α to γ transition and plasma uniformity in large area intermediate pressure N₂capacitively coupled plasma (CCP) discharges

1D particle-in-cell (PIC) simulations of a 2.5 cm gap, 1.6 Torr N₂ capacitive discharge showed an α to γ transition, characterized by a rise in density and a collapse of the sheath width when the rf sheath voltage amplitude V₁ exceeds a breakdown voltage V_B. Smaller sheath widths enhance electromagnetic (EM) effects which may negatively affect plasma uniformity and stability. We develop a 2D axisymmetric cylindrical EM fluid model including secondary emission to study the α to γ transition and plasma uniformity in a large area reactor with discharge radius R=1.8 m. For lower power and input current I_rf, the discharge is fully in the α mode with V₁<V_B for 0<r<R. At higher power and I_rf, the enhanced finite wavelength 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, in which V₁>V_B for 0<r<R, is obtainable at a frequency f=6.78 MHz but not at f=13.56MHz, due to the enhanced finite wavelength effect at the higher f. Thus, for lower f, plasma uniformity may be obtained not only at lower powers when the discharge is fully in the α mode, but also at higher powers when the discharge is fully in the γ mode.