Electric lamp and discharge devices – Discharge devices having a thermionic or emissive cathode
Patent
1998-02-04
2000-08-01
Patel, Ashok
Electric lamp and discharge devices
Discharge devices having a thermionic or emissive cathode
313336, 313351, 313495, 313497, 313346R, 313309, H01J 130
Patent
active
060971398
DESCRIPTION:
BRIEF SUMMARY
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, .apprxeq.3.times.10.sup.9 V m.sup.-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 .mu.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), 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.sub.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 the other. 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.
A further discussion of the prior art is now made with reference to FIGS. 1 and 2 of the accompanying diagrammatic drawings, in which FIG. 1 shows basic components of one field electron emission display, and FIG. 2 shows the conceptual arrangement of another field electron emission display.
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. FIG. 1 shows basic components of such a display in which a field electron emission current is extracted from points 1 by applying a positive potential to gate electrodes 2. The extracted electrons are accelerated by a higher positive potential to a patterned phosphor on conducting strips 3 on a front plate. Pixels are addressed by energising horizontal and vertical stripes in a crossbar arrangement. The device is sealed around the perimeter and evacuated.
A major problem with all point 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.
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 1991, it was reported by Wang et al (Electron. Lett., 1991, 27, pp 1459-1461) that field electron emission current could be obtained from broad area diamond films with electric fields as low as 3 MV m.sup.-1. This performance is believed 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. 1993, 29, pp 1596-159).
Coatings with a high diamond content can now be grown on room temperature substrates using laser ablation and ion beam tech
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Latham Rodney Vaughan
Taylor William
Tuck Richard Allan
Patel Ashok
Printable Field Emitters Limited
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