JP2016197621A - Thin film transistor and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin film transistor with an organic semiconductor layer, in which favorable step coverage can be obtained even when a gate insulation film is reduced in thickness by forming an edge of a gate electrode in a tapered shape, during manufacturing processes all of which are performed using printing techniques, and in which leakage currents between the gate electrode and a source electrode and between the gate electrode and a drain electrode are reduced.SOLUTION: A thin film transistor 18 includes, on an insulation substrate 10, a gate electrode 11, a gate insulation layer 12, a source electrode 13, a drain electrode 14, and an organic semiconductor layer 15. In the thin film transistor, a tapered portion 17 is formed from the highest portion of a peripheral portion of the gate electrode toward the inside and the outside of the gate electrode.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a thin film transistor using an organic semiconductor as a channel layer and a manufacturing method thereof.

  At present, a general flat and thin image display device is driven by an active matrix of a thin film transistor using amorphous silicon or polycrystalline silicon as a semiconductor layer.

  On the other hand, in recent years, attempts have been made to use a resin substrate instead of a glass substrate in order to further reduce the thickness and weight of a flat and thin image display device and improve breakage resistance.

  However, the manufacture of the above-described thin film transistor using silicon requires a relatively high temperature thermal process and is generally difficult to form directly on a resin substrate having low heat resistance.

  Therefore, development of a thin film transistor using an organic semiconductor that can be formed at a low temperature has been actively conducted.

  Organic semiconductors have the advantage that they can be patterned by a printing method. Furthermore, for thin film transistors using organic semiconductors, not only the semiconductor layer but also the electrodes and gate insulating layers can be formed by printing by selecting materials that can be formed by a printing method. is there.

  The manufacturing cost of a thin film transistor manufactured using a printing method is expected to be significantly reduced from the manufacturing cost of a thin film transistor using a silicon-based thin film manufactured by vacuum film formation / photolithography.

  The most common electrode material for thin film transistors is silver (see, for example, Non-Patent Documents 1 and 2). In order to further increase the functionality of the flat and thin image display device, it is necessary to further improve the performance of the thin film transistor. In order to increase the function, it is necessary to reduce the gate insulating film in order to further reduce the drive voltage while increasing the ON current. However, due to the thinning of the gate insulating film, when the edge (peripheral edge) of the upper surface of the gate electrode is not tapered as shown in FIG. There is a problem that a defect occurs at a leaked portion 16 at the peripheral edge of the upper surface of the gate electrode 11 that causes an increase in leakage current between the drain electrodes.

Unless defects that cause an increase in leakage current between the gate electrode, the source electrode, and the drain electrode are not suppressed, it is impossible to reduce the thickness of the gate insulating film. As a result, the on-current of the thin film transistor is reduced and the device operates. Therefore, desired characteristics cannot be obtained.
In order to solve this problem, it is known that providing a taper portion on a silver electrode, particularly a gate electrode, is an effective method. For example, a technique for manufacturing a gate electrode by an inkjet method is disclosed (see Patent Document 1).

  However, when the tapered portion of the gate electrode is manufactured using the ink jet method, there is a problem such as liquid dripping (hereinafter referred to as droplet dripping), and it is difficult to manufacture a desired tapered shape. There is a drawback.

JP 2006-303507 A

Proceedings of the National Academy of Sciences of the United States of America, Vol. 15 No. 13, 4976 (2008) Applied Physics Letters 95, 253302 (2009)

  Therefore, in view of the above problems, the present invention provides a step coverage that is good even if the gate insulating film is thinned by making the edge of the gate electrode into a tapered shape in the manufacturing process using printing. It is an object of the present invention to provide a thin film transistor having an organic semiconductor layer in which leakage current between a gate electrode, a source electrode, and a drain electrode is reduced.

  As a means for solving the above problems, the invention described in claim 1 is a thin film transistor comprising a gate electrode, a gate insulating layer, a source electrode and a drain electrode, and an organic semiconductor layer on an insulating substrate. The thin film transistor is characterized in that a tapered portion is formed from the highest portion of the peripheral portion toward the inside and outside of the gate electrode.

  The invention described in claim 2 is the thin film transistor according to claim 1, wherein the thin film transistor is a bottom gate / bottom contact type.

  The invention according to claim 3 is characterized in that the gate electrode is formed using an ink-jet ink containing at least one ultrafine metal material of silver, copper, and gold. Or it is a thin-film transistor of 2.

  The invention described in claim 4 is the thin film transistor according to any one of claims 1 to 2, wherein a main component constituting the gate electrode is silver.

The invention according to claim 5 includes a step of forming a gate electrode on an insulating substrate by a printing method,
Forming a gate insulating film by a printing method;
Forming a source and drain electrode by a printing method;
A step of forming an organic semiconductor layer by a printing method,
The step of forming the gate insulating film by a printing method is a step using an intaglio printing method using an intaglio inking method for inking an intaglio plate using an ink jet device. It is a manufacturing method.

An example will be shown in which the assumed on-current changes by providing a coffee stain-shaped taper portion free from dripping on the gate electrode. FIG. 5 shows the characteristics of the gate voltage Vgs and source / drain current Ids of a transistor in the case where a TFT is manufactured without providing a coffee stain-shaped taper portion without dripping on the gate electrode. FIG. 4 shows the characteristics of the gate voltage Vgs and the source / drain current Ids of a transistor in the case where a TFT having a coffee stain-shaped tapered portion without dripping on the gate electrode is manufactured. The gate voltage is measured from −40V to + 20V, and the source / drain voltage is −15V. The former has poor rise and low on-current characteristics. The latter exhibits good characteristics with good rise and high on-current. This improves the step coverage of the gate insulating film with respect to the gate electrode, suppresses defects that cause leakage current between the gate electrode, the source electrode, and the drain electrode, and enables the gate insulating film to be further thinned. It is shown that.

The schematic sectional drawing showing the cross-section of the thin-film transistor corresponding to Example 1 which shows one Embodiment of this invention. The schematic sectional drawing which showed typically the problem of the step coverage in the gate electrode of the conventional thin-film transistor. Schematic explanatory drawing which showed typically the printing method in the intaglio inking method. 7 is a graph showing an example of Vgs-Ids characteristics of a thin film transistor in the case where the gate electrode according to the present invention is provided with a coffee stain-shaped taper portion without dripping of liquid droplets and thus has good characteristics. The graph which shows an example of the Vgs-Ids characteristic of a thin-film transistor in case the conventional gate electrode does not provide the coffee stain shape taper part without dripping and the characteristic is not good.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the embodiments, the same components are denoted by the same reference numerals, and redundant description among the embodiments is omitted.

  In the present invention, the gate electrode is provided with a coffee stain-shaped taper portion without dripping, and the layer improves the step coverage of the gate insulating film with respect to the gate electrode, and leaks between the gate electrode, the source electrode, and the drain electrode. This is characterized in that it is a layer that can suppress the defects that cause the above and can further reduce the thickness of the gate insulating film. As shown in FIG. 1, the thin film transistor of the present invention has the following configuration.

  A gate electrode 11 on the insulating substrate 10, a gate insulating layer 12 formed on the gate electrode so as to cover the gate electrode, a source electrode 13 and a drain electrode 14 on the gate insulating layer 12, and a source electrode 13 and a drain A bottom gate / bottom contact type thin film transistor including a semiconductor layer 15 connected to an electrode 14. The gate electrode 11 forms a coffee stain-shaped taper portion free from dripping by intaglio inking. The desired shape of the tapered portion can be adjusted by the depth and width of the plate used in the intaglio inking method.

  The gate electrode is provided with a coffee stain-shaped taper that is free from dripping by the intaglio inking method (described later) to improve the step coverage of the gate insulating film with respect to the gate electrode. It is possible to suppress a defect such as a crack that causes the leakage current of the transistor, reduce the thickness of the gate insulating film, and provide a transistor with good characteristics that can obtain a high on-current. As shown in FIG. 3, the intaglio inking method uses ink 2 ejected from the inkjet head 1 to ink 3 in the grooves 8 of the intaglio 4 provided on the plate cylinder 5, and from the grooves 8 of the intaglio 4. This is a printing method in which the coffee stain shape pattern 9 is transferred 6 to the substrate 7. Since the intaglio inking method does not require plate cleaning as compared with a normal printing method, there is an advantage that material utilization efficiency such as ink is remarkably improved.

(Insulated substrate)
As the insulating substrate 10 for forming the thin film transistor 18 of the present invention, a glass substrate or a resin substrate can be used. In the case of a resin substrate, for example, polyimide, polymethyl methacrylate, polyacrylate, polycarbonate, polystyrene, polyethylene sulfide, polyethersulfone (PES), polyolefin, polyethylene terephthalate, polyethylene naphthalate (PEN), cycloolefin polymer, polyethersulfene Triacetyl cellulose, polyvinyl fluoride film, ethylene-tetrafluoroethylene copolymer resin, glass fiber reinforced acrylic resin film, glass fiber reinforced polycarbonate, fluorine resin, cyclic polyolefin resin, and the like can be used. These substrates can be used alone, or a composite substrate in which two or more kinds are laminated can be used.

(Gate electrode)
For the gate electrode 11 of the present invention, inkjet ink 2 containing ultrafine particles of at least one metal of low resistance metal materials such as Ag (silver), Cu (copper), and Au (gold) is inkjet. It can be formed by printing on the base material 7 (intaglio inking method) what is inked into the concave portion (groove portion 8) of the intaglio plate 4 of the plate cylinder 5 using an apparatus, and baking it.

  The ultrafine metal particles may be ultrafine particles prepared using an alloy of these metal materials as a starting material. The above three types of metals only need to contain the total amount as a main component, and may contain other components. In particular, in order to form a low-resistance gate electrode, it is preferable to use an ink-jet ink mainly composed of Ag (silver). In addition, when adding other characteristics in addition to low resistance, Ag (silver) or Ag (silver), Cu (copper), Au (gold), or a combination thereof Other materials may be added.

  The coffee stained tapered portion 17 free from dripping can be formed by an intaglio inking method. Here, the coffee cytein shape without dripping means that the peripheral edge of the upper surface of the rectangular parallelepiped printed material is gently higher than the central portion, and after the peak height has passed, the center and edges of the printed material The shape is lower toward the both sides of the printed portion, and the shape that forms a side surface that reaches the substrate surface on the end side of the printed material (refer to the shape of the gate electrode 11 in FIG. 1). Accordingly, a tapered portion 17 is formed from the highest portion of the peripheral edge of the gate electrode 11 toward the inside and outside of the gate electrode 11. The desired shape of the taper portion 17 can be adjusted by the depth and width of the plate used in the intaglio inking method. A conductive organic material such as PEDOT (polyethylenedioxythiophene) can also be used. The film thickness of the gate electrode 11 is preferably 0.01 μm or more and 1 μm or less. Moreover, as a ratio of the film thickness of the center part (concave part) and end part (convex part) of a coffee stain shape, about 9:10 is preferable.

(Gate insulation layer)
As the gate insulating layer 12 of the present invention, for example, a polymer solution such as polyvinylphenol, polymethyl methacrylate, polyimide, polyvinyl alcohol, parylene, fluororesin, epoxy resin, a solution in which particles such as alumina and silica gel are dispersed, Alternatively, a precursor solution of an inorganic material such as silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, tantalum oxide, yttrium oxide, hafnium oxide, zirconium oxide, titanium oxide, or the like is applied using a spin coating method, a slit die coating method, or the like. It can be formed by coating and baking.

(Source electrode, drain electrode)
The source electrode 13 and the drain electrode 14 of the present invention can be formed by printing a low resistance metal material such as Ag, Cu, Au or the like in the form of ink or paste by a transfer method and baking it. In particular, Ag in an ink form or a paste form is preferable from the viewpoint of low resistance and low cost.

(Organic semiconductor layer)
Examples of the material of the organic semiconductor layer 15 of the present invention include high-molecular organic semiconductor materials such as polythiophene, fluorenebithiophene copolymers, and derivatives thereof, and low molecular weights such as pentacene, tetracene, copper phthalocyanine, and derivatives thereof. Molecular organic semiconductor materials can be used. Moreover, carbon compounds such as carbon nanotubes or fullerenes, semiconductor nanoparticle dispersions, and the like can also be used as the material for the semiconductor layer, but are not limited thereto. These organic semiconductor materials can be dissolved or dispersed in an aromatic solvent such as toluene and used as an ink-like solution or dispersion. You may add additives, such as a suitable dispersing agent and a stabilizer, to a solvent.

  The organic semiconductor layer 15 of the present invention contains a compound that binds to metal ions. For example, a benzotrial type or triazine type compound can be mentioned.

  In the benzotriazole system, benzotriazole represented by the following formula (1) is a basic form. In addition, 1H-benzotriazole-1-methanol (formula (2)), which is an adduct of methanol, or an alkyl group on the triazole side. Examples include those added (formula (3)) and those obtained by adding an alkyl group to the benzene side (formula (4)).

Ｒ：アルキル The basic skeleton of triazine is represented by the formula (5), and examples thereof include 2,4-diamino-6-vinyl-S-triazine represented by the formula (6).

R: alkyl

  As a method for forming the organic semiconductor layer 15, known printing methods such as gravure printing, offset printing, screen printing, and inkjet method can be used. In general, since the organic semiconductor has a low solubility in a solvent, it is desirable to use flexographic printing, transfer printing, an inkjet method, and a dispenser suitable for printing a low viscosity solution.

  Specific examples of the thin film transistor according to the present invention will be described below. Note that the present invention is not limited to each embodiment.

<Example 1>
In Example 1, a thin film transistor element 18 as shown in FIG. 1 is produced.
On a polyethylene naphthalate (PEN) film to be an insulating substrate 10, a gate electrode 11 provided with a coffee stain-shaped taper portion 18 free from dripping was formed by intaglio inking using nano silver ink. The desired shape of the tapered portion 17 was adjusted by the depth and width of the intaglio used in the intaglio inking method. After baking at 180 ° C. for 1 hour, a gate electrode 11 having a thickness of 200 nm was produced. Next, polyvinyl phenol to be the gate insulating layer 12 was formed on the insulating substrate 10 including the gate electrode 11 by spin coating, and baked at 180 ° C. for 1 hour to obtain a gate insulating layer 12 having a thickness of 1 μm. . Subsequently, nano silver ink was formed as a source electrode 13 and a drain electrode 14 on the gate insulating film 12 by using a transfer method. After baking at 180 ° C. for 1 hour, a source electrode 13 and a drain electrode 14 having a thickness of 200 nm were obtained. Further, a self-assembled monolayer was formed on the source electrode 13 and the drain electrode 14 by immersing in a solution of pentafluorothiophenol diluted to 1% by weight with isopropyl alcohol for 30 minutes. Finally, in a solution in which 6,13-bis (triisopropylsilylethynyl) pentacene, which is an organic semiconductor material, is dissolved in tetralin to 2% by weight, the benzotriazole compound is mixed with the semiconductor material (solid content) in a weight ratio of 1 .5: 1, and printing is performed between the source electrode and the drain electrode so as to cover a part of the source electrode 13 and the drain electrode 14 using a relief printing method, and dried at 100 ° C. for 60 minutes to form a film. An organic semiconductor layer 15 having a thickness of 50 nm was formed.
The manufactured thin film transistor 18 had a channel length of 20 μm and a channel width of 180 μm.

  A thin film transistor having at least a gate electrode, a gate insulating layer, a source electrode and a drain electrode, and an organic semiconductor layer on an insulating substrate, wherein the gate electrode is composed of at least one metal, and the edge of the gate electrode is a droplet By providing a taper part with no coffee stain shape, the step coverage of the gate insulating film with respect to the gate electrode is good, and the gate insulating film is further thinned to provide a thin film transistor with good characteristics that can obtain a high on-current. can do. Such a thin film transistor can be used as a switching element such as flexible electronic paper and a pressure sensor.

DESCRIPTION OF SYMBOLS 1 Inkjet head 2 Ink 3 Inking 4 Intaglio 5 Plate cylinder 6 Transfer 7 Base material 8 Groove 9 Coffee stain shape pattern 10 Insulating substrate 11 Gate electrode 12 Gate insulating layer 13 Source electrode 14 Drain electrode 15 Semiconductor layer 16 Possible leak location 17 Tapered part 18 Thin film transistor

Claims (5)

  In a thin film transistor including a gate electrode, a gate insulating layer, a source electrode and a drain electrode, and an organic semiconductor layer on an insulating substrate, tapered portions are formed from the highest part of the peripheral part of the gate electrode toward the inside and outside of the gate electrode. A thin film transistor, wherein   2. The thin film transistor according to claim 1, wherein the thin film transistor is a bottom gate / bottom contact type.   3. The thin film transistor according to claim 1, wherein the gate electrode is formed using an inkjet ink containing at least one kind of ultrafine metal material of silver, copper, and gold.   The thin film transistor according to claim 1, wherein a main component constituting the gate electrode is silver. Forming a gate electrode on an insulating substrate by a printing method;
Forming a gate insulating film by a printing method;
Forming a source and drain electrode by a printing method;
A step of forming an organic semiconductor layer by a printing method,
The step of forming the gate insulating film by a printing method is a step using an intaglio printing method using an intaglio inking method for inking an intaglio plate using an ink jet device. Manufacturing method.

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