Behavior of Electrons and Negative Ions in a Capacitively-Coupled Radio-Frequency NF 3/Ar Plasma

The behaviors of electrons and negative ions in a low-pressure (100 mTorr) capacitively-coupled radio-frequency (13.56 MHz) NF3/Ar plasma have been studied at a low-power level (2–10 W) using a laser photodetachment technique combined with a microwave cavity resonance method for determining electron density. The wavelength dependent photodetachment measurement indicated that F- is the predominant negative ion species in the plasma. The negative ion density was higher than the electron density by a factor of ∼50 at a NF3 mixing ratio r as low as 1% and the negative-ion/electron density ratio further increased with increasing r mainly because of the decrease in the electron density. The overall change in the negative ion density in the studied range of 1%<r<40% was relatively small (a factor of ∼2). For small r (<∼10%), the negative ion density increased with decreasing r; the change in the electron-attachment and ion-ion recombination rate constants with varying r was investigated to explain the phenomenon.