Phase shifted harmonics for RF bias

harm1An interesting paper from Thomas Mussenbrock’s Lab using a phase shifted harmonic of the source bias in an inductive discharge to drive the bias electrode.  Their work shows that there is a synchronization of the bias field and inductive field that enhances power efficiency.

It seems clear to me that the source power needs to be coupled efficiently to maximize the plasma production and ion flux. Provided there is no other resistance in the path that is preventing the power getting to the plasma. However, the bias power is a different story.  Here maximizing the power to the electrode does not necessarily maximize the ion energy. This might seem counter intuitive but remember the plasma has an impedance and the electrode sheath is an part of the circuit.  Driving power in at a resonance may not increase ion energy but leave the ion energy very unstable.   Reconstructing a waveform to maximize the ion energy for a specific plasma impedance seems a much more reliable and stable solution.  Of course, this would require smarter matches and power sources.

Comments and questions are welcome, what do you think?

Enhanced power coupling efficiency in inductive discharges with RF substrate bias driven at consecutive harmonics with adjustable phase

November 2017
Applied Physics Letters 111(20):201601
DOI: 10.1063/1.5000144

Researchgate project



Inductive discharges with radio-frequency (RF) substrate bias are frequently used for various technological applications. We operate such a hybrid discharge with a phase-locked RF substrate bias at twice the frequency of the inductive coupling with fixed but adjustable phase between both RF sources in neon at low pressures of a few Pa. The ion flux to the substrate is found to be a function of this relative phase in the H-mode at constant RF powers as long as some residual capacitive coupling of the planar coil is present. For distinct choices of the phase, Phase Resolved Optical Emission Spectroscopy measurements show that energetic beam electrons generated by the expanding boundary sheaths (i) are well confined, (ii) are accelerated efficiently, and (iii) propagate vertically through the inductive skin layer at the times of maximum azimuthal induced electric field within the fundamental RF period. This enhances the inductive stochastic electron heating, the power coupling efficiency, and finally the ion flux.

Mike B Hopkins

Impedans Ltd

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