Growth of Polycrystalline Silicon Films at Low Temperature by Plasma Enhanced Chemical Vapor Deposition
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Fig. 1 Film deposition setup for PECVD Substrates aretomounted on thewith upper A stainless steel mesh (#100/inch) was attached the cathode thepowered spacing electrode. of 5 cm between the mesh and anode. The experiments were carried out at a constant pressure of 0.09 Tort. The substrate temperature was measured by means of a small thermocouple in contact with the substrate surface. An applied rf power of 75W made the substrate temperature by less than 8 °C. Borosilicate glass (TEMPAX, Asahi Glass Co.) with an area of 5 cm2was used as a substrate. Prior to the introduction of source gas (Sill4), the deposition chamber was evacuated by means of a high vacuum system down to a pressure of 5 x 10.6 Torr and then a soft hydrogen plasma was applied in the system for one hour at a pressure of 0.2 Torr. The thicknesses of the films deposited on glass substrates were measured by ellipsometry (Niic, EL-1001), Dek-Tak measurements and interference patterns in the visible and near IR region[4]. The average deposition rate was estimated by the deposition time and thickness. The films thicker than 2 ttm were used for the estimation of the deposition rate and X-ray diffraction spectroscopy. The crystallinity and other properties of silicon films were investigated by means of X-ray diffraction (40KV, 20mA, CuK a) (XRD), reflective high energy electron diffraction (RHEED) and Raman spectroscopy (Ar, 514.2nm, 50roW).
RESULTS AND DISCUSSION Figure 2 shows the deposition rate as a function of power at a substrate temperature of 400 °C. In low power region, the film deposition rate initially increases with applied of power and then reaches a balanced shoulder in the power range of 50-60W. When the power is furthermore increased, the deposition rate rapidly increases. In the low power region, the deposition rate is increased due to the increase of precursor formation. In the power range of 50-60 W, the constant rate seems to be caused by the balance between deposition and etching which results from the breakage of weak Si-SiHx bonds with atomic hydrogen [3,5,6]. The rapid increase of the growth rate for over 60W power can be
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Fig.3 RHEED patterns obtained by the samples of (a), (b), (c) and (d); conditions are indicated in Fig.2.
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attributed to the increase of the ionic precursors falling on the cathode. Broken bonds help the precursor to adhere onto the growing surface. To elucidate the effect of ionic flux, a mesh was attached to the cathode and the deposition rate was measured. (see Fig.2). The saturation of the deposition rate occurs around the same power as that for the mesh-free case. In the high power region, the deposition rate gradually decreases. It seems that the ionic flux does not reach the growing surface; and in the high power region, an increase of hydrogen radicals increases etching. Due to the chemical equilibrium cond
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