Comparison of Phase Diagrams for vhf and rf Plasma-Enhanced Chemical Vapor Deposition of Si:H Films
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A5.2.1
Comparison of Phase Diagrams for vhf and rf Plasma-Enhanced Chemical Vapor Deposition of Si:H Films G.M. Ferreira,1 A.S. Ferlauto,1 J.M. Pearce,1 C.R. Wronski,1 C. Ross,2 and R.W. Collins,3 1 Center for Thin Film Devices, Penn State University, University Park, PA 16802, USA; 2 Institut für Photovoltaik, Forschungszentrum Jülich, 52425 Jülich, Germany; 3 Department of Physics and Astronomy, The University of Toledo, Toledo, OH 43606, USA. Abstract Deposition phase diagrams are convenient for categorizing the evolution of the surface microstructure and phase with accumulated thickness for hydrogenated silicon (Si:H) films during plasma-enhanced chemical vapor deposition (PECVD). They can also be used to assess the electronic quality and device suitability of Si:H, based on previous correlations. In this study, phase diagrams have been applied in a comparison of Si:H PECVD using two different plasma excitation frequencies (rf: 13.56 MHz; and vhf: 60 MHz). Smooth crystalline Si (c-Si) wafer substrates have been used to obtain the surface roughness evolution with maximum sensitivity in the amorphous silicon (a-Si:H) growth regime. This study has shown that under all explored conditions of plasma power, frequency, and gas pressure, yielding deposition rates of 0.5-20 Å/s, a-Si:H exhibits improved microstructural characteristics with increasing H2-to-SiH4 flow ratio R right up to the amorphous-to-(mixed-phase microcrystalline) [a→(a+µc)] boundary of the phase diagram. For depositions at R values much lower than the a→(a+µc) transition for a thick film, vhf PECVD can provide a significant improvement in microstructural evolution over rf PECVD, for a given deposition rate. For optimum R just below the a→(a+µc) transition, however, vhf and rf a-Si:H films exhibit remarkably similar structural evolution for a given rate. Introduction In order to increase the deposition rate of Si:H by PECVD, thus increasing the throughput of solar cell production and lowering its costs, very high frequency (vhf) plasma excitation, typically within the range of 50 < f < 100 MHz, has been adopted as an alternative to conventional radio frequency (rf) excitation at f = 13.56 MHz [1-4]. Vhf excitation is generally considered to be more efficient in dissociating SiH4+H2 mixtures, thus permitting higher rate PECVD of a-Si:H and microcrystalline Si:H (µc-Si:H). The goal of this investigation has been to explore the effects of excitation frequency on Si:H PECVD in greater detail, comparing real time spectroscopic ellipsometry (RTSE) results for the microstructural evolution of f = 60 MHz (vhf) and f = 13.56 MHz (rf) Si:H materials. The information deduced from RTSE can be summarized succinctly using the deposition phase diagram concept wherein one or more transition boundaries are plotted in the plane of the H2-dilution flow ratio R=[H2]/[SiH4] and the accumulated bulk layer thickness db [5]. R is the deposition parameter exerting the strongest influence on the phase evolution and so is the natural one to be used as the diagram’s abscissa. Th
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