Ion energy distributions in pulsed large area microwave plasma

Abstract

The energy and the flux of ions impinging on surfaces exposed to low pressure plasmas are important factors which determine the chemical structure and the physical properties of the surfaces and of the thin films. In the present work, we use a large area microwave (MW) plasma reactor in which a grounded sample holder is exposed to a MW (2.45 GHz) discharge excited in different gases, such as Ar, N2, and He, at a pressure ranging from 50 to 200 mTorr. A three-grid, differentially pumped ion energy analyzer is used to measure the ion energy distribution functions (IEDF). The use of a pulsed plasma gives rise to a structured IEDF in which the mean ion energy values vary 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 analyzed in terms of the pulsed plasma global model, used to derive the characteristic time constants of plasma ignition and plasma decay. It is shown that 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 one to selectively adjust the ion energy, and thereby the surface phenomena in materials processing which are primarily influenced by ion bombardment.