Vapour deposited cone formation during fabrication of low voltage field emitter array cathodes

Abstract

A line-of-sight vapour deposition process is used to form a low-voltage field emission cathode structure on the surface of an oxide-metal eutectic composite etched to expose arrays of single-crystal tungsten fibres. A SiO₂ insulating film is first deposited, forming cone-shaped deposits on the tips of the exposed fibres. These cones act as a shadow mask for the subsequent deposition of a metal film. After removal of the cones, the metal film forms a grid-like structure with concentric apertures centered on each fibre tip. Application of a potential of less than 100 V between the metal grid and the base of the fibres produces an electric field at the fibre tips of sufficient intensity to produce field emission. The equilibrium geometry of the deposits which form on the fibre tips (cathode cones) is such that there is a linear relationship between the height of the cone and its diameter. There is also a direct correlation between the cathode cone angle, which depends on the material being deposited, and the cone angle of the insulator film. For SiO₂, the cathode cone angle was determined to be 37°, while the cone angle of the insulator film was 26°. Comparison of the theoretical values of the lateral growth velocity of the cathode cones with values determined from experimental measurements indicated that the maximum sticking coefficient was independent of the growth angle, η, for SiO₂, but varied as a function of cos η for molybdenum. A multiple deposition process was developed which permitted independent control of the interelectrode spacing and insulator film thickness.