Momentum transfer using variable gaseous plasma ion beams and creation of high aspect ratio microstructures

Intense gaseous ion beams are created from compact microwave plasmas confined in a multicusp magnetic field. The wave frequency (ω) is comparable to the electron plasma frequency (ω_pe) and ⪢ the ion plasma frequency (ω_pi); therefore, the heavier plasma (ions) are least disturbed by the high frequency electromagnetic waves. By changing the experimental gas, ion beams of different species are obtained, which expands the applicability of the ion beams. For the same applied accelerating potential, the controllability of the beam current owing to different velocities for different ionic species adds to the enhanced functionality. The ion beams are utilized to create a variety of microstructures by direct writing on metallic substrates, and microstructures of a high aspect ratio (a_r = line width/depth) in the range of 100–1000 are created by varying the ion species and writing speed. For fixed species (Ga) and low current (1 pA) focused ion beam systems, typically a_r ∼ 2.0 to 9.3 may be realized in a single beam scan. A parameter called current normalized force, defined as the momentum transfer per unit time, normalized with the beam current helps in understanding the different momentum transferred to the target sample upon impact by the ion beams of variable species. A mathematical formulation is developed to demonstrate this aspect.