Wafer edge uniformity considering ion inertia in single- and dual-frequency capacitively coupled discharges

摘要

Summary form only given. Uniformity and edge effects (the perturbation of features near the edge of the wafer) are of increasing concern in low pressure plasma processing. Edge effects are usually caused by perturbation of reactant and ion fluxes produced by wafer termination and matching to tool materials. Very often these perturbations result from subtle geometrical features, such as the spacing between the edge of the wafer and focus rings. In capacitively coupled plasmas (CCPs), and dual frequency systems in particular, these issues are intrinsically coupled with ion inertia as ions are typically able to follow the variation in the low frequency electric field but not the high frequency source. In this paper, we discuss results from a computational investigation of the consequences of tool design on edge effects in CCP reactors. The effects of the focus ring height and methods of terminating the wafer will be discussed with respect to the uniformity and directionality of the ion fluxes near the edge of the wafer. Examples will be taken from single and dual frequency systems (tens to hundreds of mtorr) in Ar and Ar/Cl₂ chemistries. As these effects are sensitive functions of the geometry and ion inertia, improvements were made to a 2-dimensional plasma hydrodynamics model that is implemented on an unstructured mesh capable of capturing a large dynamic range in scale length. The effects of ion inertia are captured by solving the full momentum equations for ions. The coupling between the dynamics of the charged and neutral transport is through the species resolved collision terms in the momentum equations for charged and neutral species. The numerical implementation of these algorithms on an unstructured mesh will be briefly described