Absolute measurements of rf fields using a retarding field energy analyser

Summary form only given. We present measurements of ion energy distribution functions obtained with a single retarding field energy analyser (RFEA) along the axis of a low pressure helicon source. We have found that for high densities, the sheath thickness becomes small and sheath effects can be measured by the RFEA: a transition from a high frequency (single peak distribution) to a low frequency (bimodal distribution) sheath has been observed in the helicon source both for the capacitive and inductive modes. The energy difference between the two peaks of the bimodal distribution is related to the rf modulation of the plasma potential and to the ratio between the ion transit time in the sheath and the rf period. While the limiting cases of high and low frequency sheaths can be dealt with analytically, most of the results obtained correspond to a middle-ground where the phase mixing effect on the distribution function of the ion transit time across the sheath must be taken into account.