Microstructural control of amorphous silicon films crystallized using an excimer laser
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Microstructural control of amorphous silicon films crystallized using an excimer laser John Viatella and Rajiv K. Singh Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611 (Received 21 January 1997; accepted 9 February 1998)
A technique for microstructural control of excimer laser-annealed silicon thin films on SiO2 substrates has been developed. By using single-crystal photolithographically etched silicon seed wafers in intimate contact with the silicon films, we have shown that it is possible to spatially control nucleation. Transmission electron micrographs show the resultant microstructure to consist of large (,1 mm) grain structures in the area surrounding the seed contact, with distinct organization not previously observed. A theoretical discussion is presented to explain the observed phenomena. Also, results from a numerical simulation are given which outline the effects of the seed wafer on the resultant microstructure of the laser-annealed film, as compared to nonseeded areas. I. INTRODUCTION
II. EXPERIMENTAL
Thin film transistors (TFT’s) fabricated from polycrystalline silicon crystallized from an amorphous state are an important part of many large area electronics-onglass applications.1 One limiting factor for fabrication is the temperature limit resulting from use of an inexpensive substrate glass, such as Corning 7059, which has a working range below 600 ±C. Laser annealing of amorphous silicon thin films has been actively investigated2 because the process results in the highest carrier-mobility values observed, when compared to furnace annealing3 and rapid thermal processing.4 However, much work remains in understanding the transformation mechanisms and thermal effects of underlying layers.5 Also, there is a desire to control the number of grain boundaries in the active channel region of devices.6 Research in this area includes the super lateral growth (SLG) phenomenon observed by Im et al.,7 in which a small experimental window exists within which exceedingly large-grained poly-Si films can be obtained using pulsedlaser annealing. In this paper, we report results of experiments designed to artificially control the microstructure of laserannealed films. Given that nucleation of a molten Si film does not readily occur at the SiO2 interface and large (as much as ,500 K) undercooling values have been observed,8 it is possible to have external nucleation points by use of special seeds made from (100) Si wafers. The seed wafers consist of photolithographically etched pyramid-shaped structures of sizes on the order of microns. By contacting these seeds with the film during laser irradiation, it is possible to control heat flow spatially and tailor the solidification dynamics. The results show that by using this method, grain sizes up to 1.5 mm can be produced, which is nearly an order of magnitude larger than the grains formed under standard laser-annealing conditions.
Film samples were prepared by low pressure c
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