Angular Current Intensity Research Articles

Enhanced angular current intensity from Schottky emitters

Even though the Schottky emitter is a high-brightness source of choice for electron beam systems, its angular current intensity is substantially lower than that of thermionic cathodes, rendering the emitter impractical for applications that require high beam current. In this study, two strategies were attempted to enhance its angular intensity, and their experimental results are reported. The first scheme is to employ a higher extraction field for increasing the brightness. However, the tip shape transformation was found to induce undesirably elevated emission from the facet edges at high fields. The second scheme exploits the fact that the angular intensity is proportional to the square of the electron gun focal length [Fujita, S. & Shimoyama, H. (2005) Theory of cathode trajectory characterization by canonical mapping transformation. J. Electron Microsc. 54, 331-343], which can be increased by scaling-up the emitter tip radius. A high angular current intensity (J(Omega) approximately 1.5 mA sr(-1)) was obtained from a scaled-up emitter. Preliminary performance tests were conducted on an electron probe-forming column by substituting the new emitter for the original tungsten filament gun. The beam current up to a few microamperes was achieved with submicron spatial resolution.

Generalization of the paraxial trajectory method for the analysis of non-paraxial rays: simulation program G-optk for electron gun characterization

The paraxial trajectory method has been generalized for the application to the cathode rays inside electron guns. The generalized method can handle rays that initially make a large angle with the optical axis with a satisfactory accuracy. The key to success of the generalization is the adoption of the trigonometric function sine for the trajectory slope specification, instead of the conventional use of the tangent. Formulas have been derived to relate the ray conditions (position and slope of the ray at reference planes) on the cathode to those at the crossover plane using third-order polynomial functions. Some of the polynomial coefficients can be used as the optical parameters in the characterization of electron sources; the electron gun focal length gives a quantitative estimate of both the crossover size and the angular current intensity. An electron gun simulation program G-optk has been developed based on the mathematical formulations presented in the article. The program calculates the principal paraxial trajectories and the relevant optical parameters from axial potentials and fields. It gives the electron-optical-column designers a clear physical picture of the electron gun in a much more faster way than the conventional ray-tracing methods.

Lens theory of non-paraxial rays for electron gun characterization

It has been attempted to characterize the cathode rays inside electron guns by utilizing the optical concepts familiar in the paraxial lens theory. We have proposed the Canonical Mapping Transformation (CMT) theory to obtain the gun optical parameters. The method is based on the Lagrange differential invariant theorem known in analytical mechanics. It has been found that major electron source properties, such as the crossover position, crossover size, and the angular current intensity, are all deducible from the four CMT optical parameters ( zco , f , Csg , and Ccg ), which in turn can be estimated by calculating the normal electron rays, whose emanation vectors on the cathode surface are in the surface normal direction. Since the normal electron rays can in many cases be regarded as paraxial, a scheme has been proposed to calculate the relevant optical parameters by a modification of the conventional paraxial trajectory calculation. It is shown that the normal electron ray of the CMT theory corresponds to (1, 0) principal trajectory in the paraxial method, g(z) . The conventional perturbation characteristic function integral method can be employed for the evaluation of the CMT aberration coefficients. Two realistic electron gun models (a single-crystal LaB6 cathode gun and a Schottky emitter source) were analyzed by use of the CMT optical parameters. Both the ray tracing of the normal electrons and the modified paraxial calculation method were employed for the analyses. It has been found that the guns with quite different nature in the source properties can well be described by the CMT optical parameters proposed. The paraxial calculations have been shown to produce accurate enough results and the authors hope their use would help electron optical column designers both in reducing the work load and in having clear physical images of their gun characteristics. © 2008 Elsevier B.V. All rights

Gun lens theory for nonparaxial trajectories by canonical mapping transformation: Characterization of general skewed rays inside electron guns by electron gun focal length

The canonical mapping transformation (CMT) theory has been generalized to nonmeridional (skewed) rays. It has been derived that with minor mathematical modification a CMT optical parameter, “electron gun focal length,” can be applied to skewed rays. The generalization makes it possible to characterize the behavior of nonparaxial electron trajectories inside the gun by a small number of optical parameters derived in the CMT theory. Important gun properties such as the crossover size and the angular current intensity are readily available from the electron gun focal length. The relevant CMT optical parameters can be estimated either by ray tracing a few representative cathode trajectories or by use of the conventional paraxial trajectory method whose applicability is usually limited to paraxial trajectories. Interpretation of the parameters by the CMT method extends their validity to nonparaxial cases. It has been found that using the rotating coordinate system in the conventional paraxial trajectory theory is equivalent to adopting the canonical direction (direction based on the canonical momentum as opposed to the kinetic momentum) in specifying the ray condition in the CMT. The “vector potential shift” must be taken into consideration in the beam profile evaluation at the crossover when nonzero magnetic field exists on the cathode surface. The beams from the off-axis points on the cathode miss the optical axis and do not form a conventional crossover with consequent reduction in the average brightness of the electron beam when the beam current is increased by accepting the electrons from increasingly off-axis emission points on the cathode.

A new evaluation method of electron optical performance of high beam current probe forming systems

A new numerical simulation method is presented for the electron optical property analysis of probe forming systems with point cathode guns such as cold field emitters and the Schottky emitters. It has long been recognized that the gun aberrations are important parameters to be considered since the intrinsically high brightness of the point cathode gun is reduced due to its spherical aberration. The simulation method can evaluate the 'threshold beam current I(th)' above which the apparent brightness starts to decrease from the intrinsic value. It is found that the threshold depends on the 'electron gun focal length' as well as on the spherical aberration of the gun. Formulas are presented to estimate the brightness reduction as a function of the beam current. The gun brightness reduction must be included when the probe property (the relation between the beam current l(b) and the probe size on the sample, d) of the entire electron optical column is evaluated. Formulas that explicitly consider the gun aberrations into account are presented. It is shown that the probe property curve consists of three segments in the order of increasing beam current: (i) the constant probe size region, (ii) the brightness limited region where the probe size increases as d approximately I(b)(3/8), and (iii) the angular current intensity limited region in which the beam size increases rapidly as d approximately I(b)(3/2). Some strategies are suggested to increase the threshold beam current and to extend the effective beam current range of the point cathode gun into micro ampere regime.

Criterion for decrease of electric field at ionization point of liquid-metal ion sources

Criterion for decrease of electric field at the ionization point of liquid-metal ion sources has been derived. The argument is based on the equation of space-charge limited current with small charge approximation and the balance of electric field stress, with liquid flow and surface tension stress. The result was such that it is possible to decrease the electric field with an increase in the applied voltage, that is, the source current, under the condition of weaker dependence of current density on the electric field than that in the conventional low temperature field evaporation theory. The dependence of the ion current density on the electric field was calculated from the dependence of the angular current intensity on the electric field, assuming postionization theory. The result indicated that the dependence became almost proportional, which is surprisingly weak for field ion emission.

Gallium liquid metal ion source (abstract)

Considerable interest has been shown in the electrohydrodynamic liquid field emission ion source, from which positive ions emitted under the action of high electric fields, and high brightness sources are being developed with a variety of applications in mind. The basic principles of such sources are discussed. The threshold voltage Vth, the ion current (I) versus extraction voltage (V) relationships under various operating temperatures To, I versus To relationship, the angular current intensity and brightness, energy spread, as well as the current changes slowly with time under a definite voltage, are presented and discussed. The experiments show that the present performance of the ion source is as follows: The threshold voltage is about 5.4 kV, the brightness equal to 1×108A/cm2 sr, and energy spread of 26 eV.

Use of the liquid metal ion source for focused beam applications

A review of the liquid metal ion source (LMIS) emission properties as they affect its performance in focused ion beam (FIB) applications is given for several pure metal sources which include Al, Ga, In, Au and Bi. Functional relationships between source emission characteristics such as energy spread, angular current intensity, virtual source size and LMIS properties such as atomic mass, source current and ionic charge are given. Experimental and Monte Carlo results are used to develop these relationships. In general best LMIS performance for FIB applications occurs at low source current and with small atomic mass LMIS. Beam noise characteristics are close to the shot noise limit.

Growth and current characteristics of stable protrusions on tungsten field ion emitters

Protrusions on field ion emitters can improve the angular current intensity of a gas field ion source by two orders of magnitude. We undertook some investigations of the growth of protrusions, here called superstrips, by cathode bombardment and their field ion current characteristics. The growth is related to surface migration activated by ion bombardment. The current measurements show that a high lateral gas supply to the superstrip and hence a low tip temperature is the most important factor in obtaining high currents from superstrip. He + angula current intensities of 8 μA/sr at total current of 10 nA have been reached at 18 K tip temperature.

Reservoir-type liquid metal ion source of Al

A liquid metal ion source of Al with a boride reservoir was developed. In this reservoir a shorter boride emitter was designed to overcome the brittleness of boride materials and some problems in supplying the Al material. This source makes possible a long continuous operation time and enhanced reliability. The fundamental performance characteristics did not change after 250 h of operation. A stable Al ion beam emission was obtained for more than 500 h and its current fluctuation was less than ±1%/3 h for a 20–30-μA source ion current. The energy spread was less than 9 eV (FWHM) for a 30-μA/sr angular current intensity.

A focused ion beam system for submicron lithography

A two-lens optical system for a focused ion beam system has been designed, using a four-electrode accelerating lens as the condenser lens and an Einzel lens as the objective lens. A 1 nA beam current in a 0.055–0.1 μm beam spot is obtained under the following conditions: 20 μA/sr angular current intensity, 10 eV beam energy spread, 30–60 kV acceleration voltage, and 4 mrad object side beam half angle. The chromatic aberration coefficient of the condenser lens is reduced by picking out the dominant design parameters and optimizing them under the design constraints. The four-electrode accelerating lens has an object-side chromatic aberration coefficient Cco=11 mm at the object side focal length fo=25 mm at infinite magnification for 30 kV acceleration voltage. The Einzel lens operating in the acceleration mode has an image-side chromatic aberration coefficient Cci=70 mm at the image-side focal length fi=50 mm. A focused ion beam system is developed using the designed lenses and a Ga ion source. A pattern as fine as 0.15 μm with excellent definition is obtained.

100 keV focused ion beam system with a E×B mass filter for maskless ion implantation

Various liquid metal alloy ion sources and a 100 keV mass separated focused ion beam system have been fabricated and their basic characteristics have been measured. These are mass spectra, energy spread and angular current intensity for ion sources, and focusing characteristics of the system. It was observed that Be–Si–Au ternary alloy ion sources produce doubly charged Be and Si ions and the importance of these ion sources is demonstrated by fabricating a GaAs JFET using a maskless ion implantation technique and by PMMA resist exposure.

B, Sb and Si Liquid Metal Alloy Ion Sources for Submicron Fabrication

B, Sb and Si liquid metal alloy ion sources have been fabricated and basic characteristics such as current-voltage relations, angular current intensity, energy distribution and mass spectra have been measured. Characteristics of ion beam focusing systems are also discussed. It was observed that the energy distribution width (FWHM) of various ion species obtained for Sb alloy sources decrease with increasing the charge to mass ratio of the ion beam. It was also observed that the mass spectra varied slightly with a total ion source current. These ion sources could be operated for more than 10 h without any change in mass spectra.

Liquid metal alloy ion sources for B, Sb, and Si

B, Sb, and Si ion sources using liquid metal alloys have been fabricated and basic characteristics such as current–voltage relations, angular current intensity, energy distribution, and mass spectra have been measured. Singly charged B ions which amount to 33% of the total ions and have an angular current intensity of ∠40 μA/sr at a total energy spread (FWHM) of ∠20 eV were obtained using B–Ni–Pt alloys. Singly charged Si and Sb ions with an angular current intensity of ∠11 μA/sr and ∠1.4 μA/sr at an energy spread of ∠20 eV were obtained by using Si–Au and Sb–Pb–Au alloys, respectively. These ion sources could be operated for more than 10 h without any changes in mass spectra.