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Collaborative Academic Papers

Control of ion flux-energy distributions by low frequency square-shaped tailored voltage waveforms in capacitively coupled plasmas

Capacitively coupled plasmas are routinely used in an increasing number of technological applications, where a precise control of the quantity and the shape of the energy distribution of ion fluxes impacting boundary surfaces is required. Oftentimes, narrow peaks at controllable energies are required, e.g. to improve selectivity in plasma etching, which cannot be realized in classical discharges. We combine experimental ion flux-energy distribution measurements and PIC/MCC simulations to provide insights into the operation and ion acceleration mechanisms for discharges driven by square-shaped tailored voltage waveforms composed of low-frequency (100 kHz) pulsed and high-frequency (27.12 MHz) signals. The formation of ion flux-energy distributions with a narrow high energy peak and strongly reduced ion fluxes at intermediate energies is observed. The position of the high energy peak on the energy axis can be controlled by adjusting the low-frequency voltage pulse magnitude and duty cycle. The effects of tailoring the driving voltage waveform by adjusting these control parameters as well as its repetition rate on the plasma operation and the ion flux-energy distribution are analysed in depth. We find, e.g. that the duty cycle regime (<40% or >60%) determines if the high energy ions form at the grounded or the powered electrode and that the duration of the pulse must exceed the ion energy relaxation time, on the order of 0.5 μs.

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Equivalent electric circuit model of accurate ion energy control with tailored waveform biasing

For atomic scale plasma processing involving precise, (an)isotropic and selective etching and deposition, it is required to precisely control the energy of the plasma ions. Tailored waveforms have been employed to bias the substrate table to accurately control this ion energy. Recent research has shown that switched-mode power converters can be used to generate this kind of waveform, with the benefit of increased energy efficiency and flexibility compared to the traditionally used linear amplifiers. In this article, an improved equivalent electric circuit model of the plasma reactor is proposed to allow simulation and bias waveform optimization. The equivalent electric circuit is analysed for different process phases, including the charge, discharge, and post-discharge phase. The proposed model is suitable for electric circuit simulation and can be used for predicting the electric waveforms and ion energy distributions. Plasma parameters are required as input for the model, thus an empirical parameter identification method based on the electrical measurements of the bias voltage and output current waveforms is introduced. Since these electrical measurements do not interact with the plasma process, the proposed parameter identification method is nonintrusive. Experiments have been carried out, which demonstrate that the proposed model and parameter identification method provide the expected accuracy.

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