Ion energy distributions in pulsed large area microwave plasma

The energy and the flux of ions impinging on surfaces exposed to low pressu re plasmas are important factors which determine the chemical structure and the physical properties of the surfaces and of the thin films. In the pres ent work, we use a large area microwave (MW) plasma reactor in which a grou nded sample holder is exposed to a MW (2.45 GHz) discharge excited in diffe rent gases, such as Ar, N-2, and He, at a pressure ranging from 50 to 200 m Torr. A three-grid, differentially pumped ion energy analyzer is used to me asure the ion energy distribution functions (IEDF). The use of a pulsed pla sma gives rise to a structured IEDF in which the mean ion energy values var y between 2 and 10 eV. The pulse frequency and the duty cycle were found to strongly affect the IEDF and the ion flux. The evolution of the IEDF is an alyzed in terms of the pulsed plasma global model, used to derive the chara cteristic time constants of plasma ignition and plasma decay. It is shown t hat the ions in the low energy portion of the IEDF originate from the time period between the individual power pulses, and their relative contribution increases with decreasing the duty time. Controlled pulsing thus allows on e to selectively adjust the ion energy, and thereby the surface phenomena i n materials processing which are primarily influenced by ion bombardment. ( C) 1999 American Institute of Physics. [S0021-8979(99)06909-1].