Electron power absorption in CF₄ capacitively coupled RF plasmas operated in the striation mode

Abstract

The electron power absorption mechanisms in electronegative capacitively coupled plasmas in CF4 are investigated using PIC/MCC simulations at a pressure of p=60 Pa, a driving frequency of f= 13.56 MHz for voltage amplitudes in the interval of Φ0=100...300 V, where pronounced self-organized density variations, i.e., striations, develop. The calculations are based on the Boltzmann Term Analysis, a computational diagnostic method capable of providing a complete spatio-temporal description of electron power absorption. The discharge undergoes an electron power absorption mode transition from the drift-ambipolar- to the striation-mode at Φ0=180 V. Although Ohmic power absorption is found to be the dominant electron power absorption mechanism in the parameter range considered, the electron power absorption mode transition can be inferred from the behaviour of the spatio-temporally averaged ambipolar power absorption as a function of the voltage amplitude. Furthermore, it is shown, that as a consequence of the presence of striations, the temporal modulation of the electron density leads to a temporal modulation of the ambipolar electric field, which is responsible for the striated structures of various physical quantities related to electrons, such as the electron temperature and the ionization source function.