Electric lamp and discharge devices – Discharge devices having a thermionic or emissive cathode
Reexamination Certificate
1999-07-30
2003-01-07
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Discharge devices having a thermionic or emissive cathode
C313S309000, C313S311000, C313S326000, C313S336000, C313S351000, C313S34600R
Reexamination Certificate
active
06504292
ABSTRACT:
FIELD OF INVENTION
This invention pertains to nanoscale conductive devices and, in particular, to metallized nanostructures particularly useful as electron field emitters and nanoscale conductors.
BACKGROUND OF THE INVENTION
Field emitting devices are useful in a wide variety of applications. A typical field emitting device comprises a field emitting assembly composed of a cathode and a plurality of field emitter tips. The device also typically includes a grid spaced relatively closely to the emitter tips and an anode spaced relatively farther from the tips. Voltage induces emission of electrons from the tips, through the grid, toward the anode. Applications include microwave tube devices, flat panel displays, klystrons and traveling wave tubes, ion guns, electron beam lithography, high energy accelerators, free electron lasers, and electron microscopes and microprobes. See, for example,
Semiconductor International,
December 1991, p.46; C. A. Spindt et al.,
IEEE Transactions on Electron Devices,
vol. 38, pp. 2355 (1991); I. Brodie and C. A. Spindt,
Advances in Electronics and Electron Physics,
edited by P. W. Hawkes, vol. 83, pp. 1 (1992); and J. A. Costellano,
Handbook of Display Tehnology,
Academic Press, New York, pp. 254 (1992), all of which are incorporated herein by reference.
A conventional field emission flat panel display comprises a flat vacuum cell having a matrix array of microscopic field emitters formed on a cathode and a phosphor coated anode disposed on a transparent front plate. An open grid (or gate) is disposed between cathode and anode. The cathodes and gates are typically intersecting strips (usually perpendicular) whose intersections define pixels for the display. A given pixel is activated by applying voltage between the cathode conductor strip and the gate conductor. A more positive voltage is applied to the anode in order to impart a relatively high energy (400-5,000 eV) to the emitted electrons. For additional details see, for example, the U.S. Pat. Nos. 4,940,916; 5,129,850; 5,138,237 and 5,283,500, each of which is incorporated herein by reference.
A variety of characteristics are advantageous for field emitting assemblies. The emission current is advantageously voltage controllable, with driver voltages in a range obtainable from “off the shelf” integrated circuits. For typical CMOS circuitry and typical display device dimensions (e.g. 1 &mgr;m gate-to-cathode spacing), a cathode that emits at fields of 25 V/&mgr;m or less is generally desirable. The emitting current density is advantageously in the range of 1-10 mA/cm
2
for flat panel display applications and >100 mA/cm
2
for microwave power amplifier applications. The emission characteristics are advantageously reproducible from one source to another and advantageously stable over a long period of time (tens of thousands of hours). The emission fluctuations (noise) are advantageously small enough to avoid limiting device performance. The cathode should be resistant to unwanted occurrences in the vacuum environment, such as ion bombardment, chemical reaction with residual gases, temperature extremes, and arcing. Finally, the cathode manufacturing is advantageously inexpensive, e.g. devoid of highly critical processes and adaptable to a wide variety of applications.
Previous cathode materials are typically metal (such as Mo) or semiconductor (such as Si) with sharp tips. While useful emission characteristics have been demonstrated for these materials, the control voltage required for emission is relatively high (around 100 V) because of their high work functions. The high control voltage increases damage due to ion bombardment and surface diffusion on the emitter tips and necessitates high power densities to produce the required emission current density. The fabrication of uniform sharp tips is difficult, tedious and expensive, especially over a large area. In addition, these materials are vulnerable to deterioration in a real device operating environment involving ion bombardment, chemically active species and temperature extremes.
Diamond emitters and related emission devices are disclosed, for example, in U.S. Pat. Nos. 5,129,850, 5,138,237, 5,616,368, 5,623,180, 5,637,950 and 5,648,699 and in Okano et al.,
Appl. Phys. Lett.
vol. 64, p. 2742 (1994), Kumar et al.,
Solid State Technol.
vol. 38, p. 71 (1995), and Geis et al.,
J. Vac. Sci. Technol.
vol. B14, p. 2060 (1996), all of which are incorporated herein by reference. While diamond field emitters have negative or low electron affinity, the technology has been hindered by emission non-uniformity, vulnerability to surface contamination, and a tendency toward graphitization at high emission currents (>30 mA/cm
2
).
Nanoscale conductors (“nanoconductors”) have recently emerged as potentially useful electron field emitters. Nanoconductors are tiny conductive nanotubes (hollow) or nanowires (solid) with a very small size scale of the order of 1.0-100 nm in diameter and 0.5-100 &mgr;m in length. Nanoconductors typically have high aspect ratios (>1,000) and small tip radii of curvature (1-50 nm). These geometric characteristics, coupled with the high mechanical strength and chemical stability, make nanoconductors attractive electron field emitters. Carbon nanotube emitters are disclosed, for example, by T. Keesmann in German Patent No. 4,405,768, and in Rinzler et al.,
Science,
vol. 269, p.1550 (1995), De Heer et al.,
Science,
vol. 270, p. 1179 (1995), Saito et al.,
Jpn. J. Appl. Phys.
Vol. 37, p. L346 (1998), Wang et al.,
Appl. Phys. Lett.,
vol. 70, p. 3308, (1997), Saito et al.,
Jpn. J. Appl. Phys.
Vol. 36, p. L1340 (1997), Wang et al.,
Appl. Phys. Lett.
vol. 72, p 2912 (1998), and Bonard et al.,
Appl. Phys. Lett.,
vol. 73, p. 918 (1998), all of which are incorporated herein by reference. Other types of nanoconductors such as Si or Ge semiconductor nanowires can also be useful because of the electrical field concentrating, high-aspect-ratio geometry, although they tend to have an outer surface of insulating oxide. See Morales et al.
Science,
Vol. 279, p. 208, (1998). Nonconductive materials form similar nanotubes and nanowires with similar favorable geometric features but are not presently used for emission or conduction. The term “nanostructures” will be used herein to encompass nanotubes and nanowires whether conductive or nonconductive.
Nanostructures are typically grown in the form of randomly oriented, needle-like or spaghetti-like powders that are not easily or conveniently incorporated into field emitter devices. Due to this random configuration, the electron emission properties are not fully utilized or optimized. Ways to grow nanostructures in an oriented fashion on a substrate are disclosed in Ren et al.,
Science,
Vol. 282, p. 1105 and Fan et al.,
Science,
Vol. 283, p. 512, both of which are incorporated herein by reference.
In the design and fabrication of efficient and reliable conductors and electron field emitters, a stable electrical continuity from the source of electrical power to the electron emitting tips is important. A combination of two conditions, i.e. that the nanostructures conduct along their entire length and that the substrate surface exhibit continuous conductivity, should be met to ensure a continuous transport of electrons to the field-emitting tips. (A third condition of electrical continuity from underneath the substrate to the top surface of the substrate should also be met if the electrical contact from the power supply is to be made on the bottom surface of the substrate). However, there are occasions when one or both conditions are not met.
Carbon nanotubes are the preferred nanostructures for electron field emission. FIGS.
1
(
a
)-
1
(
c
) are schematic molecular models showing various known configurations of carbon nanotubes. Single-wall nanotubes can be metallic with the “armchair” configuration of the carbon atoms C (FIG.
1
(
c
)). See M. S. Dresselhous et al.,
Science of Fullerines and Carbon Nanotubes,
Chapter 19, p. 758 and p. 805-809, Acdemic Press, San Di
Choi Kyung Moon
Jin Sung-ho
Kochanski Gregory Peter
Zhu Wei
Agere Systems Inc.
Lowenstein & Sandler PC
Patel Nimeshkumar D.
Santiago Mariceli
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