Alpha/Gamma Transition and Plasma Uniformity in Large Area Intermediate Pressure N₂ Capacitive Coupled Plasma (CCP) Reactors

Capacitive discharges in an intermediate pressure (0.2 to 6 Torr) range with feedstock mixtures containing N₂ are common for thin film processing in the semiconductor, display, and solar panel manufacturing industries. Such discharges are typically operated in the α-mode but at powers close to the transition to the γ-mode. In the α-mode, the hot γ electrons due to ion-induced secondary emission from the electrodes play a minimal role in the discharge while, in the γ-mode, they are essential for its maintenance. 1D particle-in-cell simulations of a 2.5 cm gap, 1.6 Torr N₂ capacitive discharge at 13.56 MHz showed an α to γ transition, characterized by a rise in density and a collapse of the sheath widths when the rf sheath voltage amplitude V₁ exceeded a breakdown voltage V_B. Smaller sheath widths enhance EM effects which may negatively affect plasma uniformity and stability. We develop a 2D cylindrical EM fluid model including secondary emission to study the effect of the α to γ transition on plasma uniformity in a large area reactor with radius R=1.8 m. At lower powers, the discharge is fully in the α-mode with V₁<V_B for 0<r<R. As the power increases, the enhanced EM effect causes V₁>V_B (γ-mode) toward the center, and V₁<V_B (α-mode) toward the radial edge, with the transition point r=r_B approaching R. A fully γ-mode discharge where V₁>V_B for all r is obtainable at 6.78 MHz but not at 13.56 MHz due to the reduced EM effect at the lower frequency.