Crystallinity Uniformity of Microcrystalline Silicon Thin Films Deposited in Large Area Radio Frequency Capacitively-coupled Reactors

Abstract

ABSTRACTThe 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.