Comment on “A study on improvement of discharge characteristic by using a transformer in a capacitively coupled plasma” [Phys. Plasmas 22, 123504 (2015)]

Abstract

In the paper [Y.C. Kim et al., Phys. Plasmas 22, 123504 (2015)], the plasma density in radio frequency (RF) capacitively coupled plasma was increased by introducing a step-down ferrite-core transformer into a conventional type-T impedance matching network. The authors ascribed the increment in plasma density to the reduction of DC self-bias. Although this correlation was roughly right, but behaviors of the DC self-bias in the asymmetric RF discharge that was non-linearly interacted with the external circuit were not discussed. In practical applications, this approach of increasing plasma density will be limited by the rapid increase in power loss in step-down ferrite-core transformer operating in high-power 13.56 MHz discharges and the still high DC self-bias governed by the asymmetric geometry.