Abstract:
A two-layer antistatic/antireflective coating technique employs a 2-step sputtering approach for first depositing an inner metallic antistatic layer on the outer surface of a glass display screen of a video display device such as a cathode ray tube (CRT), followed by deposit of an outer antireflective layer on the antistatic layer. The inner metallic antistatic layer is comprised of Ti and Cr and has a light refractive index of 2.0-2.8 and a thickness of 2-8 nm. The outer antireflective layer is comprised of SiO 2  and MgO and has a light refractive index of 1.3-1.47 and a thickness of 70-100 nm. Light transmission through the inner metallic antistatic layer may be closely adjusted as desired by precise control of the thickness of this inner layer.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the application of a surface coating to a glass display screen of a video display device and is particularly directed to the deposit of a 2-layer antistatic/antireflective coating on the display screen of a video display device such as a color cathode ray tube by means of sputtering.  
         BACKGROUND OF THE INVENTION  
         [0002]    The outer surface of a display screen, or panel, of a video display device such as a cathode ray tube (CRT) is typically provided with a multi-layer coating which performs various functions. These functions include reducing light transmission through the glass display screen/outer coating combination for improved video image contrast. In addition, an inner layer of the surface coating is electrically conductive in order to shield viewers of the video display device from low frequency electromagnetic radiation and to dissipate electrostatic charge on the display panel to neutral ground. The coating also typically provides an antireflective capability to reduce light reflection from the display screen for ease in viewing a video image on the display screen.  
           [0003]    Various approaches are employed in applying the multi-layer coating to the outer surface of a display screen. These techniques include spin coating, sometimes referred to as the wet method, vacuum vapor deposition, and sputtering. The spin coating method has been widely used with materials containing Ag—Pd or Ag—Au colloid. While the coating thus formed possesses good electrical conductivity and relatively low light reflectance, it is of relatively low quality and involves high processing costs. The spin coating approach also suffers from problems with reproducibility and control of the thickness of the coating. The vacuum vapor deposition approach involves high temperature heat treatment and is thus energy intensive and more expensive than the spin coating approach. The sputtering approach has encountered difficulties in forming at high speed a stable SiO 2  layer having a low refractive index for use in the antireflection layer. One approach for applying a light absorptive antireflective layer to a CRT display screen is disclosed in U.S. Pat. No. 5,691,044. This sputtering approach applies an inner layer of TiN to the surface of a glass substrate. The TiN layer suffers from instability at the high temperatures used for applying the multi-layer coating to the glass substrate. To improve the heat resistance of the TiN layer, an oxide barrier layer of metal nitride (TiN) is formed on the inner TiN layer. This approach requires various reacting gases such as N 2  and O 2  in the sputtering process which increases the cost and complexity of video display screen manufacture.  
           [0004]    The present invention avoids the limitations of the prior art by providing a multilayer antistatic/antireflective coating applied by sputtering to the outer surface of a video display screen which allows for precise control over the thickness of the multilayer coating as well as its light transmission characteristics.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0005]    Accordingly, it is an object of the present invention to provide a multi-layer antistatic/antireflective coating for a video display screen wherein the thickness of the coating may be precisely controlled for precise adjustment of the light transmitted through the coating.  
           [0006]    It is another object of the present invention to provide an antistatic/antireflective coating for the outer surface of a video display panel having precisely controlled conductivity as low as on the order of 103 ohms and reflectivity as low as 0.7%.  
           [0007]    The further object of the present invention is to provide an antistatic coating for the outer surface of a video display panel having a metallic composition and low conductivity, i.e., as low as 10 3  ohms.  
           [0008]    A still further object of the present invention is to provide a multi-layer antistatic/antireflective coating for a video display screen and a method of application therefor, which is metal-based and does not require the use of a reacting gas in producing and depositing the coating by sputtering.  
           [0009]    Yet another object of the present invention is to provide a sputter coating technique for depositing a multi-layer coating on the surface of a video display screen which eliminates the need for a reactive gas and allows for close control of coating conductivity and reflectance by precise control of coating thickness.  
           [0010]    The present invention contemplates a process for forming an antistatic/antireflective coating on an outer surface of a video display screen comprising the steps of: sputter-depositing on the outer surface of the video display screen an inner metallic antistatic layer having a precisely controlled thickness within a range of 2-8 nm, wherein a light refractive index of the inner antistatic layer is also precisely controlled within a range of 2.0-2.8; and sputter-depositing on the inner antistatic layer an outer antireflective layer having a precisely controlled thickness within a range of 70-100 nm, wherein a light refractive index of the outer antireflective layer is also precisely controlled within a range of 1.3-1.47. This invention also contemplates a multi-layer coating for a video display panel having the aforementioned composition. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The appended claims set forth those novel features which characterize the invention. However, the invention itself, as well as further objects and advantages thereof, will best be understood by reference to the following detailed description of a preferred embodiment taken in conjunction with the accompanying drawings, where like reference characters identify like elements throughout the various figures, in which:  
         [0012]    [0012]FIG. 1 is a longitudinal sectional view of a CRT incorporating an antireflective/antistatic coating in accordance with the principles of the present invention;  
         [0013]    [0013]FIG. 2 is a partial sectional view of a flat display screen having an outer surface coating comprised of an inner antistatic layer and an outer antireflective layer in accordance with the present invention; and  
         [0014]    [0014]FIG. 3 is a simplified combined schematic and block diagram of apparatus for applying a multi-layer antireflective/antistatic coating on the outer surface of a video display screen by sputtering in accordance with one embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    Referring to FIG. 1, there is shown a longitudinal sectional view of a color CRT  10  incorporating an antistatic/antireflective coating  32  applied by sputtering in accordance with the present invention. In the following discussion the terms “display screen”, “display panel” and “faceplate” are used interchangeably. In addition, the terms “layer” and “coating” are used synonymously. CRT  10  includes a sealed glass envelope  12  having a forward faceplate or display screen  14 , an aft neck portion  18 , and an intermediate funnel portion  16 . Disposed on the inner surface of glass display screen  14  is a phosphor screen  24  which includes plural discrete phosphor deposits, or elements, which emit light when an electron beam is incident thereon to produce a video image on the display screen. Color CRT  10  includes three electron beams  22  directed onto and focused upon the CRT&#39;s glass display screen  14 . Disposed in the neck portion  18  of the CRT&#39;s glass envelope  12  are plural electron guns  20  typically arranged in an inline array for directing the electron beams  22  onto the phosphor screen  24 . The electron beams  22  are deflected vertically and horizontally in unison across the phosphor screen  24  by a magnetic deflection yoke which is not shown in the figure for simplicity. Disposed in a spaced manner from phosphor screen  24  is a shadow mask  26  having a plurality of spaced electron beam passing apertures  26   a  and a skirt portion  28  around the periphery thereof. The shadow mask skirt portion  28  is securely attached to a shadow mask mounting fixture  30  around the periphery of the shadow mask. The shadow mask mounting fixture  30  is attached to an inner surface of the CRT&#39;s glass envelope  12  and may include conventional attachment and positioning structures such as a mask attachment frame and a mounting spring which also are not shown in the figure for simplicity. The shadow mask mounting fixture  30  may be attached to the inner surface of the CRT&#39;s glass envelope  12  and the shadow mask  26  may be attached to the mounting fixture by conventional means such as weldments or a glass-based frit.  
         [0016]    Referring to FIG. 2, there is shown a partial sectional view of a portion of the CRT&#39;s glass display screen  14  having the aforementioned phosphor layer  24  on the inner surface thereof and an outer antistatic/antireflective coating  32  on the outer surface thereof in accordance with the present invention. The glass display screen  14  of FIG. 2 is shown as being flat as the present invention is applicable with both curved display screens as shown in FIG. 1 as well as to flat display screens as shown in FIG. 2. In addition, while the present invention has been illustrated in the figures in terms of use of the outer surface of the display screen of a CRT, the present invention is not limited to use with this type of display device. For example, the antistatic/antireflective coating  32  of the present invention may be used equally as well on the outer surface of the display panel of virtually any type of self-emitting color display device, i.e., where the video image is produced by phosphor activated by energetic electrons incident thereon. Self-emitting color display devices other than CRTs include field emission displays, plasma discharge panels, vacuum fluorescent screens, and gas discharge screens. The phosphor layer  24  disposed on the inner surface of the glass display screen  14  may be in the form of a large number of discrete dots or stripes.  
         [0017]    In accordance with the present invention, the antistatic/antireflective coating  32  includes an inner antistatic layer  46  and an outer antireflective layer  48 . A conductor  50  may be attached to the inner antistatic layer  46  or to the outer surface portion of the display screen  14  for electrically coupling the display screen to neutral ground potential. In this manner, the build up of electrostatic charge on the display screen  14  is limited by discharging the electrostatic charge on the display screen to neutral ground via the electrically conductive inner antistatic layer  46 .  
         [0018]    Shown in FIG. 3 is a simplified combined schematic and block diagram of a sputter deposition apparatus  60  for applying an antistatic/antireflective coating to the outer surface of the glass display screen  62   a  of a CRT  62  in accordance with one aspect of the present invention. Sputter deposition apparatus  60  includes a dual chamber  64  comprised of a larger chamber  64   a  and a smaller chamber  64   b  which are connected together by means of a valve  65 . A conventional sputtering system (not shown for simplicity) is disposed within the vacuum chamber  64  for sputtering targets onto the outer surface of the display screen  62   a  of CRT  62 . Each of the larger chamber  64   a  and the smaller chamber  64   b  has its own vacuum gauge and valve for controlling the respective pressures therein. Thus, the larger vacuum chamber  64   a  is provided with vacuum gauges  70 ,  74 , and  84  for monitoring the pressure therein. A discharge valve  72  allows for air to enter the larger chamber  64   a  such as for performing maintenance on the larger chamber. Vacuum gauge  66  permits monitoring of the pressure in the smaller vacuum chamber  64   b , while a discharge valve  68  allows for the entry of air into the smaller chamber for inserting or removing the display screen  62   a  of CRT  62 . A diffusion pump  76  is connected to the combination of the larger chamber  64   a  and smaller chamber  64   b  via a gate  78 . Vacuum gauges  80  and  82  are also connected between the diffusion pump  76  and the combination of the larger chamber  64   a  and smaller chamber  64   b  for monitoring the vacuum level within the diffusion pump. A pair of mechanical pumps  86  and  88  are connected to the diffusion pump  76  by means of respective valves  98  and  100 . A vacuum gauge  94  is also connected between the mechanical pumps  86 ,  88  and the diffusion pump  76  for monitoring the pressure of the vacuum pumps. The combination of a pair of mechanical pumps  90  and  92  is coupled to the larger chamber  64   a  and the smaller chamber  64   b  by means of respective valves  108  and  106 . In addition, mechanical pumps  90  and  92  are coupled to the valves  106  and  108  by means of valves  102  and  104 , respectively, as well as by means of a vacuum gauge  96 . Vacuum gauge  96  allows for monitoring the pressure of the vacuum pumps  90  and  92 .  
         [0019]    The sputter deposition apparatus  60  operates in the following manner. Machanical pumps  86  and  88  are turned on for pumping the diffusion pump  76  with valves  98  and  100  in the open position. Mechanical pumps  90  and  92  are turned on for pumping the larger vacuum chamber  64   a  with valves  102 ,  104  and  108  all in the open position. Valves  98 ,  100 ,  102  and  104  are always open. When the pressure of the diffusion pump  76  and the pressure in the larger vacuum chamber  64   a  reach the working pressure, gate  78  opens and valve  108  closes. The display screen  62   a  of CRT  62  is then loaded in the smaller vacuum chamber  64   b  and valve  106  opens for pumping the smaller vacuum chamber down to the working pressure by means of mechanical pumps  90  and  92 . When the pressure within the smaller vacuum chamber  64   b  reaches the working pressure, valve  65  disposed between the larger and smaller vacuum chambers  64   a,    64   b  opens to equalize the pressure between the two chambers. The sputtering system within the smaller vacuum chamber  64   b  then deposits the sputtering targets onto the outer surface of the CRT&#39;s display screen  62   a . After coating the outer surface of the CRT&#39;s display screen  62   a  with the multi-layer antistatic/antireflective coating of the present invention, valve  65  closes and valve  68  opens for allowing air into the smaller vacuum chamber  64   b . The CRT  62  is then unloaded, or removed, from the smaller vacuum chamber  64   b  and another CRT is loaded in the smaller vacuum chamber. The above described sequence of steps is then repeated for the new CRT now loaded in the small vacuum chamber  64   b.    
         [0020]    The sputter deposition apparatus  60  of FIG. 3 permits the thickness of the inner antistatic layer  46  to be controlled with great precision. The thickness of the inner antistatic layer  46  may be controlled to within 2-8 nm. The inner antistatic layer  46  preferably includes the metals Ti or Cr. By precisely controlling the thickness of the inner antistatic layer  46 , its light refractive index may be controlled to be within the range of 2.0-2.8. The inner antistatic layer  46  is preferably provided with a low conductivity such as on the order of 103 ohms and a low reflectance on the order of 0.7%. The outer antireflective layer  48  preferably includes SiO 2  or MgO. The thickness of the outer antireflective layer  48  may also be precisely controlled so as to be within a range of 70-100 nm. By thus controlling the thickness of the outer antireflective layer  48 , its light refractive index may be precisely controlled to be within the range of 1.3-1.47.