The present invention relates to the field of diamond-like carbon-containing coatings, products coated with such coatings, and the use of such coatings on electronic devices and coatings on components for such devices. More specifically, the present invention relates to dielectric diamond-like carbon-containing coatings, comprising an amorphous matrix, that possess low secondary electron emission coefficients, coated on various substrate materials, such as electrical displays. The coatings are xe2x80x9ctunablexe2x80x9d with respect to electrical conductivity/resistivity.
Field emission displays (FEDs) are a type of thin, lightweight, flat panel information display. These displays are, in effect, flat cathode ray tubes that use matrix-addressed cold cathodes to produce light from a cathodoluminescent phosphor screen. FEDs consists of a field emission array, dielectric spacers, and a phosphor-coated (monochrome or color) faceplate with matrix-addressable electronics. The field emission array comprises electron emitters, each smaller than an individual pixel, that might employ gate electrodes. The electron emitter material may be shaped in any geometrical configuration (e.g. shaft tip, line edge, plane, etc.). Electrons are emitted into a vacuum when an electric field of sufficient strength is applied to the emitter material. The electrons are accelerated to an electron target such as the phosphor-coated screen. The phosphor then luminesces and the pixel xe2x80x9cturns onxe2x80x9d.
FEDs employ high voltage spacers, typically comprising dielectric materials such as ceramics, glass, or high temperature plastics to separate the emitting plate from the phosphor plate. The spacing between the emitter and the phosphor is very small (about 1-10 mm) and is critical to optimal display performance. The spacers must meet several requirements, such as high dielectric strength, resistance to surface flashover, low secondary electron emission, low leakage current, ability to dissipate electrostatic charge, and good mechanical strength. In addition, these materials must maintain these properties under high energy electron bombardment for extended periods. In operation, many dielectric materials are prone to surface flashover, dielectric breakdown, and poor electronic control. It has been exceedingly difficult in the field to find a material which meets the above requirements, especially the control of secondary electron emission and charging.
Dielectric spacers are used in field emission displays (FEDs) to separate the anode faceplate (screen) from the cathode material. Preferably, such spacers must possess a high dielectric strength (greater than about 106 V/cm), high electrical resistance (from about 10+8 to about 10+11 ohm-cm), high resistance to flashover, good thermal conductivity and resistance to arcing damage. Furthermore, the structural and chemical properties of the spacers must not change throughout the operational lifetime of the display (greater than about 10,000 hours).
Presently, dielectric spacers are most commonly made from bulk substrate materials, such as glass and ceramics. These materials satisfy the FEDs"" dielectric strength requirements but have limited ranges of electrical resistivity and have secondary electron emission coefficients (SEEC) typically much greater than unity (greater than 1.0), for example 2.0 to 3.5. Primary electron refer to electrons from a source, such as an electron beam, which impact a substrate surface. Secondary electron emission refers to the electrons which are emitted from a substrate surface after being impacted by primary electrons. The secondary electron emission coefficient (SEEC) is a ratio value representing the average number of secondary electrons emitted from a bombarded substrate surface for every incident primary electron on the substrate surface.
A material which meets the dielectric strength requirements for desired electrical applications, including use with FEDs, and which also has electrical resistivity values that can be predictably altered, or xe2x80x9ctunedxe2x80x9d, while also having a SEEC value of less than unity (less than 1.0), is presently unknown, but would be advantageous. The present invention relates to the unexpected results that the present coatings are much thinner than those known and provide a low secondary electron emission coefficient of less than about 1.0, while maintaining all other desirable properties, and providing for high productivity and lower cost.
Such a material as described above would also benefit other applications. Color picture tubes use either perforated shadow masks or grilles with vertical slits to direct electron trajectory to an electron target, typically a phosphor coated screen. Electrons from the tube""s electron guns pass through the mask or grille and are directed at slightly different angles to excite a red, blue, or green phosphor. Precise alignment of the electron beams is required to achieve sharp images with high contrast. Some fraction of the electrons typically fall on the mask or grille and generate secondary electrons. This may result in defocusing of the image-forming beam due to its interaction with the secondary electrons which have uncontrolled trajectories. Higher resolution images and enhanced brightness and contrast can be achieved if the production of secondary electrons is suppressed or eliminated.
Carbon-containing coatings have been applied to electrical components that are bombarded by electrons. Carbon has many distinct phases, for example, diamond, graphite, soot, etc. Each of these carbon phases has a different secondary electron emission coefficient, or SEEC, for example diamond=2.8; graphite=1.0; and soot=0.45. Certain applications, including electronic displays or other component parts incorporated into electronics under vacuum, require coatings or substrate materials having a SEEC of a specified value. Many electronics applications require coatings having extremely low SEEC values, for example,  less than 1.0 in combination with other properties such as durability, adhesion and smoothness. Certain C:H and Si:C thin films have been attempted for use with high frequency waveguides. Such films as reported by Groudeva-Zotova et al. (Diamond and Related Materials, Vol. 5, No. 10, 1987), have low SEEC values in the energy range of from 250-2000eV. The SEEC on these films is very sensitive to film composition and morphology. Also they must be annealed to lower the SEEC. Finally, the electrical resistivity cannot be tailored. In addition, coatings containing graphite in the form of Aquadag (Acheson Colloids, Port Huron, Mich., vacuum pyrolyzed graphite, and lamp black deposited by electrophoresis, have been used on high frequency electronic devices to prevent multi-pactor discharges (surface flashover). However, these films often must be applied at paint thicknesses of from 10 xcexcm to over 100 xcexcm. This creates adhesion problems and other limitations adversely affecting electrical tailorability, durability, stability and smoothness. Further, U.S. Pat. No. 5,466,431 discloses a 0.5 to 2.0 micron thick two network nanocomposite film having a high thermal conductivity and low secondary emission used as a protective coating on the grids of color TV tubes. However, such thick coatings are not only unnecessary, but are also disadvantageous for display applications. Coatings at such thicknesses have a high cost, lower overall productivity due to long deposition times, and low equipment efficiency. Such a thick film coating may also cause variations in critical physical dimensions of the substrate.
As a result, low SEEC coatings which can be applied at required thicknesses and which have no adhesion problem are not known. Coatings for electronic components, especially FEDs and cathode ray tubes, which have both a low SEEC (of less than about unity, i.e. less than about 1.0) and which have superior adhesion and are electrically tunable over a broad range would be highly advantageous.
The present invention relates to electrical devices having improved performance. Such devices comprise components having coatings made from materials that have low secondary electron emission coefficients, preferably less than about one. In a particularly preferred embodiment, the coating materials with SEECs less than about 1.0 further are electrically tunable, in terms of resistance, over a range of from 10xe2x88x922 to 1016 ohm-cm. and display their low SEEC value of less than about 1.0 over an electron energy range of from about 80 to about 10,000 eV.
In a further embodiment, the present invention is directed to a display comprising an electron target substrate and an electron source on one side of the substrate and a coating on the same side of the substrate as the electron source. In one preferred embodiment the electron target is a generally transparent substrate.
In a further embodiment, the present invention is directed to a device having an electron source and a target arranged so that electrons from the source impinge on the target, and a passive element. The target and passive element and source are positioned so that electrons from the source may impinge on the passive element, and secondary electrons emitted from the passive element impinge on the target. The surface of the passive element has a coating comprising carbon and silicon for reducing the secondary electron emission coefficient of the surface to less than about one. The target optionally comprises the coating. The coating is preferably deposited at a thickness of from about 0.02 to about 0.15 microns.
In a further embodiment of the present invention, the source comprises an electron gun and the target comprises an electroluniinescent screen.
A still further embodiment of the present invention is directed to an electrical device such as, for example a display device including a field emission display or a color television tube comprising a coating comprising carbon and silicon on a surface for reducing the secondary electron emission coefficient of the surface to less than about one.
A further embodiment of the present invention is directed to a method of improving the performance of an electrical device comprising providing an electrical device comprising an electron source, an electron target and a passive element, positioning the source, the target and the passive element so that electrons from the source may impinge on the passive element, and secondary electrons emitted from the passive element impinge on the target, and depositing on the passive element a coating comprising carbon and silicon on a surface of the passive element for reducing the secondary electron emission coefficient of the surface to less than about one.
In another embodiment, the present invention comprises an electrical component in a device comprising a substrate and a coating made from a material having a SEEC value less than or close to unity. Preferably the SEEC value of the coating is in a range of from about 1.0 to about 0.45, more preferably from about 0.9 to about 0.45, and most preferably from about 0.90 to about 0.80. The coating is further preferably electrically tunable over a range of from about 10xe2x88x922 to 1016 ohm-cm, and more preferably from about 106 to about 1010 ohm-cm.
In a further embodiment, the present invention relates to a diamond-like material comprising carbon, hydrogen, silicon and oxygen. Optionally, the material further comprises dopant elements or dopant compounds comprising elements from Groups 1-7b of the periodic table.
In a still further embodiment, the invention relates to an electronic device display comprising a substrate and a coating having a low secondary electron emission coefficient, preferably less than unity, and that is tunable in terms of electrical resistivity over a wide range, such as about 10xe2x88x922 to about 1016 ohm-cm.
Still further, the present invention relates to a method of improving the performance of an electrical component display, especially a flat panel display comprising providing an electrical component and coating the component with a material having a secondary electron emission coefficient less than unity.