Study on the Growth of Microcrystalline Silicon Films in Atmospheric-Pressure VHF Plasma Using Porous Carbon Electrode

Abstract

Using atmospheric pressure (AP) plasma excited by a 150-MHz very high-frequency (VHF) power, the growth process of hydrogenated microcrystalline silicon (μc-Si:H) films was studied. A porous carbon electrode, which had originally been developed for the high-quality Si epitaxy at low temperatures, was used for the plasma generation in the area of 30 × 100 mm2. VHF power density (PVHF) and H2/SiH4 ratio were varied as parameters under a fixed substrate temperature of 220 °C. It was shown that μc-Si:H films having good uniformities of thickness and crystallinity in the area of 20×80 mm2 were obtained without being contaminated by dusty particles. Although increasing H2/SiH4 ratio led to the improvement of both deposition rate and film crystallinity, a crystallized Si film with a Raman crystalline volume fraction of ~72% was obtained with a high deposition rate of 5 nm s−1 at PVHF = 24 W cm−2, even if hydrogen was not added to the process gas mixture (H2/SiH4 = 0). Numerical simulations of the gas flow revealed that the process gas mixture was uniformly injected into the narrow plasma gap region from the opposite side of the substrate surface and suggested that the gas residence time in the plasma at the central part of the plasma region was long enough (>5 ms) to sufficiently deplete the source SiH4, promoting the crystallization of the growing film without the necessity of H2 addition.