Crystallinity Uniformity of Microcrystalline Silicon Thin Films Deposited in Large Area Radio Frequency Capacitively-cou
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1066-A01-01
Crystallinity Uniformity of Microcrystalline Silicon Thin Films Deposited in Large Area Radio Frequency Capacitively-coupled Reactors Benjamin Strahm, Alan A. Howling, and Christoph Hollenstein Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, CH-1015, Switzerland ABSTRACT The microcrystalline silicon (μc-Si:H) intrinsic layer for application in micromorph tandem photovoltaic solar cells has to be optimized in order to achieve cost-effective mass production of solar cells in large area, radio frequency, capacitively-coupled PECVD reactors. The optimization has to be performed with regard to the deposition rate as well as to the crystallinity uniformity over the substrate area. The latter condition is difficult to achieve since the optimal solar grade μc-Si:H is deposited at the limit between a-Si:H and μc-Si:H material, where the film crystallinity is very sensitive to the plasma process. In this work, a controlled RF power nonuniformity was generated in a large area industrial reactor. The resulting film uniformity was studied as a function of the deposition regimes. Results show that the higher the input silane concentration, the more the uniformity of the crystallinity is sensitive to the RF power nonuniformity for films deposited at the limit between a-Si:H and μc-Si:H. The effect of the input silane concentration on the microstructure uniformity could be explained on the basis of an analytical plasma chemistry model. This result is important for reactor design. In reactors generating nonuniform plasma the input silane concentration has to be limited to low values in order to deposit films with uniform microstructure. To benefit from the high silane flow rate utilization fraction encountered only for higher input silane concentration, the RF power distribution has to be as uniform as possible over the whole substrate area.
INTRODUCTION Plasma enhanced chemical vapour deposition of silicon for large area microelectronics has to fulfil various objectives to achieve a successful production of devices such as TFT displays or low cost thin film photovoltaic solar cells. High deposition rate (≈10 Å/s) of silicon film, especially of microcrystalline silicon (μc-Si:H), is one of these objectives which for many years has been the centre of interest of research groups all around the world. The quality of the deposited films is of course also one of the major concerns to reach cost effective production of high-efficiency solar cells. Therefore, properties such as the crystalline fraction or the hydrogen content of the film have to be as uniform as possible (less than ± 10 %) over the whole substrate area, similarly for the film thickness uniformity. The film uniformity becomes a crucial issue when processing large area devices that nowadays can reach sizes of several square meters [1, 2]. Up to now, studies on the uniformity are mainly performed by industrial companies due to the scarcity of large area equipment in academic research laboratories. Moreover
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