Guide for Low-Temperature and High-Rate Deposition of Device Quality Poly-Silicon Films By Cat-Cvd Method
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Table I . Deposition conditions for poly-Si films. FR(SiH 4 ) 1 sccm FR(H 2) 30 sccm Substrate temperature 400 'C Catalyzer temperature 1600, 1800 'C Gas pressure 0.001 - I Torr
Fig. 1. Schematic diagram of Cat-CVD apparatus. gases were introduced into a stainless-steel chamber through nozzles at the bottom. At the upper part of the chamber, a substrate holder with a heater was installed. Substrate temperatures were measured by a thermocouple attached to the substrate holder. Distance from the nozzles to the substrate holder was about 12 cm. A catalyzer wire was set at the middle between the nozzles and the substrate holder. A coiled tungsten (W) wire with a diameter of 0.75 mm was used as the catalyzer. Catalyzer temperature (T,.t) was measured by an infrared thermometer through a quartz window. Flow rate of Sill4 , FR(SiH 4), and that of H2 , FR(H 2), were fixed to 1 sccm and 30 sccm, respectively. Gas pressure (P.) was varied between 1 mTorr and 1 Torr by adjusting the main valve between the chamber and the pump. Deposition conditions for poly-Si films are summarized in Table I . Films were deposited on fused quartz substrates. Raman scattering measurements using the exciting light of 514.5 nm wavelength of an argon ion laser were carried out at room temperature. The fraction of crystalline phase to amorphous one was evaluated from the TO phonon signal of the Raman spectra. Crystalline fraction was determined from the areal ratio of the signal due to the crystalline phase around 520 cm' to the sum of the signals due to both the crystalline phase and the amorphous phase around 480 cm'. XRD technique using Cu K X-rays was used to investigate the orientation of poly-Si grains. RESULTS AND DISCUSSION High-rate deposition of Cat-CVD poly-Si film Relationship between the gas pressure and the deposition rate is shown in Fig. 2. As the gas pressure increases up to several-tens mTorr, the deposition rate increases linearly. However, the deposition rate starts to decrease above a critical gas pressure. This critical pressure for high T, appears lower than that for low Tal. The exact reason why this unique relationship is observed is not clear, however, it can be speculated as follows. As the gas pressure increases, the number of molecules colliding with the catalyzer increases proportionally to the gas pressure, and thus, the number of decomposed species increases proportionally to the gas pressure. Since the number of sites on the surface of W catalyzer used for the decomposition-reaction is limited, the number of decomposed species saturates above a particular gas pressure. On the other hand, as the gas pressure increases, the gas-phase elimination-reaction of deposition precursors such as SiH. (n=0, 1, 2, 3) becomes more active, and also, the number of H atoms which are active for etching of amorphous region on the growing film surface increases. Probably, the deposition rate starts to decrease due to these two reasons, when the gas pressure exceeds a certain critical value. The difference between Teat = 1800 'C and
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