Unexpectedly high electron temperatures in argon:silane plasmas measured by double probe *

Nonthermal plasmas are attractive sources for nanoparticle synthesis. A double probe is utilized to measure the plasma properties of an argon:silane plasma and exhibits good reproducibility despite the chemically reactive environment. However, unexpectedly large electron temperatures, as high as 10 eV, are measured with increasing the silane mole fraction. Here, we discuss that these high electron temperatures can be explained based on the non-Maxwellian electron energy probability function (EEPF) and the fact that a double probe indicates an “apparent” electron temperature, which corresponds to the slope of the EEPF around the probe’s floating potential. We utilize a zero-dimensional global model, in which nanoparticles are modeled as quasi-negative ions with fractional negative average particle charges, to determine the reduced electric field in the plasma. First, using the plasma electronegativity as an independent parameter, the corresponding average particle charges and particle densities are found. Positive ion losses to the particles and reactor walls determine the reduced electric field, which is used to derive the EEPF for different plasma electronegativities. The apparent electron temperatures computed from this model are in good agreement with experimental results.