Single-Crystal Si Films Via a Low-Substrate-Temperature Excimer-Laser Crystallization Method
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EXPER]MENTAL METHOD The samples consisted of Si substrates with a 1.9-+tm-thick SiO2 isolation layer, on top of which 2,000-A-thick a-Si films were deposited via LPCVD. The experimental apparatus consisted of an excimer laser operating at 308 nm (XeCI), a UV projection system, and a sub-micrometerprecision translation stage. The projection system included a variable-energy attenuator for fine control of the beam energy, and a two-element imaging lens for imaging the mask features onto the sample. The UV mask used in these experiments was fabricated out of molybdenum silicide on a quartz substrate, and the pattern consisted of an array of chevron-shaped apertures on an opaque background such that only the laser radiation that passed through the narrow slits was transmitted. The relative positions of the mask, imaging lens, and sample were adjusted to yield a pattern demagnification ratio of approximately 5X. At this demagnification, the image of each chevron had a slit width of 5 gtm at the sample plane, and was 50 jim across. The samples were translated for 50 jim. Processing was carried out by iteratively (1) irradiating the sample at an energy density sufficient to induce complete melting of the Si film in the exposed areas (approximately 900 mJ/cm2 for the 2,000-A thickness), and (2) translating it relative to the beam over a distance (typically 0.75 gim) approximately one-half of the single-pulse lateral growth distance. This process was repeated over a total translation distance of 50 jim. Subsequent to processing, the majority of the samples were defect-etched (Secco etchant), and examined via optical microscopy. Some samples were left unetched so that their surface morphology could be examined using a stylus-type profilometer. RESULTS Figure 1 shows a low-magnification optical micrograph of a defect-etched SLS-processed sample. Each individual crystallized region represents the area processed by a single chevron feature on the mask, which contained a regular array of many such chevrons. All of these regions were crystallized simultaneously; the total area that can be processed is limited only by the available laser energy. The number and locations of the individual regions can be controlled through design of the mask.
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FIG. 1. Low-magnification optical micrograph of an SLS-processed film. The arrow shows the solidification direction, and the magnitude of translation. Each crystallized region was processed by an individual beamlet, and the entire area was processed simultaneously.
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FIG. 2. High-magnification optical micrographs of SLS-processed films; (a) brightfield, with dotted lines demarcating the boundaries between the single-crystal and columnar crystal regions; (b) darkfield. Figure 2(a) is a higher magnification optical micrograph (also defect-etched), showing the region crystallized by a single chevron. Figure 2(b) is the corresponding darkfield image, which gives better contrast. The dashed lines in Fig. 2(a) delineate the boundaries between the singlecrystal region (I) and regions II, w
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