Rapport final
(Anglais)
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The project aim was to test the feasibility and usefulness of Very High Frequency (VHF) plasma operation in large-area reactors suitable for volume production of solar cell panels, particularly with respect to microcrystalline silicon deposition for the new generation of IMT Neuchâtel .micromorph. solar cells. Faster deposition is necessary because the current rate of about 1 Å/s is prohibitively slow for industrial production. The goal of the project was to achieve 10 Å/s whilst maintaining a high degree of crystallinity, good adhesion (low stress), and suitable material properties such as low defect density.Two approaches for increasing the deposition rate of microcrystalline silicon were studied: an alternative plasma chemistry by addition of silicon tetrafluoride (SiF4) to standard silane / hydrogen mixtures, and an alternative plasma source using a high current DC arc.It was shown by experiment and analytical modelling that SiF4 contributes little to the deposition rate because the fluorine generated from the dissociation of SiF4 etches away the film as fast as it is deposited. The deposition rate of microcrystalline silicon is therefore determined by the silane flowrate, provided that the silane is almost completely dissociated by the plasma and the plasma composition is essentially hydrogen. The VHF technique is therefore well-suited for microcrystalline silicon deposition because of the high dissociation rates which can be obtained, for a given power, relative to the conventional 13.56 MHz frequency.Extremely fast deposition rates, up to 100 Å/s, were obtained in the high current DC arc reactor. The film was highly microcrystalline but with a defect density and uniformity insufficient for photovoltaic applications in these preliminary experiments. The rapid rate is due to the high power density in the arc compared with radio-frequency (RF) plasmas, and proves that there is no physical limitation to microcrystalline silicon deposition well above the required 10 Å/s.In conclusion, to increase the deposition rate of low defect density, uniform, microcrystalline silicon thin films, it is clear that the power density in large area RF reactorsmust be raised to several kW/m2. In this case, the high RF voltages which result in film damage from energetic ion bombardment can only be avoided by using higher VHF frequencies (above 60 MHz) which, unfortunately, cause non-uniform plasma power distribution in large area reactors. The optimum RF contact configuration for large area reactors was calculated for the design of future VHF high deposition rate reactors with 1 m2 electrodes.
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Documents annexés
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