Abstract:
An electron-emitting device including a protective layer that is formed on a catalyst layer to protect the catalyst layer from the deleterious environmental conditions before or during a cathode process. The present invention further includes a half etching process that is adapted to partially remove portions of the protective layer from the catalyst layer to etch the catalyst layer except carbon nano-tube growing portions. Portions of the protective layer still remain on the catalyst layer to protect the catalyst layer from the deleterious conditions from next cathode formation process.

Description:
FIELD OF USE 
     This invention relates to carbon nano tube based field emitters display. More particularly, this invention relates to the structure and fabrication of an electron-emitting device in which a protective layer is selectively etched during the formation of a catalyst layer which is suitable for use in a flat-panel display of the cathode-ray tube (“CRT”) type. 
     BACKGROUND 
     A Cathode Ray Tube (CRT) display generally provides the best brightness, highest contrast, best color quality and largest viewing angle of prior art computer displays. CRT displays typically use a layer of phosphor which is deposited on a thin glass faceplate. These CRTs generate a picture by using one to three electron beams which generate high energy electrons that are scanned across the phosphor in a raster pattern. 
     The phosphor converts the electron energy into visible light so as to form the desired picture. However, prior art CRT displays are large and bulky due to the large vacuum envelopes that enclose the cathode and extend from the cathode to the faceplate of the display. Therefore, typically, other types of display technologies such as active matrix liquid crystal display, plasma display and electro-luminescent display technologies have been used in the past to form thin displays. 
     Recently, a thin flat panel display (FPD) has been developed which uses the same process for generating pictures as is used in CRT devices. These flat panel displays use a backplate including a matrix structure of rows and columns of electrodes. One such flat panel display is described in U.S. Pat. No. 5,541,473 which is incorporated herein by reference. Flat panel displays are typically matrix-addressed and they comprise matrix addressing electrodes. The intersection of each row line and each column line in the matrix defines a pixel, the smallest addressable element in an electronic display. 
     The essence of electronic displays is the ability to turn on and off individually picture elements (pixels). A typical high information content display will have about a quarter million pixels in a 33 cm diagonal orthogonal array, each under individual control by the electronics. The pixel resolution is normally just at or below the resolving power of the eye. Thus, a good quality picture can be created from a pattern of activated pixels. 
     One means for generating arrays of field emission cathode structures relies on well established semiconductor micro-fabrication techniques. These techniques produce highly regular arrays of precisely shaped field emission tips. Lithography, generally used in these techniques, involves numerous processing steps, many of them wet. The number of tips per unit area, the size of the tips, and their spacing are determined by the available photo-resist and the exposing radiation. 
     Emitter tips produced by the method are typically cone-shaped with base diameters on the order of 0.5 to 1 um, heights of anywhere from 0.5 to 2 um, tip radii of tens of nano-meters. This size limits the number of tips per pixel possible for high resolution displays, where large numbers (400–1000 emitters per pixel) are desirable for uniform emission to provide adequate gray levels, and to reduce the current density per tip for stability and long lifetimes. Maintaining two dimensional registry of the periodic tip arrays over large areas, such as large TV-sized screens, can also be a problem for gated field emission constructions by conventional means, resulting in poor yields and high costs. 
     U.S. Pat. No. 4,338,164 describes a method of preparing planar surfaces having a micro-structured protuberances thereon comprising a complicated series of steps involving irradiation of a soluble matrix (e.g., mica) with high energy ions, as from a heavy ion accelerator, to provide column-like traces in the matrix that are subsequently etched away to be later filled with an appropriate conductive, electron-emitting material. The original soluble material is then dissolved following additional metal deposition steps that provide a conductive substrate for the electron emitting material. The method is said to produce up to 10 6  emitters per cm2, the emitters having a diameter of approximately 1–2 um. 
     U.S. Pat. No. 5,266,530 describes a gated electron field emitter prepared by a complicated series of deposition and etching steps on a substrate, preferably crystalline. 
     Carbon, the most important constituent element, which is combined with oxygen, hydrogen, nitrogen and the like, of all organisms including the human body, has four unique crystalline structures including diamond, graphite and carbon. Carbon nano-tubes can function as either a conductor or a semi-conductor according to the constituents of the tube. A conventional approach of fabricating carbon nano-tubes is described in an article entitled “epitaxial carbon nanotube film self-organized by sublimation decomposition of silicon carbide” (Appl. Phys. Lett. Vol. 77, pp. 2620, 1997), by Michiko Kusunoky. In the conventional approach, the carbon nano-tubes are produced at high temperatures by irradiating a laser onto a graphite silicon carbide. In this particular approach, the carbon nano-tubes are produced from graphite at about 1200° C. or more and for silicon carbide at a temperature range of about 1600° C. to 1700° C. However, this method requires a multi-stage approach of deposition of the carbon material. This method is, from a manufacturing perspective, costly and cumbersome. 
     Another conventional approach is to grow the carbon nano-tubes on a silicon substrate. This approach requires that the carbon nano-tube material be deposited at temperature higher than 700° C. to ensure a purified and defect-free vertically aligned carbon nano-tube structure. 
     Any attempt to grow the carbon nano-tube structure at temperatures on the contaminated catalyst results in a defective structure. This conventional approach also results in the inability to control the height of the carbon structure. 
       FIG. 3  is an illustration of a prior art carbon nano-tube structure. The carbon nano-tube structure shown in  FIG. 3  comprises a substrate  101  with a catalyst metal layer  240  upon which carbon nano-tube layer  310  is deposited. The catalyst layer  240  diffuses into the silicon layer  120  during the growing of the carbon nano-tube layer  310 . The carbon nano-tube layer  310  is grown by a plasma deposition and etching method at temperatures ranging from 500° C. to 900° C. The plasma density in this approach ranges from a high density of 10 11  cm 3  or more. In the structure in  FIG. 3 , the diffusion of the catalyst layer  240  into the silicon layer  11  results in a high amount of carbon material being deposited to form the nano-tube structure. 
     The catalyst layer  240  is provided to facilitate the uniform growth of the carbon nano-tubes. For non-defective carbon nano-tube formation, the conditions of the catalyst layer, such as the materials, the thickness, uniformity and surface conditions of the catalyst layer are very important. In the prior art method of growing the carbon nano-tubes, the catalyst layer is typically exposed to the chemicals and gas process that are used to fabricate the cathode prior to growing the carbon nano-tubes. This deleteriously affects the catalyst layer and could result in the contamination of the catalyst layer prior to the formation of the carbon nano-tubes. Having a contaminated catalyst layer also results in the poor growing of the carbon nano-tubes. 
     A method of forming a catalyst layer that has not been exposed to the deleterious conditions of the prior art is thus needed to promote the quality growth of carbon nano-tube. 
     GENERAL DISCLOSURE OF THE INVENTION 
     The present invention furnishes an electron-emitting device having a catalyst layer patterned to meet enable the growth and curing of carbon nano-tube structures on glass substrates. The present catalyst layer contains multiple laterally separated sections situated between electron emitting carbon nano-tube, on one hand, and underlying emitter electrodes, on the other hand. The present invention provides a catalyst layer that is formed in a way to reduce the deleterious effects that growing the carbon nano-tubes provide. The sections of the catalyst layer are spaced apart along each emitter electrode. 
     The catalyst sections underlie gate holes of the present electron-emitting device in various ways. In one general embodiment, the catalyst sections are basically configured as strips situated below the carbon nano-tubes. 
     In another general embodiment of the catalyst layers are formed on a glass substrate and the carbon nano-tubes are formed on the catalyst layer at a temperature of about 600°. An anti-diffusion barrier layer is also interlaced between the catalyst layer and other layers including a resistive layer in order to ensure a defect-free carbon nano-tube formation. 
     Embodiments of the present invention include a protective layer that is formed on the catalyst layer to protect the catalyst layer from the deleterious environmental conditions before or during the next cathode process. 
     Embodiments of the carbon nano-tubes structure of the present invention use sputtering process to deposit the catalyst layer on the glass substrate. The catalyst layer may also be deposited by an evaporation process. A post growth treatment of the carbon nano-tube structure is further performed to control the height of the structures in a plasma chemical vapor deposition environment. 
     Embodiments of the present invention further include a half etching process that is adapted to partially remove portions of the protective layer from the catalyst layer to etch catalyst layer except carbon nano-tube growing portions. Portions of the protective layer still remain on the catalyst layer to protect the catalyst layer from the deleterious conditions from next cathode formation process. 
     These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a prior art patterned catalyst layer structure. 
         FIG. 2  is a cross-sectional structural view of a prior art carbon nano-tube structure with a damage catalyst layer in a cathode structure. 
         FIG. 3  is cross-sectional view a prior art carbon nano-tube device. 
         FIG. 4  is cross-sectional structural view representing a step in manufacturing an embodiment of the carbon nano-tube device in accordance with the teachings of the present invention. 
         FIG. 5  is a cross-sectional structural view of one embodiment of the carbon nano-tube structure after the formation of a catalyst layer. 
         FIG. 6  is a cross-sectional structural view of one embodiment of a carbon nano-tube structure with a protective layer in accordance with the teachings of the present invention. 
         FIG. 7  is a cross-sectional structural view of one embodiment of the partial removal of the protective layer of  FIG. 6 . 
         FIG. 8  is a cross-sectional structural view of one embodiment of the removal of excess catalyst material from the top of the carbon nano-tube structure of  FIG. 7 . 
         FIG. 9  is a cross-sectional structural view of one embodiment of the excess protective layer material from the top of the catalyst layer in accordance with the teachings of the present invention. 
         FIG. 10  is a cross-sectional structural view of one embodiment of a carbon nano-tube cathode structure according to the teachings of the present invention. 
     
    
    
     Like reference symbols are employed in the drawings and in the description of the preferred embodiments to represent the same, or very similar, item or items. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the present invention, a vertical conductor connected in series with electron-emissive elements of an electron-emitting device is patterned into multiple sections laterally separated along each emitter electrode in the device. The electron emitter of the invention typically operates according to field-emission principles in producing electrons that cause visible light to be emitted from corresponding light-emissive phosphor elements of a light-emitting device. The combination of the electron-emitting device, often referred to as a field emitter, and the light-emitting device forms a cathode-ray tube of a flat-panel display such as a flat-panel television or a flat-panel video monitor for a personal computer, a lap-top computer, or a workstation. 
     In the following description, the term “electrically insulating” (or “dielectric”) generally applies to materials having a resistivity greater than 10 10  ohm-cm. The term “electrically non-insulating” thus refers to materials having a resistivity below 10 10  ohm-cm. Electrically non-insulating materials are divided into (a) electrically conductive materials for which the resistivity is less than 1 ohm-cm and (b) electrically resistive materials for which the resistivity is in the range of 1 ohm-cm to 10 10  ohm-cm. These categories are determined at an electric field of no more than 1 volt/μm. 
     Examples of electrically conductive materials (or electrical conductors) are metals, metal-semiconductor compounds (such as metal silicides), and metal-semiconductor eutectics. Electrically conductive materials also include semiconductors doped (n-type or p-type) to a moderate or high level. The semiconductors may be of the mono-crystalline, multi-crystalline, polycrystalline, or amorphous type. 
     Electrically resistive materials include (a) metal-insulator composites such as cermet, (b) certain silicon-carbon compounds such as silicon carbide and silicon-carbon-nitrogen, (c) forms of carbon such as graphite, amorphous carbon, and modified (e.g., doped or laser-modified) diamond, and (d) semiconductor-ceramic composites. Further examples of electrically resistive materials are intrinsic and lightly doped (n-type or p-type) semiconductors. 
     As used below, an upright trapezoid is a trapezoid whose base (a) extends perpendicular to the direction taken as the vertical, (b) extends parallel to the top side, and (c) is longer than the top side. A transverse profile is a vertical cross section through a plane perpendicular to the length of an elongated region. The row direction in a matrix-addressed field emitter for a flat-panel display is the direction in which the rows of picture elements (pixels) extend. The column direction is the direction in which the columns of pixels extend and runs perpendicular to the row direction. 
     Referring to  FIG. 4 , a substrate  401  for use in the formation of the carbon nano-tubes according to an embodiment of the present invention is shown. An emitter electrode  410  is formed on the substrate  401 . In the preferred embodiment of the present invention, the substrate  401  is glass. In one embodiment of the present invention, the substrate  401  is ceramic, silicon or quartz. 
     A resistor layer  420  is subsequently disposed on the emitter electrode  410 . The resistor layer  420  provides the carbon nanotube that are formed in the structure  400  uniform emission characteristics. A barrier layer  430  is subsequently formed on the resistor layer  420  and serves as an anti-diffusion layer for the catalyst layer upon which the carbon nano-tubes are formed. In one embodiment of the present invention, the barrier layer  430  may be formed of a metal. 
     In one embodiment, the metal may be molybdenum. In another embodiment, the metal may be titanium or titanium tungsten or titanium nitride. In one embodiment of the present invention, the barrier layer  430  may be an alloy of titanium, titanium tungsten, tungsten, titanium nitride or molybdenum. After the formation of the barrier layer  430 , the resistor layer  420 , an insulator layer  440 , gate electrode  450  and a passivation layer  460 , a gate hole is etched through these layers to form a gate hole through which the carbon nano-tubes are formed. 
     A catalyst layer  510  and  515  is subsequently formed over the barrier layer  430  as shown in  FIG. 5 . In one embodiment of the present invention, the catalyst layer  510  and  515  is formed by a sputtering deposition process. In one embodiment of the present invention, the catalyst layer  510  and  515  may be formed by an evaporation process. In one embodiment of the present invention, the catalyst layer  510  is deposited to a thickness of about 1 nm to 100 nm. In one embodiment of the present invention, the catalyst layer  510  may be made of Nickel or an alloy thereof. In one embodiment, the catalyst layer may be cobalt or iron or alloys thereof. 
     In  FIG. 6 , a structure  600  is formed by the deposition, a protective layer  620  is formed by depositing a photo-resist material over the exposed layers in structure  500  and gate hole  465 , in one embodiment, by a self aligning normal deposition, as opposed to an angle of evaporation, whereby the protective material fills the gate hole  465  and covers the catalyst layer  510 . In one embodiment of the present invention, the protective material possesses etching characteristics incompatible with the etching characteristics of the catalyst layer  510  and  515 . In one embodiment of the present invention, the protective layer  620  may be coated over the catalyst layer  510  and the exposed layers of structure  500 . 
     After the deposition of the protective material  620 , a half etching process is applied to remove portions of the protective material from the exposed layers of structure  600 . In one embodiment of the present invention, all of the protective material is removed except in the gate hole where some of the protective material is left to protect the catalyst layer  510  during the catalyst layer  515  removal step. In one embodiment of the present invention, the protective material  620  remains in the gate hole  465  to cover and protect the catalyst layer  510  from subsequent etching steps of the structure  600 . The protective material  620  remains in the gate hole  465  because the depth of the protective material in the gate hole  465  is deeper than the depth of the protective material covering the exposed layers of the structure  600 . 
     After partially etching the protective layer  620 , the catalyst layer  515  is exposed and then etched by either a dry etching process or a wet etching process. In one embodiment of the present invention, if a Ni catalyst layer material is used, the etchant to partially remove the catalyst layer  515  may comprise of H 3 PO 4 , HNO 3  or CH 3 COOH. In one embodiment of the present invention, a gas etchant is used to partially remove the protective layer  620  from the structure  600 . In one embodiment of the present invention, prior to the partial etching of the protective material  620 , the etch rate of the protective material is determined in order to determine the amount of time it takes to partially etch the protective material. In one embodiment of the present invention, the protective material may be etched at a rate of 400 sccm of O 2  to 5 mins for a protective material with a thickness of about 1.5 um. 
     Referring now to  FIG. 7 , a structure  700  is formed after the partial removal of the protective material. As shown in  FIG. 7 , portions of the catalyst layer  515  is exposed after the protective material has been partially removed. The catalyst layer  515  is subsequently removed as shown in  FIG. 8 . Removing of the exposed catalyst layer  515  does not affect the protected catalyst layer  510  which is protected by the portion of the protective material  620  that was not removed from the gate hole  465 . 
     After the excess exposed catalyst layer  515  has been removed by an etching process, the excess protective material  620  is removed through an etching process from the gate hole  465 . Etching the excess protective material  620  does not have a deleterious effect on the catalyst layer  510 . This is because the etchant used, in one embodiment of the present invention, to remove the excess protective material has no effect on the catalyst layer  510 . 
     In one embodiment of the present invention, the etchant used to etch the catalyst layer  510  has good selectively with respect to the surface materials of the protective layer  620  and the catalyst layer  510 . After the excess protective material has been etched and the catalyst layer  510  exposed as shown in  FIG. 9 , the carbon nanotube of the present invention are formed on the catalyst layer  510  as shown in  FIG. 10 . 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications that are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalent.