Development of Negative Oxygen and Fluorine Ion Sources with Helicon-Wave Discharge

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In this work, efficient negative oxygen and fluorine ion sources for materials processing have been developed.A helicon-wave discharge device for the production of negative ions was constructed. High-density oxygen and CF_4 plasmas of 10^<12>-10^<13> cm^<-3> were produced. Two kinds of plasma phases were formed in the high-density plasmas for the efficient negative ion production, namely a high-density and high-temperature plasma of 5-6 eV in the discharge-ON phase, and a high-density and low-temperature plasma of about 1 eV in the discharge-OFF phase. It was confirmed by time-of flight mass spectrometry and laser photodetachment that negative ions were produced efficiently in the afterglow of the discharge-OFF ohase. Negative ions were also efficiently produced in the outer low-temperature region of a sheet plasma, which was formed by controlling plasma parameters spatially.The dominant negative ion species in the high-density oxygen and CF_4 plasmas were atomic negative ions, O^- and F^-, respectively. The atomic negative ions increased with increase in electron density.The main production process of negative ions was dissociative attachment of slow electrons to excited molecules or molecular radicals, and the dominant decay process was mutual neutralization between negative and positive ions.Efficient negative ion production can be realized by forming a high-density and high-temperature plasma, and a low-temperature and high-density plasma. The plasmas with the two phases or regions can be formed by controlling parameters of high-density oxygen and fluorine plasmas temporally and spatially. A guiding principle of the design for efficient negative ion sources has been obtained by this work.