Cathode Geometry Research Articles

The physics of field emission in the context of vacuum microelectronics

Abstract It is well known that surface treatments and cathode geometry affect the stability and strength of field emission (FE). The development of sub-micron electron sources requires research on the strength, stability and controllability of emission from areas of a few nanometres in diameter. The influence of cathode radius and surface layers on FE has been studied theoretically showing strong deviations from Fowler-Nordheim behaviour at low voltages. The interaction of a tunnelling electron with the charge density in the emitter, is also responsible for modifications in the surface potential. There is a characteristic response time for changes in the charge density and therefore it is of importance to be able to estimate the time that an electron spends tunnelling. An approach to the problem using Stochastic Mechanics is introduced and discussed in the context of other theories of the tunnelling time. Electron transport in a semiconductor depends on the doping concentration. Results are presented on the narrow width of the FE energy distrubution and explained by the narrow energy spectrum of the supply of electrons to the surface for highly doped silicon.

Application of glow discharge electron gun to surface modification

Surface modification of a material requires a known temperature field in the treated zone resulting from the technology of specific materials processing and related with the power density distribution of a surface heat source and heat conduction. Glow discharge electron gun was developed to realise the electron beam heat source of modified power density distribution. A numerical analysis of two-dimensional heat transfer for a flux of energy with optional distribution w( r) irradiating the material surface was carried out. The selection of a proper cathode geometry, electron beam power, working distance and interaction time makes it possible to obtain predicted temperature distribution in the treated material and expected shape of modified zone. The surface modification by an electron beam of modified power density distribution is expected to provide greater selectivity and precision of the treated area.

Discharge characteristics of a saddle-field ion source

The discharge characteristics of a cylindrical, saddle-field ion source, operating with nitrogen in the oscillating mode, have been measured using a Langmuir probe capable of being traversed along the axis of the source. Measurements made closer than 30 mm from the anode plane cause the source to become unstable. Outside this limit the discharge is found to have an electron density of approximately 2*1014 m-3, rising to approximately 5*1014 m-3 close to the cathode end surface. The electron temperature rises from 2.5*105 K to 4*105 K in the same distance range. The space potential falls monotonically from 580 V, 20 mm from cathode, to 220 V in the cathode aperture. The potential distribution is similar to the vacuum potential predicted by solution of Laplace's equation, but is about 200 V higher at any given point. Measurement of the ion energy distribution shows only two peaks, one at 200 eV, the other at 63% of the anode potential, essentially exactly that predicted for the saddle-point potential by solution of Laplace's equation, It is shown that this is not an effect of cathode geometry. A model is proposed to account for these effects based on spatially selective neutralisation of the ions within the source. This can arise because the electron density along the device axis is lower than anywhere else in the ion-forming region. The spatially selective neutralisation of ions by electrons within the source proceeds with a higher cross section for N2+ than N+ leading to an ion output largely comprising N+.

Direct current-excited cw CO_2 metal waveguide laser

A novel design for a dc-excited cw CO(2) metal waveguide laser has been developed in which a slotted hollow cathode also doubles as a metal waveguide for the cavity modes. This design has been implemented in a compact structure that produces over 1 W of cw 10.6-microm radiation. The discharge characteristics, laser gain, and laser output have been studied as functions of various discharge parameters. The advantages of the transverse discharge of the slotted hollow cathode geometry include low voltage, positive impedance, rugged structure, and high optical gain. Overall efficiency is comparable with that of conventional longitudinal CO(2) lasers. The output laser modes are clean low-order Hermite-Gaussian or Airy function modes.

The Effect of Oxidant Composition on the Performance of Molten Carbonate Fuel Cells

Theoretical and experimental studies were performed to determine the effect of oxidant composition on the overpotential of molten carbonate fuel cell cathodes. Pressure level, carbon dioxide and oxygen mole fractions, the diluent species (nitrogen vs. helium), and electrolyte content were varied. The overpotential was found to increase with decreasing carbon dioxide and oxygen mole fraction in most cases, due to an increase in gas‐phase diffusional resistance within the cathode and its current collector. The effective carbon dioxide and oxygen reaction orders for current at constant overpotential depend on oxidant composition, cathode and current collector geometry, and electrolyte content. The data were rationalized by a theoretical model which assumes that the rate controlling step is the reaction of oxide ion with carbon dioxide to form carbonate ion. However, further clarification of the reaction path is desirable to facilitate performance optimization.

Operating modes of relativistic rising-sun and A6 magnetrons

The operating characteristics of a relativistic 16-vane rising-sun magnetron were investigated with particular emphasis on determining the operating regimes of different modes. The magnetron performance was studied as a function of voltage, magnetic field, cathode geometry, axial boundary conditions, and output coupling. Operation was observed in the 3 pi /8 mode at 3.3 GHz, in the pi /2 or 3 pi /8 mode at 3.5 GHz, and in the pi or 7 pi /8 mode at 4.6 GHz. A maximum power of 80 MW was emitted in the 3 pi /8 mode with an efficiency of 4.5%. Typical pulse lengths were 40-50 ns. Cold tests were performed to measure the resonant frequencies and azimuthal electric fields in the interaction space which agreed within 1-4% of theoretical calculations. The operating modes were inferred from close agreement between hot-test frequencies and cold-test results and because high-power RF emission occurred at, or just above, the Buneman-Hartree threshold calculated for these modes. The characteristics of a six-vane A6-magnetron operating in the pi and 2 pi modes were also studied. A unique transmitting-receiving system, which was used as a microwave diagnostic, is described. >

Comparison of low-voltage field emission from TaC and tungsten fiber arrays

Field emitter array (FEA) devices were constructed using thin-film fabrication techniques based on in situ eutectic composites containing either TaC or NbC fibers in a NiCr alloy matrix. The emission characteristics of the TaC devices were measured and compared to those of W–ZrO2 FEA's. Both devices produced the linear Fowler-Nordheim plots common to all field emission devices; however, the array current density of the TaC FEA's was more than two orders of magnitude lower than that of the W–ZrO2 FEA's. This was primarily due to the lower fiber density and lower field enhancement of the carbide fiber devices. If it were possible to produce cathode geometries comparable to those of the W–ZrO2 FEA's, a theoretical analysis indicates that emission performance of the TaC devices would be superior because of the lower work function of TaC. However, post-emission observations indicated that the carbide fiber emitters failed more readily than the tungsten emitters under comparable operating conditions.

6211. The space distribution of the electron beam in a glow discharge electron gun and its correlation with the cathode geometry

6211. The space distribution of the electron beam in a glow discharge electron gun and its correlation with the cathode geometry

Hollow cathode lamps as excitation sources for analytical atomic spectroscopy

Although the applications of the Hollow Cathode Discharge (HCD) are not confined to the analytical field, it is here that this low-pressure excitation source has gained most of its popularity. Atomic absorption, atomic fluorescence and atomic emission spectrometry greatly benefit from HCD properties such as high stability of discharge, sharp profile of spectral lines, low background intensity, applicability to samples in various physical forms and ease of determination for non-metals. With the obvious exception for atomic absorption spectrometry, there has not been the expected widespread utilization of the HCD over the years, nor have manufacturers been significantly attracted by the relatively low costs of production and maintainance of HCD-based instruments. One of the factors hampering a wider distribution of this source could be the somewhat limited intensity output achievable when the discharge is operated under conditions which fully preserve its basic features. Thus, the exploitation of the HCD for the analysis of trace and ultratrace elements, is severely limited. No wonder therefore that so much effort has been recently prompted for developing new versions of the HCD capable of promoting radiation intensity enhancement. The most promising of these comprise coupling with magnetic field, boost through auxiliary electric discharge, modifications of the cathode geometry, application of radiofrequency bursts, use of pulse-current regime, superposition of microwave field and separation of the atomization and excitation processes. The consequent increase in signal-tonoise ratio, already very favourable in the HCD as it is not in local thermal equilibrium, further expands its analytical potential. Some representative analytical applications of the improved HCD tubes are illustrated.

The space distribution of the electron beam in a glow discharge electron gun and its correlation with the cathode geometry

Results of experimental research on the space distributions of the energetic electron beam generated in a glow discharge electron gun are presented. A computational method to determine the cathode geometry as a function of the electron distribution on a target is given.

Substrate heating rates for planar and cylindrical-post magnetron sputtering sources

Substrate heating energies per atom deposited are reported for planar magnetron sputtering of aluminum, chromium, nickel, copper, molybdenum, indium, tantalum, tungsten and platinum in argon as well as for aluminum and chromium sputtered in O 2. Data are also reported for cylindrical magnetron sputtering of niobium, silver, tantalum, tungsten and Pb-Sn in argon as well as for molybdenum sputtered in neon, argon, krypton and xenon. The planar and cylindrical magnetron heating rates were comparable. The heating energies for argon sputtering ranged from 10–15 eV per deposited atom for the light metals to almost 100 eV atom -1 for tantalum and tungsten. The implied reactive sputtering heating energies were about 500 eV molecule -1 for Cr 2O 3 and 1150 eV molecule -1 for Al 2O 3. Special experiments were conducted to examine the contributions to the substrate heating of plasma species and ion neutralization and reflection at the cathode. The data indicate that charged plasma species do not contribute significantly to the heating but that neutralized and reflected ions play a significant role in the planar as well as the cylindrical cases despite the differences in cathode geometry.

Reactive high rate d.c. sputtering of oxides

Normally the reactive d.c. sputtering process is characterized by a transition from the metallic sputter mode to the oxide sputter mode depending on the partial pressure of the reactive gas, the current density of the cathode and various other sputtering parameters. Typically this transition is combined with a drastic drop of the deposition rate. Because of the highly insulating properties of the oxides arcing will occur on areas of the target surface where the sputter rate will be less intense, leading to an unstable sputtering process. A new design of the reactive d.c. sputtering cathode geometry for oxide sputtering has been investigated, in which the reaction of the reactive gas at the substrates is separated from the reaction with the target surface. In this configuration the target is sputtering in the metallic mode while a stoichiometric oxide is formed on the substrate. Under these circumstances, even in the case of the deposition of Al 2O 3 and SiO 2, very stable process conditions have been observed. Furthermore, the deposition rate of the oxides is determined by the sputter yield of the target material. The theoretically expected deposition rates of Al 2O 3, SiO 2 and Ta 2O 5 compare well with the measured values in the experiments. For SnO 2 a lower experimental value has been obtained although the absolute deposition rate was the highest observed. In the case of Al 2O 3 a static deposition rate of 36 Å s -1 was measured at a dissipated power of 12W cm -2.

Intense relativistic electron beam propagation in evacuated drift tubes

The propagation of a magnetically confined intense relativistic electron beam (IREB) (1 MeV, 27 kA, 20 ns FWHM) in vacuum drift tubes has been studied experimentally and theoretically. Experimental results for current propagation as a function of uniform applied magnetic field (0–1.2 T) are presented for various drift tube diameters, cathode geometries, and anode aperture sizes. An analytic model of laminar beam flow is presented which predicts the space-charge-limited current of a solid IREB propagating in a grounded drift tube. A steady-state equilibrium is examined and is compared with the experimental results.

Mass Transfer Analysis of Electrodeposition Through Polymeric Masks

A tertiary current‐distribution model is developed for predicting two‐dimensional shape change during electrodeposition through polymeric masks. The cathode geometry studied consists initially of a parallel array of microscopic, rectangular trenches, the trench bottoms being conductive metal, and the trench walls being formed from insulating mask material. Concentration polarization, activation polarization, and a stagnant diffusion‐layer treatment of convective mass transport are incorporated into the model equations. Boundary integral equation methods and Euler‐predictor, trapezoid‐corrector time integration are used to calculate the deposit‐shape history as it depends on geometrical parameters, the level of convection, and the degree of polarization.

Electrogalvanic Finite Element Analysis of Partially Protected Marine Structures

Abstract A general finite element modeling (FEM) procedure for calculating electrogalvanic field responses due to multiple anodic/cathodic interactions has been developed. Within any defined electrolytic continuum the exact geometry and location of anodes, cathodes, and paint surfaces can now be realistically incorporated in the mathematical model formulation. The anodic/cathodic interactions in the conductive electrolyte are predicted by the application of classical d.c. electric field theory for conductive continuums in conjunction with widely accepted laboratory oxidation/reduction responses for the electrodes. The electrogalvanic fields in the electrolyte are calculated using the scalar Poisson equation whereby traditional boundary conditions are prescribed in the far field of the electrolyte. In the near field of the anodes, cathodes, and the painted metallic substrate, complex boundary conditions are enforced based on empirical polarization curves and paint impedance values. The ionic current in the...

Optimization of the UV CuII laser

The cw output power of the uv CuII laser has been optimized with respect to the hollow cathode geometry, the discharge current, the fill gas pressure and the resonator mirror parameters. A maximum laser output power of 900 mW for multiline operation at 248.6, 259.1, 260.0, and 270.3 nm was achieved with 100 A discharge current, 260 V voltage and 16 mbar fillgas pressure, when a hollow cathode of 1.2 m length and 2×6 mm2 cross section was employed. The single-pass pain gl has been estimated to 7%. A hollow cathode cross section of 1.5×4.5 mm2 is suggested as an optimum geometry. In addition, some investigations on the mechanism of the laser power decay in Ne−Cu-discharges are presented.

Simultaneous Measurement of Liquid Velocity and Oxygen Tension with Catheter-Tip Electrodes - The Dual Cathode Polarographic Method

The application of a dual-cathode catheter system to the simultaneous measurement of oxygen tension (pα) and velocity (V) has been studied under conditions similar to those in human arteries. The catheter tip contained a large silver electrode (16 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and a small silver electrode (0.035 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) which were connected to a common polarizing power supply and external silver/silver chloride reference anode (774 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). The 2 mm diameter catheter was centered in a 6.4 mm diameter tube through which flow of buffered physiologic saline solution at various oxygen tensions from 154 to 736 torr was metered at velocities of 1-67 cm/s. At a fixed velocity, the limiting current outputs from both the small and large cathodes were linear with pα. Membrane-coated catheters exhibited velocity-insensitive outputs from the small cathode whenever V 5 cm/s. The large cathodes were always flow-sensitive, but the degree of sensitivity depended upon the membrane thickness. Experiments with bare large electrodes indicated that the greatest sensitivity possible with the present cathode geometry is a relative current increase of 124 percent over a velocity range of 5-67 cm/s.

Fused salt electrorefining of gadolinium: An evaluation of three electrolytes II: Deposition characteristics in LiCl-LiF-GdF 3, LiF-BaF 2-GdF 3 and LiF-GdF 3

The nature of the gadolinium deposits formed during fused salt electrorefining in three electrolytes was investigated. Different morphologies were found for the gadolinium metal deposit in the three electrolytes — a platelike morphology in 80mol.%LiCl-18mol.%LiF-2mol.%GdF 3, a block-like morphology in 75mol.%LiF-10mol.%BaF 2-15mol.%GdF 3 and a needle-like morphology in 75mol.%LiF-25mol.%GdF 3. For the LiF-BaF 2-GdF 3 salt the block-like dendrites changed to needle-like dendrites when the current was raised above 0.8 A (the initial current density was 1.6 A cm −2). The current efficiency was determined as a function of cell current and temperature for the LiCl-LiF-GdF 3 and LiF-GdF 3 electrolytes, and it was found that the efficiency for the LiF-GdF 3 electrolyte was far superior to that for the LiCl-LiF-GdF 3 electrolyte. This may be because of the wide difference in GdF 3 concentrations, i.e. 2 mol.% for LiCl-LiF-GdF 3 compared with 25 mol.% for LiF-GdF 3. Also investigated were the influences of stirring, the bath composition, the cathode geometries and the continued usage of the electrolyte.

Cathodic reduction of glucose

The cathodic reduction of D-glucose was studied on Pb, Ni, Zn and Cd cathodes at three pH values in aqueous solution. The effect of cathode geometry and flow rate were also examined. Partial reduction current densities were obtained by g.l.c. analysis of the derivatised products. Current interruption and resumption, as well as runs using pretreated and untreated cathodes, all tend to suggest the presence of a surface film formed on the cathode during reductions. From these results and a re-interpretation of the data of other workers, a mechanism is proposed involving the near-electrode surface pH change, with the Tafel slope and reaction rate further altered by the solid formed at the cathode surface.

Impedance Control in Relativistic Electron Diodes with Externally Applied Prepulse

Low impedance relativistic electron accelerators currently produce nominal 50 ns pulses that are capable of power levels near 1 Terawatt at impedances near 1 Ohm. The time-dependent diode impedance characteristic plays a major role In efficiently coupling the pulse line power to an electron beam. In an effort to establish the desired accelerator impedance match early in the pulse, experimentalists have investigated cold cathode vacuum breakdown and subsequent space charge limited emission during the ~ 100 kV machine prepulse. This machine prepulse is due to capacitive coupling across the accelerator switches, and consequently cannot be independently studied and optimized. In this paper, a technique for externally introducing a typically 100 kV, low power conditioning pulse prior to the main pulse of a low impedance relativistic electron diode is described, along with techniques for reducing the machine prepulse to less than 5 kV. For various cathode geometries, the breakdown field, closure velocity, and time-dependent impedance established by this external prepulse is measured and compared with an empirical model of space charge limited emission from a hydro-dynamically expanding plasma. 3 Experimental evidence is presented that the high current accelerator impedance is effectively controlled by the relative time delay between the start of the prepulse and the main pulse.