Electric lamp and discharge devices – Discharge devices having a multipointed or serrated edge...
Reexamination Certificate
2001-03-09
2002-09-10
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Discharge devices having a multipointed or serrated edge...
C313S336000, C313S351000, C313S497000, C313S311000
Reexamination Certificate
active
06448701
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a field emitter cell and array, in particular, to an integrally gated, self-aligned field emitter cell and array whose cathode is formed of a recently discovered class of materials of nanotubes and nanowires, collectively referred to as nanofilaments.
BACKGROUND OF THE INVENTION
Field emitters arrays (FEAs) are naturally small structures which provide reasonably high current densities at low voltages. Typically, FEAs are composed of emitter cells in the form of conical, pyramidal, or cusp-shaped point, edge or wedge-shaped vertical structures. These cells are electrically insulated from a positively charged extraction gate and produce an electron beam that travels through an associated opening in the positively charged gate.
The typical field emitter structure includes a sharp point at the tip of the vertical structure (field emitter) and opposite an electrode. In order to generate electrons which are not collected at the extraction electrode, but can be manipulated and collected elsewhere, an aperture is created in the extraction electrode. The aperture is larger (e.g., two orders of magnitude) than the radius of curvature for the field emitter.
Consequently, the extraction electrode is a flat horizontal surface containing an extraction electrode aperture for the field emitter. Such an extraction electrode is referred to as the gate electrode. The field emitter is centered horizontally in the gate aperture and does not touch the gate although the vertical direction of the field emitter is perpendicular to the horizontal plane of the gate. The positive charges on the edge of the extraction electrode aperture surround the field emitter symmetrically so that the electric field produced between the field emitter and the gate causes the electrons to be emitted from the field emitter in a direction such that the electrons are collected on an electrode (anode) that is separate and distinct from the gate. The smaller the aperture, i.e., the closer the gate is to the field emitter, the lower the voltage required to produce field emission of electrons.
The sharp point at the tip of the field emitter provides for reduction in the voltage necessary to produce field emission of electrons. As a result, numerous micro-manufacturing techniques have been developed to produce various sharp tip designs. Current techniques include wet etching, reactive ion etching (RIE), and a variety of field emitter tip deposition techniques.
Effective methods generally require the use of lithography which has a number of inherent disadvantages including a high equipment and manufacturing cost. For example, the high degree of spatial registration requires expensive high resolution lithography.
Additionally, cathode structures include very small localized vacuum electron sources which emit sufficiently high current. However, these vacuum electron sources are difficult to fabricate for practical applications. This is particularly true when the sources are required to operate at reasonably low voltages. Presently available thermionic sources do not emit high current densities, but rather result in small currents being generated from small areas. In addition, thermionic sources must be heated, and thus require special heating circuits and power supplies. Photoemitters have similar problems with regard to low currents and current densities.
Recent advancements in nanotechnology have resulted in the creation of nanofilaments including nanotubes. One such example is carbon nanotubes. These nanotubes behave like metals or semiconductors and can conduct electricity better than copper, transmit heat better than diamond, and are among some of the strongest materials known while being only a few nanometers in diameter. Nanofilaments can have small diameters, ranging down to only a few nanometers. The nanofilaments may be grown to various lengths (e.g., 100-1000 nm) yet their diameter remains uniform. The aspect ratio (length to diameter) is extremely high.
Nanofilaments in the form of nanotubes have a hollow edge which is on the order of a couple of Angstroms thick. The nanotubes may be either single, double, or multiple walled (i.e., one nanotube within a second, third or further nanotube). For a more comprehensive discussion on carbon nanotubes, see “Carbon Nanotubes Roll On,”
Physics World,
June 2000, pages 29-53.
Carbon nanotubes have been proposed as excellent candidates for use as field emitter cathodes due to: (1) the extreme sharpness of their edges and the extremely large aspect ratio, which enable the achievement of low operating voltages; (2) the resistance to tip blunting by residual back ion bombardment due to the uniform wall thickness throughout their height; (3) the relative inertness, high mechanical strength and current carrying capacity; and (4) an inherent current-limiting mechanism in the presence of adsorbed water which retards emitter burn out and destruction by arcing, a problem plaguing the present day FEAs. Nanotubes have been demonstrated in use as a cathode in a cathode lighting element in which the carbon-nanotubes act as the field-emitting cathode.
To be effective emitters, the nanofilaments need to be oriented largely perpendicular to the substrate. Recently, this property has been achieved by growing the nanofilaments on substrates under suitable conditions such as by high temperature chemical vapor deposition (CVD) on catalytic surfaces. For example, CVD has been used to form extremely vertical and uniformly grown carbon nanotubes directly above a metal catalyst substrate of patterned and oxidized iron patches. The resulting nanotubes form an ungated clump electrode which provides a stable field emission over the entire test duration of 20 hours.
On the other hand, high emission current from carbon nanotubes oriented parallel to the substrate has also been observed, which can be attributed to defects on the tube sidewalls. Nanotubes in this orientation can be expected to erode more quickly than those oriented perpendicular to the substrate by residual back ion bombardment.
However, these nanofilament electrodes are not gated and thus, have limited practical use as field emitters. In order to use nanofilaments as a field emitter, one must control the operating characteristics of the nanofilaments, i.e., the turning on and off of small selected groups (i.e. clumps) of nanofilament emitters which comprise an array of emitter cells (e.g. pixels). This control is accomplished by providing a gate electrode, whose applied voltage bias controls the turning on, turning off and the field emission current magnitude. In order to enable low voltage operation, it is necessary to provide a control gate in very close proximity to a group of nanofilament emitters.
One proposed method of forming a gated nanofilament field emitter includes pre-positioning a paste layer of the nanotubes separately on a substrate and assembling a control grid gate assembly to the paste layer of nanofilaments. This and other presently available manufacturing techniques (all non-integral) fail to provide practical (e.g., in terms of functional and economical) gating of nanofilaments, e.g., nanotube, field emitters.
One clear disadvantage of this method is that the resulting gated unit tends to be large when compared to integrally formed conventional field emitter cells, which limits the resolution. As a result of the increased emitter-grid gate separation, these grid-gated emitters require a much higher gate voltage (hundreds of volts as compared to tens of volts for integrally gated emitters) for their operation.
An additional disadvantage with presently available carbon nanotube field emitting cathodes is that the grid-type control gates and nanotube cathodes are not self-aligned with one another because the control grid gate is assembled to the nanotubes after a paste layer of nanotubes has already been formed. As a result, the gate current (e.g.: current intercepted by the gate) tens to be very high which can cause overheating. In addition
Forman Rebecca L.
Karasek John J.
Patel Ashok
The United States of America as represented by the Secretary of
LandOfFree
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