Negative ions in plasmas play a main role in the discharge kinetics. For instance they may be at the origin of plasma instability [1], or may be responsible for coalescence in the primary state of dust formation [2]. Plasma based negative ion sources can have many applications. They may be used to reduce surface charge during plasma etching and in the context of controlled fusion research, they are used to generate neutral beams to heat fusion plasma. Therefore controlling negative ion production and loss in plasmas is of primary interest in many research fields. Efficient negative ion sources use caesium deposited on surfaces to increase negative ion production. Indeed, due to its property of reducing the work function, caesium leads to a high negative ion surface production yield. Negative ions are also produced in plasma bulk through electron attachment on molecules. Up to now most of works focused on plasma bulk production and on caesiated-surface production. Few works deal with caesium free surface production while almost all low pressure plasma sources are running without caesium. The aim of the present paper is to study caesium free negative ion surface production in low pressure plasmas.

Our first study is concerned with H2/D2 plasmas and graphite material for fusion applications [3]. However, results obtained here can be extended to low pressure plasma sources used in microelectronic industry for instance, since carbon containing materials are often in interaction with H2 plasmas or even more electronegative plasmas such as oxygen, fluorine or chlorine containing plasmas.

We use a helicon reactor whether in capacitive or inductive mode. A mass spectrometer is placed in the diffusion chamber of the helicon reactor and faces a one square centimetre graphite sample. The sample is negatively biased with respect to the plasma. Positive ions (H+, H2+, H3+) bombards it and negative ions formed (H-) upon bombardment are repelled from the surface toward the plasma. Under low pressure considered here, they reach without any collision the mass spectrometer where they are analysed according to their energy. Study of negatve Ion Distribution Function (IDF) provides information on surface production mechanisms. In this talk we will discuss IDFs measurements, describe how we identify surface production mechanisms, show negative ion surface production yield dependency with positive ion flux and energy, and compare H2 and D2 plasmas.

The authors acknowledge ANR (project ITER-NIS BLAN08-2_310122)

[1] Chabert P et al 2001 Pl. S. Sci. Tech.10 478

[2] Bouchoule A et al 1996 Pure Appl. Chem.68 1121

[3] L Schiesko et al Pl. S. Sci. Tech.17 (2008) 035023