Cold atmospheric plasmas for material synthesis : particle-free thin film deposition initiated by VUV photochemistry

Deposition of high-quality, homogeneous thin films with controllable chemical structure and properties under ambient conditions is a topic of current research due to the possibility to integrate such processes in production lines without the challenge of transferring substrates into vacuum. Atmospheric plasmas are promising tools for this application. The high collision frequency at this pressure increases reaction rates but also leads to low kinetic energy of depositing species, source degradation by internal deposition, fast gas phase polymerization and ultimately to film defects like nanoparticle incorporation. This work addresses these issues by utilizing vacuum ultraviolet (VUV) radiation from a pure noble gas plasma for initiating (ionic) gas phase chemistry with deposition from different precursors.
A deposition source providing windowless separation of plasma, photons and precursor was built. Influence of photon energy could be studied as photons from helium plasma have higher energies compared to photons from argon plasma. Gas phase polymerization mainly proceeded via reactions of precursor fragment ions with neutral precursor molecules. The photons interacting with the growing film were found to influence film structure and properties. Film oxidation due to omnipresent water impurities was advantageous in case of silicon oxide deposition, where the energetic photons removed the organic fraction from the film by photodesorption, enhanced cross-linking and reduced porosity. To demonstrate the capabilities of the developed source, deposition from acetylene and silane was carried out, which very effectively form nanoparticles from negative ions in plasma. Homogeneous, particle-free films could be deposited by irradiating these precursors with photons from the atmospheric helium plasma.

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