Measurement of ion energy distributions at the powered rf electrode in a variable magnetic field

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Abstract
High-resolution energy distributions of ions, accelerated by the sheath at the powered electrode of a low-pressure 13.56-MHz gas discharge, have been measured. The observed spectra are compared to existing models. Excellent agreement between measured and calculated spectra is obtained. Detailed information on rf sheath behavior is derived from the observed energy profiles and from the measured total ion current densities towards the electrode surface. Analogous to the case of dc discharges, a decrease of sheath thickness is observed when a homogeneous variable magnetic field (0≤B≤315 G) is applied. However, the product of magnetic-field strength B and sheath thickness d is found to be independent of sheath voltage. This leads to the conclusion that in rf discharges, sheath contraction under influence of a magnetic field proceeds by a different mechanism than in dc discharges. It is suggested that the value of the product Bd is determined by the (virtually constant) temperature of the plasma electrons, rather than by the energy of secondary electrons that have been liberated from the electrode surface by ion bombardment. The decrease of sheath thickness d with magnetic-field strength B leads to a changing capacitive-voltage division of the applied generator voltage over the discharge. When the magnetic-field strength is sufficiently high, this may result in a sign reversal of the electrode self-bias voltage.