Electric lamp and discharge devices – Discharge devices having an electrode of particular material
Reexamination Certificate
1999-04-22
2003-10-07
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Discharge devices having an electrode of particular material
C313S309000, C313S310000, C313S495000, C445S050000, C445S049000, C445S051000
Reexamination Certificate
active
06630772
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to field emission devices comprising carbon nanotubes.
2. Discussion of the Related Art
Currently-used vacuum microelectronic devices include flat panel displays, klystrons and traveling wave tubes used in microwave power amplifiers, ion guns, electron beam lithography, high energy accelerators, free electron lasers, and electron microscopes and microprobes. A desirable source of electrons in such devices is field emission of the electrons into vacuum from suitable cathode materials. A typical field emission device comprises a cathode including a plurality of field emitter tips and an anode spaced from the cathode. A voltage applied between the anode and cathode induces the emission of electrons towards the anode.
One promising application for field emitters is thin matrix-addressed flat panel displays. See, for example,
Semiconductor International
, December 1991, 46; C. A. Spindt et al., “Field Emitter Arrays for Vacuum Microelectronics,”
IEEE Transactions on Electron Devices
, Vol. 38, 2355 (1991); I. Brodie and C. A. Spindt,
Advances in Electronics and Electron Physics
, edited by P. W. Hawkes, Vol. 83 (1992); and J. A. Costellano,
Handbook of Display Technology
, Academic Press, 254 (1992). A conventional field emission flat panel display comprises a flat vacuum cell, the vacuum cell having a matrix array of microscopic field emitters formed on a cathode and a phosphor coated anode on a transparent front plate. Between cathode and anode is a conductive element called a grid or gate. The cathodes and gates are typically intersecting strips (usually perpendicular strips) 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 (e.g., 400 to 5000 eV) to the emitted electrons. See, for example, U.S. Pat. Nos. 4,940,916; 5,129,850; 5,138,237 and 5,283,500, the disclosures of which are hereby incorporated by reference.
Field emission is also used in microwave vacuum tube devices, such as power amplifiers, which are important components of modern microwave systems, including telecommunications, radar, electronic warfare, and navigation systems. See, e.g., A. W. Scott,
Understanding Microwaves
, John Wiley & Sons, 1993, Ch. 12. Semiconductor microwave amplifiers are also available, but microwave tube amplifiers are capable of providing microwave energy several orders of magnitude higher than such semiconductor amplifiers. The higher power is due to the fact that electrons are able to travel much faster in a vacuum than in a semiconductor material. The higher speed permits use of larger structures without unacceptable increase in transit time, and the larger structures provide greater power.
A variety of characteristics are known to be advantageous for cathode materials of field emission devices. The emission current is advantageously voltage controllable, with driver voltages in a range obtainable from commercially available integrated circuits. For typical device dimensions (e.g. 1 &mgr;m gate-to-cathode spacing in a display), a cathode that emits at fields of 25 V/&mgr;m or less is generally desirable for typical CMOS driver circuitry. The emitting current density is desirably 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 desirably reproducible from one source to another and desirably stable over a very long period of time (e.g., tens of thousands of hours). The emission fluctuations (noise) are desirably small enough to avoid limiting device performance. The cathode is desirably 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 desirably inexpensive, e.g. having no highly critical processes and being adaptable to a wide variety of applications.
Conventional cathode materials for field emission devices are typically made of metal (such as Mo) or semiconductor material (such as Si) with sharp, nanometer-sized 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 voltage operation increases the damaging instabilities caused by ion bombardment and surface diffusion on the emitter tips and necessitates high power densities to be supplied from an external source to produce the required emission current density. In addition, the fabrication of uniform sharp tips is often difficult, tedious and expensive, especially over a large area. The vulnerability of these materials in a real device operating environment to phenomena such as ion bombardment, reaction with chemically active species, and temperature extremes is also a concern.
For microwave tube devices, the conventional source of electrons is a thermionic emission cathode, typically formed from Ir-Re-Os alloys or oxides such as BaO/CaO/SrO or BaO/CaO/Al
2
O
3
, which are coated or impregnated with metals, e.g., tungsten. These cathodes are heated to above 1000° C. to produce sufficient thermionic electron emissions (on the order of amperes per square centimeter). However, the need to heat these thermionic cathodes has the potential to create problems. Heating tends to reduce cathode life, e.g., by evaporating barium from the cathode surface. Some traveling wave tubes, for example, have lifetimes of less than a year. Heating also introduces warm-up delays, e.g., up to about 4 minutes before emission occurs, and such delays are commercially undesirable. Also, the high temperature operation requires bulky, ancillary equipment, e.g., cooling systems.
Attempts to provide improved emitter materials have recently shown carbon materials to be potentially useful as electron field emitters. 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, 1994, 2742; Kumar et al.,
Solid State Technol
., Vol. 38, 1995, 71; and Geis et al.,
J. Vac. Sci. Technol
., Vol. B14, 1996, 2060. While diamond offers advantages as field emitters due to its negative or low electron affinity on its hydrogen-terminated surfaces, further improvements are desired.
Another, recently discovered carbon material is carbon nanotubes. See, e.g., S. Iijima, “Helical microtubules of graphitic carbon,”
Nature
Vol. 354, 56 (1991); T. Ebbesen and P. Ajayan, “Large scale synthesis of carbon nanotubes,”
Nature,
Vol. 358, 220 (1992); S. Iijima, “Carbon nanotubes,”
MRS Bulletin,
43 (November 1994); B. Yakobson and R. Smalley, “Fullerene Nanotubes: C
1,000,000
and Beyond,”
American Scientists,
Vol. 85, 324 (1997), the disclosures of which are hereby incorporated by reference. Nanotubes take essentially two forms, single-walled (having tubular diameters of about 0.5 to about 10 nm), and multi-walled (having tubular diameters of about 10 to about 100 nm). The use of such nanotubes as electron field emitters is disclosed, for example, in German Patent No. 4,405,768; Rinzler et al.,
Science,
Vol. 269, 1550 (1995); De Heer et al.,
Science,
Vol. 270, 1179 (1995); De Heer et al.,
Science,
Vol. 268, 845 (1995); Saito et al.,
Jpn. J. Appl. Phys
., Vol. 37, L346 (1998); Wang et al.,
Appl. Phys. Lett
., Vol. 70, 3308 (1997); Saito et al.,
Jpn. J. Appl. Phys
., Vol. 36, L1340 (1997); and Wang et al.,
Appl. Phys. Lett
., Vol. 72, 2912 (1998), the disclosures of which are hereby incorporated by reference. Carbon nanotubes feature high aspect ratio (>1,000) and small tip radii of curvature (~10 nm). These geometric characteristics, coupled with the relatively high mechanical strength and chemical stability of the tubules, indicate the pote
Bower Christopher Andrew
Zhou Otto
Zhu Wei
Agere Systems Inc.
Harness & Dickey & Pierce P.L.C.
Patel Nimeshkumar D.
Santiago Mariceli
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