Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Reexamination Certificate
2000-06-30
2001-11-20
Mayekar, K. (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
C204S192340, C204S192350
Reexamination Certificate
active
06319367
ABSTRACT:
FIELD OF THE INVENTION
BACKGROUND OF THE INVENTION
Field emission electron sources, often referred to as field emission materials or field emitters, can be used in a variety of electronic applications, e.g., vacuum electronic devices, flat panel computer, television and other display screens, emission gate amplifiers and klystrons and in lighting.
Display screens are used in a wide variety of applications such as home and commercial televisions, laptop and desktop computers and indoor and outdoor advertising and information presentations. Flat panel displays are only a few inches thick in contrast to the deep cathode ray tube monitors found on most televisions and desktop computers. Flat panel displays are a necessity for laptop computers, but also provide advantages in weight and size for many of the other applications. Currently laptop computer flat panel displays use liquid crystals which can be switched from a transparent state to an opaque one by the application of small electrical signals. It is difficult to reliably produce these displays in sizes larger than that suitable for laptop computers or for operation over a wide temperature range.
Plasma displays have been used as an alternative to liquid crystal displays. A plasma display uses tiny pixel cells of electrically charged gases to produce an image and requires relatively high electrical power to operate.
Flat panel displays having a cathode using a field emission electron source, i.e., a field emission material or field emitter, and a phosphor capable of emitting light upon bombardment by electrons emitted by the field emitter have been proposed. Such displays have the potential for providing the visual display advantages of the conventional cathode ray tube and the depth and weight advantages of the other flat panel displays with the additional advantage of lower power consumption than the other flat panel displays. U.S. Pat. Nos. 4,857,799 and 5,015,912 disclose matrix-addressed flat panel displays using micro-tip, i.e., Spindt tip, cathodes constructed of tungsten, molybdenum or silicon. WO 94-15352, WO 94-15350 and WO 94-28571 disclose flat panel displays wherein the cathodes have relatively flat mission surfaces.
R. S. Robinson et al., J. Vac. Sci. Technolo. 21, 1398 (1983) disclose the formation of cones on the surfaces of substrates under ion bombardment. The effect was reported for various substrate materials and were generated by simultaneously sputtering a surface at high energy while seeding it with impurity atoms deposited at low energy. They also disclosed the formation of carbon whiskers up to 50 &mgr;m in length when a graphite substrate was ion-bombarded with impurities from a stainless steel target.
J. A. Floro et al., J. Vac. Sci. Technolo. A 1, 1398 (1983) disclose the formation of whiskers during relatively high current density ion bombardment of heated graphite substrates. The whiskers were disclosed to be 2-50 &mgr;m in length and 0.05-0.5 &mgr;m in diameter and to grow parallel to the ion beam. Simultaneous impurity seeding was reported to inhibit whisker growth. J. A. van Vechten et al., J. Crystal Growth 82,289 (1987) discuss the growth of whiskers from graphite surfaces under ion sputtering conditions. They note that the whiskers of smallest diameter, characteristically about 15 nm, definitely appear to be different from either diamond or the scrolled-graphite structure found in carbon fibers grown by catalytic pyrolysis of hydrocarbons. Larger whiskers with diameters ranging from 30 to 100 nm were also observed to grow in sputtering systems. The smaller diameter whiskers are constant in diameter along the length while the larger diameter whiskers may have a slight taper.
M. S. Dresselhaus et al., Graphite Fibers and Filaments (Springer-Verlag, Berlin, 1988), pp. 32-34, disclose that filaments may be grown on several types of hexagonal carbon surfaces, but not on diamond or glassy carbon.
T. Asano et al., J. Vac. Sci. Technol. B 13, 431 (1995) disclose increased electron emission from diamond films which have been deposited on silicon by chemical vapor deposition, argon ion milled to form diamond cones and then annealed at 600° C. These cones are formed if the diamond is in the form of isolated grains.
C. Nützenadel et al., Appl. Phys. Lett. 69, 2662 (1996) disclose field emission from cones etched into both synthetic boron-doped diamond and silicon by ion sputtering.
J. R. Conrad, U.S. Pat. No. 4,764,394 discloses ion implantation using a plasma of ions.
In view of this art, there is still a need for a process for readily and economically producing both small and large sized highly emitting field emission electron emitters in large quantities for use in various flat panel applications. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the detailed description which hereinafter follows.
SUMMARY OF THE INVENTION
The invention provides a process for producing a carbonaceous field emission electron emitter, which comprises:
(a) positioning a carbonaceous material in a closed chamber in contact with one of the two electrodes provided in the chamber;
(b) evacuating the chamber;
(c) generating a plasma of ions which surrounds the exposed surface of the carbonaceous material; and
(d) applying to the electrode in contact with the carbonaceous material a negative voltage relative to the other electrode in the chamber to thereby accelerate the ions in the plasma toward the carbonaceous material and provide an ion energy sufficient to etch the exposed surface of the carbonaceous material but not sufficient to result in the implantation of these ions within the carbonaceous material.
It is preferred to mask any portions of the substrate that would otherwise be exposed to the plasma and any desired portions of the carbonaceous material that are not to be exposed to the plasma. Especially preferred is a graphite mask.
If the walls of the chamber are made of an electrically conducting material, the walls can serve as one of the electrodes.
The ions used are those of an inert gas (i.e., argon, neon, krypton or xenon), oxygen, nitrogen, hydrogen or mixtures thereof. Preferably, the ions used are those of an inert gas or an inert gas with a small amount of added nitrogen.
The voltage applied is from about 100 V to about 20 kV, preferably from about 1 kV to about 10 kV. The pulse frequency of this applied voltage typically ranges from about 100 Hz to 30 kHz, preferably from about 1 kHz to about 25 kHz. The pulse width is about 5 &mgr;sec to about 50 &mgr;sec, preferably from about 5 &mgr;sec to about 20 &mgr;sec.
The pressure in the chamber during the process is from about 1×10
−5
torr (1.3×10
−3
Pa) to about 10 mtorr (1.3 Pa), preferably from about 1×10
−5
torr (1.3×10
−3
Pa) to about 10×10
−5
torr (1.3×10
−2
Pa).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The use of a plasma provides a novel process to produce a carbonaceous field emission electron emitter. This plasma-based process utilizes the ions in the plasma for bombarding the carbonaceous material and forming a multitude of small needle-like structures on the surface of the carbonaceous material. These structures have aspect ratios which are typically five-to-one or greater. The plasma surrounds the exposed surfaces of the carbonaceous material. In contrast, conventional ion beam bombardment is a line-of-sight process in the sense that only a region directly in the path of the beam is subjected to ion bombardment and therefore relatively small areas, defined by the aperture of the ion gun's line-of-sight opening, are affect. Since the plasma-based process is not line-of-sight, very large surface areas can be processed simultaneously, essentially limited only by the size of the vacuum chamber used. This provides great practical advantage by enabling the production of large area electron emitters for use in large area flat panel displays as well as for enabling the economical production of a
Coates Don Mayo
Walter Kevin Carl
E.I. duPont de Nemours and Co.
Mayekar K.
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