Plasma Uniformity and Stability in Large Area Intermediate Pressure Capacitive Coupled Plasma (CCP) Reactors with N2/NH3 Chemistry

1D particle-in-cell (PIC) simulations of an intermediate pressure N2 capacitive discharge showed an alpha to gamma transition, characterized by a collapse of the sheath width when the rf sheath voltage amplitude exceeds a breakdown voltage of about 140 V. Smaller sheath widths enhance electromagnetic (EM) effects which may negatively affect plasma uniformity and stability. One solution is to use a dual frequency drive in which the lower fundamental frequency maintains the sheath width while the higher second harmonic frequency maintains the plasma density at lower overall voltages below the breakdown. Such configurations are known to lead to electric asymmetry (EA) effects, which are a function of the relative phase between the drives. Another solution is to add an electronegative gas such as NH3 to the N2 gas to increase the power threshold needed to transition to the gamma-mode, since sheath multiplication of the hot gamma electrons must now compete with their loss through attachment. We study the EA effects by conducting 1D PIC simulations of a dual frequency (13.56/27.12 MHz) symmetrically-driven 1.6 Torr N2/NH3 discharge at various relative phases between the drives. We then develop a dual frequency 2D EM fluid model for this discharge to study the EM effects. By combining the two solutions, we may be able to find the optimum operating conditions for enhancing stability and minimizing non-uniformity.