Actions of low- and high-energy electrons on the phase transition between E- and H-modes in an inductively coupled plasma in Ar

Abstract

It is known that an inductively coupled plasma (ICP) sustained by a radiofrequency current coil has a mode transition and hysteresis characteristics of the internal plasma parameter as a function of the external plasma parameter. We focus on the contributions of low- and high-energy electrons to the phase transition between the capacitive E-mode and the inductive H-mode in an ICP. Our analysis is based on the diagnostics for a time-resolved two-dimensional net excitation rate of short-lived excited atoms, mainly produced collisionally by low- and high-energy electrons by using an intensified charge coupled device optical image. Short-lived excited atoms Ar(2p1) and Ar(2p9) with different excitation processes have been employed as optical probes in an axisymmetric configuration of the ICP chamber, driven at 13.56 MHz by an external single-turn current coil at 300 mTorr in pure Ar. The E-to-H transition is characterized by two time constants of electrons: establishment of an axisymmetric distribution by electron diffusion and the accumulation of symmetric high-density electrons in order to sustain the inductive discharge under a weak electromagnetic field. On the other hand, the H-to-E transition is strongly influenced by the presence of a long-lived excited atom (i.e. metastables).