semiconductor processing [1]. However, it is best suitable for whole surface etching and

not for patterning as a major disadvantage is the undercutting of the material below the

mask, resulting in a loss of resolution of the final features [1]. Thus, regardless of the

increased costs, industry has adopted the dry etch method to achieve the small features

needed to keep up with the demands for smaller and smaller devices [1, 3, 5].

 Plasma etching is a dry etching method and is of particular interest because of its

efficiency in producing micron-sized features of specific dimensions and extensive use in

the semiconductor industry. In general, a plasma can be described as an ionized gas that

is charge neutral. This means that there are equal numbers of free positive and negative

charges [1, 3]. The charges are generated when a voltage is applied between two

electrodes inducing a current flow to form the plasma as shown in Figure 3. The positive

charge is mostly in the form of singly ionized neutrals, including atoms, radicals, or

molecules, that have been stripped of one electron [3]. The negative charges are usually

free electrons [3]. As the diffusion of charge to the walls and recombination on the

sample surface occurs, there is a depletion of charge in the adjacent gas. Because the

electrons are light and have high energy, they diffuse the fastest, leaving a thin boundary

layer of positive charge referred to as the sheath [3]. Positive ions are accelerated

towards the surface of the substrate which has accumulated a negative charge from the

electrons that have diffused to the substrate and strike at near-normal incidence. This

causes directional etching and results in features with vertical sidewalls. Figure 4

illustrates the sheath and near-normal incidence of the ion striking to produce vertical

sidewalls.