B. Knuffman

Cold atomic beam ion source for focused ion beam applications

We report measurements and modeling of an ion source that is based on ionization of a laser-cooled atomic beam. We show a high brightness and a low energy spread, suitable for use in next-generation, high-resolution focused ion beam systems. Our measurements of total ion current as a function of ionization conditions support an analytical model that also predicts the cross-sectional current density and spatial distribution of ions created in the source. The model predicts a peak brightness of 2 × 107 A m−2 sr−1 eV−1 and an energy spread less than 0.34 eV. The model is also combined with Monte-Carlo simulations of the inter-ion Coulomb forces to show that the source can be operated at several picoamperes with a brightness above 1 × 107 A m−2 sr−1 eV−1. We estimate that when combined with a conventional ion focusing column, an ion source with these properties could focus a 1 pA beam into a spot smaller than 1 nm. A total current greater than 5 nA was measured in a lower-brightness configuration of the ion s...

Focused chromium ion beam

With the goal of expanding the capabilities of focused ion beam microscopy and milling systems, the authors have demonstrated nanoscale focusing of chromium ions produced in a magneto-optical trap ion source. Neutral chromium atoms are captured into a magneto-optical trap and cooled to 100 μK with laser light at 425 nm. The atoms are subsequently photoionized and accelerated to energies between 0.5 and 3 keV. The accelerated ion beam is scanned with a dipolar deflector and focused onto a sample by an einzel lens. Secondary electron images are collected and analyzed, and from these, a beam diameter is inferred. The result is a focused probe with a 1 standard-deviation radius as small as 205±10 nm. While this probe size is in the useful range for nanoscale applications, it is almost three times larger than is predicted by ray-tracing simulations. Possible explanations for this discrepancy are discussed.

Bright focused ion beam sources based on laser-cooled atoms

Nanoscale focused ion beams (FIBs) represent one of the most useful tools in nanotechnology, enabling nanofabrication via milling and gas-assisted deposition, microscopy and microanalysis, and selective, spatially resolved doping of materials. Recently, a new type of FIB source has emerged, which uses ionization of laser cooled neutral atoms to produce the ion beam. The extremely cold temperatures attainable with laser cooling (in the range of 100 μK or below) result in a beam of ions with a very small transverse velocity distribution. This corresponds to a source with extremely high brightness that rivals or may even exceed the brightness of the industry standard Ga+ liquid metal ion source. In this review we discuss the context of ion beam technology in which these new ion sources can play a role, their principles of operation, and some examples of recent demonstrations. The field is relatively new, so only a few applications have been demonstrated, most notably low energy ion microscopy with Li ions. Nevertheless, a number of promising new approaches have been proposed and/or demonstrated, suggesting that a rapid evolution of this type of source is likely in the near future.

Inter-ion coulomb interactions in a magneto-optical trap ion source

We have investigated the role played by inter-ion Coulomb interactions in a magneto-optical trap ion source (MOTIS). Using a Monte Carlo simulation accounting for all pair-wise ion-ion Coulomb interactions in the source, we have calculated the broadening of the transverse spatial and velocity distributions as well as the increase in emittance over a range of beam currents and extraction electric fields. Using a 7Li MOTIS, we have experimentally studied the broadening of the spatial distribution as a function of total beam current and extraction electric field by measuring the fraction of the beam current that passes through a 20 μm diameter aperture. The Monte Carlo simulations agree well with the experimental results, indicating that such simulations capture the essential physics of the source. Our results show that while Coulomb interactions can cause a significant increase in emittance in some situations, it is possible to keep the effects to an acceptable level by suitable choice of extraction field a...

High-brightness Cs focused ion beam from a cold-atomic-beam ion source

We present measurements of focal spot size and brightness in a focused ion beam system utilizing a laser-cooled atomic beam source of Cs ions. Spot sizes as small as (2.1 ± 0.2) nm (one standard deviation) and reduced brightness values as high as (2.4 ± 0.1) × 107 A m−2 Sr−1 eV−1 are observed with a 10 keV beam. This measured brightness is over 24 times higher than the highest brightness observed in a Ga liquid metal ion source. The behavior of brightness as a function of beam current and the dependence of effective source temperature on ionization energy are examined. The performance is seen to be consistent with earlier predictions. Demonstration of this source with very high brightness, producing a heavy ionic species such as Cs+, promises to allow significant improvements in resolution and throughput for such applications as next-generation circuit edit and nanoscale secondary ion mass spectrometry.

MOTIS: A Focused Ion Beam Source Based On Laser‐Cooled Atoms

We have demonstrated high resolution focused ion beams based on a magneto‐optical trap ion source (MOTIS), which takes advantage of the ultra cold temperatures of laser cooled atoms to produce high brightness, low emittance ion beams. We have created focused beams of both Cr+ and Li+ and present secondary electron micrographs obtained with these beams, demonstrating a focal spot size as low as 27 nm at a beam energy of 2 keV. This work shows that the MOTIS can be a useful source for focused ion beams that will open new opportunities for applications in materials characterization and metrology.

Atoms and plasmas in a high-magnetic-field trap

We investigate cold rubidium plasmas in a particle trap that has the unique capability to simultaneously laser‐cool and trap neutral atoms as well as to confine plasmas in magnetic fields of about three Tesla. The atom trap is a high‐field Ioffe‐Pritchard laser trap, while the plasma trap is a Ioffe‐Penning trap that traps electrons and ions in separate wells. The observed plasma dynamics is characterized by a breathing‐mode oscillation of the positive (ionic) plasma component, which feeds back on the behavior of the negative (electron) component of the plasma. At higher densities, the observed oscillations become nonlinear. The electron component has been found to undergo rapid cooling. We further report on the recombination of magnetized plasmas into Rydberg atoms in transient traps and quasi‐steady‐state traps. In transient traps, large numbers of recombined Rydberg atoms in high‐lying states are observed. In quasi‐steady‐state traps, the measured numbers of recombined atoms are lower and the binding e...

Trapping Rydberg atoms in optical lattices

We study Rydberg atoms in ponderomotive optical lattices. Unlike for ground-state atoms, for Rydberg atoms in an optical lattice the extent of the electronic wave-function can approach the lattice period. This leads to state-dependent adiabatic trapping potentials that are unique to Rydberg atoms. We first discuss a theoretical model of adiabatic lattice potentials of Rydberg atoms. Then, we use microwave spectroscopy to experimentally demonstrate and investigate the state-dependence of the adiabatic potentials of S1/2 Rydberg states of rubidium. The observed microwave spectra depend strongly on both the principal quantum number and the depth of the lattice. A semi-classical simulation is used to explain the features seen in the spectra. Based on the results, we estimate the trapping efficiency of the ponderomotive optical lattice.

Coherent population transfer of ground state atoms into Rydberg states

We demonstrate experimentally and theoretically an efficiency of /spl sime/70%for excitation from the /sup 85/Rb 5S ground state to the 44D Rydberg state, using the technique of stimulated Raman adiabatic passage (STIRAP).