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Phase-shift effect in capacitively coupled plasmas with two radio frequency or very high frequency sources Very high frequency ͑VHF͒ capacitively coupled plasma ͑CCP͒ 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.

Capacitively coupled plasma (CCP) discharges using high frequency (HF) and very high frequency (VHF) sources are widely used for dielectric etching in the semiconductor industry. A two-dimensional fluid plasma model is used to investigate the effects of interelectrode gap on plasma spatial characteristics of both HF and VHF CCPs. The plasma model includes the full set of Maxwell’s equations in their potential formulation. The peak in plasma density is close to the electrode edge at 13.5MHz for a small interelectrode gap. This is due to electric field enhancement at the electrode edge. As the gap is increased, the plasma produced at the electrode edge diffuses to the chamber center and the plasma becomes more uniform. At 180MHz, where electromagnetic standing wave effects are strong, the plasma density peaks at the chamber center at large interelectrode gap. As the interelectrode gap is decreased, the electron density increases near the electrode edge due to inductive heating and electrostatic electron heating, which makes the plasma more uniform in the interelectrode region.

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