A voltage uniformity study in large-area reactors for RF plasma deposition

Non-uniform voltage distribution across the electrode area results in inhomogeneous thin-film RF plasma deposition in large-area reactors. In this work, a two-dimensional analytic model for the calculation of the voltage distribution across the electrode area is presented. The results of this model are in good agreement with measurements performed without plasma at 13.56 MHz and 70 MHz in a large-area reactor. The principal voltage inhomogeneities are caused by logarithmic singularities in the vicinity of RF connections and not by standing waves. These singularities are only described by a two-dimensional model and cannot be intuitively predicted by analogy to a one-dimensional case. Plasma light emission measurements and thickness homogeneity studies of a-Si:H deposited films show that the plasma reproduces these voltage inhomogeneities. Improvement of the voltage uniformity is investigated by changing the number and position of the RF connections.

Study of radio-frequency plasma deposition of amorphous silicon for the improvement of solar cell production

Juliette Ballutaud

Plasma enhanced chemical vapour deposition (PECVD) of thin films such as amorphous silicon has widespread applications especially in the field of photovoltaic solar cells and thin-film transistors for flat screen production. Industrial applications require high depositions rates over large areas with a good uniformity in layer thickness. In this thesis, some aspects of PECVD in large surface, industrial type, capacitive radio frequency reactor are investigated. The aim of this work is to study the plasma process conditions to increase the deposition rate of uniform, good quality, a-Si:H layer for solar cell application in a single chamber reactor. The studies realized during this thesis have necessitated the development and the comprehension of diagnostics such as deposition rate measurement by in-situ interferometry, plasma power measurement and layer density measurement by ellipsometry. During the thesis, we have also elaborated a matching-box circuit for using process frequencies of 27.12 MHz and 40.68 MHz. Deposition of a-Si:H in small electrode gap reactor has been studied. At present industrial reactors have a standard electrode gap of 2.4 cm. We modified a reactor to reach a small gap of 1.7 cm. It appears that we have obtained faster deposition rate in the small gap reactor but non-uniformity problems increase due to edge focusing and powder effects. One solution, based on a teflon jigsaw to keep the plasma away from the edge confined spaces, is proposed to suppress focusing effect but the operation parameter space is still reduced by the powder effect. Systematic measurements of a-Si:H layer density was also done by ellipsometry. It is shown that the layer density decreases when the deposition rate increases, independently of pressure, gas flow and frequency (27.12 MHz/40.68 MHz) of the plasma. At high deposition rate, 6 Å/s, only an increase of the process temperature from 200°C to 230°C can significantly improve the layer density. We have noted also a slight improvement of layer density for layers deposited in the small gap reactor. Nevertheless industrial constraints impose a process temperature of 200°C and a standard gap reactor. By optimising the process parameters, keeping the temperature process at 200°C, good quality, uniform, a-Si:H layer were deposited at 3 Å/s on 37 cm × 47 cm glass substrates at 40.68 MHz. A particular source of non-uniformity in large area reactor has been examined. In large area reactors, a perturbation in RF plasma potential, due to the electrode edge asymmetry, propagates towards the plasma center with a characteristic damping length λ. The variation of RF plasma potential at the edge implies a variation of the deposition rate across the reactor area and then a non-uniformity of the deposited layer. A theoretical study was done to understand of this phenomenon and experimental results confirmed the model. Finally, for solar cell applications, a study of the boron cross-contamination during solar cell deposition in a single chamber process has been done. During the deposition of the intrinsic layer, for a p-i-n cell, i-layer is contaminated by the residual boron radicals present in the reactor after the deposition of the boron-doped layer. This contamination decreases cell performance. Several reactor treatments have been tested to solve this contamination problem. The effectiveness of these treatments was evaluated by secondary ion mass microscopy (SIMS) measurements. It appears that an ammonia flush or a water vapour flush of a few minutes, between the deposition of the p-layer and i-layer, reduces the boron contamination at the p-i interface. The performance of cells made with these treatments, in a single chamber process, are comparable to performance of cells done in a multi-chamber process.

EPFL2004

Investigations of radio-frequency, capacitively-coupled large area industrial reactor

Benjamin Strahm

The constant energy consumption and world-wide demography expansion add to the potential risks of ecological and human disaster associated with global warming. This makes it a necessity to develop renewable energy technologies such as photovoltaic energy. These technologies already exist, but their cost has to be reduced in order to compete with well-established energies based on natural resources such as oil, coal or natural gas. A first step has been performed successfully in recent years in the field of low cost photovoltaic (PV) solar cells. Manufacturing equipment for large area (> 1 m2) amorphous silicon thin film solar cell production is now available. Even if this type of PV cell has lower energy conversion efficiencies (≈ 9 – 10 %) than other types of cells such as crystalline silicon cells (≈ 25 %), it has lower financial and ecological costs. Nevertheless, the next generation of large area silicon thin film PV cells promise higher conversion efficiencies (≈ 12 %) and stability under light exposure. This new type of PV cell is based on microcrystalline silicon grown on large glass substrates by plasma-enhanced chemical vapor deposition from silane (SiH4) and hydrogen (H2) gas, as for the previous amorphous silicon generation. Amorphous/microcrystalline silicon PV multi-junction cells require a thick (≈ 2 µm) microcrystalline intrinsic light absorber layer because of the need to fit the photo-generated current of the two stacked cells. This increases the cost of the final product since the deposition rate of microcrystalline silicon achieved nowadays is limited to a few Å/s, making the processing time very long. The enhancement of the deposition rate while maintaining a good material quality, i.e. at the boundary between amorphous and microcrystalline growth, is difficult because the phenomena involved in the deposition of microcrystalline silicon are not well understood, and the optimization is then generally performed empirically. The plasma composition is measured using Fourier transform infrared absorption spectroscopy and optical emission spectroscopy. It is shown that the deposited films can be classified into three categories (amorphous, transitional and microcrystalline) as a function of silane concentration in the plasma, while it is impossible to do so as a function of all other process parameters (silane input concentration, RF power, pressure, etc...) if they are all varied simultaneously. This means that the common way to deposit microcrystalline silicon by strongly diluting the silane with hydrogen (< 5 %) is not unique. This is because the plasma composition does not depend only on the gas composition, but also on the fraction of silane depleted in the plasma. Analytical and numerical plasma chemistry modeling show that this is because the silane concentration in the plasma determines the species flux towards the growing film surface. Hence, in agreement with existing phenomenological models of microcrystalline growth, the lower the silane concentration in the plasma, the higher the H to SiHx flux ratio towards the surface and the higher the crystallinity. This is used to demonstrate the feasibility of the growth of microcrystalline silicon in large area reactors using radio-frequency excitation (40 MHz) even from pure silane gas. Moreover, it is shown that the optimum in terms of deposition rate and deposition efficiency tends towards pure silane and not to the common H2-diluted regime. Following this conclusion, an optimization strategy is constructed by varying only the hydrogen flow rate and the working pressure. Guidelines for the selection of the initial process parameters are given in order to achieve high deposition rate (> 10 Å/s) and high gas utilization efficiency (> 80 %) of good quality microcrystalline silicon with a high input silane concentration (> 10 – 20 %) by using the optimization strategy. Furthermore, it is shown by using time-resolved plasma composition measurement and modeling that the time necessary to reach chemical steady-state is about 1 second in large area Plasma-Box™ reactors. This time is much shorter than times reported for small laboratory reactors, which are typically of about 1 minute. It is demonstrated by using two-dimensional modeling that this is due to back-diffusion of the silane molecules from the vacuum chamber to the plasma zone in laboratory reactors, whereas the plasma fills the whole volume in large area Plasma-Box™ reactors, making the plasma composition quasi-instantaneously uniform at plasma ignition. This is of importance for the film microstructure uniformity across the thickness of the layer. In large area Plasma-Box™ reactors, it is not necessary to use strategies such as H2-dilution profiling in order to eliminate the plasma-induced amorphous incubation layer at plasma ignition, because plasma composition suitable to grow microcrystalline silicon is reached directly from ignition. Finally, it is shown that the Telegraph effect and the standing-wave, which are the two necessary and sufficient distinct electromagnetic modes to determine the electromagnetic fields in a RF reactor, affects the film thickness and microstructure uniformity. It is shown that the film thickness variation at the reactor edges due to the Telegraph effect can be eliminated by symmetrizing the two electrodes. It is also shown that the standin-gwave affects significantly the film microstructure of microcrystalline silicon deposited from discharges with a silane input concentration about 10 %, whereas for low input silane concentrations (< 4 – 5 %) the microstructure is more or less uniform. This means that the higher the silane concentration, the better has to be the design of the lens-shaped electrode to compensate for the standing-wave in large area reactors.

EPFL2007

Electromagnetic sources of nonuniformity in large area capacitive reactors

Christoph Hollenstein, Alan Howling, Laurent Fabien Hubert Sansonnens

Theory and experiment show that two electromagnetic modes are necessary and sufficient to determine the field nonuniformity within a parallel-plate rf capacitive plasma reactor. These two modes give rise to the standing wave effect and the telegraph effect. The standing wave effect is associated with high frequencies in large reactors where the reactor size is larger than about a tenth of the vacuum wavelength of the rf excitation. The telegraph effect is associated with asymmetric electrode areas, which necessitates the redistribution of rf current along the plasma to maintain rf current continuity. (c) 2006 Elsevier B.V. All rights reserved.