INFLUENCE OF THE RF EXCITATION ON THE LOW-ENERGY BROAD ION-BEAM CHARACTERISTICS

Influence of the rf excitation in a capacitively coupled filamentless broad ion beam source with radial electrode configuration on the ion beam properties has been examined. Ion energy distribution was measured by use of a retarding field energy analyzer connected to a computer controlled data acquisition system. Influence of working conditions on the average ion energy and on the ion energy spread in the beam is described. Energy spread has typical values between 8 and 25 eV. Average ion energy can change due to variations of ion source working conditions (at constant extraction voltages) up to 60 eV. Increase of both the average ion energy and ion energy spread is observed with increasing rf power coupled to the plasma (20-100 W). This correlates to the increasing amplitude of plasma floating potential oscillation. Average ion energy decreases with increasing magnetic induction (0-75 mT) and with increasing pressure in the vacuum chamber (2 X 10(-4) - 1.8 X 10(-3) mbar) due to decreasing cathode oscillation amplitudes. Average ion energy decreases with increasing distance from the broad beam axis because of chromatical and spherical aberrations of the source extraction optics. Multiple peaks in the ion energy spectrum are observed. Their occurrence can be explained by charge exchange effects in the rf oscillating plasma sheath.