Dependence of Microcrystalline Silicon Growth on Ion Flux at the Substrate Surface in a Saddle Field PECVD
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Dependence of Microcrystalline Silicon Growth on Ion Flux at the Substrate Surface in a Saddle Field PECVD Erik Johnson, Nazir P. Kherani, Stefan Zukotynski Department of Electrical and Computer Engineering, University of Toronto Toronto, ON M5S 3G4, Canada ABSTRACT The Saddle-Field Glow Discharge PECVD system emulates RF-like excitation using a semi-transparent anode and a DC power supply. It has been used to deposit high quality amorphous and microcrystalline hydrogenated silicon thin films in the past. The growth of microcrystalline material is particularly sensitive to the conditions under which it is produced. Significant levels of microcrystallinity are only produced under conditions of higher pressure and electrical isolation of the substrate surface from the grounded substrate holder. We present results of a study on the relationship between substrate electrical potential and microcrystalline growth, as quantified by Raman scattering spectroscopy, at growth pressures near the minimum required for microcrystalline growth. INTRODUCTION Microcrystalline silicon (µc-Si:H) has been extensively researched as a promising material for low-cost optoelectronics grown at low temperatures. It can be produced in a plasma discharge through dilution of silane with another gas, such as hydrogen or argon [1], or by alternatively igniting silane and hydrogen/argon discharges for differing time lengths [2]. It can also be produced using other growth methods not requiring dilution or cycling, such as hotwire/catalytic CVD [3]. The precise electromagnetic means through which a silane/hydrogen plasma is produced (DC-PECVD, RF-PECVD, VHF-PECVD, ECR, RETP) seems to have little effect on the microcrystallinity of the material produced, though each technology has different advantages (high growth rates) and challenges (scalability) that must be managed. The SaddleField PECVD (SF-PECVD) is no exception, and it had been previously reported that µc-Si:H could only be grown in the SF-PECVD system under very specific growth conditions, including electrically floating the substrates [4,5]. It was proposed that the floating condition allowed the substrate surface to charge up, and thus shield itself from the higher ion energy and flux that occur due to the dilution of silane with hydrogen. This is not, however, a desirable circumstance when trying to optimize a SF-PECVD system to grow µc-Si:H. A floating surface in plasma will adopt a potential partially based on the relative areas of the surface, the driven electrode, and the grounded walls, so the quality of film grown in the SF-PECVD will vary with the surface area of the substrate. The goal of this study was to understand the role of surface potential during the growth of µc-Si:H in the SF-PECVD, and then establish it as a design variable by externally manipulating it. Samples were grown at different pressures and surface potentials, and the microcrystallinity of the as-grown material was quantified by Raman scattering spectroscopy at low laser intensity. These results were the
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