Cluster-less Plasma CVD Reactor and Its Application to a-Si:H Film Deposition
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Cluster-less Plasma CVD Reactor and Its Application to a-Si:H Film Deposition Masaharu Shiratani, Kazunori Koga and Yukio Watanabe Department of Electronics, Graduate School of Information Science and Electrical Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan ABSTRACT Even under the conventional device-quality discharge conditions, large amount of clusters are found to exist in silane radio frequency (RF) discharges. The size and density of clusters near the substrate on the grounded electrode are 2 nm and 1011 cm-3, respectively. In order to prevent such clusters from depositing on hydrogenated amorphous silicon (a-Si:H) films, we have developed a cluster-less plasma CVD reactor which can significantly suppress cluster amount by using gas viscous and thermophoretic forces exerted on clusters, and by reducing gas stagnation region. Using the reactor, we have demonstrated deposition of ultra high quality a-Si:H films which have a significantly less concentration of Si-H2 bonds compared to conventional devicequality films.
INTRODUCTION While a-Si:H-based solar cells are promising as a clean and sustainable energy source, lightinduced degradation of a-Si:H films has been the most important issue over twenty years [1]. Recently, clusters below a few nanometers in size formed in silane RF discharges have been pointed out to be a possible cause of the light-induced degradation [2,3], while the correlation between clusters and the film quality has not been clarified yet. We have studied cluster growth mechanism and revealed many features [4,5]. By using two novel in-situ methods for measurement of size and density of clusters below 10 nm in size, we have revealed the facts that gas temperature gradient, gas flow, pulse discharge modulation and hydrogen dilution are effective in suppressing cluster growth [6,7]. In this paper, we report on existence of large amount of clusters under the conventional device-quality discharge conditions, features of a newly developed cluster-less plasma CVD reactor, and preliminary experimental results of a-Si:H film deposition obtained with this reactor.
CLUSTERS UNDER THE CONVENTIONAL DEVICE-QUALITY DISCHARGE CONDITIONS In order to reveal cluster amount under the conventional device-quality discharge conditions, experiments were carried out using a conventional parallel plate capacitively coupled reactor [6-9]. Stainless steel (SUS) plane electrodes of 100 mm diameter, the lower one of which was grounded, were placed 45 mm apart in the SUS reactor of 300 mm in diameter. 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 = 12.5 or 200 s. The grounded (GND) electrode temperature was set at 250Û&. Pure SiH4 was fed to obtain a nearly one-dimensional flow parallel to the electrode surfaces at a total flow rate of 5 or 10 sccm and at a pressure of 13.3 Pa. A5.6.1
cluster amount (cps)
cluster amount (×104 cps)
The size and density of 2 clusters were determined by the PCLLS method. Since this method has been
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