Generation and Characterization of Energetic Neutral Beams for Surface Modification

Ayilaran, Adetokunbo (2018). Generation and Characterization of Energetic Neutral Beams for Surface Modification. PhD thesis The Open University.



The aim of this research is to explore ways in which energetic neutral beams can be generated and characterized for modifying polymer substrates in a charge-less etching environment, without profile distortion.

A modified inductively coupled plasma source was constructed to characterize processing plasmas and generate a source of energetic neutral particles based upon the resonant gas phase charge-exchange mechanism. A combination of probe diagnostics and plasma imaging was used to study the electron temperature, electron density and electromagnetic structure in the capacitive and inductive modes of operation. Measured average energies in the tail of the electron energy distribution function ranged between 7 and 9 eV in a pressure range of 1 - 100 mTorr, indicating sufficient ionization. Characteristics such as an E-H transition, electron densities of up to 1018 m-3 and plasma uniformity were measured to show that the plasma source was ideal for plasma processing.

An electrostatic quadrupole mass/energy analyzer was used to determine the energy of charge-exchanged neutral particles in the range of 10 - 70 eV. It was shown that the charge-exchanged neutral energy distribution function mirrored the energy of the parent ions. Validation of these results involved the use of electron emission current modulation and the dissociative electron attachment mechanism on fast, energetic O2. It was shown that the O anion fragment carried away approximately half the energy of the fast O2 molecule. Ion flux probe measurements determined a gas phase charge-exchange neutralization efficiency of 36% along a neutralization path corresponding to half the mean free path for charge-exchange collisions.

Finally, an inductively coupled plasma etcher tool was used to ignite plasma with tailored parameters as those studied within the modified source.

Ions were extracted and neutralized via surface or gas phase charge-exchange. The neutral beam etching of polymers with a reactive mixture of O2/SF6 produced initial etch rates of up to 30 nm min-1 in a single extractor arrangement and up to 14 nm min-1 using a wafer neutralizer. Atomic force microscopy and x-ray photoelectron spectroscopy was used to measure the physical and chemical profiles of the substrates post etch. It was determined that the presence of vacuum ultraviolet photons assisted the neutral beam etching mechanism and improved the etch rate of a fluorocarbon polymer from 3.5 nm min-1 up to 4.5 nm min-1. In the case of a hydrocarbon polymer, photo-chemistry hardened the surface which suppressed the etch rate. Roughness profiles indicated smooth features in the presence of reactive chemical etching whilst profiles were rougher when the main etching mechanism was mechanical. The smoothness of the substrate surfaces post etch was comparable to polished silicon, indicating neutral beam etching as a suitable technique for surface polishing and the patterning of smooth profiles.

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