PPPS-2013: Collisional effect on the ion energy distributions outside the sheath during the afterglow of pulsed oxygen and nitrogen plasmas

摘要

The time evolution of ion energy distributions (IEDs) during the afterglow was measured in oxygen and nitrogen plasmas generated by an inductively coupled plasma source. The afterglow regimes are prepared by periodically pulsing the plasmas with a period of 1000 µs. The time-resolved retarding field energy analyzer method and Langmuir probe method were used to measure the detailed time evolution of IEDs at 5 µs intervals to investigate precisely what happens during the afterglow regime. The measured results show that in low pressure regime (a few mTorr) two-peak IEDs appear with time, following the IEDs which are generated during the rf-on time. By analyzing the force equation of the ions in a time-varying plasma potential environment, we found that these two-peak structures originate from the delayed arrivals of ions in collisionless discharge volumes. In the case of molecular gas discharges such as oxygen and nitrogen, the two-peak structures seem to last much longer with time than noble gases such as helium, argon and xenon, where electronegativity may affect diffusion time scale during the afterglow. This effect is expected to disappear under high pressure regime where ions lose their energy due to the frictional force caused by collisions between particles, and in this case, the time evolution of IEDs during the afterglow changes its form to a typical single-peak structure. The findings in this experiment can help us understand pulsed plasmas in detail, such as ‘source pulsed’ plasmas which are now widely used in semiconductor etching process.