Electronegative Plasma Reactor Engineering

Abstract

Low pressure (0.1 mtorr to 10 torr), cold (gas temperature ∼500 K), weakly ionized (degree of ionization 10⁻⁶-10⁻¹) glow discharge plasmas are used extensively in the processing of electronic materials, especially for etching and deposition of thin films1. Such plasmas also find application in surface modification (e.g., hardening, corrosion resistance), lighting, and even environmental remediation. Figure 1 is a schematic of a plasma etch process carried out in a capacitively-coupled reactor. The case of polysilicon etching in a chlorine plasma is shown as an example. The plasma is generated by applying radio frequency power between a pair of parallel plates in a low pressure chamber (Fig. 1a). The Cl₂ feedstock gas is attacked by plasma electrons to produce Cl radicals and Cl₂ ⁺ ions. Radicals diffuse or are convected by gas flow towards the wafer where they adsorb on the surface. Ions accelerate in the sheath naturally occurring over the wafer, and bombard the wafer vertical to its surface (Fig. 1b). The combination of radical and ion bombardment produces SiCl₄ product which desorbs and is removed by the gas flow. It is this directional ion bombardment which promotes anisotropic etching of microscopic features (Fig. 1c), whereby the film etches much faster in the vertical as compared to the horizontal direction. At the atomic level, ion bombardment produces a modified surface layer in which the reactant (Cl) is mixed within the silicon lattice (Fig. 1d) to a depth depending on the ion energy (∼ 10s of Å). The energy deposited by ions promotes the formation of products that are either sputtered away or desorb spontaneously in the gas phase. Figure 1 also demonstrates the disparity in length scales encountered in plasma processing. Wafers are now 300 mm in diameter, so the reactor scale is of the order of 10s of cm. The sheath thickness ranges from 0.1–10 mm depending on the Debye length and the voltage applied to the electrode.