Grain Formation in Polycrystalline Silicon Films Deposition on SiO 2 at Very Low Temperatures
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In this paper, we present some studies of grain size and morphology in high quality
polycrystalline silicon (poly-Si) films deposited at low temperatures by the hydrogen dilution 0 method utilizing electron cyclotron resonance chemical vapor deposition (ECR-CVD) at 250 C [1-5]. The largest grains were obtained with a 98% hydrogen dilution. Plan-view transmission electron microscopy (TEM) observations show that the largest grain size in the poly-Si film is about 1 jam and that the grains have a leaf-like shape. The grains look like upside-down cones when observed by cross-sectional transmission electron microscopy (XTEM) and atomic force microscopy (AFM). The crystalline fraction content of these poly-Si films as determined by Raman scattering spectra is nearly 100%. According to the X-ray diffraction patterns, the preferred orientations of the grains in the poly-Si films are and . In this paper, we describe grain formation in our poly-Si films. Our goal is explain the special geometry of the grains. A simple model is proposed to explain the formation of these grains. II. EXPERIMENTAL PROCEDURES Poly-Si films were deposited both on SiO 2 and glass substrates using the same process. Poly-Si films on SiO 2 substrates were used for TEM and AFM observation, and poly-Si films on glass substrates were used for Raman scattering and XRD measurements. Gases used for the 149
Mat. Res. Soc. Symp. Proc. Vol. 377 01995 Materials Research Society
hydrogen dilution methods were pure hydrogen gas and SiH 4 (5%)/Ar(95%) gas. The optimal values of deposition parameters, based on our experiments, are 1200W for the microwave power, 20 mTorr for the process pressure, 2500C for the substrate temperature. and 98% for the dilution ratio of H~i(Hit-SiH 4 ). Plan-view TEM imaging was used to determine the crystallinity, grain size, and grain shape of the deposited poly-Si films. XTEM images were used to provide information about grain growth of poly-Si. Atomic force microscopy (AFM) was applied to observe the surface morphology of the poly-Si films. XRD with 20=10-90' was employed to study the crystalline orientations in the poly-Si films. The broad amorphous silicon TO band located at around 480 cm"1 and the crystalline silicon TO band located at around 520 cm"1 of Raman spectra were used to determine the crystalline fraction of the poly-Si films. IIl. RESULTS AND DISCUSSION Figure I shows the plan-view TEM images and XTEM image of poly-Si deposited under the optimal conditions. The grain shape, as shown in Fig. I (a) bright field and (b) dark field images, is leaf-like and the maximum grain size is about I gim. The grains in the poly-Si film look like upside-down cones, as shown in Fig. I (c). Figure 2 shows the AFM images of poly-Si films grown by 98% hydrogen dilution ratio having different thicknesses, (a) 170 nm and (b) 1 gm. The grains are much smaller in Fig. 2 (a) than in Fig. 2 (b). The grain dimensions increase as the
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Fig. I TEM images of poly-Si film (98% hydrogen dilution) plan-view (a) bright field image and (b) dark
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