Transients using low-high pulsed power in inductively coupled plasmas

Abstract

Pulsed inductively coupled plasmas (ICPs) are widely deployed in the fabrication of semiconductor devices. Pulse repetition frequencies of up to tens of kHz are commonly used during plasma etching for the high power densities they generate during the pulse-on period, and for their unique chemistries during the pulse-off period. The use of highly attaching halogen gases produces low electron densities during the pulse-off period, and these low densities can result in instabilities, E–H transitions and ignition delays when applying power on the next pulse. To mitigate these possibilities, a low-level power environment could be maintained during ‘pulse-off’ to moderate the minimum plasma density, therefore reducing ignition delays and enhancing plasma stability. In this work, ICPs sustained by 5 kHz pulsed power using Ar/Cl2 mixtures at 20 mTorr were computationally investigated using a high-power, low-power format. For these conditions, the computed electron temperature (Te) reaches a quasi-steady state during both the high- and low-power excitation. The model predicts that within the electromagnetic skin-depth, Te spikes to a high value during a low-to-high power transition, and to a low value during a high-to-low power transition. At the same time, a few cm above the substrate, there is little modulation in Te, as electron power convected from the skin depth disperses in traversing the reactor. The positive and negative spikes, and convection of transients across the reactors, are functions of power ramping time and gas mixtures.