Mass-spectrometric ion sampling from reactive plasmas I. Apparatus for argon and reactive discharges

Abstract

An apparatus has been constructed to sample the ionic and neutral species that emerge from r.f. discharges in which chemical reactions are taking place. The results for pure argon show that the sheath potential (∼ 50 V) between the sampling orifice and the plasma is too large to be supported by the plasma electron temperature (1.9eV ≤ T e ≤ 2.3 eV, as determined by a floating double probe experiment). Most ions are sampled after at least one collision in the sheat. A 1 % mixture of vinyltrimethylsilane (VTMS) in argon yielded ions (ArH +, H 3 +, CH 3 +, C 2H 2 + and C 2H 3 +) characteristic of elementary ion—molecule reactions or dissociative charge exchange rather than primary electron impact. The most probable sources of ArH + are Ar + + H 2 → ArH + + H H 2 + + Ar → ArH + + H Ar + + VTMS → ArH + + (VTMS—H). Reactions of VTMS in the r.f. discharge appear to be very efficient.