JP5334236B2 - Field effect transistor - Google Patents

Description

  The present invention relates to a field effect transistor and an organic electroluminescence element having the field effect transistor.

  Currently, the most common field effect transistor (hereinafter sometimes referred to as “FET”) has a source electrode and a drain electrode formed on a substrate made of an inorganic material such as Si, Ga, As, and the like. And a gate insulating layer, and a gate electrode is formed on the gate insulating layer. By applying a voltage to the gate electrode, a current flows from the source electrode to the drain electrode through part of the inorganic material substrate (channel layer).

  In recent years, an organic FET using an organic semiconductor as the channel layer has attracted attention. Advantages of this organic semiconductor channel layer are (1) that it can be formed by printing that is easy to mass-produce at low cost in addition to the vapor deposition process used in conventional inorganic material substrates, and (2) it can be formed at low temperature. It is possible to use a heat-sensitive organic material substrate, and (3) since the organic semiconductor itself is flexible, a flexible, high impact resistance and lightweight FET can be formed. In particular, an organic FET having the characteristics of being lightweight and flexible is extremely preferable as an electronic component such as an RF tag, electronic paper, an organic EL, and a liquid crystal display device.

  On the other hand, since the organic semiconductor material easily deteriorates due to the presence of oxygen and moisture, it has a drawback that the lifetime of the FET is shortened. In recent years, organic material substrates have come to be used from the viewpoint of weight reduction. However, since the organic material substrate has a lower gas barrier property than that of an inorganic material substrate, moisture and gas can easily enter the transistor. There is a problem that semiconductor deterioration progresses.

Therefore, a technique for preventing the ingress of moisture and gas by the gas barrier layer has been studied. For example, Patent Document 1 discloses a technique for protecting an organic semiconductor from water or gas by providing a gas barrier layer on an organic material substrate or an organic semiconductor layer made of polyimide or polyethylene terephthalate.
JP 2006-93332 A

  As described above, the substrate and the channel layer are formed of an organic material, and a technology for protecting an organic semiconductor that is sensitive to water or gas by forming a gas barrier layer on the substrate or the channel layer is known. It was done. However, such prior art cannot always suppress the deterioration of the organic semiconductor.

  For example, in the organic semiconductor device described in Patent Document 1, polyimide or polyethylene phthalate is used as a substrate material, but these materials have poor gas barrier properties. In addition, the inorganic material gas barrier layer on the organic substrate is formed by sputtering or the like, and defects such as pinholes tend to occur at that time, and the gas barrier layer made of inorganic material may be cracked by bending. is there. Therefore, it is not possible to completely prevent the intrusion of water or gas simply by providing a gas barrier layer on the substrate made of these organic materials. As a result, deterioration of the organic semiconductor is promoted particularly when the organic FET is applied to an electronic device that is always driven, such as a display.

  In addition, a film made of polyethylene terephthalate has low heat resistance. Moreover, the thermal expansion coefficient of a general organic material is much larger than that of a metal or an inorganic material. As a result, when receiving a thermal history, a problem arises in dimensional stability or warpage occurs.

  Therefore, the problem to be solved by the present invention is to provide a field effect transistor having high dimensional stability while exhibiting sufficient gas barrier properties while having flexibility. Another object of the present invention is to provide an organic electroluminescence element including the field effect transistor.

  In order to solve the above problems, the present inventor searched for a material having high gas barrier properties and high dimensional stability as a substrate material. As a result, the present invention was completed by finding that the liquid crystal polymer is excellent as a substrate material for a field effect transistor since it exhibits extremely high gas barrier properties and dimensional stability.

  The field effect transistor of the present invention is characterized by having at least a source electrode, a drain electrode, a gate electrode, a gate insulating layer, and an organic semiconductor layer on a substrate, and the substrate is made of a liquid crystal polymer film.

  Moreover, the organic electroluminescent element of this invention is characterized by including the field effect transistor of this invention.

  Since the field effect transistor of the present invention uses a liquid crystal polymer as a substrate material and has a sufficient gas barrier property while being flexible, the organic semiconductor can be protected from moisture and gas and has a long life. Moreover, since it has high dimensional stability, there is little deformation even if it receives a thermal history. Therefore, the field effect transistor of the present invention can be applied to an organic electroluminescence element that can be used for an organic EL display device having a large screen. Therefore, the present invention is extremely useful industrially as an electronic device component.

  The field effect transistor (FET) of the present invention has at least a source electrode, a drain electrode, a gate electrode, a gate insulating layer, and an organic semiconductor layer on a substrate, and the substrate is made of a liquid crystal polymer film. .

  The liquid crystal polymer that is the substrate material for the FET according to the present invention is a heat-resistant thermoplastic resin, and includes a thermotropic liquid crystal polymer that exhibits liquid crystal properties in a molten state and a lyotropic liquid crystal polymer that exhibits liquid crystal properties in a solution state. The liquid crystal polymer used in the present invention is preferably a thermotropic liquid crystal polymer, more specifically, a thermotropic liquid crystal polyester or a thermotropic liquid crystal polyester amide.

  Thermotropic liquid crystal polyester (hereinafter simply referred to as “liquid crystal polyester”) is, for example, an aromatic polyester synthesized mainly from aromatic dicarboxylic acid and monomers such as aromatic diol and aromatic hydroxycarboxylic acid. Sometimes it exhibits liquid crystallinity. Typical examples include type I [Formula (1)] synthesized from parahydroxybenzoic acid (PHB), terephthalic acid and 4,4′-biphenol, synthesized from PHB and 2,6-hydroxynaphthoic acid. Type II [Formula (2)] and type III [Formula (3)] synthesized from PHB, terephthalic acid and ethylene glycol.

  As the liquid crystal polymer resin according to the present invention, as long as it exhibits liquid crystallinity (particularly thermotropic liquid crystallinity), for example, the unit represented by the above formulas (1) to (3) is mainly used (for example, the entire configuration of the liquid crystal polymer). It may be a copolymer type polymer having 50 mol% or more in the unit) and also having other units. Examples of the other unit include a unit having an ether bond, a unit having an imide bond, and a unit having an amide bond. Examples of the film made of liquid crystalline polyester that is particularly suitable in the present invention include “BIAC (registered trademark)” manufactured by Japan Gore-Tex.

  Examples of the liquid crystal polyester amide include the above liquid crystal polyester having an amide bond as another unit, and examples thereof include those having a structure of the following formula (4). For example, in the formula (4), the molar ratio of the unit of s, the unit of t, and the unit of u is 70/15/15.

  As the liquid crystal polymer as the substrate material, a polymer alloy containing the above liquid crystal polymer may be used. In this case, the alloy polymer mixed or chemically bonded to the liquid crystal polymer includes a polymer having a melting point of 220 ° C. or higher, preferably 280 to 360 ° C. Examples include, but are not limited to, polyetheretherketone, polyethersulfone, polyimide, polyetherimide, polyamide, polyamideimide, and polyarylate. The mixing ratio of the liquid crystal polymer and the alloy polymer is not particularly limited. For example, the mass ratio is preferably 10:90 to 90:10, and more preferably 30:70 to 70:30. A polymer alloy containing a liquid crystal polymer can also have excellent characteristics due to the liquid crystal polymer.

  In the present invention, a film is formed by using the liquid crystal polymer described above as a material to form a substrate. The thickness of the liquid crystal polymer film is preferably about 10 to 2000 μm. If it is too thin, strength and gas barrier properties may be insufficient. On the other hand, if it is too thick, film formation may be difficult. The planar shape and size may be determined in accordance with the liquid crystal display device that is the final product.

  As for the surface roughness of the liquid crystal polymer film, the three-dimensional center plane average roughness (SRa) when measured using a laser microscope is preferably 50 nm or less, more preferably 25 nm or less. Since the thickness of the electrode formed on the film is as extremely thin as about 10 to 100 nm, if the roughness is larger than 50 nm, the film thickness becomes uneven in a fine region, and the field-effect mobility and on / off ratio are reduced. There is a fear. On the other hand, even if the roughness is high, the yield can be maintained by increasing the thickness of the electrode, the semiconductor layer, etc., but this increases the manufacturing cost. However, if the roughness is 50 nm or less, a high-quality FET can be efficiently manufactured even when the thickness of the electrode or the like is appropriate.

  The surface roughness of the liquid crystal polymer film in the present invention refers to the three-dimensional center plane average roughness (SRa). The three-dimensional center plane average roughness can more accurately represent the unevenness in the plane direction.

  The surface roughness can be measured by the following method. For example, using a laser microscope such as OLS3000 manufactured by Olympus, using a semiconductor laser of λ = 408 ± 5 nm as a laser type, setting the moving resolution in the Z direction to 0.01 μm, and moving in the Z direction in the range of 192 μm × 256 μm The roughness is measured in a confocal mode of 50 times at a pitch of 0.1 μm, and the average value is obtained.

  In order to reduce the surface roughness of the liquid crystal polymer film, it is preferable to perform a surface smoothing treatment using a hot press apparatus. For example, a liquid crystal polymer film may be sandwiched between a polymer film or plate having smoothness, heat resistance, and release properties, such as a metal plate or polyimide having a mirror-finished at least one side as a release material, and then heated and pressurized. The temperature at that time is set below the melting point of the liquid crystal polymer, although it is sufficiently softened. The pressure is usually about 1 to 10 MPa, although it depends on the type of liquid crystal polymer and the heating temperature. The kind in particular of a hot press apparatus is not restrict | limited, In addition to a general flat plate press, it can also press continuously between hot rolls. In addition to the above method, known methods such as flattening by polishing and formation of a smooth layer by separately coating a solution of a liquid crystal polymer or other polymer material may be used.

  In the liquid crystal polymer film, the linear expansion coefficient in the direction parallel to the film plane is preferably adjusted to 30 ppm / ° C. or less. More preferably, it is 25 ppm / ° C. or less. The lower limit of the linear expansion coefficient of the liquid crystal polymer film is desirably 5 ppm / ° C. If the liquid crystal polymer film has a linear expansion coefficient in such a range, the difference in the linear expansion coefficient from an electrode made of metal or the like is small, so that the warpage that occurs when receiving a thermal history is reduced. . For example, the linear expansion coefficient of the liquid crystal polymer film is preferably about 5 to 20 ppm / ° C. when the metal layer is in contact, and 20 to 30 ppm / ° C. when it is in contact with the organic layer.

  The linear expansion coefficient of the liquid crystal polymer film can be measured by instrumental analysis (TMA) method. More specifically, for example, the width of the test piece is 4.5 mm, the distance between chucks is 15 mm, the load is 1 g, the temperature rising rate is 5 ° C./min. It is determined from the dimensional change of the test piece measured between 160 ° C. and 25 ° C. when it is cooled at. It is preferable that both the linear expansion coefficients of the liquid crystal polymer film in the MD direction (running direction during film production) and the TD direction (direction orthogonal to the MD direction) satisfy the above range.

  The linear expansion coefficient of the liquid crystal polymer film can be adjusted by controlling the molecular orientation. Moreover, you may adjust by addition of a filler. However, since the filler may adversely affect the surface smoothness of the liquid crystal polymer film, the linear expansion coefficient is preferably adjusted according to the stretching conditions.

  When an FET is applied to an electronic device that is forced to be constantly driven, such as a display, the substrate is required to have a higher gas barrier property because the organic semiconductor is easily deteriorated. Therefore, a gas barrier layer may be provided on at least one surface of the substrate of the present invention in order to further enhance the gas barrier property. Examples of the material of the gas barrier layer include Al, Cr, Ni, Cu, Zn, Si, Fe, Ti, Ag, Au, and Co; oxides of these metals; nitrides of these metals; oxynitrides of these metals. One of these can be selected and used, or two or more can be selected and mixed for use.

  The thickness of the gas barrier layer is usually about 5 to 1000 nm. If the thickness is less than 5 nm, gas barrier properties may not be sufficiently exhibited. On the other hand, if the thickness exceeds 1000 nm, problems such as high cost and heavy elements may occur.

  The formation method in particular of a gas barrier layer is not restrict | limited, A general well-known method can be used. For example, various PVD methods such as vacuum deposition, sputtering, and ion plating as dry processes, various CVD methods such as plasma CVD, thermal CVD, and laser CVD, and sol-gel methods, plating methods, coating methods, etc. as wet processes Can be mentioned.

  The gas barrier layer can be formed on one side or both sides of the substrate. When the gas barrier layer is provided on one side, the gas barrier layer is usually formed on the side opposite to the gate electrode layer described later. However, the gas barrier layer may be provided on the same side as the electrode. For example, the metal gas barrier layer formed on the substrate may be used as a ground electrode, or the gas barrier layer may be partially made of metal oxide and the remaining metal portion may be used as a circuit.

  As shown in FIG. 1 showing one embodiment of an FET according to the present invention, a source electrode, a drain electrode, a gate electrode, a gate insulating layer, and an organic semiconductor layer are provided on the substrate.

  Each electrode can be formed of a paste containing a metal such as Al or Cu, or a conductive material such as metal or carbon.

  The electrode formation method is not particularly limited, and a known formation method such as vapor deposition, sputtering, plating, coating, or printing may be used. The thickness of the electrode is not particularly limited, but can generally be about 50 to 1000 μm.

  A gate electrode is formed on the liquid crystal polymer substrate, and the gate electrode is covered with a gate insulating layer in order to insulate the source electrode and the drain electrode.

As the material of the gate insulating layer, those conventionally used for the gate insulating layer of FET can be used. For example, inorganic materials such as SiO ₂ , Si ₃ N ₄ , and Al ₂ O ₃ , and resins having insulating properties such as phenol resins, acrylic resins, epoxy resins, and imide resins can be used. A conventionally known method for forming the gate insulating layer can also be used. For example, it may be formed by a method such as coating, sputtering, sol-gel method, or printing.

  The thickness of the gate insulating layer is not particularly limited, but can generally be about 10 nm to 10 μm. Within this range, a short circuit between the electrodes can be surely prevented, while the gate insulating layer does not become excessively thick and no extra cost is incurred.

  On the gate insulating layer, an organic semiconductor layer serving as a current path between the source electrode and the drain electrode, that is, a channel layer is formed.

  As the material of the organic semiconductor layer, those conventionally used for the organic semiconductor layer of the FET can be used. For example, a low molecular organic semiconductor material such as pentacene, tetracene, anthracene, or phthalocyanine, or a high molecular semiconductor material such as polythiophene, polyacene, polyacetylene, or polyaniline can be used. Conventionally known methods can be used for forming the organic semiconductor layer. For example, it may be formed by a method such as vapor deposition, coating, sputtering, sol-gel method, or printing.

  The thickness of the organic semiconductor layer is not particularly limited, but can generally be about 10 nm or more and 500 μm or less. If it is this range, while being able to prevent a short circuit between electrodes reliably, an organic-semiconductor layer does not become excessively thick and extra cost does not start.

  A source electrode and a drain electrode are formed on the organic semiconductor layer. These electrodes can be formed in the same manner as the gate electrode. Alternatively, the source electrode and the drain electrode may be formed directly on the gate insulating film in advance, and then the organic semiconductor layer may be formed.

  Since the FET and the channel layer of the present invention are made of an organic material, they are lightweight and can be made thin, and are flexible. In addition, since a liquid crystal polymer film having excellent gas barrier properties is used as the substrate, the intrusion of moisture and gas into the inside is suppressed, and the dimensional stability is excellent even when subjected to a thermal history. Therefore, the FET of the present invention has a long life because deterioration of the organic semiconductor due to moisture or the like is suppressed, and is suitable as an electronic component such as a large screen organic EL display device.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

Example 1 Manufacture of FET according to the present invention A 20 μm thick polyimide film (Ube Industries, Ltd.) large enough as a release film on the top and bottom of a liquid crystal polymer film (Japan Gore-Tex, BIAC BC125) of 100 mm long × 100 mm wide × 125 μm thick And smoothing treatment was performed by heating and pressurizing at 290 ° C. and 3 MPa for 5 minutes using a vacuum hot press apparatus. The roughness (Ra) of the obtained surface smoothed liquid crystal polymer film was measured with a laser microscope (OLS3000, manufactured by Olympus). Specifically, a semiconductor laser of λ = 408 ± 5 nm is used as the laser type, the moving resolution in the Z direction is set to 0.01 μm, and the roughness in the Z direction in the range of 192 μm × 256 μm is set at a pitch of 0.1 μm. Measurements were made in a 50 × confocal mode. As a result, the roughness (SRa) was 20 nm.

  Moreover, the linear expansion coefficient of the liquid crystal polymer film was measured as follows. That is, a liquid crystal polymer film was cut into a width of 4.5 mm, fixed to an apparatus (TA Instruments, TMA2940) with a distance between chucks: 15 mm, a load: 1 g, a heating rate: 5 ° C./min. When the temperature was lowered to a temperature lowering rate of 5 ° C./min, the dimensional change of the test piece measured between 160 ° C. and 25 ° C. was measured. As a result, the linear expansion coefficient in the TD direction was 16 ppm / ° C., and the linear expansion coefficient in the MD direction was 16 ppm / ° C.

The surface smoothed liquid crystal polymer film was cut into 32 mm × 25 mm. On one side, an aluminum thin film layer having a vertical width of 30 μm × horizontal length of 25 mm × thickness of 0.2 μm was formed by vacuum deposition using a metal mask, and this was used as a gate electrode. A SiO ₂ thin film having a thickness of 100 nm was formed on the gate electrode by sputtering to form a gate insulating layer. Further, an organic semiconductor layer made of pentacene and having a thickness of 100 nm was formed on the gate insulating layer by a vacuum deposition method. A source electrode and a drain electrode made of gold having a channel width of 10 mm and a channel length of 30 μm were formed on the organic semiconductor layer using a sputtering apparatus. A sealing layer made of SiO ₂ and having a thickness of 100 nm was formed on the organic semiconductor layer on which the source electrode and the drain electrode were formed, using a sputtering apparatus.

Comparative Example 1
An FET was manufactured in the same manner as in Example 1 except that a 125 μm-thick polyimide film (Ube Industries, Upilex 125S) was used instead of the liquid crystal polymer film as the organic material substrate.

Test Example 1 Measurement of moisture permeability The moisture permeability of each organic EL element was determined using O ₂ gas as a gas according to the isobaric method (MOCON method) defined in JIS K7129B. The results are shown in Table 1. In Table 1, “LCP” indicates a liquid crystal polymer, and “PI” indicates polyimide.

Test Example 2 Measurement of Field Effect Mobility The FETs manufactured in Example 1 and Comparative Example 1 were allowed to stand at room temperature and normal pressure, a predetermined gate voltage was applied after 2 hours and 720 hours from the preparation, The field effect mobility (μ) of the FET was determined. The results are shown in Table 1.
μ = 2LI _ds / WC _i (V _g −V _th ) ²
[In the formula, L is the channel length, I _ds is the drain current value in the saturation region, W is the channel width, C _i is the capacitance per unit area of the gate insulating film, V _g is the gate voltage, and V _th is the threshold current. ]

  As shown in Table 1, the gas barrier property of the substrate made of polyimide was low, and the electric field mobility of the FET using the substrate decreased with the passage of time. On the other hand, since the gas barrier property of the liquid crystal polymer film is high, the FET having the liquid crystal polymer film as a substrate is excellent in durability, and the performance after being left in the air for 720 hours after production was hardly deteriorated. Therefore, the excellent characteristics of the FET according to the present invention were demonstrated.

It is a schematic diagram which shows one aspect | mode of the field effect transistor of this invention.

Explanation of symbols

  1: liquid crystal polymer substrate, 2: gate electrode, 3: gate insulating layer, 4: organic semiconductor layer, 5: source electrode, 6: drain electrode

Claims (3)

On the substrate, at least a source electrode, a drain electrode, a gate electrode, a gate insulating layer, and an organic semiconductor layer,
The substrate comprises a liquid crystal polymer film of type I thermotropic liquid crystal polyester represented by the following formula (1) or type II thermotropic liquid crystal polyester represented by the following formula (2) :

The surface roughness of the side forming the electrodes the liquid crystal polymer film, a three-dimensional center plane average roughness by surface smoothing process using a hot press device (SRa) with Ri Contact becomes 25nm or less, the film plane A field effect transistor characterized by having a linear expansion coefficient in a direction parallel to 1 to 5 ppm / ° C. to 30 ppm / ° C.   The field effect transistor according to claim 1, further comprising a gas barrier layer on at least one surface of the liquid crystal polymer film.   An organic electroluminescence device comprising the field effect transistor according to claim 1.

Images