A new inductively coupled plasma source design with improved azimuthal symmetry control

The geometry of an inductively coupled plasma (ICP) source impacts the plasma and processing uniformity. A reasonably uniform source design does not always guarantee a uniform plasma however, because transmission line (i.e. standing wave) effects also impact on its performance. In this work we present an ICP source design with a geometry that enables better control over the field profiles' azimuthal symmetry despite transmission line effects. The geometry is three dimensional rather than planar and consists of two layers of full and semicircular loops with the RF current generally Rowing in opposite directions. We have measured the free space magnetic fields produced by one implementation of the new source in the (r, theta) plane using a B-dot probe. The new source generated fields of higher azimuthal symmetry than the planar coil, despite a larger current variation along the new source length. We have also developed a three-dimensional electromagnetic model for ICP sources that accounts for current variations along the source length due to standing wave effects. The model showed good agreement with the measured fields for the planar coil. However, it showed less agreement for the new source design since the interaction between the loops in the different layers were not included in the model. Langmuir probe measurements showed that the new ICP source generated high density (10(11)-10(12) cm(-3)) argon and chlorine plasmas at low pressures (1-30 mTorr) at 1 cm above the wafer surface. Spatial profiles of electron temperature and ion density in a chlorine plasma at 1 cm below the dielectric window showed improved azimuthal symmetry of power deposition with the new ICP source. Polysilicon etch rate profiles on 150 mm wafers also showed improved azimuthal symmetry and uniformity with the new ICP source.