Consequences of asymmetric pumping in low pressure plasma processing reactors: A three-dimensional modeling study

Low pressure (<10s mTorr), high plasma density (>1011 cm−3) reactors are rapidly becoming the tool of choice for etching during microelectronics fabrication. Although gas injection and pumping are well characterized process parameters in higher pressure systems, the impact of the symmetry of gas injection and pumping on the uniformity of reactant fluxes to the substrate are less well known in low pressure systems. In this article, results from a three-dimensional plasma equipment model are used to investigate the consequences of asymmetric gas pumping on reactive fluxes in inductively coupled plasma reactors operating in Cl2. We find that for typical conditions (10 mTorr, 150 sccm, 400 W) the azimuthal symmetry of ion density and ion fluxes are little affected by, for example, side pumping. This results from the dominance of ambipolar electric fields which are largely determined by the uniformity of ion sources and shape of the chamber. Neutral reactant fluxes do suffer azimuthal asymmetries due to pumping, however the variations are typically <10%–15% over a pressure range of 5–20 mTorr. The magnitude of the side-to-side variations is largely determined by the relative rates of loss of reactant species to surfaces compared to pumping.