Method for manufacturing a thin film transistor

Abstract

To provide a thin film transistor having improved interface properties of a polyimide insulating film with respect to a metal oxide semiconductor layer and achieving high performance and flexibility, and a method of manufacturing a thin film transistor.SOLUTION: Provided is a thin film transistor including: a substrate 101; a gate electrode 102 formed on the substrate 101; a gate insulating film 103 formed to cover the top and side of the gate electrode 102 and made of a polyimide material; an aluminum oxide film 105 formed on the gate insulating film 103, the aluminum oxide film being an interlayer insulating film; a metal oxide semiconductor layer 104 formed on the aluminum oxide film 105; and a source electrode 106a and a drain electrode 106b formed such that tips of the source and drain electrodes face each other via a gap, on the metal oxide semiconductor layer 104.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a thin film transistor used for driving, for example, an organic EL (Electro-Luminescence) element ((OLED (Organic Light Emitting Diode)) or an LCD (Liquid Crystal Display)), and a method for manufacturing the thin film transistor. Specifically, the present invention relates to a thin film transistor provided with an oxide semiconductor layer on a gate insulating film and a method for manufacturing the thin film transistor. is It relates to a manufacturing method.

Background Art

[0002] A thin film transistor (hereinafter also referred to as TFT) is used, for example, as a transistor for driving a display device, and is known as an electronic device that drives each pixel. A TFT using a metal oxide as a semiconductor material has been manufactured and put into practical use by using a vacuum process such as sputtering.

[0003] In particular, a TFT using an oxide (IGZO) composed of In-Ga-Zn as a semiconductor material is generally known to exhibit a relatively high mobility of 5 to 10 cm 2 / Vs and is used as an electronic device for driving a display. However, when a vacuum process is used, an expensive and large-scale vacuum apparatus is required, and time and cost are also required to form a vacuum atmosphere. In addition, when a photolithography technique is used, many complicated processes are required, resulting in problems such as a decrease in efficiency and an increase in environmental load.

[0004] Therefore, a coating film forming technique has attracted attention as a technique that can easily form a film in the atmosphere without using a vacuum apparatus as much as possible. Furthermore, electronic devices such as flexible displays using plastic substrates that can greatly expand the degree of freedom in shape, such as folding and bending, are expected. That is, flexible electronic devices are required in that various shape display applications such as displays that are not easily broken even when folded or bent and wall-mounted displays can be realized.

[0005] Conventionally, many gate insulating films of TFTs use materials formed by a vacuum process such as SiO2 or SiNx. On the other hand, as an insulating film that can be formed by coating, those using polymer materials, particularly polyimide materials, which are excellent in heat resistance and mechanical and chemical properties, have attracted attention (see Patent Document 1 below and Non-Patent Document 1 below). Furthermore, this polyimide material is expected to be applied to flexible electronics in that it is superior in flexibility compared to insulating films formed by conventional vacuum processes, and in fact, attempts have been made to apply it to specific technologies (see Non-Patent Document 2 below). FIG. 6 is a schematic cross-sectional view showing an example of a TFT200a formed by a coating film formation method using a polyimide material as a gate insulating film. That is, a gate electrode 202 is laminated on a substrate 201, a polyimide insulating film 203 is coated and laminated so as to surround the upper part and side parts of the gate electrode 202, and a metal oxide semiconductor layer 204 is laminated on the polyimide insulating film 203 so as to be in contact with the interface of the polyimide insulating film 203. Furthermore, a source electrode 106a and a drain electrode 106b are laminated on the polyimide insulating film 203 and on the metal oxide semiconductor layer 204. By forming a TFT having such a configuration using a coating film formation method, it is possible to configure a TFT that is easy to manufacture and has excellent flexibility.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Non-Patent Documents

[0007]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] By the way, as in the example described with reference to FIG. 6, when manufacturing a TFT using a coating film formation technique, it is desired to apply a polyimide material to the insulating film. However, when a polyimide material is used as the gate insulating film of a TFT, it is difficult to form a chemically good interface between this gate insulating film and the metal oxide semiconductor layer, and it is difficult to obtain the characteristics of a high-performance TFT.

[0009] The present invention has been made in view of the above circumstances, and aims to improve the interface characteristics of a polyimide insulating film with respect to a metal oxide semiconductor layer, and to provide a manufacturing method of a high-performance and flexible thin-film transistor. thin That is the object of the present invention.

Means for Solving the Problems

[0012] book The manufacturing method of the thin-film transistor of the invention is A first step of forming a gate electrode on a substrate, a second step of forming a gate insulating film made of a polyimide material on the gate electrode, a third step of forming an interlayer insulating film made of aluminum oxide on the gate insulating film, a fourth step of forming an oxide semiconductor layer on the interlayer insulating film, and a fifth step of forming source / drain electrodes on the oxide semiconductor layer are performed in this order. Among these steps, at least also before the third step is performed using a coating film formation method. In the third step, a precursor solution made of the aluminum oxide is applied on the gate insulating film, and then a baking treatment is performed at a temperature of 500°C or lower to form the interlayer insulating film. is characterized by . Note that the phrase "made of aluminum oxide" does not necessarily mean that it does not contain metal oxides other than aluminum oxide, and includes cases where aluminum oxide is the main material.

Advantages of the Invention

[0013] In the TFT described above, in order to insulate the gate electrode and the metal oxide semiconductor layer, an insulating film of a polymer material such as polyimide is interposed between the two. However, in order to realize a highly reliable and high-performance TFT, it is important to improve the flatness of the surface on the insulating film side facing the metal oxide semiconductor layer and form an energetically favorable chemical bond at the interface between the gate insulating film and the metal oxide semiconductor layer.

[0014] That is, since the polyimide gate insulating film has a large surface roughness, it was difficult to form a chemically good interface with poor alignment when joined to the metal oxide semiconductor layer. However, in the present invention, an interlayer insulating film made of aluminum oxide (alumina) is interposed between the polyimide gate insulating film and the metal oxide semiconductor layer. Thus, the flatness of the surface of the insulating film can be improved by laminating the interlayer insulating film made of an aluminum oxide layer. Further, by using an interlayer insulating film of a metal oxide (aluminum oxide) having a structure similar to that of the metal oxide semiconductor, a covalent bond can be formed at the interface, and a flat and continuous interface can be formed as compared with the polyimide organic insulating film. Furthermore, since a polymer material made of polyimide is used as the main material of the gate insulating film, it is possible to impart the flexibility required in the field of electronic devices such as large-screen displays and wearable electronics, and an inexpensive electronic device that can be used for various applications can be obtained.

[0015] Also, since polyimide has high heat resistance, it can withstand the process temperature for forming the metal oxide semiconductor layer and the electrode layer. In particular, in the method for manufacturing a thin film transistor of the present invention, since at least one manufacturing step is performed using a coating film forming technique, a high-performance thin film transistor can be created for that step without using an expensive vacuum device.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Mode for Carrying Out the Invention

[0017] Hereinafter, a thin film transistor according to an embodiment of the present invention and a method for manufacturing the same will be described with reference to the drawings. First, the cross-sectional structure of the thin film transistor (hereinafter referred to as TFT) according to the present embodiment will be briefly described with reference to FIG. 1.

[0018] As shown by the cross-sectional structure of FIG. 1, the TFT100a according to the present embodiment is formed by laminating a gate electrode 102, a polyimide insulating film 103 which is a gate insulating film, an aluminum oxide film 105 which is a coating type interlayer insulating film, a metal oxide semiconductor layer 104 which is a coating type oxide semiconductor, and a source electrode 106a and a drain electrode 106b on a substrate 101. In this TFT100a, at least the polyimide insulating film 103 and the metal oxide semiconductor layer 104 are formed by a coating method, but it is preferable that each of the other films is also formed by a coating method.

[0019] Hereinafter, a method for manufacturing the thin film transistor according to the present embodiment will be described with reference to FIG. 2. First, a washed plastic film or the like is set as the substrate 101. As the plastic film, for example, a film made of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyimide, polycarbonate (PC), nanocellulose, etc. is used. By using a plastic film as the substrate 101, weight reduction can be achieved compared to the case of using a glass substrate, and portability can be enhanced. Thereby, it is possible to realize an electronic device that has flexibility and a desired shape.

[0020] Subsequently, as shown in Fig. 2(A), a gate electrode 102 is formed on the substrate 101. As the material of the gate electrode 102, conductive oxides such as ITO and IZO, metals such as Au, Al, Ag, Cr, Mo, Ti, Cu, etc., and alloys thereof can be used. Using a sputtering method or a solution coating method, etc., for example, gold, titanium, chromium, aluminum, molybdenum or alloys or laminates thereof are formed (in the example of Fig. 2(A), the sputtering method is used as the film formation method and a molybdenum alloy is used as the material), and the gate electrode 102 is formed by patterning it into the required shape. For fine pattern formation, a method of photolithography (a fine processing technique by ultraviolet exposure) is used.

[0021] Next, as shown in Fig. 2(B), a polyimide insulating film 103 is formed by using a coating method so as to surround the upper and side portions of the gate electrode 102. That is, polyamic acid, which is a precursor of polyimide, is dissolved in N-methyl-2-pyrrolidone (NMP) to prepare a precursor solution 160. By applying the obtained precursor solution 160 onto a substrate 101 and a gate electrode 102, a thin film of the precursor solution is formed (in the example of FIG. 2(B), film formation is performed using a spin coater (6000 rpm, 30 seconds)). Then, after performing a pre-bake treatment (in the example of FIG. 2(B), performed at 130° C. for 10 minutes), the thin film is subjected to a firing treatment at a high temperature of 300 to 500° C. (in the example of FIG. 2(B), firing is performed at 350° C. in a nitrogen atmosphere) to promote a dehydration and cyclization (imidization) reaction and obtain polyimide. The thickness of this polyimide insulating film 103 needs to be adjusted according to the solution concentration and also according to the number of times the solution is applied.

[0022] Next, as shown in FIG. 2(C), an aluminum oxide film (interlayer insulating film) 105 is formed on the upper part of the polyimide insulating film 103 by using a coating method (in the example of FIG. 2(C), film formation is performed using a spin coater (2000 rpm, 30 seconds)). As the material of the aluminum oxide film 105, aluminum oxide (alumina: Al2O3), which is the same metal oxide as the material of the metal oxide semiconductor layer 104, is used. Aluminum oxide has advantages such as a higher dielectric constant compared to polymer materials such as polyimide and the ability to suppress leakage current because it has a large bandgap.

[0023] Also, as the process of the coating method, first, an inorganic acid salt, which is a precursor solution of aluminum oxide, is dissolved in a solvent to prepare a precursor solution. By applying the prepared precursor solution onto the polyimide insulating film 103, a thin film of the precursor solution is formed. Then, after performing a pre-bake treatment (in the example of FIG. 2(C), performed at 130° C. for 10 minutes), the thin film of the applied precursor solution is subjected to a firing treatment (in the example of FIG. 2(C), performed at 350° C. for 3 hours) to promote the oxidation treatment and obtain the aluminum oxide film 105. Since the inorganic acid salt of aluminum generally starts to crystallize at about 300° C., which is the process temperature of the above polyimide, a uniform aluminum oxide film 105 can be formed.

[0024] From a physical property perspective, materials that can form the interlayer insulating film may include yttrium oxide, titanium oxide, tantalum oxide, niobium oxide, scandium oxide, etc. However, since these are rare metals, they are expensive, and since they belong to rare earth metals, there are concerns about their toxicity, so they are not included in the thin film transistor of the present invention.

[0025] Incidentally, heretofore, a single-layer aluminum oxide has been known to be used as a gate insulating film that can be formed by coating, and good switching characteristics can be obtained thereby. However, since aluminum oxide is an inorganic material, a single-layer aluminum oxide structure has no flexibility, and when used in applications that require bending, problems similar to those of the above-described prior art, such as cracks occurring, will occur. That is, as in the present embodiment, by laminating the aluminum oxide film 105 on the polyimide insulating film 103, it is possible to prevent cracks from occurring even when used in applications that require bending.

[0026] As described above, when using the aluminum oxide layer as a single layer, it is necessary to set an appropriate film thickness of 100 nm or more. In this case, the manufacturing time becomes long by alternately repeating the coating process and the annealing process. On the other hand, in the present embodiment, polyimide rich in flexibility is used as the main component of the insulating film layer, and the aluminum oxide film 105 can have a minimum thickness necessary for improving the interface of about several nm to 10 nm. Therefore, the interface function with the metal oxide semiconductor layer 104 can be improved without impairing the flexibility.

[0027] Next, as shown in FIG. 2(D), a metal oxide semiconductor layer 104 is formed on the upper part of the aluminum oxide film 105. Examples of the oxide constituting the metal oxide semiconductor layer 104 include In-Ga-Zn-based oxides, In-Zn-based oxides, In-Sn-Zn-based oxides, Zn-Sn-based oxides, etc. (in the example of FIG. 2(C), it is ITZO), but it is not limited thereto. The metal oxide semiconductor layer 104 is formed into a film by sputtering, solution coating, or the like, similar to the gate electrode 102 (in the example of FIG. 2(D), the sputtering method is used as the film formation method), and is patterned into a required shape. For forming a fine pattern, photolithography (a fine processing technique by ultraviolet exposure) can be used to pattern it into a desired size and shape.

[0028] Subsequently, by performing a firing process, the oxidation process is promoted to obtain an oxide semiconductor layer. The firing process in this case is, for example, in the range of 150°C to 600°C for 30 minutes to 6 hours (in the example of FIG. 2(D), it is performed at 300°C for 1 hour). In the firing process at this time, natural drying, hot air, cold air, room temperature air drying, infrared light drying, reduced pressure drying, etc. can be used. Drying by a heating device using microwaves may also be possible. Each firing process can be performed not only in the atmosphere but also in a gas atmosphere such as oxygen, nitrogen, argon, etc. From the viewpoint of promoting oxidation, it is also preferable to perform an irradiation process of energy rays such as ultraviolet light on the thin film before and after firing.

[0029] Examples of the energy rays include an excimer lamp, a deuterium lamp, a low-pressure mercury lamp, a high-pressure mercury lamp, an ultra-high-pressure mercury lamp, a metal halide lamp, a helium lamp, a carbon arc lamp, a cadmium lamp, an electrodeless discharge lamp, etc. In particular, when using a low-pressure mercury lamp, the conversion from the precursor film to the oxide film can be easily promoted. In this way, the metal oxide semiconductor layer 104 is formed.

[0030] Next, as shown in FIG. 2(E), source electrodes 106a and drain electrodes 106b are formed mainly from the upper part of the metal oxide semiconductor layer 104 to the upper part of the aluminum oxide film (interlayer insulating film) 105. As materials for the source electrode 106a and the drain electrode 106b, similar to the gate electrode 102, conductive oxides such as ITO and IZO, metals such as Au, Al, Ag, Cr, Mo, Ti, Cu, etc., and alloys thereof can be used.

[0031] That is, by using a sputtering method or a solution coating method, etc., for example, gold, titanium, chromium, aluminum, molybdenum or their alloys or laminates are patterned into the required shapes to form the source electrode 106a and the drain electrode 106b. (In the example of Fig. 2(E), the sputtering method is used as the film formation method and a molybdenum alloy is used as the material.) For fine pattern formation, it is preferable to use a method of photolithography (a fine processing technique by ultraviolet exposure). Subsequently, by performing a firing treatment, the oxidation treatment is promoted to obtain an oxide semiconductor layer. (In the example of Fig. 2(E), the firing is performed at 150 °C for 30 minutes.) Through the above procedures, the TFT 100a is formed.

Example

[0032] (Example) Hereinafter, as an example of the thin film transistor (TFT) 100a according to this embodiment, an evaluation TFT configured as shown in Fig. 1 was fabricated, and a performance evaluation was performed with an evaluation TFT according to a comparative example fabricated as described later. That is, first, a low-resistance silicon wafer with a 200-nm-thick thermal oxide film was prepared. On this silicon wafer, a gate electrode (the material is a molybdenum alloy) was patterned by a sputtering method using a metal mask.

[0033] Next, polyamic acid, which is a polyimide precursor as the coating material (manufactured by Unitika Ltd., product name: U Imide Varnish (U Imide is a registered trademark of Unitika Ltd.)), was diluted with NMP to a concentration of 10 wt% to prepare a precursor solution. This precursor solution was applied to the entire upper surface of the exposed gate electrode and substrate using the spin coating method. The spin coating settings at this time were 6000 rpm for 30 seconds. Thereafter, pre-baking was performed on a hot plate heated to 130 °C for 10 minutes, and then firing treatment was performed for 3 hours in an oven under a nitrogen atmosphere set at 350 °C to form a polyimide insulating film having an imide bond.

[0034] Next, aluminum nitrate nonahydrate (Al(NO3)3·9H2O, manufactured by Aldrich) was dissolved in pure water at a concentration of 0.3 mol / L to prepare a precursor solution for the aluminum oxide film. Thereafter, this precursor solution was stirred at room temperature for 6 hours to be in a completely dissolved state. Next, the above polyimide insulating film was subjected to hydrophilic treatment by O2 plasma treatment for 1 minute, and then the above-mentioned precursor solution was applied onto the silicon wafer on which the above polyimide insulating film was formed using the spin coating method. The spin rotation speed at this time was set at 2000 rpm and the spin time was 30 seconds. Thereafter, the one coated with the precursor solution for the aluminum oxide film was baked on a hot plate set at 350 °C for 1 hour to form an aluminum oxide film (interlayer insulating film) with a film thickness of 5 nm.

[0035] Next, a metal oxide semiconductor layer (ITZO) was patterned on the substrate in a state where the above aluminum oxide film was formed by sputtering using a metal mask. The one with the patterned metal oxide semiconductor layer formed thereon was heated on a hot plate set at 300 °C for 1 hour to activate the semiconductor layer.

[0036] Next, a source electrode / drain electrode (made of a molybdenum alloy) was patterned on the substrate on which the metal oxide semiconductor layer was formed by a sputtering method using a metal mask. Thereafter, the substrate on which the source electrode / drain electrode was patterned was heated on a hot plate set at 150°C for 30 minutes to form the source electrode and the drain electrode. Thereby, a TFT for evaluation according to the example was fabricated.

[0037] Next, a TFT for evaluation according to the comparative example was fabricated as follows. (Comparative Example) The TFT according to this comparative example is formed with only polyimide as the insulating film without providing an aluminum oxide film, as compared with the TFT according to the above example. Otherwise, the configuration and fabrication method were exactly the same as those of the above example.

[0038] (Comparison of Performance) For the TFT for evaluation according to the above example, semiconductor characteristics (gate voltage-drain current characteristics) were measured. The results are shown in FIG. 3. In FIG. 3 (and also in FIG. 4), the vertical axis represents the drain current (A) in logarithmic scale (1E-n represents 1×10 -n ). As shown in FIG. 3, the characteristic curve rises from the position where the gate voltage is approximately 0V, and an ON-OFF ratio of 10 6 or more and good characteristics were obtained. Also, the S value (threshold coefficient) indicating the switching performance was 0.49V / dec. Here, the S value is an index indicating the sharpness of the rise of the current, and represents the voltage value required for the current value to increase by one digit (the same applies to the following description).

[0039] On the other hand, for the TFT for evaluation according to the above comparative example, semiconductor characteristics (gate voltage-drain current characteristics) were also measured. The results are shown in FIG. 4. The ON-OFF ratio was 10 4 and the S value (threshold coefficient) indicating the switching performance was 2.74V / dec.

[0040] Thus, in this embodiment having an interlayer insulating film made of an aluminum oxide film on a polyimide insulating film, an ON-OFF ratio that is two digits or more better was obtained compared to the comparative example in which no aluminum oxide film was provided on the polyimide insulating film. Also, the S value is 0.49 V in this embodiment as described above, whereas in the comparative example, it is 2.74 V / dec, which is about five times that in this embodiment as described above. It is clear that this embodiment is superior in performance compared to the comparative example as the prior art.

[0041] (Comparison of interface states) Next, the results of observing the interface states of the above-described embodiment and the above-described comparative example using an atomic force microscope (AFM) were compared. That is, the states of the surfaces (interfaces) of the insulating films facing the metal oxide semiconductor layers were compared. The observation results of the interface state in the comparative example are shown in FIG. 5(A), and the observation results of the interface state in the embodiment are shown in FIG. 5(B).

[0042] In the comparative example, the surface state of the polyimide insulating film becomes the interface state, and in the embodiment, the surface state of the aluminum oxide formed on the polyimide insulating film becomes the interface state. It is shown that visually, the surface roughness is greatly improved in the embodiment compared to the comparative example. When specific numerical values were calculated from the data, in the comparative example, the average surface roughness was 0.831 nm, whereas in the embodiment, the average surface roughness was 0.344 nm. It is clear that the surface state is greatly improved in the embodiment compared to the comparative example.

[0043] (Other aspects) Also, the thin film transistor and its manufacturing method of the present invention are not limited to those of the above-described embodiment, and can be changed to various other aspects. For example, it is also possible to adopt a configuration in which other layers are interposed between the respective layers shown in the above-described embodiment.

[0044] In addition, in the above embodiment, the polyimide insulating film and the formation of aluminum oxide as the interlayer insulating film use the coating method, but it is preferable to use the coating method for the formation of each of the other layers as well. However, if necessary, it may be formed by appropriately using a vacuum method (including the method of photolithography).

Explanation of Signs

[0045] 100a, 200a Thin film transistor (TFT) 101, 201 Substrate 102, 202 Gate electrode 103, 203 Polyimide insulating film 104, 204 Metal oxide semiconductor layer 105 Aluminum oxide (alumina) film 106a, 206a Source electrode 106b, 206b Drain electrode 160 Precursor solution

Claims

【Claim 1】 A method for manufacturing a thin film transistor, comprising a first step of forming a gate electrode on a substrate, a second step of forming a gate insulating film made of a polyimide material on the gate electrode, a third step of forming an interlayer insulating film made of aluminum oxide on the gate insulating film, a fourth step of forming an oxide semiconductor layer on the interlayer insulating film, and a fifth step of forming source / drain electrodes on the oxide semiconductor layer, which are performed in this order, Among these steps, at least the third step is performed using a coating film formation method. In the third step, after applying a precursor solution made of the aluminum oxide on the gate insulating film, a firing treatment is performed at a temperature of 500 °C or lower to form the interlayer insulating film.

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