Effect of the Grain Growth Process on the Characteristics for the Excimer Laser Crystallized Poly-Si Thin Film Transisto
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poly-Si film deposited by LPCVD.
A 125-nm-
thick SiO 2 insulating film was deposited by LPCVD at 4000C. Doped poly-Si deposited by LPCVD and Al were used to form the gate electrode. Posthydrogenation was performed by hydrogen plasma.
I1
2
,.. ,., . poly-Si ELC poly S y Fig. 1. Schematic cross-section of staggered TFT.
315 Mat. Res. Soc. Symp. Proc. Vol. 403 01996 Materials Research Society
RESULTS and DISCUSSION Grain growth process Figure 2 shows the SEM images of the Secco-etched poly-Si films. The grains grow gradually and continuously with the number of shots at energy densities of 339 and 424 mJ/cm'. The mean vrain diameter increases from 32 to 60 nm at 339 mJ/cm" and from 72 to 129 nm at 424 mJ/cm as the number of shots is increased from 1 to 50. The a-Si film is crystallized into fine grains of about 30 nm at 1 shot and 521 mJ/cm 2. Then the grain diameter decreases in spite of the increase in energy density at 1 shot and the morphology changes from that at lower energy densities. It is deduced from the decrease in grain diameter that the cooling rate decreases in spite of increase in energy density. The crystallinity degradation to microcrystalline or amorphous is often observed [9] at excessive energy densities, i.e., just below the abrasion energy. However, large grains are generated selectively from the matrix of fine grains through the additional shots at 521 mJ/cm2. The grain growth process seems to change. At more than 10 shots, the residual fine grains unite with the relatively small grains seen around the large grains. The mean diameter increases to 1040 nm at 50 shots. Figure 3 shows cross-sectional TEM images of the poly-Si films irradiated at 424 (a and b) and 521 mJ/cm 2 (c and d). Columnar grains are seen in the samples irradiated even at 1 shot and 424 mJ/cm2. The grains grow laterally with increasing number of shots. In the case of higher energy density of 521 mj/cm 2, granular grains at 1 shot grow into large columnar grains with increasing number of shots. The change in grain growth with energy density can be seen in the cross-sectional structure as well. a
1 shot
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20 d 50
339 m/rcmZ
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521
Fig. 3. Cross-sectional bright field TEM images of the poly-Si films irradiated at (a) 1 shot and 424 mJ/cm2, (b) 20 shots and 424 mJ/cm2, (c) 1 shot and 521 mJ/cm 2 , and (d) 20 shots and 521 mJ/cm2 .
Fig. 2. Change with laser conditions of the Secco-etched microstructure of the excimer laser crystallized poly-Si thin film.
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Figure 4 shows the change in the oriented structure obtained by X-ray diffraction (XRD) random 0.6 patterns with grain growth process. Only two peaks, (111) and (220), are clearly observed in the 2 XRD patterns of all samples. The peak intensity 521 mJ/cm a ratio of (220) over (111) for the sample at I shot 0.4 and 424 mJ/cm 2 is almost saturated with a smaller value (0.18) than that for a random structure (0.6). C; The intensity ratio depends only slightly on the 0.2 number of shots, and progress of the grain growth, Cn 2 424 mJ/cm+ at 424 mJ/cm2 . In co
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