Methods of Suppressing Cluster Growth in Silane RF Discharges
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Methods of Suppressing Cluster Growth in Silane RF Discharges Masaharu Shiratani, Shinichi Maeda, Yasuhiro Matsuoka, Kenichi Tanaka, Kazunori Koga, and Yukio Watanabe Department of Electronics, Graduate School of Information Science and Electrical Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan ABSTRACT The effects of gas temperature gradient, pulse discharge modulation, hydrogen dilution, gas flow, and substrate materials on growth of clusters below about 10 nm in size in silane parallelplate RF discharges are studied using a high-sensitivity photon-counting laser-light-scattering (PCLLS) method. Thermophoretic force due to the gas temperature gradient between the electrodes drives neutral clusters above a few nm in size toward the cool RF electrode. Pulse discharge modulation is much more effective in reducing the cluster density when it is combined with the gas temperature gradient, and clusters above a few nm in size cannot be detected by the PCLLS method even for the discharge over a few hours. Hydrogen dilution and gas flow are also effective in suppressing growth of clusters, when the H2/SiH4 concentration ratio is above about 5 and the flow velocity is above about 6 cm/s, respectively. Cluster growth rate with a glass or Si substrate is found to be considerably higher than that without the substrate. INTRODUCTION Amorphous and microcrystalline silicon thin films are commonly produced using RF glow discharges of pure silane (SiH4) or SiH4+H2 mixtures. Since particles less than 10 nm in size (hereafter referred to as clusters) formed in the discharges have been believed to reduce the yield of fabricated devices and to degrade film quality [1,2], it is important to develop a method of suppressing growth of clusters. Recently, we have studied the effects of gas temperature gradient, pulse discharge modulation, H2 dilution, gas flow, and substrate materials on their growth in SiH4 RF discharges using a high-sensitivity photon-counting laser-light-scattering (PCLLS) method, which is applicable to the cluster size range from about 1 to 10 nm. In this article, we will report these experimental results and discuss the mechanisms bringing about the effects. EXPERIMENTAL Experiments were carried out using a conventional parallel plate capacitively coupled reactor [3-6]. Stainless steel (SS) plane electrodes of 100 mm diameter, the lower one of which was grounded, were placed 45 mm apart in the SS reactor of 300 mm in diameter. Compared to the spatial resolution (1 mm) of the PCLLS system, the electrode distance was large enough to clarify the effects of parameters such as H2 dilution on the particle growth region. A discharge frequency of 13.56 MHz was used and a power of 8-16 W (0.1-0.2 W/cm2) was supplied during a period of Ton = 0.2-250 s for a nonmodulated discharge. For a modulated discharge, periods of discharge-on and –off (hereafter referred to as RF-on and RF-off) in each modulation cycle were set at ton=5 ms and toff=0.1-10 ms, respectively. The grounded (GND) electrode temperature ranged fro
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