Negative ions in a pulsed-power inductively-coupled chlorine plasma for etching

To understand the mechanism of notch-free etching in a pulsed inductively-coupled plasma (ICP), time-resolved measurements of the chlorine negative ion (Cl-) density and the oscillation of plasma potential are performed. A new technique for measuring the absolute negative ion density is used, which is based on laser-photodetachment and electron-beam excited instabilities at plasma frequency. This novel method reveals that the dissociative electron attachment yields a 90 % of the negative ion density at the afterglow time of 40 mu s. At the same time, the time variation of plasma potential was measured to elucidate the sheath structure around a substrate using a Langmuir probe and a capacitive probe for de and rf potential variations, respectively. These results suggested that the rapid electron cooling and negative ion increase in afterglow modify a sheath structure at the substrate from electron retarding to electron acceleration, thus neutralizing positive charge on the gate oxide layer.