Tianbao Xie

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 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...

Transition metal carbide field emitters for field-emitter array devices and high current applications

We report on the use of transition metal carbides, primarily zirconium carbide, for field emission cathodes. This work encompasses three areas first, the use of carbide films as overcoatings on molybdenum field-emitter cathodes, second, the deposition of thick carbide films to form field-emitter cones directly in field-emitter arrays, and third, the use of single emitters formed from macroscopoic, single crystal carbide for use at high currents and in adverse vacuum conditions. Recent work with ZrC film overcoatings on Mo field emitters has focused on the processing steps required to obtain stable, low work function surfaces. Clean substrates are found to be essential. Emission degradation sometimes noted after exposure to air has been successfully reversed through operation in 10−5 Torr pressure of hydrogen. The most promising results applicable for use with arrays have been achieved with the direct deposition of zirconium carbide cones in blank field-emitter arrays. Thick films were deposited by physica...

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.

Emission fluctuation and slope–intercept plot characterization of Pt and transition metal carbide field-emission cathodes in limited current regimes

Our objective was to identify optimum emitter materials for block arrays and individually addressable tip arrays where emission stability and operation in a moderate vacuum is required. In general, transition metal carbide emitters are robust at high current and moderate vacuum levels due to their hard nature. Physical vapor deposition of composite emitters consisting of a carbide cone on a truncated niobium cone was used throughout. Four carbides (HfC, NbC, TiC, and ZrC) and platinum were deposited for the emitter materials in this study. All data were taken in the classically unstable pressures region of 10−7 Torr. Brief current stability measurements were collected along with current–voltage data. HfC, ZrC, and Pt demonstrated good initial stability, though all five materials were similar in emission performance. After a proprietary processing algorithm, TiC gave variations as low as ±1.5% ΔI/I at a 240 nA current level and 7.6×10−8 Torr pressure. An extensive discussion and analysis is given of Fowler...

Enhanced field emission from carbide-coated field emitters, and device applications

Abstract There is great interest in vacuum microelectronic devices using large compact field emitter arrays (FEAs), particularly flat panel displays and microwave devices. FEAs made of Mo or Si have been used in most cases. However, both materials have characteristics which limit FEA performance in devices. At LRI we have studied other FEA materials which may enable superior performance, particularly refractory carbides of transition metals such as ZrC. Our experiments demonstrate and quantify significant advantages, particularly reduced work function, more stable emission, greater tolerance of a degraded environment, high emitted current capability, and resistance to ion bombardment. With appropriate treatment and processing, it is possible to largely retain the advantages of carbides, either bulk emitters or thin coatings on Mo substrates, after exposure to air.

Proton RBS measurement of the oxygen in heavy-metal oxides

Abstract Although the stoichiometry of high-Tc superconductors is often measured using alpha-particle RBS, the small Rutherford cross section for oxygen relative to the high-Z components makes a determination of the oxygen content difficult. Above 1 MeV, the cross section for proton backscattering from oxygen becomes significantly greater than the Rutherford cross section. Studies of proton backscattering in CuO between 0.6 and 2.0 MeV show that it is possible to measure the oxygen content of high-Z metal oxides. RBS simulations using the Bragg-rule stopping power consistently underestimate the low-energy yield. Scaling the stopping power by a linear function of energy to reduce loss at low energy improves the simulation in CuO, Cu and Au. This general result suggests that the standard RBS simulation procedure may omit some relevant physics.

Nuclear resonance effects in the analysis of proton backscattering from carbon and silicon films

Abstract The use of elastic resonance backscattering of protons in the analysis of thin films is sometimes an advantage. It allows precise and accurate thickness determination even when the energy calibration of the system is marginal. The availability of reliable values for the scattering cross section in the region of the resonance is essential. We have remeasured the elastic scattering cross section of protons on C and Si in the energy region between 450 and 2200 keV at a laboratory angle of 155°. Our data are compared with earlier measurements. The usefulness of the incorporation of resonance scattering in the spectrum analysis is demonstrated.