VHF LARGE AREA PLASMA PROCESSING ON MOVING SUBSTRATES

摘要

The production of amorphous and microcrystalline silicon, for solar cells and related applications, requires large area, high-deposition rate plasma reactors. Increasing the frequency from the conventional 13.56MHz into the VHF range has demonstrated higher deposition and etch rates and lower particle generation, a reduced ion bombardement and lower breakdown of process and bias voltages. But the use of VHF for large area systems leads to some problems. The non-uniformity of deposition rates increases due to the generation of standing waves and evanescent waveguide modes on the electrode surface. Hence, one possibility to process large area substrates is the use of a one-dimensional, extended, homogeneous plasma source in combination with a moving substrate. The requirements, which result from the deposition process and from the RF-engineering, corresponds with the developed plasma source, making use of operation frequencies in the VHF-range (50-100 MHz), almost perfectly. By using a source of 550mm in length experiments were done at 81.36MHz with RF power densities of 70-180mW/cm~2, silane/ hydrogen pressures of 5-30Pa and flow rates of 10-300sccm. The measured potential distribution error was +-2%. Optical emission spectroscopy delivered discharge intensity deviations of +-3-10%. Deposition rates up to 20μm/h (60A/s) for amorphous silicon and film thickness inhomogeneities of less than +-5% were achieved (with an area of the moved substrate of 30cmx30cm). Experimental results of the film properties and first p-i-n solar cells employing these high rate i-layers are discussed as a function of the deposition parameters.