Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Reexamination Certificate
2000-05-31
2004-05-25
Ramsey, Kenneth J. (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
C313S257000, C313S292000, C313S258000, C313S609000, C445S024000, C445S025000
Reexamination Certificate
active
06741025
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to field electron emission materials, and devices using such materials.
In classical field electron emission, a high electric field of, for example, ≈3×10
9
V m
−1
at the surface of a material reduces the thickness of the surface potential barrier to a point at which electrons can leave the material by quantum mechanical tunnelling. The necessary conditions can be realised using atomically sharp points to concentrate the macroscopic electric field. The field electron emission current can be further increased by using a surface with a low work function. The metrics of field electron emission are described by the well known Fowler-Nordheim equation.
There is considerable prior art relating to tip based emitters, which term describes electron emitters and emitting arrays which utilise field electron emission from sharp points (tips). The main objective of workers in the art has been to place an electrode with an aperture (the gate) less than 1 &mgr;m away from each single emitting tip, so that the required high fields can by achieved using applied potentials of 100V or less—these emitters are termed gated arrays. The first practical realisation of this was described by C A Spindt, working at Stanford Research Institute in California (
J. Appl. Phys.
39, 7, pp 3504-3505, (1968)). Spindt's arrays used molybdenum emitting tips which were produced, using a self masking technique, by vacuum evaporation of metal into cylindrical depressions in a SiO
2
layer on a Si substrate.
In the 1970s, an alternative approach to produce similar structures was the use of directionally solidified eutectic alloys (DSE). DSE alloys have one phase in the form of aligned fibres in a matrix of another phase. The matrix can be etched back leaving the fibres protruding. After etching, a gate structure is produced by sequential vacuum evaporation of insulating and conducting layers. The build up of evaporated material on the tips acts as a mask, leaving an annular gap around a protruding fibre.
An important approach is the creation of gated arrays using silicon micro-engineering. Field electron emission displays utilising this technology are being manufactured at the present time, with interest by many organisations world-wide.
Major problems with all tip-based emitting systems are their vulnerability to damage by ion bombardment, ohmic heating at high currents and the catastrophic damage produced by electrical breakdown in the device. Making large area devices is both difficult and costly.
In about 1985, it was discovered that thin films of diamond could be grown on heated substrates from a hydrogen-methane atmosphere, to provide broad area field emitters—that is, field emitters that do not require deliberately engineered tips.
In 1991, it was reported by Wang et al (
Electron. Lett.,
27, pp 1459-1461 (1991)) that field electron emission current could be obtained from broad area diamond films with electric fields as low as 3 MV m
−1
. This performance is believed by some workers to be due to a combination of the negative electron affinity of the (111) facets of diamond and the high density of localised, accidental graphite inclusions (
Xu, Latham and Tzeng: Electron. Lett.,
29, pp 1596-159 (1993)) although other explanations are proposed.
Coatings with a high diamond content can now be grown on room temperature substrates using laser ablation and ion beam techniques. However, all such processes utilise expensive capital equipment and the performance of the materials so produced is unpredictable.
S I Diamond in the USA has described a field electron emission display (FED) that uses as the electron source a material that it calls Amorphic Diamond. The diamond coating technology is licensed from the University of Texas. The material is produced by laser ablation of graphite onto a substrate.
From the 1960s onwards another group of workers has been studying the mechanisms associated with electrical breakdown between electrodes in vacuum. It is well known (
Latham and Xu, Vacuum,
42, 18, pp 1173-1181 (1991)) that as the voltage between electrodes is increased no current flows until a critical value is reached at which time a small noisy current starts flowing. This current increases both monotonically and stepwise with electric field until another critical value is reached, at which point it triggers an arc. It is generally understood that the key to improving voltage hold-off is the elimination of the sources of these pre-breakdown currents. Current understanding shows that the active sites are either metal-insulator-vacuum (MIV) structures formed by embedded dielectric particles or conducting flakes sitting on insulating patches such as the surface oxide of the metal. In both cases, the current comes from a hot electron process that accelerates the electrons resulting in quasi-thermionic emission over the surface potential barrier. This is well described in the scientific literature e.g.
Latham, High Voltage Vacuum Insulation, Academic Press
(1995).
FIG. 1
a
of the accompanying diagrammatic drawings shows one of these situations in which a conducting flake is the source of emission. The flake
203
sits on an insulating layer
202
above a metal substrate
201
and probes the field. This places a high electrical field across the insulating layer formed by for example the surface oxide. This voltage probing has been named the “antenna effect”. At a critical field the insulating layer
202
changes its nature and creates an electro-formed conducting channel
204
. A proposed energy level diagram for such a channel is shown in
FIG. 1
b
of the accompanying diagrammatic drawings. In this model electrons
212
near the Fermi level
211
in the metal can tunnel from the metal
210
into the insulator
216
and drift in the penetrating field until they are near the surface. The high field
213
in the surface region accelerates the electrons and increases their temperature to
~
1000° C. It is not known precisely what changes occur in the region of the channel but a key feature must be the neutralisation of the “traps”
217
that result from defects in the material. The electrons are then emitted quasi-thermionically over the surface potential barrier
215
. The physical location of the source of these electrons
205
is shown in
FIG. 1
a
and, whilst a proportion of them will initially be intercepted by the particle, it will eventually charge up to a point at which the net current flow into it is zero.
It is to be appreciated that the emitting sites referred to in this work are unwanted defects, occurring sporadically in small numbers, and the main objective in vacuum insulation work is to avoid them. For example, as a quantitative guide, there may be only a few such emitting sites per cm
2
, and only one in 10
3
or 10
4
visible surface defects will provide such unwanted and unpredictable emission.
Accordingly, the teachings of this work have been adopted by a number of technologies (e.g. particle accelerators) to improve vacuum insulation.
Latham and Mousa (
J. Phys. D: Appl. Phys.
19, pp 699-713 (1986)) describe composite metal-insulator tip-based emitters using the above hot electron process and in 1988 S Bajic and R V Latham, (
Journal of Physics D Applied Physics, vol.
21 200-204 (1988)), described a composite that created a high density of metal-insulator-metal-insulator-vacuum (MIMIV) emitting sites. The composite had conducting particles dispersed in an epoxy resin. The coating was applied to the surface by standard spin coating techniques.
Much later in 1995 Tuck, Taylor and Latham (GB 2304989) improved the above MIMIV emitter by replacing the epoxy resin with an inorganic insulator that both improved stability and enabled it to be operated in sealed off vacuum devices.
All of the inventions described above rely on hot electron field emission of the type responsible for pre-breakdown currents but, so far, no method has yet been proposed to produce emitters with a plurality of conducting particle MIV emitters in a c
Bishop Hugh Edward
Tuck Richard Allan
Barnes & Thornburg
Printable Field Emitters Limited
Quarterman Kevin
Ramsey Kenneth J.
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