WO1998007066A1

WO1998007066A1 - Backlights for color liquid crystal displays

Info

Publication number: WO1998007066A1
Application number: PCT/US1997/014429
Authority: WO
Inventors: Zvi Yaniv; Nalin Kumar
Original assignee: Si Diamond Technology, Inc.
Priority date: 1996-08-16
Filing date: 1997-08-15
Publication date: 1998-02-19

Abstract

A backlight (110) for a color liquid crystal display (28) uses various techniques for activating colored phosphors (310) which emit colored light to each one of several sub-pixels (12-18) within a particular liquid crystal display pixel. Activation of the colored phosphors (310) may be performed using field emission devices, both diode and triode, a fluorescent lamp, or a high-intensity glow discharge lamp.

Description

BACKLIGHTS FOR COLOR LIQUID CRYSTAL DISPLAYS

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to backlights for use with color liquid crystal displays and specifically to a backlight. BACKGROUND INFORMATION Due to their non-emissive nature, traditional liquid crystal Flat Panel Displays

("FPD") use either reflected light or backlights (e.g. cold cathode or hot cathode fluorescent tubes) as white light sources. When a backlight is employed, the white light generated by the backlight is directed to a matrix consisting of individual liquid crystal pixels. Light entering each individual pixel of the matrix is either blocked or transmitted, depending on whether or not a sufficient electric field has been applied to that individual pixel.

In an active matrix type liquid crystal display ("LCD"), each liquid crystal pixel is directly addressable (i.e. able to be directly switched between the "on" and "off modes by the application of an electric field). Such addressing is typically accomplished with the aid of thin film transistors ("TFT"). In color active matrix LCDs, as well as in passive matrix color LCDs, individual liquid crystal pixels are grouped into formations of several sub- pixels. Within the formation, each individual sub-pixel is associated with a color filter (e.g., in a three sub-pixel formation, each sub-pixel would be associated with either a red, green or blue filter). Typical formations include three or four sub-pixels arranged in a vertical stripe, quad, triad or horizontal stripe configuration, although other configurations and filter colors are possible.

Details regarding conventional LCD configurations, conventional backlights, and the operation of conventional liquid crystal displays are well known in the art. See, for example, S.W. Depp and W.E. Howard "Flat Panel Displays" Scientific American, page 90, (March, 1993); H. Miller. "An Examination of Active Matrix Technologies and Components", Sharp LCD Application Note, pages 2-10-2-14; and A. Dragon,

"Backlighting," Sharp Application Notes, pages 2-100-2-106.

FIGURE 1 illustrates a simplified cross sectional representation of a generic conventional backlit color LCD 10 having liquid crystal sub-pixels 12, 14, 16. and 18, i.e. two red sub-pixels. 12 and 16. and two green sub-pixels, 14 and 18. The white light source, used as a backlight in the LCD configuration of FIGURE 1, is a conventional cold cathode fluorescent tube 20. Associated with this light source are light guide 22, diffuser 24 and brightness enhancement filter 26. The entire arrangement, 28, of all liquid crystal pixels and sub-pixels, is known as a liquid crystal matrix. (The liquid crystal matrix of a typical LCD can contain on the order of several hundred thousand pixels and several times more sub-pixels. For clarity, only four sub-pixels are shown in FIGURE 1). Each liquid crystal sub-pixel (12. 14. 16 and 18) has a first polarizer 30 on first glass substrate 32, a

TFT driven liquid crystal cell 34. a color filter 36, second glass substrate 38 and second polarizer 40. In sub-pixels 12 and 16, color filter 36 is a red filter and in pixels 14 and 18 a green filter.

During operation of the generic LCD 10 of FIGURE 1 , white light leaving brightness enhancement filter 26 travels to the each of the individual liquid crystal sub- pixels 12, 14, 16 and 18. Depending upon the magnitude of an electric field applied to each liquid crystal sub-pixel (by means not shown), the white light incident on first polarizer 30 is either (i) transmitted through the entire liquid crystal sub-pixel, including color filter 36 and second polarizer 40, and therefore exits polarizer 40 as colored light or (ii) blocked by operation of the liquid crystal sub-pixel. In order to block the incident light, the polarization of the light which exits first polarizer 30, through first glass substrate 32, is rotated by liquid crystal molecules (not shown) contained in liquid crystal cell 34 such that the colored light leaving color filter 36 is blocked by second polarizer 40.

A major disadvantage of a conventional backlit color LCD is that a majority of the light generated by the white light source is lost due to the less than ideal transparency associated with each of the liquid crystal display components. For example, color filters employed in LCDs typically have a light transmission efficiency (defined as the percentage of incident light that is transmitted through the filter) of between 20 and 33%. See, P. Pleshko, "Overview and Status of Information Displays" Society for Information Display, 1992 Seminar Lecture Notes, May 18. Therefore, if a red-filtered sub-pixel is in the "on"

(light transmitting) mode, at least 66% of the white light incident on the red-filtered liquid crystal sub-pixel (i.e. the non-red wavelength portions of the white light) is blocked by the filter and therefore wasted. Likewise, the green and blue-filtered liquid crystal sub-pixels, even when transmitting light, waste at least 66% of the incident white light. Moreover, as illustrated by the typical values in Table 1. the overall efficiency of the light transmission, taking into account the efficiency of each of the liquid crystal display components, is typically only around 3-4% (see. Pleshko. supra at page M-0/63). Table 1 : Conventional Backlit Color LCD

Component Transmissivity Cumulative Transmissivity

Backlight Components 0.56 0.56

Polarizer 0.8 0.4

Substrate 0.945 0.37

Color Filters 0.2 0.075

Liquid Crystal 0.65 0.049

Substrate 0.945 0.0464

Polarizer 0.80 0.037

Total 0.037

Other disadvantages associated with conventional backlit Liquid crystal displays are the relatively high cost of the color filters and an inability to generate a high intensity image at a relatively low power input (typically measured in units of lumens per watt).

One proposed alternative to conventional cold cathode or hot cathode fluorescent tubes backlights is a cathodoluminescent lamp employing thin film edge emitting devices as electron sources. See Akinwande. et al.. "Thin Film Edge Emitter Vacuum Microelectronics Devices for Lamp/Backlight Applications." Eighth International Vacuum Microelectronics Conference Technical Digest. July 30-August 3. 1995, page 418. This configuration, however, still requires color filters, with their attendant inefficiencies, to produce a multicolor image.

As a result of the foregoing, what is still needed in the art is a cost effective, low power, multi-color liquid crystal display with a high light transmission efficiency and brightness. SUMMARY OF THE INVENTION

The foregoing need is satisfied by the present invention, which is a color backlight for use with liquid crystal displays including (i) a light emitting faceplate patterned with colored-light-emitting phosphor pixels having a predetermined size, pattern and spacing, so as to cooperate with corresponding liquid crystal sub-pixels of a liquid crystal matrix, and (ii) a broad area source of electrons adapted to provide electrons to the phosphor pixels.

In one embodiment, the broad area source of electrons is a cold cathode field emitting device and the phosphor pattern is disposed on the anode of the device. Another embodiment employs either diode-configuration or triode-configuration cold cathode field emitting devices that utilize thin film diamond emitters.

Use of a field emission-based color backlight eliminates the need for expensive color filters in the LCD pixel. The elimination of the color filters increases the efficiency of light transmission by at least a factor of three. In addition, conventional backlights generate white light at a power efficiency of approximately 15 lumens per watt. Multiplying this by an overall transmission efficiency of a conventional color LCD, approximately 4% as reported in Table 1. yields an overall LCD power efficiency of 0.6 lumens per watt. In comparison, a field-emission device employing patterned colored-light-emitting phosphors can typically generate light at an efficiency of approximately 20 lumens per watt. Multiplying this by an overall LCD transmission efficiency of approximately 15-18%

(higher than a conventional LCD configuration because conventional light guide, light diffuser components and color filters components are not required) yields an overall LCD power efficiency of approximately 3.3 lumens per watt. See Table 2. This is 5.5 times greater than the conventional LCD configuration. The present invention, therefore, can provide either increased brightness at conventional power consumption levels or an equivalent brightness at a lower power consumption.

Table 2

Component Transmissivity Cumulative Transmissivity

Backlight Components 0.44 0.44

Polarizer 0.8 0.352

Substrate 0.945 0.333

Color Filters N/A 0.333

Liquid Crystal 0.65 0.216

Substrate 0.945 0.204

Polarizer 0.80 0.164

In one alternative embodiment of the present invention, the backlight for a color LCD utilizes ultraviolet (UV) bombardment of the color phosphors from a plasma produced in the same manner as a tluoresceni lamp.

In yet another alternative embodiment of the present invention, bombardment of electrons upon the color phosphors is produced from a glow discharge from a high-intensity lamp.

In yet another alternative embodiment of the present invention, photons emitted from the color phosphors are focused towards the LCD sub-pixels by a focusing lens feature embodied within the anode structure of the backlight.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. BRIEF DESCRIPTION OF THE DRAWING

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: FIGURE 1 is a cross-sectional schematic of a generic conventional color LCD employing a fluorescent backlight.

FIGURE 2 is a cross-sectional display ot a color LCD display employing a color field emission backlight assembly. FIGURES 3 is a schematic representations of the pixel-like arrangement of red (R). green (G) and blue (B) phosphors on a faceplate.

FIGURE 4 is a schematic drawing of one embodiment of the electrodes and associated electronics of the invention.

FIGURE 5 is a graph of emission site density and electrical current vs. electrical field strength for a cathode of the light source of one embodiment of the invention.

FIGURE 6 is a schematic drawing of apparatus used to form a cathode for use in the invention.

FIGURE 7 is a detailed schematic ot a fiat diode diamond FED backlight assembly.

FIGURE 8A is a schematic diagram of the surface of a field emitting device's cold cathode plate employing a mesh patterned thin film diamond field emitter.

FIGURE 8B is a cross-sectional schematic diagram of the cold cathode plate of FIGURE 8A along line A-A.

FIGURE 9A is a schematic diagram of the surface of a field emitting device's anode plate (which also serves as the faceplate) employing a mesh patterned insulator and checkerboard patterned phosphors.

FIGURE 9B is a cross-sectional schematic diagram of the anode plate of FIGURE 9A along line A— A.

FIGURE 10 is a schematic diagram of a portion of a cold cathode field emitting device-based LCD backlight assembly. FIGURE 1 1 is a schematic diagram depicting the relationship between two diamond field emitter patches, a single patterned phosphor pixel and the associated liquid crystal sub- pixel.

FIGURE 12 illustrates an alternative embodiment of the present invention.

FIGURE 13 illustrates another alternative embodiment of the present invention. FIGURE 14 illustrates focusing of colored light from a phosphor.

FIGURE 15 illustrates a data processing system in accordance with the present invention. DETAILED DESCRIPTION

In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details concerning timing considerations and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art. Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views.

All basic embodiments of the patterned-phosphor color backlight assembly include a faceplate patterned with light-emitting phosphor pixels of differing colors (e.g. a mixture of red, green and blue light-emitting phosphors though other colors may be utilized). The phosphor pixels are of a predetermined size and are arranged in a pattern that corresponds to the liquid crystal pixel arrangement of the liquid crystal matrix with which the backlight will be used. The size of the phosphor pixels may be equal to. larger than, or smaller than the corresponding liquid crystal sub-pixels. Also included in the patterned phosphor color backlight assembly is a broad area source of electrons adapted to uniformly provide electrons to the patterned-phosphor pixels of the faceplate.

FIGURE 2 schematically illustrates a portion of an LCD employing such a patterned multi-colored- light-emitting phosphor backlight assembly. In the embodiment of FIGURE 2, a multi-colored-light-emitting patterned phosphor backlight assembly 1 10 has both a broad area electron source and a light-emitting-patterned phosphor faceplate integrated into a unitary assembly. Backlight assembly 1 10 is positioned in close proximity to standard LCD pixel matrix 28. Each of the LCD matrix pixel components illustrated in FIGURE 2 has the same function as that described with respect to FIGURE 1. The difference between the configuration of a conventional color LCD shown in FIGURE 1 and that of the embodiment shown in FIGURE 2, is that light source 20, light guide 22, light diffuser 24, color filters 36, and optionally brightness enhancer 26, of FIGURE 1 have been replaced with patterned phosphor color backlight assembly 1 10. FIGURE 3 schematically represents a light-emitting patterned phosphor faceplate 300 for use in backlight assembly 1 10. Patterned red phosphor pixels (R) 310, green phosphor pixels (G) 320 and blue phosphor pixels (B) 330 are disposed in a predetermined pattern on the surface of faceplate substrate 340. The pixels of FIGURE 3 are arranged in four formations of four pixels each (two green and one each of red and blue), for a total of sixteen pixels. The size, spacing and pattern of the red, green and blue phosphor pixels are predetermined such that each color phosphor pixel provides colored light to essentially a single corresponding liquid crystal sub-pixel of an associated liquid crystal matrix. The phosphor pixel of FIGURE 3 is arranged in a quad formation and may be used with a liquid crystal matrix having sub-pixels also arranged in a quad formation.

In one embodiment, there are two green phosphor pixels, one red pixel and one blue pixel in each quad formation, and the size of each four pixel formation is on the order of 330 microns by 330 microns. The size, color, spacing and pattern of the phosphor pixels, as well as the number of pixel formations are. of course, not limited to that of FIGURE 3 but can be adapted as needed to correspond to the pixels of an associated liquid crystal matrix 28. The patterned color-phosphors could, for example, be arranged in a vertical stripe, horizontal stripe, linear stripe or other formation.

The light-emitting color phosphor pixels are composed of any suitable phosphor that will emit colored light when subjected to electrons from a broad area electron source. Examples of known phosphors include ZnO:Zn (blue-green), ZnS:Mn, La₂O₂S:Tb (green),

Y₂O₂S:Eu (red), LaOBπTb (blue) and ZnS:Ag (blue). In one embodiment, the phosphors are those used for high-voltage applications, such as in cathode ray tubes. However, the present invention can be practiced using low voltage electrons and phosphors developed for the appropriate voltage applied. See Jacobsen, et al. "Improved Performance of Low Voltage Phosphors for Field Emission Displays" SID Display Manufacturing Conference,

Late News, page 1 , Santa Clara. CA, February 2. 1995.

The phosphors can be deposited on the faceplate 340 by electrophoretic deposition of powdered phosphors, application and patterning of photosensitive slurries containing phosphors, and silk screening techniques, all of which are well known in the art. Alternatively, the phosphors may be deposited using a process described within U.S. Patent

No. 5,531,880, which is hereby incorporated by reference herein. Faceplate 340 will take on one of several various embodiments depending upon the particular electron source utilized within the backlight. Such embodiments are further described below.

The broad area electron source adapted to provide electrons to the patterned phosphor pixels can be of any suitable design, including those well known in the art, as long as it is capable of providing sufficient electrons to the patterned phosphor pixels to cause the emission of a sufficient flux of colored light from the phosphors.

Several embodiments of the broad area electron source include cold cathode field emitting devices, such as flat diamond field emission diode devices, flat diamond field emission triode devices or field emitting microtip devices. Details regarding the fabrication and operation of cold cathode field emitting devices and displays are well known in the art. See. for example, N. Kumar et al., "Development of Nano-Crystalline Diamond-Based Field-Emission Displays" SID Digest of Technical papers Vol. 25 page 43 (1994); Kumar et al, "Field Emission Displays Based on Diamond Thin Films", Society of Information Display Conference, Technical Digest. 1993. pp. 1009- 1010: Xie et al, "Electron Field

Emission from Amorphic Diamond Thin Films", 6th International Vacuum Microelectronics Conference Technical Digest. 1993. pp. 162-163: and Xie et al. "Use of Diamond Thin Films for Low Cost Field Emission Displays". 7th International Vacuum Microelectronics Conference Technical Digest. 1994. pp. 229-232; and U.S. patents 5,199,918, 5,312,514, 5,341.063. 5,399,238. 5.449,970. 5,528.099. 5.548.185. 5.536, 193, 5,543,684, and 5,252,833, each of which is hereby incorporated by reference herein.

A light source or lamp and method of causing light emission from the phosphors of this invention is described in co-pending application titled "Pulsed Cathodoluminescent Lamp," filed March 25. 1996 (Serial No. 08/621.531 ) assigned to a common assignee. Referring to FIGURE 4. cathode 220 has a cold-electron-emitting surface with low effective field emission work function. A low effective field emission work function material is defined as any material having a threshold electric field for electron emission of less than 50 Volts/micrometer. The emitting surface may be prepared by first growing on substrate 218 a metal layer. Any metal normally used in the semiconductor industry, such as molybdenum, tungsten, chromium, copper or aluminum, may be used. Preferably, a carbonaceous layer having low effective field emission work function is then deposited on the metal layer. If substrate 218 is sufficiently electrically conductive, the metal layer is not required. The carbonaceous coating may be a coating deposited by the laser ablation process described in U.S. Patent No. 4.987.007. entitled "Amorphic Diamond Material Produced by Laser Plasma Deposition." incorporated by reference herein, or may be applied by a variety of chemical vapor deposition (CVD) processes, or by any other process which produces a low effective field emission work function surface. The cathode layer 220 is selected to have high current output at low electrical field gradient and to have uniformity in emission over the surface with high emission-site density.

Assembly 210 also includes spacers 214. anode (glass) substrate 216, ITO (indium tin oxide) layer 222, and phosphor 224. Optionally, aluminum (Al) layer 225 of 100 angstroms to 5,000 angstroms may be deposited on phosphor 224, which will allow electrons from cathode 220 to pass to phosphor 224, but which will reflect photons up to the anode.

After the cathode and anode materials are in place, lamp assembly 210 is evacuated to a high vacuum and sealed. Getter 226 may then be activated to remove remaining gases from inside assembly 210. The pressure in assembly 210 may be reduced to less than 10^"5 torr. The high vacuum is to avoid positive ions forming in the gas and bombarding and destroying the cathode. Electrical leads 228 and 229 connect the anode and cathode respectively to the pulse generator 227.

Operation of lamp assembly 210 includes driving the diode assembly with high voltage pulses. The reason for driving the assembly with electrical pulses will be explained below. In operation, assembly 210 is connected to a high voltage, usually a pulse generator 227, which is adjusted to produce either positive or negative pulses having the desired voltage, frequency and pulse length (duty cycle) to produce light output at the desired level. Duty cycle of the pulses, defined as time-on divided by cycle time, may be less than 5 per cent and may be less than 1 per cent. It is possible, however, to operate the lamp with DC voltage ( 100 per cent duty cycle), but the lamp may suffer from problems as described in the following sections, particularly if the lamp is on for a significant time.

Voltage applied between the cathode and anode is in the range of 6 kilovolts to 30 kilovolts. In one embodiment, voltage is in the range of 10 kilovolts to 12 kilovolts. The gap distance between cathode and anode is preferably in the range of 0.1 mm to 10 mm, but gap distance will be selected based on emission characteristics of the cathode and other factors as described below . Electric field in the gap will normally be in the range from about 1 volt per micrometer to about 50 volts per micrometer.

The reason for application of high voltage pulses having a low duty cycle can best be understood by examination of curves showing current output versus electrical field and emission site density (ESD) of a diamond cold cathode as a function of electrical field gradient at the cathode. Such curves are provided in FIGURE 5. The data represented in these curves are obtained by the following procedure: a flat cathode is placed in a vacuum cell and at a selected distance from a transparent anode. A DC voltage is applied and current through the gap is measured as a function of voltage. While voltage is applied, a magnified view of the anode is obtained, either by placing a microscope so as to directly observe the anode or by taking a high-resolution photograph of the anode. The number of points of light on the anode is then measured over the viewing area and the average density of light emission sites is calculated.

The data shown in FIGURE 5 were obtained with a gap distance of 21 micrometers, an area of the cathode of 0.0035 cnr, pulse width of 20 microseconds, pulse frequency of

60 Hz and an area of current collection of 6.35 mm².

The design of a lamp according to the present invention begins with a selection of the light output desired. For example, assume that an output of 10,000 Ft-Lamberts of light is desired and that the efficiency of the phosphors that will be used is 20 Lumen per watt. The output of light is then about 1 1 Lumen per cm². Assume that a voltage of 10 kilovolts will be used. The electrical power input required is then 1 1 Lumen per cm² divided by 20 Lumen per watt, which is approximately 0.5 watt per cm². To produce this electrical power, the average electrical current density will be 0.5 watt per cm² divided by 10 kilovolts, or 50 microamps per cm². If one observes a graph of Emission Site Density (ESD, expressed in sites per cm²) vs. electrical current for a low effective field emission work function cold cathode made of carbonaceous material, one sees that ESD is very low at such low current density. For a lamp, a high ESD is desired. For example, using data from a cathode discussed further below, if an ESD of 10,000 sites per cm² is desired, the current will be in the range of 10 millamps per cm². To bring the electrical power in line with requirements of 50 microamps per cm" and at the same time produce the high ESD for a lamp, the duty cycle of the electrical pulses driving the lamp will be 50 microamps divided by 10 milliamps, or 0.5 per cent. For a frequency of pulses of 1 kHz, the pulse width would then be 5 microseconds.

From a graph of ESD vs. electrical field for the cathode material to be used, the electrical field to produce the desired ESD can be determined. At this electrical field, for the voltage of the pulses, the gap spacing between the cathode and anode can be determined.

Examination of the curves shows that driving the diode having the cold cathode at low electrical field strength produces only low emission site density. This results in low light output from a device using such cathode. Experiments have shown that at low site density "hot spots" are present on the cathode. This produces burning of the cathode and burning of the phosphor opposite the hot spot in the diode configuration. The solution to the problem of low site density or hot spots has been found to be the use of high-voltage pulses. Reference to FIGURE 5 shows that at high electric fields, the emission site density becomes orders of magnitude greater For example, at an electrical field of 12 volts per micrometer the emission site density was about 2800 sites per cm². At a field of 15 volts per micrometer, the emission site density had increased to about 85,000 sites per cm² However, emission current had also become much larger — increasing from about 60 microamperes to about 500 microamperes. Power consumption of the diode under DC operation per cm² of area would be [500 x 10^"" X 10 kV X 1/0.0035] 1 .4 kilowatts, which would cause severe overheating at the electrodes in a short time and require too much power. It has been found, however, that the application of high voltage pulses at low duty cycle overcomes both the problem of low emission site density and excess power consumption at the electrodes. Neglecting capacitance losses, for example, with a duty cycle of 1%, the power requirement will be in the range of 14 watts. Voltage of pulses and duty cycle are selected to produce the brightness desired from phosphor layer 24 of FIGURE 4, keeping in mind the limitation of heating of the electrodes. A duty cycle of one percent or less can produce a bright lamp using presently available phosphors having normal efficiency. The frequency of the pulses may be in the range from about 20 Hz to about 20 MHz but is selected to produce a light output that is effective for the use intended. Excess flicker or variation in intensity can easily be avoided by increasing frequency of pulses. In one embodiment, pulse frequency is from about 1 kHz to about 10 kHz. The gap distance between cathode and anode is selected based on emission characteristics of the cathode material. For the material having the properties shown in FIGURE 5, a field gradient of at least about 10 volts per micrometer may be needed to reach an acceptable operating range. This would result in a required gap distance of 1 mm when using a pulse of 10 kV. Higher electrical current would be drawn from the cathode than the level at 10 volts per micrometer shown in FIGURE 5. If a current corresponding to a field of 15 volts per micrometer were desired, for example, gap distance could be decreased to 0.66 mm for the material having properties shown in FIGURE 5. Gap distance may be varied so as to allow operation of the diode in the desired emission site density range and current range for each cathode material selected, keeping in mind the upper voltage limitation to avoid x-ray production if such rays could cause problems in operation of the lamp.

The cathode material, having current and emission site properties shown in FIGURE 5, was prepared by an apparatus shown in FIGURE 6. First, a metal layer may be grown on a substrate wafer, using magnetron sputtering, if the wafer is not sufficiently conductive.

(If it is made of glass, for example.) The wafer may then be placed in deposition system 230. Reactor 232, made of stainless steel, encloses cathode 234, anode grid 236 and substrate 238. Cathode 234 may be mounted on a copper holder adapted for the circulation of cooling water. DC power supply 240 provides electrical power through resistor 242. Gases such as hydrogen and methane are supplied from containers 244 and 246 through electronic flow controllers 248 and buffer volume 250. A variety of gases may be used, including ethyl alcohol and other carbon-containing gases which are known to produce CVD diamond. Vacuum pumps 252 maintain a pressure of about 100 to about 300 torr when the gas flow rate is maintained in the range of about 500 standard cm³ per minute (seem) through reactor 232.

Substrate 238 is placed on substrate holder 254, which may also be a second anode 256, which is usually grounded. Substrate holder 254 may be electrically isolated from the second anode 256. so that substrate 238 is insulated from second anode 256. or both may be grounded. Substrate holder 254 also includes a heating element (not shown) to heat substrate 238, normally to a temperature in the range from 700°C to about 1 100°C. A preferred operating temperature is about 900°C. The surface of substrate holder 254 includes small openings connected to one of vacuum pumps 252 which hold substrate 238 in place by suction force. Water cooling is provided by flow in the center of substrate holder 254.

Substrate 238 may be seeded by a standard procedure well known in the art and is then placed underneath grid 236 to position the substrate 238 "downstream" or out of the discharge region which will exist between cathode 234 and grid 236. By placing substrate

238 out of the plasma region, the glow discharge CVD technique can be used to grow diamond thin films on substrate 238 even if the substrate is an electrical insulator, such as glass. The distance between grid 236 and the surface of substrate 238 ranges between 0.1 and about 5 mm. Grid 236 is formed as a mesh, made from wire having a diameter of about 0.3 mm.

The wire material used may be tungsten. The mesh includes a plurality of openings, each opening having a width of about 0.1 mm to about 5 mm and a length in about the same range of dimensions. Grid 236 is heated. Heating is achieved by the discharge current. The grid temperature is increased to above 1 100°C. Grid 236 then behaves as a hot element to increase the diamond film growth rate on substrate 238. The high temperature also allows formation of film material having a structure which is effective as a cold cathode electron emitter. The grid temperature may be above 1300°C. for effective cold electron emission and may be increased to as high as about 2500°C.

Cathode 234 may be formed from a metallic plate or from a porous metallic diaphragm. The tip of cathode 234 has Rocovskey's Form, to provide a more homogenous electric field. The distance between cathode 234 and grid 236 is in the range from about 5 to 50 mm. A negative voltage is applied to cathode 234 while grid 236 is grounded. The voltage between grid 236 and cathode 234 is in the range from about 600 volts to about 1200 volts. Ionization occurs in the gaseous column between the cathode 234 and grid 236 to form a discharge.

To form an effective cathode on a silicon substrate, wherein the silicon has conductivity sufficient not to require a metal layer, first, a film of silicon oxide on substrate 238 is etched or removed. This removal step occurs at a substrate temperature of about 900°C. with hydrogen in chamber 232 at a pressure of about 50 to 300 torr. In the second stage, methane is also admitted to chamber 232 to achieve a methane concentration from about 7% to about 12% along with the hydrogen. In this stage, silicon carbide is formed from the substrate surface. The step of forming the silicon carbide increases the adhesion of the diamond thin film to silicon substrate 238. Also, the silicon carbide layer improves electron ejection from the silicon substrate into the diamond films and increases electron emission from the diamond film grown during the third stage. In the third stage, polycrystalline diamond is grown on the surface. In this stage the methane concentration in the mixture is reduced to between about 3% to about 6%. The thickness of the film is increased to about 0.3 to about 2.0 micrometers. Finally, a step of annealing is added, in which only hydrogen is placed in the reactor and temperature is maintained for a period of about 5 to 15 minutes. To achieve greater uniformity in the emission from the surface, the substrate may be moved as the film is grown. The amplitude of movement may be at least as great as the distance between mesh wires of the grid anode being used. The frequency may be from about 1 Hz to about 100 Hz.

Cathodes prepared by the method described herein are then placed in apparatus as described above and tested for their emission properties. Variables may be adjusted to achieve optimum properties for the lamp design selected. The apparatus of FIGURE 4 includes only one cathode surface. The size of this surface is limited by the area of low-effective field emission work function diamond or carbonaceous material that can be produced on a single surface. Production of wafers having low effective field emission work function diamond coatings up to about 10 inches in diameter is presently available for diamond made by laser ablation. For larger areas than available from one wafer, or for forming a cathode in a preferred pattern, a plurality of cathode areas may be used. Patterning of the cathode areas may be achieved using techniques well known in the industry.

FIGURE 7 is a cross-sectional schematic of one embodiment of a field emission- based color assembly backlight 1 10. The broad area electron source is a flat diamond field emitting device in a diode configuration. A plurality of diamond film patches 420 is in contact with metal layer 410 on glass cathode 400. Patterned phosphor pixels 310 and 320 (shown as red and green pixels, respectively, although it is understood that the entire assembly would contain numerous pixels of red. green and blue), on indium-tin-oxide film (ITO anode) 450 and glass anode plate 460. are disposed at a predetermined distance above diamond film patches 420. A spacer 430 is provided to maintain the proper distance between the elements. Each phosphor pixel. 310 or 320, is aligned with, and in close proximity to, a standard liquid crystal pixel 28. Each phosphor pixel. 310 or 320, is also aligned with an associated diamond film patch 420 such that when the appropriate electric field is applied across the field emitting diode device, electrons are emitted from the diamond patches and subsequently strike the phosphor pixels, whereupon either red or green light is emitted from the phosphor. In an alternative embodiment, diamond film patches 420 and metal lines 410 of

FIGURE 4 are replaced by a continuous sheet-like layers of diamond film and metal respectively.

During operation of the backlight, light emitted from the phosphors travels through transparent ITO film 450 and glass anode 460. and subsequently enters sub-pixels 12, 14, 16 and 18 of liquid crystal pixel 28.

Optionally, a black matrix material 470 may be coated on portions of the exposed ITO film surface not covered by the phosphor pixels 310 and 320. This black matrix coating significantly reduces the likelihood of individual phosphor-pixels emitted light (e.g. a red phosphor pixel) to enter non-corresponding liquid crystal sub-pixels (e.g. adjacent liquid crystal sub-pixels that correspond with green or blue phosphor pixels). If electrically insulating materials, such as silicon dioxide, are used with the black matrix coating, the coating also reduces loss of power caused by electrons hitting the base ITO film 450. Such black matrix materials are well known in the art of making flat panel displays.

It is also optional to coat a portion of the top surface of the faceplate (i.e. glass anode 460) with black matrix materials 480. leaving openings which correspond with the liquid crystal sub-pixel pattern. These openings further collimate the light coming from a particular phosphor pixel toward its corresponding liquid crystal sub-pixel cell, thereby decreasing stray light entering non-corresponding liquid crystal sub-pixels. The use of black matrix coatings 470 and 480, therefore, improves the color purity of the overall display.

Referring to FIGURE 14. there is illustrated a further alternative embodiment of the present invention. Illustrated is a portion of the assembly shown in FIGURE 7, illustrating one sub-pixel 12 illuminated by photons produced by phosphor 310. Since the light (photons) emitted from phosphor 310 will disburse through ITO 450 and substrate 460 in all directions, focusing lens 1401 and 1402 may be utilized separately or in combination to focus the emitted light onto sub-pixel 12. In an alternative embodiment illustrated in FIGURES 8A and 8B, for the cathode plate of a cold cathode field emitting device for use in the present invention, a diamond film 510 overlying a metal conductor 520 is disposed in a mesh pattern on the upper face of glass cathode substrate 550 (as opposed to the "patches" pattern of FIGURE 7). The mesh pattern includes openings 530 where a surface of the glass substrate is exposed. Optionally, patches of metal 540 may be disposed on the underface of glass cathode plate 520 to function as a mirror.

FIGURES 9A and 9B illustrate an embodiment of the combined anode plate and patterned phosphor faceplate for use with the cold cathode of FIGURES 8A and 8B. Mesh- patterned Si0₂ 610 is disposed on top of a layer of aluminum 615. The aluminum may be about 1 -2 microns in thickness. The aluminum layer is on the face of anode glass substrate 620 which faces a cold cathode (such as the cold cathode of FIGURES 8A and 8B). Disposed in the checkerboard openings of mesh 610 are patches of phosphor material 630. Aluminum film 615 may act as an electrical contact and also as a mirrored surface to reflect light toward the sub-pixels.

FIGURE 10 illustrates an embodiment wherein the mesh-patterned diamond film cold cathode of FIGURES 8A and 8B is combined with the anode of FIGURES 9A and 9B to provide an LCD backlight assembly 1 10. Typical spacing between the glass cathode plate 550 and glass anode plate 620 (which also functions as the faceplate) would be on the order of 1-3 mm. Typical spacing between phosphor pixels 630 is on the order of 10-200 microns. Phosphor areas are aligned with the "holes" in the cold cathode.

As discussed earlier, the size of phosphor sub-pixels 630 can be the same or different from the liquid crystal cell size depending on the application. The various factors to be considered in determining the sizes are discussed below in reference to FIGURE 1 1. Referring to FIGURE 1 1. colored light emitted from each phosphor pixel 320 scatters in all directions. Maximizing the percentage of colored light going into the liquid crystal sub-pixel 12 requires that the phosphor pixel 320 and gap D be as small as possible. However, phosphor lifetime and total light emission is increased as phosphor pixel size increases. Thus, the relative size of phosphor pixel 320, in comparison to liquid crystal sub-pixel 12, is determined by the desired light emitting efficiency and phosphor lifetime for a particular application. Larger size of the metal conductor 410 and diamond film 420 reduces the required electric emission current density.

The aforementioned field-emitter based embodiments differ from conventional field emitting color-phosphor based display structures in that the present invention does not require any of the complex elements required to individually drive and address each phosphor pixel. In addition, it is envisioned that the present invention would always be operated in the permanently "on" condition. In such a condition, a sufficient electric field to cause the emission of electrons from the diamond films, and therefore the emission of light from the phosphors, is continually maintained whenever the associated liquid crystal matrix is in operation. Additionally, light passing to sub-pixel 12 may be focused by curving (451 ) aluminum layer 450 so that light emitted from phosphor 320 is reflected back in a focused manner. Furthermore, layer 480. which may be a black matrix material, may be coated on cathode 400.

Referring next to FIGURE 12. there is illustrated an alternative embodiment of the present invention, wherein a fluorescent lamp is utilized to bombard phosphors 310 and 320 with photons (UV light). The fluorescent lamp comprises elements 1401 and 1402, which energize a mercury (Hg) vapor into a plasma, which emits UV photons to excite the phosphors, which then emit light to the sub-pixels 12. 14. 16 and 18. Please refer to 23.4: Development and Production of a Flat Fluorescent Lamp for LCD Backlighting, W. Coleman. et al.. SID 92 Digest, pp. 430-432. which is hereby incorporated by reference herein.

Herein, photons, electrons, and UV light may all be referred to as energentic particles.

Referring next to FIGURE 13. there is illustrated another alternative embodiment of the present invention whereby a high-intensity lamp employing an electron beam from a glow discharge is used to excite phosphors 10 and 320. DC or pulse generator 1303 activates cathode 1302 to emit electrons towards the phosphors. For a further discussion for the construction and operation of the apparatus illustrated in FIGURE 13, please refer to United States Patent Application Serial No. 08/XXX.XXX. entitled High Intensity Lamp, assigned to a common assignee, which is hereby incorporated by reference herein. The backlight assembly 1 10 can be used in conjunction with both passive matrix and active matrix LCD arrays In both cases the benefit of eliminating ^fhe need for expensive color filters and increasing the effiαencv ot light transmission is obtained

A representative hardware environment for practicing the present invention is depicted in FIGURE 15, which illustrates a typical hardware configuration of a workstation m accordance with the subject invention having central processing unit (CPU) 1510, such as a conventional microprocessor, and a number of other units interconnected via system bus 1512 The workstation shown in FIGURE 15 includes random access memory (RAM) 1514, read only memory (ROM) 1516. and input/output (I/O) adapter 1518 for connecting peripheral devices such as disk units 1520 and tape drives 1540 to bus 1512, user interface adapter 1522 for connecting kevboard 1524. mouse 1526, speaker 1528, microphone 1532, and/or other user interface dev ices such as a touch screen device (not shown) to bus 1512, communication adapter 1534 tor connecting the workstation to a data processing network, and display adapter 1536 for connecting bus 1512 to display device 1538 CPU 1510 may include other circuitry not shown herein, which will include circuitry commonly found within a microprocessor, e g , execution unit, bus interface unit, arithmetic logic unit, etc CPU 1510 may also reside on a single integrated circuit

Display 1538 may embody any one of the liquid crystal display embodiments described herein Although the present invention and its advantages have been described in detail, it should be understood that various changes substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims

Claims

WHAT IS CLAIMED IS:

1 A backlight adaptable for use with a liquid crystal display comprising: a plate patterned with a plurality of colored-light-emitting phosphor pixels; and a source adapted for providing sufficient energetic particles to said phosphor pixels to cause an emission of colored light

2 The backlight as recited in claim 1. wherein the source is a cold cathode field emitting device and the energetic particles are electrons.

3 The backlight as recited in claim 2. wherein the cold cathode field emitting device is a diode structure cold cathode field emitting device.

4 The backlight as recited in claim 2. wherein the cold cathode field emitting device is a triode structure cold cathode field emitting device

5 The backlight as recited in claim l wherein said colored-light-emitting phosphor pixels are of a predetermined size and disposed on said plate in a predetermined pattern adapted such that colored light emitted from a color phosphor pixel enters essentially a single predetermined liquid crystal sub-pixel of said liquid crystal display

6. The backlight as recited in 3. wherein said diode structure cold cathode field emitting device further comprises

7 a cathode structure comprising a first conductive material and an emitter material deposited on said conductive material.

8 an anode structure comprising a second conductive material deposited on a glass substrate, wherein said phosphor pixels are deposited on said second conductive material; and

9 a power supply coupled to said first and second conductive materials.

The backlight as recited in claim I . wherein the phosphors are selected from the group consisting of ZnO:Zn. zinc sulphide doped with manganese, La₂O₂S Tb, Y₂O₂S:Eu, LaOBπTb, and ZnS:Ag

10. The backlight as recited in claim 1. wherein the source is a fluorescent lamp and the energetic particles are UV photons

1 1 The backlight as recited in claim 8. wherein said fluorescent lamp includes a chamber containing a Hg gas. which is excited by a pair of elements coupled to a power source, and wherein said excited Hg gas emits I 'V photons that bombard said phosphor pixels.

12. The backlight as recited in claim 1. wherein the source comprises a lamp employing an electron beam from a glow discharge.

13. The backlight as recited in claim 5. further comprising a focusing means implemented within said plate for focusing light emitted from a phosphor pixel towards each said single predetermined liquid crystal sub-pixel of said liquid crystal display.

14. An LCD display comprising: an LCD pixel matrix comprising a plurality of sub-pixels; and a backlight assembly comprising a plate patterned with a plurality of colored-light- emitting phosphor pixels and a source adapted for providing sufficient energetic particles to said phosphor pixels to cause an emission of colored light, wherein said colored-light- emitting phosphor pixels are of a predetermined size and disposed on said plate in a predetermined pattern adapted such that colored light emitted from a color phosphor pixel enters essentially a single predetermined liquid crystal sub-pixel.

15. The LCD display as recited in claim 12. wherein the source is a cold cathode field emitting device.

16. The LCD display as recited in claim 13. wherein the cold cathode field emitting device is a diode structure cold cathode field emitting device.

17. The LCD display as recited in claim 13. wherein the cold cathode field emitting device is a triode structure cold cathode field emitting device.

18. The LCD display as recited in claim 12. wherein the source is a fluorescent lamp.

19. The LCD display as recited in claim 12. wherein the source comprises a lamp employing an electron beam from a glow discharge.

20. \ computer comprising: a processor; a storage device: an input device: an output device: a display device: and a bus coupling said processor, said storage device, said input device, said output device, and said display device. wherein said display device comprises an LCD display comprising: an LCD pixel matrix comprising a plurality of sub-pixels; and a backlight assembly comprising a plate patterned with a plurality of colored- light-emitting phosphor pixels and a source adapted for providing sufficient energetic particles to said phosphor pixels to cause an emission of colored light, wherein said colored-light-emitting phosphor pixels are of a predetermined size and disposed on said plate in a predetermined pattern adapted such that colored light emitted from a color phosphor pixel enters essentially a single predetermined liquid crystal sub-pixel.

21. The computer as recited in claim 18. wherein the energetic particles are electrons.

22. The computer as recited in claim 18. wherein the energetic particles are UV photons.

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