Patent Publication Number: US-2004053431-A1

Title: Method of forming a flexible thin film transistor display device with a metal foil substrate

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a flexible display process, and more particularly, to a method of forming a flexible thin film transistor (TFT) display with a metal foil substrate, such as an aluminum alloy foil, a titanium foil or a titanium alloy foil.  
       [0003] 2. Description of the Related Art  
       [0004] Plastic displays with plastic substrates are attractive for use in flat panel displays because such displays are lightweight, flexible and unbreakable.  
       [0005] However, during the high temperature process of forming an active device such as a thin film transistor (TFT) directly on a plastic substrate, the plastic substrate which has a glass transition temperature of less than 200° C. is subject to dimensional instability. Moreover, because of the very large difference in thermal expansion coefficients between the plastic substrate and the TFT, the thin film of the TFT may separate or crack during the repeated thermal cycles required by the fabrication process and is thus detrimental to device reliability and yield.  
       [0006] Additionally, the plastic substrate is inadequately protected from chemicals, oxygen, moisture, mechanical impact, static electricity, and other damaging factors. As a result life span of plastic-based display devices is limited.  
       [0007] Recently, a stainless steel foil, serving as a flexible substrate display material, has been investigated. Stainless steel, however, with a density of about 7.9 g/cm 3 , has the disadvantage of being heavier than plastic and hinders the development of lightweight displays. Additionally, the higher thermal expansion coefficient (about 17.3E-6/° C.) of stainless steel may affect the deposition of a thin film transistor (TFT). Therefore, it is important to discover an optimal substrate material.  
       SUMMARY OF THE INVENTION  
       [0008] The object of the present invention is to provide a method of forming a flexible thin film transistor display.  
       [0009] Another object of the present invention is to provide a method of forming a flexible thin film transistor display having a metal foil substrate, wherein the metal foil substrate is an aluminum alloy foil, a titanium foil or a titanium alloy foil.  
       [0010] In order to achieve these objects, the present invention provides a method of forming a flexible thin film transistor (TFT) display device. A metal foil serving as a flexible metal substrate of a display device is provided, wherein the metal foil is an aluminum alloy foil, a titanium foil or a titanium alloy foil. The thickness of the metal foil is 0.05˜0.8 mm. An insulation layer is formed on the flexible metal substrate. A thin film transistor (TFT) array is formed on the insulation layer. In addition, the aluminum alloy foil can include magnesium of 0.01˜1% wt and/or silicon of 0.01˜1% wt and the titanium alloy foil can include aluminum of 0.01˜20% wt and/or molybdenum of 0.01˜20% wt.  
       [0011] The present invention improves on the prior art in that it utilizes a metal foil, such as an aluminum alloy foil, a titanium foil or a titanium alloy foil, as a flexible substrate, and an insulation layer is then formed on the surface of the metal foil. Since aluminum alloy, titanium and titanium alloy have the properties of a high melting point (above 600° C.), low density, low thermal expansion coefficient, good flexibility, and high strength, aluminum alloy, titanium or titanium alloy foils are an optimal substrate material for flexible thin film transistor (TFT) display devices. Thus, the present invention is suitable for use in fabrication of flexible displays, such as flexible thin film transistor (TFT) displays. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012] The present invention can be more fully understood by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein:  
     [0013] FIGS.  1 ˜ 2  are sectional views according to an embodiment of the present invention.  
     [0014]FIG. 3 is a sectional view showing an application of the present invention to a flexible reflective liquid crystal display device; and  
     [0015]FIG. 4 is a sectional view showing another application of the present invention to a flexible organic electroluminescent display device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0016] The present invention provides a method of forming a flexible thin film transistor (TFT) display device. FIGS.  1 ˜ 2  are schematic diagrams according to an embodiment of the present invention.  
     [0017] In FIG. 1, a metal foil  100  serving as a flexible metal substrate  100  of a display device, wherein the metal foil is an aluminum alloy (Al alloy) foil, a titanium (Ti) foil or a titanium alloy (Ti alloy) foil. The thickness of the metal foil  100  is about 0.05˜0.8 mm.  
     [0018] It should be noted that, when using the Al alloy foil as the flexible metal substrate  100 , the Al alloy foil includes other beneficial elements, such as silicon (Si) and/or magnesium (Mg). For example, adding magnesium (Mg) can decrease the density of the Al alloy foil, thereby reducing weight. Adding silicon (Si) can reduce the thermal expansion coefficient of the Al alloy foil. Also, silicon (Si) can improve the mechanical property of the Al alloy foil, for example, enhancing strength, toughness and rigidity. The content of magnesium (Mg) in the Al alloy foil is preferably 0.01˜1% wt, and the content of silicon (Si) in the Al alloy foil is preferably 0.01˜1% wt.  
     [0019] Additionally, pure titanium (Ti) foil is also well suited to serve as the flexible metal substrate  100  in a thin film transistor (TFT) process because pure titanium (Ti) has a high melting point of about 1668° C., a low density of about 4.45 g/cm 3 , a low thermal expansion coefficient of about 8.4E-6/° C., and sufficient strength. Moreover, in order to enhance rigidity of the titanium (Ti) foil, a Ti alloy foil comprising aluminum of 0.01˜20% wt and/or molybdenum of 0.01˜20% wt can be utilized. Thus, the flexible metal substrate  100  comprising Al alloy foil, Ti foil, or Ti alloy foil is well suited for use in high temperature processes.  
     [0020] In FIG. 1, an insulation layer  110  with a thickness of about 500˜10000 angstroms is formed on the metal substrate  100 . As a demonstrative method of forming the insulation layer  110  on the metal substrate  100 , referring to FIG. 1, a metal oxide film  102  is formed on the surface of the metal substrate  100  by thermal oxidation. When the metal substrate  100  is an Al alloy foil, the metal oxide film  102  is an Al 2 O 3  film. When the metal substrate  100  is a Ti foil or a Ti alloy foil, the metal oxide film  102  is a TiO 2  film.  
     [0021] In FIG. 1, using deposition, an insulating film  104  is formed on the metal oxide film  102  to improve the insulation property of the insulation layer  110 . The insulating film  104  can be a SiO 2  layer, a TiO 2  layer or a SiN x  layer. That is, the insulation layer  110  can be a stack structure.  
     [0022] In FIG. 2, a thin film transistor (TFT) array  210  is formed on the insulation layer  110 . The TFT structure of the TFT array  210  can be a bottom gate type TFT or a top gate type TFT structure. In FIG. 2, the TFT array  210  with the bottom gate type TFT structure is shown, but is not intended to limit the invention. The bottom gate type TFT structure includes a gate electrode  22 , a gate insulating layer  24 , a semiconductor layer  26  and source/drain regions  28 / 30 . In order to avoid obscuring aspects of the invention, the description of the known TFT process is omitted.  
     [0023] It should be noted that the aluminum alloy substrate, the titanium substrate, and the titanium alloy substrate of the present invention all have a high melting point of above 600° C., enabling their use in conventional high temperature TFT (e.g. a-Si TFT or LTPS TFT) processes of the invention. Thus, the present invention is well suited to the fabrication of flexible displays, such as flexible thin film transistor (TFT) displays.  
     [0024] Hereinafter, a flexible reflective liquid crystal display device (RLCD) and a flexible organic electroluminescent display device (OELD) as demonstrative applications will be explained next with reference to FIGS. 3 and 4.  
     [0025] First Application  
     [0026]FIG. 3 is a sectional view showing an application of the present invention to a flexible reflective liquidcrystal display device.  
     [0027] In FIG. 3, a plurality of transparent pixel electrodes  310  are formed to electrically connect to the TFT array  210 . The transparent pixel electrode  310  may be an indium tin oxide (ITO) or indium zinc oxide (IZO) layer formed by sputtering. Then, a common electrode  330  is formed on an inner side of a transparent plastic substrate  340  opposite the metal substrate  100 . The transparent plastic substrate  340  serves as an upper substrate. The common electrode  330  may be an indium tin oxide (ITO) or indium zinc oxide (IZO) layer formed by sputtering. A display material, such as liquid crystal molecules, is then filled in a space between the metal substrate  100  and the transparent plastic substrate  340  to form a display layer  420 .  
     [0028] It should be noted that the above mentioned liquid crystal material can comprise other display materials, such as, microcapsules with an electrophoretic characteristic.  
     [0029] Second Application  
     [0030]FIG. 4 is a sectional view showing another application of the present invention to a flexible organic electroluminescent display device.  
     [0031] In FIG. 4, a plurality of anode electrodes  410  are formed to electrically connect to the TFT array  210 . The anode electrode  410  may be an indium tin oxide (ITO) electrode. Then, an emitter layer such as an organic electroluminescent layer  420  is formed on the anode electrodes  410  by, for example, evaporation. The organic electroluminescent layer  420  includes low polymer or high polymer material. A cathode electrode  430  is then formed on the organic electroluminescent layer  420 . The cathode electrode  430  may be a metal electrode. Finally, a transparent plastic substrate  440  is disposed on the cathode electrode  330 .  
     [0032] Thus, the present invention produces a novel flexible TFT display device using the metal foil, such as the aluminum alloy foil, the titanium foil or the titanium alloy foil, as a substrate. Also, since the melting point of the metal foils is above 600° C. and the thermal expansion coefficient of the metal foils is lower than conventional plastic plates, the metal foils are well suited for use in current equipment employing high temperature TFT processes, thereby improving device reliability and yield and ameliorating the disadvantages of the prior art. In addition, aluminum alloy, titanium, and titanium alloy foils are less expensive and thereby reduce manufacturing cost.  
     [0033] Finally, while the invention has been described by way of example and in terms of the above, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.