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A method of forming vertical knife-edge cold-cathode field emission electron sources with self-aligned gate electrodes and sub-micron electrode separations. The method exploits the enhancement of ion-beam erosion rates obtained in metals at oblique ion incidence, which allows the preferential remova

A method of forming vertical knife-edge cold-cathode field emission electron sources with self-aligned gate electrodes and sub-micron electrode separations. The method exploits the enhancement of ion-beam erosion rates obtained in metals at oblique ion incidence, which allows the preferential removal of a metal layer at the convex edge of a mesa 2 to create a well-defined separation between the horizontal and vertical surfaces of the metal. The horizontal surface may be used as the gate and the vertical surface as the cathode in a vacuum triode structure. Electrical isolation is obtained by forming the mesa 2 in an insulating layer or substrate 1. Isolation may be improved by removing the insulating material in the vicinity of the metal edges. Field-induced electron emission from the cathode may be obtained at low voltage based on the enhancement of the electric field at the sharp tip of the cathode.

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1. A method of forming an electrode structure comprising the steps of providing an electrically insulating substrate, the substrate having being etched by a directional process to form a mesa having an edge defined by two intersecting planes at an upper convex corner of the mesa over which is provid

1. A method of forming an electrode structure comprising the steps of providing an electrically insulating substrate, the substrate having being etched by a directional process to form a mesa having an edge defined by two intersecting planes at an upper convex corner of the mesa over which is provided a layer of conductive material and selectively removing the conductive material at the edge thereby to form two electrodes, a first electrode formed in a horizontal plane of the mesa and a second electrode formed in a vertical plane of the mesa, both electrodes being formed in the same layer of conductive material, and wherein the removal of the material is effected at the edge only and in a single step.2. A method as claimed in claim 1, further comprising the step of removing a part of the insulating substrate adjacent the edge from which the conductive material has been removed, thereby to enhance the electrical insulation.3. A method as claimed in claim 1, wherein the mesa is formed with a small radius of curvature at the junction between its horizontal and vertical surfaces.4. A method as claimed in claim 1, further comprising the step of providing an additional layer of a material different from the conductive material at the junction of the vertical plane and the lower horizontal surface of the substrate, which serves to prevent erosion of the conductive layer at that position during ion-beam bombardment.5. A method as claimed in claim 1, wherein the conductive material has a low work function suitable for electron emission.6. A method as claimed in claim 1, wherein the electrodes are formed on a region of the surface of the substrate and wherein the edge is in the form of a meander pattern, the total length of which is substantially greater than the perimeter of the region.7. A method as claimed in claim 6, wherein the meander pattern comprises a plurality of linear segments.8. A method as claimed in claim 1, wherein the conductive material is removed by ion-beam erosion.9. A method as claimed in claim 8, wherein the ion-beam erosion comprises bombardment by ions of one of: (a) an unreactive species; (b) a reactive species; and (c) a mixture of the two.10. A method as claimed in claim 1, wherein the substrate includes a first component formed of a conductive material and a second component formed of an insulating material, the insulating material being deposited on the conductive material.11. A method as claimed in claim 10, wherein the insulating material is so deposited after formation of the edge of the mesa in the surface of the substrate.12. An electrode structure formed by a method as claimed in claim 1, wherein the two electrodes are formed on a region of the surface of the substrate, the two electrodes defining a gap which extends in a meander pattern, the total length of which is substantially greater than the perimeter of the region.13. A cold-cathode field-emission electron source comprising an electrode structure as claimed in claim 12, wherein the edge comprises the junction of the vertical and horizontal planes of a mesa formed on the substrate and wherein the resulting vertical electrode comprises the cathode of the electron source and the resulting horizontal electrode comprises the gate.14. A cold-cathode field-emission electron source comprising an electrode structure as claimed in claim 12, wherein the edge comprises the junction of the vertical and horizontal planes of a mesa formed on the substrate and wherein the resulting horizontal electrode comprises the cathode of the electron source and the resulting vertical electrode comprises the gate.15. A diode comprising an electrode structure as claimed in claim 12.16. A triode comprising an electrode structure as claimed in claim 12.17. An ion source comprising an electrode structure as claimed in claim 12.18. A mass spectrometer including an electrode structure as claimed in claim 12.19. A display device comprising an electrode structure as claimed in claim 12.