Controlling plasma chemistry of atmospheric-pressure plasmas in helium with oxygen and water admixtures*

Many applications of atmospheric-pressure plasmas (APPs) rely on a controlled delivery of known amounts of reactive oxygen species (ROS). Careful control of the plasma chemistry of these devices is therefore desired, but often challenging because of the complex, non-linear relations between plasma input parameters and ROS outputs. Nevertheless, combining (advanced) diagnostics and numerical modelling can enhance our understanding of the plasma chemistry, allowing better control and performance in plasma applications. In this work, the focus is on Helium-based APPs with molecular admixtures of O₂ and/or H₂O. First, case studies on using APPs for changing the wettability of plastic and the removal of photoresist are presented. Atomic oxygen was deduced to be the most relevant ROS for both these processes. Optimisation of O density in the plasma device was subsequently undertaken, showing a 100-fold improvement for the photoresist removal process.

A more complex chemistry that is of interest for applications is He + H₂O, producing ROS such as OH and H₂O₂. We investigated the kinetics of OH and O in an RF atmospheric-pressure plasma by UV absorption spectroscopy (OH) and VUV Fourier-Transform Absorption Spectroscopy (O). Alongside these measurements, a 0D plasma-chemical kinetics model was used to compare the experimental results and understand the reaction pathways. These investigations showed a strong coupling between the O and OH densities with specific production and destruction pathways changing with increasing water content. In addition, at low water content, small amounts of oxygen impurities significantly change the chemistry with a sharp increase in O density, showing that adding admixtures to the feed gas rather than relying on the ambient atmosphere, enhances control of the plasma chemistry. Finally, to further improve control of the mix of ROS in an APP, we investigated plasmas with both O₂ and H₂O admixtures. An independent control of OH, through the H₂O content, and O, through the O₂ content was observed.