ION VELOCITY DISTRIBUTIONS IN HELICON WAVE PLASMAS - MAGNETIC-FIELD AND PRESSURE EFFECTS

Consideration of ion transport in high density, low pressure plasma sy stems is important for meeting process requirements in the manufacturi ng of ultra-large-scale integrated circuits. The ion energy and angula r distributions at the boundary between the plasma and the wafer, the sheath, influence etching selectivity, linewidth control, plasma-induc ed damage, and microscopic etching uniformity. These distributions, in turn, are easily altered by changing the magnetic field profile and/o r the neutral gas pressure. Using Doppler-shifted laser-induced fluore scence, metastable ion velocity distribution functions in helicon-wave -excited Ar plasmas are measured. Two magnetic field configurations ar e examined. For a magnetic ''mirror,'' where the field exhibits a maxi mum and a saddle point in the source, the plasma is observed to be asy mmetric and nonuniform: this leads to broadened velocity distributions and significant ion drift from one region of the plasma to another. A s the pressure is increased in the mirror field configuration, the tra nsverse ion ''temperature'' exhibits a maximum as a function of pressu re and, when etching is ion-flux limited, either decreasing or increas ing the pressure should result in improved linewidth control. The plas ma is more symmetric when the magnetic field is reversed in the source and again downstream. With this double cusp configuration, the transv erse ion temperature decreases monotonically with pressure, and improv ed linewidth control in the ion-flux limit would be obtained by operat ing at higher pressure.