Effects of a sheath boundary on electron energy distribution in Ar/He dc magnetron discharges

Abstract

In this study, the effects of a sheath boundary on electron energy distribution and discharge characteristics in an unbalanced and planar-type dc magnetron sputtering system are investigated. The anode sheath potential is changed by applying dc bias voltages to the substrate. The electron energy distribution functions (EEDFs) are measured in argon and helium discharges using a single Langmuir probe in conjunction with the ac superposition method. The evolutions of the EEDFs are first observed in argon at 3 mTorr and then in helium at 30 mTorr. The results show that, as the substrate bias voltage decreases to high negative voltage, the EEDF transition from the bi-Maxwellian to the Maxwellian in the downstream region occurs at a specific bias voltage that depends on the operating gas. The major factors that affect the EEDF formation are investigated. In particular, the concept of total electron bounce frequency is introduced to represent the change of the sheath boundary condition. The observed EEDF transition is explained by comparing it with the plasma characteristic frequencies calculated from the measured EEDFs. As a result, the bi-Maxwellian distribution observed at the small substrate bias voltage is attributed to the low electron–electron collision frequency and the different loss mechanisms of two electron groups: the ambipolar diffusion loss of low-energy electron group confined by low plasma potential and the direct thermal loss of high-energy electron group, providing the electron current that compensates for the discharge current in a steady state.