Nonthermal plasmas generated under atmospheric pressure (AP) have been receiving increased attention in direct plasma technology applications for thin film deposition. This is because the atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) is expected to realize low-cost and high-throughput processing with open air systems, which are of prime importance for various industrial applications. A large number of studies have been reported on the preparation of thin films using various types of AP plasma sources such as corona, dielectric barrier and AP glow discharges excited by pulsed or low-frequency power sources that can produce a nonequilibrium AP plasma. Most of the reported films using these common AP plasma sources have been related to polymers, oxides, and carbon materials. On the other hand, by virtue of the low ion energy due to the high collision frequency, AP-plasma process can have a nature of soft or gentle processing in addition to high-rate processing. Therefore, AP-PECVD also has a potential to form good-quality functional thin films, such as high-purity semiconductor or insulator thin films, which may be applicable for electronic devices. Although the development of AP-PECVD technology for such applications are attractive in the future advanced industry, the reports on these applications are limited. The reason may be related to the fact that the high collision frequency in AP plasma enhances secondary reactions in the gas phase to generate dust particles which will deteriorate the film quality, and also limits mass transport, which leads to poor uniformity of the resulting film. In the present article, the authors review the present status of AP low-temperature plasma processes, bearing in mind their application for high-purity functional thin films including silicon and related materials. The authors first summarize recent progress in the use of common AP plasma sources for direct PECVD processes. To grasp the present status of AP-PECVD technique, the authors have picked up popular materials for AP-PECVD, such as carbon, oxides, and other inorganic materials as well as silicon and related materials. Although there already exists a plenty of good review articles dealing with PECVD using common AP plasma sources, works on reviewing PECVD using radio-frequency (RF) and very-high-frequency (VHF) excitations of AP plasma seem to be insufficient. RF and VHF excitations of AP plasma are capable of generating continuous oscillating glow discharges without unstable streamers and filaments, which will be important to form uniform and dust-free films. So, secondly, the authors discuss the key distinguishing features of PECVD using RF and VHF excitations of AP plasma from the common AP plasma sources. Finally, they describe examples of the application of AP-VHF plasma to the preparation of silicon and related thin films.

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The low-temperature growth of epitaxial Si films by atmospheric pressure plasma chemical vapor deposition (AP-PCVD) was investigated. A 150 MHz very high frequency (VHF) power supply was used to generate an atmospheric pressure plasma of gas mixtures containing He, H 2 , and SiH 4 . Two types of electrode (i.e., cylindrical rotary and porous carbon electrodes) were used in plasma generation. When a cylindrical rotary electrode was used, polycrystalline Si growth was inevitable at the film edge on the upstream side. This is due to the variation in deposition rate along the gas flow direction, which is extremely high at the plasma/atmosphere interface on the upstream side. To solve this problem, we developed a novel porous carbon electrode where process gas molecules are directly supplied into the plasma region through a porous carbon plate a distance (0.8 mm) away from the substrate surface. Using such a porous carbon electrode, we successfully grew a defect-free epitaxial Si film on the entire surface of a 4 in. Si wafer at 600 C. The average growth rate was 0.25 -0.3 mm/min, which is as high as that obtained by thermal CVD at 900 C. The epitaxial Si films grown at 600 C were characterized by various methods, including transmission electron microscopy, atomic force microscopy, secondary ion mass spectrometry, and selective etching. The influence of adsorbed impurities in the porous carbon material on the quality of epitaxial Si films was also investigated.

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Epitaxial Si films have been grown at low temperatures (500 -700 C) by atmospheric pressure plasma chemical vapor deposition using a cylindrical rotary electrode. The variation in Si film quality across the entire deposition area has been investigated. It is found that defect-free epitaxial Si growth is possible at 500 C with sufficient plasma power in the central part of the film. At film edges, however, there are narrow regions of polycrystalline growth. From the deposition experiments under simplified boundary conditions (laminar gas flow and a uniform electric field in the plasma region), it is suggested that a large number of reactive precursors are generated when SiH 4 molecules enter the plasma region, and the growth rate at the film edge on the upstream side is too high for the growth of single crystals. On the other hand, polycrystalline growth at the film edge on the downstream side is related to particle formation.

To prepare polycrystalline silicon (poly-Si) films at low temperatures (<400°C) with high deposition rate, we propose a chemical transport method using atmospheric-pressure pure hydrogen plasma, called the atmospheric-pressure enhanced chemical transport method. In this method, high-pressure (200–760Torr) stable glow plasma of pure hydrogen was generated by a 150MHz very high frequency power between the two parallel electrodes less than 2mm apart. One of the electrodes is composed of the cooled Si solid source and the other the heated substrate (200–400°C). According to the temperature dependence of hydrogen etching rate of Si, SiHx species are mainly generated at the cooled Si solid source by hydrogen atoms. These species are again decomposed in the plasma, transported to the substrate to form Si films. In the present experiments on poly-Si film formation, a high deposition rate of 300nm∕min was achieved at 400°C, and nearly ideal utilization efficiencies (>93%) of Si solid source was realized in every condition. Si grains formed on a (001) Si substrate revealed anisotropic morphology elongated along ⟨110⟩ directions, and most of them had columnar structures epitaxially grown to the thickness of 2μm even at 200°C.

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