Reactive Ion Etching (RIE) uses a combination of chemical and physical reactions to remove material from a substrate; it is the simplest process that is capable of directional etching. A highly anisotropic etching process can be achieved in RIE through the application of energetic ion bombardment of the substrate during the plasma chemical etch. The RIE process thus provides the benefits of highly anisotropic etching due to the directionality of the ions bombarding the substrate surface as they get accelerated towards the negatively biased substrate, combined with high etch rates due to the chemical activity of the reactive species concurrently impinging on the substrate surfaces. The synergistic effect of ion bombardment on increased etch rates in the presence of chemically active species was first demonstrated and explained by Coburn and Winters, where they showed the significantly higher silicon etch rate in the presence of both Ar+ ion beam and XeF2 gas compared to either the Ar+ ion beam or the XeF2 gas only as illustrated in Figure 1. In the RIE process, the ions carry sufficient energy to break the chemical bonds of the atoms in the substrate that they impinge upon lowering the activation energy for the chemical etching reactions and thus increasing the reaction rates with the reactive neutrals that are also incident on the substrate surface. In certain etching chemistries, there may be reaction byproducts that are formed on the surface and act as inhibitors for the chemical etch processes. The energetic ions also act to remove such reaction byproducts from the surface through physical sputtering exposing the underlying material to be removed by the chemically reactive species. Thus RIE is sometimes also referred as Ion-Enhanced Etching or Reactive and Ion Etching. The highly directional nature of ions bombarding the surfaces leads to the anisotropy in the etch process as illustrated in the Figure 2.