Facetting Growth of Low Temperature Polycrystalline Silicon by Ecr-Cvd with Hydrogen Dilution Method
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dilution. Based on this observation and the experimental results reported, a grain formation model is proposed to explain this peculiar phenomenon. Moreover, the roles of hydrogen dilution in the surface reactions kinetics, grain orientation determination and grain size enhancement are discussed. EXPERIMENT A PlasmaQuest electron cyclotron resonance chemical vapor deposition system with downstream gas inlet was used for thin film deposition. The polycrystalline silicon films were deposited on bare Si wafer, thermal oxidized Si wafer and coming 7059 glass substrates. Gases used for the hydrogen dilution method were pure hydrogen gas (upstream) and SiH 4/Ar (1:19) mixed gases (downstream). The deposition parameters for grain formation studies are 92-99% for the hydrogen dilution ratio (H 2/(SiH 4+H 2)), 250°Cfor the substrate temperature, 1200W for the microwave power and 20 mtorr for the process pressure. Plane-view TEM was used to determine the crystallization, grain size, grain shape and grain orientation. Cross-sectional TEM was used to provide information about grain growth of the silicon films. AFM topographic images for the poly-Si films were acquired in air with the PSI Autoprobe AFM/STM system. RESULTS AND DISCUSSION To investigate poly-Si grain formation, the deposition process was interrupted after 10, 30, 50, and 75 minutes, respectively. Fig. 1 shows the plane-view TEM dark field images of polycrystalline silicon films with different thickness (different deposition time) under the same deposition condition. As is clear seen, the poly-Si deposition exhibited 3-dimensional island growth, and the grain size is strongly dependent on the film thickness. The grain shape, as shown in Fig.2(a), is leaf-like and the maximum grain size is about 1p m. Fig.2(b) shows the corresponding diffraction pattern of single grain in Fig.2(a). From the diffraction pattern, we can identify that the longer side of the leaf-like grain is , and the shorter side is . The X-ray diffraction spectra (fig.3) show that the dominant crystal textures are and orientations. The leaf-like two-fold symmetrical grain shape and the corresponding crystallography diffraction pattern indicate the orientation of largest grain is < 110>. The dark field TEM image also shows the pyramid-like facets shape. Considering the effect of orientation on deposition rate and symmetry (fig.4), the possible facets orientation should be . The AFM images of low temperature poly-Si thin films deposited on bare Si wafer and the thermally oxidized Si substrate are shown in Fig.5. Except the difference of the ex-situ chemical cleaning processes, all growth parameters (microwave power: 1200W, substrate temperature: 250'C, process pressure: 20 mtorr, hydrogen dilution ratio: 98%, deposition time: 90 minutes) are the same. Their surface morphology statistic information are listed in table I. We can easily find that the surface morphology of poly-Si thin films deposited on these two different kinds of substrates are almost the same. Another unexpected phenomena which observe
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