Pulsed plasmas as a method to improve uniformity during materials processing

Abstract

Continuous wave operation of inductively coupled plasma (ICP) reactors as used for materials processing and which have geometrical or flow asymmetries may produce asymmetric species densities, temperatures, and fluxes. Flow asymmetries that produce nonuniformities in plasma conductivity initiate a positive feedback loop. In this feedback loop, asymmetries in conductivity are reinforced by the resulting nonuniform power deposition, which causes electron impact ionization to also be nonuniform. In this article, we discuss results from a computational investigation of long-term transients during pulsed operation of ICPs and their consequences on side-to-side asymmetries in plasma properties. During pulsed operation, diffusion of charged species during the afterglow between pulses smoothens these asymmetries prior to the next power pulse. The power deposition during subsequent pulses is more symmetric and this reduces the positive feedback. The improvement in uniformity afforded by pulsing is determined by the intrapulse plasma dynamics, and so is a function of the pulse repetition frequency, duty cycle, and feedstock gas. Improvements in the azimuthal uniformity of reactant densities were obtained in Ar and Cl2 plasmas in an asymmetrically pumped reactor using pulsed power. As dissociative attachment dominates in the afterglow of Cl2 pulsed plasmas, it provides a more uniform sink for electrons compared to ambipolar diffusion and different systematic behavior is obtained compared to argon.