Invited Paper AS-MoA1
Status and Prospects for ICP Focused Ion Beams

Monday, October 28, 2013, 2:00 pm, Room 204

Milling speeds with a gallium focused ion beam (FIB) are often much too slow for many sample preparation and surface engineering applications. For example, cross-sectioning stacked-die semiconductor devices, prototyping micro-mechanical structures and delayering IC’s for circuit mapping are growing applications that require a milling rate that far exceeds that provided by the gallium FIB.

In the more general area of direct-write surface engineering, milling with nanometer precision is limited to volumes of <10⁴um³ when using gallium FIB systems. Also, engineered devices must generally be tolerant of high gallium concentrations being implanted in the near-surface region. These are major restrictions when fabricating micromechanical devices.

Furthermore, elemental secondary ion mass spectrometric imaging (SIMS-Imaging) has been limited to a lateral resolution of 200nm when using an oxygen focused ion beam for high sensitivity surface analysis. Many areas of material science could benefit from an ability to image trace level surface chemistry with <20nm resolution. Example applications include, sub-cellular imaging of trace metals in the brain for neurodegenerative disease studies, analysis of trace element segregation in metal alloys and studying isotope distributions in meteorites.

Here, we review inductively coupled plasma ion source technology that can provide a focused ion beam capable of milling silicon at a rate of >5000um³/s with <4um milling resolution and <25nm imaging resolution with 30keV xenon ions. The latest generation ICP-FIB, readily operates as a high brightness source of not only any inert ion, but also positively and negatively ionized oxygen for trace element SIMS imaging.

By transferring energy to plasma electrons via a radio frequency induction field, it is possible to create a plasma state without a cathodic electrode. This method of plasma creation can create energy normalized beam brightness values that now exceed 1x10⁴ Am^-2sr^-1V^-1. This high brightness can be attained with long lifetimes (>>2000 hours), stable beam current (<±0.5% drift per 30 minutes) and an axial energy spread for the extracted ion beam of 5-6eV and for a broad array of ion species.

At Oregon Physics, we have developed the Hyperion^TM inductively coupled plasma ion source that is already capable of generating smaller probe diameters (Xe⁺) than the liquid metal ion source (LMIS, Ga⁺) FIB at beam currents in excess of 20nA. When operated with oxygen, imaging resolution, source lifetime and current stability are significantly higher than provided by a duoplasmatron.

This paper presents FIB and SIMS data from this new ion source technology, to exemplify the impact on surface science. The operating principles of the inductively coupled plasma source, the properties of the ion beam(s) being created and the projected future for this technology are also described.