Heating mechanisms of capacitively coupled discharges H2, 2 Pa H2, 120 Pa Capacitively coupled discharges electrons accelerated by the electric field between electrodes ωpi < ω ωpe (electrons are fast, they are able to follow the field ions are not able to follow the electric field) plasma size λ Capacitively coupled discharges – structure Bulk plasma electrons present during the whole RF period high conductivity, weak electric field inductive character σ = ne2 m (ν + iω) + iωε0 Sheaths strong electric field with a DC component repels electrons capacitive character nonlinear nature SHEATH GLOW sheath UP MATCHING NETWORK 13.56 MHz Ohmic (collisional) heating σ = ne2 m(ν2 + ω2) (ν − iω) + + iωε0 Pohm ≈ 1 2 j2 m e2 V ν n dV namely in the bulk plasma (high n) needs collisions – depends on pressure -0.1 -0.05 0 0.05 0 200 400 600 800 x[m] t [ns] 0 5 10 0 1 2 3 4 5 6 7 8 9 [eV] t [µs] Stochastic heating sheaths repel electrons electrons reflected from expanding sheath gain energy, electrons reflected from collapsing sheath lose energy more electron-sheath collisions during the expansion energetic electron beams SHEATH GLOW sheath UP MATCHING NETWORK 13.56 MHz Field reversal During fast sheath collaps electrons may be unable to react on the electric field. Sheath (positive) charge may be higher than electrode (negative) charge. Electrons are accelerated towards the sheath. SHEATH GLOW sheath UP MATCHING NETWORK 13.56 MHz − − − − − − −+ + + + + ++ + + + γ-heating Few (∼1 %) ions emit an electron from an electrode (potential emission) The electron is accelerated towards bulk plasma and obtains energy sufficient for ionization. At sufficient pressure – electron avalanche + E + Plasma Electrode