In-situ Measurements of Cluster Volume Fraction in Silicon Thin Films Using Quartz Crystal Microbalances
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In-situ Measurements of Cluster Volume Fraction in Silicon Thin Films Using Quartz Crystal Microbalances Yeonwon Kim1, Kosuke Hatozaki1, Yuji Hashimoto1, Hyunwoong Seo2, Giichiro Uchida1, Kunihiro Kamataki3, Naho Itagaki1, Kazunori Koga1 and Masaharu Shiratani1 1
Graduate School of Information Science and Electrical Engineering, Kyushu University, 744
Motooka, Nishi-ku, Fukuoka 819-0395, Japan. 2
Center of Plasma Nano-interface Engineering Graduate School of Information Science and
Electrical Engineering, Kyushu University. 3
Center for Research and Advancement in Higher Education, Kyushu University
ABSTRACT We have carried out in-situ measurements of cluster volume fraction in silicon films during deposition by using quartz crystal microbalances (QCM’s) together with a clustereliminating filter. The cluster volume fraction in films is deduced from in-situ measurements of film deposition rates with and without silicon clusters using QCM’s. The results show that the higher deposition rate leads to the higher volume fraction of clusters. INTRODUCTION Silane plasmas used for hydrogenated amorphous silicon (a-Si:H) thin film deposition have been studied over three decades [1-8]. A wide range of studies have shown that a low defect density a-Si:H (1015-1016 defects per cm3) films can be prepared, when SiH3 radicals are the main deposition precursors, a substrate temperature high enough (150-250oC) to provide high mobility to radicals on the surface, and promote cross-linking reactions leading to the elimination of excess hydrogen, typically 10% atomic hydrogen in the films [9]. Beside the improvement of the material properties, cost reduction is another key issue from industrial application viewpoint. This often implies increasing the deposition rate, which can be achieved by increasing the RF power, the pressure in the reactor, etc. However, such increase in deposition rate from typically 0.1 nm/s up to a few 10 nm/s generally results in the formation of clusters [10-13], and eventually in the deterioration of the a-Si:H film properties. In other words, clusters formed in silane plasma can be incorporated into films and such incorporation may change considerably their electronic and optical properties depending on their size, structure, and volume fraction [14-18]. Recently we have developed a cluster-suppressed plasma chemical vapor deposition (CVD) method by which clusters can be significantly suppressed in the upstream region due to high gas flow velocity enough to drive clusters toward the downstream region [19,20]. By using the method, we successfully deposited cluster-free a-Si:H films of 4.7x1015 cm-3 in stabilized defect density at a deposition rate of 3.0 nm/s [21]. The films show high stability against light exposure. This motivates us to develop a cluster-eliminating filter utilizing the different sticking probability of clusters and radicals [22]. Here we report in-situ measurements of cluster volume fraction (Vf) in silicon films. It is achieved by comparing deposition rates of the films with and with
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