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

TLDR

The source’s angular intensity is roughly 1000 times higher than a liquid‑metal ion source, enabling high‑current beams where spherical aberration limits LMIS‑FIB performance. The study aims to enhance the source’s brightness and energy spread so that its figure‑of‑merit approaches that of a liquid‑metal ion source. The authors characterized the virtual source size, angular intensity, mass spectra, and energy spread using argon and xenon plasmas. The magnetically enhanced inductively coupled plasma source achieves reduced brightness of 5.4×10³–9.1×10³ Am⁻²sr⁻¹V⁻¹, energy spreads of 7–10 eV, sub‑25‑nm spots at 30 keV, sub‑100‑nm imaging resolution, and current densities surpassing LMIS‑FIB above 50 nA, with excellent lifetime and stability.

Abstract

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.