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.