Ion drag effects in inductively coupled plasmas for etching

Abstract

Ion drag refers to Coulomb momentum transfer collisions between a directed flux of charged particles and a second charged species. It is an important force in determining the motion of negatively charged dust particles in plasma processing reactors. The same ion drag mechanism acts between the directed flux of positive ions moving towards the boundaries of a plasma etching reactor and negative ions being accelerated by electrostatic forces towards the center of the plasma. In this letter, we discuss the parameter space for inductively coupled plasma etching reactors in which ion drag forces on negative ions influence their transport using results from a two-dimensional plasma equipment model. We find that ion drag forces on negative ions are important at high plasma densities and low ion temperatures. Under these conditions, the large positive ion flux, coupled with a large Coulomb cross section, creates an ion drag force which may dominate the electrostatic forces on negative ions. The end result is that negative ions may accumulate near the sheath edge.