William A. Mackie

Transition metal carbides for use as field emission cathodes

Field emission characteristics of clean ZrC, HfC, and TaC cathodes are reported. High current density emission, greater than 1×108 A/cm2, is discussed, and a method for determining the cathode changes leading to this high current emission condition is proposed. A close‐spaced triode designed for testing individual emitters is described, and results are reported. The effective thermionic work functions of clean and partially oxygen covered surfaces of ZrCx specimens of two different bulk compositions are reported and discussed. Clean values of 3.5 and 3.4 eV were observed for ZrC0.92 and ZrC0.86 specimens, respectively, at 1500 K. With adsorbed oxygen, values as low as 3.4 and 3.2 eV, respectively, were observed for these surfaces at 1500 K.

Robust high current field emitter tips and arrays for vacuum microelectronics devices

We seek to identify the most promising materials and geometries for field emitter tips and arrays (FEA) in applications requiring stable high current and conductance in a moderate vacuum. Mo FEAs have been used successfully in relatively high current (50–100 mA) TWTs. But molybdenum has some undesirable characteristics. For several years we have studied field emission from refractory carbides, particularly ZrC, and found that ZrC surfaces have very desirable characteristics for stable high current field emission. We calculate thermal effects and instabilities for five emitter materials and conclude that W or Mo emitters coated with a thin coating of ZrC have the highest current capability, particularly for dc emission. We then calculate β factors for various field emission microdiodes and triodes. We conclude that broad cone angle ZrC/Mo emitters have the highest conductance capability. We then discuss several factors which cause FEA performance to fall short of theoretical calculations in practice. We th...

Field emission from hafnium carbide

An electron source for a high resolution SEM/TEM application should produce a high brightness, have a minimal energy distribution, and should be highly stable. In an application in which the resolution is limited by chromatic aberrations, one can improve the performance over a commonly used thermal field emission source, such as a ZrOW Schottky emitter, by using a cold field emission source (CFE).

Field emission from ZrC films on Si and Mo single emitters and emitter arrays

Field emission from ZrC films deposited on Si and Mo single emitters and field emitter arrays (FEAs) has been studied. For single emitters, the results show dramatic improvements in emitter performance by reducing work functions—on the order of 1 eV—and increasing stability. For FEAs, deposition of a ZrC film reduced the operating voltage 30%–50% at an emission current of 1.0 μA/tip and increased the emission stability.

Field emission from ZrC and ZrC films on Mo field emitters

We have been working for several years on field emission measurements from transition metal carbides. These studies have covered emission primarily from HfC and ZrC in the form of solid carbide emitters, thin film carbide overcoatings on single tip field emitters, and film coatings on field emitter arrays (FEAs). These carbide materials have work functions approximately 1 eV lower than Mo or Si, making them attractive candidates for low voltage microelectronic field emitter arrays. Uses for arrays or single emitters of these carbides include video displays, microwave applications, high current or small spot electron sources, and cold cathodes for operation in poor vacuums. Since Mo is one common FEA emitter material, we used it for an emitter substrate in this study. Our aim is to determine improvements in I(V) characteristics, emission stability, capability of emission at high currents and in poor vacuum or tube conditions, and the ability to withstand exposure to atmospheric pressure without degradation. Individually, fabricated Mo field emitters were dosed via plasma vapor deposition from a high-purity ZrCx source. The deposited film used for high currents were generally not heated but tested as deposited by field emission microscopy examination and determination of I(V) characteristics. The results of these experiments indicate that work function reductions of the order of 1 eV can be achieved along with the ability to emit at higher current levels. Stable high current emission has also been obtained in the 0.5 mA range with ZrC films on Mo. This represents a tenfold increase in the obtainable emission levels compared to emission from a clean Mo substrate. These data are also compared to field emission data from bulk ZrC emitters. Some observations of high current emission in μTorr vacuum levels are also presented. The mechanisms for these improvements are discussed along with the possible role of stoichiometry.We have been working for several years on field emission measurements from transition metal carbides. These studies have covered emission primarily from HfC and ZrC in the form of solid carbide emitters, thin film carbide overcoatings on single tip field emitters, and film coatings on field emitter arrays (FEAs). These carbide materials have work functions approximately 1 eV lower than Mo or Si, making them attractive candidates for low voltage microelectronic field emitter arrays. Uses for arrays or single emitters of these carbides include video displays, microwave applications, high current or small spot electron sources, and cold cathodes for operation in poor vacuums. Since Mo is one common FEA emitter material, we used it for an emitter substrate in this study. Our aim is to determine improvements in I(V) characteristics, emission stability, capability of emission at high currents and in poor vacuum or tube conditions, and the ability to withstand exposure to atmospheric pressure without degradation...

Single-crystal zirconium carbide as a high-temperature thermionic cathode material

Experiments to determine the feasibility of using transition metal carbides as stable thermionic cathodes at temperatures exceeding 2000 K are discussed. The primary emphasis here is electron emission and thermal stability properties of single crystals of ZrC. To obtain accurate emission versus temperature data, the spectral emissivity at 0.65 mu m of single-crystal ZrC specimens was measured in the temperature range 1200 >

OS3: Photoemission from gold thin films for application in multiphotocathode arrays for electron beam lithography

Photoemission is a promising approach to electron sources for electron beam lithography because of the ease with which various shapes or arrays of independently modulated sources can be fabricated. However, most high-quantum-efficiency photoemitters are extremely sensitive to even partial monolayers of contamination, and therefore require some combination of differential pumping systems and photoemitter surface protection after activation. Here we propose to use a high-power 257 nm laser in combination with the relatively high work function and low quantum efficiency of gold films to produce practical multicathode electron sources for electron beam lithography. Gold films have the offsetting advantages that their photoemission characteristics are relatively reproducible and stable even in contaminating environments. It is possible, therefore, to prepare and handle them in air as well as operate them in less demanding vacuum environments. It is shown that a back-illuminated 15 nm gold film on a quartz or s...

High current density field emission from transition metal carbides

Abstract We report on fabrication, performance and modeling of field emission cathodes made from refractory transition metal carbides. The primary carbides under study include HfC, TaC, and ZrC. Single crystal carbide specimens were prepared by arc floating zone refinement from sintered stock. Emission patterns are shown along with emission stability data. Work function ordering of various crystal planes determined by means of field emission microscopy is reported and comparisons are made with results from thermionic projection microscopy. Typically, the carbides show that ∅210,310 2400 h are reported. Stable pulse field emission of > mA for a single emitter at pulse lengths of 10 μs and duty factors of 0.01-0.001 are reported. Single-pulse currents as high as 48 mA were observed from a single emitter tip. Current densities have been calculated to be in the high 108 A/cm2 range.

Field emission from carbide film cathodes

We report here on experiments with the deposition of ZrC onto individual prefabricated W and Mo emitters and on field emission arrays using Mo and Si emitters. With observed field emission current densities greater than 1×108 A/cm2 and work functions approximately 1 eV lower than Mo or W, carbides make good candidates for low voltage microelectronic field emitter arrays. In addition, evaporation of ZrC films onto Mo and W single field emitter cathodes improves their emission stability and greatly improves beam confinement. Mo and W field emitters were individually fabricated and an appropriate quantity of ZrC was evaporated onto the emitter surface from a high‐purity ZrC evaporation source. Several emitters prepared by this method have been tested. The uniformity of the emission pattern was checked by in situ field emission microscopy, both before and after carbide deposition, in order to verify that the surface of the emitter had been cleaned and smoothed. The deposited film was subjected to a variety of...

Hafnium carbide films and film-coated field emission cathodes

We have previously reported on field emission improvements in turn-on voltages and emission stability using ZrC films as coatings on Si and Mo single emitters and emitter arrays. However, during our emission studies of bulk carbides, HfC was found to be slightly superior. We now report on work in progress investigating HfC films and HfC film coated field emission cathodes. Uses for arrays of these field emission cathodes range from video displays to microwave applications. This article deals with physical vapor deposition of HfC, absolute work function measurements, and electron emission properties of these film surfaces. This work demonstrates improvements by using HfC films over films of ZrC and an associated surface work function lowering of more than 1 eV in some instances compared to the clean surface.