Organic thin film transistor and method of manufacturing the same

[0001] This application claims priority to Japanese Patent Application No. 2003-193110 filed Jul. 7, 2003 which is hereby expressly incorporated by reference herein in its entirety.

[0002] 1. Field of the Invention

[0003] The present invention relates to an organic thin film transistor and in particular to a method of controlling the threshold voltage of an organic thin film transistor.

[0004] 2. Related Art

[0005] Thin film transistors (TFTs) have been commercialized as switching elements in active matrix liquid crystal displays and the like, and are fabricated using amorphous or polycrystalline silicon as a semiconductor.

[0006] In recent years, much attention has been placed on organic semiconductor materials as the semiconductor material for TFTs. Organic semiconductors can be easily formed in a thin film using simple techniques such as spin coating and vacuum vapor deposition, and there is also the advantage that the manufacturing process can be carried out at a lower temperature than for conventional TFTs in which amorphous or polycrystalline silicon is used. If the process temperature is reduced, it becomes possible to form a TFT on a plastic substrate that normally has low heat resistance, which has great potential for reducing the weight and cost of displays and also for greater variety in the applications of TFTs due to the flexibility of plastic substrates.

[0007] However, when TFTs have hitherto been developed using organic semiconductor materials, it has been difficult to control the threshold voltage by doping with impurities in the same way as when manufacturing a TFT that uses amorphous or polycrystalline silicon, and this has been an obstacle for commercialization.

[0008] Threshold voltages are described by an article by Jiyoul Lee et al (see APPLIED PHYSICS LETTERS, Vol. 80, 2925-2927 (2002), for example), but a technique for freely controlling threshold voltages is not described.

[0009] It is an object of the present invention to provide an organic thin film transistor whose threshold voltage can be easily controlled, without changing the material forming an organic semiconductor film, by providing a threshold voltage controlling film between the gate insulating film and the organic semiconductor film, and a method of manufacturing the same.

[0010] An organic thin film transistor according to the present invention includes a gate electrode, a gate insulating film, a source electrode, a drain electrode, and an organic semiconductor film, and a threshold voltage controlling film between the gate insulating film and the organic semiconductor film.

[0011] According to the present invention, the threshold voltage controlling film is provided between the gate insulating film and the organic semiconductor film so that it is possible to easily change the threshold voltage without changing the material forming the organic semiconductor film.

[0012] The thickness of the threshold voltage controlling film may be equal to or less than 3 nm.

[0013] By forming the threshold voltage controlling film of a superthin film, when designing the construction of the transistor and during the manufacturing process, it is possible tb carry out handling in approximately the same state as when there is no threshold voltage controlling film, so that there are few, if any, restrictions due to the provision of the threshold voltage controlling film. Also, it is possible to form the threshold voltage controlling film with an extremely small amount of material, which is extremely economical.

[0014] The threshold voltage controlling film may be chemically absorbed by at least one of the gate insulating film and the organic semiconductor film. By carrying out chemisorption, it is possible to form a precise and strong film that is a superthin film and functions extremely effectively.

[0015] The threshold voltage controlling film may be formed from a silane compound. A silane compound is easily chemically absorbed by the surface of an oxide, such as SiO₂ and Al₂O₃ favorably used as the gate insulating film or a surface that has been made hydrophilic by a simple hydrophilic treatment, and so can favorably form a precise, strong superthin film (a monomolecular film). Here, the hydrophilic treatment is a process that forms a hydroxide group (—OH) on the surface.

[0016] The silane compound may include at least one trifluoromethyl group (—CF3). This is effective in shifting the threshold voltage towards the plus (positive) end of the axis.

[0017] The silane compound may include at least one amino group NH₂). This is effective in shifting the threshold voltage towards the minus (negative) end of the axis.

[0018] The organic semiconductor film may be formed from at least one substance selected from a group including organic low molecular weight materials such as pentacene and oligothiophene, organic high molecular weight materials such as polythiophene, metal complexes such as phthalocyanine, fullerene materials such as C₆₀, C₇₀, and metallofullerene, and carbon nanotubes.

[0019] A method of manufacturing an organic thin film transistor including a gate electrode, a gate insulating film, a source electrode, a drain electrode, and an organic semiconductor film according to the present invention comprises a step of forming a threshold voltage controlling film between the gate insulating film and the organic semiconductor film.

[0020] The thickness of the threshold voltage controlling film may be equal to or less than 3 nm.

[0021] The threshold voltage controlling film may be formed from a compound with a functional group capable of being chemically absorbed by at least one of the gate insulating film and the organic semiconductor film.

[0022] A compound forming the threshold voltage controlling film may be a silane compound.

[0023] The silane compound may include at least one trifluoromethyl group (—CF₃). This is effective in shifting the threshold voltage towards the plus (positive) end of the axis.

[0024] The silane compound may include at least one amino group NH₂). This is effective in shifting the threshold voltage towards the minus (negative) end of the axis.

[0025] The organic semiconductor film may be formed from at least one substance selected from a group including low organic molecular weight materials such as pentacene and oligothiophene, organic high molecular weight materials such as polythiophene, metal complexes such as phthalocyanine, fullerene materials such as C₆₀, C₇₀, and metallofullerene, and carbon nanotubes.

[0026] The method of manufacturing may include a step of carrying out a hydrophilic treatment on at least a base surface for the threshold voltage controlling film before the threshold voltage controlling film is formed.

[0033] Preferred embodiments of the present invention will now be described with reference to the drawings.

[0034] Organic Thin Film Transistor

[0035] The construction of an organic thin film transistor according to the present embodiment is described below

[0036] FIG. 1 is a cross-sectional view schematically showing the construction of an organic thin film transistor according to an embodiment of the present invention.

[0037] The organic thin film transistor according to the present invention includes a gate electrode 12, a gate insulating film 14, a source electrode 16, a drain electrode 18, an organic semiconductor film 20, and a threshold voltage controlling film 22 that are provided on a substrate 10, with the threshold voltage controlling film 22 being provided between the gate insulating film 14 and the organic semiconductor film 20. Using this threshold voltage controlling film 22, it is possible to control the threshold voltage (V_th) of the organic thin film transistor without changing the material that composes the organic semiconductor film 20.

[0038] Manufacturing Process

[0039] The manufacturing process of the organic thin film transistor according to the present embodiment is described below.

[0040] FIGS. 2A to 2D are cross-sectional views schematically showing a method of manufacturing the organic thin film transistor according to an embodiment of the present invention.

[0041] There are no particular limitations on the substrate 10, and it is possible to use a P-type or N-type single crystal silicon substrate to which boron (B), phosphorous (P), antimony (Sb), or the like has been added as a dopant, a glass substrate, a quartz substrate, or a plastic substrate of polymethyl methacrylate, polyether sulfone, polycarbonate, or the like. In the present embodiment, a single crystal silicon substrate doped with an impurity is used as the substrate 10, with the doped part forming the gate electrode 12.

[0042] First, as shown in FIG. 2A, the gate insulating film 14 is formed on the substrate 10.

[0043] There are no particular limitations on the method of forming the gate insulating film 14, and the surface of the substrate may be oxidized by a thermal oxidization method to form silicon dioxide (SiO₂) or an insulating film of SiO₂, Al₂O₃, or the like may be formed by sputtering or a vacuum coating method, such as chemical vapor deposition (CVD).

[0044] The thickness of the gate insulating film 14 is 100 to 800 nm, for example.

[0045] Next, as shown in FIG. 2B, the source electrode 16 and the drain electrode 18 are formed.

[0046] There are no particular limitations on the materials of the source electrode 16 and the drain electrode 18, which can be formed using metal electrodes, metal oxide electrodes, and carbon electrodes. For example, when fullerene (C₆₀) is used as the organic semiconductor film 20, platinum (Pt), gold (Au), silver (Ag), copper (Cu), aluminum (Al), indium-tin oxide (ITO) and the like can be favorably used.

[0047] The thickness of the source electrode 16 and the drain electrode 18 is in a range of 50 to 300 nm, for example.

[0048] The source electrode 16 and the drain electrode 18 are formed by the following method. A conductive thin film is formed on the gate insulating film 14 by a vacuum coating method and then a lithographic technique is used to form a predetermined pattern of the source electrode 16 and the drain electrode 18.

[0049] Next, as shown in FIG. 2C, the threshold voltage controlling film 22 is formed.

[0050] The threshold voltage controlling film 22 controls the threshold voltage characteristics of the organic semiconductor film 20 so as to become a desired value, and a silane compound expressed by the general formula R¹(CH₂)_mSiR²_nX_3-n (where m is a natural number and n is 1 or 2) can be used, for example. For a silane compound expressed by this general formula, in the case where “X” is a halogen, an alkoxy group, or the like, the compound can be easily applied to an oxide surface of SiO₂, Al₂O₃, or the like that is favorably used as the gate insulating film 14 by chemisorption to form a precise and strong superthin film (monomolecular film), with the end group R¹ being disposed on the surface of the threshold voltage controlling film 22. The threshold voltage controlling film 22 acts so as to control the threshold voltage of the organic thin film transistor. More specifically, by changing R¹, it is possible to control the threshold voltage characteristics of the organic semiconductor film 20. As examples, hydrogen (—H), methyl group (—CH₃), trifluoromethyl group (—CF₃), amino group (—NH₂), mercapto group (—SH), and the like can be used as R¹.

[0051] There are no particular limitations on the method of manufacturing the threshold voltage controlling film 22, and as examples the threshold voltage controlling film 22 may be formed using a vapor phase method such as CVD or a method, such as spin coating or dipping, that uses a liquid phase.

[0052] Before the threshold voltage controlling film 22 is formed, at least the surface that forms the base (here, the gate insulating film 14) for the threshold voltage controlling film 22 may be subjected to a hydrophilic treatment to facilitate chemisorption of the material forming the threshold voltage controlling film 22 by the base surface. Such hydrophilic treatment can use vacuum UV light with a wavelength of 5 to 200 nm or oxygen (O₂) plasma.

[0053] After the threshold voltage controlling film 22 is formed, rinsing with an alcohol such as ethanol or 2-propanol, or ultrapure water or the like may be carried out as necessary to remove adhering matter that is not required.

[0054] So long as the desired transistor characteristics are obtained, the threshold voltage controlling film 22 may be formed only in a partial region between the gate insulating film 14 and the organic semiconductor film 20 and does not need to be formed across the entire region. Also, so long as the obtained transistor characteristics are not problematic, the threshold voltage controlling film 22 may be formed at places aside from the region between the gate insulating film 14 and the organic semiconductor film 20, such as on the source electrode 16 and on the drain electrode 18.

[0055] Next, as shown in FIG. 2D, the organic semiconductor film 20 is formed on the substrate.

[0056] At least one material selected from a group including organic low molecular weight materials such as pentacene and oligothiophene, organic high molecular weight materials such as polythiophene, metal complexes such as phthalocyanine, fullerene materials such as C₆₀, C₇₀, and metallofullerene, and carbon nanotubes can be used as the organic semiconductor film 20.

[0057] Vapor deposition, spin coating, casting, and the like can be used as the method of forming the organic semiconductor film 20.

[0058] As examples, a method such as lithography, coating with a mask, an ink jet method can be used to pattern the organic semiconductor film 18.

[0059] First Experiment

[0060] Test specimens (organic thin film transistors) were constructed as follows. An N-type single crystal substrate was used as the substrate and this was set as_the gate electrode. A 300 nm thermal oxide film was formed on this substrate and the source electrode and drain electrode were formed on this using gold (Au). The thickness of the gold was 100 nm. The following three types of silane compound (a), (b), and (c) were respectively used to form different threshold voltage controlling films.

• • [0061] (a) CF₃(CH₂)₉Si(OC₂H₅)₃
  • [0062] (b) CH₃(CH₂)₇Si(OC₂H₅)₃
  • [0063] (c) NH₂(CH₂)₃Si(OC₂H₅)₃

[0064] The threshold voltage controlling films were formed with silane compounds (a) and (b) by CVD and with silane compound (c) by dipping.

[0065] The organic semiconductor film was then formed by coating the threshold voltage controlling film with fullerene (C60) by molecular beam epitaxy (MBE). The degree of vacuum during formation of this film was 1×10⁻⁹ torr). The vapor deposition speed was 0.15 Å/s and the substrate temperature was 110° C.

[0066] FIG. 3 is a graph showing the relationship between the drain current I_D and the gate voltage V_G for the case where the drain voltage V_D of the three test specimens was set at 80V. In FIG. 3, the vertical axis shows the square root of the drain current I_D. As shown in FIG. 3, it can be seen that the characteristics shift leftwards as the silane compound changes in the order (a), (b), (c) (shown in FIG. 3 as “F”, “Me”, and “NH₂”, respectively). For each compound, the intersection between the dotted line extrapolated on a left side of the linear part and the horizontal axis is the threshold voltage (V_th), and it was observed that the value of V_th decreases in the order (a), (b), (c). This shows that it is possible to control the V_th of a thin film transistor (C₆₀-TFT) fabricated using C₆₀ with the threshold voltage controlling film.

[0067] Second Experiment

[0068] Test specimens (organic thin film transistors) were constructed as follows. An N-type single crystal substrate was used as the substrate and this was set as the gate electrode. A 300 nm thermal oxide film was formed on this substrate and the source electrode and drain electrode were formed on this using gold (Au). The thickness of the gold was 100 nm. The following three types of silane compound (a), (b), and (c) were respectively used to form different threshold voltage controlling films.

• • [0069] (a) CF₃(CH₂)₉Si(OC₂H₅)₃
  • [0070] (b) CH₃(CH₂)₇Si(OC₂H₅)₃
  • [0071] (c) NH₂(CH₂)₃Si(OC₂H₅)₃

[0072] The threshold voltage controlling films were formed with silane compounds (a) and (b) by CVD and with silane compound (c) by dipping.

[0073] The organic semiconductor film was then formed by coating the threshold voltage controlling film with pentacene (C₂₂H₁₄) by molecular beam epitaxy (MBE). The degree of vacuum during formation of this film was 1×10⁻⁹ torr). The vapor deposition speed was 0.15 Å/s and the substrate temperature was 30° C.

[0074] FIG. 4 is a graph showing the relationship between the drain current I_D and the gate voltage V_G for the case where the drain voltage V_D of the three test specimens was set at 80V. In FIG. 4, the vertical axis shows the square root of the drain current I_D. As shown in FIG. 4, it can be seen that the characteristics shift leftwards as the silane compound changes in the order (a), (b), (c) (shown in FIG. 4 as “F”, “Me”, and “NH₂”, respectively). For each compound, the intersection between the dotted line extrapolated on a right side of the linear part and the horizontal axis is the threshold voltage (V_th), and it was observed that the value of V_th increases in the order (a), (b), (c). This shows that it is possible to control the V_th of a thin film transistor (pentacene-TFT) fabricated using pentacene according to the threshold voltage controlling film.

[0075] Modifications

[0076] The above embodiments can be modified as follows.

[0077] As shown in FIG. 5, the drain electrode 18 may be provided on an organic semiconductor film layer via a gate insulating film. In this case, compared to the construction shown in FIG. 1, there is the advantage of a larger degree of freedom for the substrate.

[0078] Also, as shown in FIG. 6, the source electrode 16 and the drain electrode 18 may be formed on the organic semiconductor film 20. In this case, compared to the construction shown in FIG. 1, there is the advantage that there is greater mobility since it is harder for the drain electrode 18 and the source electrode 16 to be affected by the material of the organic semiconductor film 20.

[0079] The present invention is not limited to the embodiments described above and can be modified in a variety of ways without departing from the scope of the invention.

[0080] As described above, according to the present invention, by providing a threshold voltage controlling film between a gate insulating film and an organic semiconductor film, it is possible to easily control the threshold voltage without changing the material used to form the organic semiconductor film.