Field emission cathode having an electrically conducting materia

Details Field emission cathode having an electrically conducting materia Field emission cathode having an electrically conducting materia

1996-08-23

1998-06-30

Dombroske, George M.

Electric lamp and discharge devices

Discharge devices having a multipointed or serrated edge...

313336, 313351, H01J 105

Patent

active

057739218

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a field emission cathode device of an electrically conducting material and with a narrow, rod-shaped geometry or a knife edge to achieve high amplification of the electric field strength, such that the electron-emitting part of the field emission cathode has cylindrical molecules. The invention also relates to a method for producing such a field emission cathode device.
Field emission means the emission of electrons from the surface of an electric conductor under the action of an electric field exceeding 10.sub.9 V/m. In practice, such field strengths are realized at sharp edges or tips, where the field strength is amplified. High vacuum is necessary to avoid gas discharges.


DESCRIPTION OF THE PRIOR ART

Field emission cathodes are used, for example, in field electron microscopes, in electron accelerators, in high-power switches (OS DE 39 24 745 A1) and in field emission diodes and field emitter arrays for vacuum microelectronics (thus for example Busta, Vacuum microelectronics--1992, Journal of Micromechanics and Microengineering, 2 (1992), pp. 53-60, and Iannazzo, A survey of the present status of vacuum microelectronics, Solid State Electronics, 36 (1993), pp. 301 to 320). A tungsten wire can be used as the field emission cathode, whose tip becomes so fine by etching that it can no longer be seen in an optical microscope. Also by etching, the ends of carbon fibers can be made sufficiently fine (Heinrich, Essig, Geiger, Appl. Phys. (1977) 12, pp. 197-202) to serve as a field emission cathode.
In vacuum microelectronics, field emission cathodes generally are produced by the methods of microprocess technology, by etching and sputtering, using lithographically produced masks (see Busta, Vacuum microelectronics--1992, Journal of Micromechanics and Microengineering, 2 (1992), pp. 53-60). By this method, one can produce conical tips with a radius of curvature of a few nm or wedge-shaped cutting edges with comparable radii of curvature. As materials for the cathode, one can use, for example, molybdenum, lanthanum hexaboride, hafnium, diamond-like carbon (B. C. Djubua, N. N. Chubun, Emission properties of Spindt-type cold cathodes with different emission cone material, IEEE Transactions on Electron Devices, 38 (1991) No. 10, pp. 2314-2316).
A disadvantage in the use of tips and edges, which have been produced by the known methods, is that the electron stream declines with operating time, since the tips or edges are destroyed by the positive ions of the unavoidable residual gas in the system. The like applies to field emission cathodes which are produced by sputtering techniques. The reason for this primarily is that the material structure of the emission tips is not uniquely defined. Thus, the geometry and microstructure of the tip and thus the work function of the electrons can vary within such wide limits that the electron streams from several tips, which were produced in one process, can differ by orders of magnitude, and furthermore change with operating time.
Furthermore, field emission cathodes for vacuum microelectronics cannot be produced in their optimal geometry by the prior art. Field strength calculations for various geometries of the tips show that the best shape of a field emission cathode is a narrow rod (Utsumi, Vacuum microelectronics: What's new and exciting, IEEE Transactions on Electron Devices 38 (1991), pp. 2276-2283). The present methods of microstructure technology can produce at most wedge-shaped tips in a defined manner.
Carbon nano-cylinders were observed for the first time in an electron microscope by Iijima (Nature, 354 (1991), p. 56). They can now be produced in large quantities, for example at the cathode of a visible arc (Iijima, Materials Science and Engineering, B19 (1993), pp. 172-180). In the presence of iron or cobalt, one can produce single-shell carbon nano-cylinders. Theoretical calculations show that, depending on the helicity of the hexagonal ring structure, the walls of the carbon nano-cylinders are electrically conducting or semiconducting (S

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