Control of plasma uniformity in a capacitive discharge using two very high frequency power sources

Very high frequency (VHF) capacitively coupled plasma (CCP) discharges are being employed for dielectric etching due to VHF's various benefits including low plasma potential, high electron density, and controllable dissociation. If the plasma is generated using multiple VHF sources, one can expect that the interaction between the sources can be important in determining the plasma characteristics. The effects of VHF mixing on plasma characteristics, especially its spatial profile, are investigated using both computational modeling and diagnostic experiments. The two-dimensional plasma model includes the full set of Maxwell equations in their potential formulation. The plasma simulation results show that electron density peaks at the center of the chamber at 180 MHz due to the standing electromagnetic wave. Electrostatic effects at the electrode edges tend to get stronger at lower VHFs such as 60 MHz. When the two rf sources are used simultaneously and power at 60 MHz is gradually increased, the ion flux becomes uniform and then transitions to peak at electrode edge. These results are corroborated by Langmuir probe measurements of ion saturation current. VHF mixing is therefore an effective method for dynamically controlling plasma uniformity. The plasma is stronger and more confined when the 60 MHz source is connected to the smaller bottom electrode compared to the top electrode.