Advanced control of plasma characteristics by sidewall impedance tuning in 60 MHz capacitively coupled plasma systems

In 60 MHz high-frequency capacitively coupled plasma (CCP) systems, precise control of spatial plasma distribution near chamber sidewalls is essential for process uniformity. While previous simulation-based studies suggested that sidewall impedance affects plasma behavior, this work experimentally validates the concept. A ceramic sidewall with an embedded copper antenna was connected to an external tunable circuit (0.2 μH inductor in parallel with a ~100 pF capacitor), enabling impedance tuning from capacitive to inductive. This tuning was effective at 60 MHz but negligible at 13.56 MHz, confirming frequency dependence.



Experiments were conducted in argon plasma at 500 W under 50, 100, and 200 mTorr to evaluate pressure and impedance of sidewall effects. Diagnostics included electron density profiles, ion energy distribution functions (IEDFs), and optical emission spectroscopy (OES). Calibrated VI probes were installed at the powered top electrode (TE), grounded bottom electrode (BE), and the tunable sidewall to analyze voltage and current waveforms. From this, we extracted ΔV_TE–BE and ΔV_Wall–BE to infer current paths and power deposition.



A localized density peak (~+3 to +4 mm) was observed near the baffle, confirming active modulation via impedance tuning. VI and OES data showed consistent correlations among RMS values, intensity, and density. The impact increased with pressure. When ΔV _TE–BE > > ΔV_Wall–BE, power favored the TE, reducing edge density. The opposite condition increased sidewall density. This provides a practical method for tuning plasma uniformity in high-frequency CCP systems.