Variable gaseous ion beams from plasmas driven by electromagnetic waves for nano-micro structuring: a tutorial and an overview of recent works and future prospects

Abstract

A compact microwave plasma has been employed as an ion source for focused ion beam applications, that can provide non-toxic ions and facilitate rapid processing of materials without introducing any metallic contamination. A variety of microstructures with high aspect ratio (line width/depth) (∼100–1000) relevant to the energy and current regimes, are created on copper thin films using 26 keV Ne, Ar and Kr ion beams. A mathematical formulation is developed to calculate the impact of the ion beams, which act as energetic projectiles falling onto the target sample, by defining a new parameter called ‘current normalized force’ which is the total momentum transferred per unit time, normalized with the beam current. Capillary guiding of the plasma ion beams has demonstrated beam self-focusing which can be employed to further reduce the beam source size (plasma electrode aperture) for demagnification. Particle-in-cell (PIC) simulations are performed to interpret the experimental results of self-focusing. Hysteresis in beam current with extraction voltage (ion energy) is observed and the hysteresis area is used to calculate the dissipated charge from the beam during capillary transmission. The effect of plasma and beam parameters on focal dimensions has been investigated, and a unique feature of enhanced nonlinear demagnification is observed when the aperture size of the plasma electrode is reduced to below the Debye length. Submicron focusing of plasma ion beams is observed by minimizing the space charge effects and reducing the plasma electrode aperture (source size).