Enhancement of the negative ion flux to surfaces from radio-frequency processing discharges

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Abstract
The relative flux of negative ions from rf discharges through CF4 and F2‐He has been studied using mass spectrometry. The negative ion flux impinging on a surface in contact with a glow discharge is usually quite small because of the sheaths formed to contain the more mobile electrons. In the afterglow, however, these sheaths decay on the time scale of the electron density loss rate while negative ions can remain in the discharge region significantly longer. As a consequence, the negative ion flux to a surface in contact with the glow discharge can rise by a very large amount in the afterglow when the sheaths have diminished. It is shown here that the flux of the negative ions can be enhanced by factors of 50 to greater than 1000 by square wave amplitude modulating the rf discharge. This enhancement depends upon the dc self‐bias voltage and the modulation frequency as well as the particular negative ion. Data are presented for both CF4 and F2‐helium discharges, along with the correlation to simple models.

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