JP2020027897A - Thin film transistor, manufacturing method thereof, and image display device - Google Patents

Abstract

To provide a thin film transistor which has low electrical contact resistance between a semiconductor layer and source/drain electrodes and has good patterning properties of a semiconductor layer, can be easily manufactured, has good transistor characteristics, a manufacturing method of the thin film transistor, and an image display device including the thin film transistor.SOLUTION: A thin film transistor includes an insulating substrate, a gate electrode, a gate insulating layer, source and drain electrodes, and a semiconductor layer, and the source electrode and the drain electrode each include a first metal layer and a second metal layer stacked on the side surface of the first metal layer.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a thin film transistor, a method for manufacturing a thin film transistor, and an image display device.

  At present, the mainstream of semiconductor materials is silicon-based (Si-based). As a method for forming a semiconductor layer using a silicon-based material, a method is generally used in which silicon is formed by a dry method such as sputtering or CVD, and then patterned by photolithography.

  On the other hand, research on transistors using organic semiconductors (organic transistors) has been actively conducted from the viewpoints of flexibility, weight reduction, and cost reduction (see Non-Patent Document 1). In general, when an organic semiconductor is used, a printing process that is a wet method can be performed. By using this printing technique, there are advantages in that the cost for equipment and manufacturing is lower than that of the photolithography method, and that a plastic substrate can be used because no vacuum or high temperature is required.

  Its application field is wide, and it is expected to be applied not only to flexible displays such as thin and lightweight electronic paper but also to RFID (Radio Frequency Identification) tags and sensors.

  For these reasons, transistors using organic semiconductors have attracted attention at present.

  In a transistor including an organic semiconductor, a bottom-contact transistor in which an organic semiconductor layer is formed over a substrate over which a gate electrode, a gate insulating layer, and a source / drain electrode layer are formed is used. The bottom-contact transistor has an advantage that the organic semiconductor layer is less likely to be chemically and physically damaged since there is no electrode layer formation process after the organic semiconductor layer is formed.

  However, a transistor using an organic semiconductor has the above advantages, but has a problem that TFT characteristics are low because a large electrical contact resistance is interposed between the organic semiconductor layer and the electrode.

  In order to solve such a problem, a method of using a stacked structure of an oxide layer / metal layer for a source / drain electrode has been studied. In that method, an attempt is made to lower the charge injection barrier between the electrode layer and the organic semiconductor layer by using a material having a good charge injection property for the oxide layer (Patent Document 1).

  Further, attempts have been made to control the wettability and work function of the metal electrode surface by forming a thin organic layer by modifying the surface of the metal electrode with, for example, an organic compound having a thiol group at the terminal. Patent Literature 2 discloses a TFT using an electrode surface treatment agent having a functional group that forms a chemical bond with a metal.

  In addition, noble metals such as gold (Au) having a large work function have been used as a material for the source / drain electrodes. However, gold is expensive, difficult to pattern, and has a large area such as a transistor array. Not suitable for forming

  Further, in the case of laminating the electrode layers, there is a problem that the difficulty of the process is increased, for example, the number of steps is increased and the accuracy of alignment is required.

  Further, when a printing method is used for the semiconductor layer, it is important to control the wettability of the source / drain electrodes and the insulating film layer because the wettability of each layer is significant.

JP 2010-003747 A JP 2008-060117 A

Francis Garnier, et al. , "All-Polymer Field-Effect Transistor Realized by Printing Technologies", Science Vol. 265, 1684 (1994)

  Therefore, the present invention has been made in view of such a problem, and its object is to make it easy to fabricate since the electrical contact resistance between the semiconductor layer and the source / drain electrodes is low and the patterning property of the semiconductor layer is good. An object of the present invention is to provide a thin film transistor having favorable transistor characteristics, a method for manufacturing the thin film transistor, and an image display device including the thin film transistor.

  One embodiment of the present invention for solving the above problem is a thin film transistor including an insulating substrate, a gate electrode, a gate insulating layer, a source electrode and a drain electrode, and an organic semiconductor layer, Each of the drain electrodes is a thin film transistor including a first metal layer and a second metal layer stacked on a side surface of the first metal layer.

  Further, the second metal layer may include any of silver, gold, platinum, and palladium.

  Further, the first metal layer may include molybdenum or an alloy of molybdenum.

  Further, the second metal layer may be subjected to a surface modification treatment.

  Also, different surface modification treatments may be applied to each of the first metal layer and the second metal layer.

  Another embodiment of the present invention provides a step of forming a gate electrode over an insulating substrate, a step of forming a gate insulating layer, a step of forming a source electrode and a drain electrode, and forming an organic semiconductor layer. Forming a source electrode and a drain electrode. The step of forming a first metal layer and then stacking a second metal layer on a side surface of the first metal layer includes the steps of: Performing a surface treatment on the metal layer and the second metal layer.

  Another aspect of the present invention is an image display device including an image display medium using the above-described thin film transistor.

  Further, the image display medium may be an electrophoretic body for displaying by an electrophoretic method.

  Another aspect of the present invention is a sensor device using the above thin film transistor.

  According to the present invention, a thin film transistor which has low electric contact resistance between a semiconductor layer and a source / drain electrode, has good patterning properties of a semiconductor layer, can be easily manufactured, and has good transistor characteristics, and a method for manufacturing a thin film transistor And an image display device including the thin film transistor.

Sectional view of the thin film transistor according to the first embodiment of the present invention Sectional view of a thin film transistor according to a second embodiment of the present invention Sectional view of a thin film transistor according to a third embodiment of the present invention Schematic diagram showing a method for forming a source electrode and a drain electrode of a thin film transistor of the present invention. Sectional view of a thin film transistor according to a comparative example

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings, but the present invention is not limited thereto.

  FIG. 1 is a sectional view of a thin film transistor 100 according to the first embodiment of the present invention. The thin film transistor 100 includes a substrate 1, a gate electrode 2, a gate insulating film 3, a source electrode 4, a drain electrode 5, a semiconductor layer 6 laminated between the source electrode 4 and the drain electrode 5, 6 and a protective layer 7 laminated on the protective layer 6. Each of the source electrode 4 and the drain electrode 5 includes a first metal layer 8 to be described later and a second metal layer 9 formed on the side surface of the first metal layer 8. Next, a method for manufacturing the thin film transistor 100 and each configuration will be described.

  First, an insulating substrate 1 is prepared. The material used for the substrate 1 is not particularly limited, but a flexible plastic material such as polyethylene terephthalate (PET), polyimide, polyethersulfone (PES), polyethylene naphthalate (PEN), polycarbonate, and glass such as quartz. There are a base material and a silicon wafer.

  Next, a gate electrode 2 is formed on the substrate 1. The material used for the gate electrode 2 is not particularly limited, but a metal such as Al, Cr, Au, Ag, Ni, Cu, or Mo, or a transparent conductive film such as ITO can be used. The gate electrode 2 may be formed by a photolithography method after vapor deposition or sputtering, but a printing method (screen printing, flexo printing, gravure printing, offset printing, reverse offset printing, or the like) may be used. it can. When printing is used, silver (Ag) ink, nickel (Ni) ink, copper (Cu) ink, or the like can be used. In particular, flexographic printing and reverse offset printing are suitable as a method for forming the gate electrode 2 since thin film printing is possible and excellent in flatness.

  Next, a gate insulating film 3 is formed on the substrate 1 and the gate electrode 2. The gate insulating film 3 is made of an oxide-based insulating material such as silicon oxide (SiOx), aluminum oxide (AlOx), tantalum oxide (TaOx), yttrium oxide (YOx), zirconium oxide (ZrOx), and hafnium oxide (HfOx). Polyacrylates such as silicon nitride (SiNx), silicon oxynitride (SiON) and polymethyl methacrylate (PMMA), resin materials such as polyvinyl alcohol (PVA) and polyvinyl phenol (PVP), and polysilsesquioxane (PSQ) Such organic / inorganic hybrid resins can be used, but are not limited thereto. These may be a single layer, a laminate of two or more layers, or a composition whose composition is inclined toward the growth direction.

  The method for forming the gate insulating film 3 includes a vacuum deposition method, an ion plating method, a sputtering method, a laser ablation method, a plasma CVD method, a photo CVD method, a hot wire CVD method, and other vacuum film forming methods, a spin coating method, A wet film formation method such as a die coating method and a screen printing method can be used as appropriate depending on the material.

  Next, the source electrode 4 and the drain electrode 5 including the first metal layer 8 and the second metal layer 9 are formed on the gate insulating film 3. As a material of the first metal layer 8, a metal material such as aluminum (Al), molybdenum (Mo), or a molybdenum alloy such as MoNb can be used.

  The first metal layer 8 can be formed by, for example, a photolithography method. The pattern formation portion is protected by a resist, and unnecessary portions are removed by etching. After the etching step, the second metal layer 9 is formed, and thereafter, the resist on the first metal layer 8 is stripped.

  As a material of the second metal layer 9, silver, gold, platinum, or palladium can be used. These noble metal-based metals are preferable because they have a large work function and a small hole injection barrier to an organic semiconductor. Further, the contact resistance can be further reduced by forming the surface modification layer. The method of forming the second metal layer 9 is not particularly limited, but a method of immersing the second metal layer 9 in an aqueous solution of a second metal salt can be used.

  The second metal layer 9 is formed by laminating only on the side surface, and particularly, by selecting a noble metal-based metal having a large work function, the hole injection barrier to the organic semiconductor material is reduced, and the contact resistance is reduced. . In addition, a surface modification treatment layer having high hydrophobicity different from that of the second metal layer 9 can be formed on the first metal layer 8 serving as the upper surface, and the printability of the subsequently formed semiconductor layer 6 is improved. It becomes possible.

  For forming the surface modification layer 10 on the first metal layer 8, various materials that have affinity for the metal layer or oxygen atoms and can be subjected to surface modification can be used. Among the materials that can be used for the organic thin film layer, preferably, a material known as a self-assembled monolayer (SAMs) agent is suitable.

  A surface modification layer may be provided on each of the first metal layer and the second metal layer. FIG. 2 is a cross-sectional view of a thin film transistor 101 according to the second embodiment including a source electrode 4 and a drain electrode 5 each having a surface modification layer 11 provided on a side surface of a second metal layer 9. Further, FIG. 3 shows a third example in which the surface modification layer 10 is provided on the upper surface of the first metal layer 8 and the source electrode 4 and the drain electrode 5 in which the surface modification layer 11 is provided on the side surface of the second metal layer 9. 1 shows a sectional view of a thin film transistor 102 according to the embodiment.

  Examples of the material used for the surface modification layer 10 include a fluorine atom, a carboxylic acid having a phenyl group, a phosphonic acid, and a silane coupling agent, but are not limited thereto. In order to control the printability of the semiconductor layer 6, it is preferable that a fluorine atom having high hydrophobicity is contained.

  Examples of the material of the surface modification layer 11 formed on the side surface of the second metal layer 9 include a thiol compound containing a fluorine atom, but are not limited thereto. By using such a surface modification material, hole injection into an organic semiconductor can be improved, and electrical contact resistance can be reduced.

  The surface modification layers 10 and 11 on the first metal layer 9 and the second metal layer 10 were formed by forming the first metal layer 9 and the second metal layer 10 in a solution of the organic thin film layer material described above. This can be done by immersing the substrate. After that, heat treatment may be performed.

  Next, a semiconductor layer 6 is formed on the gate insulating film 3, the source electrode 4, and the drain electrode 5 so as to connect the source electrode 4 and the drain electrode 5. Examples of the material of the semiconductor layer 6 include low molecular organic semiconductor materials such as pentacene, tetracene, phthalocyanine, perylene, thiophene, benzodithiophene, anthradithiophene, and derivatives thereof, carbon compounds such as carbon nanotubes, and polythiophene. However, high molecular organic semiconductor materials such as polyallylamine, fluorenebithiophene copolymer, and derivatives thereof can be used, but are not limited thereto. As a method for forming the semiconductor layer 6, a known method such as a letterpress printing method, a reverse offset printing method, an inkjet printing method, a screen printing method, a spray coating method, and a spin coating method can be suitably used. Also, flexographic printing is preferable because it is easy to increase the area. In addition, the shape of the semiconductor layer 6 is not particularly limited, and may be an island shape such as a dot or a stripe shape parallel to the source wiring.

  Next, a protective layer 7 is formed on the semiconductor layer 6. The protective layer 7 is formed to protect the semiconductor layer 6. The protective layer 7 needs to be formed so as to cover at least a region overlapping the channel portion of the semiconductor layer 6.

  Examples of the material of the protective layer 7 include inorganic materials such as silicon oxide, aluminum oxide, tantalum oxide, yttrium oxide, hafnium oxide, hafnium aluminate, zirconium oxide, and titanium oxide, and polyacrylates such as PMMA (polymethyl methacrylate). Insulating materials such as PVA (polyvinyl alcohol), PVP (polyvinyl phenol), and fluorine-based resin are exemplified, but not limited thereto.

  As a method for forming the protective layer 7, known methods such as letterpress printing, reverse offset printing, ink jet printing, screen printing, spray coating, and spin coating can be preferably used. It is preferable to use a low-cost printing method with a small number of steps. In particular, screen printing is preferable because the application range of ink viscosity is wide, ink material selectivity is high, ink use efficiency is high, and area enlargement is easy. Also, flexographic printing is preferable because it is easy to increase the area. The shape of the protective layer 7 is not particularly limited, and may be an island shape such as a dot or a stripe shape parallel to the source wiring.

<Image display device>
By arranging a plurality of thin film transistors 100, 101, and 102 in a matrix, a thin film transistor array can be obtained. The thin film transistor array can be used for an image display device in combination with an image display medium. As an image display medium, each image display medium such as an electrophoretic reflective display device, a transmissive liquid crystal display device, a reflective liquid crystal display device, a transflective liquid crystal display device, an organic EL display device, and an inorganic EL display device is used. Can be. As an image display device, an electronic paper, an organic EL display device, or a liquid crystal display device can be used.

  Further, the thin film transistors 100, 101, and 102 of the present invention can be used for sensor devices such as a pressure sensor, an optical sensor, a chemical sensor, and a biosensor. Hereinafter, description will be made based on embodiments.

  The result of manufacturing the thin film transistor array substrate according to each embodiment of the present invention and evaluating the same is shown. Polyimide (manufactured by Ube Industries) was used as a material of the base material 1 of the thin film transistor 103. A 100 nm aluminum film was formed on the base material 1 by a sputtering method, and a gate electrode 2 was formed by photolithography using a positive resist, etching, and resist peeling. Next, an acrylic resin was applied on the base material 1 on which the gate electrode 2 was formed by a die coater and dried at 180 ° C. for 1 hour to form a gate insulating film 3.

Next, a method for forming the source electrode 4 and the drain electrode 5 will be described with reference to FIG.
First, as shown in FIG. 4A, a MoNb layer 30 having a thickness of 100 nm was formed on the gate insulating film 3 by a sputtering method.
Next, as shown in FIG. 4B, a positive resist 31 was applied by spin coating. Next, as shown in FIG. 4C, exposure and development were performed, and the resist 31 was patterned to obtain a patterned resist layer 32. Next, as shown in FIG. 4D, the MoNb layer 30 was etched, and the first metal layer 8 was patterned. Next, as shown in FIG. 4E, the second metal layer 9 was formed on the side surface of the first metal layer 8 by immersion in an aqueous solution containing 0.1 mM silver. Next, as shown in FIG. 4F, the resist 31 was peeled off. Next, as shown in (g) of FIG. 4, a second surface modification layer 11 was formed on the side surface of the second metal layer 9 by immersion in a pentafluorothiophenol solution (manufactured by Tokyo Chemical Industry). Next, as shown in FIG. 4H, perfluorooctanephosphonic acid was dissolved in isopropyl alcohol to prepare a 5 mmol solution, immersed for 1 hour, and baked at 140 ° C. for 1 hour on a hot plate to form a first surface modification. Layer 10 was formed. Thus, the source electrode 4 and the drain electrode 5 were formed through the steps shown in FIGS.

  Next, as a semiconductor material, 6,13-bis (triisopropylsilylethynyl) pentacene (TIPS-pentacene) (manufactured by Aldrich) dissolved in tetralin (manufactured by Kanto Chemical) at 2% by weight was used as an ink. Semiconductor 6 was formed by a printing method. Since the first surface modification layer 10 was hydrophobic, the semiconductor material could be printed well without spreading on the upper surfaces of the source electrode 4 and the drain electrode 5.

  Next, a fluororesin was applied on the semiconductor layer 6 by flexographic printing, and dried at 100 ° C. to form a protective layer 7.

  The transistor characteristics of the manufactured thin film transistor 103 were measured. The measurement was performed using a semiconductor parameter analyzer B1500A (manufactured by Agilent).

As a result of the measurement, in the thin film transistor 100 according to the first embodiment was thus produced, on-off ratio is I _on the I _off ratio of the current value of the transistor can be confirmed that indicated 10 ^4, and the good transistor characteristics Was.

A thin film transistor 101 was manufactured in the same manner as in Example 1 except that the first surface modification layer 10 of the source electrode 4 and the drain electrode 5 was not provided. Since there is no first surface modification layer 10, the patterning of the semiconductor layer 6 but had spread to the first metal layer 8 side, the on-off ratio could be confirmed 10 ^4, and the good transistor characteristics.

A thin film transistor 102 was manufactured in the same manner as in Example 1 except that the first surface modification layer 10 and the second surface modification layer 11 of the source electrode 4 and the drain electrode 5 were not provided. Because it does not form a surface modification layer, high electrical contact resistance, but the on-off ratio becomes 10 ^3, it was possible to verify the satisfactory transistor characteristics.

Comparative example

  As a comparative example, a thin film transistor described below was manufactured. FIG. 5 shows a cross-sectional view of a thin film transistor 500 according to a comparative example. The thin film transistor 500 according to the comparative example is the same as the first embodiment except that the method of forming the source electrode 21 and the drain electrode 22 is different. Specifically, MoNb was formed to a thickness of 100 nm by a sputtering method, and was exposed to light using a positive resist, developed, etched, and stripped of resist to form a source electrode 21 and a drain electrode 22.

As a result, large electrical contact resistance between the semiconductor layer and the source and drain electrodes, small I _on, off ratio 10 ^2, and good transistor characteristics were not obtained.

<Image display device>
When the thin film transistors 100 to 102 manufactured in Examples 1, 2, and 3 are arranged in a matrix to form a thin film transistor array, and an electrophoretic image display medium sandwiched between the thin film transistors and a counter electrode is driven, display switching is also a problem. It was confirmed that the display was possible without any problem.

  When the thin film transistors 500 manufactured in the comparative example were arranged in a matrix to form a thin film transistor array, and an electrophoretic image display medium sandwiched between the thin film transistor and a counter electrode was driven, the on / off ratio was small, and a favorable display was obtained. could not.

<Evaluation of electrical contact resistance>
The contact resistance between the semiconductor layer and the source / drain electrode of each of Examples 1, 2, 3 and Comparative Example was evaluated. In the evaluation, the output characteristics were measured using a semiconductor parameter analyzer B1500A (manufactured by Agilent), and the contact resistance was calculated from the results of the output characteristics by using the TLM method (Transfer Line Method). Contact resistance measurements, ¹⁰⁵ [Omega] cm "++" and not more than "+" and those of less than 10 ⁵ greater than [Omega] cm to 10 ⁶ [Omega] cm, what is 10 ⁶ [Omega] cm or more - Table 1 as "" Shown in Table 1 also shows the respective on / off ratios and the determination of the results. For on-off ratio, if it is 10 ⁴ or more indicated by "++" as having a good and stable transistor characteristics, as on-off ratio was transistor characteristics 10 ³ indicated by "+" in less than 10 ³ In some cases, "-" indicates that the transistor characteristics were not good. As shown in Table 1, the one in which the second metal layer 9 was formed and the surface modification layer was formed showed the best characteristics. Even when the surface modification layer was not formed, good characteristics were exhibited by forming the second metal layer 9.

  The present invention is useful for an image display device, an RFID tag, a sensor, and the like.

Reference Signs List 1 substrate 2 gate electrode 3 insulating film layer 4 source electrode 5 drain electrode 6 semiconductor layer 7 protective layer 8 first metal layer 9 second metal layer 10 first surface modification layer 11 second surface modification layer 21 source electrode 22 drain electrode 30 sputtered metal film 31 resist 32 patterned resist 100, 101, 102, 500 thin film transistor

Claims (9)

An insulating substrate, a gate electrode, a gate insulating layer, a source electrode and a drain electrode, a thin film transistor including an organic semiconductor layer,
The thin film transistor, wherein each of the source electrode and the drain electrode includes a first metal layer and a second metal layer stacked on a side surface of the first metal layer.   The thin film transistor according to claim 1, wherein the second metal layer includes any of silver, gold, platinum, palladium, and copper.   The thin film transistor according to claim 1, wherein the first metal layer includes molybdenum or an alloy of molybdenum.   The thin film transistor according to claim 1, wherein a surface modification treatment is performed on the second metal layer.   The thin film transistor according to claim 4, wherein a different surface modification treatment is applied to each of the first metal layer and the second metal layer. A step of forming a gate electrode on an insulating substrate, a step of forming a gate insulating layer, a step of forming a source electrode and a drain electrode, and a step of forming an organic semiconductor layer,
Forming the source electrode and the drain electrode,
After the first metal layer is formed, a step of laminating a second metal layer on a side surface of the first metal layer, and performing a surface treatment on the first metal layer and the second metal layer And a method for manufacturing a thin film transistor.   An image display device comprising an image display medium using the thin film transistor according to claim 1.   The image display device according to claim 7, wherein the image display medium is an electrophoretic body for displaying by an electrophoresis method.   A sensor device using the thin-film transistor according to claim 1.