Grain Boundary Location-Controlled Polt-Si Films for Tft Devices Obtained Via Novel Excimer Laser Process
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903 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
predetermined regions of the Si film so as to allow controlled lateral solidification to initiate from the other, partially melted portions of the film and proceed into the completely melted regions. Experimental Method A photolithographically patterned Si0 2 layer on top of a 1000A-LPCVD a-Si film was chosen to induce spatial variation in the amount of energy absorbed by the Si film, which was on top of a thickly oxidized (2 gm) Si substrate (Figure 1). A particular SiO 2 layer thickness of approximately 500A was selected so as to induce the antireflective coating effect [6] while minimizing the conductive cooling of the a-Si film by the capping layer during the initial heating period. The samples were irradiated inside the hot-stage equipped vacuum chamber with a 30-nanosecond, 308-nanometer XeCl excimer laser pulse. Si films with stripe patterned capping oxide (stripe widths of 2 gm and 5 gim, separation distance of 1.5 gim) were irradiated at various energy densities and substrate temperatures. After irradiation, the microstructures of irradiated films were analyzed using Transmission Electron Microscopy (TEM). As previously observed [41, comparison of irradiation experiments involving Si films with and without the continuous oxide layer showed a substantial difference in2 critical incident energy densities required to completely melt the Si film (280 mJ/cm and 350 mJ/cm 2 at room temperature for capped and uncapped samples, respectively). Results TEM micrographs illustrated in Figure 2 show the most prevalent microstructures of crystallized Si films with a 2-gm-wide patterned capping layer (separated by 1.5-jm cap-free region) and which were irradiated at various incident energy densities at a substrate temperature of 150'C. Figure 2a shows the typical small-grained poly-Si films that are obtained when the incident energy density corresponds to the value that induces partial melting in both the capped and uncapped films (i.e., within the low-energy-density regime for both configurations). Careful inspection of the capped and uncapped portions of the film reveals that slightly larger grains are obtained, for this particular energy density, under the capped region than within the uncapped region.
500 AS1i02 1000 A LPCVI
a-Si Film
S102
S102
a-Si
I
Oxidized Si Substrate
Figure 1. Schematic of the sample configuration.
904
a
b
C
Figure 2. Planar view bright field TEM images of single-pulse-irradiated Si films with oxide stripes on top (2-gm-wide/1.5-gm-separated) at various energy densities (with a substrate temperature of 1500 C) (a) 190 mJ/cm 2 , (b)310 mJ/cm 2 , and (c) 385 mJ/cm2 . Figure 2b shows the GLC microstructure, which is obtained when the incident energy density is such that it leads to complete melting of the oxide-capped film but leads only to partial melting of the uncapped films (i.e., corresponds to the low-energydensity regime for the uncapped films, but corresponds to the high-energy-density for the capped f
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