Virtual Source Size Research Articles

A high brightness source for nano-probe secondary ion mass spectrometry

The two most prevalent ion source technologies in the field of surface analysis and surface machining are the Duoplasmatron and the liquid metal ion source (LMIS). There have been many efforts in this area of research to develop an alternative source [ S.K. Guharay, J. Orloff, M. Wada, IEEE Trans. Plasma Sci. 33 (6) (2005) 1911; N.S. Smith, W.P. Skoczylas, S.M. Kellogg, D.E. Kinion, P.P. Tesch, O. Sutherland, A. Aanesland, R.W. Boswell, J. Vac. Sci. Technol. B 24 (6) (2006) 2902–2906] with the brightness of a LMIS and yet the ability to produce secondary ion yield enhancing species such as oxygen. However, to date a viable alternative has not been realized. The high brightness and small virtual source size of the LMIS are advantageous for forming high resolution probes but a significant disadvantage when beam currents in excess of 100 nA are required, due to the effects of spherical aberration from the optical column. At these higher currents a source with a high angular intensity is optimal and in fact the relatively moderate brightness of today's plasma ion sources prevail in this operating regime. Both the LMIS and Duoplasmatron suffer from a large axial energy spread resulting in further limitations when forming focused beams at the chromatic limit where the figure-of-merit is inversely proportional to the square of the energy spread. Also, both of these ion sources operate with a very limited range of ion species. This article reviews some of the latest developments and some future potential in this area of instrument development. Here we present an approach to source development that could lead to oxygen ion beam SIMS imaging with 10 nm resolution, using a ‘broad area’ RF gas phase ion source.

Direct Images of the Virtual Source in a Supersonic Expansion

Direct images of the virtual source in a supersonic expansion of helium are presented. The images were obtained using a Fresnel zone plate with free-standing zones, 540 microm in diameter and with an outermost zone width of 50 nm. The general method can be extended to other beams, including seeded beams. Measurements were carried out at absolute source pressures ranging from 11 to 171 bar using a 10 microm nozzle with a source temperature of 320 K. The size of the virtual source was found to be strongly dependent on pressure, changing from a diameter of 67+/-6 microm at an absolute nozzle pressure of 11 bar to 180+/-9 microm at 171 bar. The virtual-source brightness displays a maximum at an absolute nozzle pressure of around 30 bar. This phenomenon occurs because of two competing effects: As the pressure increases, the total flux also increases, but at the same time the virtual source broadens. We modeled the expansion process by calculating the velocity distribution with solutions from the Boltzmann equation to estimate the location of the quitting surface where the frequency of interatomic collisions is assumed negligible. Realistic potentials have been used to calculate the cross section for atomic collisions and, for the velocity distribution perpendicular to the center streamline, a proper scaling with distance derived from the continuum expansion model has been introduced. A good agreement between experiments and model has been found and we discuss its approximation limits. For instance, backscattering effects are not included in the calculations and at present we cannot exclude that they also contribute to a broadening of the virtual-source size for the highest pressure regime. The results presented here are important for improving the understanding of the supersonic expansion process. The experimental method might eventually be used as a new way to test molecular and atomic interaction potentials.

Cathode ray tube type electron gun as a source for multibeam electron lithography

The authors have investigated the potential of using a dispenser cathode in space charge limited regime for employment in an electron beam lithography electron source. The space charge limitation guarantees stable and uniform emission even if there are small work function variations or bumps and depressions on the surface. Employment of a dispenser cathode in the space charge limited regime enables high beam currents and splitting of the electron beam into many sub-beams for parallel multibeam electron lithography. In the reported experiment, the electron beam is split into 194 sub-beams. The reduced brightness, defined as current divided by normalized emittance, was measured at different cathode temperatures and extraction potentials for a cathode ray tube type electron source equipped with an I-type dispenser cathode. In the central 25 sub-beams, reduced brightness values of up to 106Am−2sr−1V−1 were observed. Such a high reduced brightness in combination with a high total emission current (up to 20mA) indicates potential application in electron beam lithography systems. In accord with theory, the experimentally observed reduced brightness is directly proportional to the emission current density. It was found, however, that the brightness drops if the emission current density is increased beyond the level where the emitter leaves the space charge limited regime. Within the space charge regime, increasing reduced brightness as a function of increasing current density is found to be caused by a decreasing virtual source size, while the angular current density remains nearly invariant.

Appendix to “Coulomb interactions in Ga LMIS” by Radlička and Lencová

This appendix comments on the input data and simulation results of Radlička and Lencová (RL), reported in the preceding paper. These simulations relate to the effects of space-charge on the full-width at half-maximum (FWHM) of the ion energy distribution from a gallium liquid metal ion source (LMIS), and on its virtual source size. Corrections are necessary to the input data, relating to LMIS shape, that were supplied by the present author. It is clear from RL's work that their simulations, when applied to the corrected shape data, would generate very good agreement between experimental results and numerical simulations concerning the FWHM. This work confirms that an inappropriate hydrodynamic condition has been used in past modelling programmes for LMIS shapes and current/voltage characteristics; consequently, some quantitative results from these old programmes must be considered unreliable in detail, especially at low emission currents, although qualitative trends have been correctly exhibited.

Comparison of parameters for Schottky and cold field emission sources

Total energy distribution (TED) measurements were carried out for point electron sources operating in the cold field (T=300K) and Schottky (T=1800K) emission regimes. The full width at half maximum (FWHM) values of the TED’s for both emission regimes were found to increase significantly above the respective theoretical values as the emitter radius (a) was decreased and as the angular current density (I′) was increased. This increase in the FWHM arises from the stochastic electron-electron interactions in the beam commonly known as the Boersch [Z. Phys. 139, 115 (1954)] effect. A method was devised to extract the magnitude of the Boersch effect from the experimental TED’s. The TED’s were investigated as a function of I′ and a. In addition, the reduced brightness for both emitters was calculated from the virtual source size and I′ values as a function of the FWHM values.

High brightness inductively coupled plasma source for high current focused ion beam applications

A high brightness plasma ion source has been developed to address focused ion beam (FIB) applications not satisfied by the liquid metal ion source (LMIS) based FIB. The plasma FIB described here is capable of satisfying applications requiring high mill rates (>100μm3∕s) with non-gallium ions and has demonstrated imaging capabilities with sub- 100-nm resolution. The virtual source size, angular intensity, mass spectra, and energy spread of the source have been determined with argon and xenon. This magnetically enhanced, inductively coupled plasma source has exhibited a reduced brightness (βr) of 5.4×103Am−2sr−1V−1, with a full width half maximum axial energy spread (ΔE) of 10eV when operated with argon. With xenon, βr=9.1×103Am−2sr−1V−1 and ΔE=7eV. With these source parameters, an optical column with sufficient demagnification is capable of forming a sub-25-nm spot size at 30keV and 1pA. The angular intensity of this source is nominally three orders of magnitude greater than a LMIS making the source more amenable to creating high current focused beams, in the regime where spherical aberration dominates the LMIS-FIB. The source has been operated on a two lens ion column and has demonstrated a current density that exceeds that of the LMIS-FIB for current greater than 50nA. Source lifetime and current stability are excellent with inert and reactive gases. Additionally, it should be possible to improve both the brightness and energy spread of this source, such that the (βr∕ΔE2) figure-of-merit could be within an order of magnitude of a LMIS.

Development of scanning x-ray fluorescence microscope with spatial resolution of 30nm using Kirkpatrick-Baez mirror optics

We developed a high-spatial-resolution scanning x-ray fluorescence microscope (SXFM) using Kirkpatrick-Baez mirrors. As a result of two-dimensional focusing tests at BL29XUL of SPring-8, the full width at half maximum of the focused beam was achieved to be 50×30nm2 (V×H) under the best focusing conditions. The measured beam profiles were in good agreement with simulated results. Moreover, beam size was controllable within the wide range of 30–1400nm by changing the virtual source size, although photon flux and size were in a trade-off relationship. To demonstrate SXFM performance, a fine test chart fabricated using focused ion beam system was observed to determine the best spatial resolution. The element distribution inside a logo mark of SPring-8 in the test chart, which has a minimum linewidth of approximately 50–60nm, was visualized with a spatial resolution better than 30nm using the smallest focused x-ray beam.

Source brightness and useful beam current of carbon nanotubes and other very small emitters

The potential application of carbon nanotubes as electron sources in electron microscopes is analyzed. The resolution and probe current that can be obtained from a carbon nanotube emitter in a low-voltage scanning electron microscope are calculated and compared to the state of the art using Schottky electron sources. Many analytical equations for probe-size versus probe-current relations in different parameter regimes are obtained. It is shown that for most carbon nanotube emitters, the gun lens aberrations are larger than the emitters’ virtual source size and thus restrict the microscope’s performance. The result is that the advantages of the higher brightness of nanotube emitters are limited unless the angular emission current is increased over present day values or the gun lens aberrations are decreased. For some nanotubes with a closed cap, it is known that the emitted electron beam is coherent over the full emission cone. We argue that for such emitters the parameter “brightness” becomes meaningless. The influence of phase variations in the electron wave front emitted from such a nanotube emitter on the focusing of the electron beam is analyzed.

Analytical model of a gas phase field ionization source

High resolution focused ion beam technology is based on the use of field ionization sources. The most widely used source by far is the liquid metal ion source (LMIS) and most focused ion beam systems today employ it. The gas phase field ionization source (GFIS) is hardly employed today except in field ionization microscopy, but present day technology would allow its unique properties vis a vis the liquid metal ion source, including smaller virtual source size and energy spread, and the ability to produce ions from H, He, and heavier noble gases, to be exploited. If the GFIS is used as an ion source for a focusing column it would be useful to be able to calculate its emission properties (current versus voltage) as a function of emitter shape (a parameter not variable in a LMIS). Such a calculation had not been done precisely based on realistic emitter shapes. In this work, a theoretical description of the mechanism for ion production in a GFIS is presented. The comparison of the result with the experimental data for a H2–Ir GFIS is given, which shows reasonable agreement. With the present model, when used with the model for the optical properties of the GFIS (to be published), a focused ion beam (FIB) optical designer can make reasonable predictions about the properties of a GFIS-based FIB. Based on the modeling, we predict that a sub 1nm beam of He+ should be possible with ∼1pA current.

SU-FF-T-294: Monte Carlo Simulations of the Dosimetric Characteristics of a New Multileaf Collimator

Purpose: The aim of the work was to investigate the dosimetric characteristics of a new multileaf collimator (160MLC™, Siemens) with the help of Monte Carlo (MC) simulations during the design phase. Method and Materials: The MLC was implemented in the MC code Geant4. For the simulation of the 6 MV treatment beam an experimentally validated phase space and a virtual source model were used. For the simulation of the geometry in Geant4 the jaws and the two leaf packages were implemented with the help of CAD data. First, transmission values for different tungsten sinters were extracted using the simulation codes Geant4 and BEAMnrc and compared to experimental measurements. In a second step, high resolution simulations were performed to detect the leakage at depth of maximum dose. The 20%–80% penumbra along the leaf travel direction was determined for different 10×10 cm2 fields shifted along the x-axis. The simulated results were compared with measured data obtained with a prototype. Results: The simulation of the transmission values for different tungsten sinters showed a good agreement with the experimental measurements (within 2.0%). This gave an accurate estimation of the absorption coefficient for various leaf materials. Simulations with varying source sizes showed that the leakage and the penumbra depended very much on this parameter: e.g. source sizes of 2 mm and 4 mm result in the interleaf leakages below 0.3% and 0.75% respectively. The results for the leakage and the penumbra are in good agreement with the measurements. Conclusion: This study showed that Geant4 is appropriate for the investigation of the dosimetric characteristics of a multileaf collimator. In particular we could quantify the leakage and the penumbra and evaluate the influence of the beam parameters such as the virtual source size. Conflict of Interest: Research supported by Siemens Oncology Care Systems.

Interference fringes observed in electron emission patterns of a multiwalled carbon nanotube

A capped multiwalled carbon nanotube with clean surface gives field emission patterns consisting of six pentagonal rings which correspond to pentagons located at the tip end. One or a few bright streaks are also observed at the boundaries of neighboring pentagons. The spacing of streaks is inversely proportional to the square root of the accelerating voltage. Namely, the spacing changes with the wavelength of emitted electrons according to Young’s interference equation. The visibility of streaks increased with the accelerating voltage, which can be explained in terms of a concept of a virtual source size. These experimental results suggest that the streaks are no more than Young’s interference fringes for which the adjacent pentagons behave as double slits.

Brightness of carbon nanotube electron sources

The virtual source sizes of individual multiwalled carbon nanotube electron emitters were investigated with a point projection microscope. The average radius of the virtual source size was found to be 2.6 nm, which does not correspond to the standard model of a field emitter. Instead, a model based on a flattened cap or an open cap seems to provide a more realistic description. The broadening effect of Coulomb interactions on the virtual source was calculated. The reduced angular current density was measured at the maximum current at which stable emission was obtained and arrived at an average of 30 nA sr−1 V−1. The reduced brightness values obtained for two emitters were (2.5±1)×109 and (1.3±0.5)×109 A m−2 sr−1 V−1, respectively. These values are an order of magnitude larger than the values of state-of-the-art commercial sources.

High brightness electron beam from a multi-walled carbon nanotube.

Carbon nanotubes can act as electron sources with very rigid structures, making them particularly interesting for use as point electron sources in high-resolution electron-beam instruments. Promising results have been reported with respect to some important requirements for such applications: a stable emitted current and a long lifetime. Two parameters of an electron source affect the resolution of these instruments: the energy spread of the emitted electrons and a parameter called the reduced brightness, which depends on the angular current density and the virtual source size. Several authors have measured a low energy spread associated with electron emission. Here we measure the reduced brightness, and find a value that is more than a factor of ten larger than provided by state-of-the-art electron sources in electron microscopes. In addition, we show that an individual multi-walled carbon nanotube emits most current into a single narrow beam. On the basis of these results, we expect that carbon nanotube electron sources will lead to a significant improvement in the performance of high-resolution electron-beam instruments.

High-Brightness Unstable-Resonator Lasers Fabricated with Improved Dry-Etching Technology for Ultra-Smooth Laser Facets

In this article, we present the fabrication and investigation of high-brightness semiconductor lasers. Details about the epitaxial growth together with results of broad-area lasers are shown. For the fabrication of the dry-etched laser mirrors, a multilayer etch mask together with an optimized chemically-assisted ion-beam etching (CAIBE) process have been developed leading to ultra-smooth laser facets. With this technique, unstable-resonator lasers with curved mirrors have been fabricated. The devices were tested in pulsed and continuous wave operation, exhibiting high optical output powers. Corrected far fields were measured and the corresponding virtual source size was calculated to be in the range of 2 µm leading to a brightness of 60 MW/(cm2 sr). This was achieved in cw operation using only the main lobe intensity of the uncoated single-facet output power.

Performance of multicusp plasma ion source for focused ion beam applications

The multicusp plasma ion source has found many uses such as in ion implanters, ion projection lithography, and injectors for particle accelerators. This source is favorable for such applications because of its low ion temperature, small energy spread, and good stability. These properties also offer the tantalizing possibility of utilizing such a source to create a noncontaminating focused ion beam (FIB) system for semiconductor fabrication line use and advanced photomask repair applications. We have made measurements of the brightness of this ion source and have done some preliminary applications work with a Kr+-enabled FIB system for mask repair and ion milling. A specially designed multicusp plasma ion source has been used to measure focused beam spot size versus current relationships for several inert gas ion species. From these data the axial brightness of the ion source was ascertained. For the extraction of Kr+ from the source we find an angular intensity of 12 μA/sr and a virtual source size of 16.9 μm, giving an approximate value for the image-side brightness (at 30 keV) of 1650 A/cm2 sr. We find good sputtering rates for the Kr+ beam, and the Aerial image measurement system measurements show essentially 100% transmission of 193 nm light through photomask quartz that had been implanted with a significant dose of krypton.

Axially symmetric electron beam trajectory simulation

An axially symmetric simulator for the Schottky emission gun has been developed using the boundary-fitted coordinate transformation method. The domain decomposition method is successfully employed with multi-layer overlapping, which allows complicated electrode structures to be modeled and the electric potential computation to converge quickly. The angular intensity distribution of Swanson’s Schottky emission gun is analyzed, and good agreement is seen with his experimental data. The simulation results show that angular intensity and virtual source size are remarkably dependent on the real emitter size.

A liquid metal ion source in a high energy microprobe setup

We describe first experiments with a new arrangement of the Bochum superconducting solenoid microprobe using a single ended electrostatic accelerator and the implementation of a high brightness Ga liquid metal ion source. In this setup the accelerator and the microprobe components are mounted on a common optical bench which is mechanically decoupled from the laboratory building via a separate basement. Care had to be taken of the ion optical adaptation of the source to the accelerator tube in order to preserve the source brightness in the entire experimental setup. The emittance characteristic of the Ga ion beam was determined directly at the location of the microprobe via automatic emittance scanning using the computer controlled slit system of the setup. By this means the parameters of the unfocused beam could be measured for both the accelerated case (315keV) and the unaccelerated case (30keV). It could be shown that the observed brightness of the source behind the extraction optics is about three orders of magnitude less than values quoted in the literature (∼106A m−2 rad−2 eV−1) which were deduced from the virtual source size and the angular current density of the ion beam at the source tip. The parameters of the focused beam are presented.

Ion projection lithography: Status of the MEDEA project and United States/European cooperation

Structure and targets of the European MEDEA project on ion projection lithography as well as related U.S./European cooperation are explained. By assuming 10 μm virtual source size and 1 eV (full width half maximum) energy spread calculations for a multielectrode electrostatic ion–optical system (1.25 m between ion source and stencil mask, ≈1.8 m between mask and wafer) we realize the possibility of 100 nm resolution (line and space) over an exposure field of 22×22 mm2 even when using the MONTEC model for calculating the stochastic blur and when running 3.3 μA He+ ion beam current through the ion–optical column, thus more than twice exceeding target specifications. Thus, for 100 nm resolution and 50% pattern density the raw throughput is ≈12 cm2/s corresponding to >75 WPH (pattern within 80% of 300 mm wafer area).

Emission characteristics of an Au–Ge liquid metal ion source

A liquid metal ion source (LMIS), based on an Au 27 at.% Ge eutectic alloy was investigated and the mass spectrum and angular distributions of different ion species determined. A strong influence of the total emission current and the angle with respect to source axis on the mass spectrum was observed. The process of ion emission from the ion source was simulated and the virtual source size, beam spot size, and the energy and angular distributions of the ions estimated. The effect on the emission characteristics of Coulomb interaction between ions was also investigated.

Calculation of the electron-optical characteristics of electron beams transmitted into vacuum from a sharp tip-thin foil junction

The electron-optical characteristics of a novel electron source consisting of a sharp tip-thin foil tunnel junction are calculated, taking into account the tunnel junction, electron transport through the freestanding metal foil, and transmission across the opposing vacuum emission surface. A tunable high-pass energy filter is obtained, via adjustment of the tunnel bias voltage, enabling monochromatization of the electron beam. The dependence of the vacuum emission current, energy spread, reduced brightness, and virtual source size on the tunnel bias voltage are evaluated for a constant tunnel junction current of 10 nA and a foil thickness of 5 nm. Because the dimensions of the tunnel junction are comparable to the electron wavelength, diffraction plays an important role. As a result, the reduced brightness and vacuum emission current are related via the expression B=Iemission (2 me/h2). First, the source may be operated at a tunnel bias voltage for which the energy spread approaches the value for a room-temperature field-emission source (0.2 eV), with a vacuum emission current of 1 nA and a reduced brightness of 7×108 A m−2 sr−1 V−1. By careful adjustment of the tunnel bias voltage to the foil work function value it is possible, in principle, to contain 50% of the beam current within an energy spread of 100 meV at a total vacuum emission current of 0.1 nA and a reduced brightness of 7×107 A m−2 sr−1 V−1. The virtual source size in this case is approximately 1.4 nm. The energy spread may be decreased even further, down to the room-temperature thermionic limit, at the expense of vacuum emission current and, consequently, reduced brightness.