Abstract:
Substrate structures for display devices and fabrication methods thereof The substrate structure comprises a substrate, an interfacial layer disposed on the substrate, and a patterned paste layer applied on the interfacial layer, wherein a contact angle of the interface between the patterned paste layer and the interfacial layer exceeds 35 degrees.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to display devices and fabrication methods thereof, and in particular, to substrate structures for display applications and fabrication methods thereof. 
         [0003]    2. Description of the Related Art 
         [0004]    Field emission display (FED) devices are panelized conventional cathode ray tube (CRT) displays. By using screen printing technology, large scale FED devices can be achieved. Conventional large scale FED devices have many advantages such as low volume, light weight, low power consumption, excellent image quality, and applicability 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]      FIG. 1  is cross section of a conventional field emission display device. In  FIG. 1 , a field emission display device  10  comprises a pair of opposing parallel substrates  11  and  12 . The lower substrate  11  comprises a cathode electrode, gate line and electron field emitter  13  thereon. The upper substrate  12  comprises an anode electrode  14 . A phosphor layer  15  is applied on the anode electrode  14 . When a bias is applied between the cathode and anode electrodes, electrons emit from field emitter (or cathode electrode) toward the anode electrode and then bombard phosphor layer  15  exciting visible light. Large scale FED devices can be used as a back light, referred to as field emission back light unit (FE-BLU). Conventional FE-BLU and FED, however, require a lithographic process to achieve high resolution patterned electrodes. If the electrodes can be patterned by screen printing, intricate exposure apparatus, development apparatus and consumption of developer can be saved, thus reducing production cost. 
         [0006]      FIGS. 2A-2D  are cross sections showing the lithographic fabrication steps of a conventional FED device. Referring to  FIG. 2A , a lower substrate  22  such as transparent glass substrate is provided. A conductive layer  21  is deposited on the lower substrate  11 . 
         [0007]    Referring to  FIG. 2B , the conductive layer  21  is patterned by lithography. For example, a photo resist (not shown) is applied on the conductive layer  21 . A mask  51  is disposed on the photo resist exposed under a UV light source. After developing, the conductive layer  21  is etched and patterned, as shown in  FIG. 2C . The patterned conductive layer comprises a cathode pattern  24  and a gate line pattern  36 . 
         [0008]    Referring to  FIG. 2D , a carbon nanotube field emitter  25  is subsequently formed on the cathode pattern  24 . For example, a carbon nanotube paste is screen printed on the cathode pattern  24 . After photo spacers and ribs are formed on the lower substrate, the lower substrate and upper substrate are assembled, completing fabrication of the FED device. 
         [0009]    Conventional screen printing technology uses a squeegee to press paste through a patterned screen, thereby transferring the pattern to a substrate. Thick film screen printing technology is a well-developed technology for reducing cost and mass production in conventional electronic industries. Resolution of thick film screen printing, however, is limited by screen meshes and spread of patterned paste, hindering high resolution printing. For example, referring to  FIG. 3 , a paste pattern  120  is transferred onto a substrate  110  by screen printing. Since the interface between the paste pattern  120  and the substrate  110  includes low contact angle a, spread of the paste pattern  120  occurs leading to low resolution. More specifically, the contact angle between cathode paste pattern and the glass substrate is very small, thus spread of the cathode paste pattern on the glass substrate deteriorates. Further, if the viscosity of the paste is low, the printed pattern line width can be twice as wide as the pattern line width on the screen mesh, reducing line width resolution. Thus, eliminating paste spread to improve line width resolution from several hundreds of micrometers to several tens of micrometers in resolution is desirable. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    Accordingly, substrate structures for display applications are provided by interposing an interfacial layer between the paste pattern and the substrate to prevent spread of the paste pattern and to achieve high density, high resolution FED devices. 
         [0011]    The invention provides a substrate structure, comprising: a substrate, an interfacial layer disposed on the substrate, and a patterned paste layer applied on the interfacial layer, wherein a contact angle of the interface between the patterned paste layer and the interfacial layer exceeds 35 degrees. 
         [0012]    The invention further provides a substrate structure, comprising a substrate, an interfacial layer disposed on the substrate, a patterned paste layer applied on the interfacial layer, a dielectric layer disposed on the patterned paste layer, and a gate electrode disposed on the dielectric layer, wherein a contact angle of the interface between the patterned paste layer and the interfacial layer exceeds 35 degrees. 
         [0013]    The invention further provides a substrate structure, comprising a substrate, an interfacial layer disposed on the substrate, a patterned paste layer applied on the interfacial layer, a patterned insulating wall structure disposed on the interfacial layer dividing a plurality of pixel regions, and a fluorescent layer disposed in each pixel region covering the patterned paste layer, wherein a contact angle of the interface between the patterned paste layer and the interfacial layer exceeds 35 degrees. 
         [0014]    The invention still further provides a method of fabricating a substrate structure. A substrate is provided. A surface treatment process is performed on the substrate to change the polarity of the substrate. A patterned paste layer is applied on the treated surface of the substrate, wherein a contact angle of the interface between the patterned paste layer and the treated surface of the substrate exceeds 35 degrees. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0016]      FIG. 1  is cross section of a conventional field emission display device; 
           [0017]      FIGS. 2A-2D  are cross sections showing the lithographic fabrication steps of a conventional FED device; 
           [0018]      FIG. 3  is a schematic view of a paste pattern transferred onto a substrate by screen printing which includes a low contact angle α leading to low resolution; 
           [0019]      FIGS. 4A-4C  are cross sections showing fabrication steps of a substrate structure for a field emission back light unit (FE-BLU) according to an embodiment of the invention; 
           [0020]      FIG. 5  is a cross section of a CNT-FED device according to an exemplary embodiment of the invention; 
           [0021]      FIGS. 6A-6C  are cross sections showing fabrication steps of a substrate structure for a plasma display panel (PDP) according to another embodiment of the invention; and 
           [0022]      FIG. 7  is a cross section of a PDP device according to another exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The following description is of the best-contemplated 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. 
         [0024]    The invention is directed to a substrate structure for display applications. An interfacial layer is disposed on a substrate to prevent spread of electrode paste patterns on the substrate. The interfacial layer can improve surface tension of the electrode paste and reduce wettability between the electrode paste patterns and the substrate. The contact angle between the electrode paste patterns and the interfacial layer is preferably greater than 35°, more preferably greater than 40°. Since the interfacial layer can prevent spread of electrode paste, the contact angle between the electrode paste pattern and the interfacial layer is greater the contact angle between the electrode paste pattern and the substrate. Compared with printing an electron paste pattern of 50 μm line width and 50 μm line interval on a glass substrate, the contact angle can increase by at least 15° due to addition of the interfacial layer. Moreover, a substrate structure with a high resolution electron paste pattern of 17 μm line width and 83 μm line interval can further achieved due to addition of the interfacial layer. 
         [0025]      FIGS. 4A-4C  are cross sections showing fabrication steps of a substrate structure for a field emission back light unit (FE-BLU) according to an embodiment of the invention. Referring to  FIG. 4A , a substrate  210  such as a transparent glass substrate or a flexible substrate is provided. An interfacial layer  220  or formed on the substrate  210 . The interfacial layer  220  can prevent spread of subsequent printed electrode paste patterns and improve surface tension of the electrode paste patterns. The contact angle between the electrode paste patterns and the interfacial layer is preferably greater than 35°, more preferably greater than 40°. Any interfacial layer which can increase the contact angle by at least 15° is suitable for preventing spread of the electrode paste pattern. 
         [0026]    Referring to  FIG. 4B , an electrode patterns including a cathode electrode pattern  224  and a gate line pattern  222  are formed on the interfacial layer  220 . For example, a patterned conductive paste layer is screen printed on the interfacial layer  220 . Since the difference in wettability between the patterned conductive paste layer and the interfacial layer  220  is apparent, the surface tension of the patterned conductive paste layer on the interfacial layer  220  is strong, resulting in a high contact angle between the patterned conductive paste layer and the interfacial layer  220 . Compared with printing a patterned conductive paste layer directly formed on the glass substrate, the contact angle can increase at least 15° due to addition of the interfacial layer. 
         [0027]    Referring to  FIG. 4C , a carbon nanotube field emitter  225  is formed on the cathode electrode pattern  224 . For example, a carbon nanotube paste is screen printed on the cathode electrode pattern  224 . The interface between the carbon nanotube paste and the cathode electrode pattern  224  includes a high contact angle to prevent spread of the carbon nanotube paste pattern on the cathode electrode pattern  224 . Subsequently, a photo spacer and a continuous rib are formed on the substrate structure. The substrate structure is assembled with a corresponding upper substrate, thus, fabrication of a FED or a FE-BLU is complete. 
         [0028]    According to embodiments of the invention, the interfacial layer can be transparent or opaque. The interfacial later can comprises conductive or metallic materials. The interfacial layer and the electrode paste pattern can be co-fired for process simplification. Note that any material which can increase the contact angle at least 15° is suitable for the interfacial layer to prevent spread of the electrode paste patterns. 
         [0029]    The interfacial layer can comprise insulating materials, such as SiO 2 , SiO y , SiN x , SiC, B 2 O 3 , Al 2 O 3 , SrBaTiO 3 , ZnS, ZrO 2 , BST, PZT, HfSiO z , HfO 2 , ZnO or Polyimide. The interfacial layer can alternatively comprise Pb, Zn, B, Si, or Bi, or oxides thereof which are sintered at low temperature with high transparency and flatness. Moreover, the interfacial layer can alternatively comprise conductive material such as Ag, Cu, Au, Pd, Pt, CNT, or other electrode materials which can serve as an interface between an electrode and an electrode field emitter. The interfacial layer can alternatively comprise a green tape. The green tape can preferably comprise a silicide, a boride, a metal oxide, a metal nitride, or combination thereof Moreover, the patterned paste layer comprises an emitter paste, phosphor paste, conductor paste, dielectric layer paste, or binder layer paste. For example, the emitter paste may comprise carbon nanotube (CNT), diamond like carbon (DLC), graphite, PdO, or TiO W . The conductor paste may comprise a metal paste (e.g, Ag, Au, Cu, Pt, or Pd), or conducting polymer (e.g., PEDOT or polyaniline). The dielectric paste may comprise SiO 2 , SiO y , SiN x , SiC, B 2 O 3 , ZnO, ZnS, ZrO 2 , BST, PZT, HfSiO z , HfO 2 , or polyimide. The interfacial layer can alternatively comprise a sintered silicon oxide, aluminum oxide, or combinations thereof. Note that a surface improvement process can be performed on the substrate. For example, the interfacial layer can be formed on a sand blasted substrate to remedy a damaged substrate surface to increase contact angle. 
         [0030]    Accordingly, the interfacial layer for use in the present invention is not limited to those types described above, and may be of the other types if applicable to the present invention. Several materials with different surface tension and wettability can be chosen to serve as an electrode comprising a high contact angle with an electron field emitter thereon. The straightness and resolution of the screen printing can be improved due to the interfacial layer. Those skilled in the art will appreciate that other substrate structures, such as FE-BLU, CNT-FED structures and plasma display panels (PDP), are also applicable to the invention. 
         [0031]      FIG. 5  is a cross section of a CNT-FED device according to an exemplary embodiment of the invention. In  FIG. 5 , a CNT-FED device  500  comprises a lower substrate  501  and an upper substrate  502 . A wall structure  550  or a rib structure separating the lower and upper substrates with a predetermined gap G. The lower and upper substrates are sealed in vacuum. An interfacial layer  505  is disposed on the lower substrate  501 . A patterned cathode structure  510  is formed on the interfacial layer  505 . A CNT thick film  515  is disposed on the patterned cathode structure  510  to serve as an electron field emitter. A dielectric layer  520  surrounding the patterned cathode structure  510  is disposed on the lower substrate  501 . A gate electrode  530  is disposed on the dielectric layer  520 . 
         [0032]    An anode electrode  560  is disposed on the upper substrate  502 . Red, green, and blue fluorescent layers  575  are alternately disposed on the anode electrode  560 . A black matrix  570  is disposed between the red, green, and blue fluorescent layers  575 . 
         [0033]      FIGS. 6A-6C  are cross sections showing fabrication steps of a substrate structure for a plasma display panel (PDP) according to another embodiment of the invention. Referring to  FIG. 5A , a substrate  610  such as a transparent glass substrate or a flexible substrate is provided. An interfacial layer  620  is formed on the substrate  610 . The interfacial layer  620  can prevent spread of subsequently printed electrode paste patterns and improve surface tension of the electrode paste patterns. The contact angle between the electrode paste patterns and the interfacial layer is preferably greater than 35°, more preferably greater than 40°. Any interfacial layer which can increase the contact angle by at least 15° is suitable for preventing spread of the electrode paste patterns. 
         [0034]    Subsequently, a patterned cathode electrode  630  or data electrode is formed on the interfacial layer  620 . For example, a patterned conductive paste layer is screen printed on the interfacial layer  620 . Since the difference in wettability between the patterned conductive paste layer and the interfacial layer  620  is apparent, the surface tension of the patterned conductive paste layer on the interfacial layer  620  is strong, resulting in a high contact angle between the patterned conductive paste layer and the interfacial layer  620 . Compared with printing a patterned conductive paste pattern layer directly formed on the glass substrate, the contact angle can increase by at least 15° due to addition of the interfacial layer. 
         [0035]    Referring to  FIG. 6B , a patterned continuous rib structure  640  is formed on the interfacial layer  620  dividing a plurality of pixel regions. For example, a photoresist layer is formed on the interfacial layer  620  and then patterned. Alternatively, the photoresist layer can be directly screen printed on the interfacial layer  620 . The interface between the patterned continuous rib structure  640  and the interfacial layer  620  includes a high contact angle to prevent spread of the patterned continuous rib structure  640  on the interfacial layer  620 . 
         [0036]    Referring to  FIG. 6C , a fluorescent layer  650  is formed in each pixel region and covering the patterned cathode electrode  630 . The substrate structure is assembled with a corresponding upper substrate, thus, fabrication of a PDP device is complete. 
         [0037]      FIG. 7  is a cross section of a PDP device according to another exemplary embodiment of the invention. In  FIG. 7 , a PDP device  700  comprises a lower substrate  601  and an upper substrate  690 . A wall structure  640  or a rib structure separates the lower and upper substrates with a predetermined gap G. The lower and upper substrates are sealed in vacuum or optionally filled some insert gases sequentially. An interfacial layer  620  is disposed on the lower substrate  610 . A patterned cathode structure  630  is formed on the interfacial layer  620 . A fluorescent layer  650  is formed in each pixel region and covers the patterned cathode electrode  630 . 
         [0038]    The upper substrate  690  comprises an anode electrode structure including a scan electrode  680   a  and a sustain electrode  680   b.  A dielectric layer  670  is disposed on the upper substrate  690  covering the scan electrode  680   a  and the sustain electrode  680   b.  A passivation layer  660  such as an MgO layer is disposed on the dielectric layer  670 . 
         [0039]    Accordingly, the invention is advantageous in that an interfacial layer which can control surface tension between a glass substrate and a patterned paste is formed on a substrate structure. The interfacial layer can change wettability between the glass substrate and the patterned paste. Since the interfacial layer can maintain surface tension between the glass substrate and the patterned paste, the contact angle increases due to the interfacial layer. A high contact angle can prevent the spread of the patterned paste, thereby reducing the interval of line patterns and increasing resolution. Moreover, the interfacial layer can be a highly transparent material to meet requirements for FE-BLU. A sand blast pretreatment may be needed on the glass substrate. The sand blasted glass substrate, however, comprises a low contact angle, leading to spread of the patterned paste. The interfacial layer can be formed on the substrate treated by sand blasting to remedy damage due to the sand blasting. 
         [0040]    Compared with printing an electron paste pattern of 50 μm line width and 50 μm line interval on a glass substrate, the contact angle can increase by at least 15° due to addition of the interfacial layer. Moreover, a substrate structure with a high resolution electron paste pattern of 17 μm line width and 83 μm line interval can further be achieved due to addition of the interfacial layer. 
         [0041]    While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. 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.