Effect of driving frequency on plasma property in radio frequency and very high frequency magnetron sputtering discharges

Abstract

Ion energy distributions (IEDs), electron energy distributions (EEDs) and other plasma parameters of magnetron sputtering discharges driven by 13.56, 27.12 and 60 MHz sources were investigated by a retarding field energy analyzer and Langmuir probe measurements. An increase in driving frequency leads to an increase in ion energy and the evolution of IEDs from a uni-modal distribution at the 13.56 MHz discharge toward a bi-modal distribution at 27.12 MHz, and a multi-modal distribution at the 60 MHz discharge. For IEDs near the target surface, this evolution is related to the ion acceleration and the charge transfer collisions between Ar atoms and Ar+ ions in the presheath, while for IEDs at the substrate, the evolution depends on the ratio of the ion transit time across the sheath to the radio frequency period. The increase in driving frequency also leads to the evolution of EED function from a Maxwellian type at the 13.56 MHz discharge toward a bi-Maxwellian type at the 27.12 MHz discharge and a Druyvesteyn-like type at the 60 MHz discharge due to the change in the generation and loss mechanisms of electrons. In addition, increasing the driving frequency can lead to a higher electron temperature and a lower electron density. Therefore, the driving frequency becomes an effective tool to control the plasma properties of magnetron sputtering discharges.