Abstract:
Methods for fabricating field emission display devices. A first substrate is provided. A cathode structure is formed on the first substrate. A surface treatment procedure is performed on the first substrate with cathode structure thereon. A second substrate opposing the first substrate is provided and assembled in vacuum with a wall rib therebetween. The surface treatment procedure includes free radical oxidization and a supercritical CO 2  fluid cleaning.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to field emission display (FED) devices, and in particular to methods for fabricating field emission display devices. 
         [0003]    2. Description of the Related Art 
         [0004]    Field emission display (FED) devices are panelized conventional cathode ray tube (CRT) displays. Using screen printing technology, large scale FED devices can be achieved. Conventional larger scale FED devices provide low volume, light weight, low power consumption, excellent image quality, and are applicable to a variety of electronic and communication devices. Carbon nanotube or other nano-scale field emitters have benefits such as low threshold field, high emission current density, and high stability due to lower threshold voltage, higher light efficiency, higher viewing angle, and lower power consumption. 
         [0005]    Compared with conventional large scale display devices, CRT displays have excellent display quality but occupy a large amount of space. Projection TVs occupy less space but offer poor display quality. Plasma display panel (PDP) displays exhibit lighter, thinner features and can be fabricated by screen printing, nonetheless, they require high power consumption. 
         [0006]    Field emission display (FED) devices are self-emitting display devices including an array of micro vacuum tube field emitters. In operation, electrons are emitted from field emitters by biasing control voltage on the gate electrode while maintaining high voltage on the anode such that the emitted electrons bombard the phosphor with large amounts of energy. The field emitters are conventionally formed by semiconductor thin film process to provide an emitter array on the cathode substrate. The field emitters are typically inorganic materials such as Mo, W, Si, or the like. Field emitters formed by semiconductor thin film process, however, require high cost apparatus and are difficult to achieve on a large scale. 
         [0007]      FIG. 1  is a cross section of a conventional field emission display device  10 , comprising a lower substrate  11  and an opposing upper substrate  12  with a specific gap G therebetween supported by a wall structure. The lower substrate  11  and upper substrate  12  are sealed in a vacuum. A patterned cathode structure  13  is disposed on the lower substrate  11 . A field emitter  14  is disposed on the cathode structure  13 . The patterned cathode structure  13  is surrounded by a dielectric layer  15  with a gate electrode  16  thereon. 
         [0008]    An anode electrode  17  is disposed on the upper substrate  12 . A phosphor layer comprising red  18 R, green  18 G, and blue  18 B elements is disposed on the anode electrode  17 . A black matrix (BM)  19  is interposed among the phosphor layer with red  18 R, green  18 G, and blue  18 B elements. 
         [0009]    To simplify production processes and achieve large scale display, thick film screen printing is employed to fabricate large scale field emission display devices. Conventional thick film screen printing method, however, forms stacked materials as cathode structure on the lower substrate. The stacks are co-fired or sintered at the same temperature. Some impurity residues may remain on the surface of the electron emission layer, creating porous structure, affecting field emission efficiency. 
         [0010]    U.S. Pub. No. 2005/0062195, the entirety of which is hereby incorporated by reference, discloses an adhesive film attached on the field emitters of the lower substrate. The adhesive film is released from the field emitters of the lower substrate, thereby removing impurity residues from the surface and improving electron emission alignment to vertical field. 
         [0011]      FIGS. 2A-2B  are cross sections of a method for fabricating a FED device using an adhesive film attached on the field emitters of the lower substrate. In  FIG. 2A , a substrate  35  with a cathode electrode structure  40  thereon is provided. Patterned isolation structure  50  and gate electrode  60  are formed on the cathode electrode structure  40 . A field emission structure  70 A is attached on the cathode electrode structure  40  using an adhesive tape  30  as shown in  FIG. 2B . The field emission structure  70 A, however, exhibits degraded field emission efficiency. Moreover, the adhesive tape  30  cannot be reused, increasing production cost. The surface of the field emitters may be damaged during release of the adhesive tape  30 . The organic residue from the adhesive tape  30  may result in the field emitter arching at high operating voltages, degrading properties of the FED devices. 
         [0012]    In another conventional method for improving field emission uniformity, the surface of the field emitters is rubbed. The field emitters are well-aligned and provide improved electron emission alignment to vertical field. The roller used in the rubbing, however, may leave residual dust or impurities on the surface of the field emitters, which can result in the field emitter arching at high operation voltage, degrading properties of the FED devices. 
         [0013]    Another conventional method for improving field emission uniformity is provided by sandblasting the surface of the field emitters. The field emitters are bombarded by high energy small rigid particles to remove impurities. Some particles may, however, remain, degrading properties of the FED devices. 
         [0014]    U.S. Pat. No. 6,890,230, the entirety of which is hereby incorporated by reference, discloses a fabrication method for a field emission display device utilizing laser activation to normalize orientation of carbon nanotubes.  FIGS. 3A-3B  are a cross section of a conventional method of laser activation to create carbon nanotube (CNT) emitters with uniform orientation. In  FIG. 3A , a field emission display device comprises a lower substrate  110  with a cathode  120  thereon. A CNT thick film  130  is formed on the cathode  120  as a field emitter. An upper substrate  160  is disposed opposing the lower substrate  110 . An anode  150  is disposed on the upper substrate  160 . A voltage controller  140  applies bias between the cathode  120  and the anode  150 , thereby controlling the field emission display device. A laser source  170  passing through the upper substrate  160  and anode  150  radiates the CNT thick film  130  to activate the field emitter.  FIG. 3B  is a cross section of the field emission display device activated by laser treatment of  FIG. 3A . 
         [0015]    The field emission display device activated by laser treatment can, however, be damaged by undesirable heating. For example, the upper substrate  160 , anode  150 , dielectric layer and gate electrode may be damaged by laser heating. Moreover, if the laser treatment is performed after the field emission display device is assembled, it is difficult to address and align the laser source, inter alia, for high definition FED devices, resulting in intricate fabrication procedures and reduced throughput. 
       BRIEF SUMMARY OF THE INVENTION 
       [0016]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
         [0017]    Accordingly, the invention is related to a surface treatment method for FED devices. By thoroughly removing impurities and contaminants from the field emitters, uniformity of the field emission display device is improved. High-efficiency environmentally friendly surface treatment methods are provided. A plurality of substrates can be treated simultaneously without producing additional contaminants, thereby preventing arching due to high operation voltage and improving stability of the FED device in a high vacuum. 
         [0018]    The invention provides a method for fabricating a display device. A first substrate is provided. A cathode structure is formed on the first substrate. A surface treatment is performed on the cathode structure. A second substrate is provided opposing the first substrate with a rib wall structure therebetween, assembled in a vacuum. 
         [0019]    The invention further provides a method for fabricating a field emission display. A first substrate is provided. A cathode structure comprising a cathode electrode, a field emitter on the cathode electrode, and a gate electrode is formed by screen printing on the first substrate, wherein the field emitter comprises a carbon nanotube (CNT), a carbon nanofiber (CNF), graphite, palladium oxide (PdO), polysilicon, diamond film, or carbon nitride (C x N y ). A surface treatment is performed on the cathode structure. A second substrate is provided opposing the first substrate with a rib wall structure therebetween, assembled in a vacuum. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0021]      FIG. 1  is a cross section of a conventional field emission display device; 
           [0022]      FIGS. 2A-2B  are cross sections schematically illustrating a method for fabricating a FED device using an adhesive film attached to the field emitters of the lower substrate; 
           [0023]      FIG. 3A  is a cross section of a conventional method of laser activation to create carbon nanotube (CNT) emitters with uniform orientation; 
           [0024]      FIG. 3B  is a cross section of the field emission display device activated by laser treatment of  FIG. 3A ; 
           [0025]      FIG. 4A  is a fabrication flowchart of a FED panel according to an embodiment of the invention; 
           [0026]      FIG. 4B  is a flowchart showing the surface treatment and activation of  FIG. 4A ; 
           [0027]      FIGS. 5A-5C  are cross sections showing fabrication of a substrate structure for a field emission display (FED) device according to an embodiment of the invention; 
           [0028]      FIGS. 6A-6B  are schematic views illustrating free radical oxidization treatment and supercritical CO 2  fluid treatment of the cathode substrate according to an embodiment of the invention; and 
           [0029]      FIG. 7  is a cross section of a CNT-FED device according to an exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    The following description is of the mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0031]    The invention is related to an FED panel and surface treatment methods thereof. The cathode substrate is activated by methods combining free radical oxidization and supercritical carbon dioxide fluid cleaning to improve uniformity and stability of the FED panel. A plurality of cathode substrates can be treated simultaneously to purify and modify surface properties of the field emitters without producing potential contaminants. Furthermore, surface properties of carbon nanotube powders can be modified according to a embodiment, thereby improving uniformity and stability of the FED panel. 
         [0032]      FIG. 4A  is a fabrication flowchart of a FED panel according to an embodiment of the invention. In step  310 , a lower substrate of the FED panel is formed. In step  320 , an upper substrate of the FED panel is formed. In step  330 , the lower substrate and the upper substrate are assembled and sealed in a vacuum, thus the field emission display device is completed. 
         [0033]    Step  310  of forming a lower substrate of the FED device comprises synthesizing field emitter powders (ex. CNT) (step  301 ) by, for example, arc discharge, chemical vapor deposition (CVD), or laser ablation. The field emitter powders are gathered in a container. The field emitter powders are mixed into a field emitter paste in step  303 . Next, in step  304 , a patterned cathode structure is formed by screen printing the field emitter paste on a substrate. Surface treatment and activation (step  305 ) are performed on the patterned cathode structure. The patterned cathode structure is sintered or fired (step  306 ) to complete the lower substrate of the field emission display (FED) device. 
         [0034]    Step  320  of forming an upper substrate of the FED device comprises forming a conductive layer or electrode on a substrate (step  312 ). Next, in step  314 , a patterned anode structure is formed on the substrate and sintered (step  316 ). A fluorescent layer is formed on the anode structure to complete the upper substrate of the field emission display (FED) device. 
         [0035]      FIG. 4B  is a flowchart showing the surface treatment and activation of  FIG. 4A . The surface treatment and activation comprises loading a cathode structure substrate in a reaction chamber (step  410 ). Subsequently, a free radical oxidization surface treatment (step  420 ) is performed. The step of free radical oxidization surface treatment can optionally comprise UV treatment ( 425   a ), O 3  treatment ( 425   b ), or UV/O 3  treatment ( 425   c ). After the free radical oxidization surface treatment, the cathode structure substrate is transferred to a supercritical CO 2  fluid reaction chamber in step  430 . Subsequently, a supercritical CO 2  fluid cleaning treatment is performed. The cathode structure substrate is loaded in a supercritical CO 2  fluid reaction chamber. After the pressure and temperature of the supercritical CO 2  fluid reaction chamber and addition ratio of the modifier are set, the supercritical CO 2  fluid is conducted into the chamber to clean cathode structure substrate (steps  440  and  450 ). After the cleaning step is completed, the pressure and temperature of the reaction chamber are reduced followed by removal of the cathode structure substrate from the supercritical CO 2  fluid reaction chamber (steps  460  and  470 ). 
         [0036]    The physical properties of supercritical fluid are similar to transition between gas phase and liquid phase. The supercritical fluid exhibits low viscosity, high diffusion coefficient, and low surface tension similar to gas phase, but further high density like liquid phase. Chemical properties of the supercritical fluid differ from gas phase and liquid phase, such as the supercritical CO 2  fluid, thereby becoming organically soluble. The organic solubility of the supercritical CO 2  fluid depends on temperature and pressure of the supercritical fluid. The organic solute in the supercritical CO 2  fluid is precipitated with temperature and pressure reduction, producing gas phase CO 2  which is recyclable. 
         [0037]      FIGS. 5A-5C  are cross sections showing fabrication steps of a substrate structure for a field emission display (FED) device according to an embodiment of the invention. Referring to  FIG. 5A , a substrate  510  such as a glass substrate or a flexible substrate is provided. A conductive layer  512  is formed on the substrate  510 . 
         [0038]    Referring to  FIG. 5B , the conductive layer  512  is patterned into a cathode electrode pattern  513  and a gate line pattern  514  by, for example, lithography or etching. Alternatively, a patterned conductive layer  512  can be screen printed on the substrate  510 . 
         [0039]    Referring to  FIG. 5C , a field emitter  515  is formed on the cathode electrode pattern  513  by, for example, carbon nanotube paste screen printing, completing fabrication of the substrate with cathode structure. Note that the formation of the field emitter  515  can optionally comprise screen printing, micro-contact printing, ink-jet printing, electrophoresis deposition (EPD), or chemical vapor deposition (CVD). Furthermore, the field emitter can comprise a carbon nanotube (CNT), a carbon nanofiber (CNF), graphite, palladium oxide (PdO), polysilicon, diamond film, or carbon nitride (C x N y ). 
         [0040]      FIGS. 6A-6B  are schematic views illustrating free radical oxidization treatment and supercritical CO 2  fluid treatment of the cathode substrate according to an embodiment of the invention. Referring to  FIG. 6A , the cathode substrate for the FED device is irradiated by a UV light source with a wavelength in a range of 185-254 nm. Preferably, the wavelength of the UV light source is 185 nm or 254 nm in about 3 min. The distance between the cathode substrate and the UV light source is about 0.2 cm. Alternatively, O 3  can be conducted into the process chamber during UV irradiation, or simply conduct O 3  gas performing free radical oxidization. 
         [0041]    Subsequently, referring to  FIG. 6B , the cathode substrate for the FED device is transferred into a processing chamber  650  full of supercritical CO 2  fluid  620 . After gas phase to supercritical fluid phase transition, the supercritical CO 2  fluid becomes organically soluble. Operating pressure of the supercritical CO 2  fluid is preferably controlled at about 3000 psi, and that of the supercritical CO 2  fluid is preferably controlled at about 50° C. The supercritical CO 2  fluid cleaning lasts about 5 min. More preferably, an additional modifier such as 7% n-propanol can improve the cleaning capability of the supercritical CO 2  fluid. 
         [0042]      FIG. 7  is a cross section of a CNT-FED device according to an exemplary embodiment of the invention. In  FIG. 7 , a CNT-FED device  700  comprises a lower substrate  701  and an upper substrate  702 . A wall structure  750  or a rib structure separates the lower and upper substrates by a predetermined gap G. The lower and upper substrates are sealed in a vacuum. The lower substrate  702  includes a patterned cathode structure  710 . A CNT thick film  715  is disposed on the patterned cathode structure  710  to serve as a field emitter. A dielectric layer  720  surrounding the patterned cathode structure  710  is disposed on the lower substrate  702 . A gate electrode  730  is disposed on the dielectric layer  720 . 
         [0043]    An anode electrode  706  is disposed on the upper substrate  702 . Red, green, and blue fluorescent layers  775  are alternatively disposed on the anode electrode  706 . A black matrix  770  is disposed between the red, green, and blue fluorescent layers  775 . 
         [0044]    The invention provides a surface treatment method comprising free radical oxidization and supercritical CO 2  fluid cleaning. The surface treatment method is applicable with FED devices comprising a horizontal triode structure, a vertical triode structure, or an undergate triode structure. The disclosed treatment deeply cleans the field emitter without leaving impurities or contaminants, resulting in increased brightness and improved display uniformity. 
         [0045]    While the invention has been described by way of example and in terms of the embodiment, it is to be understood that the invention is not limited thereto. To 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.