JP2008235581A - Organic thin film transistor and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin film transistor protective film for improved durability of a thin film transistor in which an organic material is used for a semiconductor active layer, as well as its manufacturing method. <P>SOLUTION: The thin film transistor comprises a substrate, a gate electrode, an insulating layer, a source electrode, a drain electrode, an organic semiconductor layer, and a protective layer. The protective layer is formed of at least one layer of thin film containing smectite sheet silicate compound being excellent in barrier characteristics. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a thin film transistor, and particularly relates to a technique for improving a thin film transistor (TFT) using an organic material for a semiconductor layer.

  Organic thin-film transistors, whose active layers are thin-film transistors composed of organic semiconductor materials, have the advantage that they are suitable for production on flexible substrates and can be applied to highly productive manufacturing processes such as printing methods. Therefore, it is said that the compatibility with the integrated circuit technology of an electronic device that is used in a portable environment and is supplied at a low price, such as a portable display, an electronic tag such as an electronic price tag and an electronic tag, etc. Has gathered expectations.

  In order to develop an organic thin film transistor that exhibits such performance, an element constituent material that is formed by coating from a solution such as a printing process and that has flexibility and high impact resistance is required. Today, various organic semiconductor materials have been developed as materials for semiconductor active layers that satisfy the above requirements. However, materials exhibiting excellent semiconductor characteristics have a problem that they are vulnerable to the surrounding environment such as oxygen or water. ing. For this reason, it is necessary for the thin film transistor to be provided with a protective film for enhancing environmental resistance. However, when a transistor is manufactured on a flexible substrate by a coating process, a protective film is also required to be manufactured by a coating process, but a material that can provide high protective film characteristics without damaging the semiconductor layer constituting the active layer. In addition, the manufacturing method is difficult to obtain, and there is a problem that the protective film does not perform its function sufficiently.

  As a protective film for a device using an organic semiconductor material, a protective film for an organic EL device has been actively developed. A typical example is a technique in which a thin film of silicon nitride or silicon dioxide is formed on an organic EL element by a CVD method or the like (see “Patent Document 1” below). In particular, as a technique for producing a protective film with the aim of improving productivity in device production and introducing a solution process, a method using a thin film made of an organic polymer material such as polyimide by spin coating is known. . In particular, for the purpose of enhancing the compatibility with the element on the flexible substrate, as a protective film that increases the bending stress resistance, an inorganic material and an organic material are alternately laminated to reduce the stress of the protective film in the element. There is a proposal of a technique to be performed (see “Patent Document 2” below). Among the protective film functions, in particular, as a technique for improving moisture permeation resistance, there is a report of a method of dispersing phosphorus pentoxide, silica gel or the like as a moisture adsorbent in a protective layer (see “Patent Document 3” below). ).

  The above-mentioned development technology example is a technology for improving the durability of the organic EL device among the devices using the organic semiconductor material. However, when the structure of the device is different, the technology for forming the protective layer is also different. . In the case of an organic EL element, since the protective layer is mainly formed on the electrode after the element is manufactured, compatibility with the electrode surface is a major factor in manufacturing the protective layer. On the other hand, in the case of a thin film transistor, in an element structure called a bottom contact or top contact that is frequently used today, the outermost layer of the element has a structure in which the source and drain electrodes and the semiconductor layer are exposed. For this reason, compatibility with these constituent materials is important for forming the protective layer. In particular, since the semiconductor layer is exposed, protection of the semiconductor from moisture and oxygen is important for improving the durability of the device.

  As a development of a protective film for improving the performance of an organic thin film transistor, there is a report of a technique for forming a protective film by laminating a silicon oxide film, alumina, silicon nitride, epoxy resin or the like (see “Patent Document 4” below). . In addition, there is a report of a method of adapting a multi-layered film of a water-repellent fluorine-based polymer material and an inorganic oxide for the purpose of improving the protection performance. In this case, there is a report that the off-state current of the transistor performance can be reduced (see “Patent Document 5” below). As an example of a protective film in which a solution process is introduced with the aim of improving productivity, there is a report of a method of coating a thin film of polyvinyl alcohol or a fluorine-based hydrocarbon compound (see “Patent Document 6” below). In this case, it has been a problem to provide sufficient process resistance to the semiconductor layer.

As mentioned above, thin film transistors using organic semiconductors in the active layer are characterized by superiority in molding, processability, and productivity. However, in order to use them with high durability, they have excellent functions. It is necessary to coat a protective layer having When a protective film is formed on a thin film transistor, it is important to form the protective layer on two types of materials, ie, a semiconductor layer and an electrode. It has become very difficult. Furthermore, it is extremely difficult to provide such a protective film with a sufficient barrier property, and there is a problem that the thin film transistor cannot be provided with sufficient durability.
JP-A-2005-166400 JP 2003-187959 A Japanese Patent Laid-Open No. 11-329719 JP 2005-191077 A JP 2006-326661 A JP 2006-41317 A

  The present invention provides a thin film transistor protective film for improving durability of a thin film transistor using an organic material for a semiconductor active layer, and a method for manufacturing the same.

  In order to suppress the deterioration of the organic thin film transistor, the present inventors can protect the organic thin film transistor as long as a material having excellent barrier properties can be formed on the organic thin film transistor even if it is difficult to reduce the film thickness. Under the expectation that the effect as a layer can be obtained, a technique for forming layers of materials having excellent barrier properties on an organic thin film transistor has been studied. As a result, the present invention has been achieved.

  That is, according to the present invention, a thin film transistor having a substrate, a gate electrode, an insulating layer, a source electrode, a drain electrode, an organic semiconductor layer, and a protective layer, wherein the protective layer has at least one layer of smectite group layered silicate A thin film transistor characterized by being formed of a thin film containing a salt compound is provided.

  Further, according to the present invention, the thin film forming the protective layer is composed of a composite in which the smectite layered silicate compound is dispersed in the matrix, and the thin film is formed from a solution state such as coating, coating, printing, and the like as a raw material. There is provided a method for producing an organic thin film transistor, characterized by being coated by a liquid phase process.

  Since the thin film transistor of the present invention is excellent in gas barrier properties and moisture permeation resistance against oxygen or the like, the durability of the device is improved. Moreover, since it can be coated on a plastic substrate, it is easy to manufacture, and can be made into a film element, a large-area element, a flexible element, and has high impact resistance.

The present invention will be described in detail below.
Referring to a typical example of the present invention, in a thin film transistor having a gate electrode 20, a dielectric layer 30, a source or drain 40, a semiconductor layer 50, and a protective layer 60 on a substrate 10 as shown in FIG. A thin film transistor is provided in which the layer 60 is formed of a thin film containing a smectite group layered silicate compound having an excellent barrier property.

  In FIG. 1, the stacking order of the source or drain 40 and the semiconductor layer 50 is reversed, and the semiconductor layer 50, the source or drain 40, and the protective layer 60 are formed on the dielectric layer 30. You may take the structure.

  The protective layer 60 used in the thin film transistor of the present invention may be formed as a single layer thin film as long as a thin film containing at least one smectite group layered silicate compound is used, or different thin films are laminated. It may be formed of a multilayer thin film. When formed with a multilayer thin film, the thin film containing the smectite group layered silicate compound may be disposed at any position in the layer to be laminated. More preferably, it is used when the outermost layer is formed at a position close to the outer layer. Further, a plurality of layers of the multilayer may be formed of a thin film containing a smectite group layered silicate compound.

（式中のＡは、ナトリウム、カルシウム又はリチウムを示し、それらが単一として構成しているか又は混在して構成しているものを示す。Ｂは、アルミニウム、マグネシウム、リチウム、鉄、亜鉛、銅、ニッケル若しくはクロムから選択される金属又はこれらの内の２種の混合金属を示す。Ｓｉは、ケイ素又はアルミニウムが混在したケイ素を示す。ＯＨは、水酸基を示す。ｍは、２又は３である。ｎは、任意の正の有理数である。）
これらの代表的な化合物としては、イオナイト、サポナイト、ヘクトライト、ソーコナイト、スチーブンサイト、スインホルダイト、モンモリロナイト、バイデライト、ノントロナイト、ボルコンスコアイト等が挙げられるが、これに限定されるものではない。 The smectite group layered silicate compound used in the present invention is composed of those represented by the following general formula.

(In the formula, A represents sodium, calcium, or lithium, and indicates that they are configured as a single or mixed. B is aluminum, magnesium, lithium, iron, zinc, copper Represents a metal selected from nickel or chromium or a mixed metal of two of them, Si represents silicon or silicon mixed with aluminum, OH represents a hydroxyl group, and m represents 2 or 3. N is any positive rational number.)
Examples of these representative compounds include, but are not limited to, ionite, saponite, hectorite, soconite, stevensite, swinholderite, montmorillonite, beidellite, nontronite, and bolcon score. .

The protective layer 60 of the thin film transistor used in the present invention is composed of a composite containing at least one smectite group layered silicate compound, but the matrix material used in this case is not particularly limited. An organic polymer can also be used, and an inorganic material can also be used. In general, organic polymer materials suitably used include polyvinyl alcohol, polymethyl methacrylate, polyimide, parylene, polyvinyl phenol, polyvinyl chloride, polyparaxylene, polypropylene, polyacrylonitrile, polycarbonate, polystyrene, polyethylene, polyethylene terephthalate, ethylene Examples include vinyl alcohol copolymers, fluorinated polymers, amorphous polyamides, photosensitive acrylic resins and epoxy resins, derivatives thereof, and blended materials thereof. Furthermore, commercially available acrylic UV curing resins (NSC2020; (Nippon Seika Co., Ltd.), Desolite (registered trademark) “Z7501; (manufactured by JSR), UVX2540; (manufactured by Toagosei Co., Ltd.)), fluorine polymer Cytop (registered trademark), manufactured by Asahi Glass Co., Ltd., etc. In addition, as inorganic materials, oxides such as SiO ₂ , nitrides such as SiN, and hydrocarbons in the SiO ₂ skeleton are contained. Glass resin, silicon oxynitride, glassy carbon and the like are used, but are not limited thereto.

  The thin film transistor protective layer 60 used in the present invention is composed of a composite containing at least one smectite group layered silicate compound, but the concentration of the smectite group layered silicate compound with respect to the matrix substance is not particularly limited. . The concentration generally used is 2 wt.% To 100 wt.%, But it is desirable that the concentration be as high as possible within the range where high film quality can be obtained. In the case where a thin film can be formed without the presence of a matrix substance, the constituent material of the thin film is a smectite group layered silicate compound of 100 wt.%, And the matrix substance may not be used.

  The thin film transistor protective layer 60 used in the present invention is composed of a composite containing at least one smectite group layered silicate compound. In this case, the method of mixing the smectite group layered silicate compound in the matrix material is particularly preferable. It is not limited. As a method suitably used, a method in which the interlayer of the smectite layered silicate compound is modified with an organic material such as a quaternary amine to increase the solubility in a solvent (for example, JP-A-7-187656). reference). Dispersion takes place either by dissolving the matrix material directly in the solution or by mixing with another solution in which the matrix material is dissolved. As a dissolution method, a method such as physical stirring, dispersion by ultrasonic waves, and heating under heating is used as necessary.

  The thin film transistor protective layer 60 used in the present invention is composed of a composite containing at least one smectite group layered silicate compound. In this case, a thin film containing a smectite group layered silicate compound is formed in a solution state. A liquid phase process as a raw material is used, but this liquid phase process is not particularly limited. Generally, methods that are preferably used are coating methods such as spin coating, dip coating, bar coating, spray coating, etc., but when printing is applied only to a specific site, printing such as screen printing, gravure printing, flexographic printing, etc. A method, an inkjet method, a drop cast method, or the like can also be used. The solvent used at this time is not particularly limited, and any solvent may be used as long as it does not damage the underlayer. It can be appropriately selected depending on the type of the base.

  The thickness of the thin film transistor protective layer 60 used in the present invention is not particularly limited. Generally used thickness is 10 nm or more and 5 μm or less, but preferably 20 nm or more and 2 μm or less.

In the thin film transistor according to the present invention, an organic semiconductor material is used for the semiconductor layer 50. The composition is not particularly limited, and may be composed of a single substance, or may be composed of a mixture of a plurality of substances. Furthermore, it can also be constituted by a layered structure of several substances. The following are known as organic semiconductor materials exhibiting excellent characteristics so far.
Anthracene, tetracene, pentacene or derivatives thereof substituted at the terminal. α-sexual thiophene. Perylenetetracarboxylic dianhydride (PTCDA) or a derivative substituted at its terminal. Naphthalenetetracarboxylic dianhydride (NTCDA) or a derivative substituted at its terminal. Copper phthalocyanine or a derivative whose terminal is substituted with fluorine or the like. A derivative in which copper of copper phthalocyanine is substituted with nickel, titanium oxide, fluorinated aluminum or the like, or a derivative in which each terminal is substituted with fluorine or the like. Fullerene, rubrene, coronene, anthradithiophene or derivatives with substituted ends thereof. Polymers of polyphenylene vinylene, polythiophene, polyfluorene, polyphenylene, polyacetylene, or derivatives in which the terminal or side chain thereof is substituted.

  The method for manufacturing the semiconductor layer 50 used in the present invention is not particularly limited, and any method may be used. In general, vapor deposition methods such as vacuum evaporation are often used, but from the viewpoint of simple and low cost production, spin coating, dip coating, bar coating, screen printing, gravure printing, flexographic printing, offset printing are used. In addition, a printing method such as ink jet printing, which is prepared by mixing a material with a solvent and applying the solution from a solution is applied. Also. A printing method called soft lithography such as microcontact printing or micromolding can also be applied.

  The substrate 10 used in the present invention is not particularly limited, and any substrate may be used. In general, glass substrates such as quartz are preferably used, but polycarbonate (PC), polyimide (PI), polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polyethersulfone (PES), polyarylate Plastic film substrates such as (PAR) and polyetherketone (PEEK), ceramic films such as green sheets, and flexible film substrates such as metal foil films can also be used. In order to improve gas barrier properties and moisture permeability resistance, it is desirable that a protective film is coated on the film substrate.

  As materials for the electrodes 20 and 40 used in the present invention, metals such as gold, silver, platinum, palladium, aluminum and copper, and compound conductors such as ITO are often used, but are not limited thereto. is not. The manufacturing method is not particularly limited, and any method may be used. A generally used method is plated wiring or the like, but a wet manufacturing process applied or attached from a solution such as gravure printing, screen printing, and ink jet printing is also applicable. In this case, conductive organic materials such as thiophene conductive polymer (PEDOT), polyaniline, and derivatives thereof can be used in addition to silver paste, gold paste, and carbon paste. It is also possible to apply a dry manufacturing process different from the above, such as a vacuum deposition method or a sputtering method.

The material forming the dielectric layer 30 used in the present invention is not particularly limited as long as it does not melt with the material constituting the semiconductor active layer 50, and any material may be used. Generally, SiO ₂ or the like is preferably used, but a material having a large dielectric constant can also be used in order to obtain a more effective electric field effect. For example, Al ₂ O ₃ , ZrO ₂ , Ta ₂ O _5, La ₂ O _{3 and the} like can be mentioned, but are not limited thereto. In order to impart flexibility of the element, polymer dielectrics such as polymethyl methacrylate, polyimide, polystyrene, polyparaxylene, polyvinylidene fluoride, polyvinylphenol, pullulan, and the like can also be used.

Examples The present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.
An n-type silicon substrate grown with a silicon thermal oxide film of 200 nm as an insulating layer is subjected to ultrasonic cleaning for 15 minutes with a diluted neutral detergent diluted with pure water (Inoue Seieisha: Pure Soft) for 15 minutes. Then, ultrasonic cleaning was performed for 15 minutes in pure water to remove impurities. Furthermore, ultraviolet irradiation cleaning was performed for 20 minutes in an oxygen atmosphere using an ultraviolet-ozone cleaner. On the substrate thus cleaned, gold was vacuum-deposited as a source and drain electrode 40 using a nickel mask so as to have a width of 100 μm and a thickness of 0.05 μm. At this time, the source-drain spacing (channel length) is 20 μm, and the channel width is 5000 μm. Thereafter, a pentacene thin film was formed as the semiconductor active layer 50 by a vacuum deposition method. Pentacene was purified by sublimation purification 5 times. The vacuum deposition conditions were that the substrate was fixed above the deposition boat, the substrate temperature was adjusted to about 30 ° C., and the degree of vacuum was reduced to 2 × 10 ⁻⁶ Torr. Thereafter, vacuum deposition was performed to a thickness of 30 nm at a rate of 2 nm per minute. Further, a protective film 60 was formed thereon. The protective film 60 is formed by spin-coating a 2 wt.% Aqueous solution of polyvinyl alcohol (PVA) at 1000 rpm, drying at 80 ° C. for 1 hour, cooling to room temperature in a desiccator, and then adding 2 wt. An aqueous solution containing PVA having the same concentration as that of the functional clay (Ionite; manufactured by Mizusawa Chemical Co., Ltd., SWP; manufactured by Corp Chemical) was spin-coated at 1000 rpm, and the same drying as described above was performed. The device characteristics were evaluated under humidity control. In the device thus manufactured, when a gate bias was applied from the gate electrode, the current flowing between the source and the drain was measured at room temperature under humidity control. As an element for performance comparison, an element in which no protective film was installed and a thin film transistor that was installed using only a PVA thin film that did not contain ionite in the protective film were produced in the same manner as described above.

  FIG. 2 shows changes in the output characteristics of the transistor when placed in an environment at a temperature of 25 ° C. and a humidity of 98%. When no protective film was installed at all, the increase in output current due to the application of the gate voltage was hardly observed after standing for 10 minutes, and the transistor characteristics could not be obtained. On the other hand, when the PVA single film was installed on the thin film transistor, the transistor characteristics were obtained by leaving it in an environment with a humidity of 98% for 10 minutes. However, the operation was not stable, noise increased, leakage current increased, and performance degradation was observed. On the other hand, when the PVA film containing ionite was used as the protective film, the output characteristics hardly changed even after 10 minutes in a 98% humidity environment.

保護膜を設置しない場合には、トランジスタ特性が得られないほどに劣化してしまうために、ゲート電圧を印加してもドレイン電流は増大せず著しく小さくなっている。その変化率は、１００分の１程度までに減少した。ＰＶＡ保護膜を設置した薄膜トランジスタに関しては、トランジスタ特性は得られ、ゲート電圧印加によりドレイン電流の増幅は得られるが、漏洩電流が著しく増加するために、出力ドレイン電流も著しく増加している。初期特性に対して、約４倍もの電流値となった。これらに対して、イオナイトを含有させたPVA保護膜を設置した薄膜トランジスタでは、高湿処理を施してもその特性がドレイン電流はほとんど変化しなかった。 Table 1 below shows the rate of change of the drain current value with respect to the initial characteristics when a gate voltage of 20 V and a drain voltage of 15 V are applied.

If no protective film is provided, the transistor characteristics deteriorate so that the transistor characteristics cannot be obtained. Therefore, even when the gate voltage is applied, the drain current does not increase and becomes extremely small. The rate of change decreased to about 1/100. With respect to a thin film transistor provided with a PVA protective film, transistor characteristics can be obtained, and a drain current can be amplified by applying a gate voltage. However, since a leakage current is remarkably increased, an output drain current is also remarkably increased. The current value was about four times that of the initial characteristics. On the other hand, in the thin film transistor provided with the PVA protective film containing ionite, the drain current hardly changed even when the high humidity treatment was performed.

  The thin film transistor according to the present invention can be produced by a coating process on a plastic substrate with low heat resistance and gives a high durability. Enables large area and flexible elements. As a result, the present invention can be used for mass production of electronic devices such as flexible displays, electronic packing tags, electronic posters, and electronic papers that are highly required for impact resistance, weather resistance, portability, and low cost.

The typical sectional view of an example of the element structure of the thin-film transistor in the present invention. The characteristic comparison of the thin-film transistor produced in Example 1 of this invention. (1) Characteristics of a transistor without a protective film. (2) Characteristics of a thin film transistor provided with a PVA protective film. (3) Characteristics of a thin film transistor provided with a PVA protective film containing ionite as a smectite group silicate compound. Initial transistor output characteristics and transistor output characteristics after high humidity treatment (left for 10 minutes in an environment of humidity 98% and temperature 25 ° C.)

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Substrate 20 Gate electrode 30 Dielectric layer 40 Drain or source electrode 50 Semiconductor active layer 60 Protective layer

Claims (4)

  A thin film transistor comprising a substrate, a gate electrode, an insulating layer, a source electrode, a drain electrode, a semiconductor layer and a protective layer, the semiconductor layer comprising an organic material, and the protective layer comprising at least one smectite group An organic thin film transistor formed by a thin film containing a silicate compound. 2. The organic thin film transistor according to claim 1, wherein the smectite group silicate compound is composed of a material represented by the following general formula.

(In the formula, A represents sodium, calcium, or lithium, and indicates that they are configured as a single or mixed. B is aluminum, magnesium, lithium, iron, zinc, copper Represents a metal selected from nickel or chromium or a mixed metal of two of them, Si represents silicon or silicon mixed with aluminum, OH represents a hydroxyl group, and m represents 2 or 3. N is any positive rational number.)   3. The organic thin film transistor according to claim 1, wherein the smectite group silicate compound forms a layered structure in the thin film forming the thin film. 4. A method for producing an organic thin film transistor, wherein the thin film forming the protective layer according to claim 1 is applied by a liquid phase process.

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