Ivor Brodie

Field-emitter arrays for vacuum microelectronics

An ongoing program on microfabricated field-emitter arrays has produced a gated field-emitter tip structure with submicrometer dimensions and techniques for fabricating emitter arrays with tip packaging densities of up to 1.5*10/sup 7/ tips/cm/sup 2/. Arrays have been fabricated over areas varying from a few micrometers up to 13 cm in diameter. Very small overall emitter size, materials selection, and rigorous emitter-tip processing procedures have contributed to reducing the potential required for field emission to tens of volts. Emission current densities of up to 100 A/cm/sup 2/ have been achieved with small arrays of tips, and 100-mA total emission is commonly produced with arrays 1 mm in diameter containing 10000 tips. Transconductances of 5.0 mu S per tip have been demonstrated, indicating that 50 S/cm/sup 2/ should be achievable with tip densities of 10/sup 7/ tips/cm/sup 2/. Details of the cathode arrays and a variety of performance characteristics are discussed. >

Field-emitter arrays to vacuum fluorescent display

A thin vacuum-fluorescent display utilizing a matrix-addressable array of groups of Spindt-type field-emission emitter tips is described. Development of the display required expanding the cathode fabrication technology to cover 5-in-diameter areas uniformly with field-emitter arrays. Each color element of the display is addressed by a group of a few hundred emitter tips operating in parallel. Electrons emitted from the tips are proximity-focused on each color element. Because each pixel is essentially a miniature cathode-ray tube (CRT), the device has the potential to produce brightness similar to those on a CRT in a panel that is 3.3 in square (8.4 cm) in area and 0.15 in (4 mm) thick. >

Scandate Dispenser Cathodes With Sharp Transition and Their Application in Microwave Tubes

We report on a new cathode which is based on die-pressed matrices made from scandia-doped tungsten powder using the liquid-solid doping technique. Transition temperatures from space charge to temperature limited emission are as low as 840 °Cb(W) for emission of 5 A/cm2. Transitions are sharper than previously published scandate cathodes and compare favorably with M and ternary alloy cathodes. We present emission curves on several cathodes, show comparisons with other types of cathodes, and offer tentative explanations for the improvement. We discuss their suitability in microwave tubes at loadings of 5-10 A/cm2.

A New Model for the Mechanism of Operation of Scandate and Refractory Oxide Cathodes

Refractory oxide thermionic cathodes are modeled as a top layer of high-band-gap semiconducting cubic nanocrystals with surface donor sites activated by a monolayer of barium dispensed from a substrate. An interpretation of the common experimental observation of an apparent deviation from Childs law for the space-charge-limited region for scandate cathodes in simple diodes as a temperature-dependent resistance of the top layer of the cathode supports this model. Using a previously developed phenomenological theory based on the uncertainty principle, it is shown that the work function depends on the number of donor sites per unit area of the nanocrystals that comprise the top layer and the diameter of the metal atomic component of the oxide. The model is applied to provide quantitative values for the basic semiconductor properties of a top-layer scandate cathode from published experimental data.

Scandate cathode with sharp transition

We report on a new cathode which is based on die-pressed matrices made from scandia-doped tungsten powder. Transition temperatures from space charge to temperature limited emission are as low as 840° Cb(W) for emission of 5 Amps/cm2. Transitions are sharper than previously published scandate cathodes and compare favorably with M and ternary alloy cathodes. We present emission curves on several cathodes, show comparisons to other types of cathodes, and offer tentative explanations for the improvement. We discuss their suitability in microwave tubes at loadings of 5-10 Amps/cm2.

The nature of the emitting surface of scandate cathodes

Experimental evidence to support the concept that the low temperature electron emission from scandate cathodes arises from a dispersed layer of scandium oxide nano-crystals on a tungsten substrate that are activated by a monolayer of adsorbed barium atoms. Particularly convincing are images of the cathode surface from an electron emission microscope.

High-performance field-emission electron gun using a reticulated vitreous carbon cathode

Proof-of-concept experimental results stemming from beam simulations for a microfocus electron gun are presented. The simulations demonstrate the potential to produce 4 mA of current through a 40-μm-diameter spot, at an energy of 30 keV, emitted from a 1-mm-diameter cathode with low energy spread and high brightness. The experimental realization, scaled down for practicality, but consistent with and confirming the higher-energy simulation, produced 2 μA of current with an approximately 28 μm spot size at an energy of 9.3 keV. The electrons originated from an Ar+-ion-treated reticulated vitreous carbon (RVC) field-emission cathode shaped as an approximately 1-mm-diameter disk. The primary application for this work is a highly monochromatic microfocus x-ray source for use in phase-contrast imaging, although other beam applications exist. The use of an Ar+-ion-irradiated RVC cathode allows high, stable current at low electric field, superior to what is achievable using field-emitter arrays or carbon-nanotube...

17.1: A new model for the mechanism of operation of scandate and refractory oxide cathodes

The experimental evidence that lead to a new model for the scandate cathode is discussed followed by a description of the model itself. In the full paper the equations governing this model will be derived and used to provide quantitative estimates for the semiconductor and surface properties of a top layer scandate cathode using experimental data and the lattice parameters of scandia. The model is applicable to other metal oxides.

The work function of the crystal faces of metals

A new method for calculating the work function of the crystal faces of elemental metals from first principles, using the uncertainty principle and the concept of a surface 2-D electron gas, is developed. Results for tungsten and aluminum are presented and shown to be in good agreement with experimental values.

P4-22: Electrostatic focusing of electrons off a large-area reticulated vitreous carbon (RVC) field emission source

An electrostatic- focused electron source with an Ar+-ion irradiated reticulated vitreous carbon (RVC) field-emission cathode is designed and simulated. The electron beam from a 1 mm emissive surface is focused to a 30 µm diameter spot, retaining low energy spread and high brightness, yet high total current, for application to phase-contrast x-ray imaging.