Paper PS2-WeM4
Effects of Very High Frequency Source Mixing and Inter-electrode Gap on Plasma Characteristics

Wednesday, October 22, 2008, 9:00 am, Room 306

Capacitively coupled plasma (CCP) discharges are widely used for dielectric etching in the semiconductor industry. Very high frequency (VHF) power sources are being employed to generate plasmas for dielectric etching due to VHF’s various benefits including low plasma potential, high electron density, and controllable dissociation. Electromagnetic effects tend to make the spatial and temporal behavior of VHF plasmas complex with a rich set of new physics. If plasmas are generated using multiple VHF sources, one can expect interaction between the sources and plasma characteristics to be different from those due to individual frequencies. We investigate the effects of VHF frequency mixing on plasma characteristics in this presentation. The study is done for a range of inter-electrode gaps. Both computational modeling and experiments are utilized. Our plasma model includes the full set of Maxwell equations in their potential formulation. The equations governing the vector potential are solved in the frequency domain after every cycle for multiple harmonics of the driving frequency. Current sources for the vector potential equations are computed using the plasma characteristics from the previous cycle. The coupled set of equations governing the scalar potential and drift-diffusion equations for all charged species are solved implicitly in time. Plasma simulation results show that electron density is usually higher in the center of the chamber at high frequencies due to a standing electromagnetic wave. Electrostatic effects at the electrode edges tend to get stronger at low VHF frequencies. Electron energy distribution function (EEDF) appears to be two-temperature Maxwellian. Ion saturation current measurements using a Langmuir probe show that ion saturation current, and hence plasma density, peaks in the chamber center at high VHF frequencies. As power at low VHF frequencies is added, ion saturation current increases at the edge of the electrodes and electrostatic effects become stronger. Even at high VHF frequencies, inductive heating at the electrode edges becomes strong for small inter-electrode gaps. This tends to increase electron density at electrode edge relative to the chamber center. As the gap is increased, the plasma is able to diffuse to the chamber center. The electromagnetic effects that dominate near the chamber center become more important than electrostatic effects.