Electron–sheath interaction in capacitive radio‐frequency plasmas

Recent experimental and theoretical work on the interaction of electrons with a moving radio‐frequency (rf) sheath has shown that this interaction is an important electron heating mechanism in capacitive rf plasmas. It is usually assumed in the theoretical treatment of rf sheaths that electron inertia can be neglected and the velocity distribution in the sheath vicinity remains close to Maxwellian. This assumption is examined critically and limits to its applicability are derived from a model of the sheath structure. The amplitude of the oscillating sheath voltage V_rf must satisfy eV_rf/k_BT_e ≲ (1/√8π)√m_i/m_e if electron loss is to remain thermal. Two mechanisms enhance electron loss: field reversal at the electrode and distortion of the velocity distribution near the moving sheath edge. The latter is important if the maximum sheath velocity in an rf cycle approaches a critical value υ_sm = ῡ_e/2 where ῡ_e is the thermal velocity. Particle simulations of the electron–sheath interaction are used to study the distortion of the electron velocity distribution in the sheath vicinity and the details of the electron loss to the wall. Plasma oscillations can be excited near the expanding sheath edge when the sheath modulation amplitude is large, and this may have an important influence on the electron velocity distribution in the plasma.