VHF Large Area Plasma Processing on Moving Substrats
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VHF LARGE AREA PLASMA PROCESSING ON MOVING SUBSTRATS J. KUSKE , U. STEPHAN, R. TERASA1, H. BRECHTEL1, A. KOTTWITZ1 Forschungs- und Applikationslabor Plasmatechnik GmbH Dresden, D-01217 Dresden, Gostritzer Straße 61-63, F.R. Germany 1 Semiconductor and Microsystems Technology Laboratory, Dresden University of Technology, D-01062 Dresden, Mommsenstraße 13, F.R. Germany
ABSTRACT The production of amorphous and microcrystalline silicon, e.g. for solar cells, requires large area, high-deposition rate plasma reactors. Increasing the frequency from the conventional 13.56MHz up to VHF has demonstrated higher deposition and etch rates and lower particle generation, a reduced ion bombardement and lower breakdown, process and bias voltages. But the use of VHF for large area systems leads to some problems. The non-uniformity of deposition rate increases due to the generation of standing waves and evanescent waveguide modes at the electrode surface. 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, using deposition frequencies in the VHF-range (50-100 MHz), almost perfectly. Using a source of 550mm length experiments were done with 81.36MHz at RF power densities of 70-180mW/cm², 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 errors of ±3-10%. Deposition rates up to 20µm/h for amorphous silicon (60Å/s) and film thickness inhomogenities less than ±5% were achieved (with an area of the moved substrate of 30cm×30cm). Experimental results of the film properties will be discussed in relation to the deposition parameters and compared with complementary experiments, carried out on a small scale equipment with excitation frequencies up to 165 MHz. INTRODUCTION Increasing the frequency of discharge increases the growth rate of amorphous and microcrystalline silicon. But there are two essential problems. Higher frequencies lead to nonuniformities of the film thickness due to the generation of standing TEM waves and evanescent waveguide modes (TE waves) at the electrode surfaces. The plasma power will also be limited due to high power losses on contacts and transmission lines. We show that by using a one dimensional extended plasma source in combination with moving substrates, impedance transformers and an optimized power coupling, these problems are minimized [1].
A5.5.1
EXPERIMENT The experiments were carried out in two deposition equipments, a large area, non-resonant one dimensional extended plasma source (ODPS) and a small scaled equipment – the resonant helical resonator (helix). Depositions of amorphous and microcrystalline silicon were done at the one dimensional plasma source in a static mode (no substrate motion) and in a dynamic mode (with substrate motion) at
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