Dependence of Fluorocarbon Plasma Chemistry on the Electron Energy Distribution Function

Recent diagnostics of high-density plasmas suggest a difference in the electron energy distribution function (EEDF) between a surface wave plasma and an inductively coupled plasma, which may be a main reason for the difference between the radical compositions of the two types of fluorocarbon plasmas at the same electron density. In order to account for the experimental result, a simple model is described based on a bi-Maxwellian EEDF with a bulk-electron temperature T b and a tail-electron temperature T t (>T b). For simplicity, the conventional C4F8 etching gas is replaced by feeding CF3, CF2 and CF radicals into a vessel. A set of particle-balance equations is formulated with wall reactions (radical sticking, charge neutralization) and inelastic electron collisions are taken into account. Numerical solutions for the discharge in 2 mTorr C4F8/18 mTorr Ar show that the plasma becomes less dissociative and CF x radical rich (F radical poor) with increasing T t and decreasing T b. For example, the radical density ratio of CF x /F at the electron density of 3×1011 cm-3 is two times larger in the bi-Maxwellian EEDF of T b=1.5 eV and T t=5.0 eV than in the `single' Maxwellian EEDF of T b=2.75 eV.