Use of an auxiliary electrode to control plasma properties at the wafer edge in inductively coupled plasmas with a substrate bias

In plasma etching of microelectronics devices, a continuing challenge is maintaining uniform fluxes of radicals and ions, and ion energy and angular distributions (IEADs) to the edge of the wafer. In plasma etching of high aspect ratio features requiring ion energies of several keV, ions incident onto the wafer should have trajectories that are normal to the wafer. At the edge of the wafer, the transition between the wafer and structures surrounding the wafer (often called the focus ring) produce curvature to the sheath responsible for accelerating ions into the wafer. The result is an angular tilt away from the normal of incident ions onto the edge of the wafer. In this paper, we discuss results from a computational investigation of using an auxiliary electrode (E2) surrounding the primary bias electrode (E1) to control sheath curvature at the edge of the wafer. The test system is an inductively coupled plasma sustained in an Ar/O2 mixture with a 5 MHz bias applied to the substrate E1. We found that with optimum placement of and voltage applied to the auxiliary electrode E2, IEADs incident onto the wafer could be made uniform to the edge of the wafer. The use of the auxiliary electrode E2 does affect the characteristics of E1, for example, modulating the DC bias and power deposition; however, these effects are small. The use of E2 does bring additional complexities and costs, which should be considered in optimizing the system.