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Characterization of the Energetic Impact of Electrosprayed Nanodroplets on Inert Materials


The energetic bombardment of covalently bonded materials by electrosprayed nanodroplets causes sputtering, topographic changes and amorphization of the target's surface. The goal of this thesis is to investigate these phenomena using a variety of semiconductor materials and dielectric liquids. The electrosprays are characterized via time-of-flight spectrometry to determine the nanodroplet charge-to-mass ratio which, together with the acceleration voltage, yield the impact velocity, stagnation pressure, and kinetic energy of the projectiles. The damage caused by the beams on the surfaces of single-crystal targets such as Si, SiC, InAs, InP, Ge, GaAs, GaSb and GaN is characterized with different tools including a mechanical profilometer, an atomic force microscope, and scanning and transmission electron microscopy, measuring the sputtering yield (ejected atoms per projectile's molecule), sputtering rate, surface roughness, and the morphology of the surface affected by the beam. These figures of merit are quantified in terms of the projectile's size, molecular mass and kinetic energy. Nanodroplets are efficient sputtering projectiles, with maximum sputtering yields of 11.5 and 25.1 for the technological important but difficult to etch SiC and GaN. The maximum sputtering rates for SiC, GaN and GaSb are 720, 1750 and 2380nm/min. The surface roughness and sputtering yields typically increase with the projectile's kinetic energy for all targets, and exhibit sharp maxima for nanodroplets with high molecular mass. The very different sputtering by droplets that are macroscopically similar, and the strong dependence of the impact phenomenology on molecular mass, indicate that nanodroplet sputtering is intrinsically a molecular scale phenomenon, dominated by the transfer of energy under non equilibrium conditions, and hence not amenable to modeling with a continuum formulation. For the case of single-crystal silicon the influence of the projectile's velocity on amorphization is studied. The impacts of nanodroplets on a single-crystal silicon wafer at kinetic energies exceeding a threshold amorphatize a thin superficial layer, of thickness comparable to the droplet diameter. This thesis contributes to develop a new generation of energetic projectile sources in a previously unavailable particle size range, producing beams which can be electrostatically focused into submicrometric spots, similar to focused ion beams, or multiplexed for broad-beam batch fabrication.

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