Electric lamp and discharge devices – Discharge devices having a multipointed or serrated edge...
Reexamination Certificate
2003-01-13
2004-02-03
Clinger, James (Department: 2821)
Electric lamp and discharge devices
Discharge devices having a multipointed or serrated edge...
C315S169100
Reexamination Certificate
active
06686680
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to electron emission devices. More particularly, it relates to a method and apparatus for improving the performance of field emitter devices by detecting the emission of electrons at excessively positive potentials and regulating the current produced at the excessively positive potentials.
2. Description of Related Art
Field emission is a tunneling process where electrons move from a solid, through a thin potential barrier, into vacuum without changing energy. The field emitted current increases as a function of the electric field at the emitter surface. A macroscopic field emitter tip requires a voltage typically greater than 100V and often more than 1000V to cause emission. The electronic, chemical, and geometric properties of the emitter surface also have a substantial effect on the field emission current. These properties can change as a result of field emission, especially when adsorbed and reacted atoms are present on the emitting surface.
When electrons tunnel from states at energies below E
F
, electronic energy is released as the empty state is filled. If this electronic energy is large enough and is directly coupled to chemical bonds, the bonds can break, thus releasing atoms to the vacuum, stimulating atomic motion on the surface, and/or causing chemical reactions. Chemical bonds typically have energies of 2-5 eV, so emission from energies more than 2 eV below E
F
can potentially stimulate these changes.
A similar electronic mechanism occurs when positive charge and very high local electric fields are created as electrons tunnel out of insulating or semi-conducting material. If the electric fields become too high, local breakdown may result. The field emission characteristics of each emitter typically change continuously during operation as a result of this electronic excitation. If atoms are released into vacuum as a result, arcs can occur at the field emission site.
Such arcs release the energy stored in the charged capacitance formed by the high voltage emitter, potentially causing significant physical damage. Although the probability of direct coupling to a bond may be low, it is possible that filling a single low energy state could break a bond. In contrast, large current densities are typically required to heat the emitter to a point where bonds may be broken. Thus, the emission current required to cause such thermal effects is often much higher than the currents at which failures are found to occur.
The electronic energy released after tunneling increases as the energy of the initial state becomes more positive (lower electron energy). Cleaning a metallic emitter surface of foreign atoms (and keeping it clean during operation) typically reduces the low energy emission and increases the maximum emission current which can be produced without causing an arc. Cleaning can be accomplished by heating the emitter to very high temperature in ultra high vacuum or by applying a very large negative electric field so as to field-desorb the surface atoms.
Cleaning may also occur spontaneously during emission because of the electronically-stimulated reactions mentioned above, or due to bombardment by ions created by the emitted electrons. However, subsequent contamination generally occurs within a few hours or minutes even when the emitter is maintained in ultra-high vacuum.
Field emitter arrays are micro-fabricated arrays of many small field emission structures (cells) and are known in the art. Each individual cell includes an emission site on the substrate and an aperture in a conducting layer (called the gate) deposited over a dielectric layer. The size of the apertures is typically about 1 micron, but may be much smaller. The distance between cells is typically 3-4 times the aperture diameter, but may be larger. A large electric field is created at the emission site when a positive voltage is applied to the gate with respect to the emitter.
An FEA typically requires an emitter-gate voltage of at least 10V and sometimes more than 100V to cause emission. In many applications, operation of the arrays must occur in relatively poor vacuum, and most arrays cannot be heated to temperatures high enough to remove adsorbed surface atoms. Thus, emission typically occurs from surfaces covered with adsorbed atoms, and the electronic properties of the adsorbed atoms frequency dominate the emission properties.
Because the area of a single cell is small compared to the area required to make an external connection, only a limited number of connections to the array are practical. Thus, in typical state of the art arrays a large number of cells (~10,000) share the same electric connection to an external voltage source.
Ideally, the field emitter arrays would be able to provide total currents nearly equal to the number of cells in the array multiplied by the current a single cell can produce. However, the cells typically do not have uniform emission properties and will fail if the emission current is excessive. Thus, only a small number of cells contribute to the emission current, so the arrays do not produce nearly as much current as they might if the emitters were more uniform.
The emission current can also vary with time and from place to place over the array as a result of spatial and temporal non-uniformity in the physical and chemical properties of the emitting sites. This variation is undesirable for many applications.
One known method of forcing the emission currents from each of the individual cells in an array to be more equal is to place a current-limiting circuit element, typically a resistance, in series with each emitter. If the resistances are large enough, the voltages developed across the resistors dominate the emission properties of each cell. Thus, the emission current can be nearly as uniform from cell to cell as are the resistances. This sort of scheme is workable for some applications such as displays requiring relatively small current densities and frequencies.
However, the voltages developed across the resistors change the energy of the emitted electrons, increasing the energy distribution (energy spread) of the beam, which is undesirable for many applications. The resistors also reduce the transconductance (dI/dV) and frequency response of the arrays. Although more complex current-limiting circuits can reduce such problems, any circuit that changes the potential of the emission site will increase the energy spread of the emitted electron beam.
FIG. 1
shows a cross sectional view of a single cell within a prior art field emitter array (FEA). Although a single cell is shown herein for the sake of simplicity, an overall FEA includes many of these cells, fabricated in a planar array. An emitter structure
3
is created on a conductive substrate
2
(or a conductive layer on an insulating substrate) in such a way that when a voltage source
12
is connected between the conductive gate layer
8
and the substrate
2
, a field emission current is induced at the emission sites
4
of the emitter
3
.
The emitter structure
3
is often pointed in shape in order to create a region of enhanced electric field at the intended emission site. The gate layer
8
is separated from the substrate
2
by an insulating layer
6
, such as, for example, silicon dioxide. Normally, the emission current passes through a first aperture
10
(hereinafter “gate aperture
10
”) and is collected at a location having a potential of at least several volts more positive than the emission site. The diameter of the gate aperture
10
is typically on the order of 1 micron. To ensure that most of the field emission current passes through the gate layer
8
, the gate layer
8
is preferred to have rotational symmetry about a vertical axis, and the emission site
4
is preferred to be located on the axis of symmetry.
In order to make the electric field at the emission site
4
relatively independent of the voltages applied to external electrodes, the exposed face of the gate aperture
10
facing the emitter
Hsu David S. Y.
Shaw Jonathan L.
Clinger James
Hunnuis Stephen T.
Karasek John J.
The United States of America as represented by the Secretary of
Tran Chuc
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