The use of SF₆ as a Plasma Processing Gas

Abstract

Power electronics and high temperature operation capable micro electro mechanical (MEM) device fabrication, using wide bandgap semiconductors, require development of plasma processing tools where both ion energy and radical fluxes can be controlled to obtain reasonable etch rates with minimal surface damage. Recent work indicates that reactive ion etching (RIE) process optimization can be achieved primarily through a consideration of plasma electrical properties [1,2], which may lead to improved material removal performance. The concept involves modifying the electronegative character of strongly attaching etchant gas discharges through dilution with an electropositive gas species such as Ar, N₂, or even a weakly electronegative gas such as O₂. This gas mixture changes the radio frequency (RF) current-voltage (I-V) phase shift from a capacitive maximum of -90° to a value in the range of -45°, corresponding to the transition from sheath to bulk dominated discharge regimes. The changes in I-V phase shifts increase power deposition efficiency in the plasma, which leads to a greater production of the ions and radicals required for material removal. The optimal discharge condition is a function of pressure, fractional dilution, and electronegativity of the total gas mixture. Several authors have demonstrated various aspects of this phenomenon [3-6]. However, Sobolowski, Langan, and Felker [1], and Langan et. al.[2] have conclusively correlated RF electrical measurements with optical emission data and dielectric (SiN, SiO₂) etch rates for NF₃, CF₄, and C₂F₆ diluted with Ar, He, O₂, N2, and N2O. Based upon their observations, and simple models of the bulk and sheath regions of a parallel plate RF discharge, they proposed this RIE discharge optimization scheme to be generic in nature and applicable to any RF discharge utilizing electronegative gases.