Ion flux, ion energy distribution and neutral density in an inductively coupled argon discharge

Abstract

The dependence of ion flux, ion energy distribution and neutral density of a planar radiofrequency (RF) driven inductively coupled plasma source on pressure and power is analysed using a plasma monitor and a Faraday cup. The ion flux is about 7 mA cm-2 at 5 Pa and 300 W and increases as RF power and argon pressure increase. The ion energy distribution consists of a single peak with a full width at half maximum of 3 eV for a discharge power in the range from 50 to 300 W and for a pressure in the range from 0.5 to 5 Pa. This indicates that inductive coupling mainly drives the discharge while capacitive coupling between coil and plasma is weak. A significant decrease in Ar neutral density is observed when the plasma is ignited. The Ar depletion increases with increasing RF power and increasing Ar base pressure and reaches 30% at 5 Pa and 300 W. The contributions of the different mechanisms resulting in an Ar depletion are estimated and compared. The decrease in neutral density cannot be explained by the ionization of Ar atoms only but is significantly attributed to the heating of Ar atoms by collisions with energetic particles. The increase in neutral gas temperature is estimated and found to be in reasonable agreement with measurements of the gas temperature reported previously by other groups.