The reactive neutral and charged species generated from the plasma react with the

substrate to form volatile species. For example, in Cl2 chemistry, Cl and Cl2 neutrals and

charged Cl radicals form AsClx volatiles with a GaAs substrate. Chemical etching

occurs when the volatile species evaporate, reducing the substrate thickness usually

isotropically. In addition, the electrostatic fields created by the particles of the plasma

may accelerate charged species so that substrate material is removed due to the ion

bombardment, mechanical etching which tends to be anisotropic. Non-volatile, or less

volatile, etch products (GaClx) may also be removed through ion bombardment. It is the

relationship between the physical sputtering and the chemical reaction mechanisms that

greatly influence the dimensions of the features, both of which can be influenced by etch

parameters that will be discussed later in this paper. Selectivity, the ratio of etch rates

between different materials when exposed to the same environment [6], also is an

important factor in feature dimensions. Chemical etching is more selective than

mechanical etching and is inherently sensitive to differences in bonds and the consistency

of the substrate [3]. The degree of selective etching of one material relative to others can

most easily be manipulated through changes in plasma chemistry [6]. Materials may also

exhibit selectivity of one crystallographic plane over another as in GaAs. For GaAs in

Cl2 plasma, the relative etch rates of the crystal planes are As-rich

[111]>[100]>[110]>Ga-rich [111] [3].

 Gold Plasma Etching

 Gold has been determined to be relatively inert during plasma etching and

therefore has been used as a mask material when creating nanostructures, IC circuits, and

other microelectronic devices [7,8,9]. If gold is etched, it tends to be physically sputtered