Abstract
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 source, demonstrating the possibility of a high current mode of operation.
Highlights
Focused ion beams (FIBs) are used for a variety of nanomachining, sample preparation, and sample analysis tasks
The gallium liquid metal ion source (LMIS) is the most commonly used because of its simplicity and robustness, gallium ions are not ideal for a number applications because of their tendency to contaminate or otherwise destroy the sample. Another source that performs very well in imaging applications is the gas field ion source (GFIS),6 which produces a few-picoampere helium ion beam that can be focused to a sub-nanometer probe size
Providing a steady flux of neutral atoms avoids the diffusion limit, allowing much higher ion currents, while laser cooling in the transverse direction provides the cold temperatures necessary for a high brightness, as long as the atom beam axis is well-aligned with the ion beam axis
Summary
Focused ion beams (FIBs) are used for a variety of nanomachining, sample preparation, and sample analysis tasks. The gallium LMIS is the most commonly used because of its simplicity and robustness, gallium ions are not ideal for a number applications because of their tendency to contaminate or otherwise destroy the sample Another source that performs very well in imaging applications is the gas field ion source (GFIS), which produces a few-picoampere helium ion beam that can be focused to a sub-nanometer probe size. In a laser-cooled photoionization source, the high brightness is achieved through the extraordinarily cold, microkelvinrange temperatures attainable through laser cooling. Providing a steady flux of neutral atoms avoids the diffusion limit, allowing much higher ion currents, while laser cooling in the transverse direction provides the cold temperatures necessary for a high brightness, as long as the atom beam axis is well-aligned with the ion beam axis. It may offer additional analytical capabilities by improving the performance of site-specific SIMS.
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