ION ENERGY-DISTRIBUTIONS IN A DC BIASED RF DISCHARGE

We measured quasisimultaneously the energy distributions of positive i ons at the powered rf and grounded electrode of a parallel plate 13.56 MHz discharge using an energy selective mass spectrometer. The result ing ion energy distributions reflect the discharge potential condition s expected from a capacitive plasma sheath model. By means of an exter nally supplied de bias of the powered electrode we are able to influen ce the potential structure and to control ion energy and ion flux inde pendently. The ratio between mean ion energy and mean sheath thickness reflects the effect of collisions on the ion energy distributions and enables estimates of sheath thickness and bulk plasma parameters to b e made which are compared with values obtained by Langmuir probe measu rements. We are able to demonstrate that changes in sheath potential a lso affect, via secondary electrons, the ionization regime in the disc harge and this can be utilized to control the species composition in t he discharge. (C) 1997 American Institute of Physics.