Electric lamp or space discharge component or device manufacturi – Process – With assembly or disassembly
Reexamination Certificate
2000-04-21
2004-11-23
Williams, Joseph (Department: 2879)
Electric lamp or space discharge component or device manufacturi
Process
With assembly or disassembly
C445S050000
Reexamination Certificate
active
06821175
ABSTRACT:
This invention relates to field emission devices and in particular to methods of manufacturing addressable field electron emission cathodes. Preferred embodiments of the present invention aim to provide low manufacturing cost methods of fabricating multi-electrode control and focusing structures.
It has become clear to those skilled in the art that the keys to practical field emission devices, particularly displays, are arrangements that permit the control of the emitted current with low voltages. The majority of the art in this field relates to tip-based emitters—that is, structures that utilise atomically sharp micro-tips as the field emitting source.
There is considerable prior art relating to tip-based emitters. 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, pp3504-3505, (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. Many variants and improvements on the basic Spindt technology are described in the scientific and patent literature.
An alternative 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. Again many variants have been described.
A major problem with all tip-based emitting systems is 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. Furthermore, in order to get low control voltages, the basic emitting element, consisting of a tip and its associated gate aperture, must be approximately one &mgr;m (one micron) or less in diameter. The creation of such structures requires semiconductor-type fabrication technology with its high associated cost structure. Moreover, when large areas are required, expensive and slow step and repeat equipment must be used.
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.
In 1988 S Bajic and R V Latham, (
Journal of Physics D Applied Physics
, vol. 21 200-204 (1988)), described a low-cost 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 (1995) Tuck, Taylor and Latham (GB 2304989) improved the above MIMV 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.
The best examples of such broad-area emitters can produce usable electric currents at fields less than 10 V&mgr;m
−1
. In the context of this specification, a broad-area field emitter is any material that by virtue of its composition, micro-structure, work function or other property emits useable electronic currents at macroscopic electrical fields that might be reasonably generated at a planar or near-planar surface—that is, without the use of atomically sharp micro-tips as emitting sites.
Electron optical analysis shows that the feature size required to control a broad-area emitter is nearly an order of magnitude larger than for a tip-based system. Zhu et al (U.S. Pat. No. 5,283,501) describes such structures with diamond-based emitters. Moyer (U.S. Pat. No. 5,473,218) claims an electron optical improvement in which a conducting layer sits upon the broad-area emitter to both prevent emission into the gate insulator and focus electrons through the gate aperture. The concept of such structures was not new and is electronoptically equivalent to arrangements that had been used in thermionic devices for many decades. For example Winsor (U.S. Pat. No. 3,500,110) described a shadow grid at cathode potential to prevent unwanted electrons intercepting a grid set at a potential positive with respect to the cathode. Somewhat later Miram (U.S. Pat. No. 4,096,406) improved upon this to produce a bonded grid structure in which the shadow grid and control grid are separated by a solid insulator and placed in contact with the cathode. Moyer's arrangement simply replaced the thermionic cathode in Miram's structure with an equivalent broad-area field emitter. However, such structures are useful, with the major challenge being methods of constructing them at low cost and over large areas. It is in this area that preferred embodiments of the present invention make a contribution to the art.
In Hoole A C F et al “Directly patterned low voltage planar tungsten lateral field emission structures”,
Journal of Vacuum Science and Technology
: Part B, Vol 11, No. 6, 1 Nov. 1993, Pages 2574-2578, XP000423379, there is disclosed a combination of low-resolution and high-resolution exposure steps. This is to overcome a problem with a high resolution device having an insufficiently small field of view. No attention or teaching is given as to the low cost fabrication of field emission cathodes having gate electrodes formed over cathode electrodes.
EP 0 795 622 A1 discloses a method for forming a field emission device. This involves vacuum deposition and is concerned with controlled ion bombardment of a precursor of a multi-phase material to form layers of different properties. It does not address the matter of forming field emission devices at low cost, and in particular, provides no teaching as to how desired accuracy of alignment can be achieved at low cost. Amongst many other things, it shows a fairly typical field emitter arrangement in which a structure that may be regarded as a gate electrode, comprising an insulating layer and a conducting layer, is disposed over a structure that may be regarded as a cathode electrode, comprising a field emitting layer between two conducting layers.
Preferred embodiments of the present invention aim to provide cost-effective field emitting structures and devices that utilise broad-area emitters. The emitter structures may be used in devices that include: field electron emission display panels; high power pulse devices such as electron MASERS and gyrotrons; crossed-field microwave tubes such as CFAs; linear beam tubes such as klystrons; flash x-ray tubes; triggered spark gaps and related devices; broad area x-ray sources for sterilisation; vacuum gauges; ion thrusters for space vehicles; particle accelerators; lamps; ozonisers; and plasma reactors.
According to one aspect of the present invention, there is provided a method of manufacturing a field electron emission cathode having at least one cathode electrode which comprises a field emitting layer between first and second conducting layers, and at least one gate electrode which overlies said cathode electrode and comprises an insulating layer and a third conducting layer, wherein said method comprises the steps of:
a. depositing on an insulating substrate to form by low resolution means, a sequence of said first conducting layer, field emitting layer and second conducting layer to form said cathode electrode;
b. depositing on said cathode electrode to form by low resolution means, a sequence of said insulating layer and third conducting layer, to form said gate electrode;
c. coating the structure thus formed with a photoresist layer;
d. exposing said photoresist layer by high resolution means to form at least one group of emitting cells, the or each said group being located in an area of overlap between one said c
Jones Peter Graham Adpar
Tuck Richard Allan
Barnes & Thornburg LLP
Printable Fields Emitters Limited
Williams Joseph
LandOfFree
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