A new inductively coupled plasma source design with improved azimuthalsymmetry control

Abstract

The geometry of an inductively coupled plasma (ICP) source impactsthe plasma and processing uniformity. A reasonably uniform source design doesnot always guarantee a uniform plasma however, because transmission line(i.e. standing wave) effects also impact on its performance. In this work wepresent an ICP source design with a geometry that enables better control overthe field profiles' azimuthal symmetry despite transmission line effects. Thegeometry is three dimensional rather than planar and consists of two layers offull and semicircular loops with the RF current generally flowing in oppositedirections. We have measured the free space magnetic fields produced by oneimplementation of the new source in the (r,θ) plane using a B-dotprobe. The new source generated fields of higher azimuthal symmetry than theplanar coil, despite a larger current variation along the new source length. Wehave also developed a three-dimensional electromagnetic model for ICP sourcesthat accounts for current variations along the source length due to standingwave effects. The model showed good agreement with the measured fields for theplanar coil. However, it showed less agreement for the new source design sincethe interaction between the loops in the different layers were not included inthe model. Langmuir probe measurements showed that the new ICP sourcegenerated high density (1011-1012 cm-3) argon and chlorineplasmas at low pressures (1-30 mTorr) at 1 cm above the wafer surface. Spatialprofiles of electron temperature and ion density in a chlorine plasma at 1 cmbelow the dielectric window showed improved azimuthal symmetry of powerdeposition with the new ICP source. Polysilicon etch rate profiles on 150 mmwafers also showed improved azimuthal symmetry and uniformity with the new ICPsource.