Electric lamp and discharge devices – Electrode and shield structures – Point source cathodes
Patent
1995-02-03
1997-07-29
Patel, Nimeshkumar
Electric lamp and discharge devices
Electrode and shield structures
Point source cathodes
313309, 313351, 445 51, 445 46, H01J 902
Patent
active
056524749
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to cold cathodes, which are devices which, without external heating and on application of a relatively small voltage, emit electrons into a vacuum. The invention includes a method of preparation, and also new cold cathodes whose emission characteristics are improved, in some cases by an order of magnitude, over any silicon cathodes described in the literature.
There are two main approaches to forming cold cathodes. One is by the production of negative electron affinity surfaces, and the other by forming material into small pyramids or columns, each with a very sharp point, on the surface of a wafer. This invention is concerned with the latter technique, the provision of sharp tips on a surface.
In order to emit electrons by field emission, the cathode tips must be very sharp, particularly if low operational voltages are required. The electrons are attracted to an anode and a metal gate usually held 0.1 .mu.m to 0.5 .mu.m away is normally used to switch the electron beam on and off. A diagram of a vacuum triode is shown in FIG. 1 and illustrates one possible arrangement of a device. A field emitter is fabricated of metal or semiconductor 10, and includes a cathode tip 12. A metal gate 14 is held around the top of the cathode tip by an insulating layer 16 (of an oxide) and a metal anode 18 is held above the cathode by a further insulating layer 20. When a positive potential difference is applied between the base 10 and the gate 14, an electric field is generated at the tip 12 which allows electrons to tunnel from the cathode material to a vacuum 22. The field at the tip and so the number of electrons emitted are controlled by the gate potential. This basic unit is usually integrated into a very large array, for example as shown in FIG. 2. This comprises a silicon base 24 having a profiled upper surface with silicon pyramids 26. An overlying layer of insulator 28 1 .mu.m thick is itself overlain by a metal grid 30, both gated to reveal the pyramids. The pyramids are shown 10 .mu.m apart, but the packing density of units into the array will depend on the particular application.
The field emission triode shown in the Figures may be used to perform similar functions to a transistor, and there are many applications which have been suggested for vacuum microelectronic devices which may lead to the development of a whole new industry. Possible applications include flat panel displays; superfast computers and memories; a new class of electron sources with large current densities, low extraction voltages, integral focussing and deflection, optical excitation and possibly multiple beams from a single chip; very high frequency amplifiers operating in the GHz range; sub-picosecond electronic devices and high power fast switches; in scientific instrumentation such as electron microscopes and in high radiation environments; for millimeter wave amplification and microwave sources for radar; as pressure sensors; and in electron beam processing of materials and for high gradient accelerators.
The properties which must be successfully developed for the evolution of vacuum microelectronics technology are cold emission, low voltage operation, high current density and small size and compatibility with present-day devices. Low emission noise, long life and uniformity are also required. Developing a fabrication method which gives reproducible cathode geometry and emission, controlling and understanding the physical processes at the emitter surface and practical aspects relevant to real devices, e.g. noise, life time and packing requirements, have all proved to be problems and are taking longer to resolve than expected. This invention focuses on improving the current from and operating voltage of individual cathodes, and also the reproducibility of emission from different individual cathodes; the current density and operating voltage of an array of cathodes should be improved comparably.
Field emitter arrays were first fabricated in 1961. These were of molybdenum and since that ti
REFERENCES:
patent: 5085746 (1992-02-01), Musselman et al.
patent: 5525857 (1996-06-01), Gnade et al.
Kovbasa et al, "Shaping of fine-tip emitters by electrochemical etching", SOV. Phys. Tech. Phys., vol. 20, No. 6, Jun. 1975 Jun. 1975.
Boswell Emily
Wilshaw Peter Richard
British Technology Group Limited
Patel Nimeshkumar
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