(Invited) Role of Trace Impurities in Microwave-Excited Atmospheric Pressure Plasmas: Application to 3D Nano-Printing
Authors: Belmonte Thierry , Noël Cédric , Kabbara Hiba 
Published: November 2022

Because rare gases like helium, neon or argon have first-excited levels at high energy (respectively 19.8, 16,62 and 11.5 eV), impurities with much lower energy levels are more easily excited. Thus, if their concentrations are high enough (hundreds of ppm or more), these impurities drive the behavior of the discharge, leading for instance to emission spectra where no lines of rare gas are visible.
Modelling these discharges is a challenge because of the complexity of the chemical behavior of these trace elements. In microwave discharges, several problems arise among which one finds the role of electron-electron collisions, super-elastic collisions and the possible non-local heating of the electron energy distribution function. Based on a collisional–radiative model proposed to describe the behavior of a helium plasma sustained in a resonant cavity at atmospheric pressure, we will discussed these different aspects. The role of impurities will be described next. The level of dry air used as impurity typically varies from 0 to 1500 ppm, which corresponds to the most commonly encountered range in atmospheric pressure discharge experiments. Results clearly show that the plasma chemistry and consequently the discharge evolution is highly affected by the concentration level of impurities (from tens of thousands of ppm) in the mixture.
Next, the use of these plasmas for 3D-nanoprinting will be detailed. This technology, named DISCRIBE, might be used for drawing electrical contacts, building pillars as part of metasurfaces or even creating 3D parts of clocks in watchmaking industry. A plasma-assisted printer, operated at atmospheric pressure, was developed to write patterns with sub-micrometric resolution. A reactive gas is injected through a needle within an atmospheric argon microwave plasma. Forming sub-micrometric needles (down to 100 nm in diameter) by pulling glass capillaries heated locally by laser is simple as this technology is readily available in the medical field. By plasma-enhanced chemical vapor deposition, a coating is formed on a substrate (conveniently a silicon wafer, but any other substrate can be used) with a spot size that almost corresponds to the needle aperture. Then, when the substrate moves, a pattern is formed accordingly. The writing speed is dependent on the dynamic deposition rate, which can be as high as 2 nm·m·s-1.
In the specific case of acetylene used as precursor, the shape of the hydrogenated-carbon spot is strongly dependent on the amount of oxygen-containing impurities present in the microwave argon plasma. The influence of the plasma composition and the local hydrodynamic flow pattern are essential to control the deposition conditions and these aspects will be discussed. In particular, the distance between the needle exit and the substrate turns out to be a key parameter in the deposition process. DISCRIBE offers the advantage of all PECVD processes: it allows deposition at atmospheric pressure and reduced temperature of multi-materials (semiconductors, metals, ceramics, plasma polymers) ony any kind of substrate.