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Published June 1, 1993 | Published
Journal Article Open

Limits to ion energy control in high density glow discharges: Measurement of absolute metastable ion concentrations


Unprecedented demands for uniformity, throughput, anisotropy, and damage control in submicron pattern transfer are spurring development of new, low pressure, high charge density plasma reactors. Wafer biasing, independent of plasma production in these new systems is intended to provide improved ion flux and energy control so that selectivity can be optimized and damage can be minimized. However, as we show here, an inherent property of such discharges is the generation of significant densities of excited, metastable ionic states that can bombard workpiece surfaces with higher translational and internal energy. Absolute metastable ion densities are measured using the technique of self-absorption, while the corresponding velocity distributions and density scaling with pressure and electron density are measured using laser-induced fluorescence. For a low pressure, helicon-wave excited plasma, the metastable ion flux is at least 24% of the total ion flux to device surfaces. Because the metastable ion density scales roughly as the reciprocal of the pressure and as the square of the electron density, the metastable flux is largest in low pressure, high charge density plasmas. This metastable ion energy flux effectively limits ion energy and flux control in these plasma reactors, but the consequences for etching and deposition of thin films depend on the material system and remain an open question.

Additional Information

© 1993 American Institute of Physics. Received 8 September 1992; accepted 2 February 1993. Travel and living support for J. Margot was kindly provided through a grant from the Ministere des Affaires Lnternationales de Quebec. Discussions with J. Lawler and C. Woods from the University of Wisconsin and M. Lieberman from U. C. Berkeley are gratefully acknowledged. We are also grateful to Lucas Labs for providing a helicon antenna which we modified slightly and to F. Skiff of the University of Maryland for bringing Ref. 36 to our attention and for many stimulating conversations.

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