Ninth Internationl Conference on Plasma Surface Engineering (PSE) Garmisch-Patenkirchen

Negative Ions in Argon-Oxygen Discharges

H. M. Katsch, C. Manthey, A. Wagner and H. F. Döbele
Institut für Experimentelle Physik, Universität Duisburg-Essen, D-45117 Essen, Germany

The temporal behavior of negative oxygen ions in oxygen/argon mixtures is investigated in the afterglow of a pulsed inductively excited modified GEC reactor which can be operated at pressures down to 1 Pa. The negative ion densities are measured by laser photodetachment of negative ions in combination with a Langmuir probe as well as by absorption of photons due to photodetachment with a cavity ringdown technique. An unexpected high density of negative oxygen ions was found at high plasma densities. The main loss mechanism for negative oxygen ions at plasma densities above 1011 cm-3 is recombination with positive ions. Measurement of the atomic oxygen densities with two-photon laser induced-fluorescence spectroscopy show, that collisions of the atomic atoms with the negative ions are a minor loss process for the negative ions. Also mass spectrometry measurement of O2--negative ions indicate, that the loss of negative ions due to collisions with low metastable oxygen molecules, O2(a1Δg), are less important.

According to predictions of common models it is unlikely that the density of the negative ions is high at such plasma densities. With increasing argon fraction and increasing plasma density an increase of the absolute O-minus density is observed, in contrast to the predictions of global models, see e.g.[1]. It is, therefore, necessary to invoke an additional production channel for the negative ions in order to explain the observed high negative ion density. Appearance mass spectroscopy measurements show an increase of highly excited oxygen molecules with increasing argon fraction. It is, therefore, likely that additional negative ions are generated by dissociative attachment of highly excited oxygen molecules [2], [3].

Comparison of the experimental findings with an improved global model show strong evidence that highly exited Rydberg oxygen molecules, generated by collisions with metastable argon atoms [4], lead to large amount of negative ions. We will present examples that negative oxygen ions can influence the oxidations depth of Silicon and may also influence the structure formation of transition metal oxides in case of magnetron sputtering.

[1] Lee C and Lieberman M A 1995 J. Vac. Sci. Technol. A 13(2) 368-80.
[2] Hayashi D and Kadota K 1998 J. Appl. Phys. 83(2) 697-702.
[3] Ding W X, et al. 1999 Plasma Sources Sci. Technol. 8(3) 384-91.
[4] Rickey D and Krenos J 1997 J. Chem. Phys. 106(8) 3135-45.

This project is funded by the 'Bundesminister für Bildung und Forschung' BMBF (FKZ 13N8052).