Modeling of ionization of argon in an analytical capacitively coupled radio-frequency glow discharge

Abstract

A hybrid Monte Carlo-fluid model has been developed for the description of electrons, argon ions, and fast argon atoms in a capacitively coupled radio-frequency (rf) glow discharge used in analytical spectroscopy. Typical operating conditions are about 6 Torr pressure and 10 W electrical power. The discharge cell is rather small and is characterized by a much smaller rf-powered electrode than grounded electrode, which yields a high dc bias voltage. The electron density at these conditions is in the order of 1013 cm−3. The computation time to simulate all these electrons with a Monte Carlo or a particle-in-cell method was found to be too long. Therefore, the electrons are subdivided in two groups. The fast electrons emitted from the rf electrode, as well as the ones formed by ionization with sufficiently high total (=kinetic+potential) energy for further ionization, give rise to so-called γ ionization; these are described with a Monte Carlo method. The slow electrons, which can, however, be heated again by the fluctuating electric field, give rise to so-called α ionization; they are described with a fluid approach, which also treats the argon ions. Moreover, the fast argon ions and atoms are treated with a Monte Carlo model in the rf sheath. Typical results of this model include the electrical characteristics (i.e., dc bias and rf amplitude voltages, electrical current, potential, and electric field distributions), the electron densities and mean energies, the ionization rates due to the electron impact α and γ ionization and fast argon ion and atom impact ionization, and the relative contributions of these ionization mechanisms to the overall ionization.