Ulf Stephan

Deposition Conditions for Large Area PECVD of Amorphous Silicon

The production of amorphous silicon devices usually requires large area, high-deposition-rate plasma reactors. Non-uniformity of the film thickness at high power and deposition rate is found to be an important factor for large area deposition. Increasing the radio frequency from the conventional 13.56 MHz up to VHF has demonstrated advantages for the deposition of a-Si:H films, including higher deposition rates and lower particle generation. The use of VHF for large area deposition leads to the generation of standing waves and evanescent waveguide modes at the electrode surface and on the power feeding lines. Thereby increasing the non-uniformity of the film thickness. The uniformity of the film thickness for an excitation frequency strongly depends on the deposition parameters e.g. pressure, input power, silane flow and the value of load impedances. With increasing exciting frequencies the range of deposition parameters for obtaining uniform films narrows. Subsequently it is shown that for a large-area plasma-box reactor (500 × 600 mm 2 plate size) with a double-sided RF electrode, the non-uniformity of the film decreases due to a homoge-neization of the electrode voltage distribution by using multiple power supplies and load impedances on the end of the RF electrode. The uniformity errors decrease from ±20% to ±2.4% (27.12MHz) and from ±40% to ±5.9% (54.24MHz). Experimental results of the film uniformity will be discussed in dependence on excitation frequencies and the deposition parameters.

VHF Large Area Plasma Processing on Moving Substrats

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/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 errors of ±3-10%. Deposition rates up to 20µm/h for amorphous silicon (60A/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.

A new helix plasma source for VHF PECVD of a-Si:H

Abstract The helix type plasma source is a resonator consisting of a cylindrical conductor, which is closed on one side by a bottom plate and an inner conductor of spiral form (helix) measuring λ 4 . This plasma source works at 30 to 200 MHz without a matching network. Undoped a-Si:H films were deposited using a He up-stream excitation of SiH4, injected near the substrate in a 20–80 Pa pressure regime. For all runs high gas flow rates of 500–1000 sccm He, with 93 sccm SiH4, were used to minimize contamination within the resonator; the wafer platen temperature was 350°C and the substrate was grounded. The helix plasma source is capable of obtaining deposition rates of 1.0 to 2.8 nm/s; the samples prepared had a high Tauc gap, Eg = 1.82–1.95 eV.