Characterization of transmission line effects and ion-ion plasma formation in an inductively coupled plasma etch reactor

The plasma and processing uniformity are greatly affected by the gas flow distribution and the source geometry in inductively coupled plasma (ICP) etch reactors. However, a reasonably uniform source design, along with uniform gas distribution, does not always guarantee uniform plasma, because transmission line (i.e. standing wave) effects also impact its performance. In this work, we demonstrate that the gas flow distribution can have a major impact on both the plasma density profiles and etch rate uniformity at low pressures where one might expect diffusion to make gas flow distribution less important. We also present an ICP source design with a geometry that enables better control over the field profiles azimuthal symmetry despite transmission line effects. B-dot probe measurements of the free space electromagnetic fields for the new source and a comparably dimensioned standard planar coil showed improved azimuthal symmetry for the new source. 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 electromagnetic field profiles obtained from the model showed good agreement with the measured field profiles. Langmuir probe measurements showed that the new ICP source generated high density (1011--1012 cm-3) plasmas at low pressures with significantly improved azimuthal symmetry of power deposition and plasma generation. In addition, polysilicon etch rate profiles on 150 mm wafers also showed improved azimuthal symmetry and uniformity with the new ICP source. The new source was then used to investigate chlorine discharge properties and their spatial profiles in continuous wave (CW) and pulsed operation. Time-resolved Langmuir probe measurements showed that electron-free or "ion-ion" chlorine plasma forms during the afterglow (i.e. power-off) due to electron attachment. Such electron-free plasma can provide both positive and negative ion fluxes to a substrate potentially reducing charge-up damage as well as enabling negative ion assisted processing. Finally, negative ion and positive ion assisted etch were compared by applying a DC step bias during the afterglow.