L. W. Swanson

Fundamental limits to imaging resolution for focused ion beams

This article investigates the limitations on the formation of focused ion beam images from secondary electrons. We use the notion of the information content of an image to account for the effects of resolution, contrast, and signal‐to‐noise ratio and show that there is a competition between the rate at which small features are sputtered away by the primary beam and the rate of collection of secondary electrons. We find that for small features, sputtering is the limit to imaging resolution, and that for extended small features (e.g., layered structures), rearrangement, redeposition, and differential sputtering rates may limit the resolution in some cases.

Scanning tunneling microscope liquid‐metal ion source for microfabrication

The use of a liquid‐metal ion source (LMIS) in a scanning tunneling microscope (STM) embodiment is described. Operation of the LMIS at emitter/target spacings of the order of 100 nm or less is discussed as are also attempts to create microfabrication features on GaAs and Si substrates. The dependence of the threshold voltage for LMIS operation on spacing follows a square root relationship as would be expected from the Taylor theory. Very stable operation of the LMIS was realized at spacings corresponding to threshold voltages <200 V. The use of a proximity focused LMIS allows an exceedingly high current density to be achieved. For example, at 0.1 μm spacing of the emitter from the target and assuming an emission half‐angle of 20° a target current density of ∼2300 A/cm2 can be achieved for a Ga LMIS operating at 0.1 μA total current. At closer spacings target current densities are correspondingly greater. Combining the LMIS with an STM embodiment makes possible nanometer fabrication carried out at speeds l...

A low‐current liquid metal ion source

Liquid metal ion sources (LMIS’s) of gallium operating at 300 K and at currents down to 1 nA have been investigated. The mechanism of operation of the LMIS at low currents is believed to be that of the usual Taylor cone mode. Values of the full width at half‐maximum of the energy spread, in the very low current range, 3 nA–1 μA, at 300 K did not decline below 4.5 eV as the emission current was reduced. Angular intensity measurements were also obtained in the emission current range.

Long‐lifetime, reliable liquid metal ion sources for boron, arsenic, and phosphorus

Operation of liquid–metal ion sources based on palladium alloys that contain boron, arsenic, and phosphorus (singly or in combination) was studied. These sources, when run on refractory metal needles and heater ribbons, have exhibited high angular intensity (1.5–5 μA/sr), long lifetime (>150 h), low energy spread (<15 eV), and stable operation with extracted currents down to 2 μA.

A combination electron/ion field emission source

A combination electron/ion field emission source will be described. For ion emission the source was operated as a conventional indium or gallium liquid metal ion source (LMIS), thereby giving a beam of principally In+ or Ga+ ions with the usual range of angular intensities and total emission currents. In the case of indium it was possible to ‘‘freeze in’’ the field stabilized Taylor cone formed during LMIS operation by carefully reducing the voltage to the threshold value and then rapidly reducing both the voltage and the emitter temperature. Field electron emission was then obtained by reversing the voltage polarity. The dc electron emission angular distribution generally overlapped that observed in the LMIS mode. This process, which was determined to be reversible and reproducible, will be discussed in detail.

A comparison of boron emission characteristics for liquid metal ion sources of PtB, PdB, and NiB

The relative abundance, angular intensities, and energy distribution of B+ emitted from liquid metal ion sources of Ni4B6, Pd4B6, and Pt4B6 alloys on carbon are measured as functions of total emission current. Evidence is presented that shows the NiB system to possess the highest concentration of B in the liquid phase which, in turn, leads to the highest electric field at the emitting site for this system. While the NiB system exhibits the highest angular intensity for the B+ species, it also displays the highest value of the energy spread of B+.

Off-axis emission properties for the extended Schottky electron source

The total energy distribution (TED) and reduced brightness from the ZrO∕W(100) Schottky electron source are extended to positions 4.4° off of the optical axis of the emitter. The faceted nature of the stable end form precludes a monotonic variation in emission properties with the beam angle. Both the full width at half maximum of the TED and the reduced brightness of the source increase by 37% and 18%, respectively, at an off-axis beam angle, consistent with the facet edge of the emitting (100) plane and at a constant current angular density of 0.5mA∕sr and 1800K source temperature. Similarly, the angular magnification and work function increase by 27% and 6%, respectively, as the beam angle increases from 0° to the facet edge at 4.4°.

Angular distribution and energy spread measurements of Ga clusters emitted from an isotopically pure liquid metal ion source

By using an isotopically pure liquid metal ion source (LMIS) of gallium on a tungsten needle it has proven possible to determine the full width at half‐maximum (FWHM) energy spread of Ga clusters up to Ga+4 as a function of both emission current and emission angle. A surprising result was that the FWHM of the dominant Ga+ peak remained constant with emission angle, while that of the cluster ions declined markedly. This parallels the behavior of the current associated with each Ga cluster species—the decline of each cluster ion species with angle is much faster than for either of the two monomeric species, Ga+ or Ga++, indicating a common mechanism for the formation of the latter two species. It is tempting to attribute the rapid falloff with emission angle of the cluster ions amounts and FWHM to a droplet origin for these species. Droplet emission is know to be strongly concentrated in the axial region of the LMIS.

Contrast in ion induced secondary electron images

Previous reports have discussed properties of ion induced secondary electron images (ISE) where it has been generally accepted that ISE and electron induced secondary electron images yield complementary material or elemental contrast [1,2]. In addition, it has also been reported that secondary electron (SE) emission in ISE images decreases with increasing atomic number [3,4]. In this report, anomalies to the previous work are presented. In particular, it is observed that the ISE contrast obtained from 30 keV Ga+ focused ion beam (FIB) imaging does not follow a monotonic variation as a function of atomic number, and in some cases, the contrast increases as atomic number increases.

The angular dependence of the emission characteristics for a Pd2As liquid‐alloy ion source

An extensive study of the Pd2As liquid‐alloy ion source (LAIS) as a function of angle has been carried out. The main goal of this investigation was to gain a better understanding of the mechanism of ion formation and the emission characteristics of the Pd2As LAIS. A point of interest was the calculation of the angular dependence of the As+++ and As++ intensities. These results could then be used in connection with the post‐ionization theory to predict the angular dependence of the electric field. The investigation showed that up to angles of 20°–30°, the cluster species of PdAs and the monomer Pd and As ions exhibited little variations in intensity with angle at a current of 10 μA. Beyond these angles, they showed a rapid falloff in intensity. The emission from the source was quite stable at temperatures of 1080–1160 K, lasting for around 40–60 h at a maximum current of 20 μA. The amount of As which was emitted in the form of different species ranged from 24% at 4 μA to about 26% at 20 μA. The total compo...