Patent Publication Number: US-6660324-B2

Title: Viewing screen for a display device and method for the fabrication thereof

Description:
This is a divisional of application Ser. No. 09/520,911 filed Mar. 8, 2000, now U.S. Pat. No. 6,400,072, which is hereby incorporated by reference, and priority thereto tar common subject matter is hereby claimed. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains to the area of viewing screens and methods for fabricating viewing screens for display devices and, more particularly, to a viewing screen for a field emission display and method for the fabrication thereof. 
     BACKGROUND OF THE INVENTION 
     Methods for fabricating viewing screens having black surround on a glass substrate are known in the art. It is known in the art to fabricate a black surround material using glass binders and pigments. These materials are known to have linear thermal expansion coefficients within a range of about 10×10 −6  to 12×10 −6 ° C. −1 . The prior art black surround materials are adequate for the combinations of temperature and type of glass substrate utilized in prior art methods for fabricating viewing screens. For example, prior art methods typically expose the black surround and the glass substrate to temperatures of up to 550° C. 
     However, it may be desirable to utilize higher temperatures, at which the prior art combinations of black surround and glass substrate may be inadequate. For example, it is believed to be desirable in the fabrication of field emission displays to utilize process temperatures up to about 600° C. First, the glass substrate must be able to withstand such temperatures. Furthermore, the black surround-substrate interface must not crack during the heat treatments. 
     However, prior art viewing screens may not be adequate for repeated high temperature treatments. For example, they may have temperature tolerances that are less than these higher temperatures. Soda lime silicate is a typical glass substrate, which can tolerate temperatures up to only 540° C. Furthermore, even if the glass can withstand the higher temperature, a mismatch of the thermal expansion coefficients of the black surround material and the glass substrate can undesirably result in the cracking of the black surround-substrate interface. 
     For example, it is known to use borosilicate glass for the glass substrate in field emission displays. It is believed to be desirable to increase processing temperatures up to about 600° C. Borosilicate glass can withstand these higher processing temperatures. However, borosilicate glass has a thermal expansion coefficient equal to about 4×10 −6 ° C. −1 , which is appreciably less than that of the standard thick-film black surround. Thus, if the standard thick-film black surround is used on the borosilicate glass, the black surround-substrate interface cracks during high temperature thermal cycling. 
     Accordingly, there exists a need for an improved method for fabricating a viewing screen for a display device, which provides a viewing screen that maintains its physical integrity at temperatures up to at least 550° C. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a bottom plan view of a preferred embodiment of a viewing screen fabricated in accordance with the method of the invention; and 
     FIG. 2 is a cross-sectional view of a preferred embodiment of a display device having the viewing screen of FIG. 1, in accordance with the invention. 
    
    
     It will be appreciated that for simplicity and clarity of illustration, elements shown in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the drawings to indicate corresponding elements. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is for a viewing screen for a display device. The viewing screen of the invention has a black matrix, which includes a black surround, a ductile metal, and lead titanate. The black matrix is affixed to a glass substrate that has a thermal coefficient of expansion within a range of 3.5×10 −6 -4.5×10 −6 ° C. −1 . One benefit of the viewing screen of the invention is the occurrence of little or no cracking of the black matrix during repeated thermal treatments of the viewing screen at temperatures less than about 600° C. This benefit is realized by the method of the invention for fabricating a viewing screen, which includes the step of adding to a black surround paste a ductile metal paste and lead titanate particles. 
     FIG. 1 is a bottom plan view of a preferred embodiment of a viewing screen  100  fabricated in accordance with the method of the invention. Viewing screen  100  includes a glass substrate  110  and a black matrix  111 , which is affixed to glass substrate  110 . Black matrix  111  preferably has a thickness within a range of 2-6 micrometers. Black matrix  111  further defines phosphor vias  112 . A cathodoluminescent phosphor  114  is disposed in each of phosphor vias  112 . 
     In accordance with the invention, glass substrate  110  preferably has a thermal coefficient of expansion within a range of 3.5×10 −6 -4.5×10 −6 ° C. −1 . Borosilicate and aluminosilicate glasses exhibit thermal expansion coefficients within this range. 
     In accordance with the invention, black matrix  111  includes a black surround, a ductile metal, and lead titanate. The concentration of the ductile metal is sufficient to provide elastic and non-elastic stress relief within black matrix  111 , and the concentration of lead titanate is sufficient to control crack propagation in black matrix  111 . Preferably, the concentration of the ductile metal is sufficient to provide elastic and non-elastic stress relief within black matrix  111  upon repeated heating to a temperature within a range of about 450-600° C., and the concentration of lead titanate is sufficient to control crack propagation in black matrix  111  upon repeated heating to a temperature within a range of about 450-600° C. 
     The black surround of black matrix  111  is preferably made up of about 10 wt % ruthenium oxide and about 90 wt % a glass. The glass of the black surround most preferably has a firing temperature that is less than 600° C., so that firing occurs at a temperature below the critical temperature of glass substrate  110 . Preferably, the glass of the black surround is a low melting point solder glass composed primarily of lead oxide, bismuth trioxide, and silica. Most preferably, the glass of the black surround has about 87.5 wt % lead oxide, about 12.5 wt % bismuth trioxide, and trace silica. 
     The ductile metal of black matrix  111  is preferably silver. Gold can alternatively be employed for the ductile metal component. 
     An exemplary final composition of black matrix  111  is: about 7.1 wt % ruthenium oxide, about 21.4 wt % silver, about 4.8 wt % lead titanate, and about 66.7 wt % glass. 
     A method for fabricating viewing screen  100  in accordance with the invention includes the steps of providing a black surround paste, adding to the black surround paste a ductile metal paste, and adding to the black surround paste lead titanate particles. In general, the amount of ductile metal paste is sufficient to realize elastic and non-elastic stress relief within black matrix  111 , and the amount of lead titanate particles is sufficient to control the propagation of cracks in black matrix  111 . 
     The black surround paste has a vitreous solder glass, a pigment, a solvent, and a binder. Preferably, the black surround paste has greater than 50 wt % vitreous solder glass, 5-30 wt % pigment, 10-35 wt % solvent, and 5-30 wt % binder. Most preferably, the black surround paste has about 54 wt % vitreous solder glass, about 6 wt % pigment, within 30-35 wt % solvent, and within 5-10 wt % binder. 
     The vitreous solder glass preferably has a firing temperature equal to less than 600° C. Preferably, the glass is a low melting point solder glass composed primarily of lead oxide, bismuth trioxide, and silica. A preferred vitreous solder glass for the black surround paste has about 85-90 wt % lead oxide, about 10-15 wt % bismuth trioxide, and trace-5 wt % silica. Most preferably, the vitreous solder glass for the black surround paste has about 87.5 wt % lead oxide, about 12.5 wt % bismuth trioxide, and trace silica. 
     An exemplary pigment for use in the black surround paste is ruthenium oxide. Exemplary solvents for use in the black surround paste are alpha terpineol, butyl carbitol acetate, trimethylpentanediol monoisobutyrate, and the like. Exemplary binders for use in the black surround paste are acrylic resin, ethyl cellulose, and the like. 
     The ductile metal paste preferably has greater than 50 wt % ductile metal, 10-35 wt % solvent, 5-30 wt % binder, and 1-10 wt % glass. Most preferably, the ductile metal paste has 51-54 wt % ductile metal, 30-35 wt % solvent, 5-10 wt % binder, and 6-9 wt % glass. Preferably, the step of adding to the black surround paste a ductile metal paste includes the step of adding to the black surround paste a silver metal paste. Alternatively, a gold metal paste can be used. Exemplary solvents for use in the ductile metal paste are alpha terpineol, butyl carbitol acetate, trimethylpentanediol monoisobutyrate, and the like. Exemplary binders for use in the ductile metal paste are acrylic resin, ethyl cellulose, and the like. The glass of the ductile metal paste is a low melting point solder glass composed primarily of lead oxide, bismuth trioxide, and silica. For example, the glass of the ductile metal paste can have about 87.5 wt % lead oxide, about 12.5 wt % bismuth trioxide, and trace silica. 
     Preferably, the amount of ductile metal paste added to the black surround paste is selected such that, if viewing screen  100  were repeatedly heated from room temperature (about 25° C.) to a temperature within a range of about 450-600° C. and then cooled naturally, elastic and non-elastic stress relief within black matrix  111  would be realized. 
     The step of adding to the black surround paste lead titanate particles preferably includes the step of adding to the black surround paste lead titanate particles having a mean particle diameter equal to about 1 micrometer. Furthermore, the amount of lead titanate particles is preferably selected such that, if viewing screen  100  were repeatedly heated from room temperature to a temperature within a range of about 450-600° C. and then cooled naturally, control of the propagation of cracks in black matrix  111  would be realized. 
     After the addition to the black surround paste of the ductile metal paste and the lead titanate particles, the black surround paste is deposited on glass substrate  110 , which is then patterned and heated to a temperature and for a duration sufficient to affix the black surround paste to glass substrate  110 . 
     Another method for fabricating a viewing screen for a display device in accordance with the invention includes the step of adding to the black surround paste a ductile metal paste, such that the black surround paste has a weight within a range of 75-85% of the combined weight of the black surround paste and the ductile metal paste. The ductile metal paste has a weight within a range of 15-25% of the combined weight of the black surround paste and the ductile metal paste. This method further includes the step of adding to the black surround paste lead titanate particles having a weight within a range of 1-5% of the combined weight of the black surround paste and the ductile metal paste. 
     Yet another method for fabricating a viewing screen for a display device in accordance with the invention includes the step of adding to a black surround paste a ductile metal paste and lead titanate particles, wherein the ductile metal paste and lead titanate particles are provided in amounts sufficient to realize an extent of cracking in black matrix  111  upon repeatedly heating to a temperature within a range of about 450-600° C. that is at least 95% less than the extent of cracking exhibited by an unimproved black matrix upon repeatedly heating to a temperature within a range of about 450-600° C. The unimproved black matrix is made only from the material of the black surround paste; its fabrication does not include the steps of adding a ductile metal paste or adding lead titanate particles to the black surround paste. The unimproved black matrix has a thickness equal to the thickness of black matrix  111 . 
     For example, a viewing screen was fabricated, wherein the black surround paste included 54 wt % vitreous solder glass, 6 wt % ruthenium oxide pigment, 30 wt % trimethylpentanediol monoisobutyrate solvent, and 10 wt % acrylic resin binder. The vitreous solder glass included about 87.5 wt % lead oxide, about 12.5 wt % bismuth trioxide, and trace silica. The ductile metal paste was a silver metal paste having 54 wt % silver, 6 wt % vitreous solder glass, 5 wt % acrylic resin binder, and 35 wt % trimethylpentanediol monoisobutyrate. The combination of the black surround paste and ductile metal paste included 75 wt % black surround paste and 25 wt % silver metal paste. The weight of the lead titanate particles was equal to 5% of the combined weight of the black surround paste and ductile metal paste. An additional amount of trimethylpentanediol monoisobutyrate solvent was added to the mixture of the black surround paste and silver metal paste. The weight of the additional solvent was equal to about 10% of the combined weight of the black surround paste and the ductile metal paste. 
     The black surround paste was batched, mixed using an ultrasonic horn, and allowed to roll for about 24 hours. Then, the black surround paste was deposited by screen printing on a substrate made from aluminosilicate glass. The film was patterned to form the phosphor vias. Thereafter, the black surround paste and glass substrate were heated at a temperature of about 520° C. for 55 minutes, thereby affixing the black surround paste to the glass substrate, and then allowed to cool naturally. 
     For comparison purposes, an unimproved black matrix was formed on an aluminosilicate glass substrate. The unimproved black matrix was made by depositing on the substrate a film of the black surround paste, which included 54 wt % vitreous solder glass, 6 wt % ruthenium oxide pigment, 30 wt % trimethylpentanediol monoisobutyrate solvent, and 10 wt % acrylic resin binder. The vitreous solder glass included about 87.5 wt % lead oxide, about 12.5 wt % bismuth trioxide, and trace silica. The method for making the unimproved black matrix did not include the steps of adding a ductile metal paste or adding lead titanate particles to the black surround paste. The thickness of the deposited film was made equal to the thickness of the film used to make the black matrix of the invention. The glass substrate having the film of the black surround paste was heated at 520° C. for 55 minutes and then allowed to cool naturally. 
     To compare the extent of cracking, the viewing screen made in accordance with the invention was heated to 450° C. for 55 minutes and then allowed to cool naturally to room temperature. This firing cycle was repeated two more times. The viewing screen having the unimproved black matrix was similarly heated to 450° C. for 55 minutes and then allowed to cool naturally to room temperature. Only one firing cycle was performed on the unimproved viewing screen. Visual inspection of the black matrices using an optical microscope at 75×magnification revealed prolific cracking and peeling in the unimproved black matrix, whereas the black matrix that was made In accordance with the invention exhibited only nominal cracking. 
     FIG. 2 is a cross-sectional view of a preferred embodiment of a display device  115  having viewing screen  100  of FIG. 1, in accordance with the invention. In the preferred embodiment of FIG. 2, display device  115  is a field emission display. Display device  115  includes a cathode plate  120 , which opposes viewing screen  100 . 
     Cathode plate  120  includes a substrate  122 , which can be made from glass, silicon, and the like. A cathode  124  is disposed upon substrate  122 . Cathode  124  is connected to a first voltage source  130 . A dielectric layer  125  is disposed upon cathode  124 , and further defines a plurality of emitter wells, each of which contains an electron emitter  128 . 
     The display device described herein is directed to a field emission display device employing Spindt tip emitter structures. However, the scope of the invention is not intended to be limited to field emission display devices or to devices having Spindt tip emitter structures. The invention can be embodied by a viewing screen and display device that employ an electron source and a cathodoluminescent phosphor for generating the display image. For example, the invention can be embodied by a cathode ray tube display device. Also, in a field emission display device, the electron emitter structure can be a structure other than a Spindt tip, such as an edge emitter, wedge emitter, or surface emitter. 
     As further illustrated in FIG. 2, cathode plate  120  includes a gate electrode  126 , which is disposed on dielectric layer  125  and is connected to a second voltage source (not shown). Application of selected potentials to cathode  124  and gate electrode  126  cause electron emitters  128  to emit electrons for activating phosphors  114 . 
     Viewing screen  100  is spaced apart from cathode plate  120  by a frame  116  to define an interspace region  118 . The fabrication of display device  115  includes the step of affixing viewing screen  100  to frame  116 . The affixant can be a glass frit, which requires a sealing temperature of about 450° C. The beneficial properties of viewing screen  100  allow it to be heated to this sealing temperature without creating cracks in black matrix  111 . 
     During the operation of display device  115 , a potential is applied to phosphors  114  for attracting thereto electrons emitted by electron emitters  128 . A third voltage source  132  is connected to viewing screen  100  for providing this anode potential. 
     In summary, the invention is for a viewing screen for a display device. The viewing screen of the invention is a combination of a black matrix, which includes a black surround, a ductile metal, and lead titanate, and a glass substrate, which has a thermal coefficient of expansion within a range of 3.5×10 −6 -4.5×10 −6 ° C. −1 . The viewing screen of the invention can be repeatedly heated to a temperature within a range of about 450-600° C. and thereafter cooled without cracking the black matrix. The method of the invention for fabricating the viewing screen includes the steps of adding to a black surround paste a ductile metal paste and adding to the black surround paste lead titanate particles in amounts sufficient to realize the beneficial cracking properties of the viewing screen of the invention. 
     While we have shown and described specific embodiments of the present invention, further modifications and improvements will occur to those skilled in the art. For example, the invention is embodied by a viewing screen that includes phosphors, which are activated by ultraviolet light, rather than electrons. We desire it to be understood, therefore, that this invention is not limited to the particular forms shown, and we intend in the appended claims to cover all modifications that do not depart from the spirit and scope of this invention.