Nucleation and Growth of Silicon Microstructures by Direct-Laser Writing
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NUCLEATION AND GROWTH OF SILICON MICROSTRUCTURES BY DIRECT-LASER WRITING
David E. Kotecki* and Irving P. Herman* * Physics Department, Lawrence Livermore National Laboratory, Livermore CA. 94550 and Department of Applied Science, University of California, Davis/Livermore, Livermore, CA. 94550 ** Department of Applied Physics, Columbia University, New York, N.Y. 10027 ABSTRACT This paper reports on a study of the nucleation phase of thin film growth during pyrolytic deposition by direct-laser writing. A Monte Carlo computer simulation is used to model the initial surface dynamics occurring during the deposition of silicon from silane (SiH 4) on strongly binding substrates such as silicon, and on weakly binding substrates such as silicon dioxide (Si0 2) and silicon nitride (Si 3N4). Results show that for high peak temperatures (_Ž 1200 K) and weakly binding substrates, the most probable location for the initiation of thin film growth is radialy displaced from the center of the locally laser-heated region of the surface. At low peak temperatures, thin film nucleation is found to begin at the center of the locally laser-heated surface for all substrates. INTRODUCTION Direct-laser writing (DLW) has become an important process for the direct fabrication of microstructures [1-4]. One critical element in thin film deposition is the nucleation phase, in which a supersaturated concentration of adatoms on the surface combine to form stable nuclei from which nucleus coalescence and growth proceeds. Practical application of DLW requires understanding the role of nucleation during thin film formation. The morphology and internal structure of deposited material, such as whether it is crystalline, polycrystalline, or amorphous, is strongly influenced by the nucleation phase of the growth process. In polycrystalline materials, the resulting grain size and grain boundaries influence the optical and electrical properties of the material. Also, when depositing materials onto weakly binding substrates, such as silicon onto substrates of silicon dioxide, (Si0 2), and silicon nitride, (Si3N4), the nucleation phase may be the rate limiting step in the deposition process. During DLW, the physical conditions which prevail axe radically different from those encountered during other thin film deposition processes such as sputtering, evaporation, molecular beam epitaxy and large-area chemical vapor deposition (CVD). In pyrolytic deposition by DLW, a focussed laser induces a temperature gradient across the surface with a spatial variation on the order of one micron. Parent molecules adsorbed onto the surface thermally dissociate into the atom being deposited plus a volatile product which eventually leaves the surface. Since the probability of molecular dissociation is temperature dependent, the formation of deposited atoms is also spatially varying over this one micron-dimension region. The most notable differences in thin films produced by DLW compared to those made by other deposition processes are the localized nature of the deposit and the
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