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The positive ions incident on the substrate plane in a planar diode discharge system are observed with an electrostatic deflection energy analyzer and a quadrupole mass spectrometer. Several targets have been rf sputtered in rare-gas discharges and the mass spectra that are obtained indicate that the sputtered species, which leave the target predominantly as neutrals, are subsequently ionized in the discharge by Penning ionization rather than by electron-impact ionization or other ion-molecule reactions.
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Coburn et al. (1971) studied this question.
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