Variation of the electron energy distribution with He dilution in an inductively coupled argon discharge

Abstract

We present experimental evidence of different behaviors of plasma parameters depending on changes in the electron energy distribution (EED), caused by an electron heating mechanism and electron-neutral collision processes in an Ar/He mixture inductively coupled plasma. At a low gas pressure of 3 mTorr, where the electron neutral collision frequency nu(m) is much smaller than the driving frequency omega(RF), the EEDs evolved from a bi-Maxwellian distribution to a Maxwellian distribution, due to the efficient heating of low energy electrons when the He flow rate increased at a fixed total gas pressure. The plasma density slowly decreased with the He flow rate portion ([He]/[Ar] + [He]) in a range of 0%-70%, while the plasma density largely decreased in the He flow rate portion of 70%-100%. On the other hand, at a high gas pressure of 350 mTorr where nu(m) >> omega(RF), the EEDs evolved from a Druyvesteyn-like distribution to a Maxwellian distribution, due to a cooling of low energy electrons and an increase in the population of high energy electrons, when the mixing ratio of the He gas is increased. In this case, plasma density abruptly decreased for a He flow rate ratio of 0%-30%. This result directly shows that the EEDs significantly affect the different variations of plasma parameters, even in the case of the same mixing ratio of the gases.