Electromagnetic (EM) and Electric Asymmetry (EA) Effects in Dual Frequency Collisional Nitrogen Capacitive Discharges

Intermediate pressure capacitive discharges containing nitrogen gas, are 

common in the thin film processing industries, and are typically operated in 

the α-mode, but at powers close to the transition to the γ-mode, 

characterized by the collapse of the sheath width and the clamping of the 

sheath voltage at the breakdown voltage. For a given voltage, higher frequency 

drives can maintain discharges at higher densities, but at the cost of 

enhancing electromagnetic (EM) effects leading to plasma non-uniformity. 

A possible solution is to operate the discharge with both the fundamental 

drive to maintain the sheath width and a supplemental second harmonic drive to 

enhance the density. Even in symmetrically-driven discharges, this 

configuration can lead to electric asymmetry (EA) effects. We study these EA   

effects by conducting one-dimensional (1D) particle-in-cell (PIC) simulations 

of a dual frequency (13.56/27.12 MHz) symmetrically-driven 1.6 Torr nitrogen 

discharge at various relative phases between the frequencies. We then develop 

a dual frequency two-dimensional (2D) EM fluid sheath model for this discharge,

taking the EA effects into account, in order to study the EM effects of this 

configuration.