Spatiotemporal analysis of the double layer formation in hydrogen radio frequency discharges

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Abstract
Radio-frequency discharges at 13.56 MHz in pure H2 and CH4-SiH4 mixtures highly diluted in H2 are investigated by spatially and spatiotemporally resolved emission spectroscopy. Spatial emission profiles of the excited species exhibit double layers near the electrodes in pure hydrogen and hydrogen-controlled discharges. Spatiotemporal analyses give complementary information showing that the emitting layers, which result from atomic and molecular excitations, occur at different times in the radio-frequency cycle. The mechanisms involved in the double layer formation are interpreted by simplified numerical models based on both a fluid model and a particle-in-cell-Monte Carlo model. The results of the models are in good agreement with the spatiotemporal representations of the excitation rate obtained experimentally. The formation of the double layers is analysed as resulting from the high drift velocity of the hydrogen ions. This behaviour seems to be characteristic of radio frequency discharges in hydrogen.