Gallium Liquid Metal Ion Source Research Articles

Investigation of the milling characteristics of different focused-ion-beam sources assessed by three-dimensional electron diffraction from crystal lamellae.

Three-dimensional electron diffraction (3DED) from nanocrystals of biological macromolecules requires the use of very small crystals. These are typically less than 300 nm-thick in the direction of the electron beam due to the strong interaction between electrons and matter. In recent years, focused-ion-beam (FIB) milling has been used in the preparation of thin samples for 3DED. These instruments typically use a gallium liquid metal ion source. Inductively coupled plasma (ICP) sources in principle offer faster milling rates. Little work has been done to quantify the damage these sources cause to delicate biological samples at cryogenic temperatures. Here, an analysis of the effect that milling with plasma FIB (pFIB) instrumentation has on lysozyme crystals is presented. This work evaluates both argon and xenon plasmas and compares them with crystals milled with a gallium source. A milling protocol was employed that utilizes an overtilt to produce wedge-shaped lamellae with a shallow thickness gradient which yielded very thin crystalline samples. 3DED data were then acquired and standard data-processing statistics were employed to assess the quality of the diffraction data. An upper bound to the depth of the pFIB-milling damage layer of between 42.5 and 50 nm is reported, corresponding to half the thickness of the thinnest lamellae that resulted in usable diffraction data. A lower bound of between 32.5 and 40 nm is also reported, based on a literature survey of the minimum amount of diffracting material required for 3DED.

Estimation of needle height using relative parallax between two SIM images in single-column focused ion beam instrument

Gallium liquid metal ion source focused ion beam (FIB) instruments are widely used for cross-sectional sample preparation of specific areas of interest for scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Most FIB instruments are equipped with a metal needle for manipulation of the sample in the sample chamber most commonly as a micro-sampling probe for preparing TEM samples. Automatic needle manipulation has not yet been achieved for a conventional single-column FIB instrument without an SEM column because a reliable method for measuring needle height has not been established yet. This study investigated the use of scanning ion microscope (SIM) image processing to measure needle height from the sample surface in a conventional single-column FIB instrument without an SEM column. The needle height is measured using the relative parallax between two SIM images in which the needle is observed from two directions using a conventional aligner without the use of any special devices. It was demonstrated that the relative parallax was proportional to the needle height. The estimation precision was about several 10 μm, which is sufficient for automatic manipulation and safe touchdown of a needle in practical micro-sampling.

Noble gas ion beams in materials science for future applications and devices

Abstract

Liquid metal alloy ion sources—An alternative for focussed ion beam technology

Today, Focused Ion Beam (FIB) processing is nearly exclusively based on gallium Liquid Metal Ion Sources (LMIS). But, many applications in the μm- or nm range could benefit from ion species other than gallium: local ion implantation, ion beam mixing, ion beam synthesis, or Focused Ion Beam Lithography (IBL). Therefore, Liquid Metal Alloy Ion Sources (LMAIS) represent a promising alternative to expand the remarkable application fields for FIB. Especially, the IBL process shows potential advantages over, e.g., electron beam or other lithography techniques: direct, resistless, and three-dimensional patterning, enabling a simultaneous in-situ process control by cross-sectioning and inspection. Taking additionally into account that the used ion species influences significantly the physical and chemical nature of the resulting nanostructures—in particular, the electrical, optical, magnetic, and mechanic properties leading to a large potential application area which can be tuned by choosing a well suited LMAIS. Nearly half of the elements of the periodic table are recently available in the FIB technology as a result of continuous research in this area during the last forty years. Key features of a LMAIS are long life-time, high brightness, and stable ion current. Recent developments could make these sources feasible for nano patterning issues as an alternative technology more in research than in industry. The authors will review existing LMAIS, LMIS other than Ga, and binary and ternary alloys. These physical properties as well as the fabrication technology and prospective domains for modern FIB applications will similarly be reviewed. Other emerging ion sources will be also presented and their performances discussed.

High resolution and high density ion beam lithography employing HSQ resist

In the early stages of focused ion beam (FIB) development, ion beam lithography (IBL) employing organic resists showed potential advantages over electron beam lithography (EBL) (most notably less proximity effects and higher sensitivity [1,2]). However, EBL has always been more popular for various reasons (e.g., instrument capabilities, well known process technology, higher resolution, absence of potential shot noise effects, no unintended doping of the irradiated substrate area [3]). We will present and detail our results obtained with a gallium (Ga) liquid metal ion source (LMIS) nanofabrication system studying the resolution limits related to this kind of IBL. We will show that minimum feature sizes below 10nm and periodicity of 30nm are possible in a 6nm hydrogen silsesquioxane (HSQ) layer. Our experiments have revealed that the minimum reachable feature sizes are limited by a phenomenon called ''discontinued lines''[2,4]. Our results indicate that statistical dose fluctuations do not dominate this effect.

Soot and Ash Deposition Characteristics at the Catalyst-Substrate Interface and Intra-Layer Interactions in Aged Diesel Particulate Filters Illustrated using Focused Ion Beam (FIB) Milling

The accumulation of soot and lubrication-derived ash particles in a diesel particulate filter (DPF) increases exhaust flow restriction and negatively impacts engine efficiency. Previous studies have described the macroscopic phenomenon and general effects of soot and ash accumulation on filter pressure drop. In order to enhance the fundamental understanding, this study utilized a novel apparatus, that of a dual beam scanning electron microscope (SEM) and focused ion beam (FIB), to investigate microscopic details of soot and ash accumulation in the DPF. Specifically, FIB provides a minimally invasive technique to analyze the interactions between the soot, ash, catalyst/washcoat, and DPF substrate with a high degree of measurement resolution. The FIB utilizes a gallium liquid metal ion source which produces Ga+ ions of sufficient momentum to directionally mill away material from the soot, ash, and substrate layers on a nm-μm scale. As the FIB cuts into the sample, uncovering intra-layer details, the coupled high resolution SEM imaging and energy dispersive x-ray (EDX) analysis provide both morphological and chemical data. This tool was applied to investigate soot and ash accumulation in the DPF, with a specific focus on characterizing interactions between the soot/ash/DPF interfaces, such as soot penetration into the ash layer, as well as soot and ash accumulation in the DPF surface pores. In particular, ash and soot particle size, layer pore structure, and the extent of penetration or intra-layer mixing, are all parameters directly impacting DPF pressure drop, which may be quantified using this technique. The work in this study leveraged existing databases of aged DPFs containing various levels of soot and ash, originating from field trials and controlled laboratory testing. Results obtained with this technique provide a fresh and complementary perspective, as well as additional details useful to understand the macroscopic observations of the combined ash and soot effects on diesel particulate filter pressure drop.

Ion beams in SEM: An experiment towards a high brightness low energy spread electron impact gas ion source

A next generation ion source suitable for both high resolution focused ion beam milling and imaging applications is currently being developed. The new ion source relies on a method of which positively charged ions are extracted from a miniaturized gas chamber where neutral gas atoms become ionized by direct electron impact. The use of a very small gas chamber and a very narrow electron beam (<100 nm) allows for a very small ionization volume, which, in turn, yields a small virtual source size and low energy spread. The authors estimate that using a high current density electron beam from a Schottky electron gun the reduced brightness of this source can exceed that of the Gallium Liquid Metal Ion Sources and the energy spread can be well below 1 eV at an optimal gas pressure and gas chamber spacing while producing more than 1 nA of usable ion beam current. In a proof-of-concept study, the authors have produced ions of helium, argon, xenon, and air from a prototype gas chamber using an electron probe inside a scanning electron microscope. Using micro-channel plates and a phosphor screen, ion beam patterns have been acquired demonstrating that a beam of ions can be produced from a miniaturized gas chamber. The authors have measured up to several hundreds of pico-amperes of ion current in a Faraday cup using an input electron probe current of ∼14 nA with 1 keV incident energy. The authors have also verified that the ion beam current is dependent on the incident electron beam energy, gas chamber bias voltage, and the gas pressure inside the ionization chamber.

High Spatial Resolution Time-of-Flight Secondary Ion Mass Spectrometry for the Masses: A Novel Orthogonal ToF FIB-SIMS Instrument withIn SituAFM

We describe the design and performance of an orthogonal time-of-flight (TOF) secondary ion mass spectrometer that can be retrofitted to existing focused ion beam (FIB) instruments. In particular, a simple interface has been developed for FIB/SEM instruments from the manufacturer Tescan. Orthogonal extraction to the mass analyser obviates the need to pulse the primary ion beam and does not require the use of monoisotopic gallium to preserve mass resolution. The high-duty cycle and reasonable collection efficiency of the new instrument combined with the high spatial resolution of a gallium liquid metal ion source allow chemical observation of features smaller than 50 nm. We have also demonstrated the integration of a scanning probe microscope (SPM) operated as an atomic force microscope (AFM) within the FIB/SEM-SIMS chamber. This provides roughness information, and will also allow true three dimensional chemical images to be reconstructed from SIMS measurements.

The neon gas field ion source—a first characterization of neon nanomachining properties

At the Charged Particle Optics Conference (CPO7) in 2006, a novel trimer based helium gas field ion source (GFIS) was introduced for use in a new helium ion microscope (HIM), demonstrating the novel source performance attributes and unique imaging applications of the HIM (Hill et al., 2008 [1]; Livengood et al., 2008 [2]). Since that time there have been numerous enhancements to the HIM source and platform demonstrating resolution scaling into the sub 0.5nm regime (Scipioni et al., 2009 [3]; Pickard et al., 2010 [4]). At this Charged Particle Optics Conference (CPO8) we will be introducing a neon version of the trimer-GFIS co-developed by Carl Zeiss SMT and Intel Corporation. The neon source was developed as a possible supplement to the gallium liquid metal ion source (LMIS) used today in most focused ion beam (FIB) systems (Abramo et al., 1994 [5]; Young et al.,1998 [6]). The neon GFIS source has low energy spread (∼1eV) and a small virtual source size (sub-nanometer), similar to that of the helium GFIS. However neon does differ from the helium GFIS in two significant ways: neon ions have high sputtering yields (e.g. 1 Si atom per incident ion at 20keV); and have relatively shallow implant depth (e.g. 46nm in silicon at 20keV). Both of these are limiting factors for helium in many nanomachining applications. In this paper we will present both simulation and experimental results of the neon GFIS used for imaging and nanomachining applications.

Low-Energy-Spread Ion Bunches from a Trapped Atomic Gas

We present time-of-flight measurements of the longitudinal energy spread of pulsed ultracold ion beams, produced by near-threshold ionization of rubidium atoms captured in a magneto-optical atom trap. Well-defined pulsed beams have been produced with energies of only 1 eV and a root-mean-square energy spread as low as 0.02 eV, 2 orders of magnitude lower than the state-of-the-art gallium liquid-metal ion source. The low energy spread is important for focused ion beam technology because it enables milling and ion-beam-induced deposition at sub-nm length scales with many ionic species, both light and heavy. In addition, we show that the slowly moving, low-energy-spread ion bunches are ideal for studying intricate space charge effects in pulsed beams. As an example, we present a detailed study of the transition from space charge dominated dynamics to ballistic motion.

Brightness measurements of a gallium liquid metal ion source

The virtual source size of a liquid metal ion source is an order of magnitude larger than the size of the region from which the ions are emitted at the source. This source size has a direct effect on the reduced brightness and, hence, on the performance of these sources. The variation of the virtual source size of a gallium liquid metal ion source as a function of the angular current density at the source has been measured. This was done by measuring the source image size from images of a pencil lead sample taken with an FEI focused ion beam system. The measurements indicate that the virtual source size grows from about 50–80nm when the emission current increases from 1to10μA. The experimental data on the virtual source size are compared with the theory on stochastic Coulomb interactions in the source region. On the basis of these measurements the authors show that the reduced brightness deteriorates with an increasing angular current density. The maximum reduced brightness measured was 1×106A∕(m2srV).

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.

Production of a helium beam in a focused ion beam machine using an electron beam ion trap

Gallium liquid-metal ion sources that have been introduced in the late 1970s have allowed the development of a new class of micro- and nanofabrication tools collectively denominated as focused ion beam (FIB) machines. To investigate the potential of a helium beam in such a FIB instrument the authors have tested a room-temperature electron beam ion trap coupled with a high resolution FIB machine. In this letter they present their first results in target imaging using a helium beam with a resolution that allows to account for a beam diameter in the submicrometer range.

Development of an instrument for simultaneous detection of positive and negative scanning ion images

We have developed an instrument that can simultaneously detect positive and negative scanning ion images of the same location on the same sample. It consists of a primary ion irradiating column, sample chamber, detection system for secondary ions and data processing and imaging system. The scanning ion probe uses a gallium liquid metal ion source with a sealed-cone-type reservoir. Positive and negative secondary ions sputtered from the same point on a sample by the scanned gallium ion beam are simultaneously extracted and converted into electrons at two separate ion–electron converters. The electrons are then accelerated toward scintillators to generate light signals, which are then detected by means of photomultipliers. The output signals of the negative and positive ion detector are then simultaneously processed by the data acquisition system in synchronization with the primary ion scanning signal.

Direct current and pulsed operation of contaminated liquid metal ion sources

Gallium liquid metal ion sources (LMISs) have been exposed to common gas and liquid phase laboratory and vacuum system contaminants. Minor changes in the direct current and pulsed operation of the LMISs were observed after contaminant exposure. Time-of-flight mass analysis of the ion emission revealed that contaminant species are primarily field evaporated with the gallium substrate. Low vapor pressure contaminants have been observed to constitute a significant portion of the total emitted ion current.

Applications of focused ion beam microscopy to materials science specimens

Focused ion beam (FIB) systems based on high-brightness gallium liquid-metal ion sources became commercially available in the late 1980s, although even today such instruments are relatively rare outside the somewhat enclosed world of semiconductor manufacturing. The use of FIB systems as precision sectioning tools down to a submicron scale and their ability to deposit metals and insulators on a micron scale is well documented. Recently, FIB systems have achieved spatial resolution rivaling that of the standard scanning electron microscope, giving them respectable capability as an imaging tool in addition to their sectioning and deposition capabilities. This improved resolution and novel FIB contrast mechanisms combined with the capability of FIB to produce in-situ stress-free bulk cross-sections and precision cross-sectional transmission electron microscopy specimens has recently attracted great interest among materials scientists. Examples of the use of FIB in materials science, both as a specimen preparation tool and as a microscope in its own right, are illustrated.

A time-of-flight spectrometer for investigations on liquid metal ion sources

A time-of-flight spectrometer with differential pulse sweeping was developed for the mass and energy analysis of ions emitted from liquid metal ion sources. The system was tested with a gallium liquid metal ion source. The angular intensity of the total source current, the mass spectra and the energy spread for , and as function of source current have been measured. A mass and energy resolution of about 700 has been obtained independent of the ion mass.

Minimum emission current of liquid metal ion sources

The inability of liquid metal ion sources (LMIS) to operate at low dc emission currents limits their performance. We briefly describe a model that explains why LMIS have a minimum dc emission current (Imin) and also predicts Imin as a function of the temperature and the properties of the liquid metal. The model predicts that Imin=217+0.744T(nA) for gallium LMIS, where T is the temperature (K). Measurements of Imin for gallium LMIS between 30 and 890 °C are in reasonable agreement with the model. A better fit to this data, however, is given by Imin=1187 exp(−0.026/kT)(nA) where k is Boltzmann’s constant (eV/K). Below 30 °C, Imin drops precipitously—values as low as 380 nA have been measured at temperatures as low as 25.8 °C. This drop is attributed to a supercooling effect that is not accounted for in the model. Imin is also calculated for 17 pure-elemental LMIS at their melting points, and found to vary from 10 nA for mercury to 1.0 μA for aluminum. Imin is measured to be much lower for bismuth LMIS than for gallium LMIS, as predicted by the model, although difficulties with the bismuth LMIS have allowed only an upper limit of Imin⩽77 nA to be measured. The model also suggests possibilities for improving ion sources by reducing or eliminating Imin.

Layout of a nuclear microprobe beamline using a liquid metal ion source

The Bochum solenoid lens microprobe will be installed in a new configuration which can be fed both by the 4 MV Dynamitron tandem accelerator and by a new 500 kV accelerator of extremely low energy-spread ( ΔE E ⋍ 10 −5 ). Apart from a conventional duoplasmatron ion source, a commercial gallium liquid metal ion source (LMIS) will be implemented in this accelerator. Microprobe optics will benefit from the high brightness of the LMIS (∼ 10 5 Am −2sr −1eV −1), thus enabling an increased lateral resolution. Ion optical ray tracing, simulating the accelerator tube and the solenoid lens, has allowed one to specify the beam parameters required at the accelerator entrance to yield a micrometer focus at the target position. Using this information and further simulations, the accelerator injection optics can be particularly optimized for the new microbeam line.

Energy spread in liquid metal ion sources at low currents

In previous work, attempts to lower the energy spread in liquid metal ion sources (LMIS) by resorting to low current operation have rarely been successful. We have found the energy spread of gallium LMIS to be ≊4.5 eV full width at half-maximum for emission currents from 0.45 μA down to 13 nA. Our experimental results suggest that this occurs because emission <0.45 μA is formed by pulses of height 0.45 μA and an appropriate duty cycle. Several observations support this hypothesis: (1) For sources that had a minimum current, below which they could not be made to operate, the minimum current was always 0.45 μA at room temperature, regardless of apex radius or surface roughness. (2) Emission ≤0.45 μA from blunt sources at room temperature always showed pulses of height 0.45 μA, and a clear transition to dc at 0.45 μA. The pulse height and transition current rose with source temperature. (3) Emission ≤0.45 μA from sharp sources always showed pulsing from the lowest currents up to the current at which the amplifier bandwidth of 1 MHz was exceeded. (4) The angular intensity of emission on axis was in constant proportion to emission current up to 0.45 μA, and in declining proportion at higher currents.