AVS 61st International Symposium & Exhibition
    Plasma Science and Technology Friday Sessions
       Session PS1-FrM

Paper PS1-FrM10
A Global Model for Ignition Delay of Pulsed Electronegative Plasmas

Friday, November 14, 2014, 11:20 am, Room 305

Session: Plasma Sources
Presenter: Lei Liu, University of Houston
Authors: L. Liu, University of Houston
S. Sridhar, University of Houston
D.J. Economou, University of Houston
V.M. Donnelly, University of Houston
Correspondent: Click to Email
A Faraday shield can be employed to minimize capacitive coupling in inductively coupled plasmas (ICP), to obtain ion energy distributions with a tight energy spread. However, in the presence of a Faraday shield, it is challenging to operate a pulsed electronegative plasma with a long afterglow duration, as most of the electrons are lost, and re-ignition requires large electric fields produced by high-voltage capacitive coupling. Our experimental studies have shown that by using a dual plasma source, consisting of a main pulsed ICP in tandem with an auxiliary continuous wave ICP, Faraday-shielded pulsed plasmas in electronegative gases can be produced even with long afterglow duration (1000 µs). However, an ignition delay was observed with duty cycles >60%. A global (spatially averaged) model with chlorine chemistry was developed to describe the mechanism of ignition delay. The flux of charged species from the auxiliary ICP was included in both particle and energy balance equations. Predicted results of ignition delay for different duty cycles, auxiliary ICP powers and main ICP powers agreed with experimental observations. The observed ignition delay increasing with increasing duty cycle is counter-intuitive. One would expect that, as the duty cycle increases, and the afterglow time correspondingly decreases (for constant pulse period), the electron density at the end of the afterglow would be higher, making it easier to re-ignite the plasma, i.e., shorter delay time, in contrast to observation. Once the lower plasma power is off for > ~5µs it appears that there is a critical electron density in the lower plasma (ncr), equal to or below the electron density when the upper (cw) plasma is operating alone (no power in the lower plasma). When both plasmas are powered, the afterglow of the lower plasma keeps decaying for as long as the electron density is above ncr. This is because the rate of plasma production is then lower than the rate of plasma loss. As soon as the electron density falls below ncr, the rate of plasma production starts exceeding the rate of plasma loss, and the electron density starts increasing, eventually re-igniting the ICP. Now, the higher the duty cycle, the higher the electron density at the end of the afterglow, and the longer it takes for that electron density to decay to ncr, resulting in longer delay times.