as raw material. At this time, the film-formation temperature was 350° C.
• Further, on thegate insulating film130, theoxide layer140fmade of In—Ga—Zn-0 oxide which forms thesemiconductor layer140 was film-formed with a thickness of 30 nm by reactive DC sputtering. At this time, the ratio of oxygen was 5% to argon. Then, theoxide layer140fwas patterned into a prescribed pattern going across thegate electrode120 by using oxalic acid of 2%, and thus thesemiconductor layer140 was formed.
• Further, as illustrated inFIG. 11C, the SiO₂film150fthat forms thechannel protecting layer150 was deposited with a thickness of 200 nm by PE-CVD of TEOS. At this time, the film-formation gas was a mixed gas of O₂and TEOS, and the film-formation temperature was 350° C. After that, this SiO₂film150fwas patterned into a prescribed pattern covering theside face140sand theupper face140uof thesemiconductor layer140 except regions that form later thesource contact region141 and thedrain contact region142, and thus thechannel protecting layer150 was formed.
• Mask exposure and back face exposure using thegate electrode120 as a mask were performed in combination for photolithography at the time of patterning of the SiO₂film150f.The etching at this time was performed by RIE using CF₄gas.
• After that, annealing was performed at 350° C. for one hour under an air atmosphere to remove damages in thesemiconductor layer140 due to the PE-CVD process for film-formation of the SiO₂film150f.
• Next, as illustrated inFIG. 12A, a SiN film that forms apassivation film181 was deposited by PE-CVD. The film-formation temperature was 250° C.
• Then, contactholes141hand142hwere formed at positions corresponding to thesource contact region141 and thedrain contact region142, respectively. At this time, the contact hole122hfor extracting thegate electrode120 was also formed simultaneously.
• Then, as illustrated inFIG. 12B, theMo film166, theAl film167, and theMo film168 that form thesource electrode161 and thedrain electrode162 were film-formed with thicknesses of 10 nm, 300 nm, and 50 nm, respectively, by sputtering to form the stackedfilms160f,and thestacked films160fwere patterned into a prescribed pattern by using a mixed acid to form thesource electrode161 and thedrain electrode162. Thereby, the configuration of thethin film transistor15ais completed. Further, thestacked films160fof theMo film166, theAl film167, and theMo film168 are provided on the stacked films of theAl film121fand theMo film122fthat form thecontact unit123, and thus thecontact unit123 is formed.
• Thus, thethin film transistor15aaccording to this example is formed.
• In the above, by the heating of 160° C. or higher during film-formation of the SiN film that forms thepassivation film181, oxygen leaves theoxide layer140fthat is not covered with thechannel protecting layer150, covered with the SiN layer, i.e., thepassivation film181, and made of In—Ga—Zn—O oxide. Therefore, theoxide layer140fbecomes low resistive. Since this portion is low resistive even when the SiN film is etching-removed, the serial resistance between thesource electrode161 and thedrain electrode162, and the channel section can be kept low. The channel section is protected by thechannel protecting layer150 that is formed of the SiO₂film150fcontaining oxygen, and thus oxygen does not leaves theoxide layer140fat this portion, where high resistance can be maintained.
Second Embodiment
• FIG. 13 is a schematic cross-sectional view illustrating the configuration of a display device according to a second embodiment of the invention.
• The portion of the thin film transistor illustrated in the drawing is illustrated as a cross-section corresponding to the cross-section taken along line A-A′ ofFIG. 1A.
• As illustrated inFIG. 13, adisplay device51 according to the second embodiment of the invention includes: thethin film transistor11aaccording to the first example of the first embodiment; apixel electrode140dconnected to thedrain electrode162 of thethin film transistor11a;and anoptical element300 that produces at least one of a change in optical characteristics and light emitting by an electric signal provided to thepixel electrode140d.
• Although thethin film transistor11ais used in this specific example, any of thethin film transistors11,11a,12,13,13a,14,15, and15aof specific examples according to the first embodiment may be used.
• Thepixel electrode140dis made of oxide that forms thesemiconductor layer140 of thethin film transistor11a,and has lower electric resistance than thesemiconductor layer140. That is, thepixel electrode140dis formed by theoxide layer140fthat forms thesemiconductor layer140, and thepixel electrode140dand thesemiconductor layer140 are the same layer. A lower resistance value is required at the portion of thepixel electrode140dthan at thesemiconductor layer140. Accordingly, as described below, a scheme is used by which theoxide layer140fused for thesemiconductor layer140 becomes low resistive at the portion of thepixel electrode140d.
• Further, in this example, an organic EL element is used as theoptical element300. That is, aCu phthalocyanine layer191 with a thickness of 25 nm that forms a hole injecting layer, an α-NPD (N,N′-di(1-naphthyl)-N,N¹-diphenylbenzidine)layer192 with a thickness of 35 nm that forms a hole carrying layer, an Alq3 (tris(8-hydroxyquinoline)aluminum)layer193 with a thickness of 50 nm that forms a light emitting layer, aLiF layer194 with a thickness of 0.6 nm, and anAl layer195 with a thickness of 150 nm that forms a cathode are provided in sequence on thepixel electrode140dto form an organic EL layer. That is, in this example, theoptical element300 emits light by an electric signal provided to thepixel electrode140d.Liquid crystal or the like that produces a change in optical characteristics such as birefringence, optical activity, scattering, and absorption by an electric signal provided to thepixel electrode140dalso may be used as theoptical element300.
• Although glass sealing is further performed on the structure illustrated inFIG. 13 to obtain a display panel with high reliability, it is omitted here.
• Since thedisplay device51 according to this embodiment uses any of the thin film transistors according to the first embodiment of the invention, even if a heat treatment for formation of the passivation film and the like after forming the thin film transistor is performed, it can provide a display device using a thin film transistor that uses an oxide semiconductor and suppresses characteristic variation due to the heat treatment.
• Furthermore, since theoxide layer140fused for thesemiconductor layer140 can be used to form thepixel electrode140d,the number of processes for formation of thepixel electrode140ddoes not increase and this configuration can provide high productivity.
• That is, thesemiconductor layer140 that forms the channel of thethin film transistor11ais covered with thechannel protecting layer150, and oxygen hardly leaves at this portion of theoxide layer140f.In contrast, since theoxide layer140fthat forms thepixel electrode140dis not covered with thechannel protecting layer150, oxygen leaves easily by the heat treatment at this portion. By using this configuration, theoxide layer140fthat forms thepixel electrode140dcan be made low resistive selectively, while theoxide layer140fhaving the same material as thesemiconductor layer140 that forms the channel of thethin film transistor11ais used for thepixel electrode140d.
• Thus, the concentration of oxygen contained in thepixel electrode140dis lower than the concentration of oxygen contained in thesemiconductor layer140, and thereby the electric resistance of thepixel electrode140dis made lower than thesemiconductor layer140.
Fourth Example
• A method for manufacturing a display device according to this embodiment will now be described as a fourth example.
• FIGS. 14A to 14C are sequential schematic cross-sectional views illustrating a method for manufacturing a display device according to the fourth example of the invention.
• FIGS. 15A and 15B are sequential schematic cross-sectional views continuing fromFIG. 14C.
• Also in these drawings, a portion of the thin film transistor is illustrated as a cross-section corresponding to the cross-section taken along line A-A′ ofFIG. 1A.
• As illustrated inFIG. 14A, first, theAl film121fand theMo film122fwere film-formed with thicknesses of 100 nm and 30 nm, respectively, on themajor face111 of theglass substrate110gthat is the insulatinglayer110 by sputtering, and were patterned into a prescribed pattern to form thegate electrode120. Photolithography was used for the patterning, and a mixed acid of phosphoric acid, acetic acid, nitric acid, and water was used for the etching. At this time, thecontact unit123 of thegate electrode120 of the thin film transistor was also formed simultaneously. Thecontact unit123 is also formed of stacked films of theAl film121fand theMo film122f.
• Next, as illustrated inFIG. 14B, the SiO₂film130fwas deposited with a thickness of 200 nm as thegate insulating film130 by PE-CVD using TEOS as raw material. At this time, the film-formation temperature was 350° C.
• Further, on the SiO₂film130f,an In—Ga—Zn—O oxide film was film-formed with a thickness of 30 nm as theoxide layer140fthat forms thesemiconductor layer140 and thepixel electrode140dby reactive DC sputtering. At this time, the ratio of oxygen was 5% to argon. Then, theoxide layer140fwas patterned into a shape that connects thepixel electrode140dand a prescribed channel pattern across thegate electrode120 by using oxalic acid of 2%, and thus the shapes of thesemiconductor layer140 and thepixel electrode140dwere formed.
• Further, as illustrated inFIG. 14C, the SiO₂film150fthat forms thechannel protecting layer150 was deposited with a thickness of 200 nm by PE-CVD using TEOS. At this time, the film-formation gas was a mixed gas of O₂and TEOS, and the film-formation temperature was 350° C. After that, annealing was performed at 350° C. for one hour under an air atmosphere to remove damages in theoxide layer140fdue to the PE-CVD process at the time of formation of the SiO₂film150f.
• After that, the SiO₂film150fwas patterned into a prescribed pattern covering theside face140sand theupper face140uof thesemiconductor layer140 except regions that form later thesource contact region141 and thedrain contact region142, and thus thechannel protecting layer150 was formed. At this time, the SiO₂film150fon thepixel electrode140dwas etched to be removed to expose theoxide layer140fthat forms thepixel electrode140dfrom thechannel protecting layer150.
• At this time, the etching of the SiO₂film150fwas performed by RIE using CF₄. Continuing the etching of the SiO₂film150f,the SiO₂film130fthat forms thegate insulating film130 was etched until theAl film121fand theMo film122fthat form thecontact unit123 of thegate electrode120 are exposed by using thesemiconductor layer140 and the resist for shape-patterning thechannel protecting layer150 as a mask,.
• Then, as illustrated inFIG. 15A, theMo film166, theAl film167, and theMo film168 that form thesource electrode161 and thedrain electrode162 were film-formed with thicknesses of 10 nm, 300 nm, and 50 nm, respectively, by sputtering to form the stackedfilms160f.After that, thestacked films160fwere patterned into a prescribed pattern by using a mixed acid to form thesource electrode161 and thedrain electrode162. Thereby, thethin film transistor11aand thecontact unit123 are completed.
• After that, as illustrated inFIG. 15B, a SiN film that forms thepassivation film181 was deposited with a thickness of 100 nm by PE-CVD. Using a temperature of approximately 230° C. for film-formation of the SiN film can provide thepassivation film181 with sufficient barrier properties.
• Further,banks182 having a prescribed shape excluding thepixel electrode140dand thecontact unit123 were formed by using a photosensitive transparent resin. Photosensitive acrylic or photosensitive polyimide may be used as the photosensitive transparent resin. The heating temperature is 230° C., for example. After forming thebank182, the SiN film, i.e., thepassivation film181 was etched to be removed by using thebank182 as a mask.
• After that, a light emitting unit of an organic EL was formed betweenbanks182. That is, theCu phthalocyanine layer191 with a thickness of 25 nm that forms a hole injecting layer, the α-NPD layer192 with a thickness of 35 nm that forms a hole carrying layer, theAlq3 layer193 with a thickness of 50 nm that forms a light emitting layer, theLiF layer194 with a thickness of 0.6 nm, and theAl layer195 with a thickness of 150 nm that forms a cathode layer were film-formed by using a deposition apparatus.
• Thus, thedisplay device51 illustrated inFIG. 13 is fabricated.
• Here, on a portion of theoxide layer140fwhich forms thepixel electrode140d,the SiN film is formed at a film-formation temperature of 230° C., and thereby oxygen leaves theoxide layer140fat a portion of theoxide layer140fwhich forms thepixel electrode140dto make the portion low resistive. On the other hand, theoxide layer140fabove thegate electrode120 which forms thesemiconductor layer140 is covered with thechannel protecting layer150 made of the SiO₂film150fcontaining oxygen, and therefore is not made low resistive even by the heating of 230° C. which is the film-formation temperature for the SiN film. Further, also in the heat treatment during subsequent formation of thebank182, oxygen further leaves thepixel electrode140dat the portion of thepixel electrode140din theoxide layer140fto make the portion further lower resistive. On the other hand, high resistance is maintained at theoxide layer140fabove thegate electrode120 which forms thesemiconductor layer140.
• Thus, in thedisplay device51 and the method for manufacturing the same according to this embodiment, the same material (theoxide layer140f) as the material of thesemiconductor layer140 that forms the channel of thethin film transistor11ais used for thepixel electrode140d.While theoxide layer140fthat forms thesemiconductor layer140 of the channel section is covered with thechannel protecting layer150 containing oxide, theoxide layer140fthat forms thepixel electrode140dis not covered with thechannel protecting layer150 and covered with, for example, a SiN film. Thereby, theoxide layer140fthat forms thepixel electrode140dis made low resistive selectively.
• Thus, thedisplay device51 and the method for manufacturing the same according to this embodiment can omit processes for formation of another film for formation of thepixel electrode140d,and provide a display device using an oxide semiconductor which suppresses characteristic variation due to the heat treatment and a method for manufacturing the same.
Fifth Example
• A display device52 (not illustrated) according to a fifth example of the invention makes the resistance different between thesemiconductor layer140 and thepixel electrode140dby controlling the film structure of theoxide layer140fthat forms thesemiconductor layer140.
• That is, on a basis of the experiments of the inventors, it was found that the electric resistance at an oxide semiconductor layer depends on the film structure of the oxide semiconductor layer, as well as on the concentration of oxygen contained in the oxide semiconductor layer. The film structure of the oxide semiconductor layer changes with, for example, the smoothness of the surface of the under layer at the time of film-forming the oxide semiconductor layer.
• For example, in the case where the surface of thegate insulating film130 that forms the under layer of thesemiconductor layer140 is a rough face, thesemiconductor layer140 made of oxide and film-formed on the surface becomes a columnar structure. When the cross-section of thesemiconductor layer140 is observed with a SEM (scanning electron microscope) or TEM (transmission electron microscope), columnar grains having sizes of approximately 10 nm to 30 nm are observed while thesemiconductor layer140 has an amorphous state in crystalline state.
• On the other hand, in the case when the surface of thegate insulating film130 of the under layer is smooth, thesemiconductor layer140 becomes not a columnar structure but a uniform film structure, and no specific grain is observed by the observing method mentioned above.
• Although depending on film-formation conditions, it was found that thesemiconductor layer140 presents the columnar structure when the surface roughness (unevenness) of thegate insulating film130 that is the under layer is 10 nm to 5 nm (not less than 5 nanometers and not greater than 10 nanometers), for example. Further, it was found that thesemiconductor layer140 presents a uniform film structure when the surface roughness of thegate insulating film130 is smooth and 1 nm to 0.1 nm (not less than 0.1 nanometers and not greater than 1 nanometer), for example. The grain size in the case of the columnar structure is 10 nm to 30 nm, for example.
• The electric resistance in the columnar structure having large grain sizes is relatively lower than the electric resistance in the uniform film structure. That is, the resistivity of thesemiconductor layer140 depends on the morphology of the under layer of thesemiconductor layer140. If the film structure of thesemiconductor layer140 is the columnar structure and has a large grain size, the resistivity is relatively low.
• For example, while in the case of the columnar structure, the resistance value is 0.1 to 10 Ωcm, in the case of the uniform film structure, the resistance value can be 1×10⁸Ωcm or more.
• By applying these experimental results, the distribution of electric resistance in the display device52 according to this embodiment can be controlled by controlling the grain size of thesemiconductor layer140.
• FIGS. 16A to 16D are sequential schematic cross-sectional views illustrating a method for manufacturing the display device according to the fifth example of the invention.
• Also in these drawings, a portion of the thin film transistor is illustrated as a cross-section corresponding to the cross-section taken along line A-A′ ofFIG. 1A.
• As illustrated inFIG. 16A, first, theAl film121fand theMo film122fwere film-formed with thicknesses of 100 nm and 30 nm, respectively, on themajor face111 of theglass substrate110gthat is the insulatinglayer110 by sputtering, and were patterned into a prescribed pattern to form thegate electrode120. Photolithography was used for the patterning, and a mixed acid of phosphoric acid, acetic acid, nitric acid, and water was used for the etching. Thecontact unit123 of thegate electrode120 of the thin film transistor was also formed simultaneously. Thecontact unit123 is also formed of stacked films of theAl film121fand theMo film122f.
• Next, as illustrated inFIG. 16B, the SiO₂film130fwas deposited with a thickness of 200 nm as thegate insulating film130 by PE-CVD using silane and TEOS as raw material. The film-formation temperature is 350° C.
• After that, a resist130rin which only a portion forming the channel of the thin film transistor was opened was formed, and the SiO₂film130fcorresponding to the channel portion was RIE-processed with a mixed gas of Ar and CF₄.
• Thereby, as illustrated inFIG. 16C, a smoothedface130gis formed at a portion not covered with the resist130r.That is, RIE processing is performed on a region of the SiO₂film130fnot covered with the resist130r.This region is made smoother than the surface morphology of the SiO₂film130f(namely, the SiO₂film130fafter film-formed by PE-CVD as it is) on which the RIE processing is not performed, and becomes the smoothedface130g.By etching the SiO₂film130fwith a depth of approximately 10 nm or more by the RIE processing, the surface of the SiO₂film130fis sufficiently smoothed.
• The smoothedface130gof the SiO₂film130fhas a surface roughness of approximately 1 nm to 0.1 nm, for example, and a portion of the SiO₂film130fon which the surface processing for smoothing is not performed has a surface roughness of 10 nm to 5 nm, for example.
• Further, as illustrated inFIG. 16D, on the SiO₂film130fincluding this smoothedface130g,an In—Ga—Zn—O oxide film was film-formed with a thickness of 30 nm as theoxide layer140fthat forms thesemiconductor layer140 and thepixel electrode140dby reactive DC sputtering. The ratio of oxygen was 5% to argon.
• On the basis of the investigation on the film structure of thisoxide layer140f,it was found that theoxide layer140finregions140nabove portions except the smoothedface130ghad a columnar structure with grain sizes of approximately 10 nm to 30 nm. In contrast, it was also found that theoxide layer140fin aregion140mabove the smoothedface130ghad not a columnar structure but a uniform structure with an unobvious film structure.
• Thus, by controlling the smoothness of the surface of thegate insulating film130 that forms the under layer, the film structure of theoxide layer140ffilm-formed thereon can be changed selectively, and thereby a high resistance region and a low resistance region can be formed selectively in theoxide layer140f.
• For example, while the resistance value is 0.1 Ωcm to 10 Ωcm in the case of the columnar structure with a rough under layer, the resistance value can be 1×10⁸Ωcm in the case of the uniform structure with a smooth under layer.
• After that, theoxide layer140fwas patterned into a shape that connects thepixel electrode140dand a prescribed channel pattern across thegate electrode120 by using oxalic acid of 2%, and thus the shapes of thesemiconductor layer140 and thepixel electrode140dwere formed. Thereby, thesemiconductor layer140 with a high resistance is formed of theoxide layer140fcorresponding to the smoothedface130g,and thepixel electrode140dwith a low resistance is formed of theoxide layer140fcorresponding to the region except the smoothedface130g.When thepixel electrode140dand one of thesource electrode161 and thedrain electrode162 of the thin film transistor are electrically connected by theoxide layer140f,the gate insulating film130 (SiO₂film130f) that forms the under layer of the connecting portion is made a not-smoothed region to become a low resistance portion.
• After that, the processes described in regard toFIG. 14C toFIG. 15B, for example, are performed, and then the display device52 according to the example is fabricated. However, a description is omitted.
• Thus, in the display device52 and the method for manufacturing the same according to the example, thepixel electrode140dis formed of oxide used for thesemiconductor layer140, and has lower electric resistance than the semiconductor layer. That is, the same material as thesemiconductor layer140 that forms the channel, i.e., thesame oxide layer140fis used for thepixel electrode140d.However, the film structure is different.
• That is, theoxide layer140fat thepixel electrode140dmay have a columnar structure. The grain size of the columnar structure is 10 nm to 30 nm (not less than 10 nanometers and lot greater than 20 nanometers), for example. On the other hand, theoxide layer140fthat forms the channel may have a uniform film structure, and in this case, no grain is observed. Thereby, thepixel electrode140dcan be made relatively lower resistive than thesemiconductor layer140 corresponding to the channel.
• As mentioned above, the smoothness of the surface of thegate insulating film130 that is the under layer can be selectively controlled, and thereby a portion of theoxide layer140fwhich forms thepixel electrode140dcan be made selectively a columnar structure and theoxide layer140fthat forms the channel can be made a uniform film structure.
• That is, the display device52 further includes a film (theoxide layer140f) that is provided under thepixel electrode140dand made of the same material as thegate insulating film130. This film has been described as thegate insulating film130 in the above description.
• Further, a surface of thegate insulating film130 on thegate electrode120 on a side of thesemiconductor layer140 is smoother than a surface of the film mentioned above (gate insulating film130) provided under thepixel electrode140don a side of thepixel electrode140d.
• The surface roughness of the surface of thegate insulating film130 on thegate electrode120 on a side of thesemiconductor layer140 may be 0.1 nm to 1 nm (not less than 0.1 nanometers and not greater than 1 nanometer), for example. Thereby, theoxide layer140fthereon has a uniform film structure and high resistance.
• The surface roughness of the surface of thegate insulating film130 under thepixel electrode140d(the above-mentioned film formed of the same material as the gate insulating film130) on a side of thepixel electrode140dmay be 5 nm to 10 nm (not less than 5 nanometers and not greater than 10 nanometers), for example. Thereby, theoxide layer140fthereon has a columnar structure and low resistance.
• Although the smoothedface130gis formed by RIE in the above, the forming method is optional. Conversely, the portion of thepixel electrode140dmay be exposed and surface processing that increases roughness of the surface thereof may be performed.
• That is, in the method for manufacturing the display device according to this embodiment, before the forming theoxide layer140f(the layer of oxide), surface processing that selectively changes the smoothness of the surface of thegate insulating film130 may be further performed. The performing the surface processing may includes making the surface of thegate insulating film130 in a region (a third region) on which thepixel electrode140dis formed relatively rougher than the other portions (a surface of the gate insulating film in a region except the third region). That is, the surface where the surface processing is performed may be a portion corresponding to thesemiconductor layer140 that forms the channel, or a portion corresponding to thepixel electrode140d.
• Thedisplay devices51 and52 according to this embodiment may be a matrix-type display device in which thin film transistors and pixel electrodes are arranged in a matrix form
• FIGS. 17A and 17B are circuit diagrams illustrating equivalent circuits of other display devices according to the second embodiment of the invention.
• That is,FIGS. 17A and 17B illustrate equivalent circuits of two kinds of display devices of active matrix type using organic EL.
• As illustrated inFIG. 17A, adisplay device60 of active matrix type using organic EL according to this embodiment includes a first transistor Tr1 for pixel selecting, and a transistor for pixel driving DTr that is connected to apower source line320 and drives an organic EL layer302 (optical element300). The gate of the first transistor Tr1 is connected to ascanning line210, and the source is connected to asignal line220. Any of the thin film transistors according to embodiments of the invention may be used for the first transistor Tr1 and the transistor for pixel driving DTr.
• Further, as illustrated inFIG. 17B, anotherdisplay device61 of active matrix type using organic EL according to this embodiment includes a first to a fourth transistor Tr1 to Tr4 for pixel selecting and a transistor for pixel driving DTr. The gate of the second transistor Tr2 is connected to annth scanning line210_n, and the gates of the first transistor Tr1 and the fourth transistor Tr4 are connected to an (n-1)th scanning line210_n-1. The source of the second transistor Tr2 is connected to thesignal line220. Any of the thin film transistors according to embodiments of the invention may be used for the first to the fourth transistor Tr1 to Tr4 and the transistor for pixel driving DTr.
• Since thesedisplay devices60 and61 of active matrix type using organic EL use any of the thin film transistors according to embodiments of the invention, a display device with high performance and good manufacturability is obtained by the thin film transistor using an oxide semiconductor which suppresses characteristic variation due to the heat treatment.
• FIG. 18 is a circuit diagram illustrating an equivalent circuit of another display device according to the second embodiment of the invention.
• As illustrated inFIG. 18, in one element of adisplay device62 of active matrix type according to the second embodiment of the invention, aliquid crystal layer301 that forms theoptical element300 is an electric load placed between thepixel electrode140dand acounter electrode310, and is connected in parallel to an auxiliary capacitance Cs that is formed byauxiliary capacitance electrodes240. Theauxiliary capacitance electrodes240 are connected to anauxiliary capacitance line230. Thepixel electrode140dis connected to thesignal line220 via athin film transistor21. Thegate electrode120 of thethin film transistor21 is connected to thescanning line210. Thegate electrode120 of thethin film transistor21 is switched to ON or OFF sequentially by thescanning line210 to write a desired charge in theliquid crystal layer301, and thus thedisplay device62 performs displaying.
• Since thedisplay device62 of active matrix type using liquid crystal uses any of the thin film transistors according to embodiments of the invention, a display device with high performance and good manufacturability is obtained by the thin film transistor using an oxide semiconductor which suppresses characteristic variation due to the heat treatment.
• Thus, thedisplay devices60,61, and62 according to this embodiment include: a plurality of thin film transistors according to embodiments of the invention which are arranged in a matrix form; ascanning lines210, connected to each gate electrode of the thin film transistors; thesignal lines220, connected to another of thesource electrode161 and thedrain electrode162 of the thin film transistors; apixel electrode140dconnected to the other of each source electrode and each drain electrode of the thin film transistors; and anoptical element300 that emits light by an electric signal provided to thepixel electrodes140d.
• Thus, the display device according to this embodiment further includes: a plurality of thin film transistors according to embodiments of the invention and a plurality of thepixel electrodes140d,each of thepixel electrode140dconnected to one of eachsource electrode161 and eachdrain electrode162 of the thin film transistors, the plurality of the thin film transistors and the plurality of thepixel electrodes140dbeing arranged in a matrix form; a plurality of thescanning lines210, each of the plurality of thescanning lines210 being connected to each of thegate electrodes120 of each of the plurality of the thin film transistors; and a plurality of thesignal lines220, each of the plurality of thesignal lines220 being connected to another of thesource electrode161 and thedrain electrode162 of each of the plurality of the thin film transistors. Theoptical element300 produces at least one of a change in optical characteristics and light emitting by an electric signal provided to each of thepixel electrodes140d.
• At this time, thepixel electrode140dmentioned above may be formed of theoxide layer140fthat forms thesemiconductor layer140 of the thin film transistor, and may have lower electric resistance than thesemiconductor layer140.
• However, the invention is not limited thereto, but the configuration of thepixel electrode140dis optional as far as using any of the thin film transistors according to embodiments of the invention. Here, as described previously, thepixel electrode140dmay be made of theoxide layer140fthat forms thesemiconductor layer140 of the thin film transistor and may have lower electric resistance than thesemiconductor layer140. This can omit processes for forming another conductive film as thepixel electrode140d,and provide advantage.
Third Embodiment
• A third embodiment of the invention is a method for manufacturing a thin film transistor.
• That is, it is a method for manufacturing a thin film transistor that includes: asubstrate110g;agate electrode120 provided on thesubstrate110g;agate insulating film130 provided on thegate electrode120; asemiconductor layer140 provided on thegate insulating film130, thesemiconductor layer140 being formed of oxide; asource electrode161 and adrain electrode162 provided on thesemiconductor layer140, thesource electrode161 being opposed to one end of thegate electrode120, thedrain electrode162 being opposed to another end of thegate electrode120, the another end being opposite to the one end, thedrain electrode162 being apart from thesource electrode161; and achannel protecting layer150 provided between the source and drainelectrodes161,162 and thesemiconductor layer140. Portions of the manufacturing method will now be described.
• FIG. 19 is a flow chart illustrating a method for manufacturing a thin film transistor according to the third embodiment of the invention.
• As illustrated inFIG. 19, in the method for manufacturing the thin film transistor according to this embodiment, first, thegate electrode120 is formed on thesubstrate110g(step S110).
• Then, thegate insulating film130 is formed on the gate electrode120 (step S120).
• Then, thesemiconductor layer140 is formed on the gate insulating film130 (step S130).
• Subsequently, thechannel protecting layer150 is formed covering at least a part of aside face140sof thesemiconductor layer140 above the gate electrode120 (step S140). That is, for example, thechannel protecting layer150 is formed so as to cover at least part of theside face140sof thesemiconductor layer140 except regions of thesemiconductor layer140 connected to thesource electrode161 and thedrain electrode162. At this time, anupper face140uof thesemiconductor layer140 is also covered.
• Subsequently, a heat treatment is performed on thesemiconductor layer140 and thechannel protecting layer150 at a temperature not less than 160° C. (step S150).
• After the performing the heat treatment, thesource electrode161 and thedrain electrode162 are formed on thesemiconductor layer140 and the channel protecting layer150 (step S160). That is, for example, thesource electrode161 and thedrain electrode162 are formed on regions of thesemiconductor layer140 which are to be connected to thesource electrode161 and thedrain electrode162, respectively.
• That is, in the manufacturing method mentioned above, the method described in regard toFIGS. 2A to 2C andFIGS. 3A and 3B may be used.
• According to the manufacturing method mentioned above, damages in thesemiconductor layer140 which occur during film-formation of thechannel protecting layer150 can be recovered by the heat treatment in step S150. Performing step S160 after this heat treatment (step S150) does not cause damages in thesource electrode161 and thedrain electrode162. Furthermore, at least part of theside face140s(andupper face140u) that could be a current path in thesemiconductor layer140 is protected by thechannel protecting layer150 at the time of this heat treatment (step S150). Thereby, oxygen does not leave thesemiconductor layer140 and a decrease of the resistivity can be prevented. This can provide a method for manufacturing a thin film transistor using an oxide semiconductor which suppresses characteristic variation due to the heat treatment.
• In step S140 mentioned above, thechannel protecting layer150 may be formed covering at least the part of theside face140sof thesemiconductor layer140 which is placed between anextension line161p(a first extension line) and anextension line162p(a second extension line). Theextension line161 is an extension line of a side (a first side) of a region (a first region) where thesource electrode161 is to be formed. Theextension line162 is an extension line of a side (a second side) of a region (a second region) where thesource electrode162 is to be formed. The first side is opposed to the region (the second region) where thedrain electrode162 is to be formed and the second side is opposed to the region (the first region) where thesource electrode161 is to be formed. Thus, thechannel protecting layer150 may be formed covering at least the part of theside face140sof the semiconductor layer between anextension line161p(a first extension line) of a first side of a first region and anextension line162p(a second extension line) of a second side of a second region, thesource electrode161 being to be formed in the first region, thedrain electrode162 being to be formed in the second region, the first side being opposed to the second region, the second region being opposed to the first region. Thereby, the current path mentioned above can be cut off efficiently.
Fourth Embodiment
• A fourth embodiment of the invention is a method for manufacturing a display device. That is, it is a method for manufacturing a display device that includes: a thin film transistor including: asubstrate110g;agate electrode120 provided on thesubstrate110g;agate insulating film130 provide on thegate electrode120; asemiconductor layer140 provided on thegate insulating film130, thesemiconductor layer140 being formed of oxide; asource electrode161 and adrain electrode162 provided on thesemiconductor layer140, thesource electrode161 being opposed to one end of thegate electrode120, thedrain electrode162 being opposed to another end of thegate electrode120, the another end being opposite to the one end, thedrain electrode162 being apart from thesource electrode162; and achannel protecting layer150 provided between the source and drainelectrodes161,162 and thesemiconductor layer140; apixel electrode140dconnected to one of thesource electrode161 and thedrain electrode162; and anoptical element300 producing at least one of a change in optical characteristics and light emitting by an electric signal provided to thepixel electrode140d.Portions of the manufacturing method will now be described.
• FIG. 20 is a flow chart illustrating a method for manufacturing a display device according to the fourth embodiment of the invention.
• As illustrated inFIG. 20, in the method for manufacturing a display device according to this embodiment, first, thegate electrode120 is formed on thesubstrate110g(step S310).
• Then, thegate insulating film130 is formed on the gate electrode120 (step S320).
• Then, anoxide layer140f(a layer of oxide) that forms thesemiconductor layer140 and thepixel electrode140dis formed on the gate insulating film130 (step S330).
• Subsequently, thechannel protecting layer150 covering at least a part of a side face of theoxide layer140f(the layer of the oxide) above thegate electrode120 and exposing theoxide layer140fin a region (a third region), thepixel electrode140dbeing to be formed in the third region (step S340). That is, for example, thechannel protecting layer150 is formed so as to cover at least part of aside face140s(andupper face140u) of theoxide layer140fexcept regions (for example the first region and the second region mentioned above) of thesemiconductor layer140 connected to thesource electrode161 and thedrain electrode162, and to expose theoxide layer140fin a region (the third region) in which thepixel electrode140dis to be formed.
• Subsequently, a heat treatment on theoxide layer140fand thechannel protecting layer150 at a temperature not less than 160° C. is performed to decrease the electric resistance of theoxide layer140fnot covered with thechannel protecting layer150 to form thepixel electrode140d(step S350).
• After the performing the heat treatment, thesource electrode161 and thedrain electrode162 are formed on thesemiconductor layer140 and the channel protecting layer150 (step S360). That is, thesource electrode161 and thedrain electrode162 are formed on regions of thesemiconductor layer140 which are to be connected to thesource electrode161 and thedrain electrode162, respectively.
• That is, in the manufacturing method mentioned above, the method described in regard toFIGS. 14A to 14C andFIGS. 15A and 15B may be used.
• The method for manufacturing a display device according to this embodiment can omit processes for forming another film that forms thepixel electrode140d,and provide a display device of high productivity using an oxide semiconductor which suppresses characteristic variation due to the heat treatment, and a method for manufacturing the same.
• In step S340 mentioned above, thechannel protecting layer150 may be formed covering at least the part of theside face140sof the semiconductor layer between anextension line161p(a first extension line) of a first side of a first region and anextension line162p(a second extension line) of a second side of a second region, thesource electrode161 being to be formed in the first region, thedrain electrode162 being to be formed in the second region, the first side being opposed to the second region, the second region being opposed to the first region. Thereby, the current path mentioned above can be cut off efficiently.
• FIG. 21 is a flow chart illustrating another method for manufacturing a display device according to the fourth embodiment of the invention.
• As illustrated inFIG. 21, in the other manufacturing method, first, thegate electrode120 is formed on thesubstrate110g(step S410).
• Then, thegate insulating film130 is formed on the gate electrode120 (step S420). Thegate insulating film130 is formed in a region (the third region) where thepixel electrode140dis to be formed as well as on thegate electrode120. That is, a portion of thegate insulating film120 is used as the under layer of thepixel electrode140d.
• Then, surface processing that selectively changes the smoothness of a surface of thegate insulating film130 is performed (step S421). That is, for example, a portion of thegate insulating film130 which forms the under layer of thesemiconductor layer140 serving as the channel is processed by RIE to be smoothed.
• At this time, thegate insulating film130 that forms the under layer of thepixel electrode140dis protected with, for example, a resist so that the surface does not to be smoothed. Further, a desired portion other than thepixel electrode140dmay not be smoothed to fabricate an optional conductive region which can be used as a wiring section, for example.
• After that, a layer of the oxide mentioned above is formed on the gate insulating film130 (step S430). That is, theoxide layer140fthat forms thesemiconductor layer140 and thepixel electrode140dis formed. Thereby, a portion of theoxide layer140fwhich forms thepixel electrode140dbecomes a columnar structure and relatively low resistive, for example.
• This can omit processes for forming another film that forms thepixel electrode140d,and provide a display device of high productivity using an oxide semiconductor and a method for manufacturing the same.
• Prior to the forming theoxide layer140fof step S330 described inFIG. 20, the surface processing of step S421 mentioned above that selectively changes the smoothness of a surface of thegate insulating film130 that forms the under layer of theoxide layer140fmay be further performed.
• That is, between step S320 and step S330 illustrated inFIG. 20, the smoothness of surface is made different between: thegate insulating film130 under thesemiconductor layer140 that forms the channel; and thegate insulating film130 under thepixel electrode140d.For example, the surface processing mentioned above may be surface processing that makes the surface of thegate insulating film130 in a region on which thepixel electrode140dis formed relatively rougher than the other portions.
• Hereinabove, embodiments of the invention are described with reference to specific examples. However, the invention is not limited to these examples. For example, one skilled in the art may appropriately select specific configurations of components of the thin film transistor, the method for manufacturing the same, the display device, and the method for manufacturing the same from known art and similarly practice the invention; and such practice is included in the scope of the invention to the extent that similar effects thereto are obtained.
• Further, any two or more components of the specific examples may be combined within the extent of technical feasibility. Such combination is included in the scope of the invention to the extent that the spirit of the invention is included.
• Moreover, all thin film transistors, methods for manufacturing the same, display devices, and methods for manufacturing the same that can be obtained by an appropriate design modification by one skilled in the art based on the thin film transistor, the method for manufacturing the same, the display device, and the method for manufacturing the same described above as embodiments of the invention are also within the scope of the invention to the extent that the spirit of the invention is included.
• Furthermore, one skilled in the art may arrive at various alterations and modifications within the idea of the invention. Such alterations and modifications should be seen as within the scope of the invention.

Claims (20)

1. A thin film transistor comprising:

an insulating layer;

a gate electrode provided on the insulating layer;

a gate insulating film provided on the gate electrode;

a semiconductor layer provided on the gate insulating film, the semiconductor layer being formed of oxide;

a source electrode and a drain electrode provided on the semiconductor layer, the source electrode being opposed to one end of the gate electrode, the drain electrode being opposed to another end of the gate electrode, the another end being opposite to the one end, the drain electrode being apart from the source electrode; and

a channel protecting layer provided between the source and drain electrodes and the semiconductor layer, the channel protecting layer covering at least a part of a side face of a part of the semiconductor layer, the part of the semiconductor layer being not covered with the source electrode and the drain electrode above the gate electrode.

2. The transistor according toclaim 1, wherein the channel protecting layer covers at least the part of the side face of the semiconductor layer between an extension line of a side of the source electrode opposed to the drain electrode and an extension line of a side of the drain electrode opposed to the source electrode.

3. The transistor according toclaim 2, wherein the channel protecting layer further covers an upper face of the semiconductor layer between the extension line of the side of the source electrode opposed to the drain electrode and the extension line of the side of the drain electrode opposed to the source electrode.

4. The transistor according toclaim 1, wherein the channel protecting layer contains oxygen.

5. The transistor according toclaim 1, wherein the semiconductor layer includes an amorphous oxide which contains Zn.

6. The transistor according toclaim 5, wherein the amorphous oxide further contains at least one of In and Ga.

7. A method for manufacturing a thin film transistor, the thin film transistor including:

a substrate;

a gate electrode provided on the substrate;

a gate insulating film provided on the gate electrode;

a semiconductor layer provided on the gate insulating film, the semiconductor layer being formed of oxide;

a source electrode and a drain electrode provided on the semiconductor layer, the source electrode being opposed to one end of the gate electrode, the drain electrode being opposed to another end of the gate electrode, the another end being opposite to the one end, the drain electrode being apart from the source electrode; and

a channel protecting layer provided between the source and drain electrodes and the semiconductor layer,

the method comprising:

forming the gate electrode on the substrate;

forming the gate insulating film on the gate electrode;

forming the semiconductor layer on the gate insulating film;

forming the channel protecting layer covering at least a part of a side face of the semiconductor layer above the gate electrode;

performing a heat treatment on the semiconductor layer and the channel protecting layer at a temperature not less than 160° C.; and

forming the source electrode and the drain electrode on the semiconductor layer and the channel protecting layer after the performing the heat treatment.

8. The method according toclaim 7, wherein the channel protecting layer is formed covering at least the part of the side face of the semiconductor layer between a first extension line of a first side of a first region and a second extension line of a second side of a second region, the source electrode being to be formed in the first region, the drain electrode being to be formed in the second region, the first side being opposed to the second region, the second region being opposed to the first region.

9. A display device comprising:

a thin film transistor including:

an insulating layer;

a gate electrode provided on the insulating layer;

a gate insulating film provided on the gate electrode;

a semiconductor layer provided on the gate electrode, the semiconductor layer being formed of oxide;

a source electrode and a drain electrode provided on the semiconductor layer, the source electrode being opposed to one end of the gate electrode, the drain electrode being opposed to another end of the gate electrode, the another end being opposite to the one end, the drain electrode being apart from the source electrode; and

a channel protecting layer provided between the source and drain electrodes and the semiconductor layer, the channel protecting layer covering at least a part of a side face of a part of the semiconductor layer, the part of the semiconductor layer being not covered with the source electrode and the drain electrode above the gate electrode;

a pixel electrode connected to one of the source electrode and the drain electrode, the pixel electrode being formed of the oxide and having a lower electric resistance than the semiconductor layer; and

an optical element producing at least one of a change in optical characteristics and light emitting by an electric signal provided to the pixel electrode.

10. The device according toclaim 9, wherein the pixel electrode is not covered with the channel protecting layer.

11. The device according toclaim 9, wherein a concentration of oxygen contained in the pixel electrode is lower than a concentration of oxygen contained in the semiconductor layer.

12. The device according toclaim 9, wherein the oxide in the pixel electrode has a columnar structure.

13. The device according toclaim 9, further comprising

a film provided under the pixel electrode and formed of the same material as the gate insulating film,

a surface of the gate insulating film on a side of the semiconductor layer being smoother than a surface of the film on a side of the pixel electrode.

14. The device according toclaim 13, wherein surface roughness of the surface of the film on the side of the pixel electrode is not less than five nanometers and not greater than ten nanometers.

15. The device according toclaim 14, wherein the surface roughness of the surface of the gate insulating film on a side of the semiconductor layer is not less than 10.1 nanometers and not greater than one nanometer.

16. The device according toclaim 9, further comprising:

a plurality of the thin film transistors and a plurality of the pixel electrodes, each of the plurality of the pixel electrodes being connected to one of the source electrode and the drain electrode of each of the plurality of the thin film transistors, the plurality of the thin film transistors and the plurality of the pixel electrodes being arranged in a matrix form;

a plurality of scanning lines, each of the plurality of the scanning lines being connected to each of the gate electrodes of each of the plurality of the thin film transistors; and

a plurality of signal lines, each of the plurality of the signal lines being connected to another of the source electrode and the drain electrode of each of the plurality of the thin film transistors.

17. A method for manufacturing a display device,

the display device including:

a thin film transistor including:

a substrate;

a gate electrode provide on the substrate;

a gate insulating film provide on the gate electrode;

a semiconductor layer provided on the gate insulating film, the semiconductor layer being formed of oxide;

a source electrode and a drain electrode provided on the semiconductor layer, the source electrode being opposed to one end of the gate electrode, the drain electrode being opposed to another end of the gate electrode, the another end being opposite to the one end, the drain electrode being apart from the source electrode ; and

a channel protecting layer provided between the source and drain electrodes and the semiconductor layer;

a pixel electrode connected to one of the source electrode and the drain electrode; and

an optical element producing at least one of a change in optical characteristics and light emitting by an electric signal provided to the pixel electrode,

the method comprising:

forming the gate electrode on the substrate;

forming the gate insulating film on the gate electrode;

forming a layer of the oxide on the gate insulating film;

forming the channel protecting layer covering at least a part of a side face of the layer of the oxide above the gate electrode and exposing the layer of the oxide in a third region, the pixel electrode being to be formed in the third region;

performing a heat treatment on the layer of the oxide and the channel protecting layer at a temperature not less than 160° C. to decrease an electric resistance of the layer of the oxide not covered with the channel protecting layer to form the pixel electrode; and

forming the source electrode and the drain electrode on the semiconductor layer and the channel protecting layer after the performing the heat treatment.

18. The method according toclaim 17, further comprising:

performing a surface processing selectively changing smoothness of a surface of the gate insulating film before the forming the layer of the oxide.

19. The method according toclaim 18, wherein the performing the surface processing includes making the surface of the gate insulating film in the third region rougher than a surface of the gate insulating film in a region except the third region.

20. A method for manufacturing a display device,

the display device including:

a thin film transistor including:

a substrate;

a gate electrode provided on the substrate;

a gate insulating film provided on the gate electrode;

a semiconductor layer provided on the gate insulating film, the semiconductor layer being formed of oxide;

a source electrode and a drain electrode provided on the semiconductor layer, the source electrode being opposed to one end of the gate electrode, the drain electrode being opposed to another end of the gate electrode, the another end being opposite to the one end, the drain electrode being apart from the source electrode; and

a channel protecting layer provided between the source and drain electrodes and the semiconductor layer;

a pixel electrode connected to one of the source electrode and the drain electrode; and

an optical element producing at least one of a change in optical characteristics and light emitting by an electric signal provided to the pixel electrode,

the method comprising:

forming the gate electrode on the substrate;

forming the gate insulating film on the gate electrode;

performing surface processing selectively changing smoothness of a surface of the gate insulating film; and

forming a layer of the oxide on the gate insulating film.

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