Mass Spectrometry Studies of Hydrogen Ions Energy Distributions in an ECR- based Large Volume Plasma Source

Abstract

Plasma is produced in a Large Volume Plasma Source (LVPS; dia. = 1 m, height = 1m) using CW microwaves (= 400 - 600 W, 2.45 GHz), in a compact ECR plasma source (CEPS) attached to LVPS, at hydrogen gas pressures = 1 - 3 mTorr. Plasma expands along the CEPS magnetic field into LVPS. A Hiden Analytical HPR 60 molecular beam mass spectrometer (MBMS) is used to measure the H^- ion energy distribution functions (IEDFs) in the downstream plasma. Previous plasma characterization studies in LVPS indicated favourable downstream plasma conditions for volume production of H^- ions. Measurements conducted with the MBMS probe aligned facing the plasma flow = 80 cm downstream, gave typical H^- count rates = 3 x 10^5 counts /s, at = 400 W, = 1 mTorr, along with a distinct high energy tail (<= 20 eV). These and other results are analyzed in detail. The positive ion spectrum showed the H_3^+ count to be consistently high in all cases (= 60-70 %); the counts for H_2^+ and H^+ were =30-35 % and a =few %. Combining the Langmuir probe (LP) and MBMS data it is possible to determine the approximate densities in front of the MBMS probe aperture. At = 500 W and = 2 mTorr, one finds: n_(H^+) = 9.6 x 10^9 cm^(-3), n_(H_2^+) = 1.7 x 10^10 cm^(-3) and n_(H_3^+) = 4.3 x 10^10 cm^(-3). The corresponding H^- density, = 80 cm downstream is n_(H^-) = 3.9 x 10^8 cm^(-3). Accounting for all H^- losses due to scattering and destruction, one finds the effective mean free path for H^- loss to be = 12.4 cm. Noting that H^- formation takes place about = 10 - 30 cm downstream of the source exit, the approximate average H^- density in the formation zone is determined as = 5.5 x 10^10 cm^(-3). This value is remarkably encouraging for H^- production in volume mode, considering the large chamber volume and area, as well as the very moderate power used for the experiments.