Paper PS-ThA2
Electron Energy Distribution Measurements in Dusty Non-thermal Plasmas

Thursday, October 25, 2018, 2:40 pm, Room 104A

Dusty plasmas are a peculiar class of plasmas characterized by the presence of charged solid particles. Understanding the properties of these environments, ever-present in laboratory discharges, represent a crucial requirement for the engineering and optimization of several plasma-based processes employed in industrial manufacturing, such as thin film fabrication and etching. Langmuir probe measurements represent a well-established method used for the investigation of the properties of plasma discharges, such as the electron density, the ion density, the electron temperature and the electron energy distribution function (EEDF). In dust-rich plasmas, however, the application of the Langmuir probe method is quite challenging as the dust particles quickly form an insulating film on the probe surface which may hinder a reliable measurement. In this contribution, Langmuir probe measurements are performed in an inductively coupled RF Ar-H₂ primary plasma which is dosed with conductive nanoparticles produced in a secondary RF plasma reactor. To avoid the formation of an insulating coating, graphitic carbon nanoparticles, obtained in the secondary reactor from the dissociation of C₂H₂ in an Ar-H₂ plasma, are used for this study. The conductive graphitic nanoparticle coating formed on the probe tip does not negatively impact EEDF measurements in a pristine Ar-H₂ plasma, allowing a more forgiving environment in which to study the effect of dust on plasma properties. The EEDF is obtained through the Druyvesteyn method, via the second-derivative of I-V probe characteristics. Electron densities and temperatures are obtained from the EEDF measurements, while ion densities are calculated from the I-V characteristics. The role of process parameters such the nanoparticle density and the primary plasma input power is carefully mapped. The nanoparticle density is measured through the mass injection rate into the primary reactor, allowing for the particle charge to be measured across the parameters. In the dust-free pristine Ar-H₂ plasma, a transition in the primary ion is observed as a function of the applied RF power: H₃⁺ appears to dominate at low powers, transitioning to Ar⁺ at higher values. In dusty environments, the measured plasma power is much lower than in pristine, prompting H₃⁺ as a likely choice for the primary ion in ion density calculations. As expected from theory and previous literature, nanoparticles act as electron sinks, reducing the electron density inside the plasma volume, resulting in an increased electron temperature to maintain ionization events; contrary to theory, however, the electron temperature increases with increasing input plasma power.