Time-resolved measurements of the E-to-H mode transition in electronegative pulse-modulated inductively coupled plasmas

Abstract

Time-resolved measurements of electronegative pulse-modulated inductively coupled plasmas (ICPs) were carried out using various measurement techniques. In order to explain the experimentally observed results, it is proposed that the structure of an Ar∕CF4 plasma during an afterglow dynamically changes, passing through three stages when the period of the afterglow is long enough: (1) The first stage is the initial afterglow where the electron temperature suddenly decreases due to inelastic collision with CF4. Electron density decreases and the density of fluorine negative ions increases by electron attachment, but the sheath potential still exists and the negative ions are confined in the bulk plasma region. Since charge neutrality should be maintained, the density of positive ions is almost constant during this stage. (2) The second stage is the intermediate afterglow where the plasma consists mainly of negative and positive ions but sheath potentials remain, reducing the negative ion flux from the plasma. The sheath potential gradually disappears and the densities of all the charged particles (electrons and positive and negative ions) decrease because of increased loss to the wall surface. (3) The third stage is where the sheath structure has disappeared completely, and the plasma consists of mainly positive and negative ions and losses are dominated by ambipolar diffusion. The presence of these stages during the afterglow is very important in understanding the behavior of pulsed ICPs with E-to-H mode transitions. The state of an afterglow plasma at the moment power is reapplied determines the discharge characteristics of the pulsed ICPs (e.g., the appearance of an E mode, the duration of the E mode, stability of the plasma).