Low-Temperature Formation of Device-Quality Polysilicon Films by cat-CVD Method
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Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
Heater
Table I. The deposition conditions of poly-Si films. FR(SiH 4) FR(H 2) Substrate temperature Catalyzer power Catalyzer temperature Gas pressure
0.5, 1.0, 1.5 sccm 30 seem 300 cC 1050 W 1600'C 1.2 mTorr
Fig 1. Schematic diagram of cat-CVD apparatus. 1.2 mTorr. The flow rate of SiH 4, FR(SiH 4), is varied using values of 0.5, 1.0 and 1.5 sccm, while the flow rate of H 2, FR(H 2), is fixed at 30 sccm. Deposition conditions of poly-Si films are summarized in Table I. The properties of another poly-Si film, which is deposited with FR(SiH 4) = 1.0 sccm and FR(H 2) = 30 sccm and annealed at 900 C in N 2 for lh is studied for comparison. Structural properties of poly-Si films are studied by the X-ray diffraction (XRD) technique using Cu K at X-rays and by the Raman scattering spectroscopy using the excited light of 514.5 nm wavelength of an argon ion laser. The cat-CVD poly-Si films are preferentially oriented along (220) direction. The grain size of poly-Si films is estimated using Scherrer's formula [11] from the FWHM of (220) peak. The fraction of crystalline to amrphous phase is evaluated from the TO phonon-made signal of the Raman spectra. Crystalline fraction is determined from the areal ratio of the signal due to the crystalline phase at 520 cm- to the sum of the signals due to both the crystalline phase and the amorphous phase at 480 cm". The transmission electron microscopy (TEM) is also used to observe the crystalline structure. Optical properties of poly-Si films are characterized by the absorption coefficient a, which is measured by optical transmittance measurement. Electrical properties of poly-Si films are characterized by the carrier mobility a and the barrier height O B at grain boundary. The mobility at of poly-Si films is obtained using the Van der Pauw method. The barrier height d) B is derived from the temperature dependence of ju. Additionally, to confirm the quality of cat-CVD poly-Si, a "ITT is finally fabricated by using cat-CVD films. RESULTS AND DISCUSSION Structural properties of poly-Si films Figure 2 shows the XRD patterns for cat-CVD poly-Si films. For films deposited with FR(SiH4 ) = 0.5 and 1.0 sccm and annealed poly-Si films, diffraction signals of (220) peak are clearly observed although for the films deposited with FR(SiH 4 ) = 1.5 sccm no diffraction signal is observed. The grain size is estimated to be about 370 A for FR(SiH 4) = 0.5 sccm, about 270 A for FR(SiH 4 ) = 1.0 sccm and about 740 A for annealed sample, respectively. The Raman spectra for cat-CVD poly-Si films are shown in Fig.3. For the film deposited with FR(SiH 4) = 0.5 sccm and annealed sample, TO photon signal at 520 cm4 is clearly observed and that at 480 cmn is scarcely observed. However, with an increase in FR(SiH 4) from 0.5 sccm to 1.5 sccm, the signal at 520 cmn becomes small and that at 480 cma becomes large. These films consist of the mixture of crystalline and amorphous phases. Crystalline fraction estimated is about 70 % for FR(SiH
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