(1) Field of the Invention
The invention relates to field emission stuctures, and more particularly to structures and methods of manufacturing field emission devices having two-part emitters.
(2) Description of the Related Art
Emission of electrons from conductive material is known to occur in the vicinity of an electric field, through such processes as Fowler-Nordheim tunneling. It is desirable to reduce the field strength required to induce electron emission. This is accomplished primarily by (1) the use of pointed structures at the location of emission, and (2) by using emitting materials with a low work-function. FIG. 1 shows a typical field emitting tip structure, which is utilized in such applications as electron microscopes and field emission displays (FEDs). A conical emitter 16 having a sharp tip 18 is formed on a conductive layer 10. This layer can be used as a conductive path formed on a glass or silicon substrate (not shown). For FEDs, the emitter is metal deposited by evaporation process, or alternately may be formed of silicon using well-known processes from the semiconductor industry including photolithography, deposition and etching. A conductive film 14 is separated from the substrate by a dielectric layer 12. The application of a voltage differential between conductive layers 14 and 10 induces electron emission from tip 18.
A reduction of the field strength necessary to create emission from the field emitter is desirable for several reasons. In an FED, for example, power consumption, driver circuit complexity and cost are lowered by reducing the driving voltage. The voltage must also be low enough so that dielectric breakdown does not occur in dielectric layer 12, which has a typical thickness of about 1 micrometer.
The use of one low work-function material for a field emitter is described in U.S. Pat. No. 5,258,685 (Jaskie et al.), and is shown in FIG. 2. A field emitter 16 is provided, on which a diamond coating 22 is formed, where the diamond coating is fabricated by implanting carbon ions which act as nucleation sites for the diamond film. Diamond deposited in an amorphic form has an extremely low work-function of -0.2 eV. Using the method disclosed by Jaskie et al. has several drawbacks, however. For instance, whereas the field emitter 16 may have had a sharp tip as formed, the formation of the diamond film 22 will reduce this sharpness and require a higher driving voltage. In addition, the use of this diamond process is likely to form a carbon film over the un-implanted area. The undesirable carbon growth along the top 26 and sidewall 28 of gate layer 14, and along the sidewall of dielectric 12, could lead to an undesired short-circuit condition between the conductive layers 14 and 10.